Slow-Drift Pitch Motions and Air-Gap Observed From Model Testing With Moored Semisubmersibles

Low-frequency pitch motions of a moored semisubmersible in irregular sea states are analyzed. Physical mechanisms and significance to air-gap problems are addressed. Excitation from wave drift and from moorings/risers is primarily considered, Effects from current and wind are also addressed. Related challenges in deepwater model testing of semis with truncated moorings are discussed. Motion and air-gap data from two previously performed model tests are analysed. Catenary moorings in 335m water depth and in 1100m water depth, respectively, are considered. Model scales are 1:55 and 1:150, respectively. Observed slow-drift pitch components are of the same magnitude level as the wave-frequency components. Comparisons to coupled numerical analysis models are made. Wave drift moment coefficients calibrated empirically according to experiments were used, since the original coefficients gave too low results. The final comparisons show good agreement for the 1:55 case. For the 1:150 case, fairly good agreement is found, but some deviations are observed and believed to be due to poorer wave repeatablity. Tests with truncated moorings at half of the two actual depths were also included, for a check of methods for deepwater model tests performed at reduced depths and combine with numerical analysis (hybrid verification). The importance of proper experimental reproduction at reduced depths, of full-depth pitch and air-gap, is addressed. The results show that with the actual truncation designs, reasonable agreements are obtained, but use of the scale 1:150 seems to give too large uncertainties due to the poorer wave repeatability.Copyright © 2007 by ASME