Moring Failure Detection PDF
Moring Failure Detection PDF
Moring Failure Detection PDF
RR1097
Research Report
© Crown copyright 2017
Prepared 2010
First published 2017
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to the Information Policy Team, The National Archives, Kew, London
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Safety Executive (HSE). Its contents, including any opinions and/or
conclusions expressed, are those of the authors alone and do not
necessarily reflect HSE policy.
HSE Books
1
Mooring
Loremfailure
ipsumdetection
dolor sitsystems
ametfor
floating offshore installations
consectetuer adipiscing elit
Mooring Integrity Joint Industry Project Phase 2
GL Noble Denton
No 1 The Exchange
62 Market Street
Aberdeen AB11 5JP
2
CONTENTS
SECTION PAGE
1 PREFACE 6
2 INTRODUCTION 7
2.1 PURPOSE 7
2.2 DEFINITION 7
2.3 ABBREVIATIONS 9
3 WHY HAVE A SYSTEM? 10
3.1 EXISTING RULES AND GUIDANCE 10
3.2 FACTORS TO BE CONSIDERED 12
4 SYSTEM SPECIFICATION 14
4.1 GENERAL CONSIDERATIONS 14
4.2 TENSION MEASUREMENT 18
4.3 POSITION 20
5 CURRENTLY AVAILABLE SYSTEM TYPES 22
5.1 DIRECT TENSION MEASUREMENT 22
5.2 TENSION / ANGLE MEASUREMENT 28
5.3 TENSION / STRESS MEASUREMENT 34
5.4 SONAR 36
5.5 VISUAL 37
6 SYSTEMS UNDER CONSIDERATION 39
6.1 HULL MOUNTED SONAR 39
6.2 SEABED SONAR 41
6.3 OSCAR 42
6.4 DEPTH SENSOR 43
6.5 MULTI-SENSOR UNIT 45
7 CONTRIBUTING SUPPLIERS 46
REFERENCES 54
3
FIGURES
4
Figure 6-7: Operation of Depth Sensor Arrangement (Courtesy of QinetiQ) 44
Figure 7-1: Summary of contributing suppliers Mooring Failure Detection system experience 46
5
1 PREFACE
This sub-report has been produced under the auspices of Phase 2 of the GL Noble
Denton lead Mooring Integrity Joint Industry Project (JIP), which has been
sponsored by the following companies:
6
2 INTRODUCTION
2.1 PURPOSE
The purpose of this report / activity is to provide the industry with a summary of the
state of the industry as regards mooring failure detection systems, including:
• What regulations exist?
• What guidance exists?
• What systems are being marketed?
• What systems are being considered as potential developments?
This report reviews the published guidance available in the industry in order to
provide a summary of the considerations those seeking to install a mooring failure
detection system should take into account.
This report is aimed at Operators and those involved in designing, verifying and
integrity management of mooring systems for offshore floating production units.
The extent of published guidance is quite limited and there is little prescriptive
regulation, either as to whether a Mooring Failure Detection system should be
fitted, or if one is fitted to what standard it should conform. Even within the limited
guidance and regulation there is little agreement between different organisations on
either of these subjects.
As the subject of mooring integrity is generally a growing discipline within the
industry, with more and more Operators realising that their mooring systems
represent a significant risk, this Report provides a collation that each Operator may
utilise to determine what may be optimal for its own floating production units as
regards Mooring Failure Detection systems.
This Report deals solely with the issue of Mooring Failure Detection systems which
is a very small part of Mooring Integrity. It is recommended that Guidances and
other available literature on actions to prevent loss of integrity, and procedures in
the event of single or sequential mooring line failures, since they are closely related
subjects, are also reviewed by Operators. At no time should a Mooring Failure
Detection system be considered an alternative to having a robust Mooring Integrity
Management System.
2.2 DEFINITION
A Mooring Failure Detection system is one that monitors one or more parameters
in connection with a mooring system in order to alert the personnel on board
whether any one of the mooring lines has failed. The same result can be
achieved by Mooring Failure Detection systems that look at the reverse of this,
ie to provide a continuous confirmation that all mooring lines are intact.
7
In practice many Mooring Failure Detection systems fitted to floating production
units are just one aspect of a larger and more inclusive mooring monitoring system
that has additional purposes including:
• Verifying design analyses and assumptions;
• Correlation of fatigue analyses;
• Continuous integrity monitoring.
There is however one key difference from the mooring monitoring functions of such
“larger” systems. A Mooring Failure Detection system is one that is a safety
system, one that provides important and immediate information to the personnel on
board as regards the level of risk they are exposed to right now, including risk to the
unit itself and potentially to the environment – from failed risers - and other
installations in the vicinity from a drifting unit.
This difference can be highlighted for those offshore installations that are within a
Safety Case regime, as the Mooring Failure Detection system may be considered a
Safety Critical Element, ie a Mooring Failure Detection system can often be
considered as a system that can “prevent, or limit the effect of, a major accident”
[1], where a major accident includes “any event involving major damage to the
structure of the installation or plant affixed thereto or any loss in the stability of the
installation” [1], which clearly includes the mooring system. As a Safety Critical
Element the Mooring Failure Detection system must therefore be “suitable” and
must be kept “in good repair and condition” [1], and this suitability is typically in the
UK determined by a Performance Standard.
