New Touch Down Zone (TDZ) Solutions For Steel Catenary
New Touch Down Zone (TDZ) Solutions For Steel Catenary
New Touch Down Zone (TDZ) Solutions For Steel Catenary
March 2008
Granherne Project Number: J51018
Granherne Inc
601 Jefferson
Houston, TX 77002-7900, USA
Tel: +1 (713) 753 7200
Fax: +1 (713) 753 8499
CONTENTS
FRONT PAGE
CONTENTS
ABBREVIATIONS
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7.0 UPSET END SOLUTION 7-1
7.1 Approach 7-1
7.2 Design Development 7-1
7.3 Qualification Status 7-2
7.4 Qualification Need/Assessment 7-3
7.5 Qualification Work with Tenaris Tamsa, Veracruz 7-4
7.6 Qualification Work with V&M Deutschland, Dusseldorf 7-8
7.7 Conclusions 7-10
CP Cathodic Protection
ID Internal Diameter
LF Low Frequency
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NS North Sea
OD Outside Diameter
PP Poly Propylene
UT Ultrasonic Testing
WF Wave Frequency
WT Wall Thickness
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SCR JIP Summary Report to MMS_Rev 0.doc 3/28/2008
1.0 INTRODUCTION
This report provides a summary of the work done in a Joint Industry Project (JIP) during
2004 to 2007 [1-6] to develop design solutions and undertake qualification tasks for four
alternatives with the potential to improve the fatigue performance by a factor of 10 or more
at the touch down zone (TDZ) of steel catenary risers (SCRs). The design enhancements
developed in the JIP would enable increase in the selection of the SCR design for
applications (production and export risers) under severe operating conditions, harsh
environment, and floating systems with high motions. Through significant qualification
tasks undertaken in this JIP, progress has been made to bring these TDZ design solutions
to project ready state (or increase their technology readiness level, TRL) for their
consideration at the front end engineering design (FEED) stage.
The JIP objectives, scope, participants are identified and brief notes on the work
undertaken in four stages of the JIP are provided. The qualification program undertaken
for each solution varied and in some cases it also included manufacturing of samples,
laboratory and full-scale fatigue testing, and post-failure evaluation. The major
qualification tasks undertaken for each solution are identified.
The major effort in this JIP has been on the development and qualification of alternative
solutions with competing suppliers, thus the design details of solutions, manufacturing
processes, and test results will remain confidential for a 10 years period. Thus, such
details are not included in this summary report. They will be part of the JIP report set,
which will be submitted to the JIP Participants as per the JIP Agreement(s) signed with all
companies.
2.1 Background
With increase in the number of deepwater and ultra-deepwater fields being developed
using tie-back of subsea wells to floating production units (FPUs), the use of steel
catenary riser (SCR) has significantly increased since its first application in 1993 as an
export riser on the Auger Tension Leg Platform (TLP) in the Gulf of Mexico (GOM). SCR
provides a simpler and cost effective riser system solution. Its application as a tie-back
riser for production, water injection, gas injection, and export has largely been from
deepwater TLPs and SPAR floating production units. In recent years, the SCR design
has been used in some regions with semi-submersible and tanker based Floating
Production Storage Offloading (FPSO) units, which in some cases required significant
changes in the hull design. An illustration of alternative riser systems available and SCR
tie-back to semi-submersible is given in Figure 2.1. The TDZ region of SCR, which is
estimated to have higher fatigue damage from riser and vessel motion is identified.
Semisubmersible FPS
Mooring
Mooring System
System
HP
HP // HT
HT
Jumper
Jumper
SCR,
SCR, CCR,
CCR, or
or
Flexible
Flexible Catenary
Catenary
The use of SCR with these floating units in deepwater and ultra-deepwater is challenged
due to high fatigue damage estimated in offshore and onshore welds at the SCR touch
down zone (TDZ). The severity level varies with a combined effect of various site-specific
In general, the benefits from the first three approaches listed above are reported to be
less than 3 times the fatigue life estimates for conventional SCR without them. In some
projects, the design modifications at SCR TDZ were implemented and the fatigue life
improvement for solutions such as thick forged ends welded onshore is estimated to be
of the same order as for first three approaches above. The CRA clad layer has recently
been used in one application, on the inside of riser sections at SCR TDZ, to guard against
corrosion acceleration of fatigue at welds [8].
Alternative designs for catenary riser have also been evaluated by some with change in
the riser pipe material (composite, titanium) [9, 10], hybrid designs (titanium and steel), or
by changing riser shape near the seabed through provision of significant buoyancy (lazy
wave steel catenary riser) [11-13], with potential to improve both fatigue and strength
characteristics of the riser at TDZ and above. These alternative catenary riser designs,
with potentially higher costs associated in comparison with the traditional SCR designs,
are reported to have been evaluated for harsher environment and more challenging
applications. Another alternative design recently evaluated by some companies considers
an application of heavy weight coating over part of the riser length above the TDZ [14] to
improve the behavior of SCR and also help improve the fatigue life estimates.
So far, however none of these alternative designs have found applications due to lack of
qualification data for these solutions besides consideration of other technical and
economic factors for site-specific development projects.
The overall goal of the JIP is to improve the Technology Readiness Level (TRL) of
alternative SCR TDZ design solutions with the potential to provide a significant increase in
the fatigue life estimates. With the development of these design solutions, the JIP aims to
achieve the following objectives:
Reduce the overall field development cost from use of SCR as a tieback riser
solution with floating production units in deepwater and ultra-deepwater
Enable application of SCR design with higher motion floating production units
Enable application of SCR design for sour service, HPHT, and large diameter
risers
The above goals were met by development of the 4-Stage JIP work plan, as shown in
Figure 2.2, to identify, evaluate, develop, and undertake qualification tasks for alternative
TDZ design solutions to significantly improve the fatigue performance of riser sections.
