Fiber Rope GN June21
Fiber Rope GN June21
Fiber Rope GN June21
These Guidance Notes supersede the ABS Guidance Notes on the Application of Synthetic Ropes for
Offshore Mooring, 1999. The main purpose of these new Guidance Notes is to reflect the latest technology
developments and industry practice for applications of fiber ropes in offshore mooring systems.
These Guidance Notes provide detailed guidance for three fiber materials: polyester, HMPE (high modulus
polyethylene), and aramid (aromatic polyamide). This does not exclude the use of other fibers in the design
of mooring systems, provided that good engineering practice is followed, all relevant fiber properties are
considered and justification for the use is adequately documented. Designers of mooring system are
encouraged to consult fiber rope experts and manufactures when other rope materials are considered.
The June 2021 edition aligns the tension and fatigue design criteria with the ABS Guide for Position
Mooring Systems.
These Guidance Notes become effective on the first day of the month of publication.
Users are advised to check periodically on the ABS website www.eagle.org to verify that this version of
these Guidance Notes is the most current.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 ii
GUIDANCE NOTES ON
THE APPLICATION OF FIBER ROPE FOR OFFSHORE
MOORING
CONTENTS
SECTION 1 General................................................................................................10
1 Scope............................................................................................10
2 Definitions..................................................................................... 11
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 iii
6 Stiffness Values for Preliminary Design........................................ 24
7 Determination of Stiffness Based on Test Data............................ 24
8 Mooring Analysis Procedure.........................................................24
8.1 Major Conclusions from Parametric Studies....................24
8.2 Analysis Procedure Based on the Static-Dynamic
Model............................................................................... 24
8.3 Analysis Procedure Based on the Upper-Lower
Bound Model....................................................................24
9 Mooring Analysis Examples..........................................................25
10 Creep............................................................................................ 25
11 Fatigue..........................................................................................25
11.1 Tension-Tension Fatigue..................................................25
11.2 Axial Compression Fatigue..............................................26
12 Torque Compatibility..................................................................... 26
12.1 Permanent Mooring......................................................... 27
12.2 MODU Mooring................................................................27
13 Delayed Preloading...................................................................... 27
14 MODU Mooring Considerations....................................................28
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 iv
FIGURE 2 Typical HMPE Creep Curve ................................................ 31
FIGURE 3 Typical HMPE Creep Rate Curve.........................................31
FIGURE 4 Impact of Load and Temperature on Creep Rate ................32
FIGURE 5 GOM Water Temperature Distribution .................................32
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 v
6 Splice Qualification....................................................................... 47
7 Particle Ingress Resistance.......................................................... 47
7.1 Test Procedure.................................................................47
7.2 Inspection and Testing..................................................... 48
8 Torque Match with Steel Wire Rope............................................. 49
9 HMPE Creep Rate Verification..................................................... 49
10 Aramid Axial Compression Fatigue.............................................. 49
10.1 Test Procedure.................................................................49
10.2 Data Reporting.................................................................50
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 vi
4 Rope Production Report............................................................... 55
5 Testing, Inspection, and Certification............................................ 56
5.1 General............................................................................ 56
5.2 Inspection, Examination, and Testing during Rope
Production........................................................................56
5.3 Inspection of Completed Rope Product........................... 56
5.4 Examination and Inspection of Terminations................... 56
5.5 Determination of Finished Rope Length.......................... 56
6 Marking......................................................................................... 56
APPENDIX 1 References..........................................................................................63
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 vii
2.1 Dynamic Stiffness............................................................ 66
2.2 Quasi-static Stiffness....................................................... 67
3 Major Conclusions from Parametric Studies for Polyester
Mooring.........................................................................................69
3.1 Stiffness Model................................................................ 69
3.2 Dynamic and Quasi-static Stiffness................................. 69
3.3 Fatigue Analysis.............................................................. 70
4 Polyester Mooring Analysis Example........................................... 70
4.1 Determination of Dynamic Stiffness.................................70
4.2 Determination of Quasi-static Stiffness............................73
4.3 Determination of Upper and Lower Bound Stiffness........73
4.4 Strength Analysis Example..............................................74
4.5 Fatigue Analysis Example............................................... 75
5 HMPE Mooring Analysis Example................................................ 76
5.1 HMPE Creep Analysis Example...................................... 76
5.2 HMPE Creep Rupture Analysis Example........................ 76
5.3 Example of HMPE Quasi-Static Stiffness........................ 77
6 Guidance for Dynamic Stiffness Test Matrix................................. 78
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 viii
1 General......................................................................................... 79
2 Example 1 - Fixed Number of Subropes.......................................79
2.1 Assumption:..................................................................... 79
2.2 Procedure:....................................................................... 79
3 Example 2 - Fixed Size of Subrope.............................................. 80
3.1 Assumption:..................................................................... 80
3.2 Procedure:....................................................................... 80
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 ix
SECTION 1 General
1 Scope
The main purpose of these Guidance Notes is to describe criteria for design, material, testing,
manufacturing, installation and subsequent survey of fiber ropes to be used as mooring components in
offshore mooring systems. The secondary purpose of these Guidance Notes is to highlight differences
between fiber rope mooring systems and typical steel mooring systems, and to provide guidance on how to
handle these differences during system design and installation.
In view of the influence of rope properties on mooring system performance, these Guidance Notes include
details of how rope testing, mooring analysis and installation can be integrated to provide a consistent
mooring system design methodology. In this matter, these Guidance Notes cover the following aspects:
Where the mooring design, construction and installation details are similar or equivalent to steel wire/chain
mooring systems, no further guidance is included in these Guidance Notes. These Guidance Notes are not
intended to provide a comprehensive manual on all aspects of mooring design, construction and
installation since these details are adequately covered by other recognized standards, such as API RP 2SK.
The publication of these Guidance Notes reflects the growth in offshore mooring applications for fiber
ropes and the need for a consolidated written guidance. These Guidance Notes summarize industry
experience and common practices in application of fiber ropes for offshore mooring and provides a general
guidance to check the integrity of fiber ropes application.
These Guidance Notes applies to fiber ropes used in the mooring system of both permanent and temporary
offshore installations such as:
Therefore, these Guidance Notes should be used in conjunction with the latest ABS publications as
follows:
i) ABS Rules for Building and Classing Mobile Offshore Units (MOU Rules)
ii) ABS Rules for Building and Classing Single Point Moorings (SPM Rules)
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 10
Section 1 General 1
iii) ABS Rules for Building and Classing Floating Production Installations (FPI Rules)
iv) ABS Guide for Certification of Offshore Mooring Chain (Chain Guide)
These Guidance Notes are not intended to cover general marine applications of fiber ropes, such as
berthing and mooring lines at piers and harbors, towing hawsers on tugs, mooring hawsers on Single Point
Moorings (SPMs) and Tension Leg Platform (TLP) tendons.
2 Definitions
Aged Rope: The rope that has been subjected to preloading and subsequent environmental loads to reach a
fully bedded in condition.
Amplitude to Diameter Ratio (A/D): The ratio of VIM amplitude to the diameter of a Spar or column of a
deep draft semisubmersible.
Aramid Rope: Rope made of aromatic polyamide fiber, which has higher strength and stiffness than
polyester rope. The issue of axial compression fatigue needs to be addressed.
Average Breaking Strength: The average of the results of several rope break tests.
Axial Compression Fatigue: A failure mode for fiber rope such as aramid under low tension or
compression.
Bedding-In: The loading process of compaction of internal rope components to reduce construction stretch.
Creep Model: A model that generates creep and creep rupture design curves for ropes based on yarn test
data, particularly applicable to HMPE ropes.
Creep: The increase in rope length under sustained tension or cyclic loading.
Creep Regime: The time regime that can be clearly distinguished by a different behavior of the creep rate
for an HMPE rope.
Creep Rupture: Failure of fiber rope, such as HMPE, due to continuous creep over time under a specific
load and temperature.
Design Service Life: The intended life for the mooring system of a specific project. The design service life
for the mooring system can be the same as or different from that for the floating unit.
Dynamic Stiffness: The ratio of change in load to change in strain in a rope under cyclic loading, typically
normalized by MBS.
Elongation: The change in length between two gage marks, separated by a known distance (gage length) as
tension is applied to the rope or as tension is maintained over time.
Fiber Finish: A designation of the process and finish used on a fiber for a particular purpose (e.g. “marine
finish”).
Fiber Grade: A designation of the quality of a particular fiber, indicating adherence to tolerances for
properties.
Fiber Type: A designation given by the fiber producer which indicates the manner in which a particular
fiber has been drawn or spun, processed, and treated with various finishes and/or oils.
FPI: Floating Production Installations as defined in the ABS FPI Rules [1].
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 11
Section 1 General 1
FPSO: Floating Production Storage and Offloading Unit as defined in the ABS FPI Rules [1].
Frequency Domain (FD) Analysis: An analysis method that considers system responses in terms of
frequency rather than time. The analysis will produce responses such as dynamic tension or motion
responses in a form of statistical values (standard deviation, significant, and maximum, etc.).
FSO: Floating Storage and Offloading Unit as defined in the ABS FPI Rules [1].
Group Approval: Approval for a group of different sizes of rope of the same design based on one or two
rope tests.
HMPE Rope: Rope made of high modulus polyethylene fiber, which has higher strength and stiffness than
polyester rope. The issue of creep needs to be addressed.
Jacket: A braided or plastic covering which is placed over the rope, subrope, or individual strand for
protection and to hold the rope structure together.
Lay Length: The length along the axis of a rope in which a strand makes one complete spiral around the
rope axis.
Low Frequency (LF) Response: The tension or motion dynamic response that has a period close to the
natural period of the moored system, typically in the range of 100 to 400 seconds.
