In this paper; a dynamic coupling model which incorporate floating rig-riser-wellhead system; is established aims to study the hysteresis effect occurs in a drift-off process of dynamic positioning (DP) system. The proposed model and calculation procedure are verified by performing a model experiment in a 10m water depth basin. A case study of the proposed model for an application in a simulated South China Sea environmental condition is performed and presented by this paper; the internal solitary wave is especially considered. The hysteresis effect is discussed and studied quantitatively by this paper. Some key findings and suggestions are provided in the end.
DP systems are widely employed on deep-water and ultra-deep water drilling platforms to keep the floating structures' position. A typical DP system is commonly comprised of power system; thruster; control system; position reference system; etc. Any component failure may result in the loss of position (Liu; 2017). As a result; the floating rig will drift off from the original equilibrium position due to the influence of environmental loads. During the process; the lower marine riser package (LMRP) needs to be disconnected timely to avoid riser being damaged and broken; subsea wellhead and blowout preventor (BOP) damage in the case of excessive drift-off displacement (Chen; 2008; Shaughnessy; 2010). Due to inertia effect and large length characteristics of riser in deep-water drilling; there is a time lag when the load caused by floating rig drift-off transmit from riser top to wellhead. This phenomenon is named as "hysteresis effect". This time lag has been noticed long ago in petroleum industry as well. Chung (1980) formulated a transient analysis procedure for the numerical determination of nonlinear transient motion of pipes. They found a typical phase lag of the pipe bottom response to the ship or pipe top. Besides; in deep ocean mining; this is also a critical issue for equipment touchdown (Chung; 1981). However; this lag time is always ignored by designers in the previous work in offshore drilling engineering; which may induce redundant design of drilling riser emergency evacuation response plan and consequently increase the offshore oil and gas drilling costs.
