Methods of calculating environmental loads are introduced, including wave loads, current loads and wind loads. Especially for wind load calculation, a method considered wind force direction correction and shielding coefficient among different parts of drillship is introduced. In this wind load calculation method, three (3) correction coefficients determined by wind tunnel test are brought in to improve the precision of wind load calculation. To find out the best solution with minimum power consumption of DP system, a nonlinear constrained optimization problem is built and solved by SQP method (Sequence Quadratic Programming). Based on the environmental load calculation method and capacity calculation method, calculation of a DP3 deepwater drillship with no less than six (6) thrusters is carried out. Furthermore, to verify the DP capacity calculation, a time-domain simulation of DP capacity calculation is also carried out to obtain the drillship positioning precision, motion track, and also the time signal of all thrusters including force signal and direction signal. And then the verification is carried out by comparing the average value of the signal and capacity result.
As the offshore exploitation is going further into deep sea, offshore structures such as drillship are to be used in harsh marine environment, including strong wind, current and waves. Especially for wind, it can make great effect on DP system of offshore structures which is designed to keep them in position. Because of the wind effect among all parts on drillships such as deckhouse, derricks, cranes, pipe racks and etc, it is difficult to calculate accurately and quickly. As the wind load is the calculation input of DP capacity calculation, the DP capacity of offshore structures may be misled and unfortunately it is very difficult to verify. Wind tunnel test is a common method for offshore structure, for example, Boonstra.H. and Leynse.C. carried out a wind tunnel test for Semisubmersible Platform. But for early design phase, wind tunnel test is too expensive.