Manufacturing Specification: A document which completely describes the process of making the rope,
including instructions for each step of the manufacturing process.
Material Certificate: A document prepared by the manufacturer and the fiber producer to certify that the
type and grade of fiber material, the properties of the yarn, and the material used in rope production are
those specified in the Rope Design Specification.
Material Chemical Composition: The generic designation of a specific chemical composition and process
of material used in the fiber (i.e., nylon, polypropylene, Aramid, high-modulus polyethylene).
Material Specification: A document, which completely describes the fiber material used in the rope,
including the material chemical composition, the fiber producer, the fiber type and grade, and the yarn test
properties.
Minimum Bend Radius (MBR): Minimum radius to which the fiber rope can be bent without damage to the
rope construction (including, as applicable, the jacket).
Minimum Break Strength (MBS): The wet breaking strength guaranteed by the rope manufacturer for a
specific rope.
MODU: Mobile Offshore Drilling Unit, a floating drilling vessel that engages in exploratory drilling.
Mooring Line: A mooring component which consists of chain, wire rope, fiber rope or a combination of
them to connect the floating unit with the anchor for stationkeeping.
Non-Torque Component: Mooring component for which twist is not generated or negligible due to tension
variation, such as chain and spiral strand. Polyester rope is generally non-torque but can be a torque
component by design.
Non-Torque-Matched Approach: The approach in which a non-torque fiber rope is connected to a torque
component such as 6-strand wire rope.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 12
Section 1 General 1
Parallel Construction: The most commonly used type of fiber rope construction for offshore moorings
consisting of parallel subropes held together by a braided jacket.
Particle Ingress: Penetration of soil particles into the load bearing fiber core.
Permanent Mooring: The mooring system for a floating platform that has a long design service life,
typically 20 years or more.
Polyester Rope: Rope made of polyester fiber, which is the most widely used fiber rope for offshore
mooring.
Post-Installation Rope: The rope that has been subjected to a specific preload during installation.
Pre-Installation Rope: The rope that has not been subjected to a specific preload.
Preloading: A procedure applying a specific load to induce bedding in, thus reducing construction stretch
and increasing rope stiffness during mooring installation.
Pretension: The tension initially set in the mooring lines for normal operation.
Production Rope Sample: A rope sample removed from production or selected after production for the
purpose of testing.
Prototype Rope Sample: A rope sample fully complying with the rope design specification made for the
purpose of testing either before an order is placed or before regular rope production begins for an order.
Quality Assurance Manual: A document which completely describes the Manufacturer’s quality control
and assurance program.
Quality Control Data Sheet: A document which lists the important parameters in setting up and
accomplishing a designated step of the rope making and assembly process, including normal values and
tolerances.
Quality Control Report: A document prepared at the completion of rope making and assembly which
includes the completed quality control data sheets, material certificates, and inspection reports.
Quasi-Static Stiffness: The static stiffness, which is reduced to account for the rope creep under an
environmental event.
Rope Assembly Interface: Any physical connection which is a permanent part of the rope assembly (e.g.,
thimble) which is used to interconnect rope assemblies or to connect a rope assembly to another tension
member (e.g., a wire rope or chain) or hardware (e.g., an anchor, a buoy, or a platform). [Note: this
excludes shackles and other detachable links.]
Rope Assembly Length: The distance between the assembly interface points as measured at a defined
tension and by a method agreed to by the Purchaser and the Manufacturer.
Rope Assembly: The rope, its terminations, and any other accessory gear such as thimble.
Rope Construction: The manner in which the fibers, yarns, strands and subropes are assembled together in
making the rope.
Rope Design Specification: A document which describes the design of the rope, including the numbers and
arrangements of strands, the strand pitch, the material chemical composition, and the manufacturing
method.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 13
Section 1 General 1
Rope Fiber Area: The total cross-section area of load-bearing fiber in the rope, which is determined by
dividing the weight of fiber per unit length by the fiber density.
Rope Production Report: A document which describes the rope product, including rope design,
termination design, and assembly length, and which includes the material certificates, material test results,
and the various data sheets.
Rope Termination: The method (e.g. splice, potted socket, wedged socket) by which the rope is attached to
the assembly interface.
Rope Yarn: The largest yarn-like component of a strand generally formed by twisting intermediate yarns
together.
Rotation Property: The relative rotation between one end and the other end of a rope of unit length caused
by application of tension.
Rotation: The tendency of the unrestrained end of a rope to rotate about its axis when tension is applied.
Segment: A length of chain, steel or fiber rope with terminations that can be connected to provide the
required length of a mooring line.
Soil Filter: A barrier incorporated in fiber rope for blocking ingress of soil particles.
Splice: A termination type which is normally formed by passing the rope around a spool or similar
attachment, and then separating the rope into strands or sub-ropes and tucking these strands or sub-ropes
back into the rope structure.
Static Stiffness: The ratio of change in load to change in strain in a rope under slowly varying tension for a
period of time, typically normalized to MBS.
Static-Dynamic Model: A stiffness model where the elongation under mean and cyclic load are represented
by different slopes in a load versus elongation curve.
Stiffness Model: A simplified representation of the complex fiber rope load versus elongation behavior.
Stiffness: The ratio of change in load to change in strain in a rope in units of force such as kN or kips.
Stiffness is typically normalized by the MBS in this document.
Strain: The ratio of elongation to the gage length over which the elongation takes place.
Strand: The largest component, which is twisted, braided, or otherwise assembled together to form the
finished sub-rope.
Subrope: The largest component, which is assembled together to form the finished rope.
Termination Specification: A document which completely describes the design of the termination and the
process of making that termination, including materials and steps for making or assembling the
termination.
Test Insert: A short segment typically 10 m to 15 m long, placed at the top of the fiber mooring line, which
can be taken out for testing and inspection.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 14
Section 1 General 1
Three-Slope Model: A stiffness model defined by 3 different slopes for mean load, LF, and WF dynamic
load in a load versus elongation curve.
Time Domain (TD) Analysis: A dynamic analysis method that considers system responses as a function of
time. The analysis produces dynamic tension or motion responses in a form of time history.
T-N Curve: A fatigue design curve that defines the relation between the mooring line tension range and
number of cycles to failure.
Torque Component: Mooring component for which twist is generated due to tension variation, such as 6-
strand or 8-strand wire rope.
Torque-Matched Approach: The approach in which a fiber rope is designed to match the torsion
characteristics of a torque component such as 6-strand wire rope.
Two-Slope Model: A stiffness model defined by 2 different slopes for mean load and dynamic load
(combined LF and WF) in a load versus elongation curve.
Upper-Lower Bound Model: A simplified stiffness model where the rope stiffness is defined by the
maximum and minimum value for a specific rope.
Vortex Induced Motion (VIM): The vessel motions of a Spar or a deep draft semisubmersible induced by
vortex shedding under current.
Wave Frequency (WF) Response: The tension or motion dynamic responses that have periods of waves,
typically in the range of 4 - 30 sec.
Wire Rope Construction: Rope construction resembling steel wire rope either as subrope or as full rope.
Yarn Break Strength: The average breaking load from yarn break tests.
Yarn Creep: The characteristic of the yarn to undergo a time related non-recoverable increase in length
when subjected to sustained load.
Yarn Elongation: The average elongation at break from several yarn break tests.
Yarn-on-Yarn Abrasion Property: The average number of cycles of tested yarns to failure at designated
loads in yarn-on-yarn abrasion tests.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 15
SECTION 2 Scope and Procedure for Design and Analysis
1 General
This Section provides general guidance for the design and analysis of mooring systems incorporating fiber
ropes. Requirements as specified in the ABS Rules for Building and Classing Floating Production
Installations (FPI Rules) [1] and API RP 2SK (2005) [2] are generally applicable unless otherwise
provided in these Guidance Notes. The purpose of design verification is to confirm that the proposed
mooring system satisfies the specified design conditions, Rules, Guides and other related standards.
TABLE 1
Documentation for Design, Testing, Manufacturing, and Survey
6 Torque Match with Steel Wire Rope Test Rope Manufacturer or Mooring Designer
8 HMPE Creep and Creep Rupture Models and Basis Fiber Producer
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 16
Section 2 Scope and Procedure for Design and Analysis 2
3 Mooring Configuration
A mooring system with fiber ropes can be configured as either a taut-leg or a catenary system. The choice
depends on many considerations that are beyond the scope of these Guidance Notes. A taut-leg mooring
(TLM) has a smaller mooring footprint than the conventional catenary mooring system. This can be
particularly important for the field layout of production installations and in congested development areas.
The taut leg mooring systems also differ from conventional catenary mooring systems in which the anchor
must resist substantial vertical load.
Fiber ropes offer several advantages over their steel counterparts in the design of mooring systems.
However, unlike steel wire ropes, fiber ropes should be treated with due consideration of particular fiber
specific characteristics. Some general discussion of material properties with regard to mooring design
considerations is included in various sections.
The choice of fiber material depends on the nature of the application and on the level of confidence in the
material. Currently, polyester is widely used for offshore mooring applications due to its low stiffness
which induces less tension during design storm, good resistance to axial compression, good strength to
weight ratio and good creep resistance. HMPE and aramid have better strength to weight ratios and are
stiffer than polyester. However, HMPE can be subject to creep, potentially leading to a creep rupture
problem, and aramid can be subject to axial compression fatigue failure under low tension or compression.
These issues should be properly addressed in the design.
