Motion Optimization Of Semi-Submersibles
Authors: Minkenberg H.L. , van Sluijs M.F. 
Published: October 2008

Offshore operations have been performed successfully during the last decades, in which time period it was established that semisubmersibles are superior to conventional ship-shaped barges in view of minimum down-time requirements. To meet these requirements it is necessary to reduce the motions, which can be attained by optimization of the dimensions or the shape of the underwater hulls. For optimization purposes there is a need for theoretical methods in order to reduce costly and time-consuming model experiments. Such a method has been developed at N.S.M.B., which makes it possible to calculate wave forces, moments and the resulting motions of semisubmersible platforms of arbitrary shape. The paper outlines the fundamentals of the method.

A comparison has been made between computed heave and model heave measurements for four cross-section types of lower hulls of a simplified platform in beam waves. Both results show a very good agreement. The results indicate that heave will be strongly influenced by the cross-section of the lower hulls in particular with respect to wave period. The paper concludes with a motion comparison for an actual platform, which will be optimized for two operational areas where prevailing weather conditions differ substantially.

Introduction

Because of an increased demand for energy and minerals, the search for natural resources is in a phase of great expansion. Up to 1938, oil, gas and minerals were searched for on land or at inland waters.

In the last decades, however, these activities were extended to offshore areas. From the start, exploration and production was restricted to rather shallow water in which one could operate from fixed platforms, using conventional shore-based facilities. The performance of fixed platforms is only to a minor degree affected by waves, wind and current; the resulting deflections are too small to be detrimental. At present, activities are successfully performed further offshore in deeper water and rougher seas where only floating structures can be used.

Utilization of conventional equipment has diminished, since it became impractical in view of its limitations in storm seas. The more expensive equipment and the higher costs for transportation of the products, necessitated designing floating units from a point of view of minimum down time. The limiting factors for platform performance are, in general, the motions of the structure induced by combined external forces.

The forces acting upon floating structures can be split into a static part caused by steady wind, current and wave-drifting forces, and a dynamic part resulting from wave action. The consequences of the static forces can be counteracted by a well designed multipoint mooring or a dynamic positioning system. Oscillatory motions, which are induced by the dynamic part of the wave forces, can only be held within permissible limits by an appropriate selection of the platform submerges hulls.

Since minimum downtime is of prime importance, a platform should be designed with long natural motion periods to avoid or minimize resonance phenomena. Long natural motion periods, outside the range of the dominant wave periods, can solely be realized by floaters having a small ratio between waterplane area and displacement.