MARINE & STRUCTURAL
2.17 THRUSTERS .1 Applications
Thrusters may be fitted to FPSOs for several different reasons:
a) To maintain heading in vessels fitted with an amidships turret that do not have natural weathervaning.
b) To provide a major part of station-keeping capability in a deeper water application.
c) To fine-tune heading to minimise roll motions so that the process is settled, for crew comfort and to assist shuttle tanker operations.
Thrusters may also be useful in fire or platform abandonment scenarios where the vessel can be rotated to clear fire or smoke from around production areas and living quarters and to provide a lee side for lifeboat launch.
The configuration of thrusters and power requirement differs for the above applications.
In the case of a) and b), they are safety critical items, and so the configuration and the reliability and the availability of the electrical supplies have to be carefully considered.
In type (a), the thrusters are transverse normally tunnel-mounted with sufficient power to 'unwind' the vessel before the limits of the drag chain hose transfer system on the turret are reached.
In type (b), the thrusters are a combination of transverse thrusters to maintain heading and longitudinal thrusters (which could be main propulsion in a converted vessel) to reduce the environmental load transmitted to the mooring system.
In type (c), the thrusters are transverse stern-mounted, either tunnel or azimuthing. The power requirements are relatively small, since the thrusters are used only to fine-tune heading in moderate sea-states, and not to achieve major weathervaning. The normal uses are where the wind and current are not collinear with waves or swell, causing heavy rolling. Thrusters may be used to fine-tune the FPSO heading to reduce rolling. During shuttle tanker connection and loading, the thrusters may be used to reduce fish tailing and slow down changes in FPSO heading following a change in tidal strength or direction.
2.17.2 Design considerations
Thruster sizing and type selection is a function of the duty requirements above, and the environmental loads the thrusters have to overcome. The environmental loading and thrust requirements can be assessed using the methods in section 2.3 of these Design Guidance Notes or more accurately from wind tunnel and model tank testing. Thrust requirements need to be increased for the effects of water inflow speed due to high currents.
The required power rating can be deduced from manufacturers' data or initially as 100 HP to provide 1.3metric tonne thrust. Power requirements should be increased where necessary to cover continuous ratings.
Thruster redundancy depends on the duty requirement of the thrusters and requires an assessment of the probability and consequences of failure and the reduced capability of the system.
For new-build FPSO hulls of dumb barge configuration, it will be easier to fit an azimuthing thruster(s) in a cut-up stern rather than incorporate a tunnel. For converted vessels, a tunnel-mounted thruster(s) may be more easily fitted in the afterbody skeg.
Thruster type selection should take account of maintenance at sea requirements and allow working parts to be withdrawn into the hull for inspection and repair rather than diver-assisted keel hauling.
2.18 OPERATIONAL CONSIDERATIONS
Design of the FPSO should take account of the demands of long-term service with in-situ inspection and maintenance and the requirements of performing this safely with minimum interruption to production activity.
An integrity management plan should be developed in parallel with the design of hull structure/systems and the mooring system. This document will cover inspection strategy, the frequency of inspection, criteria for acceptance of results and plans for repair or replacement of failed items with a permissible time limit for these to be implemented.
Design safety factors, fatigue lives, redundancy and corrosion margins should be compatible with the inspection regime in the integrity management plan.
Hull design should enable safe means of access to all tanks and void areas for inspection and maintenance at sea. Consideration should be given to locating stiffeners in double skin wing tanks and in double bottom where possible to facilitate inspection without entering cargo spaces. Access to pumps and valves for change-out should be provided.
Tank arrangements should provide a sufficient number of tanks to enable production operations to continue at close to optimum efficiency while selected tank(s) are cleaned for inspection and maintenance. Tank venting arrangements should facilitate purging of selected tanks for entry.
The International Chamber of Shipping & Oil Companies International Marine Forum publication International Safety Guide for Oil Tankers & Terminals (ISGOTT) (ref.54) is a reference for many items relevant to tanks operations.
Other useful reference sources are the Tanker Structures Co-operative Forum Condition Evaluation & Maintenance of Tanker Structures (ref.55), which deals mainly with corrosion, and their Guidance Manual for the Inspection and Condition Assessment of Tanker Structures (ref.56) which deals with weld defects and possible repair details.
The mooring integrity plan should include a reliable means of monitoring the integrity of each mooring line and anchor. The permissible delay until a failed line is replaced should also be covered. This will depend on the time of year and the probability of environmental loads exceeding the reduced capacity of the mooring system. Results from DNV DEEPMOOR and ND/MCS Integrated Mooring and Riser Design Study suggested a time limit of 3 months.