Computation of environmental loads towards an accurate dynamic positioning capability analysis

Dynamic positioning (DP) systems enable vessels keep station or maintain a predetermined track under specified environmental and operational conditions. This ability is required for many offshore operations such as drilling, supplying, heavy lifting, pipe laying and diving support. In order to establish the maximum environmental conditions in which a DP vessel can maintain its position and heading for a proposed thruster configuration, a DP capability analysis (DPCAP) must be carried out. The most crucial step in a DPCAP is to estimate the environmental loads due to the wind, current and waves in an accurate, consistent and efficient manner. In this paper semi-empirical methods and a RANS based analysis tool compared in order to determine the reliability of such approaches for the purpose of early assessment of wind, wave and current loads. The main focus of this approach is to cross validate the semi-empirical and CFD based procedures so that the initial rough estimates on the loads could be refined accurately as well as to provide a comprehensive example of a reference work for the literature. Typical DP capability polar plots for an offshore supply vessel are presented to illustrate the application of the computational analysis procedure.