The basics of mooring leg design.
The function of a mooring system
The primary function of the mooring system of a floating production unit (FPU) is to keep the excursions of the unit within acceptable limits for the riser system. Exceeding these limits could damage the risers, releasing the crude oil that they contain. To perform their function, mooring systems must possess adequate strength to withstand the loads imposed by waves, wind and current. The majority of mooring systems have been designed against the 100-year return period storm condition, ensuring the mooring has adequate strength with an appropriate safety factor against failure. The safety factor applied under the 100-year conditions actually aims to ensure that the mooring system can withstand the 10,000 year return period storm without failing, as discussed in a previous blog (Design extremes!).
Impact of water depth
The mooring legs of FPUs in shallow water, whether turret moored or spread moored, are generally composed of chain. The weight of the chain is used to providing a restoring force as the unit drifts off station under the action of wind, waves and current. The mooring legs are arranged in groups, or bundles, and hang from the FPU in a catenary arrangement. The grouping of mooring legs provides clear corridors between the bundles, which are used to route flow lines and risers to the FPU. This arrangement helps avoid clashes between the mooring legs and risers, and minimising the subsea facilities exposure to damage if a mooring leg fails and falls to the seabed.
As the unit moves off station, mooring legs are lifted on the weather side of the system, and laid down on the lea. This results in an imbalance in the suspended weight of the mooring lines which generates a restoring force, acting to pull the unit back onto its station. If required, the restoring force can be increased by increasing the linear weight of the mooring leg. This is most commonly achieved by increasing the diameter of the mooring chains, but the effect can also be achieved by fixing discrete clump weights along the length of the chain, or having a double chain segment in the mooring leg.
Whilst this approach to station keeping is effective in water depths up to about 300m, the use of weight alone to control excursions becomes ineffective at greater depths, and the suspended weight of the mooring legs becomes excessively high. As a result, alternative types of mooring system have been developed and deployed for FPUs in deeper water.
Although there is no universally agreed definition of the terms, ‘deep water’ is generally applied to water depths in the range of 400 to 1,500m, with depths in excess of 1,500m being termed ‘ultra-deep’. As FPU operations have moved into deeper waters, taut mooring systems based on the elastic response of steel wire ropes and synthetic ropes have been deployed as an alternative to chain-based catenary mooring systems. The legs of these systems stretch as the FPU moves of station, generating a restoring force without the large change in mooring leg geometry that is characteristic of catenary systems. Taut mooring systems are lighter than chain systems and are able to limit excursions within the capabilities of the riser systems more cost effectively than could be achieved using catenary systems. The legs of taut moorings are still grouped into bundles to provide corridors for routing of flow lines and risers.
Designing with redundancy
All permanent mooring systems, whether for shallow or deep water, are designed with a degree of redundancy. The majority of operating systems have been designed to allow continued operation with the failure of any single mooring leg, ie a single leg failure is a design case and is treated as a component failure rather than a system failure. In this approach, after failure of a single leg, there remains an adequate factor of safety against failure for the remaining intact legs.
However, originating from the regulations for permanent moorings in the Norwegian sector of the North Sea, designs that can accommodate two broken legs have also been deployed on several floating production units. This approach reflects a concern that if one mooring leg has failed, then adjacent legs, which have the same design and have experienced similar loading, may also be on the verge of failure. At present there is no consensus in the industry whether the additional cost of this approach is warranted, and its application is still limited.
Fatigue damage
In addition to withstanding extreme design loads, mooring systems must have adequate fatigue resistance to allow them to operate for the design life of the production unit. In general, the fatigue lives of wire ropes and synthetic ropes are long, and can achieve factors of safety well in excess of the requirements of the design codes. However, mooring systems composed primarily of wire or synthetic rope still use a section of chain at the top of the mooring leg to allow the required leg pre-tension to be achieved by adjusting the length through a chain stopper.
This top section of chain is particularly sensitive to fatigue, experiencing the phenomenon of out-of-plane bending (OPB) fatigue as well as the tension-tension fatigue experienced by the rest of the mooring leg. The industry was alerted to OPB fatigue following the failure of three mooring legs on the Girassol deep water off-loading buoy in 2002. These failures occurred less than one year after the installation of the buoy. Subsequent investigation revealed that the last chain link on the chain stopper was bending in its weak plane before it started to slide over the adjacent link. The interlink rotation is inhibited in part by the high pre-tension associated with deep water systems, with the consequence that there is a practical limitation on the magnitude of pre-load that can be used if excessive fatigue damage is to be avoided. This limitation can increase the number of mooring legs that would otherwise be needed so as to maintain excursions within acceptable limits without using excessive pre-load.
OPB fatigue is likely to be a consideration in the design of mooring systems for floating offshore wind systems, which will generally incorporate sections of chain a the top of the mooring legs.
For more information on the design of mooring systems, please get in touch or take a look at my article 'Mooring in Deep Water’, Marine Technology, pp48 to 53, April 2014.
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