Some of the published guidance for Mooring Failure Detection systems is based on
Safety Case regimes, but should be considered applicable for all locations,
especially as there is so little other guidance available. It is clearly understood by
any prudent Operator that the absence of a prescriptive regulation should not
be interpreted as a licence to ignore the subject.
8
2.3 ABBREVIATIONS
9
3 WHY HAVE A SYSTEM?
ABS: No
BV: No
DNV: Yes
LRS: Yes
API: No
ISO: Yes
O&GUK: Yes
In more detail:
“For deep water moorings, taut systems, fibre rope moorings, and other
cases where the verification of line pre-tensions cannot be achieved by
conventional methods, a permanent load monitoring device shall be
fitted on each line, for the control of line pre-tensions at the time of the
installation and of periodical surveys.
10
NB There is further guidance that other mooring systems may achieve
tension measurement by calculations, requiring the position of the
anchors and line lengths to be known within acceptable tolerances and
that the unit’s position is known and continuously monitored.
11
O&GUK [8] (ii)
Other sources of mooring related Rules and Guidances have been searched, such
as Norwegian (eg Norsok) and USA (eg MMS / CFR) but no separate reference to
Mooring Failure Detection (or Monitoring) systems has been found, other than with
a reference to the above documents.
The UK HSE has previously studied the subject of monitoring of mooring lines (for
example refer to OTR 96 018 which is a brief summary of the state of the industry
in 1996 – refer to paragraph 5.1), when clearly the available methods were
considerably more limited than described in this present report. The more up to
date HSE guidelines are contained within the O&GUK guidelines, reference [8].
12
• There have been a number of multiple line failures, including several that have
resulted in the floating production unit drifting [9]. To date, most fortunately, the
industry has been extremely lucky that no substantial loss of life has occurred
from any of these incidents, but it is clear that a drifting production vessel is a
risk to its own personnel and to any on another facility that it may drift on to;
• Some FPSOs have suffered a line failure but have not known about it for a
period of at least weeks. During this time the risk to the Installation – of further
line failures - was higher than any one was aware of and higher than the
safeguards in place were designed for;
• The mooring system for many floating production units is designed with factors
of safety close to those specified by their chosen design codes. Hence if one
line does fail, it is typical that production would at least initially stop, and then be
limited to specified threshold weather conditions;
• The majority of failure modes identified for floating unit mooring systems are
those that can be considered common to all mooring legs [8]. Generally a
mooring system has the same, or similar, components throughout each mooring
leg, and if one component experiences a problem, then the same component
on other legs has to be considered suspect.
13
4 SYSTEM SPECIFICATION
While a few of the relevant Rules and Guidances do provide a requirement for
floating production units to be provided with a Mooring Failure Detection system (as
above in Section 3), there is however less guidance for the methodology that
should be used.
Certainly there are a variety of standards for the electronic and software elements
of monitoring and associated alarm systems, including for safety critical systems 1
However few references have been found of a descriptive methodology of a
Mooring Failure Detection system; most of the references already given above
generally use words such as “suitable” or “applicable”, without any definition.
Tension measurement and position determination are the two most recurring
themes, although it should be noted that all but two of the references above (see
Section 3.1) are not specific as to the type of Mooring Failure Detection system and
as given in Sections 5 and 6 below, a number of other systems are available (refer
to Section 5).
General issues, and then more specific ones concerning the two most common
system types, are given below:
4.1.1 At which Points of a Mooring Catenary will the System Detect a Failure?
Identifying the priorities for the individual mooring system may be important
therefore in choosing the right type of Mooring Failure Detection system.
For a simple catenary mooring, such as shown below, the majority of failures are
likely to occur at or near the top connection, and around the touchdown area.
However the inclusion of non-continuous components such as shackles, or a
considerably more involved mooring such as combined chain/wire, chain/rope,
pennant buoys etc can easily skew the risks for an individual mooring system.
1
Including:
• ABS “Guide for Automatic or Remote Control and Monitoring for Machinery and Systems (Other than Propulsion) on Offshore
Installation”, October 2008
14
It is recommended that reference is made to [8] for a systematic way to identify
mooring integrity risks.
Top connection
Catenary
Ground Touchdown
A careful systematic approach to cause and effect for the alarms should be carried
out, noting that a false alarm may cause a production shut-down and resumption
which will have some associated risk – yet the Mooring Failure Detection system is
intending to reduce risk.
15
Involvement of the users can be a vital step to having the useful display. In some
cases this may be a simple traffic light (green, yellow, red) but somewhere available
there should also be more detailed information so that those with additional
knowledge and understanding can refer to it. Care should be taken that a simple
traffic light arrangement does not mask any aspect by giving too little information.
The type of alarm - audible, visual – and particularly the method of acknowledging
the alarm, can be key to the user’s acceptance of the system.