The work plan included the following four stages of development:
STAGE II
GATE-1 CASE STUDIES Implementation
of TDZ Solutions
STAGE III
GATE-2 CONCEPTUAL DESIGN AND
QUALIFICATION EVALUATION
OF FOUR SOLUTIONS
STAGE IV
GATE-3 FINAL
QUALIFICATION
REPORTS
TESTING & FEA OF
FOUR SOLUTIONS
The Stage II case study analyses aimed at estimating the design changes required at the
SCR TDZ for each selected solution, which helped form the basis for undertaking design
development and qualification work in Stages III and IV. The work done for case studies
is discussed in Section 4.
The Stage III design development and qualification assessment aimed at improving the
overall understanding of each solution and identifying specific components or issues that
need qualification data from previous applications or undertaking qualification tests of
analyses.
The contributing participants joined Phase 2 of the JIP, which included work done in
Stages 3 and 4. The contributing participants and their associated companies worked
with the Granherne project team in the development of design, assessment, and
qualification tasks.
In addition to Granherne and the contributing participants listed above, some qualification
tasks were done by the following companies (or R&D units of participating companies):
In addition to the Contributing Participants listed above, discussions were also held with
the following companies during Stage I (Comparative Assessment task) to obtain details
of products, and for a few solutions some of the following companies did specific work to
develop details:
The JIP project team included a total of 12 engineers from Granherne, Houston and KBR,
Houston and it was lead by Dr. Rajiv Aggarwal, JIP Manager.
In addition to the Granherne team, the project teams of the 5 Contributing Participants and
sub-contractors as listed above became part of the overall Qualification Teams for
solutions. The contributing participants teams were lead by the following:
RTI Energy Systems, Spring, Texas: Dr. Carl Baxter; Dr. Ron Schutz
VAM, Houston & Aulnoye-Aymeries: Mr. Jacky Massaglia; Dr. Celine Sches
3.1 General
A total of 15 solutions were identified for comparative assessment. These solutions are
grouped under 4 categories as given below:
I) Variations to Carbon Steel Material Change at SCR TDZ:
a) Titanium segment
b) Composite segment
c) Flexible pipe segment
d) Aluminum Alloy segment
e) Clad steel pipe
f) Duplex and Super Duplex
II) Add-ons to Conventional Carbon Steel Riser Sections at TDZ
a) Lightweight coating
b) Buoyancy modules
c) Sleeves or clamps
III) Variations to Offshore Pipe Welding - Components to Fit Solutions
a) Thick forged ends welded onshore
b) Upset end pipe
c) Weld-on threaded connectors
d) High strength threaded and coupled connectors
e) Flanges
IV) Alternative Configurations
a) Low Lazy Wave SCR
The selection criteria included various factors such as technical feasibility, application limits,
potential improvement in fatigue life, installability, overall cost and schedule, design maturity
and history of application, complexity of solution, qualification status, near term applicability,
and number of suppliers. Weighting factors were allocated to the criteria and a ranking was
developed, which was then reviewed with the JIP participants.
The SCR TDZ design solutions evaluated with having a potential for high value in terms of
increase in estimates of fatigue life at TDZ, and with an increased likelihood for their near
term application in deepwater projects, were selected for detailed work in Stages II to IV of
the JIP. Several of the solutions listed in Section 3.2 were excluded from further
consideration for the following reasons:
Complications and interface issues with carbon steel pipe above (and beyond) TDZ
The selected solutions, from the Stage-I comparative assessment, and the product suppliers
that joined as JIP Contributing Participants are listed as follows:
Thick light weight coating over conventional X-65 steel riser sections with Socotherm,
Escobar, Argentina
Steel riser sections with integral threaded connectors with VAM, Houston, USA, and
V&M Tubes, Aulnoye-Aymeries, France
Steel riser sections with upset ends with V&M Deutschland, Dusseldorf, Germany
Steel riser sections with upset ends with Tenaris Tamsa, Veracruz, Mexico
The above 4 solutions with 5 product manufacturers/suppliers were then evaluated in Stage
II to estimate the likely design changes at TDZ in two regions. In Stages III and IV, the
design development and qualification tasks were undertaken in co-operation with these
companies.
4.1 General
In Stage II of the JIP, case studies were undertaken in two deepwater offshore regions to
estimate the need and extent of TDZ design enhancement required to obtain desired
increase in fatigue life estimates from each applicable solution. This section presents a
summary of the basis used in case studies and analysis performed, and presents a
summary of improvement in fatigue life estimates from each solution.
The estimates given in this section were used to initiate dialogues with product
manufacturers/suppliers to obtain detailed information about products/solutions and
undertake assessment, design development, and qualification tasks.
The riser design basis considered in three case studies undertaken in the JIP is
summarized in Table 4.1. The GOM case studies considered production and export
SCRs tied to a semi-submersible vessel whereas the WOA case study considered
production SCR tied to a FPSO. In GOM production riser case study, the operational
conditions considered were 10,000 psi internal pressure and sour service case, in order to
evaluate the extreme case application scenarios. In WOA production riser case study, the
data for a field in relatively less water depth (2,460 ft) with non-sour service was
considered to evaluate the need for extreme application cases.
Strength, fatigue, and VIV analyses were done in case studies for TDZ design solutions
that could be supplied by product manufacturers for riser diameters and thicknesses
considered. Time domain irregular wave analysis was performed using Riflex, and
included both wave frequency (WF) and low frequency (LF) vessel motions. Fatigue
damage was estimated using Rainflow Counting method. Simplified VIV analysis using a
single critical current was done to estimate VIV response.
