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10.6 SPECIAL CONSIDERATIONS FOR FPSOs AND DYNAMICALLY POSITIONED VESSELS
Most FPSOs operating in UK waters are turret-moored, and either weathervane naturally with the wind, waves and current, or use thrusters to control the vessel heading. A naturally weathervaning vessel has no control over its heading or motions, whereas a thruster-controlled vessel has the ability to choose its heading (within limits). For the latter, the heading is normally chosen in order to minimise the wave motions, and this normally means heading into the waves.
Dynamically positioned drillships and other offshore construction vessels also often operate with thruster heading control, with the heading invariably selected to minimise the wave induced vessel motions (unless the drilling or construction task demands some other fixed heading).
Whichever heading control strategy is adopted, the vessel’s wave induced motions (and therefore helideck downtime) are sensitive to variations in the vessel’s heading relative to waves. The heading of a naturally weathervaning vessel depends on the relative strengths and directions of the wind, wind-generated waves, swell and current. Swell and wind-generated waves can come from very different directions, and especially complex heave, roll and pitch motions may occur if swell onto the beam of the vessel occurs at the same time as a wind-generated sea onto the bow. The vessel roll response in head-sea conditions is sensitive to the amount of wave directional spreading, and a multi-directional wave model may have to be used to obtain reliable estimates of maximum roll response in these circumstances. Despite the complexity, all these effects can be taken into account at the helideck design assessment stage (see Section 10.11).
The ability of a thruster-assisted FPSO, or other dynamically positioned vessel, to turn to a desired heading can be used operationally to minimise helideck downtime due to both wave motions and aerodynamic effects. It can be used during flight operations to:
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ensure that wave induced motions at the helideck are minimised, and/or
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relative wind headings leading to downdraft, turbulence or hot gases over the helideck are avoided.
Consequently a thruster-assisted FPSO or dynamically positioned vessel with relatively poor inherent wave induced motions and helideck aerodynamic limitations may nevertheless be operated in such a way that good helideck operability is achieved. The benefit of this can also be taken into account at the helideck design assessment stage (see Section 10.9). However, it should also be recognised that a sudden loss of heading control during helicopter operations is likely to result in a rapid increase in vessel motions (especially roll) with potentially catastrophic consequences for a helicopter on the deck. This roll motion problem will be particularly severe for vessels with high-mounted helidecks. Consequently, heading control should not be relied upon unless the heading control system has adequate redundancy and capacity to bring the vessel back onto heading, and the risk of loss of heading control has been shown to be adequately low.
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