Existing Control Rooms can be very crowded, especially on converted tankers not
designed to have so many control systems. Integrated control systems where all
system displays are on one set of common screens saves this problem, but the
ease of bringing up the required information should be considered – noting that all
going well the Mooring Failure Detection system is not one that will often come up.
The distance from the sensors to the Control Room may also require some
consideration, degradation of signals over distance, interference with low power
signals, difficulty of routeing wires and getting bulkhead penetrations, may all be
factors.
Of typical difficulty is also getting the signals from the sensors on turret moored
vessels. The mooring system is fairly geo-stationary and the vessel rotates around
it. The turret is typically built with various electrical slip rings but typically
insufficient in number, especially for a retrofit. Hydro-acoustic transmission is a
common choice to overcome this, but this does add to the total cost of the system,
perhaps more cost than the actual Mooring Failure Detection system itself.
Modern technology also still provides a potential cost barrier due to battery life.
The electrical consumption of underwater equipment, and the ability to access
these for battery changeout, can be a significant life cost issue.
Use of divers or ROV vessel can be expensive but may not be avoidable on retrofit
systems.
16
4.1.6 Confirming the Presence of all Moorings within the Required Time Frame
Not all Mooring Failure Detection methods have a positive check, but may only
activate on failure; such a system that cannot give high confidence it is functioning
may not be applicable for high integrity risk mooring systems.
A self-check function that the electronics of the system are working correctly can be
useful to help eliminate spurious alarms. The limits of this self-check should be
clearly stated in any operations manual.
This may be affected by the amount of moving parts, robustness of the design,
proven track record of the components.
Duplication of equipment can ensure that there is a back-up in the event that one
sensor fails, and while both are working they can each act as a check on the other.
17
Another alternative is to have a system that provides two different means of
deriving the same information. For example a lost mooring warning may be
generated by a reduced tension, but also by an excessive offset or a constantly
increasing surge speed. One installed FPSO system generates tension both by
direct tension measurement (as in Section 5.1 below) and by angle-derived tension
(as in Section 5.2 below) and thus the loss of one system would not compromise
the compliance with safety criticality. See also Figure 5-17 below for chain angle
measurement checking a measured vessel position.
If a system does fail, how accessible is it for replacement; how easy is it to switch to
a back-up system, etc. There can be a significant cost implication; if the Mooring
Failure Detection system is safety critical then it should be working at least to the
extent required by its actual, or implied, Performance Standard; thus it may not be
acceptable to leave a repair until the next scheduled in-water inspection.
Bespoke systems that have to developed will typically take longer from Purchase
Order to delivery than an existing “off-the-shelf” system or components. This may
be a factor in deciding the most applicable system, both for initial supply but also for
the sourcing of spare parts or further software development.
One of the references [4], and to some extent another reference [5] (see Section
3.1 above), specifically require a tension monitoring system to be provided.
18
• The refresh rate of the tension measurement needs to be at sufficiently small
intervals to identify any peak tension experienced. As full dynamic computer
modelling of a mooring system shows, the tension in a mooring leg can peak
very steeply, and a fast refresh rate is required to identify this;
• Not all tensions can be directly equated to mooring line declination. On at
least one turret moored FPSO, horizontal waves along the chain have been
identified in certain conditions. As the wave passes any point the tension in
the chain is reduced, perhaps even to zero, yet the chain declination is
essentially unchanged;
• The dynamic response times of some mooring components, such as fibre
ropes, and the drag of these through the water, can also lead to a time lag in
mooring declinations responding to the change in tension;
• Where the length of the mooring leg is very long compared to the water depth,
failure of the leg well away from the floating unit may not result in a substantial
angle change at the connected end;
• A small error in the inclination measurement can also lead to a substantial
error in tension calculation. This graph is an example from a specific FPSO
(horizontal axis in whole degrees of departure angle, vertical axis as whole
percentage error.
• High tension can cause a mooring leg to fail if it reaches its minimum break load
(mbl), and the level to set this alarm is fairly easily identified by the design mbl,
with an allowance for any measured loss of diameter and typically with the
application of an appropriate factor of safety. However it is low tension that can
be an indication of a mooring line failure and the level at which to set a low
tension alarm may not be as easy to choose. For example, in a mooring leg
moving dynamically the analysed low tension may for example be 40te; thus the
low tension warning may be set at a margin below this to avoid spurious alarms.
But if the chain fails along the seabed, the weight of chain hanging from the
floating unit may be more than 40te and thus no simple alarm would be
activated. Consideration may therefore need to be given to a more intelligent
19
system that monitors more than one parameter (eg position/offset which is
equated to a tension band or mooring leg declination), or perhaps considers the
peak or mean rate of change of tension if tension is the only parameter;
• No reference has been found to specify an appropriate accuracy of tension
monitoring systems. It is known that at least one FPSO Operator has used a
figure of 10% on the basis that the alarm limits set into that system would still
activate within this range. Not all means of tension measurement provide a
linear response to increasing tension. And further, the accuracy of the tension
measuring equipment may vary over time;
• For confidence in a tension measurement system, being able to check at least
one point on the tension “graph” is helpful; this may be the zero point, or a
similar result may be achieved by having two or three tension measurement
devices either in series or in parallel;
• There are many companies that traditionally provide tension measurement
devices that are used in mooring systems. However the provision of a tension
reading by itself is of low value as regards a Mooring Failure Detection system;
what is critical is having the correct management of the tension readings, and a
useful user interface that typically provides not just an instantaneous value or a
history over time, but low alarms designed to warn of a potential line failure,
trends, rate of change, potentially correlation with position, etc.