Figure 4.1 illustrates variation in fatigue life estimates over SCR length, and clarifies that
the low fatigue life is estimated over a short riser length at TDZ. Thus, design change (or
application) of solutions at the TDZ is required over a short length as shown in Figure 4.2,
which also illustrates variation in SCR shape with vessel motion (far/mean/near position).
1.0E+11
1.0E+10 Life on SCR ID
1.0E+09 Life on SCR OD
Min. Fatigue Life (Years)
1.0E+08
1.0E+07
1.0E+06
1.0E+05
1.0E+04
1.0E+03
Min. Life on OD = 760 Years
1.0E+02
Min. Life on ID = 59 Years
1.0E+01
1.0E+00
0.0 500.0 1000.0 1500.0 2000.0 2500.0 3000.0
Distance Along SCR from Bottom End (m)
Far Position
-200
Near Position
-400
Mean Position
Water D epth (m)
-600
-800
-1000
Titanium
-1200
-1400
- 500 0 500 1000 1500 2000 2500
Horizontal Distance (m)
Table 4.2: Estimated Fatigue Life Improvement Factors from Case Studies
The base case analyses for case studies showed that significant fatigue damages would
occur over only a small length at TDZ, and only for this length design changes in riser
sections are required to improve the fatigue life estimates. Thus, for this JIP work, a
length of 320 ft (100 m) was considered for titanium, integral connector and upset end
solutions, and a length of 1,000 ft (305 m) was considered in case of the thick, light-weight
coating solution. The exact riser length requiring application of these solutions in the
critical fatigue zone, for a specific practical application in a specific region, would require
further review of installation tolerances and other site-specific factors.
5.1 General
The qualification of solutions or improvement in TRL comprised of work under Stages III
and IV as shown in Figure 2.2, which varies for each product/solutions developed for TDZ
design enhancement. This required joint effort by Granherne and Contributing
participants.
More recently industry has increased emphasis on the qualification of new products, and
of existing products in new applications or environment, to improve their technology
readiness level to improve their potential consideration in projects and FEED studies.
Qualification approach for couplings is given in various DNV and ISO standards, but
systematic qualification procedures for new technology were initially proposed by DNV
through their RP A203 [15]. In addition, similar efforts for qualification procedures have
been initiated by DeepStar, API, and ISO committees for development of specific
deepwater technologies.
The general qualification approach considered for the JIP work is presented in this
section.
5.2 Approach
The qualification approach applied in the SCR TDZ JIP includes 6 key steps as shown in
Figure 5.1.
Design
Design Development
Development
Assessment,
Assessment, TRL
TRL
Qualification
Qualification Plan,
Plan, Specs,
Specs, QCP
QCP
Manufacturability
Manufacturability
Testing,
Testing, FEA,
FEA, Post-failure
Post-failure
Evaluation
Evaluation
Qualification
Qualification Reporting
Reporting
The qualification approach varied for each solution due to their being at different TRL at
start of the JIP, and some solutions having prior experience from different applications.
The qualification work was undertaken jointly with solution/product manufacturers to
obtain maximum benefit from this JIP in improving TRL of each solutions with a specific
manufacturer/supplier.
System description and evaluation included identification and development of system level
design and addressing product design, manufacturability or applicability to riser sections
at TDZ, installability, maintainability, schedule and cost estimates. In addition, for
connection of a solution to SCR steel riser sections, the system evaluation included
selection and design development of specific connection, e.g., welded, threaded or flange
connection, and detailed evaluation and qualification analysis of solutions/products
subjected to loads at SCR TDZ.
Functional and performance specifications were established through case study analysis,
industry standards and guidelines, and lessons learnt from previous applications of a
product in other situations. These specifications were then used in the development of
design and undertaking qualification testing and analysis tasks with product
manufacturers.
Through an overall review of SCR design with TDZ enhancement, the failure modes and
mechanisms of the solution and its sub-components were identified. Industry experience
from different applications, where available for a solution, was used to rank and identify
critical failure modes requiring specific qualification activities with product/solution
manufacturers/suppliers. This activity provided clarity to the development of a
qualification program for each solution, which was then agreed with the JIP Participants
and selected solution manufacturer/supplier.
The development of solutions and their qualification undertaken in the JIP required
involvement of experts dealing with metallurgy and materials, welding technology, fracture
mechanics besides design, analysis, construction and installability assessment. For each
solution, a team of experts from product manufacturers participated in the design
development and qualification.
6.1 Approach
This solution consists of application of thick light weight coating instead of normal
insulation coating, over a small length at TDZ of a SCR with welded (onshore and
offshore) riser sections. The light weight coating makes the TDZ sections at/near neutral
buoyant stage, which results in reduced bending stress range at riser sections/welds at
TDZ and corresponding increase in the estimates of fatigue life at SCR TDZ welds.
The design, installation, qualification assessment, and qualification testing and analysis
program undertaken in the JIP is identified in following sub-sections.
In this solution, the weld type and details, and the fatigue S-N curve used remain the
same as for a conventional SCR design with X-65 or X-70 grade steel. The general
execution plan for coating application, transportation, and installation remains similar to
the SCR with insulation coating, except that due to having some riser sections with thick
light-weight coating the installation of TDZ sections will be feasible by J-lay installation
vessels. For a case where reeled installation is feasible for conventional SCR design, the
installation vessels available with both reeling and J-lay tower provide an alternative by
installing riser sections with normal insulation coating from reels and TDZ riser sections
with thick light-weight coating from J-lay tower.
The characteristics and design of thick light-weight coating are similar to the proven five
layer syntactic polypropylene insulation coating. The overall density of thick light-weight
coating is 41.24 lb/ft3 (660 kg/m3) compared to normal insulation density of 49.36 lb/ft3
(790 kg/m3). The five layers of this coating system shown in Figure 6.1 are as follows:
Solid polypropylene
Syntactic polypropylene
Solid polypropylene
The first three layers in this design are for anti-corrosion. The fourth layer is for buoyancy
and thermal insulation, and its conventional design is varied by introduction of higher
percentage of hollow glass microspheres to obtain lower density, in addition to achieving
required thermal insulation properties and hydrostatic pressure resistance. The function
of the fifth layer is to provide protective coating (top coat), and its thickness at TDZ can be
varied for applications in regions with potential for higher wear and abrasion.