For taut-leg mooring systems, the fiber rope axial stretch provides load elongation characteristics which
the catenary geometry traditionally provides in the conventional steel system. The lower elasticity of
polyester makes it suitable for certain deep water TLMs. Other fibers such as HMPE and aramid may be
more suitable for applications where frequent handling is required or for ultra-deep water TLM
applications. Comparison of alternative fiber ropes in terms of their mechanical properties alone is not
sufficient for design; the relative merit of each fiber rope can only be assessed through comparison of
mooring system performance obtained from a detailed mooring analysis.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 17
SECTION 3 Polyester Mooring Design and Analysis
Guidance for the top and bottom steel section for permanent moorings is provided below. The same
guidance can be considered for MODU moorings, but some of the guidance may not be directly applicable
to them because of the temporary nature of the MODU operation.
● The type of marine growth. Some marine growths are more harmful than other marine growths
● The type of soil filter. Some soil filters are more efficient in blocking marine growth than others
● Protective coating. Penetration of marine growth into the load bearing fiber can be blocked by
effective protective coating
In addition to above considerations, there are other factors to be considered for the length of the top steel
section, such as installation tolerances on anchor locations, fabrication length tolerance, ground chain
length tolerance and ground chain below mudline tolerance, removal of inserts, planned platform
movements for drilling or spreading out catenary riser fatigue damage by shifting the touchdown point
where fatigue damage is most severe.
2 Stiffness Characteristics
Polyester ropes as well as other fiber ropes are made of materials with visco-elastic properties, so their
stiffness characteristics are not constant and vary with the load duration and magnitude, the number and
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 18
Section 3 Polyester Mooring Design and Analysis 3
frequency of load cycles, and the loading history. In general, polyester mooring lines become stiffer after a
long time in service. Historical loading above a certain level may lead to a permanent increase of the rope
length and results in a softer mooring system if no re-tensioning is performed. Because of this complex
rope behavior, it is not possible to develop models that represent the precise stiffness characteristics of the
rope. Currently the industry relies on some simplified models that capture the most important
characteristics and at the same time yield conservative prediction of line tensions and vessel offsets.
3 Stiffness Model
The static-dynamic model [4] is recommended as the primary model for the following reasons:
An alternative to the static-dynamic model is the upper-lower bound model [6], which has been used by the
industry for a long time because of its simplicity and unavailability of a better model. The accuracy of this
model depends on selection of the upper and lower bound values, and improper selection of these values
often leads to too conservative or non-conservative predictions. These Guidance Notes provide some
guidance on establishing the upper and lower bound values for this model.
Other models have been used by the industry. They can be acceptable if they reflect the basic elongation
behavior of polymer material and produce realistic predictions. However, these Guidance Notes provide no
guidance for these models, and it is up to the designer to provide evidence for the validity of the model.
The static-dynamic model was developed to account for this fiber rope elongation behavior. In this
model, the static stiffness is utilized for the initial region of the loading curve up to the mean load.
Afterwards, the dynamic stiffness is used to predict the cyclic part of the loading (3/3.1.2 FIGURE
1). This model more accurately simulates the actual conditions faced by a fiber rope mooring out
at sea. A mooring line under a severe environment typically experiences a steady mean load and
dynamic loads oscillating around the mean load. 3/3.1.2 FIGURE 1 represents a 2-slope model
where LF and WF dynamics are combined. Separating the LF and WF dynamics will result in a 3-
slope model. An industry study [5] indicates that the difference between predictions from the 2-
slope and 3-slope model is small, and therefore the simpler 2-slope model is recommended.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 19
Section 3 Polyester Mooring Design and Analysis 3
where ΔF is the change in load, Δε is change in strain, and EA is the stiffness or the modulus
times the cross-sectional area of the rope. The stiffness is usually expressed in units of force such
as kN or kips. Equivalently, a non-dimensional stiffness Kr can also be expressed as:
EA
Kr = MBS (3.2)
In addition, these Guidance Notes deal with two types of stiffness, dynamic and static and the non-
dimensional dynamic stiffness is denoted as Krd and the non-dimensional static stiffness is
denoted as Krs.
FIGURE 1
Static-Dynamic Stiffness Model
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 20
Section 3 Polyester Mooring Design and Analysis 3
FIGURE 2
Upper-Lower Bound Stiffness Model
The upper-lower bound model has been widely used in the industry due to its simplicity and unavailability
of a better model. However, it does have certain shortcomings such as:
● There is no systematic method to determine the upper and lower bound stiffness, and therefore these
values are often arbitrarily determined
● Polyester rope has a very complicated stiffness property, which is a function of load type, amplitude,
duration, and history. Using 2 limiting values to represent the complicated behavior often results in
overly conservative or non-conservative analysis results, depending on the design parameter being
considered. To avoid this situation, many designers use some intermediate values, but again the
selection of these values is rather arbitrary, resulting in more confusion over this issue.
Although the upper-lower bound model is a simple and widely used model, determination and use of the
upper and lower bound values requires careful consideration.
● The model reflects the basic elongation behavior of polymer material as discussed in 3/3.1.1
● The model produces realistic line tension and vessel offset predictions under various conditions
● The model properly accounts for line length changes due to creep and permanent elongation
4 Dynamic Stiffness
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 21
Section 3 Polyester Mooring Design and Analysis 3
where
There is a trend in the industry to take out the tension amplitude and loading period in the equation,
claiming their impact is negligible. This leaves a simplified dynamic stiffness model depending on mean
load only. Investigations reveal that this simplified model is a poor fit to the test data. To have a general
model for all polyester ropes, all 3 parameters should be kept, unless data are available to justify the use of
a simpler formulation.
● For sinusoidal loading such as Spar VIM in the lock-in region, T is the maximum tension amplitude
for strength and fatigue analysis
● For strength analysis under stochastic loading such as storm loads or Spar VIM in the transition
region, T is 0.5 times the maximum tension amplitude.
● For fatigue analysis under wave loading, T is negligible
1) Pre-installation: The rope has not been preloaded and therefore has the lowest stiffness
2) Post-installation: The rope has been preloaded during installation and achieved an initial bedding
in
3) Aged: The rope has experienced severe loadings beyond the preload to reach a fully bedded in
condition
Available test data indicate that the difference between dynamic stiffness for post installation and aged
rope is small. Therefore dynamic stiffness for the aged rope can be used conservatively for the project, and
testing for both post-installation and aged rope is not necessary. Dynamic stiffness data for the pre-
installation rope are not available at this point.
5 Static Stiffness
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 22
Section 3 Polyester Mooring Design and Analysis 3
connecting the point at the pre-tension with the point at the end of the creep plateau for the load level. The
creep, which is a function of load or storm duration, can be represented by a linear function of log time.
Therefore the quasi-static stiffness can be determined by Equation 3.4.
FIGURE 3
Definition of Quasi-Static Stiffness
where
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 23
Section 3 Polyester Mooring Design and Analysis 3
elastic elongations are properly accounted for. A specialized test program is required to provide
appropriate test data for this approach. These Guidance Notes provide no guidance for this model.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 24
Section 3 Polyester Mooring Design and Analysis 3
run. Again the pre-tension for both runs should be the same. To achieve this, the line length or anchor
location may have to be adjusted in the second run when the lower bound stiffness is changed to the upper
bound stiffness. This procedure applies to both FD and TD analysis.
10 Creep
Polyester ropes are not subject to significant creep at loads normally experienced in mooring applications
and thus are not normally subject to failure due to creep rupture. Therefore creep or creep rupture analysis
is not required for mooring design. However, mooring line adjustments may be needed during design
service life due to rope creep, and sufficient upper chain segment length should be retained to allow future
line adjustments. Estimate of future line adjustments can be carried out using the creep rates at the creep
plateaus from the quasi-static stiffness test.
11 Fatigue
NRM = K (3.5)
where
N = number of cycles
R = ratio of tension range (double amplitude) to MBS
M = 5.2 (slope of T-N curve)
K = 25,000 (intercept of T-N curve)
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 25
Section 3 Polyester Mooring Design and Analysis 3
FIGURE 4
Polyester Fatigue Design Curve
As per the ABS FPI Rules [1], tension-tension fatigue life design criteria can be summarized in the
following table for permanent installations. For temporary installations, fatigue analysis can be waived
provided that inspection of the mooring is conducted according to API RP 2I [14].
TABLE 1
Fatigue Life Factor of Safety
Inspectable Areas 3
12 Torque Compatibility
Torque compatibility should be considered between polyester rope and other components such as chain and
wire rope. There are 2 torque categories for mooring components:
● Torque component: Twist is generated due to tension variation, such as 6-strand or 8-strand wire rope
● Non-torque component: Twist is not generated or negligible due to tension variation, such as chain and
spiral strand. Polyester rope is generally non-torque but can be a torque component by design.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 26
Section 3 Polyester Mooring Design and Analysis 3
Laboratory testing demonstrates that a 6- or 8-strand wire rope’s fatigue performance, when connected
with a non-torque polyester rope, could be significantly degraded, although the scale effect of such testing
is yet to be quantified. Following is some guidance to address this issue.
If this approach is used, a torque match test should be conducted (see 3/12.1).
13 Delayed Preloading
The preloading operation to achieve initial bedding-in of the polyester ropes should be carried out
immediately after the mooring installation. If this operation is delayed for an extended period during which
severe environments can be encountered, a mooring analysis should be conducted using the stiffness
values for a pre-installation rope. Vessel offsets and motions from this analysis should be used to check
that riser stress and fatigue life are sufficient to provide riser integrity under this condition.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 27
Section 3 Polyester Mooring Design and Analysis 3
● The requirements for the top and bottom steel section can be different (Subsection 3/1)
● Polyester rope for MODU moorings is often not preloaded or preloaded with a much lower tension.
This may result in a very soft rope when it is new, but the rope becomes stiffer as it ages. This should
be taken into consideration when conducting stiffness test and mooring analysis. This is particularly
important for deepwater operations where stroking out of the riser slip joint is possible due to large
vessel offset.
