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Assessment can be made in a qualitative manner by expert review of the installation topsides and helideck design in conjunction with information on the prevailing wind directions. This may be appropriate in the very early stages of design, and it may be possible to make an upper estimate of the helideck downtime on this basis. However, in most cases it is preferable to obtain a quantitative measure using flow assessment (Section 10.9.2.2), the wind climate (Sections 10.9.3 and 10.9.4), and a calculation of the helideck downtime (Section 10.9.6).
10.9.2.2 Detailed Flow Modelling using Wind Tunnels and/or CFD
Wind tunnel testing and CFD are the principal tools available for predicting the flow field around a helideck.
Wind Tunnel Tests
The main objectives of wind tunnel tests in the context of helideck design are to predict the mean velocity and turbulence intensity components as well as the mean and peak temperature rises for a range of wind angles and heights above the helideck. Comparison of the results can then be made with the design guidance.
The model scale should be sufficiently large to incorporate an adequate level of geometrical detail to reproduce the correct local flow features around the platform. Typical model scales that can achieve this are in the range of 1:100 to 1:200. At these scales the discrepancies in flow patterns between full-scale and model-scale are generally small.
The model scale should, however, be sufficiently small to minimise the blockage of the model on the wind tunnel flow. A high blockage would result in the airflow over the platform being adversely affected by the walls of the wind tunnel. It is recommended that the frontal area of the model should not exceed 10% of the cross sectional area of the tunnel working section.
The wind tunnel should accurately simulate boundary layer velocity and turbulence profiles representative of the full-scale marine atmospheric wind flow. Target profiles often used in offshore studies have been defined by NMD [Ref: 65]. Wind tunnels designed to simulate atmospheric boundary layers tend to have very long working sections to enable the boundary layer to be developed and controlled. Such wind tunnels should also have a reasonable length of working section continuing downsteam of the model to enable measurements of decaying temperature or turbulence to be made at least one platform diameter downwind.
In modelling buoyant hot gas plumes, it is necessary to match the ratios of the exhaust density to ambient density, the exhaust velocity to wind speed and the plume inertia force to gravitational force to maintain similarity between the model scale and full-scale exhausts. The latter ratio links velocity with buoyancy and implies that the model test velocities have to be scaled as the square root of the model scale (Froude scaling). For example, for a model scale of 1:100, a full-scale wind speed of 10 m/s is represented by a model test wind speed of 1 m/s. This scaling requirement imposes a practical limit on the model scale for a specific wind tunnel facility, and the ability to run at low speeds with good stability is often important.
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离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 2(20)