The case studies of production and/or export risers for GOM and WOA, which are
discussed in Section 4, estimated need for up to 9 thick light weight coating to obtain
required fatigue performance. Case studies analyses indicated that the length of
application of thick light-weight coating over steel riser sections at TDZ could range from
1,000 ft (305 m). In case of the North Sea applications, the required length of thick LWC
at TDZ would be longer. These estimates formed the basis for defining the qualification
work for this solution.
Prior to this JIP, the 5-layer syntactic PP insulation coating was qualified by Socotherm for
5 inch (127 mm) thick application and it has been used in recent deepwater projects for
varying application thicknesses. In this case, the coating density was conventional. The
value from this solution was also evaluated by others through analytical studies for
improvements at TDZ for studies undertaken for the GOM and the NS [16].
Thus, it is important to understand issues associated with the application of thick coating
(up to 13 thick) with light-weight density. The key issues identified below were
considered in the development of qualification program for thick light weight coating:
Identifying feasibility of thick LWC application over a range of pipe diameter, pipe
thickness
In order to provide confirmation to the above, application and testing of thick light weight
coating over riser pipes was required and it formed the basis for improving the
qualification status and TRL of this solution.
A qualification program was developed with Socotherm Americas for application and
testing of thick, light-weight coating on riser sections at their coating plant in Escobar,
Argentina. The qualification work started with evaluation of the design, construction,
installation issues and identification of critical failure modes. Then specifications and
quality control plan were prepared for application and testing work of thick, light-weight
coating. Various laboratory and full scale tests were performed. The demonstration of
coating application and tests were presented to the JIP Participants during a plant visit in
November 2005.
Socotherm applied 8.75 inch (222 mm) thick, light-weight syntactic PP coating on eight
10.75 inch (273 mm) diameter 40 ft (12.19 m) long riser sections and applied FBE coating
on four pipes for the qualification testing work. In addition, they applied 4 inch (101.6
Thick 4th layer with higher percentage of hollow glass microspheres and their
impact on performance
Installability assessment
Larger (length, volume) field joints for riser sections with thick coating
The following laboratory tests were performed to establish properties of thick coating
(8.75 thick), and some of these tests were undertaken before and after the full scale
tests:
The following full scale tests were undertaken for steel pipe coated with 8.75 inch (222
mm) thick syntactic PP:
The stinger roller test (as shown in Fig. 6.2) resulted in no cracking or disbonding on the
coating surface. The full scale bending test was performed on a riser section with a field
joint in the middle, as shown in Fig. 6.3, and it also resulted in no cracks or damage on the
coating surface and the field joint, after 4 cycles of bending/straightening around the
former with 240 ft (70 m) radius. Additional laboratory tests done on samples taken from
a field joint showed that required properties were maintained. In addition, Socotherm
demonstrated application of field joint coating using solid polyurethane, and repair
techniques used in coating application yards.
Figure 6.2. Stinger Roller Test Figure 6.3. Full Scale Bending Test
6.6 Conclusions
The feasibility of application of thick, light-weight coating on steel pipe has been confirmed
through the tasks listed in Sections 6.4 and 6.5, and this has been achieved by extending
the known technology and procedures for insulation coating established by Socotherm
through applications in several deepwater projects. The application and testing were
done for 8.75 inch (222 mm) thick lightweight coating due to limitations of maximum
diameter (pipe with coating) at the Socotherm plant at Escobar, Argentina. Socotherm
confirmed that the coating application for 13 inch (330 mm) thickness or more can be
achieved at another plant using the coating application process demonstrated for 8.75
inch (222 mm) thick coating. The variations in the design, due to additional thickness
The laboratory and full scale tests proved the ability to achieve required properties for
installation and long term in-service performance. Through this qualification testing effort,
the feasibility of application of very thick light-weight coating with the ability to retain
required insulation, mechanical, and system properties has been confirmed.
7.1 Approach
This solution comprises of weldable riser sections, 40 ft long X-65 or X-70 grade, with
thicker sections at both ends. This reduces the axial and bending stresses (and stress
ranges) at the welded connection at thick ends of two adjacent riser sections. The
increase in the fatigue life estimates from analytical predictions corresponds to the
reduced stress range at thick ends. Through this approach, in some cases the fatigue life
estimates could increase by a factor of 10 over conventional SCR riser sections with
uniform thickness.
The riser sections with thick ends could be manufactured by upsetting or by machining
from a thicker pipe. In this JIP, the upsetting approach was undertaken for development
of riser sections with upset ends. The design development, installation, qualification
assessment, and qualification program undertaken in the JIP with two major
manufacturers is discussed in the following sub-sections.
The case studies presented in Section 4 estimated need for increase in the pipe wall
thickness at welds by 39% for the GOM case and by 20% for the WOA case to obtain the
required fatigue performance. The case studies estimated that such riser sections (with
thick ends or upset ends) would be required at TDZ over a length of about 320 ft (100 m).
These estimates of likely need for increase in the pipe wall thickness and the length of
application (or number of riser sections with upset ends) became the basis for
communication with various manufacturers and for undertaking design development,
assessment, and qualification tasks.
The manufacturing limits for upsetting with regard to pipe diameter and wall thickness,
and the implications of increase in thickness on cost were identified with each
manufacturer. Based on this, specific cases were selected for qualification with two
manufacturers as discussed further in this section.