● The practice for torque compatibility is different (Subsection 3/12)
● Determination of dynamic and quasi-static stiffness values is based on a few design parameters, such
as mean tension, tension amplitude, duration of the environmental event, natural period of the moored
system, etc. MODU operations are typically of short duration and in different water depths and
environments, and therefore these design parameters are often uncertain. Simplified and conservative
assumptions should be used to determine these parameters.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 28
SECTION 4 HMPE Mooring Design and Analysis
The guidance for polyester mooring design and analysis is generally applicable to HMPE moorings. The
major issue of HMPE is its tendency to creep, which should be properly addressed, as discussed in the
following sections.
TABLE 1
Typical Rope Weights and Sizes for 10,000 kN Break Strength
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 29
Section 4 HMPE Mooring Design and Analysis 4
FIGURE 1
Comparison of Static and Dynamic Stiffness of Three Fiber Materials
2 HMPE Creep
The major issue of HMPE is its tendency to creep, which should be addressed in the design of permanent
moorings. One of the main concerns with HMPE's high creep rate is the potential for failure via creep
rupture. As a HMPE rope creeps under tension, eventually it stretches to the point of complete failure.
Another concern is the need for re-tensioning because of HMPE's high creep rate. Furthermore the high
creep rate can lower the quasi-static stiffness over long storm duration. Factors affecting HMPE creep
behavior are fiber type, applied load, time, and temperature, as discussed below.
Similar considerations should also be given to MODU moorings, but the concern for creep and creep
rupture is less for MODU operations because of the short duration of these operations.
● Regime I “primary creep”: In this regime the amorphous realignment takes place, and the high creep
rate at start reduces to a plateau level at the end. The strain is reversible with the use of an elastic and
delayed elastic component.
● Regime II: “steady state creep”: In this regime the sliding of molecular chains takes place. The creep
rate increases slightly because under a constant load the yarn stress actually increases slightly as creep
continues. For practical purpose the creep rate can be considered constant for this regime. The strain is
called “plastic creep”, which is irreversible.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 30
Section 4 HMPE Mooring Design and Analysis 4
● Regime III: “tertiary creep”: In this regime molecular chains start to break. High strains will start to
cause necking in the filaments and will increase the local stress that further accelerates the strain until
breakage.
FIGURE 2
Typical HMPE Creep Curve
FIGURE 3
Typical HMPE Creep Rate Curve
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 31
Section 4 HMPE Mooring Design and Analysis 4
FIGURE 4
Impact of Load and Temperature on Creep Rate
It can be seen that creep rate increases with increasing temperature. 4/2.2 FIGURE 5 shows the mean
annual water temperature as a function of water depth for GOM [19]. To avoid excessive creep, HMPE
ropes should be placed at a depth where the creep performance meets the design criteria.
FIGURE 5
GOM Water Temperature Distribution
● Regime I is of short duration compared with design service life and therefore creep in this regime can
be neglected
● The creep in Regime II is estimated using a constant creep rate for time, but the creep rate is still a
function of applied load and temperature
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 32
Section 4 HMPE Mooring Design and Analysis 4
● Creep rates as a function of yarn stress and temperature relevant to the project are available. These
rates are typically generated based on yarn test data and a creep model
● Information about the rope design and unit mass is available so yarn stress can be converted to rope
tension (%MBS) by a conversion factor for a particular rope
● The strain in Regime II is defined as plastic creep that is irreversible, and therefore creep is
cumulative. The annual cumulative creep strain, Gt can be calculated by the following equation for a
specific temperature:
Gt = ΣℎiHi (4.1)
where
i) The long-term environmental events can be represented by a number of discrete design conditions.
Each design condition consists of a reference direction and a reference sea state characterized by
significant wave height, peak spectral period, spectral shape, current velocity, and wind velocity.
The probability of occurrence of each design condition should be specified.
ii) For each design condition, determine the mean tensions for all mooring lines.
iii) Compute the annual creep strain from one design condition (one sea state in one direction) using
Equation 4.1.
iv) Repeat iii) for all sea states and directions and compute the annual creep strain Gt, which is the
sum of creep strain from all sea states and directions.
v) The predicted total creep strain of the mooring line for a design service life M(year) is:
G = MGt (4.2)
Special attention should be given to the high current event such as the Loop current event in the Gulf of
Mexico, which can impose high steady loads of long duration on the floating structure. Such an event
should be included in the design conditions for creep analysis.
● Regime III is of short duration compared with design service life and therefore creep rupture is
conservatively assumed at the start of Regime III.
● Creep rupture time as a function of yarn stress and temperature relevant to the project are available.
This information is typically generated based on yarn test data and a creep model
● Information about the rope design and unit mass is available so yarn stress can be converted to rope
tension (%MBS) by a conversion factor for a specific rope
● Similar to the Miner’s rule for fatigue analysis, the annual cumulative creep rupture damage ratio B
can be calculated by the following equation:
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 33
Section 4 HMPE Mooring Design and Analysis 4
where
The predicted creep rupture life for the mooring component is 1/B.
i) The long-term environmental events can be represented by a number of discrete design conditions.
Each design condition consists of a reference direction and a reference seastate characterized by
significant wave height, peak spectral period, spectral shape, current velocity, and wind velocity.
The probability of occurrence of each design condition should be specified.
ii) For each design condition, determine the mean tension for all mooring lines.
iii) Compute the annual creep rupture damage from one design condition (one seastate in one
direction) using Equation 4.3.
iv) Repeat iii) for all seastates and directions and compute the total annual creep rupture damage Bt,
which is the sum of creep rupture damage from all seastates and directions.
v) The predicted creep rupture life of the mooring line is:
Unlike fatigue damage that is mainly caused by cyclic loading from waves, creep rupture damage can be
significantly contributed by all environmental parameters including wind, waves, and current. Special
attention should be given to the high current event such as the Loop current event in the Gulf of Mexico,
which can impose high steady loads of long duration on the floating structure. Such an event should be
included in the design conditions for creep rupture analysis.
3 Quasi-Static Stiffness
Creep has much higher impact on the quasi-static stiffness for HMPE than for polyester. Equation 3.4 in
3/5.1 should be modified to the following equation for HMPE:
where
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 34
Section 4 HMPE Mooring Design and Analysis 4
4 Fatigue
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 35
SECTION 5 Aramid Mooring Design and Analysis
The guidance for polyester mooring design and analysis is generally applicable to aramid moorings. The
major issue for aramid is its susceptibility to failure from axial compression fatigue under low tension,
which should be properly addressed, as discussed in the following sections.
i) Improved rope design in the areas of splice, marine finish, and jacket
ii) Establishing proper minimum tension criteria and analysis procedure
iii) Conducting axial compression fatigue test to provide for adequate resistance to axial compression
fatigue failure
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 36
Section 5 Aramid Mooring Design and Analysis 5
ii) Axial compression fatigue test procedure: tension range, number and frequency of test
cycles
iii) Residual strength after test
iv) Basis for the design criteria and test procedure
3 Tension-Tension Fatigue
Tension-Tension fatigue design curve cannot be generated at this point because of lack of data. Fatigue
analysis should be focused on the upper chain segment, and the tension range for the upper chain should be
based on proper stiffness values for the aramid rope.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 37
SECTION 6 Design and Analysis for Other Fiber Ropes
There are fiber materials other than polyester, HMPE, and aramid that can be considered for mooring
applications. These Guidance Notes cannot give specific guidance for them at this point because of lack of
technical data. Some of the guidance in this document may be applicable to other fiber materials, but the
user should exercise caution and sound judgment in such applications.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 38
SECTION 7 Summary of Design Criteria
3 HMPE Creep
● Creep strain limit for design service life: 10%
● Factor of safety against creep rupture: 5 (creep is monitored) or 10 (creep is not monitored)
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 39
SECTION 8 Testing of Rope
1 General
This Section provides guidance for conducting rope tests to determine fiber rope properties.
TABLE 1
Test Requirements
Note:
* Required for rope preset on seabed and reuse of an accidentally dropped rope
** Required for fiber rope connected with torque wire rope (6- or 8-strand) for permanent mooring
TABLE 2
Group Approval
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Section 8 Testing of Rope 8
i) Group approval is effective for no more than 5 years from the test date
ii) One sample: qualify ropes within ±20% of MBS of the test sample
iii) Two samples: qualify ropes between 80% MBS of the small and 120% MBS of the large test
sample. The 2-sample approach is not applicable to dynamic stiffness test.
iv) Group approval applies only to ropes with same design parameters as indicated below:
● Subrope construction
● Yarn type
● Number of layers in eye configuration
● D/d ratio for hardware
● Shape of hardware bearing surface
● Splice lengths (number of strand tucks and tapered tucks)
● Chafe protection material and application in the eye
● Soil filter material and design (for “Particle Ingress Resistance” test only)
TABLE 3
Test Sample Requirements
8/8 Torque Match with Steel Wire Rope No Yes Same length as
steel wire rope
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 41
Section 8 Testing of Rope 8
Notes:
1 All samples should be full rope samples with 2 exceptions for tests of parallel construction ropes. Subrope
samples can be used for these 2 tests, and the subrope MBS can be taken as the full rope MBS divided by the
number of subropes.
2 The sample length is the length between the bearing points (8/2.1 FIGURE 1). D is sample diameter
3 The samples should be terminated in the same manner as the production with exception for 2 tests. For these two
tests the terminations should be of sufficient strength to safely withstand at least 70% of the rope MBS. The
sample length should be at least 5 m. The gage marks should be no closer than 3 times rope diameter from the last
tuck of rope splices.
4 In general, the sample should not have been previously tensioned to more than 5% of its estimated breaking
strength nor have been maintained under steady or cyclic tension except as noted. This criterion does not apply to
the test for used ropes.
