Offshore structures floating at sea should use their dynamic positioning (DP) system to maintain position and heading against environmental loads, including wave loads, current loads, and wind loads. It is difficult to calculate environmental loads accurately and quickly, especially for wind loads due to the shielding effect of different parts on offshore structures. To improve the accuracy of wind load calculation, a new method considering shielding effect is proposed. With the new method, calculated wind force becomes much closer to wind tunnel test than the traditional method. As input data of DP capacity analysis, the environmental loads have critical impact on the design of DP system. A static method and a time-domain simulation method of DP capacity are also proposed, and a case study of drillship is carried out. The results of both static analysis and time-domain simulation of DP capacity show that the new wind load calculation method has improved the accuracy of environmental load calculation and DP capacity analysis.
Introduction
As the offshore oil and gas exploitation is going further into deep sea, offshore structures are to be used in harsh marine environments, including strong wind, current, and waves. Especially for wind, it can make great effect on the dynamic positioning (DP) system of offshore structures, which is designed to maintain position and heading. Once the DP system is not able to keep the offshore structures’ position and heading, it will cause an accident such as oil leakage and oil pollution in the ocean. Because of the wind shielding effect among all parts on offshore structures, such as deckhouse, derricks, cranes, pipe racks, the wind load is difficult to calculate accurately and quickly.
Computation of environmental loads towards an accurate dynamic positioning capability analysis
