Time-Domain Research on Integrated Coupling Model of MODU, Drilling Riser and Wellhead

The coupling effects between MODU and slim drilling riser are of primary importance in analysis of dynamic response of drilling riser system. However,, the effect of MODU on drilling riser is simplified in conventional model in a way of only considering RAO of MODU in frequency-domain, ignoring the response of MODU on current and wave in time-domain. In practical, the fitness-for-purpose assessment of drilling riser and wellhead is over-conservative by using conventional model, which will induce redundant design of wellhead strength, MODU capacity requirement, and drilling riser emergency evacuation response plan, consequently increase design and operating cost. As a result there is a requirement for more refined methodologies and finite element models to evaluate the coupling effect between MODU and drilling riser in time-domain. This paper proposes a new model, an integrated coupling model incorporated MODU, drilling riser and wellhead system, in which the response of MODU on current and wave in time-domain is considered, In addition, a case study of the new model accounting for South China Sea environmental loading is presented in this paper. A number of conclusions drawn from the case study are outlined. Firstly, it is found that drilling riser has an inhibiting effect on the movement of MODU under wave and current loading, the more MODU drift-off, the stronger restoring force acts on MODU. This will actually improve drilling riser capacity against MODU drift-off. Secondly, hysteresis effect is found between drilling riser and wellhead stress peak and MODU drift-off peak. There are some lag between each other, When the MODU drift-off reaches its peak, the stress of drilling riser and wellhead does not reach its peak. It is shown in a case study, the peak stress of drilling riser lags 40s behind the peak MODU drift-off, and the peak stress of wellhead lags 60s behind the peak MODU drift-off. As a conclusion the integrated coupling model is a more refined model to simulate the interaction between MODU and drilling riser system in time domain, which enhances the accuracy of drilling riser and wellhead fitness for purpose assessment and lead to a more optimal design of drilling riser system.