5 The entire sample including terminations should be soaked in fresh water for 4 hours (subrope) or 12 hours (full
rope) before testing. The sample should be tested as soon as practical after being removed from the water. If there
is a delay of more than 16 hours after soaking, the sample should be soaked again for 4 hours (subrope) or 12
hours (full rope).
FIGURE 1
Typical Test Setup
For the “Elongation and Stiffness” (Subsection 8/5) and “Torque Match with Steel Wire Rope” (Subsection
8/8) test, the test machine should be equipped with an elongation or rotation measuring device and data
collection system capable of measuring and recording the elongation or rotation over the gage length. The
accuracy of these measurements should be established according to the guidance provided in Appendix A
of CI Standard 1500-2 [21]. The data collection system should be capable of recording sufficient data
points during a load cycle.
Preferably, the entire rope section including terminations is immersed in fresh water during the test.
Alternatively, the entire length of rope between ends at terminations can be sprayed with fresh water
during the test at a minimum rate per minute calculated by the following formula:
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 42
Section 8 Testing of Rope 8
where
Special attention should be paid to testing with dynamic loads such as fatigue and splice qualification test.
When rope temperature indicates a risk of overheating, the following measures should be considered:
i) Stop the test and resume the test after the temperature is lowered to an acceptable level
ii) Increase the period of the dynamic load
iii) Increase the water flow
● Method A: The rope is made up with a fixed number of subropes. In this case, the subrope size
changes as the full rope size changes.
● Method B: The rope is made up with subropes of the same size. In this case, the number of subrope
changes as the full rope size changes.
In general, the test and data interpretation procedures given below apply to both Method A and B unless
noted in the following sections.
4.1.1 Procedure A
i) A cycling tension between 1% and 50% of the rope MBS should be applied 10 times at a
period of 12 - 35 sec.
ii) On the last cycle, the rope is pulled to failure, at a loading rate of approximately 20%
MBS per minute.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 43
Section 8 Testing of Rope 8
iii) Record the breaking force (maximum force applied to the rope). Record the location
where the rope broke (e.g., between splices, at end of a splice, at crotch of a splice, in
back of an eye, or other breaking locations).
4.1.2 Procedure B
i) A tension of 50 % of the rope MBS should be applied at a rate of 10% MBS per minute
and held for 30 min.
ii) The tension should be reduced to 10% of the rope MBS, at a rate of 10% MBS per
minute.
iii) A cycling tension between 10% and 30% of the rope MBS should be applied 100 times at
a period of 12 to 35 sec.
iv) On the last cycle, the rope is pulled to failure, at a loading rate of approximately 20%
MBS per minute.
v) Record the breaking force (maximum force applied to the rope). Record the location
where the rope broke (e.g., between splices, at end of a splice, at crotch of a splice, in
back of an eye, or other breaking locations).
The above procedure applies to each project and group approval is generally not acceptable for MBS. However, if after the
first project, the same rope (same design and MBS) is produced for other applications, the number of tests can be reduced
according the following guidance:
2) Number of break tests for subsequent applications: 1 additional test for every 25 segments including test inserts
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 44
Section 8 Testing of Rope 8
1) The test should be done on at least two rope samples. The test should also precede all rope tests
intended to determine design rope properties for elongation, quasi-static stiffness, and dynamic
stiffness.
2) Detailed test procedure is as follows:
a) Tension the rope to 2% MBS. Measure and record the initial rope length.
b) Increase the tension to the specified pre-tension and hold at this tension for at least two
hours. Record the elongation at 1, 10, and 100 minutes, and at the end of the duration.
c) Increase the tension to the specified preload tension and hold for the planned duration
during installation at this tension (at least one hour). Record the elongation at 1, 10, and
100 minutes, and at the end of the duration.
d) Decrease the tension to pre-tension and hold at this tension for at least 6 hours. Record
the elongation (at least) at 1, 10, and 100 minutes, and at the end of the duration.
3) Report the difference in length from the end of step 2a to the end of step 2d above. This is the
permanent post-installation elongation.
The data from this test can also be used to determine the quasi-static stiffness for pre-installation rope.
1) Increase the tension from pre-tension to 30% MBS at a rate of approximately 10% MBS per
minute and hold at this tension for 100 minutes. Record the extension at 1, 10 and 100 minutes.
2) Increase the tension from 30% to 45% MBS at a rate of approximately 10% MBS per minute and
hold at this tension for 100 minutes. Record the elongation at 1, 10, and 100 minutes.
3) Increase the tension from 45% to 60% MBS at a rate of approximately 10% MBS per minute and
hold at this tension for 100 minutes. Record the elongation at 1, 10, and 100 minutes.
4) Reduce the tension from 60% MBS to pre-tension at a rate of approximately 10% MBS per minute
and hold at this tension for at least 200 minutes. Record the elongation at 1, 10, and 100 minutes
and at the end of the duration.
The data from this test should be used to determine quasi-static stiffness for the post-installation rope
(A2/2.2). For HMPE, additional holding time may be required in steps 1 through 3 to make sure that the
sample has entered Regime II in 4/2.1 FIGURE 3.
1) Increase the tension from pre-tension to 65% MBS at a rate of approximately 10% MBS per
minute and hold at this tension for 100 minutes. Record the elongation at 1, 10 and 100 minutes.
2) Apply 1000 cycles of dynamic load with a tension range of 35% to 65% MBS and a period of 12
to 35 seconds.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 45
Section 8 Testing of Rope 8
3) In the last cycle reduce the tension from 65% MBS to pre-tension at a rate of approximately 10%
MBS per minute and hold at this tension for 100 minutes. Record the elongation at 1, 10, 100
minutes.
An alternative to the above procedure for fully bedding in the rope is to use a rope sample that has been
subjected to the dynamic loading in the splice qualification test (Subsection 8/6).
The quasi-static stiffness test outlined in 8/5.2 should be repeated after above loading, and the data from
this test can be used to determine quasi-static stiffness for aged rope.
TABLE 4
Dynamic Stiffness Test Matrix for General Applications
1 15 5 10 20 250
2 20 3 17 23 12 to 35
3 20 3 17 23 250
4 23 16 7 39 250
5 30 16 14 46 12 to 35
6 30 16 14 46 250
7 30 28 2 58 250
8 35 8 27 43 12 to 35
9 35 8 27 43 250
10 40 30 10 70 12 to 35
11 40 30 10 70 250
12 50 20 30 70 12 to 35
13 50 20 30 70 250
14 60 10 50 70 250
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 46
Section 8 Testing of Rope 8
v) The test for the whole matrix should be continuous without significant interruption. If
necessary, a short pause can be placed between each test case
5.4.3 Determination of Dynamic Stiffness
i) Dynamic stiffness should be calculated by Equation 3.1 using the peak and trough points
of the cycle, and the strain should be based on the average rope length during the cycle.
ii) For each test case, calculate the dynamic stiffness for each cycle, and report the average
stiffness of the last three cycles as the dynamic stiffness for the test case.
6 Splice Qualification
This test is intended to demonstrate that the splice of the rope is properly designed and made. The test
procedure is as follows:
i) Apply either one of the following cyclic loadings at a period of less than 1 minute per cycle.
i) The soil grading should contain 30% to 40% clay (less than 2 microns) and 50% to 60%
silt (2 to 63 microns). An example of soil grading is shown in 8/7.1.2 FIGURE 2.
ii) Horizontal (back and forth) and vertical (pick up and drop) movements (minimum 1 m)
should be imposed on the rope sample at least 20 times each to simulate rope movements
during installation and impact of a dropped rope
iii) After the simulations, the rope should stay in the tank for at least 24 hours
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 47
Section 8 Testing of Rope 8
FIGURE 2
Example Soil Grading
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 48
Section 8 Testing of Rope 8
i) Equal length full rope samples of the fiber rope and the wire rope should be connected and
arranged in a test machine with both outer ends fixed.
ii) The samples should be loaded to 2% of the wire rope MBS, and the length of each rope and the
lay length of the wire rope should be measured.
iii) The samples should be cycled 10 times between 2% and 20% of wire rope MBS at a period of 20
to 30 sec.
iv) The samples should be held at the 20% wire rope MBS and the connection between the two ropes
should be provided with means to measure any rotation of the connection, such as a lever.
v) The samples should be cycled 10 times with a load range of 10% to 30% of wire rope MBS at a
period of 20 to 30 sec., and the angular rotation of the connection for each cycle should be
recorded.
The average cyclic degree of rotation per lay length of the wire rope should be calculated and should not
exceed 5° as specified in 3/12.1.
i) The test should be performed for a constant load and a constant temperature. The load level and
temperature should be selected to be relevant to the project where the rope will be applied, so that
the creep rate in Regime II (4/2.1) can be established with confidence.
ii) The duration is such that the sample shows a constant creep rate with time for at least 24 hours,
and the load level and temperature should be kept as constant as possible within the duration.
Load and elongation should be recorded during the entire duration at least hourly.
iii) The creep rate is obtained from the strain versus time data over the end of the test period (e.g., the
last 24 hours), which should be compared with the creep rate generated by the creep model for the
specific load and temperature.
iv) The total strain measured during Regime I of the test should also be reported.
i) Cycle the rope from a trough tension of 1% of MBS to a peak tension of 20% of MBS at a period
of less than 1 minute per cycle for at least 2,000 cycles.
ii) Tension the rope to break, using the test procedure of 8/4.1, to determine residual strength.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 49
Section 8 Testing of Rope 8
If the residual strength is greater than 95% MBS, then the rope is considered to pass the axial compression
fatigue test.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 50
SECTION 9 Testing of Yarn
Yarn testing should be conducted according to Subsections 9/1 to 9/3 before rope production. Some yarn
properties should be verified during production according to Subsection 9/4.
