An experimental study of the motion response in regular waves of a semisubmersible under damage conditions

Stone, Barry Michael (1986) An experimental study of the motion response in regular waves of a semisubmersible under damage conditions. Masters thesis, Memorial University of Newfoundland.

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Abstract

The determination of motion response characteristics, and therefore operating limits, of semisubmersibles in normal even keel conditions has been extensively reported in the literature. However, the extension of this work to vessels which have undergone some form of damage leading to loss of buoyancy and abnormal heel and trim angles is limited. -- to establish the motion response of a typical semisubmersible in both even keel and damage conditions, and resulting changes relative to severity and direction of damage, a model study using a 1/100 scale model of a moored, four column, twin pontoon semisubmersible has been conducted. For each wave direction of head, beam and quartering seas, tests were undertaken at five angles of trim and heel: even keel; two towards (windward damage), and two away (leeward damage) from the waves in 7 m regular seas with periods of 7 to 25 sec. In all cases six degrees of freedom motion response was obtained. -- The RAO curves for small angles of trim and heel show little change from even keel, operating draft. However, at large angles, with pontoons and deck structure piercing the water surface, substantial increases in roll and, particularly pitch motion, occurred over a band of wave periods of 9 to 13 sec. Over this band all motions contained not only the wave frequency but also a significant subharmonic component at half the wave frequency. Under these conditions leeward damage consistently produced the largest motion. The most extreme motion measured resulted in a pitch RAO of 2.9 or 19.8° for a wave height of 6.9 m at a wave period of 12 sec. in quartering seas. -- Further work using realistic irregular seas to obtain additional insight into nonlinear effects over the subharmonic or parametric resonance frequency band has been recommended.

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