Use of Multibeam Sonar for Real-Time
Integrity Monitoring of Mooring Lines,
Risers and Umbilicals on an FPSO
Angus Lugsdin
Tritech International Limited
Abstract
For new build Floating, Production, Storage and Offloading Vessels (FPSOs) with increased
focus on asset integrity management or to quantify remnant life and the potential for field life
extension of existing FPSOs; operators require technologies that enable them to safely
monitor the integrity of subsea infrastructure and identify problems before they occur.
It is widely accepted across the industry that General Visual Inspection (GVI) has many
limitations and that operators should not rely on GVI alone to detect potential mooring line
failure or identify issues with the risers. There is a recognised need, if not always a
requirement for technology that can compliment GVI to provide real-time or near real-time
status information to the operator.
Through the clever use of sonar technology, operators are now able to access real-time
information on the presence, position and therefore integrity of the mooring chains, risers and
umbilicals beneath an FPSO. If positioned correctly it is possible for sonar technology to
provide a complete 360° monitoring solution beneath an FPSO without the need for an array
of additional sensors mounted to each mooring chain or riser. This approach removes the
need for regular intervention for maintenance and servicing by Remotely Operated Vehicles
(ROVs) and compliments annual GVI checks. Use of such technology also allows the
operator to readily distinguish between mooring line failure and instrumentation failure,
something that is difficult to achieve with other technological approaches.
Correctly deployed sonar technology can provide autonomous alarms in the event of mooring
line failure or bend stiffener failure, in addition to providing the operator with early warning
of a future failure. Sonar data, when processed may be used for fatigue analysis, the data
being used to reduce conservatism in the assumed design.
The unique technology described in this paper was developed originally for BP (Aberdeen) in
response to an open requirement for riser integrity monitoring solutions. Systems have been
successfully trialled and deployed on an FPSO in the North Sea where it has provided
around-the-clock integrity monitoring capability and successfully detected the failure of a
bend stiffener.
The short case study that follows provides an overview of the technology development,
testing, deployment and successful operation and an update on the latest developments.
Introduction
Over the past decade a number of studies and Joint Industry Projects (JIP) have been carried
out to investigate how to improve and better manage the integrity of moorings and riser
systems used by FPSOs. As highlighted in a number of these reports, given the safety critical
nature of mooring lines and the consequences of a ruptured riser, one might imagine that they
would be heavily instrumented with automated alarms which would be activated in the case
of failure. In practice many FPSOs are not equipped with such instrumentation or alarms.
This is often due to a lack of consensus in regulation or lack of understanding of how best to
address the problem. For example how does an operator distinguish between mooring line
failure and instrumentation failure without direct intervention, for turrets where the mooring
chains are permanently locked off under the hull?
The main form of monitoring continues to be GVI by ROV which is performed by all
operators most commonly on an annual basis. The main limitation with annual GVI is that it
is only a snapshot and where it detects failure it is after the event. GVI also has a number of
other limitations that include;
-
Environmental restrictions – inspection is dependent upon suitable environmental
operating conditions
Difficult to identify small areas of damage
Difficult or impractical to operate in a confined or enclosed space
Requires ROV support or Diver Support Vessel (DSV) with associated costs
Marine growth often masks damage
It is difficult to inspect all targets
These issues relate not only to mooring integrity but to the integrity of risers. A recent report
(MCS Kenny, 2010) highlighted that ancillary device failure, which in the main relates to
bend stiffener failure be it due to design flaws, material degradation or mechanical
interference with the vessel, accounts for approximately 8% of worldwide flexible pipe
failures or damage, of which there were 315 cases in the report period, i.e. 25 cases caused by
the failure of bend stiffeners during the 10 year reporting period.
Real-time or close to real-time integrity monitoring of both mooring lines and risers can
complement existing inspection techniques and schedules, providing far greater confidence in
the ongoing structural integrity and reduce ongoing inspection costs.
Requirements for monitoring technologies
Despite the ‘safety critical’ nature of mooring lines, the extent of the published guidance
regarding mooring failure detection systems or systems to detect the Structural Integrity (SI)
of flexible pipe is quite limited. Even with the limited guidance and regulation there is little
agreement between different organizations or class societies on the subject of whether
mooring failure detection systems and riser monitoring systems should be fitted, what they
should be capable of and to which standard they should conform to.
