Research On The Local Failures Of The Flexible Members On Deepwater Tlps Based On Multi-body Dynamics

Tension Leg Platform(TLP)is one major type of offshore structures which are used in deepwater oil and gas field development.A whole TLP generally includes three parts: the hull,which is the main body of the platform floating on the sea;tendons,which are the crucial part to form tension legs,fastening the hull downward to wellhead on seabed,in general,each leg consists of 2 or 3 tendons;and risers,the specific type corresponding to a TLP is Top Tension Riser(TTR),used for oil drilling or production transfer.In addition,to maintain its top tension,each TTR is connected with a tensioner system to the hull.In the recent years,the local failure responses of flexible members on TLPs,e.g.tendons and risers,have been focused on in the field of international ocean engineering.However,there is no TLP which is constructed independently by our country and currently in operation.Thus,researching the above problem is greatly significant for our country’s development on deepwater equipment design and manufacture.Firstly,to solve the local mooring failure of TLPs,a tendon failure mathematical model and a corresponding simulation method are proposed,based on the 3D potential flow theory and panel element method.A TLP is chosen as the target structure.The proposed model,method,and calculation are verified by contrasting its conventional hydrodynamic analysis results.Then,the coupling hydrodynamic responses are predicted around the time of tendon one-time failure and progressive failure.The environment loads in different directions and the different positions of failed tendons are considered.The motion responses of the hull,the tension of residual tendons and TTRs are given in the analysis results.The study shows: the motion and structural responses following a local mooring failure have obvious difference with those under the full moorings;each time history after the failure can be divided into transient stage and steady stage,and the responses in the two stages have different amplitudes of variation under different environmental conditions;in an extreme sea state,a local mooring failure at a key position on TLPs can develop into a global mooring failure.Secondly,with regard to the strong coupling dynamic responses of the tensioner between the hull and the riser,a coupled TLP hull-tensioner dynamic model and a corresponding numerical modeling method are presented,based on multi-body dynamics.A 4-cylinder hydraulic pneumatic tensioner(HPT)is also modeled,and it is validated by testing its vertical and lateral restoring forces.Besides,a detailed TTR stack-up model is applied to the TTR global performance analysis,and a simplified TTR array model based on the calculation of a central riser is also proposed.Different dynamic response time histories of the tensioner are obtained by using the coupled dynamic model and making hydrodynamic analyses under regular waves and irregular waves.The research indicates: under a periodic wave load,the multi-cylinder tensioner has not only vertical dynamic responses,but also lateral dynamic responses,and the cylinders at different positions have different response time histories;as the parameters of the wave excitation change,the coupled motion of the hull and the risers becomes different.Finally,to deal with the local failure responses of the tensioner,a tensioner local failure mathematical model and a simulation method are given in the study,based on the above coupled hull-tensioner dynamic model.The response time histories are simulated before and after the tensioner failure,by controlling the hydraulic pressure tension component in the failed cylinder.The coexistence of the healthy and failed risers is considered during the numerical modeling,and different environmental conditions are also taken into account in the calculation.In the results,the response behaviors of different cylinders of the failed tensioner,the hull motion,and the global deformation of the risers are illustrated.The conclusions can be drawn: the process of the HPT’s local failure can be divided into a healthy stage,a transient stage,and a steady stage;when an accidental local failure of the HPT occurs,the tension and stroke responses are still far from the designed limits to induce a progressive failure;enhancing the reliability of the hydraulic pneumatic system to avoid the initial accidental failure is the key to reduce the loss due to the tensioner local failure.