Floating production, storage and offloading vessels (FPSO) that have a turret positioned relatively far back from the bow can be seriously affected by thruster system failure. In these circumstances, the FPSO would be expected to take a significant angle to the environment and thus attract large loads, which could overstress the mooring system. Mooring system design is strongly dependent upon the robustness of the thruster system. A thruster system failure that escalates to a mooring system failure compromises crew safety, threatens continuity of production, and jeopardizes vessel survivability.

The degree of robustness required in the design of both the thruster system and the mooring system is determined by the magnitude of the consequences of failure, which in turn is dependent upon the operating status.

Even though FPSOs with similar designs operate in the North Sea, their design philosophy has historically been developed in an ad-hoc manner, with no consistency either in the operating practices or in the matrix of load cases used. Furthermore, most FPSO station-keeping designs do not establish the limiting environmental conditions at which the FPSO has to transfer from an operating state to a state of suspended production so that the consequences of failure can be properly managed.

FPSO in the northern North Sea

One such FPSO has recently been designed for deployment in the northern North Sea. With a displacement in excess of 200,000 metric tons, it is one of the largest in the world and will operate in the Atlantic margin. Its sheer scale makes a robust station-keeping system key.

The FPSO’s station-keeping system is a combination of thrusters and catenary mooring lines that allow the hull to weathervane around the central axis of the turret. The purpose of weathervaning is to align the hull with the predominant wave conditions, which reduces the hull environmental loads and impact on the mooring system. In addition, weathervaning ensures the hull is oriented to minimize vessel motions. As the FPSO turret is located as far back as a third of the length between perpendiculars, weathervaning can only be achieved with assistance from the thruster system.

When environmental conditions are reached that result in line tensions meeting or exceeding a predetermined limit, production is stopped and the topsides depressurized, while the risers remain at operating pressure. These environmental conditions are pre-calculated, with the GPS-based position/excursion of the FPSO being the determining factor for shutting in production.

In this state, the FPSO is expected to ride out the extreme environmental conditions defined by 100-year and 10,000-year return periods while satisfying the relevant safety factors. The design is such that the risers do not have to be depressurized under most circumstances. Under the worst failure (a.k.a. “blackship”) conditions, production is shut down. In exceptional circumstances, risers may also be depressurized.

In one-line-broken conditions, production can continue in this “defective” station-keeping state provided the same line tension limits are in place as when all of the lines were intact. In these conditions, however, it is also necessary to establish the environmental conditions at which it is necessary to depressurize the risers because a further line failure or thruster system failure could result in unacceptable consequences.

Power for the thruster system

The thruster system is powered by the four main generators. The generators are usually powered by fuel gas, but in the event of production being curtailed, power automatically switches to “buy back” gas. If this is not available, two of the gas turbines automatically switch to liquid fuel, and there is sufficient capacity to power the other operations and the active heading control (AHC) system.

Ensuring the essential generators are running and in standby mode means the FPSO can regain power in considerably less than two minutes, (minimizing the chances of the FPSO getting broadside onto the waves), i.e., in the time needed to close the breaker on the essential generator and begin loading. The generators have sufficient power to regain and maintain heading in 100-year conditions.

No single point of failure should lead to blackout (as required by DNV POSMOOR 1996). The power management system will primarily deliver power to the AHC with due consideration given to the other life support services.

Consequence classes linked to operating philosophy

The rules and regulations detailed in ISO 19901-7 Offshore Norway Annex require that the selection of the design cases and safety factors take into account the various operating states of the FPSO because the different operating states result in different consequences of station-keeping failure.

To assist FPSO operators, environmental limits ought to establish the limit at which the FPSO should move from its primary production service to the position where its main objective would be to safely ride out potentially hazardous weather or sea conditions.

A number of operating states that result in increasingly severe consequences as the FPSO moves from survival to standby and then on to operating states have been identified.

The design philosophy is to make certain that the increased consequences are managed through increasing levels of safety factors in the design. This FPSO will never be in consequence class one (CC1) because risers are expected to remain connected at all times. Therefore, extreme environmental conditions should be tested against safety factors appropriate for CC2.

It is important to realize that in CC2 conditions, the riser will hold the operating pressure. Furthermore, in standby condition, there will at least be two barriers in place at the wells and two in the turret directly after riser hangoff.

These barriers significantly reduce the consequences of catastrophic station-keeping failure and permit the consideration of this situation as CC2. There are no plans for the FPSO to produce in the 100-year environmental conditions, so the vessel does not need to be designed to the more conservative CC3 safety factors.

Blackship scenario

Unfortunately, there are no well established design requirements for the blackship scenario as far as mooring systems are concerned. However, ISO 19901-7 does require that the allowable thrust for the redundancy check thruster condition shall be “equal to the available effective thrust after accounting for the worst failure as determined by the failure, modes, and effects analysis.”

A review of historical data established that the blackship scenario has a probability of occurring once every seven years, which is similar to the likelihood of a single-line failure. Recovery from blackout conditions, however, takes only minutes as opposed to months in the case of line failure, and therefore it is appropriate to consider the failure to be equivalent to the two-line failure conditions as detailed by ISO 19901-7 Offshore Norway Annex.

Some operators require this assessment to be made against 100-year return conditions but with safety factors that would be lower than those required by ISO for assessment against 10-year return conditions.

While these safety factors were taken in the main from ISO 19901-7, safety factors for the 10,000-year return case and the blackout case were identified by inspection and analogy with two-line-failed conditions.

It is recommended that system reliability studies be conducted to confirm that this package of measures results in a thruster-assisted mooring system that is robust enough to manage the consequences of system failure.

Preparing FPSOs for the future

While designs of station-keeping systems for FPSOs have progressed rapidly in recent years, the specific issue associated with designing a thruster-assisted mooring system, where the thruster system is critical to the weathervaning capability of the FPSO, has not hitherto been addressed systematically.

The non-passively weathervaning characteristic of this FPSO case study instigated a systematic and thorough review of the station-keeping design process.

The end result is the development of a matrix of design cases that can be used to design station-keeping systems for future FPSOs with similar degrees of dependence on the thruster system. This will ensure the operating advantages gained from the use of thrusters are maintained while assuring the safety of the FPSO in case of thruster system failure.