Wind, Wave, and Current Loads on Semisubmersibles

Abstract

Prediction of environmental forces on offshore structures is indispensable in the design and engineering stage. Evaluation and dimensioning of mooring and/or dynamic positioning systems require accurate information on both the mean forces and the dynamic loading. For semi-submersibles the complicated non-streamline geometry was normally prohibitive for calculating such environmental loads. Based on systematic wind tunnel tests, model tests and diffraction analysis a practical calculation procedure has been developed recently. The method is based on a component building block approach with special attention paid to component interaction and lift force effects. INTRODUCTION Floating units are used more and more for offshore exploration and production. For deep water floating production is an obvious choice, but also for moderate depths floaters are becoming more attractive. Reduction of pay-load requirements due to flexible risers, subsea completion, multi-phase pumps and decrease of production equipment make floating production an attractive alternative to fixed platforms. Floating systems are also capable to move from one field to another. Besides the exploration and production facilities, an increasing fleet of vessels is engaged in installation, workover, maintenance and support work. For all these structures the station keeping ability is of prime importance for their operation. Passive systems such as moorings, and active systems such as dynamic positioning are used to prevent drift of the vessel due to waves, wind and current. For the drift of a vessel basically the average components of the wind, wave and current loads are of importance. Due to the large mass of the vessel and the restoring capacity of its stationing system, the vessel normally shows a resonant response for low frequency excitation. Such excitation may originate from wind gusts, second order wave forces and changes in current. Since the damping of the system is small in this frequency region, the amplitudes of the low frequency motions may be large. Dynamically controlled systems may counteract or dampen these motions. For this purpose mooring systems are equipped with controlled thrust in 'DP assistt- mode of operation. The direcr wave induced motions of the vessel are normally not restricted by the stationing system. The related wave forces cannot be counterbalanced by the system and on the other hand the resulting oscillatory motions are of the same magnitude as the wave amplitude. As outlined above, the wind, wave and current loads are of prime importance for the station keeping performance of floating structures. Detailed knowledge and computational tools to determine these forces are therefore indispensable for the design, engineering and operation of stat ion keeping systems. To compute the motions of a floating platform kept on station by means of its mooring system or by controlled thrust, a time step simulation is required. Nonlinearities in the mooring characteristics and the control system prevent frequency domain calculations. Concerning the fluid loading, normally the superposition approach is applied to the reactive forces (due to oscillations in calm water) and the exciting forces (due to a captive vessel in waves). The reactive forces are then normally incorporated in the equations of motion. The wave exciting forces, wind and current and other loads are then accounted for in the right-hand side of the equations of motion. For tanker type ships the number of shape parameters affecting the wind, wave and current loads are limited.

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