This paper discusses data which indicate deteriorative trends in mooring chains on offshore floating production systems (FPS). A global data set collected from the measurements taken on chains on 29 FPS shows a clear trend towards increased elongation of mooring chains near the touch-down point at the seabed. Elongation also occurs in chains near the surface at the interface with the vessel and turret. At the midpoint there is less deterioration through elongation.
Data were collected using an optical caliper chain measuring system deployed by ROV to take measurements of chains while they are in the water. This in situ methodology is both cost- and time-efficient because it eliminates the need to recover chains for dry inspection. It also reduces the risk of mooring damage during the retrieval and reinstallation.
When used in combination with other tools that assess the looseness of chain link studs and that provide 3D video visual display, this inspection methodology can improve the management of mooring systems and reduce the risk of a moorings failure, thereby increasing the operational integrity of FPS.
The integrity of underwater moorings is critical to the viability of floating offshore production installations. Industry operators, regulators and certifiers have recognized that a failure of the moorings of on an FPS can have serious consequences for assets, production outputs, the environment and safety of crew.
The UK Health and Safety Executive (HSE) ranks multiple mooring line failure of FPSO facilities as Risk Category 1, its highest hazard rating. A study by The UK Offshore Operators Association (UKOOA) found that the average failure rate for an FPSO moorings system in the UK is once every 5.4 years1. In addition to FPSOs, moorings failures also have serious implications for other floating installations such as Spars, FPUs, MODUs and CALM Buoys. Certifying authorities such as DNV, Lloyds, ABS, and Bureau Veritas require periodic inspection of chains as a component of vessel reclassification. And an international Joint Industry Project that was convened in 2003 to study the issue has released recommendations on improving moorings integrity of floating production systems2.
The implications of failure are wide ranging. They include everything from reduced or suspended production to vessel damage through collision, oil spills and endangered safety of crew on board floating production installations and vessels in the vicinity. In order to avoid failure operators must verify the operational integrity, or condition, of mooring components. By doing this, ongoing deterioration can be identified and monitored to ensure that appropriate actions are taken before the condition becomes critical and failure imminent.
Dry inspections require the recovery of mooring chains from water for assessment on the deck or on shore. While chain recovery is a necessity for mobile drilling units (MODUs), this measure is neither cost-effective nor time-efficient as an inspection methodology for permanently and semi-permanently moored installations such as FPSOs.