The design development included establishing requirements for automatic welding and
ultrasonic testing (UT) operations, through discussion with major welding, UT and
installation contractors, and establishing limits for upsetting with two manufacturers
undertaking development of upset end solution. The following are important in the
development of upset end and transition zone design:
Radii of curvature from taper transition to upset end and original pipe
Manufacturing feasibility
Finite element analysis (FEA) was performed to optimize transition zone design. Special
attention was paid to the stress concentration factor (SCF) values at the transition. This
solution provides significantly improved fit-up at welds, which is reduced to the tooling
tolerance. Thus, the SCF at welds is reduced and the fatigue life estimates are increased.
Upsetting process is commonly used in the industry for casing and riser joints with
threaded ends. Steel grades with higher carbon content are normally used for these
applications. The process has not been used so far for weldable pipe of SCR quality.
The steel risers with thick ends used in deepwater risers and flowlines were with forged
and machined ends welded onshore at ends of riser or flowline sections. However, by this
approach the increase in fatigue life is estimated to be less than a factor of 3,
corresponding to the variation in fatigue life estimates for the onshore welds compared to
the offshore welds for a conventional SCR.
The upset end solution developed in this JIP is for a weldable case using X65 steel grade
riser sections with integral upset ends. The riser sections with thick upset ends are
feasible to manufacture by upsetting through forging, or by machining of a thick section.
For qualification effort in this JIP, the manufacturing process using a die to upset a riser
section at both ends was selected. This has not been qualified so far and, thus, there is a
need to undertake qualification tasks of upset end solution, which is feasible to weld
offshore at upset ends, for pipe diameters and thicknesses used in the deepwater
production and export SCRs.
The qualification work for this case in the JIP was undertaken with two manufacturers:
TenarisTamsa, Veracruz, Mexico and V&M Deutschland, Dusseldorf, Germany (VMD).
The important steps in the development and qualification of this solution are to establish
the design requirements, prove the manufacturing process and tolerances, and undertake
material, weldability and fatigue tests. Specifications and quality plans for manufacturing
of samples, welding, and testing procedures were prepared with each company.
The design, analysis, and assessment work undertaken in this JIP helped to identify the
most important qualification tasks as follows:
Manufacture the upset end pipe and confirm that required material and mechanical
properties are achieved, and sour service performance standards are met
Perform welding of upset ends for both weldability and full scale fatigue tests using
recommended procedures
Undertake weldability test to confirm that the steel at upset end is weldable
Undertake fatigue tests to confirm the fatigue behavior of upset end transition
In order to achieve the above for riser sections with very thick upset ends, adjustments
may be needed in the X65 steel chemical composition and the manufacturing process
(heat treatment, quenching) during the upsetting process.
NACE corrosion tests hydrogen induced cracking (HIC) and sulfide stress
cracking (SSC)
Microstructure evaluation
GOM case: 10.75 inch OD x 1.25 inch WT (273.1mm x 31.8mm) Pipe body, 1.80
inch WT (45.7 mm) upset ends, steel grade X65 for sour service application
WOA case: 10.75 inch OD x 0.866 inch WT (273.1mm x 22.0mm) Pipe body,
1.102 inch WT (28 mm) upset ends, steel grade X65 for non-sour service
For both cases, the upset transition was designed using FEA, and specific alloy design
and heat treatment parameters were used to obtain the desired micro-structural
characteristics in both pipe body and heavy wall upset sections, which resulted in
excellent mechanical properties, and met the strength and corrosion requirements.
TenarisTamsa undertook manufacturing of a large number of pipes for two design cases,
and performed all qualification tests identified earlier, including weldability and full scale
fatigue tests [6]. The manufacturing process followed proven measures undertaken by
Special dies were manufactured and procured for this work by TenarisTamsa. The WOA
case required 1 set of tooling, whereas the GOM case with WT of 1.75 at upset end
required 3 stage upsetting process. Upset end pipes were manufactured for both sizes as
required from the JIP case studies. (see Fig. 7.1).
Figure 7.1. Steel Pipe as Upset and as Machined GOM Design Case
(Source: TenarisTamsa, Veracruz)
After upsetting, heat treatment, ID/OD quenching and tempering steps are key in this
process to obtain desired mechanical properties and micro-structure. This required a
through evaluation/design by Tenaris of quenching (transforming austenite into martensite
and harden steel) and tempering (reheating - lowers strength but increases ductility and
toughness).
The objective of fatigue test was to establish the fatigue performance at upset transition,
thus TenarisTamsa developed and qualified a procedure to improve the weld fatigue so
The weldability test required characterization of the HAZ for two cases subject to different
combinations of heat input and interpass temperature using an API RP2Z Bevel. All tests
passed or exceeded requirements, including hardness HV10 below 248 for sour service
case.
Figure 7.2. Welding at Upset End Using Orbital GTAW-P Arc Machines
(Source: TenarisTamsa, Veracruz)
In case of girth welds for fatigue testing, an additional consideration was made to develop
stronger welds to make sure that during the fatigue tests no failure occurs at weld.
TenarisTamsa achieved this by careful selection of welding consumables, and butt welded
using GTAW and SAW procedures, with an interpass temperature between 170 deg to
260 deg C. Various mechanical tests (hardness, tensile) and CTOD were performed.
Figure 7.3. Full Scale Fatigue Testing of Upset SCR TDZ Welded Joints
(Source: TenarisTamsa, Veracruz)
The fatigue tests were done at 3 different stress ranges and failures were recorded at 1
million to 17 million cycles and higher cycles in case of run-outs. These tests
demonstrated that the fatigue performance at upset end transition is better than the base
metal target and runout S-N curves, such as DNV B-1 curve [17], used for presentation of
results.
The focus of these tests was on establishing fatigue performance at upset end transition.