4.3 POSITION
The references are slightly more helpful as regards positioning systems, but a
number of issues are still often relevant when choosing a positioning system for the
purpose of detecting mooring failures.
4.3.1 Accuracy
The following quantified references have been found:
20
20
15
Offset (m)
10
0
03:11
03:13
03:15
03:20
03:10
03:12
03:14
03:16
03:17
03:18
03:19
T ime (hh:mm)
If the positioning system is satellite derived then Operators should consider utilising
the full potential of the system with a refresh interval of circa 0.6 second;
• The refresh rate is also important when considering the offset limitations that
the unit may have. Typically a FPSO with a limiting offset for riser integrity of
perhaps only 35 m, may find that the time taken to reach this limiting offset if
the moorings were to fail when the vessel is moving astern with the weather at
its maximum speed may be a shorter time than if the moorings were to fail
when the vessel was at its maximum offset but stationary in the water, ie just
before it starts surging forward again;
• The location of the position measurement can be important. Any positioning
system measures the location of a specific location, typically its antenna. If
that antenna is at a substantial distance from the point of interest, perhaps
typically the turret centre, then the achieved accuracy at the turret centre relies
on:
o a correct determination of the antenna position offset (noting that the
drawings of many FPSOs are not entirely accurate);
o the accuracy of the FPSO heading; if this heading is just 1 degree in
error, this equates to an error of 1.75 m if the antenna offset is 100 m.
The temptation of placing a satellite unit above the aft accommodation –
perhaps 200 m away from the turret of a typical FPSO conversion -
could easily result in an unacceptable error of the turret centre position
unless the vessel heading is measured with a suitable level of accuracy
(perhaps with such as a twin DGPS antenna).
21
5 CURRENTLY AVAILABLE SYSTEM TYPES
Categorising Mooring Failure Detection system “types” is not straightforward for at
least two reasons:
• There are not such a plethora of them available that they can be statistically
split. This limitation and the lack of published standards has allowed individual
Operators to make their own specifications and/or individual suppliers to
specify their provided systems. All of the systems described below are
individual;
• Many of the available systems are not “simple” systems that measure a single
parameter and provide straightforward minimum and maximum threshold
alarms, but are more intelligent, with perhaps more than one parameter being
measured and with more sophisticated alarm logic.
In just about every example below the system is more than a Mooring Failure
Detection system but also provides a measure of monitoring. Also just about every
example below is capable of additional inputs and further configuration; this may be
to enhance the capability of the Mooring Failure Detection system, or to improve its
capability as a Mooring Monitoring system.
The suppliers of Mooring Failure Detection systems that have kindly contributed to
this Report are given in Section 7 with a brief description of their capabilities and
experience.
This is the most common type of Mooring Failure Detection system on the market.
The basic measurement device is typically an instrumented pin in a shackle or
other connecting link so that strain is effectively measured in bending; or it may be
a specifically designed link so that a strain gauge is measuring axially along the
mooring line in the area of maximum strain, or it may be use of compression cells
under chain stopper plates or winch footings. The following are example drawings
or photos of such devices:
22
Figure 5-1: Example Tension Measuring Devices (Courtesy of BMT SMS)
23
Figure 5-3: Close-up of a Load Measuring Shackle (Courtesy of WME)
Figure 5-4: Intelligent Load Shackle with Acoustic Transmitter (Courtesy of J+S Ltd)
24
Figure 5-5: Compression Cells Fitted to a Chain Stopper showing FE Analysis for Location
Choice (Courtesy of Scan Sense)
Typically a number of load measuring devices within the one unit will be included to
allow for redundancy; thus at least some of the above examples include for three
strain gauges within the one pin, so that they may all be compared to each other in
order to provide confidence (the system may provide for adjustment of the
allowable tolerance between them or it may just flag an error if this is too large) and
to provide redundancy.
An alternative means of tension measurement in chain is to accurately measure the
distance across a studless chain link. As a link has tension placed on it, the two
parallel sides of the link pull in towards each other.
A device clamped to each of the two sides of the link allows the reduction in
distance to be measured and the tension to be derived. As the measurement is in
compression, and the unit spring loaded, the clamps can be made of a suitable
non-metallic material. The unit can then be linked to a data storage and hydro-
acoustic transmitter for the information to be sent back to the floating unit.
25
Figure 5-7: Chain Width Tension Measuring Device (Courtesty of Pulse)
NB: the left hand image may need more slack on the wiring links to allow for link movement
For full accuracy this system does require onshore calibration as actual link shapes
and chain material affect the link behaviour. The amount of bedding in and wear
between links will also affect the tension measurement accuracy as the stress
pattern at the ends of the link change.
The system is designed for retro-fit installation by either divers or ROV.
