Hydrodynamic interactions and relative motions of floating bodies in waves

Abstract

Floating structures play an important role in sea transportation as well as exploration, drilling and production of oil and gas around the world. Hydrodynamic interaction of two platforms floating close to each other affects their motion responses which hinder the loading, unloading and production operation of the floating structures. The hydrodynamic interaction between two floating bodies may lead to large amplitude motions due to low damping in low frequency. This behavior induces enormous loading in mooring system and thus possibility of the mooring system failure increases. In this research, 3D source distribution panel technique was used to find a numerical solution in frequency domain based on the potential flow theory. It was developed to compute the motions of offshore structure. For computations of the motions and relative motions of two floating offshore structures, HydroStar (Hydrodynamic analysis software developed by Bureau Veritas) was also used. In order to validate the results, model tests were carried out at the Towing Tank, Universiti Teknologi Malaysia for a semisubmersible in the vicinity of a Tension Leg Platform. The tests were conducted in a head sea condition at regular wave. Due to the limitation of water depth in the towing tank, vertical forces of the moorings were ignored and horizontal forces were modeled by horizontal mechanical spring connectors. The results obtained from the developed code, HydroStar and experimental results for a semisubmersible show a very good agreement which proves the efficiency and effectiveness of the 3D source distribution technique. Computations were also carried out for two bodies at different separation distances, wave headings and wave frequencies by using HydroStar. The results show an increase in the relative motion owing to the sheltering effect. It implies that decreasing the distance would increase the probability of accidents. From extensive numerical computations, experiments, it is concluded that a minimum gap must be maintained in order to perform safe operation. An empirical formula is also proposed from the current research for finding the minimum gap between the two structures.