Coupled Hydrodynamic Analysis of an AHTS and a Box Structure in Waves

摘要

The installation design of small structures using an AHTS(anchor handling and towing ship) is addressed, through thecoupled motion analysis of the two structures connected by aninstallation cable, in the frequency domain, using the WAMITprogram. This problem is sometimes analyzed by imposing theAHTS stern motions on the top of the installation cable, anuncoupled analysis. We will show that in some situations thepresence of the small structure will influence the AHTS motions,mainly the pitch motion, and should be taken into account.The influence of some parameters on the solution will beaddressed, like the depth of the submerged small box structure,and the cable stiffness. The converged results will bepost-processed in order to get short term statistical responses ofthe line tensions as a function of the sea-states, using a Jonswapspectrum to represent the sea environment. Varying thesignificant wave height and peak period, we will be able todefine a window in this domain where the marine operation willbe safe with regard to the line tensions.The suspended structures weight around lOOtf, while theinstallation cables have an operational maximum tension around300tf. The most critical design condition will occur when thedynamic maximum tension in the installation line gets close tothe structure self weight, as the cable will not be allowed to getslack.