Good: The helideck cantilevered over the port side of the vessel gives a clear approach and overshoot path that is free of obstructions and should be largely clear of turbulence for head winds. There will also be good visual cues for the pilot.
Bad: The highly offset or overhanging helideck location means that vessel roll motion will be manifest at the helideck as heave motion and, depending on the roll characteristics and wave conditions experienced, might severely limit helicopter operability. A shuttle tanker can connect to the stern of the FPSO, and when present is likely to violate the helideck obstruction-free sector.
Good: Good visual cues and clear approach path for head winds.
Bad: The helideck is mounted relatively low, and in the wake of the main superstructure. As a result landing helicopters are likely to experience turbulence, and a sharp reduction in wind speed leading to loss of lift. Any shuttle tanker connected to the stern of the FPSO is likely to violate the helideck obstruction-free sector and the 5:1 falling gradient.
10.9 METHODS OF DESIGN ASSESSMENT
10.9.1 Introduction
The environmental effects described in this section are influenced by the wind and wave conditions experienced by the offshore installation. Clearly these weather conditions vary from day to day in a largely unpredictable way.
However, wind speeds and wave heights are both amenable to statistical analysis, and data can be obtained which describe their statistical properties. These data can be used with information about the flow patterns around the platform, and the platform wave motions to:
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Estimate the likely helideck operational downtime
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Locate the helideck in the best location on the installation to minimise helideck downtime
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Determine the best compromises between conflicting requirements.
The following sections outline methods of assessing the installation properties (from experience, from wind tunnel tests and other modelling methods - see Sections 10.9.2.1 and 10.9.2.2), the key statistical properties of the offshore ocean climate (see Sections 10.9.3 and 10.9.4), and how they can be used together to estimate operability and inform the design process (Section 10.9.6), and in reporting any likely operating limitations to helicopter operators (see Section 10.10).
10.9.2 Wind Flow Assessment
10.9.2.1 Expert Visual Inspection
The main factors that influence the wind flow conditions over the helideck are the prevailing wind direction and the location of the helideck relative to this direction. Ideally, the helideck should be located so that, for the prevailing wind direction, it is upwind of major obstructions such as drilling derricks and gas turbine exhausts. In this way, for the majority of the time, the turbulent wake flows and high temperature gas plumes will be blown away from the helideck and away from the helicopter flight path.
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