Importance of Wind Loads for Floating System Designs

Abstract

Wind represents a driving factor for floating system designs, not only for stability, but also for platform global performance including offset, heel and airgap, and thus for mooring, riser/umbilical, structure, topsides equipment and personnel habitability. Wind load evolves as metocean criteria, topsides layouts, design requirements, and estimation methods, and can be in continuous development from concept, design, execution to operation phases. The impact needs to be considered from operability to survivability of floating systems. A case study of semi-submersibles in worldwide regions will be considered. It is first developed a semi-submersible in the Gulf of Mexico for hurricanes up to 1,000-year return period. Wind speeds in other representative locations worldwide, i.e., the North Sea, and offshore Northwest Australia, are then applied to reconfigure the semi-submersible hull and mooring. The impacts of wind loads are evaluated by changing the wind speed and associated wind profile, but maintaining the other design parameters. The North Sea case is for relatively less severe winds of up to 10,000-year storms, while the offshore Northwest Australia case is for strong tropical winds of up to 10,000-year cyclones. The hull configuration, including main dimension, displacement, GM, and mooring sizing, are compared for the worldwide cases. For the Gulf of Mexico case, wind load is not only crucial for stability in 100-year environment, but also for survivability in 1,000-year hurricane. Comparably, wind load governs the North Sea case for stability although its 100-year wind speed is less severe, while 10,000-year cyclone wind is critical to survivability for the offshore Northwest Australia case. For mooring system, wind contributes to the majority of environmental loads, especially for the offshore Northwest Australia case. It is presented as a result of wind speed, and can be used to interpret uncertainty in wind load estimation from empirical method, model testing, CFD and design requirements. This paper evaluates the impact of wind loads for floating system designs from operating, extreme to survival conditions, identifies the governing factor in worldwide regions, and provides a view of systematic approach to the integrated system.