26
The outputs from these tension measuring devices are then displayed on a user
interface screen. These will typically provide an instantaneous value as well as a
historical trace.
As a minimum alarms must include a minimum and a maximum, but experience
suggests that a more sophisticated electronic check can be useful. For example, in
a multi-legged mooring, the failure of one individual mooring line will result in not
only the tension of that leg having a sudden step reduction, but the adjacent legs
having a lesser gradient step but nevertheless an increase as the unit re-positions
itself to balance the loadings from the remaining legs. The sudden drop in the one
leg may be obvious (although not necessarily so, refer to Section 4.2 above) but
could either be a mooring failure or a failure of that loadcell; however only the
former will give rise to step changes on some or all of the other moorings. A
Mooring Failure Detection system that detects these step changes (eg against a
tension change gradient parameter) may avoid spurious alarms and potential
unnecessary production shutdowns.
Figure 5-8: Example User Interface Showing Tension Readouts and Traces (Courtesy of
Synectica)
As in the case of the above example other sensors can be added to this. Above
are shown met data as a means of display so that a user can interpret the line
tensions with the wind, waves and current.
27
A further correlation can be made between position and tension in many systems.
ie if position is accurately monitored as well as tension, then a correlation between
these two can be displayed. The following example uses a pre-calculated look-up
table to correlate “range/tension”, and if this goes above or below limits then it turns
red and an alarm is sounded.
Figure 5-9: Example User Interface with tension, Position and Correlation Alarm (Courtesy of
Ilex)
The above system also provides additional information to the user by giving vessel
motion data and based on thresholds identified from dynamic analysis alerts the
user if any of these go beyond their expected ranges.
To most floating production unit offshore operators the declination of a mooring line
is not a concept that they are familiar with, nor aware of what the typical or
acceptable angles would be. But tension is something that people understand and
thus it may be more user friendly to display a tension to a user, than it is an angle.
In many cases (but see comments in Section 4.2) a specific tension in a mooring
line can be associated with a specific declination by the use of catenary equations.
Thus measurement of angle in general can be used to display the much more
useful tension figure.
28
Figure 5-10: Example of Angle vs Tension Correlation (Courtesy of WME)
For the systems being marketed angle measurement is achieved by the use of
electronic inclinometers. Historically mechanical inclinometers, often with green,
yellow and red zones, were used but experience suggests that these fail with time,
and at times give spurious readings. Electronic inclinometers offer considerably
more accuracy and more reliability.
The inclinometers are normally fitted underwater because the mooring lines on
turrets are generally submerged. For systems where the lines are above water the
housing for the inclinometer can be substantially less robust and hence the cost
considerably reduced; the method of battery changing can also be considerably
eased. Underwater the units require a substantial housing and a means of
securing to the mooring line.
The choice of attachment point to the mooring line has been chosen to date to be on
the trumpet, or stopper housing, at the floating unit connection point. This means
that the actual attachment can be very robust (eg welded).
29
Figure 5-11: Subsea Dual Axis Inclinometer (Courtesy of BMT SMS)
30
Figure 5-13: Acoustic Inclinometers (left) and Receiver (right) (Courtesy of Pulse)
The batteries of the above units can be replaced after recovery by ROV or diver.
The unit also contains a data-logger and an acoustic modem for transmission of the
data back to the floating unit.
There is clearly an alternative of locating the inclinometer on a chain link, but the
following issues can arise:
• Galvanic potential difference between metals should be avoided; effectively
the clamp arrangement should be insulated;
• If the clamp does slip then it may interfere with the integrity of that link or the
one below it;
• Detailed design of a clamp can be made more difficult because every chain
link has a different diameter and shape on the flash butt weld side;
• Using both sides of a chain link has the difficulty that the distance between
them does change with tension;
• Motion of the chain link may be locally extreme and care should be taken that
the clamp and the equipment are not subject to contact from the adjacent
links.
The data is then displayed on a user interface; this interface is effectively the same
as one used for a direct tension measurement. The example below shows the
history and absolute reading and a time trace which can be configured over two
different periods; plus a traffic light system at the bottom of the page.
31
Figure 5-14: Example User Interface Screen (Courtesy of Cybernetix)
32
The screen below also combines a traffic light system with more detailed
information and a battery level indicator.
And the screen below shows the trend of the mooring line angles against each other
for ease of comparison for a visual, and a software, check of variations between the
lines. Note that in this example angle is not equated to tension.
33
Some systems can provide a useful self-check facility based on the mooring line
angles which can be used to derive a vessel position, and comparing these to the
actual measured vessel position.
In a similar way to measurement of chain link width (see Section 5.1), a sensor is
placed on a link, connected to a means of transmitting that data back to the floating
unit.
34
Figure 5-19: Stress Probe Arrangement on Chain (Courtesy of TSC)
The location of the probe is important, and in this case is on the inside surface of
one of the parallel sides that represents a high stress point.
700
650
StressProbe Value
600
550
500
450
0 50 100 150 200 250 300 350
Load in tonnes
Figure 5-20: StressProbe Example 114mm Chain Calibration Curve (Courtesy of TSC)
This system has been designed to be retrofitted on existing units using divers, and
without the need to specially clean the chain other than remove marine growth.
