Nonlinear dynamic analysis and fatigue damage assessment for a deepwater test string subjected to random loads

The deepwater test string is an important but vulnerable component in offshore petroleum exploration,and its durability significantly affects the success of deepwater test operations.Considering the influence of random waves and the interaction between the test string and the riser,a time-domain nonlinear dynamic model of a deepwater test string is developed.The stress-time history of the test string is obtained to study vibration mechanisms and fatigue development in the test string.Several recommendations for reducing damage are proposed.The results indicate that the amplitude of dynamic response when the string is subjected to random loads gradually decreases along the test string,and that the von Mises stress is higher in the string sections near the top of the test string and the flex joints.In addition,the fatigue damage fluctuates with the water depth,and the maximum damage occurs in string sections adjacent to the lower flex joint and in the splash zone.Several measures are proposed to improve the operational safety of deepwater test strings：applying greater top tension,operating in a favorable marine environment,managing the order of the test string joints,and performing nondestructive testing of components at vulnerable positions.