The samples should be loaded to break in accordance with ASTM D 885 “Tire Cords, Tire Cord Fabrics,
and Industrial Filament Yarns Made From Man-Made Organic-Based Fibers” [21], BS EN ISO 2062
“Methods of Determination of Breaking Strength and Extension” [24], or an equivalent method. The
testing method to be used should be identified in the rope design documentation. The same method is then
to be used whenever the yarn is tested.
The average yarn dry breaking strength and dry elongation should be determined and recorded.
● 20 to 30 mN/tex;
● 35 to 45 mN/tex;
● 55 to 60 mN/tex.
Note:
Tex is a unit for expressing linear density, equal to the weight in grams of 1 kilometer of yarn, filament, fiber, or other textile
strand. 1 kg/m = 1,000,000 tex.
A minimum of 8 yarns should be tested for each level. The results at each load level should be obtained
and reported in accordance with the procedure in CI 1503. The mean number of cycles to failure as defined
in CI 1503, at each load level should be above the minimum number of cycles given in 9/3.2 FIGURE 1
for that level. The number of cycles to failure, N, shown in 9/3.2 FIGURE 1, is given by the equation:
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 51
Section 9 Testing of Yarn 9
where Ftest is the test load, expressed in millinewtons per tex (mN/tex).
FIGURE 1
Minimum Requirement for Yarn-on-Yarn Abrasion Test
i) At least one yarn sample should be taken randomly from each 2 MT of material and tested for
yarn size, dry break strength, and dry elongation to break.
ii) At least one wet yarn-on-yarn abrasion test should be performed from each 20 MT of material.
Each test should be carried out at least at one load level, on a minimum of four yarns, with an
acceptance level not less than the one given in 9/3.2 FIGURE 1 for that level.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 52
SECTION 10 Rope Design
1 General
The most commonly used type of fiber rope for offshore moorings consists of parallel subropes held
together by a braided jacket (parallel construction). The subropes consist of strands in helical (laid) or
braided arrangement. The strands are made up with yarns. Rope constructions resembling steel wire rope
are also used, either as subropes or as full rope (wire rope construction). Soil filter is typically incorporated
between the jacket and the rope core to block harmful soil particles. Fiber ropes are terminated with
spliced eyes.
Most fiber ropes are non-torque type, i.e. the rope does not exert torque when loaded. When a fiber rope is
connected to a torque steel wire rope such as 6-strand wire rope, a “torque-matched” rope is sometimes
used (Subsection 3/12).
3 Rope Jacket
Rope jacket should be sufficiently dense to protect the rope from mechanical damage during handling and
service. It should be permeable for water to flood the rope core. Visible marking such as colored strands or
brightly colored longitudinal stripes should be incorporated in the rope jacket for monitoring rope twist
during installation.
4 Soil Filter
Soil filter should be effective to prevent ingress of particles exceeding 5 microns, based on standard
filtering test such as ASTM D 4751 [26]. For ropes preset on the seafloor or reuse of rope accidentally
dropped on the seafloor, the requirement and testing specified in Subsection 12/3 should be met.
5 Termination
Fiber ropes should be terminated with spliced eyes. For terminations other than spliced eye, detailed design
and testing information should be submitted for approval on a case by case basis. Important factors for
spliced eyes are D/d ratio and protection of the rope eye. If the spliced eyes are fitted on thimbles, the
thimble should be a neat fit on the connecting (shackle or H-link) pin, and the root diameter should be
specified.
Rope jacket and soil filter should be restored to cover splice after splicing. Protective cloth should be
provided between the splice eye and the termination hardware that fits through the eye. Such cloth should
provide low friction and high wear resistance. The splice should be covered by elastomeric material such
as polyurethane coating to protect against chafing. The coating can be omitted for test samples.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 53
Section 10 Rope Design 10
6 Rope Continuity
Strand or subrope should be manufactured continuously for each rope segment without interchange or
splice. Long rope segments consisting of strand splices will be specially considered based on detailed
information of design, testing, and manufacturing.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 54
SECTION 11 Rope Production and Certification
This Section provides guidance on manufacturing the rope, including design documentation, quality
control and assurance, termination, assembly, and product documentation.
3 Material Certification
The rope manufacturer should certify that the fiber material used in making the rope is that specified in the
Rope Design Specification. The yarn producer should certify the type and grade of fiber material,
including finish designation, merge number, and other identifying information. Either the rope
manufacturer or the yarn producer should certify the following yarn properties, using the test methods
specified in Section 9:
● Yarn size
● Dry break strength
● Dry elongation to break
● Dry creep
● Wet yarn-on-yarn abrasion
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 55
Section 11 Rope Production and Certification 11
length was determined, and the quality control report. The rope production report should be available for
examination at the rope manufacturer’s offices. One copy of this Rope Production Report should be
shipped with the rope and one copy of this Report should be submitted to the attending ABS Surveyor.
5.1 General
This Subsection provides guidance on inspection, testing, and certification of the finished rope product.
Specific survey requirements during prototype testing and production are summarized in Section 14.
The inspection should be carried out by the attending ABS Surveyor. Testing of components and samples
of the produced rope should be witnessed by the attending ABS Surveyor.
The attending Surveyor may carry out reasonable inspections of the rope making and assembly processes
and question production and quality assurance personnel prior to, during, and after rope production and
assembly. He may at any time review the applicable Yarn Specifications, Manufacturing Specifications,
Termination Specifications, prototype rope test results, quality control check lists, material certificates, and
fiber certificates.
At any time during rope production or termination, the attending Surveyor may take reasonable quantities
of yarn samples from production and have them tested, in accordance with Section 9.
The Attending Surveyor should thoroughly inspect the assembled rope, after application of terminations
and as appropriate before or after the application any other appliances and accessories. Traceable rope
assembly markings in accordance with Subsection 11/6 should be identified during the inspection.
6 Marking
Each fiber-rope assembly should be marked at each end with a durable and unique identifier traceable to
appropriate certification with at least the following information:
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 56
Section 11 Rope Production and Certification 11
● Manufacturer identification
● Order and part number
● Rope MBS
● Month and year of production
● ABS certification number
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 57
SECTION 12 Handling and Installation
1 General
This Section provides guidance for handling and installation of fiber ropes. In general, the guidance in
Appendix G of API RP 2SM (May 2007) [28] should be followed to minimize damage during handling
and installation. Other guidance is included in the ABS FPI Rules [1] and the applicable sections of the
ABS MOU Rules [3].
i) The rope passes the test for particle ingress resistance as outlined in Subsection 8/7, and
ii) The rope is retrieved quickly and inspected according to API RP 2I. There is no damage
exceeding the API RP 2I discard criteria.
4 Preloading Operation
The preloading operation to remove as much permanent elongation as possible and to increase stiffness of
the rope should be carefully planned before installation. The preload level and duration should be
determined based on a number of considerations including amount of permanent elongation to be removed,
limitation of preloading equipment, and time required to complete the preload operation. The preload
duration should not be less than one hour. The preload level and duration achieved and rope elongation
should be recorded for each step and compared with the expected values.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 58
SECTION 13 Surveys During and After Installation
1 General
Surveys during and after installation should generally be based on the following documents:
A typical fiber rope mooring system consists of steel components at the floating vessel and anchor ends,
and therefore inspection procedures for fiber rope moorings and steel moorings are closely related. The
inspection objective, type, and schedule established for steel moorings in the above documents are
generally applicable to fiber rope moorings. The following sections address only inspection schedule and
additional issues unique to fiber ropes.
The decision to retire a fiber rope during a survey should be based on the fiber rope discard criteria
specified in API RP 2I [14].
Monitoring of fiber rope mooring may require additional information, such as rope elongation, time period
between re-tensioning, removal and testing of inserts if applicable, and inspection techniques. Methods to
acquire this additional information should be included in the operations, maintenance, and in-service
monitoring plan for ABS review.
2 Permanent Mooring
The mooring line should be inspected for any external damage by ROV (Remotely Operated Vehicle) or
diver. Twist can be verified at installation by ROV/diver monitoring of the marking that runs externally on
the jacket. Particular attention should be made to the condition of fiber ropes terminations. Other design
aspects which should be verified immediately following hook-up are the fiber rope near surface
termination position and the preloading. Estimated elongation should be recorded for all lines during the
preloading operation. The purpose of the survey is to establish the initial condition, which will be
compared with future inspection results.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 59
Section 13 Surveys During and After Installation 13
installation date. The survey is limited to above water components. In addition to requirements
specified in Part 7 of the FPI Rules [1], these Guidance Notes recommend the following:
● The Surveyor should review the records of anchor leg re-tensioning caused by creep, and
confirm with designer that adequate chain/wire segments are available for further re-
tensioning due to creep such that the fiber rope does not come into contact with the chain
stoppers, fairleads, etc., and stays below the water surface. See 3/1.1 for determination of
lengths of chain/wire segments.
● The Surveyor should verify that recorded values of creep are in accordance with the
anticipated design values. Any deviance from design values should be justified by the
designer, and appropriate remedial action should be taken accordingly.
● The pre-tension of mooring lines should be within the designer recommended limits. It should
be noted that the measurement of catenary angles as indicated in the FPI Rules may not be
sufficient for TLMs (Taut Leg Moorings). Other means should be used to determine the
mooring line tensions to the satisfaction of the attending Surveyor.
2.2.2 Special Periodical Surveys
A Special Periodical Survey should be completed within five (5) years after the date of build or
after the crediting date of the previous Special Periodical Survey. The survey scheme should
include methods and techniques used to verify that the system is operating as designed. In
accordance with Part 7 of the FPI Rules, Special Periodical Survey should include a dry-docking
or underwater inspection (ROV or diver), and all components of mooring system should be
examined to the satisfaction of the attending Surveyor. In addition, particular attention should be
given to the examination of the following for fiber ropes:
3 MODU Mooring
For MODU moorings, reference should be made to the applicable sections of the ABS MOU Rules [3].