35
5.4 SONAR
With a sonar head located below the hull of a vessel, a head that is able to
generate a 360 degree view, images at the correct range from the head can be
generated like a radar picture.
The image will show all objects, both risers and moorings, and the echoes can be
logged electronically, so that the expected envelope of any particular echo can be
alarmed, to activate either if the echo is outside of the location envelope, or if there
is no echo within that envelope.
36
The right hand side of the display above gives a historical representation of
individual echoes over a chosen period. The processing software uses specialised
range and bearing calculations to derive precision single point positions for each
target, so that this system can be used for tracking over time and to predict possible
critical or failure conditions.
5.5 VISUAL
Deployment of cameras can provide a high confidence method of checking that all
the moorings are in place if a suitable deployment tube is available. Systems to
date are for deployment from time to time and no permanently deployed systems
have been identified.
The deck equipment involved consists of a Control System, digital video recording
system, an electrically driven winch and a sheave; all built to a Zone 1 rating
suitable for the area typically around a turret where hydrocarbons may be present.
2
A multibeam sonar can view the whole compass simultaneously, compared to a scanning head which
has to mechanically rotate around. The former is also solid state while the latter is cheaper, but with
some moving parts. “Profiling” describes a swathe of sound energy which is very thin in one plane (for
this application in the vertical plane) and very wide in the other (ie all around the compass).
3
Internal Wood Group Engineering report after field testing in water.
37
Figure 5-23: Example of Deck Equipment for a Deployable Camera System (Courtesy of Ocean
Tools)
A camera and pan/tilt and control unit is lowered into the water. Lights with
adjustable intensity are included so that the optimal conditions can be achieved.
An extension in excess of 1 metre below the bottom of the tube can be achieved.
The user can then view all the elements, filing the video and images as applicable.
Such a system can obviously easily determine whether all the moorings are
present, and provide an inspection methodology for the visible sides. This type of
system may not be able to identify small changes in angle or rotation depending on
the skill of the user.
38
6 SYSTEMS UNDER CONSIDERATION
In addition to the systems actively being marketed as listed above, a few others
have been identified that are currently being considered either by suppliers or by
Operators.
These include:
• Hull mounted sonar (giving mooring line declination, hence tension)
• Seabed sonar (giving catenary shape, hence including tension)
• Optical Scanning for ropes
• Depth sensors
• Multi-sensor units
A number of other types of system have been suggested at various times over the
last few years, but there is no knowledge of any of these being seriously considered
by suppliers or Operators.
Over the next few months, the JIP would be happy to update this Report with new
information if it can be made available.
A sonar head mounted under the hull of a floating unit looking downwards, or at an
inward angle, will locate the mooring lines a short distance outboard of their
connection point. These echoes will be strong as the distance will be limited and
the angle of incidence close to perpendicular.
39
With the xyz location known of the mooring line connection, and also the xyz of the
sonar head, the distance to each mooring line can be accurately measured, and with
the input of vessel heading (or use of a directional sonar head) the angle of the
mooring line as it leaves its connection point can be calculated. As with inclinometer
measurement of mooring line angle the tension can then be derived in most
circumstances, and displayed on a graphical user interface such as exampled in
Sections 5.1 and 5.2 above.
One of the advantages of this system is that battery packs are not required as the
sonar heads are mounted on the hull and fixed wiring for power and signal can be
routed direct back to the floating unit. Use of divers, possibly ROV, is still likely to
be necessary for initial deployment, subject to the available structure on the hull for
attachment of the sonar heads.
Figure 6-3: Hull Mounted Sonar Bracket (Shown Inverted from Actual Use) (Courtesy of Wood Group)
This particular mounting bracket extends for 3.5m from its mounting point in order to
“see” the mooring lines correctly. The bracket is fitted with two slots, one for use
with a ROV removable sonar head, the other for potential future use of a camera or
other system.
40
6.2 SEABED SONAR
A sonar head array on the seabed can provide simultaneous images of all mooring
lines, and coupled with existing imaging technology, can provide a catenary shape
and location; this in turn can then be used to calculate the mooring line tension, as
well as confirming that all the mooring lines are present.
This system has advantages that being mounted on the seabed its deployment is
fairly straightforward, potentially even from the floating unit itself, and that battery
life can be extended as there is in practice little restriction on the size of the unit;
additionally access by ROV is fairly straightforward, either for substitution or indeed
latching on of a lifting sling for recovery.
A full 360 degree spread of sonar beams is not necessary as the seabed unit is
fixed in azimuth once landed, as are the mooring lines. This can allow
simultaneous mapping of all mooring lines rather than sequential images with a
time lag between them.
Transmission of the measured data back to the floating unit still has to be achieved,
and this can be done using existing hydro acoustic modem technology.
41
6.3 OSCAR
OSCAR – Optical Scanning Apparatus for Ropes - has been developed to predict
potential future failure of polyester and similar taut mooring lines.