A number of regulatory bodies have retreated on providing guidance to operators, moving
towards a less prescriptive approach, pushing the onus back to the operator to demonstrate
fitness for purpose. This has added further confusion and has created the opportunity for
operators to pick and choose which guidelines to follow depending upon the geographical
location, budget constraints and limited regulations being enforced. Whilst operating
companies recognise good practice these standards may not always be applied as rigorously
as possible.
The UK Health & Safety Executive (HSE) has taken the approach, providing guidance that
states that “Operators should have in place suitable performance standards for the time taken
to detect a mooring line failure”. They go on to add that ‘It is clearly not appropriate to rely
on annual ROV inspection to check if a mooring line has failed’, however stop short of
providing any guidance as to an alternative.
Although designed to cope with the failure of a single mooring line, multiple mooring line
failures could put lives at risk both on the drifting unit and on the surrounding installations.
There is also the potential risk of pollution let alone the damage to the reputation of an
operator in the event of an incident that may have been avoided with the implementation of a
more encompassing integrity management programme.
This risk has started to lead to an increased appetite amongst operators to further improve
monitoring and inspection techniques to enable them to assess on an ongoing basis the
current structural integrity of mooring lines and risers.
This need is further enhanced by operators desire to quantify remnant life and the potential
for field life extension for existing assets as many early FPSOs near the end of their design
life.
Where a technology can be used to reliably provide structural integrity monitoring, the
operator is also able to;
-
Monitor the high risk part of a structure
Justify a reduction in GVI and associated costs
Provide detailed data that through analysis can help in the cost reduction for new
build structures and improved planned maintenance of existing installations
Satisfy current and future regulations; and finally
-
Improve the overall safety of offshore operations through the addition of an early
warning capability.
It is however worth pointing out that no single technology or method by itself can provide a
complete picture of the integrity of any mooring system or flexible pipe system and many
methods and technologies that have been examined by the recent JIP reports fall into the
category of specialist solutions to meet a specific demand or project requirement. That being
said, technologies or methods that provide complete overviews and that complement existing
methods providing useful information not previously available, are more likely to be readily
adopted by the industry. The most effective of these are likely to be those that can work
autonomously in the background with minimal operator intervention and in which the
operator may have confidence in.
As the number of operators who take an interest in these monitoring technologies rises, an
increasing number of solutions will become available. The best of those are likely to be the
technologies that can integrate with existing and future technologies and assimilate
information into a central workflow or display, providing the operator with a single real-time
overview of mooring and flexible pipe integrity with automated alarm capability and the
ability to analyse past event data providing background information. These systems should
require very little user intervention, minimal maintenance and minimal ongoing costs.
Early technology approaches
Recognising the emerging need for technologies that would compliment annual GVI,
operators have tried a variety of inspection technologies including mechanical scanning
sonars. These have been deployed, normally through spare I-tubes on a compact deployment
bracket which are winched down through the tube. On reaching the target depth the sonar
traducer rotates through 360° imaging all visible targets. Although a significant improvement
to relying solely on annual GVI, the scanning sonar is only taking a snapshot at a particular
moment in time and can only tell if the moorings lines and bend stiffeners are in their
expected positions.
As it takes time to scan through 360°, if movement occurs before the scan is complete the
resultant image may be blurred. As scanning sonars use a stepping motor to scan through
360° they are not so suitable for permanent or long term deployment.
If a mooring line breaks in the mud at the touchdown point it will still have some
tension/catenary and thus the change in the screen appearance and resolution may not be
sufficient to indicate that a line has failed using scanning sonar.
A line could fail and not be detected before the next scheduled scan, during which time a
severe storm could develop, putting increased strain on the remaining lines.
Figure 1 - Screen photo of ST-1000 scanning sonar image (Courtesy of Shell)
Developing the technology
Tritech (then SRD Ltd) responded to an open invitation to develop a real-time riser integrity
monitoring system. The requirement to monitor mooring lines or anchor chains was added as
the requirement expanded. The primary objective was to develop a system to measure the
positions of each riser and to calculate the offset of the measured point from a fixed or
expected point beneath the turret. The data could be used on its own as an independent
measurement and alarm system or when taken together with other environmental variables
such as sea state, wind direction etc provide a mechanism for detailed analysis of the riser
and mooring line performance under varying conditions.