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 60
Section 13 Surveys During and After Installation 13
Regular inspection of fiber mooring ropes may be feasible, while the fiber rope moorings are recovered
and before they are redeployed at a new location. In general before a fiber rope is reinstalled it should be
carefully inspected for damage to the jacket, rope core, terminations, and termination hardware. Such
inspection can be performed during recovery of the moorings on board the recovery vessel(s), or it can be
performed at a base port facility.
After a severe storm, which is defined as environmental conditions approaching the design storm
conditions, inspection of the fiber mooring line should take place immediately or at the next rig move.
In the areas of tropical cyclone (hurricane, typhoon, etc.), MODUs may encounter environmental loads
much higher than the design loads, and mooring failures are possible. Rigorous mooring inspection is more
critical for operations in these areas to address the integrity of the mooring system and minimize the
probability of mooring failures. Also guidance is needed to address the reuse of the components from a
mooring damaged by a tropical cyclone. Appendix B of API RP 2I [14] should be followed for additional
guidance for MODU mooring inspection in these areas.
4 Test Insert
i) The use of test inserts increases the number of terminations, which are potential weak points in the
mooring.
ii) Currently there is no standard methodology to apply the test insert data to the rest of the mooring
line. The data gathered from a short segment placed at a particular location may not be
representative of other rope segments.
iii) There may be a considerable risk of damage to the mooring lines, risers, umbilicals or other
infrastructure in the water column and on the seafloor during test insert retrieval or replacement
operations, especially for operations that place test inserts several hundred feet below the water
surface and adjacent to or between other mooring lines, risers, and umbilicals. Such operations
require careful planning and execution in order to minimize the risk of damage to equipment and
injury to personnel.
For these reasons, the potential benefits of test inserts should be carefully weighed against the potential
adverse impact for each project before decisions of placing or retrieving test inserts are made.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 61
SECTION 14 Requirement for Witness by ABS Surveyor
This Section specifies survey, testing, inspection, and production to be witnessed by ABS Surveyor
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 62
APPENDIX 1 References
[1] ABS Rules for Building and Classing Floating Production Installations, 2014.
[2] API RP 2SK: Design and Analysis of Stationkeeping Systems for Floating Structures, 3rd Edition, 2005.
[3] ABS Rules for Building and Classing Mobile Offshore Units, 2008.
[4] Francois, M. and P. Davies, Fiber Rope Deep Water Mooring: A Practical Model for the Analysis of Polyester
Mooring Systems, Rio Oil and Gas Conference IBP24700 (2000)
[5] ABS Technical Report TR-2010-03, “Polyester Rope Stiffness modeling, Testing, and Analysis”, ABS JIP Phase 1
report, June 2010.
[6] Engineers Design Guide for Deepwater Fiber Moorings. 1999, NDE/TTI.
[7] Karel Devos, Carina De Plukker, Peter Van den Berghe, Experiences in Rope Design for Offshore renewable Energy
Projects, Proceedings of the 16th Offshore Symposium, February 2010, Houston, Texas
[8] Chaplin C.R., Del Vecchio C. J. M., “Appraisal of Lightweight Moorings for Deep Water Applications”, OTC 6965,
1992.
[9] Casey, N.F. and S.J. Banfield, Full-Scale Fiber Deepwater Mooring Ropes: Advancing the Knowledge of Spliced
Systems, OTC 14243 (2002)
[10] Kwan, Chi-Tat (Tom), Polyester Rope Stiffness Modeling, DeepStar CTR 6403 Final Report, February 2004
[11] Francois, M. and P. Davies, Characterization of polyester Mooring Lines, OMAE2008-57136, OMAE, 2008
[12] Huntley, M. B., Polyester Mooring Rope: Length Determination and Static Modulus, MTS 2006
[13] Flory, J. and S.J. Banfield, Durability of Polyester Ropes used as Deepwater Mooring Lines, MTS Oceans 2006
[14] API RP 2I: In-service Inspection of Mooring Hardware for Floating Structures, 3rd Edition, 2008.
[15] Sharples, Malcom, “Post Mortem Failure Assessment of MODUs During Hurricane Ivan,” U.S. Minerals
Management Service Report No. 0105PO39221, 2006
[16] Davies, P., et al., Mooring in Deep Water: Material Choices, Present and Future, UDET 2002
[17] Davies, P., et al., Synthetic Mooring Lines for Depths to 3000 Meters, OTC 14246 (2002)
[18] Vlasblom, M.P., Bosman, R.L.M., Predicting the Creep Lifetime of HMPE Mooring Rope Applications, MTS
Oceans 2006
[19] Joseph Forrest, Ettore Marcucci, and Paul Scott, Geothermal Gradients and Subsurface Temperatures in the Northern
Gulf of Mexico, Search and Discovery Article #30048 (2007)
[20] C. H. Chi, E. M. Lundhild, T. Veselis, and M. B. Huntley, “Enabling Ultra-Deepwater Mooring with Aramid Fiber
Rope Technology”, OTC 20074, 2009.
[21] CI Standard: “Test Method for Fiber Rope”, CI 1500-02, the Cordage Institute, Hingham, MA. May 2006.
[22] BS ISO 18692:2007, “Fiber Ropes for Offshore Stationkeeping - Polyester”, 2007.
[23] ASTM Test Method D 885: “Tire Cords, Tire Cord Fabrics, and Industrial Filament Yarns Made From Man-Made
Organic-Based Fibers”, American Society for Testing and Materials, Conshohocken, PA.
[24] BS EN ISO 2062, “Methods for Determination of Breaking Strength and Extension”, 1995.
[25] CI Standard: “Test Method for Yarn-on-Yarn Abrasion”, CI 1503-00, the Cordage Institute, Hingham, MA, 2000.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 63
Appendix 1 References A1
[26] ASTM D 4751 –04: Standard Test Method for Determining Apparent Opening Size of a Geotextile, 2004
[27] OCIMF: Guidelines for the Purchasing, Prototype testing and Production of SPM Hawsers, 2000.
[28] API RP 2SM: Recommended Practice for Design, Manufacture, Installation, and Maintenance of Synthetic Fiber
Ropes for Offshore Mooring, 1st Edition, 2001, and the 2007 Addendum.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 64
APPENDIX 2 Supporting Information and Examples
● Stiffness values for preliminary design before more accurate data are available
● Basis for some guidance, for example results of parametric studies and guidance for dynamic stiffness
test matrix
● Examples on how to apply some guidance
This Appendix is not intended to be used as part of the Guidance Notes. The purpose of this Appendix is to
provide some background information and examples for better understanding and application of the
Guidance Notes. The user should be aware that not all the information here is applicable to a specific
project, and therefore should be cautious in using this information.
TABLE 1
Dynamic Stiffness Coefficients for Preliminary Design
Note:
Refer to Section 3, Equation 3.3 for the definitions of these coefficients
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 65
Appendix 2 Supporting Information and Examples A2
TABLE 2
Dynamic Stiffness Values for Preliminary Design
2 About 0.5 31 26
3 Over 0.7 27 23
TABLE 3
Quasi-static Stiffness Values for Preliminary Design
Post-installation 10 13 15
Aged 13 15 18
The coefficients for the dynamic stiffness equation can be obtained by multiple regression analysis for the
data in the left four columns. For this data set, the four coefficients, α, β, γ, δ, from multiple regression
analysis are 27.5, 0.25, -0.59, -1.65 respectfully. The value of R2 is 0.96 from the regression analysis
indicating a good fit of the data to the equation (R2 greater than 0.8 indicates a good fit, based on guidance
for Excel regression analysis). Once the dynamic stiffness equation is determined, dynamic stiffness values
can be calculated for various design conditions. It should be noted that γ and δ are negative in this case
indicating tension amplitude and loading period tend to soften the dynamic stiffness.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 66
Appendix 2 Supporting Information and Examples A2
TABLE 4
Example Dynamic Stiffness Test Data
26.7 20 5 1.15 14
29.7 25 5 1.15 14
29.6 40 10 1.15 14
31.4 50 10 1.15 14
An equation for quasi-static stiffness can be derived for each load level.
FIGURE 1
Example Quasi-static Stiffness Test Data
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 67
Appendix 2 Supporting Information and Examples A2
A2/2.2 TABLE 5 provides the creep data at 45% MBS, and A2/2.2 FIGURE 2 presents a plot of the creep
data and a linear regression line for the data. The slope of the regression line, 0.225, is the creep coefficient
C in the quasi-static stiffness equation, which is:
TABLE 5
Creep Data at 45% MBS
45 1 0.00 0
45 10 1.00 0.25
FIGURE 2
Determination of Creep Coefficient for Quasi-Static Stiffness
Quasi-static stiffness testing was also conducted for an aged rope, and three quasi-static stiffness equations
can be obtained by the same procedure. A2/2.2 FIGURE 3 presents a plot of the six equations, which can
be used conveniently for design. For practical purpose, the load level can be considered the mean load for
the seastate, and the time can be considered the duration of the sea state.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 68
Appendix 2 Supporting Information and Examples A2
FIGURE 3
Quasi-Static Stiffness Design Chart
● Model sensitivity: upper-lower bound model, 2-slope (static/dynamic) and 3-slope (static/LF/WF)
model.