This utilises a specially developed Optical Fibre Strain Transducer (OFST) to detect
strain at any position along the length of the mooring line into which it is embedded,
or potentially, attached. The sensor accommodates the significant difference in
elastic modulus of the mooring line and the optical fibres.
The OFST has been demonstrated to be able to detect local strain in real-time.
42
6.4 DEPTH SENSOR
A one-time fit surveillance sensor may be a possible solution to some scenarios:
This unit remains passive and un-powered until required for use; to avoid long term
degradation of conventional batteries, a sea-water activated battery (such as
magnesium – silver chloride as existing technology) can be used. The key rupture
disc is also existing technology with a typical burst tolerance of +/- 5%.
By correctly choosing the activation depth range, taking account of wave pressures
and dynamic pressures from mooring line motions, line failures both above and
below the unit can be detected, as shown in the following sketches.
43
Typical initial arrangement
44
6.5 MULTI-SENSOR UNIT
This concept derives from a view of the Mooring Failure Detection and Monitoring
systems currently available, and trying to optimise the extent of the information
made available to the offshore personnel, and for the Operator’s design and
integrity personnel. One supplier’s view is that the best sensors are those that can
be used to provide trending data, or measured data, so that a quantification of risk
of failure can be made.
This optimum is seen as several sensor units on a mooring line catenary, each unit
fitted with redundant inclinometers, accelerometers and strain gauges, with further
catenary shape defined from a multi-beam sonar. The envisaged system requires
no changes in batteries with power being supplied through piezo-electric cells, and
the data is wirelessly transmitted back to the floating unit by modulating the return
sonar signal.
Such a system when proven certainly satisfies the criteria listed in Section 4.1
above as those considered by Operators, and indicates that at least one supplier is
seeking to use the latest technology within the mooring integrity industry – an action
that should be complimented and hopefully encouraged.
45
7 CONTRIBUTING SUPPLIERS
The Table below provides a summary of the Suppliers capabilities as described by
themselves within the Mooring Failure Detection system categories as given in
Section 5 above.
It should be noted that this categorisation is somewhat simplified, does not take full
account of systems where multi sensor types are utilised and without doubt not all
systems within one nominal category can be considered the same; further many
are developing continuously as experience is gained.
BMT SMS
BPP Tech
Cybernetix
Fugro
Ilex
Monitor
Oceantools
Pulse
Synectica
Tritech
TSC
Figure 7-1: Summary of contributing suppliers Mooring Failure Detection system experience
46
7.1.1 BMT Scientific Marine Services
BMT SMS designs, builds and installs custom tension and load monitoring systems
for spread moorings, tendon moorings, hawser moorings, top tension risers, steel
catenary risers and hybrid riser towers. The BMT mooring monitoring systems
provide more than just simple failure detection, and are frequently integrated with
environmental monitoring systems and platform motions. The BMT systems are
designed and built to the customer’s specifications.
Contact:
Mr Thomas Johnson (President)
BMT Scientific Marine Services Inc
9835 Whithorn Drive
Houston
Texas 77095
USA
Tel: +1 281 8588090
Em: info@scimar.com
www: scimar.com
7.1.2 BPP-TECH
BPP-Tech prides itself on being able to provide bespoke real-time data acquisition
and management systems for offshore operations. Examples in operation include
Riser Management and Umbilical Monitoring Systems, Subsea Extreme Event
Monitoring, Pipeline flow assurance monitoring, Polyester mooring line monitoring
(OSCAR), FPS vessel and mooring monitoring (POSMON and CHAMON). BPP-
TECH continues to be responsible for development of novel sensors and novel
application state-of-the-art technologies.
BPP-TECH has supplied many such systems including several for FPS moorings.
Contact:
Tony Kenyon (Marketing Manager)
BPP-TECH
City Tower, Level 7
40 Basinghall Street
London
ED2V 5DE
UK
Tel: +44 845 217 7000
Em: london@bpp-tech.com
www: bpp-tech.com
47
7.1.3 Cybernetix SA
Contact:
Mr Pascal Hervois
Oil & Gas Business Unit
Technopole de Chateau-Gombert
306, rue Albert Einstein
B.P. 94
13382 Marseille Cedex 13
France
Tel: +33 4 91 21 77 00
Em: pascal.hervois@cybernetix.fr
www: cybernetix.fr
The measurement of offshore structures for the oil and gas industry has been
undertaken by Fugro Structural Monitoring for many years, can be applied to the full
range of structural, riser and mooring systems, and can be integrated into a Vessel
Performance Monitoring System, the data – including metocean data - made
available to all users, including onshore personnel.
Fugro have installed many monitoring systems including several offshore mooring
systems.
Contact:
Ray McGlynn
Business Development Manager
Fugro Structural Monitoring
1 Queenslie Court
Summerlee Street
Glasgow
G33 4DB
UK
Tel: +44 141 7748828
Em: r.mcglynn@geos.com
www: geos.com/services/monitoring/offshore
48
7.1.5 Ilex Computing Ltd
Ilex is an established software solution company primarily for the offshore survey,
exploration and construction industry, with many years experience of integrating
real-time data acquisition and control systems with client oriented software
solutions that have a record of being both cost effective and on-time. T-Guard is a
FPSO/Tanker position and attitude monitoring system interfacing with DGPS,
pitch/roll, tension, metocean and other sensors providing audible and visual alarms
where systems have exceeded tolerance levels set by the user.