As an online tool for riser monitoring it was identified that the primary area of interest was
the bend radius of the riser as it passes through the bend stiffener. By making the assumption
that the deflection point of the riser is fixed and with knowledge of the position of the riser at
the point of acoustic measurement, the radius may be calculated. The system may then be
used to generate alarms when the measured bend radius falls outside the predicted tolerances.
The same is true for mooring lines where an expected position envelope is provided. In the
event that a mooring line fails or moves outside the calculated tolerance, an alarm may be
generated.
Tritech developed the sonar technology RAMS™ which was subjected to extensive tests,
both in a controlled environment within a test tank and in the field aboard the Teekay
Petrojarl Foinaven FPSO in the North Sea.
The objective of the tank tests was to demonstrate and test the measurement precision of the
technology. The original test used six sections of smooth surfaced cylindrical ducting
suspended in a test tank to simulate risers and lengths of steel recreational anchor chain to
simulate mooring lines, albeit the diameter of the chains was significantly smaller than that
used for FPSO moorings. The ducting was positioned to correspond to the locations of the
risers found in one quadrant of the Schiehallion FPSO turret as directed by the client. The
prototype RAMS™ system deployed in the tank test used a linear receiver array with a
receive aperture of 120°. The sonar was positioned at one side of the tank so as to
simultaneously illuminate the six riser targets and the mooring chains. The received echoes
were then processed to obtain precise range and bearings from the monitoring system to each
of the targets and from these, X & Y coordinates of the targets were calculated.
Figure 2 - Plan view of test tank with targets
The risers targets were capable of being moved in 1cm increments across the tank using a
moveable bar and peg system. The targets were measured both under static and dynamic
conditions. For the static measurements, the moveable bar was first set at position 0 and the
position of the riser was measured by the sonar over a period of approximately 10 minutes.
The riser position was then reset at 50cm and 100cm locations and the riser measurement
positions repeated. From the statistical analysis it could be concluded that under static
conditions the position of the riser could be calculated to within 10mm or better.
Figure 3 - Test tank
Figure 4 - Test tank showing riser targets
To illustrate the dynamic performance of the system, the riser target was moved by 10cm and
the position of the riser monitored over a period of 30 minutes. The results showed that it
took a significant time for the riser target to settle after being moved, with each small
movement during the oscillation being measured. The results show that the system was still
able to detect sub-millimetre movements of the riser after 17 minutes of settling time. In
order to help further quantify the results, a section of 11” riser from the Schiehallion was
suspended in the test tank and given a small push. This section of riser weighed
approximately 260kg, which was much greater than the approximate weight of 5kg for each
of the ducting sections used. This showed that the period of oscillation lasted approximately 4
minutes with an oscillation period of approximately 4 seconds.
It should be noted that the graph in Figure 5 shows the X position of the riser only. The
oscillations do not appear evenly spaced around the mean position due to the fact that the
risers did not just swing from their supporting ropes but also oscillated around a position
approximately mid way along their length. The graphs are a composite of both of these
oscillations.
Figure 5 - X position versus time for 11” riser dynamic test
Following the measurement of individual targets, all targets were measured simultaneously as
shown in Figure 6.
Figure 6 - Simultaneous measurement of all risers
The window on the bottom left of Figure 6 shows the acoustic image output by the
monitoring system test software. Each of the six risers can be seen. The window on the
bottom right hand side shows both the calculated positions of the risers (red circles) and the
expected area in which the risers should be found (blue circles). The window on the top left
shows an online graphical output of the difference between the expected and calculated
positions of the risers. The pink line corresponds to the moveable riser and it can be seen that
this riser is slowly oscillating around a position that is approximately 40cm away from its
expected position.
Throughout the tests it was observed that the intensity of the echo reflected from the riser
sections was significantly dependant on the vertical angle of the riser. The prototype system
utilised a fixed angle transmitter. The effect on the vertical riser angle on the echo is
illustrated in Figure 7.
