MODEL STUDIES OF THE MOTION RESPONSE OF A DAMAGED 4 COLUMN SEMISUBMERSIBLE IN REGULAR AND IRREGULAR WAVES

A brief review of the stability requirements of semisubmersibles is presented. Results of the dynamic response of a damaged, twin pontoon, four column semisubmersible at a scale of 1:100 are discussed. The tests involved both regular and irregular waves with the model oriented in head, beam and quartering directions for both intact as well as damage conditions. Four damage conditions representing partial damage to one column were simulated: two in windward (positive) direction and two in leeward (negative). The "light damage" condition represented about 9% flooding of the damaged column, while for the "moderate damage" the flooding was equivalent to about 18%. For moderate damage conditions, regular wave studies showed that the motions are essentially nonlinear, although for light damage conditions this cannot be said with certainty. Model tests showed certain asymmetry in the motions of the damage semisubmersible with respect to the position of the damaged column. Moderate damage conditions seemed to produce significant subharmonic response of the vessel in a frequency range which is twice the natural frequency of the vessel in heave. These observations were confirmed from the results of irregular wave studies. Irregular wave studies showed that the quartering sea pitch and roll motions in windward damage conditions are as significant as those in the leeward conditions which was not the case for regular wave studies. The energy due to the motion of the semisubmersible was concentrated in the frequency of 0.7-1.3 Hz, which corresponds to the energetic range of the normal sea state. The natural frequencies of the vessel in damaged condition in pitch, roll and heave are higher than the corresponding frequencies in the intact condition of the vessel. The natural frequency in heave, for both intact and damage conditions, is higher than either those of pitch and roll in similar conditions. These natural frequencies in pitch and roll begin to approach that of the natural frequency in heave as the damage condition increases. This is true irrespective of the position of the damaged column or the sea state. The value of the natural frequency in heave itself increased much more slowly with increase in damage condition. It was inferred that the nonlinear wave pressure term played only a minor role in the asymmetry of motions of the vessel, while the mooring characteristics had a more dominant influence. © 1990.