Feature ArticleReal-Time Monitoring of FPSO Mooring Lines, Risers
By Angus Lugsdin
Business Development Manager, RAMS
Tritech International Ltd.
Illustration of Tritech RAMS deployed through a fixed internal turret on an FPSO.
With the exception of those with a disconnectable turret, FPSOs are unable to move off-station and must therefore withstand all weather conditions in situ. A large number of FPSOs are anchored to the seabed through a turret that remains in a fixed orientation and allows the vessel to weathervane around it. The mooring lines, production risers and umbilicals are connected to the turret and therefore remain stationary as the FPSO weathervanes. The moorings are classed as category 1 safety critical systems.
FPSO Design Considerations
During the design phase of the FPSO mooring system, a wide range of factors are considered, including location and environmental conditions. Although they are designed to withstand 100-year storms, very few FPSO mooring systems have much reserve capacity above what is required to withstand them.
Furthermore, it is important to consider that even if an FPSO has been in location without major incident for 20 years and is approaching the twilight years of its design life, it still needs to be able to withstand 100-year storm conditions to maintain class and, therefore, insurance. Therefore, the longer an FPSO is in the field, the higher the probability that it will encounter extreme weather and the more likely the mooring system will fail.
These failures occur on average once every five to six years, although, as operators attempt to extend the field life of FPSOs, it is conceivable to expect the number of incidents to increase. The number of recent incidents of FPSO mooring failures, such as those involving the Gryphon, Banff and Volve FPSOs, highlight what seems to be a growing trend.
Mooring Line Failure
The deterioration of mooring lines over time can lead to an increase in the failure of single or multiple lines. If the deterioration or failure of a single mooring line is not detected, multiple line failure can occur in the next instance of severe weather as a result of increased load on the remaining lines exceeding the design limits. While most FPSOs are designed to cope with the failure of a single line, in the event of subsequent line failures, the increasing loads tend to result in even more lines failing. Multiple line failure may result in a FPSO breaking away from the moorings and drifting off-station into the middle of the field.
The cost of a single mooring line failure is significant when the expense of anchor handling tugs, ROV and dive support vessels, replacement parts and lost production are taken into account. These costs have been estimated at a minimum of £2 million for a 50,000-barrel-per-day (bpd) FPSO in the North Sea and many times that for a larger production FPSO operating in a more remote and less serviced area of the world. These costs are, however, nothing compared to those resulting from a catastrophic riser failure caused by loss of station, especially for a pressurized production riser, which would not only damage the reputation of the operator but immediately affect production and lead to possible safety and environmental issues. Industry experts estimate that the cost is in excess of $1 billion and still counting from the incident with the Gryphon FPSO offshore Aberdeen, Scotland, that occurred in February 2011, when it lost position and drifted 180 meters off-station after four mooring lines failed, resulting in significant damage to the subsea infrastructure. Fortunately, there was no loss of hydrocarbons, and no one was hurt. It is clear that the potential cost of not detecting such a failure is far greater than the cost of implementing a real-time monitoring system.
The Case for Regular Inspection and Monitoring
Regular inspection can reduce risk through early detection of issues, and real-time monitoring gives the with the ability to detect the failure of a single line and, in some cases, provides an early-warning system of a change of angle, tension or position, which could be precursors to line failure.
Despite the criticality of the mooring systems, with regulations varying across class societies and codes of practice, integrity management approaches seem to consist only of subsea ROV inspections or diver inspections, with very few FPSOs fitted with any form of mooring line monitoring systems. Where an FPSO is fitted with a monitoring system, these tend to be overcomplicated, poorly understood or unused, and do not provide any real-time capability or allow the operator access to historical data that would aid in assessing asset performance and integrity.
Access to the area beneath the chain table at the interface between the FPSO and mooring line or riser (one of the key areas where failures can occur) is often limited to ROV inspection due to the risk of diving beneath an FPSO. ROV inspection is itself limited by weather windows and access.
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Angus Lugsdin is Tritech International's business development manager for RAMS. He has more than 14 years experience in developing, operating and marketing all types of sonars, including side scan, imaging and multibeam. Lugsdin is a graduate of the Department of Maritime Studies at the University of Wales, Cardiff.