Thirteen (13) of these tests failed in pipe main body at some distance from upset
transition, 3 were run-out, and 2 were with cracks in each in upset length and in weld.
The failures were originated both by internal and external surface initiations, which were
further studied through fractography.
The JIP Participants visited TenarisTamsa mill and R&D center at Veracruz to review and
see various processes used and demonstration of some tests, including full scale fatigue
test. From these reviews it became clear that the extensive qualification program
undertaken has resulted in bringing both thin and thick wall upset end riser sections in X-
65 steel grade, including sour service applications, to project ready stage.
VMD manufactured for the JIP work, a total of ten (10) X-65 grade seamless pipes, 10.75
inch (273 mm) in diameter and 40 ft (12.2 m) long, and then prepared samples of riser
sections with upset ends by upsetting, testing, and CNC machining. VMD used an
existing die from previous application, for 10.75 inch OD x 1.033 inch WT (273 mm x
26.25 mm) pipe body and it was upsetted and machined to obtain 1.25 inch (31.75 mm)
WT at upset ends. The upset transition design was developed using FEA and
consideration of welding and installation of riser sections with upset ends. Discussions
were held with major welding and UT contractors, and installation contractors to develop
upset end design.
VMD implemented their standard approach for upsetting developed from previous
applications for manufacturing and supply of pipes with threaded connectors, and made
variations where necessary for weldable upset end solution for the JIP. Various tests
listed earlier were performed to ensure required mechanical and corrosion properties are
obtained. Fig. 7.4 shows two stages in the upset end manufacturing process: upset end
using a die and design shape obtained after CNC machining.
The weldability tests were performed per API RP 2Z to establish toughness of the heat
affected zone (HAZ). The properties of HAZ depend on heat input during welding and the
base material chemical composition. The welding procedure specifications were prepared
by Salzgitter Mannesmann Forschung GmbH, Duisburg, Germany (SZMF) for welds
produced with low 15.24 to 20.32 kJ/inch (0.6 to 0.8 kJ/mm) and high 63.5 to 76.2 kJ/inch
(2.5 to 3.0 kJ/mm) heat input, and welding was performed by various procedures to
achieve required welds for weldability and fatigue tests. The welding being performed
using mechanized GMAW method is shown in Fig. 7.5.
The weldability tests included CTOD testing of HAZ at its straight fusion line, Charpy
transition curves, and tensile tests. The toughness values were measured at both fusion
line and in the visible HAZ boundary material.
A total of 12 small scale strip fatigue tests and 3 full scale fatigue tests were undertaken
with VMD, based on available facilities at R&D center at SZMF, to establish fatigue
performance at the upset transition. This approach provides increased number of data
points, and the results are normalized to account for effect of undertaking these tests at a
different stress ratio (R-ratio) from that used in the tests to develop the base metal S-N
curve (such as DNV B-1 S-N curve). The small scale strip fatigue tests have shown
fatigue performance of upset end transition beyond the target S-N curve for the base
metal. With learning from initial small scale tests, full scale fatigue tests using 4-point
rotating bending machine were undertaken at SZMF under constant amplitude loads.
These tests were planned to continue up to about 2 million cycles.
The JIP Participants and study team visited the VMD Plug Mill at Dusseldorf and SZMF
research center in January 2006. The work done by VMD confirmed that it is feasible to
manufacture upset end pipes with moderate to high thickness increase at upset end. In
the case of X65 riser section with a very thick upset end, multiple passes of upsetting
process and additional variation in X-65 chemical composition may be required to obtain
higher thickness at upset end with required properties.
The design and qualification work undertaken with TenarisTamsa and VMD confirmed that
the it is feasible to manufacture riser sections with upset ends at both ends for pipes with
diameter up to 16 inch (406 mm) and the wall thickness could be increased by 50%,
depending on pipe diameter and WT. The qualification tasks confirmed feasibility to
manufacture riser sections with upset ends by implementation of single stage upsetting
process or for thick upsets with multi-stage upsetting process.
The upset end solution requires manufacturing and procurement of upsetting dies for
specific riser size, thus increasing the delivery time from placement of order. This solution
will thus require early placement of orders as long lead items.
8.1 Approach
The integral threaded connector design developed by V&M Tubes (VAM) for applications
in high fatigue zones of riser systems was considered in the JIP for application at SCR
TDZ. The integral connector design evaluated in the JIP aims at providing fatigue
performance similar to the base metal through elimination of weld.
The VAM connector design evaluated in this JIP is manufactured using high grade steel,
P110, and has smaller OD compared to weld-on connectors used for high fatigue zones of
riser sections. The riser sections are manufactured in 40 ft (12.2 m) length, with integral
threaded connectors (pin or box) at each end, for diameters up to 13-5/8 inch (346 mm)
by upsetting of pipe ends or by machining a thick walled pipe. It features easy stabbing
and quick completion of connection assembly from stabbing to final torque. In case of
sour service application, use of C110 grade steel is proposed.
Internal and external metal/metal (M/M) seal to provide highest pressure integrity
The integral connector design has evolved over time from OCTG (oil country tubular
goods) small diameter production casing applications. VAM Riser integral connector is an
improved design with higher fatigue performance, thus it is considered suitable for
application at SCR TDZ.
The industry experience with alternative threaded and coupled (T&C) connector design is
mostly through applications in top tensioned risers [18]. New designs with higher fatigue
resistance from riser sections with T&C connectors have also been developed recently by
V&M [19] with some features for fatigue design enhancement features that are similar to
those considered in the integral threaded connector design evaluated in this JIP.
There are cases of using weld-on threaded connectors, welded onshore to steel sections,
in top tension riser sections with need for higher fatigue performance. But the
improvement in the fatigue performance from weld on connectors is similar to that
estimated for an onshore weld compared to an offshore weld.