Figure 7 - Effect of riser angle on sonar echo
The angled riser reflects a large amount of the acoustic energy away from the receiver
transducer thus reducing the accuracy of the measured result. It was concluded that this effect
could be minimised using a beam steerable transmitter, which was subsequently implemented
in the system design. This allows the system to steer the transmit beam in the vertical
direction to provide a perpendicular reflection from the riser or mooring chain. This not only
provides a more accurate measurement of the position of the riser but also provides
information relating to the vertical angle of deflection of the riser at the point of
measurement.
The tests also concluded that use of a conformal 360° receiver transducer would allow the
accurate detection and measurement of all risers and mooring chains beneath an FPSO if the
targets had sufficient angular separation between them.
Following the successful tank test trials, the technology was further developed with the unit
redesigned incorporating a conformal receive array, shown in Figure 8 that would allow for
real-time 360° monitoring of multiple targets beneath an FPSO chain table. This revised
prototype, at the request of BP was subjected to field trials onboard the Foinaven FPSO, West
of Shetland in the North Sea, where it was used to provide continuous feedback on bend
stiffener status and real-time data that could be used for failure prediction through
fatigue/cycle analysis. The RAMS™ system was also tested as a backup to the existing
anchor tension monitoring system.
Figure 8 - RAMS™ sonar head with 360° conformal receive array
The field trial showed that the RAMS™ system was able to successfully detect and monitor
all risers, umbilicals and mooring chains concurrently. Riser and mooring chain movement
were clearly defined with a measurement resolution of ~1mm. During the trial no accurate
speed of sound measurement was available, so it was not possible to calibrate the sonar
system, however if such a measurement was available it would have been possible to track
and define movement of the risers to less than the 10mm accuracy required. Update rates of
over 10Hz were achieved during the trial, indicating that such data could be used for detailed
analysis of riser movement even in calm conditions.
Figure 9 shows the position of the 10” risers plotted as a position from the average position
achieved.
Figure 9 - 10" riser movements over 1 minute period
Apart from R5, the amplitude of the movements of all the 10” risers are very similar being
within a +/-1.5cm cluster.
Following successful field tests, the RAMS™ system was permanently deployed on the
Foinaven FPSO, where it has since been used to successfully detect failures.
Detecting a failure
During analysis of RAMS™ data from the Foinaven FPSO it was noticed that movement of
an 8” riser was significantly in excess of the movement of neighbouring risers. Analysis
showed that a number of events and changes had occurred over the course of a few days in
November 2009. At the time, although the real-time automated alarm system capability was
in development, it had yet to be fully implemented onboard the Foinaven FPSO. Instead data
was sent back to Tritech for analysis on a monthly basis. The data was analysed in reverse,
i.e. played back from the end of the month. This way in the event of a change occurring, the
operator could be notified immediately upon detection rather than waiting until the whole
month’s data had been analysed. This allowed the operator to take immediate action in the
event that a failure or other suspicious incident had been identified; reducing the amount of
time elapsed after such an incident. This method has subsequently been improved upon with
the implementation of a fully automated alarm system that notifies the operator immediately
onboard the FPSO in the event of an incident occurring.
Figure 10 shows the system display prior to the event occurring. Up until midday on 16 th
November 2009, the position and movement of riser R6 is as expected.
Figure 10 - Riser 6 prior to bend stiffener failure
Soon after midday, a short loud acoustic noise is observed by the RAMS™ and the riser
target became masked by a target of a much larger diameter (~1m). A number of free moving
targets were observed after the event. The larger target was seen to move towards the centre
of the turret and the relative movements of the target were also significantly larger than the
original riser movements observed prior to the event as shown in Figure 10.
Riser R6 became visible again a few days later, however was displaced from its expected
position and the movement independent from the other 8” risers.
It was concluded that the excessive movement observed was a direct result of a bend stiffener
failure. Riser R6 became visible again only after the bend stiffener had slipped further down
the riser so as not to obscure the target. The bend stiffener failure was later confirmed when it
was recovered by an ROV.
Figure 11 - Riser 6 movement post bend stiffener failure
In the event of a similar incident occurring again, it would be picked up automatically by the
system and an alarm sounded, allowing the operator to take immediate action as required.