● Effect of environment: 100-year, 10-year, one-year return period and Loop current, GOM and Brazil
● Effect of water depth: 2,000 ft, 6,000 ft, and 10,000 ft
● Effect of vessel type: Spar (WF dominating) and FPSO (LF dominating)
● Same or different stiffness for all mooring lines (investigating the practice of using the stiffness for the
most heavily loaded line for all mooring lines)
● Selection of stiffness for fatigue analysis
The conclusions from these studies are presented below, which can serve as guidance for practical and
conservative fiber rope mooring analysis.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 69
Appendix 2 Supporting Information and Examples A2
the project. The highest dynamic stiffness is used for all seastates. This can significantly simplify
the mooring analysis, avoiding using too many stiffness values in the analysis.
iii) The dynamic stiffness values for FPSO are generally lower because of larger tension amplitude
due to LF motions.
iv) For a Spar under the Loop current environment, the mean tension due to current drag is
dominating. Tension amplitude due to VIM is relatively small. Dynamic stiffness can be high
because of high mean tension and low tension amplitude.
v) Offset of spar under Loop current can be critical and therefore quasi-static stiffness should be
carefully chosen to give conservative offset for the riser design.
Since the loading is stochastic from storm environment, the maximum tension amplitude should
be reduced by a factor of 0.5 (3/4.2). The calculated dynamic stiffness values for various
conditions are summarized in A2/4.1.1 TABLE 7. It can be seen that the stiffness values are not
significantly different, and the highest dynamic stiffness 33 MBS can be conservatively used for
all environments and all mooring lines. If the stiffness assumed in the preliminary analysis is
significantly different from the calculated stiffness values, iterations may be needed. Since this is
not the case for this example, iteration is not necessary.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 70
Appendix 2 Supporting Information and Examples A2
FIGURE 4
Spar Mooring Pattern and Environmental Directions
TABLE 6
Estimated Tension for Storm Environments
100-year Intact 32 10
Damaged 42 11
10-year Intact 25 6
Damaged 34 8
1-year Intact 20 1
Damaged 25 1.2
TABLE 7
Dynamic Stiffness for Storm Environments
Intact Damaged
100-year WF 31 33
LF 29 31
10-year WF 30 32
LF 28 30
1-year WF 30 32
LF 29 30
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 71
Appendix 2 Supporting Information and Examples A2
TABLE 8
Dynamic Stiffness for Fatigue Environments
100-year WF 34
LF 32
10-year WF 32
LF 30
1-year WF 31
LF 29
Since the loading is sinusoidal under VIM lock-in condition, no reduction is needed for the
maximum tension amplitude (3/4.2). The calculated dynamic stiffness values for various
conditions are summarized in A2/4.1.3 TABLE 10. It can be seen the stiffness values for the inline
direction are high due to high mean tension and low tension amplitude. The calculated stiffness
values are significantly different, and using the highest value 38 MBS for all conditions can be
very conservative. Using different stiffness for different conditions should be considered if more
accurate results are desired.
TABLE 9
Estimated Mooring Line Tensions under Spar VIM
Inline Intact 44 4
Damaged 64 3
Perpendicular Intact 47 9
Damaged 68 12
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Appendix 2 Supporting Information and Examples A2
TABLE 10
Dynamic Stiffness for Spar VIM
Inline LF 33 38
Perpendicular LF 30 34
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 73
Appendix 2 Supporting Information and Examples A2
TABLE 11
FD Analysis Results
The responses are WF dominating. Assuming Rayleigh distribution for peak values for a 3-hour
storm, the following maximum responses are calculated based on Equation 5.2 and 5.4 of API RP
2SK :
TABLE 12
TD Analysis Results
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 74
Appendix 2 Supporting Information and Examples A2
The above results represent only one realization. The design value should be the average of at
least 5 realizations.
TABLE 13
Example Fatigue Life Prediction
Note:
Refer to Section 3 , Equation 3.5 for the definitions of K and M.
TABLE 14
Comparison of Fatigue life Prediction
Case No. Stiffness Model Quasi-static Krs Dynamic Krd Chain Fatigue Life (year)
A2 2 Slope 15 28 137
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 75
Appendix 2 Supporting Information and Examples A2
i) The analysis is performed for the most loaded line assuming all the environments coming from
one direction (For more accurate analysis, the procedure in 4/2.3.1 should be used).
ii) The annual environment can be represented by 10 weather bins as shown in A2/5.1 TABLE 15
iii) The creep rate at 20°C can, for example, be represented by the following equation, which is based
on curve fitting to data presented in [18]:
Rc = 4 × 10−11 × Tm
4 . 54
where
The analysis results are presented in A2/5.1 TABLE 15, which indicates:
TABLE 15
Creep Analysis Results
i) The analysis is performed for the most loaded line assuming all the environments coming from
one direction (For more accurate analysis, the procedure in 4/2.4.1 should be used).
ii) The annual environment can be represented by 10 weather bins as shown in A2/5.2 TABLE 16
iii) The creep rupture time at 25°C can, for example, be represented by the following equation, which
is based on curve fitting to data presented in [18]:
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 76
Appendix 2 Supporting Information and Examples A2
Tr = 2 × 1012 × Tm
−6 . 25
where
The analysis results are presented in A2/5.2 TABLE 16, which indicates:
TABLE 16
Creep Rupture Analysis Results
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 77
Appendix 2 Supporting Information and Examples A2
FIGURE 5
Example Quasi-Static Stiffness for HMPE
i) The test matrix should include variation in loading period, mean tension, and tension range.
ii) The wave frequency period should be in the range of 12 to 35 seconds.
iii) The low frequency period should be close to the natural period of the moored floating unit.
iv) Several levels of mean tension and associated tension amplitude should be selected to represent a
reasonable range of storm environments. The tension amplitude in the test is the maximum tension
amplitude in the analysis, and tension range can be taken as tension amplitude times 2.
v) If VIM (vortex induced motion) under high current may occur, the matrix should include cases
representing VIM loadings. VIM may occur for Spar and deep draft semisubmersible hull forms
and may have different amplitude to mean tension ratio from storm conditions. For example VIM
loading for Spar typically has high mean tension and low tension amplitude.
vi) The number of data should be sufficient for multiple regression analysis to obtain dynamic
stiffness coefficients.
vii) The maximum tension in the test should be less than 75% MBS.
viii) The ratio Tamp/Tmean should have significant variation to yield good results from regression analysis.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 78
APPENDIX 3 Alternative Procedure to Determine MBS
1 General
For ropes of parallel construction with MBS greater than 2000 MT, a procedure for a combination of
subrope and full rope tests can be considered, as illustrated in the following examples. This procedure
should be used only when serious limitations such as unavailability of large test machine are encountered.
ABS has not developed detailed test specification and specific acceptance criteria for this procedure, and
only general guidance is provided in this Appendix. Should situation dictate that this procedure must be
used, detailed testing specifications should be developed based on the principles provided here or other
valid principles. Also documentation supporting the reliability of the testing and data analysis procedures
should be submitted in a timely manner to allow review well in advance of their implementation.
2.1 Assumption:
A rope has an MBS of 2600 MT and the test machine can only test ropes to 2000 MT. The rope has N
subropes.
2.2 Procedure:
1) Conduct subrope and full rope break tests for ropes of the same design. The minimum number of
tests is shown in A3/2.2.1 TABLE 1. Let:
TABLE 1
Alternative Method A to Determine MBS
Rope No. MBS (MT) No. of Full No. of Subrope Conversion Method for F
Rope Test Test Factor F
Total 9 16
Note that MBS for rope No. 1 and No. 3 should be greater than 50% and 75% of the target MBS, respectively.
MBS for rope No. 2 should be close to the average MBS of rope No. 1 and No. 3.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 79
Appendix 3 Alternative Procedure to Determine MBS A3
2) Conduct regression analysis for the conversion factors F1, F2 and F3 (total 9) and determine F4
based on evaluation of test data and the statistical values (mean, mean minus SD, mean minus
2SD) from the regression analysis for all rope samples.
3) The estimated break load for the target rope is:
Lf = F4 × N × Ls (A3.2)
The MBS will be accepted if the estimated break load is greater than the MBS.
4) This method applies only to ropes with same design parameters:
● Number of subropes
● Sub-rope construction
● Yarn type
● Number of layers in eye configuration
● D/d ratio for hardware
● Shape of hardware bearing surface
● Splice lengths (Number of strand tucks and tapered tucks)
● Chafe protection material and application in the eye
3.1 Assumption:
A rope has an MBS of 2600 MT and the test machine can only test ropes to 2000 MT. The rope has 26
subropes (N= 26).
3.2 Procedure:
1) Conduct subrope and full rope break tests for ropes of the same design. The minimum number of
tests is shown in A3/3.2.1 TABLE 2 (refer to Equation A3.1).
TABLE 2
Alternative Method B to Determine MBS
Rope No. MBS (MT) N No. of Full No. of Conversion Method for F
Rope Test Subrope Test Factor F
1 13 2 F 1 (2) Tested
2 17 3 F 2 (3) Tested
3 20 4 F 3 (4) Tested
Total 9 4
Note that number of subropes for rope No. 1 and No. 3 should be greater than 50% and 75% of that for the target
rope, respectively. Number of subropes for rope No. 2 should be close to the average number of subropes of rope
No. 1 and No. 3.
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 80
Appendix 3 Alternative Procedure to Determine MBS A3
2) Conduct regression analysis for the conversion factors F1, F2 and F3 (total 9) and determine F4
based on evaluation of test data and the statistical values (mean, mean minus SD, mean minus
2SD) from the regression analysis for all rope samples.
3) The estimated break load for the target rope is:
Lf = F4 × 26 × Ls (A3.3)
The MBS will be accepted if the estimated break load is greater than the MBS.
4) This method applies only to ropes with same subrope and full rope design parameters as indicated
below:
Subrope parameters
● MBS
● Subrope construction
● Yarn type
ABS GUIDANCE NOTES ON THE APPLICATION OF FIBER ROPE FOR OFFSHORE MOORING • 2021 81