Wind Forces on a Semisubmersible Equipped With Alternative Drilling Derricks

ABSTRACT The paper presents the results of a wind tunnel study that was carried out with a model of GVA 5000 - a fully enclosed semisubmersible drilling unit - equipped with alternative derrick designs. The tests were carried out in a simulated natural ocean wind in order to determine the total overturning wind forces and moments to be used for stability analysis and anchoring force analysis. The study comprises five alternative derrick designs: An open lattice derrick with full set back, a semi shielded derrick, a totally shielded derrick, a shielded derrick with round corners and shielded derrick with circular sections. Results are presented both for upright and heeled conditions. Also two different helidecks and a more open deck arrangement were investigated. The influence of the Reynolds number has been accounted for on the basis of theoretical considerations. The research work can be applied by designers and operators of semisubmersible drilling rigs in the assessment of the influence of the derrick on the wind forces. This is of interest in connection with the analysis of stability and anchoring forces. The results are presented in non-dimensional form in tables and figures quantifying the wind load coefficients for the tested conditions. It is concluded that the derrick accounts for a considerable part of the drag force and wind overturning moment of a semisubmersible and the investigation reveals to what extent these forces and moments can be affected by a change in derrick design. INTRODUCTION In the search for oil and gas great interest has in recent years been paid by national and international offshore operators to hostile areas with low air temperatures. Such waters are the continental shelves offshore Alaska, Labrador and Newfoundland, and Northern Norway. Characteristic for drilling operations in these areas compared to what the industry has been used to from the environmental point of view are:low air temperatures causing icing and freezing problems,high wind speeds which in combination with the low air temperatures drastically increase the chilling effects,rough waters with high waves both as extreme values and seasonal average values. In the contacts between the waves and the offshore structure the wave crests are smashed to spray, and the dropplets in combination with the cold strong wind are well spread around the structure and freeze to ice on contact with it, andlow visibility due to darkness and fog. The drilling unit has to be designed and built for safe and efficient operations during environmental conditions like these. Some of the measures which are taken to improve the abilities of drilling rigs are to enclose the working areas and to design the external shape of structure including smooth coverings without obstructions. The coverings of the working areas maintain the high degree of personal and equipment efficiency despite the low air temperatures and strong winds, but it also improves the personnel safety by preventing from down falling ice from derrick and crane booms.