Contact:
Dick Edwards
Ilex Computing Ltd
34 Lowther Road
Norwich
Norfolk
NR4 6QW
UK
Tel: +44 845 2414027
Em: support@ilexuk.com
www: ilexuk.com
Monitor has been trading for 13 years and for all of that time have specialised in
Load Monitoring for offshore structures including semi-submersibles, jack-ups,
floating storage units and bespoke systems. Monitor also provides other control
and monitoring systems for offshore facilities. Over the years Monitor have proven
they can provide safe and reliable systems, enjoying co-operative relationships with
major offshore contractors.
Contact:
Brian Sinclair
Managing Director
Monitor Systems Scotland Limited
3 Merkland Road East
Aberdeen
AB24 5PS
UK
Tel: +44 1224 621073
Em: brian@monitor-systems.co.uk
www: monitor-systems.co.uk
49
7.1.7 OceanTools Ltd
OceanTools has supplied one FPSO system and is currently building a second.
Contact:
Kevin Parker
Managing Director
OceanTools Ltd
Unit 11
The Technology Centre
Aberdeen Science and Energy Park
Aberdeen
AB23 8GB
UK
Tel: +44 1224 709606
Em: kevin@oceantools.eu
www: oceantools.eu
Pulse design and supply monitoring systems that combine both instrumentation and
software to measure the response of structures, including mooring and risers.
Offshore installation and on-going support is provided as well as data processing
and management. “Moorassure” is used to confirm the integrity and performance
of catenary mooring systems by monitoring lines angles to deduce their mean
tensions; this can be integrated with “Integricuff”, a retrofitable dynamic chain
tension sensor.
Pulse has more than 10 years experience and more than 100 structural monitoring
projects; specifically Pulse has supplied two mooring line monitoring systems to
date.
Contact:
Hisham Sheriteh
Pulse Structural Monitoring
1-7 Cherry Street
Woking
Surrey
GU21 6EE
Tel: +44 1483 774910
Em: hisham.sheriteh@pulse-monitoring.com
www: pulse-monitoring.com
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7.1.9 Synectica
Synectica are a technology business serving the international offshore oil and gas
industry, specialists in the design, development and supply of integrated computer
systems for data acquisition, monitoring and analysis, together with real-time,
advisory and decision support capabilities. The Mooring Advisory System (MAS)
integrates a vessel position with mooring monitoring information with additional
display and computational capabilities.
Contact:
Martin Delaney
Synectica Limited
PO Box 2161
Woodford Green
Essex
IG8 7GR
UK
Tel: +44 208 504 2829
Em: martin@synectica.com
www: synectica.com
7.1.10 TRITECH
Contact:
Matt Winfield
Survey Business Development Manager
Tritech International Ltd
Peregrine Road
Westhill Business Park
Aberdeen
AB32 6JL
Tel: +44 1224 744111
Em: matt-winfield@tritech.co.uk
www: tritech.co.uk
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7.1.11 TSC
Contact:
Raymond Karé
TSC Inspection Systems
6 Mill Square
Featherstone Road
Wolverton Mill
Milton Keynes
MK12 5RB
UK
Tel: +44 1908 317444
Em: ray@tscinspectionsystems.com
www: tscinspectionsystems.com
Other acknowledgements:
The following companies are acknowledged in various Figures throughout the above, but are
not necessarily complete Mooring Failure Detection system suppliers:
J+S Ltd,The Lombard Centre, Kirkhill Place, Dyce, Aberdeen, UK, AB21 0GU
Tel: +44 1224 773425 Em: oilandgas@jands.co.uk www: jands.co.uk
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This report is intended for the sole use of the person or
company to whom it is addressed and no liability of any
nature whatsoever shall be assumed to any other party
in respect of its contents.
GL NOBLE DENTON
53
REFERENCES
[1] “The Offshore Installations (Safety Case) Regulations 2005”; SI 2005 No. 3117; see
also associated suite of regulations
[2] ABS “Guide for Building and Classing Floating Production Installations 2009”
[3] BV NI 493 “Classification of Mooring Systems for Permanent Offshore Units 2004”
[5] LRS “Rules and Regulations for the Classification of a Floating Offshore Installation at
a Fixed Location”, April 2008
[6] API RP 2SK “Design and Analysis of Stationkeeping Systems for Floating Structures”
3rd Ed Oct 2005
[7] ISO 19901-7:2005 “Petroleum and natural gas industries – Specific requirements for
offshore structures – Part 7: Stationkeeping systems for floating offshore structures
and mobile offshore units”
[9] JIP FPS Mooring Integrity (Phase 1), Noble Denton Report No: A4163/MGB/rev.1
54
55
Published by the Health & Safety Executive 07/17
Mooring failure detection systems for
floating offshore installations
Mooring Integrity Joint Industry Project Phase 2
RR1097
www.hse.gov.uk