The installation design of small structures using an AHTS (anchor handling and towing ship) is addressed, through the coupled motion analysis of the two structures connected by an installation cable, in the frequency domain, using the WAMIT program. This problem is sometimes analyzed by imposing the AHTS stern motions on the top of the installation cable, an uncoupled analysis. We will show that in some situations the presence 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 be addressed, like the depth of the submerged small box structure, and the cable stiffness. The converged results will be post-processed in order to get short term statistical responses of the line tensions as a function of the sea-states, using a Jonswap spectrum to represent the sea environment. Varying the significant wave height and peak period, we will be able to define a window in this domain where the marine operation will be safe with regard to the line tensions. The suspended structures weight around 100tf, while the installation cables have an operational maximum tension around 300tf. The most critical design condition will occur when the dynamic maximum tension in the installation line gets close to the structure self weight, as the cable will not be allowed to get slack.
Offshore marine operations often involve the deployment of small structures, like a manifold, a suction pile or some kind of special pump on the seafloor. These structures have in general a total weight of the order of 1500KN, and in order to carry out this installation, it is common practice to avoid the use of large units, like a semi-submersible crane vessel, and to use an AHTS instead, keeping lower installation costs.