This location dependence is particularly marked for ships and ship-shaped installations such as FPSOs. The pitching motion of a ship is such that the vertical heave motion experienced by the helideck will generally be much greater if it is located at the bow or stern, and will be least if it can be located amidships. Bow mounted helidecks can also be particularly vulnerable to damage from green seas unless mounted high above deck level.
Helidecks are also often located off the vessel’s centreline. In some cases they are cantilevered over the side (which provides the benefit of an unobstructed falling 5:1 gradient over at least 180o). In this case, downtime due to wave motions will generally tend to increase because of greater helideck heave motions caused by roll.
Semi-submersible drilling or production platforms, tension leg platforms and spar buoys tend to have smaller motions at lower frequencies, and whilst the helideck location on a spar or semi-submersible will have an effect on performance, this is much less important than for a ship-shaped vessel.
However, the location of the helideck is generally determined by factors other than the need to minimise heave motions. In the case of an FPSO or drillship, for example, the central deck area is generally occupied by processing or drilling equipment. The helideck also has to be conveniently located for access by personnel, who are generally accommodated either near the bow or stern. As the helicopter is likely to be the ‘primary means of escape’ the helideck needs to be close to the ‘temporary refuge’, which is usually incorporated into the accommodation.
Figure 10.8 illustrates how wave motion downtime for a helideck typically varies with its location along the length of a large ship (in this case: an FPSO) when operating in a reasonably harsh environment. Maximum downtime occurs when the helideck is located at the bow or stern, and minimum downtime when the helideck is amidships. Variations in downtime in this case are a direct consequence of variations in predicted heave motions.
Figure 10.9 illustrates how the helideck location affects wave motion downtime on a small ship (e.g. a diving support vessel) operating in a moderate sea environment. Once again, downtime tends to be greatest at the bow and least amidships, although there is relatively little variation over the aft part of the ship. In this case there is a marked difference between levels of downtime occurring when the helideck is at the vessel’s bow and stern.
This asymmetry in the downtime curve is not due to any marked difference between the vessel motions at bow and stern, but is rather a direct consequence of the more stringent motion limits for a helideck located at the bow of a small ship than for a helideck at the stern. This more stringent requirement is because both helicopter and ship will normally be facing into wind, and pilots landing on bow helidecks will therefore have poorer visual cues to assist their landing.
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