The RAMS™ technology
The RAMS™ technology is based on Tritech’s proprietary multibeam sonar technology and
allows the operator to visualize the area directly beneath the turrets chain table in a 360°
plane, creating an instantly updating radar type display. Unlike mechanically scanning sonars
that scan the beam through 360° to complete a scan, the time taken to complete the scan
being dependent upon desired resolution and scan range, the multibeam sonar technology
developed provides an instant snapshot, providing precise range and bearing measurements to
all visible targets, in this case mooring lines, risers and umbilicals. No longer having to ‘scan’
to build up a 360° picture the multibeam sonar provides sub-centimetre positional accuracy of
all targets up to 15 times a second.
An illustration of a RAMS™ sonar deployed beneath an FPSO is shown in Figure 12.
Figure 12 - Illustration of RAMS sonar deployed beneath an FPSO
During planning and installation phases, the operator provides detailed positional, exit
azimuth and declination information for each of the targets. This is converted into a search
grid in the software, as shown in Figure 13, to which the sonar returns are automatically
compared against in real-time. In the event a target is missing or has moved outside its
maximum allowable design envelope, the system can automatically trigger an alarm notifying
the operator.
Figure 13 - RAMS software search grid based on operator supplied model
In addition, unlike individual sensors mounted on each mooring chain or riser, the multibeam
technology is dual purpose, allowing the operator to install the sonar at a position beneath the
turret from where it can monitor the mooring lines and risers concurrently. This allows for
real-time statistical analysis and historical comparison of targets against their neighbours and
to quickly determine if one target is behaving differently to the rest of the targets beneath the
turret. This data provides the operator with an instant alarm in the event of a single line
failure or information that may indicate a potential problem, in effect an early warning
system. No additional sensors are needed, and the sonar can be safely installed and
maintained by recovering and deploying through the turret, removing the need for battery
changes or issues regarding transmission of data packages from individual sensors.
Continuing development
In addition to detecting mooring line and bend stiffener failure in real-time, it has been found
that when correlated with environmental and motion information, RAMS™ data can be
analysed to allow the operator to validate mooring line performance against design criteria,
assisting with future designs and more accurately predicting in-field life and time between
failures. A number of studies have shown that systems that can record the motion and offset
of the vessel and environmental conditions are useful in determining the extent of extreme
and fatigue conditions that riser and mooring systems experience. There have been
documented cases where the use of such data has been used to reduce conservatism in the
assumed design and justify extended operations.
The RAMS™ system has been developed to allow data inputs from other sensors, e.g. motion
reference units, onboard GPS and weather stations to name a few, that allows all data to be
assimilated providing a single output and operator display. As already acknowledged in this
paper, as there is no single technology or method that by itself can provide a complete picture
of the integrity of any mooring system, the RAMS™ system can allow for input from other
non real-time sensors, perhaps existing strain gauges or tension measurement systems, which
when combined with the RAMS™ capabilities can provide the operator with a complete
understanding of the dynamics of the mooring or flexible pipe system.
Tritech have recently enhanced the system capabilities with the addition of a visibility and
modelling software module. This allows operators to ascertain if a RAMS™ system would
benefit their operations by simulating the sonar performance beneath an FPSO turret as part
of the FEED stage of a project. It also allows operators of existing FPSOs to ascertain if
RAMS™ could be retrofitted to an existing asset through a spare I-tube to provide a real-time
monitoring solution, enhancing the safety as part of a life extension program.
Figure 14 - Example of visibility study shadow plot
Conclusion
RAMS™ is a mooring failure detection system and a riser integrity monitoring system
combined. However first and foremost, it is a safety system that can provide important and
immediate information to the personnel onboard as regards the risk they are exposed to at that
moment, including risk to the unit itself and to the environment from failed risers and other
installations in the vicinity of a drifting unit.
This is particularly critical for those offshore installations that are within a safety case regime
as the ‘Mooring Failure Detection’ system may be considered a ‘Safety Critical Element’, i.e.
it can ‘prevent or limit the effect of a major accident’. As a safety critical element, the
mooring failure detection system must therefore be suitable and must be kept in good repair
and condition.
The sonar technology can be used to complement existing methods and technologies and with
planning may be used as a central portal into which all mooring and riser integrity data may
be combined and displayed.
Acknowledgements
Thanks to BP for allowing Tritech to provide details of offshore trials, details of the bend
stiffener failure in November 2009 and for allowing Tritech to publish data from the incident
in this paper and accompanying presentation.