All helideck designs of this type should be fully assessed and tested to demonstrate their passive fire safety performance and structural integrity. A competent authority should verify them prior to acceptance as a design option.
(Photo courtesy of John Burt Associates Limited)
9.4 SUPPORT STRUCTURE
9.4.1 Introduction
The helideck support structure connects the helideck to the main structural steelwork of the offshore installation or vessel. It needs to be designed to transmit all the helicopter landing loads, environmental, and live and dead loads derived from the design of the helideck structure to the primary structure of the installation or the vessel.
9.4.2 Materials
It is quite common for the helideck support structure to be fabricated from carbon steel, whilst the helideck will often be fabricated from aluminium sections. Where an aluminium helideck is used in conjunction with a carbon steel structure, then adequate electrical isolation of aluminium from steel must be maintained.
Sufficient information on the materials and the isolation methods used, either in manuals, or by placards or paint schemes fixed to the structure, should be provided to users for subsequent operations and maintenance. The designer is advised to consult with corrosion experts to provide a connection that satisfies anti-corrosion requirements, as well as providing a suitable structural connection.
Materials used in the fabrication should conform to a suitable code. For example, carbon steel should conform to a code such as BS 7191 Weldable Structural Steel for Fixed Offshore Structures [Ref: 57], latest edition.
9.4.3 Design
Helideck support structures should be designed to carry all the loads imposed on the helideck through to the primary structure of the installation or vessel. Helideck loads derive from the parameters of the helicopter for which the helideck is intended (landing impact forces and wheel spacing), the deck weight, plus environmental loads (wind, snow and ice), and inertial loads due to installation or vessel movement (where applicable). Additionally, the effects of live loads and loads arising from parked helicopters (tied down) should be evaluated (See also Section 9.6).
The designer of the support structure should ensure that all appropriate load cases have been applied to the helideck, and that the resulting maximum load cases are used in the design of the support structure.
Similarly, it is important that the load cases are accurately transposed to the design conditions for the primary structure to which the support structure will be connected.
The helideck support structure will also be used as the supporting structure for appurtenances such as drainage and service lines to the helideck above. Therefore, the selection of section types, and acceptability of these, need to be considered in the design process.
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