Abstract
Low-frequency pitch motions of a mooredsemisubmersible in irregular sea states are analyzed. Physicalmechanisms and significance to air-gap problems areaddressed. Excitation from wave drift and from moorings/risersis primarily considered, Effects from current and wind are alsoaddressed. Related challenges in deepwater model testing ofsemis with truncated moorings are discussed.Motion and air-gap data from two previously performedmodel tests are analysed. Catenary moorings in 335m waterdepth and in 1100m water depth, respectively, are considered.Model scales are 1:55 and 1:150, respectively. Observed slowdriftpitch components are of the same magnitude level as thewave-frequency components.Comparisons to coupled numerical analysis models aremade. Wave drift moment coefficients calibrated empiricallyaccording to experiments were used, since the originalcoefficients gave too low results. The final comparisons showgood agreement for the 1:55 case. For the 1:150 case, fairlygood agreement is found, but some deviations are observed andbelieved to be due to poorer wave repeatablity.Tests with truncated moorings at half of the two actualdepths were also included, for a check of methods fordeepwater model tests performed at reduced depths andcombine with numerical analysis (hybrid verification). Theimportance of proper experimental reproduction at reduceddepths, of full-depth pitch and air-gap, is addressed. The resultsshow that with the actual truncation designs, reasonableagreements are obtained, but use of the scale 1:150 seems togive too large uncertainties due to the poorer waverepeatability.