VAM has an ongoing in-house testing program on such connectors with focus on
qualifying manufacturing route and performing qualification tests. VAM has undertaken
significant tests for their other new riser designs that include some of the features
incorporated in the integral connector solution.
Development of installation plans for mechanically connected pipelines and risers has
been undergoing in a DeepStar sponsored study [20] and its outcome would provide
additional basis and identify need for use of integral connectors qualified in this JIP.
In this JIP, the overall system design issues from use of riser sections with integral
connectors were evaluated and focus was kept on the design of connector and cross over
segment between P110 and X65 riser sections, and understanding the associated failure
modes. The failure modes were identified and detailed analysis and testing work
available with VAM were discussed. The qualification tasks undertaken with VAM, which
are specific to this JIP comprises of the following:
Detailed FEA of VAM Riser integral connector for TDZ loads determined from
GOM case study for production riser
Design and detailed FEA of a cross-over segment between VAM Riser integral
connector and X-65 welded riser section for strength, sealability, and SCF
estimates
The details of the manufacturing route, quality control plan, and test reports generated in
this program were provided by VAM for review and discussion under the JIP qualification
task, and to identify if any additional testing work is required. These test reports were
presented to the JIP Participants for review.
The sealability tests were done for combined loads per ISO 13679 [21] standards and
failure tests were performed to check that the connection is better than the pipe. Full
scale survival fatigue tests using resonant bending fatigue machine (Fig. 8.1) were
completed for a total of 6 samples at Aulnoye-Aymeries R&D center.
The installability of TDZ segments and feasibility of making such connections from J-lay
tower were reviewed in detail with major installation contractors and connection tool
suppliers, to identify feasible approaches. These reviews confirmed that it is feasible to
make offshore connections for SCR TDZ riser sections with threaded ends (pin or ox) at
weld stations of some J-lay installation barges, which would be used for installation of rest
of SCR sections. However, a tool needs to be developed for rotation of the upper riser
section.
Detailed FEA of VAM Riser integral connector and cross-over segment were performed
for both operating and extreme loads at SCR TDZ, estimated for the GOM case study, to
understand the behavior of threaded connections subjected to 100 year and 1,000 year
seastates. The analysis results showed adequate strength and low SCF estimates.
The JIP Participants and the study team visited VAM research center for Premium
Connections, and its corrosion and metallurgy laboratories, at Aulnoye-Aymeries, France
and VAM machining and threading plant at Reisholz, Germany in January and September
2006. They witnessed the fatigue test in progress.
8.5 Conclusions
The JIP work for integral connector solution has confirmed its technical feasibility as SCR
TDZ design enhancement solution, by its ability to provide a significant increase in the
fatigue life estimates. The design and performance of its X-over segment (connection
between P110 riser sections with integral connectors and conventional X-65 riser
sections) was confirmed through FE analysis. This solution is feasible to manufacture up
to 14 inch (356 mm) diameter riser sections. Use of P110 steel grade is qualified for non-
sour service applications and the use of C110 grade steel is suggested for sour service
applications. Additional qualification effort is required for corrosion-fatigue behavior of
high strength steel.
This work has shown that the VAM Riser integral connector has undergone tasks to
significantly improve its technology readiness level (TRL) and is available for
consideration in FEED stage of projects for site-specific evaluation and development.
This alternative has the potential as a high-value design solution for SCR TDZ riser
sections and it could provide a significant level of improvement in the fatigue life
estimates.
9.1 Approach
The overall assembly of titanium segment and connections to X-65 conventional SCR
sections is illustrated in Figure 9.1.
The titanium riser sections at TDZ are Ti-Grade 23 or 29 for non-sour service, and Ti-
Grade 29 for sour service applications. The GOM case study for sour service case,
analyzed with about 320 ft (100 m) length of Grade 29 titanium segment at SCR TDZ,
estimated an improvement in the fatigue performance by a factor more than 400 (see
Table 4.2), in comparison to the conventional X-65 grade welded riser sections.
The overall system design of titanium segment at the SCR TDZ, manufacturing methods,
and feasible sizes were reviewed, and installability assessment was done to identify
specific options. An evaluation of the overall system was done to identify the critical
failure modes and specific issues to focus in the JIP.
The details of the SPO compact flange [22] and qualification test reports were obtained
from Vector International and were reviewed for SCR TDZ specific issues. Such flanges
are already commonly used to connect titanium TSJs to steel riser sections, which are
subjected to fatigue, internal pressure, and high tension loads.
The important design consideration of the SPO compact flange is to achieve preloading of
bolts and three-part sealing system as follows:
Inner metal-to-metal seal at the flange heel adjacent to the bore, to ensure no gap
between the flanges at the bore during operations and to protect the primary seal.
Flexible metal ring located in a seal ring groove as the primary seal, which is
pressure energized by the deformation of the seal flanges towards seal centerline,
and also when subjected to pressure either from inside or outside.
The preloading of bolts and flange face to face contact after makeup enables eliminate the
risk of load induced seal damage. The integrity of the primary seal can be determined by
the use of a test port to the primary seal groove, which can be pressure tested.
The qualification status or past experience with key components and operations were
identified and some of the companies providing such products were contacted to obtain
details from past qualification or development work.
Extensive qualification testing for titanium pipes has been previously undertaken by
several companies and manufacturers in Norway and also by RTI Energy Systems [23] for
titanium pipes and riser components. Items such as welding, corrosion, fatigue, surface
finishes and their effects on S-N curves were examined. RTI Energy Systems undertook
additional qualification testing in connection with the preparation of DNV recommended
practice for titanium risers [24].
Under the Norwegian DEMO 2000 program, significant qualification work for material,
corrosion, welding, rubber coating was undertaken [9, 14]. The heavy-weight coating
solution, Vikoweight, by Trelleborg Viking has been qualified for bondability on titanium in
the DEMO 2000 project [9]. In case of TDZ enhancement of a production SCR, heavy-
weight coating is not required for operating conditions.
