Hydrodynamic Aspects of Semi-submersible Platforms

In this thesis the hydrodynamically induced forces on a floating platform are analysed. As a result of this analysis a calculation method is developed to predict the forces which influence the motions and the strength of such a platform. For this method it is supposed that the submerged part of the platform can be subdivided in typical elements such as spheres, ellipsoids and cylinders. It is proven that the wave excited forces on these elements can be determined when the dimensions and the added mass of the elements are known. This, however, only holds when the dimensions of the elemental part are smaller than one fifth of the wave length. The results obtained by this approximation differ 3% at most from, the results of exact calculations. When compared with model test results the difference will be 5% at most. Next it is supposed that the hydrodynamic properties of each element of the submerged construction are not influenced by neighbouring elements. It then will be possible to calculate the total added mass of the platform and the total wave excited forces on it. From a comparison between the results obtained by means of these calculations and model test results of an existing platform (Staflo type of Shell UA Ltd.) it can be concluded that from a practical point of view good results are obtained by the calculation method discussed. Once the added mass and the wave excited forces are known the platform motions are known for the frequencies outside the range of resonance. In order to calculate a motion near its natural period the damping is subdivided in potential damping and viscous damping. The potential damping is determined by means of the relation between this damping and the wave excited forces on the construction. The viscous damping is determined by summation of the viscous damping of each of the elements of the construction. In the last chapter it is shown how the method derived in this thesis for the calculation of the wave excited forces and the reaction forces, can be used for the design of platform dimensions from a point of view of minimum vertical motions in waves. This point can be of importance for both the safety and the economy of the operation of the platform.