Procedures for welding of titanium riser sections offshore, on an installation vessel, have
not been qualified, but welding onshore of titanium riser sections has been qualified
through the DEMO 2000 project.
Significant FEA of SPO Compact Flange connections have been done by both Vector
International and RTIES for designs applied in various riser TSJs to confirm strength and
fatigue integrity of this mechanical connection.
The important areas requiring detailed assessment in the JIP were identified as follows:
Electrical Isolation requirements between steel and titanium riser sections for
certain conditions
FEA of titanium-to-steel SPO compact flange connection at the ends of titanium segment
was done by Vector International for this JIP. The focus of this analysis was to evaluate
its performance when subjected to the loading conditions at SCR TDZ, through performing
functionality and SCF analysis, and addressing the bolt fatigue by developing a transfer
function between nominal pipe stress range and bolt stress range. The analysis proved
that the titanium-to-steel connection would meet the performance requirements in its
application at SCR TDZ.
Through the assessment work done in the JIP, it was identified that for most applications,
the currently available non-isolating designs would be adequate. But for dynamic
application cases with hot sour flows with high water cut, a new design of an isolation
system between steel and titanium riser sections would be required.
S-N fatigue tests on grit blasted Grade 29 titanium pipe strip specimens at EWI,
Columbus, Ohio.
The above tests proved that the grit blasting applied to titanium pipe OD surface by
Socotherm does not deteriorate fatigue life while safely meeting established fatigue
design curve; nor does it compromise titanium seawater corrosion resistance.
9.5 Conclusions
The work undertaken in this JIP has shown a practical approach for application of titanium
segment at TDZ, and its connection with conventional X-65 riser sections. Titanium
segment solution can be manufactured up to 36 inch (914 mm) diameter by extrusion
process. For sour-service applications, Grade 29 titanium sections shall be used, and for
non sour-service applications Grade 23 or 29 titanium sections be used. However for
higher temperature service above 75oC, Grade 29 titanium sections must be used.
The SCR TDZ JIP initiative has been successful in increasing the technology readiness
level (TRL) of 4 alternative design solutions undertaken to solve the industry concern with
SCR fatigue performance at touch down zone (TDZ), when used with floating production
units in deepwater and ultra-deepwater. This issue becomes more severe in case of SCR
applications with high motion floating units, larger diameter export risers, and production
risers with sour service and HPHT fluids, and for applications in fields with lesser water
depths or firmer seabed.
The JIP work has lead to the development and qualification, or increase in TRL, of
multiple design solutions selected with the potential to significantly increase the fatigue
performance at TDZ. These solutions are grouped as below:
The X-65 grade solutions are welded solutions, and the case studies showed that in
some applications these solutions could provide an increase in the fatigue life estimates
by a factor of 10. Whereas, the high strength solutions replace X-65 grade riser sections
with welds by high strength riser sections with threaded or flange connections, or by
titanium welds (onshore) with high fatigue performance (better S-N results). The case
studies showed that the high strength solutions could provide significantly higher fatigue
performance (beyond factor of 10 increase) at the SCR TDZ. Thus, these four design
solutions have potential to provide a significant change from other approaches considered
by the industry to remedy low fatigue life estimates at SCR TDZ. The application of high-
strength solutions could also be extended above TDZ to provide a strength + fatigue
solution and enable further increase in selection of SCR design in projects.
In general, the above solutions would apply to both sour and non-sour service
applications, with feasible riser sizes (D, t) defined by manufacturing or application (e.g.,
coating) limits per the facilities of product manufacturers. Thick light weight coating (LWC)
can be applied on all pipe sizes, thus it provides the only cost-effective solution for large
diameter export risers compared to other solutions developed in the JIP. The feasibility of
manufacturing riser sections with thick ends by upsetting process is identified to be up to
16 OD and the limiting wall thickness at upset end is defined by the weldability offshore,
which may vary with installation contractors. The riser sections with integral connectors
are feasible to manufacture up to 14 OD riser pipes, whereas titanium riser sections
could be manufactured up to 36 OD.
The installation of SCR TDZ sections with these solutions will be feasible by conventional
J-lay installation, a more common approach used for deepwater SCRs. There may be
potential for reeling+J-lay installation for certain sizes.
Being design solutions they would enable development of an optimum SCR TDZ design,
or an integrated FPS hull and riser design for site-specific requirements with minimal
impact on the overall SCR cost and schedule. These solutions provide high value in
terms of improved reliability of SCR and have potential to provide increases savings in the
overall platform design and benefits to the riser integrity program.
The case studies undertaken in this JIP were for the Gulf of Mexico and West of Africa
regions, but the applications at TDZ of solutions developed will be similar in other regions.
In some regions, such as the North Sea the riser length at TDZ requiring design
enhancement by these solutions would be longer compared to those estimated from the
JIP case studies. The improvement in fatigue life and the associated cost to implement
these solutions will vary for different solutions and depend on a number of field specific
parameters identified earlier.
The qualification work undertaken within this JIP included design, analysis, manufacturing
(or application) and testing, and also dealt with construction, transportation, installation,
and in-service performance issues, which all have helped in providing clarity for their
potential applications in projects. With significant development effort undertaken in this
JIP, progress has been made to significantly improve their TRL status and increase the
probability of their consideration/use in projects for site-specific applications at the SCR
TDZ in all regions.
The qualification requirements could vary among various operating companies, in line with
variations in their new technology/concept selection procedures. The qualification work
performed in this JIP is significant and in most cases enables them to be classified as
project ready solution by companies. However, some operating companies based on their
internal evaluation procedures may consider need for additional testing or analysis.
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