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As a frequently occurring and widely distributed environmental load, an internal solitary wave (ISW) always appears with a large amplitude and may result in the instability of deepwater structures and offshore accidents. In this study, a self-developed program was established to describe the dynamic behavior of a coupled platform–mooring–riser system. The response of the platform is based on the rigid body dynamics theory, that of the risers/mooring lines is obtained by the lump mass method, and the coupled effect between them is achieved by a semi-coupled analysis. Two types of riser configurations, steel catenary risers (SCRs) and lazy-wave steel catenary risers (LWRs), in the coupled system were compared under only ISWs and combined excitation of ISWs and random waves. The numerical results were proven to be reliable through validation against previous literature. The results show that different riser configurations have little influence on the platform response or mooring top tension. Both riser configurations exhibit a second-order deformation mode along the riser length and a half-wave shape with time. Additionally, a mutation in the horizontal tension, shear force, and bending moment occurs near the ISW interface. Compared with SCRs, LWRs always produce larger motion characteristics and responses with time differences owing to their complex configuration. • A coupled platform-mooring-riser numerical model has been described through a self-developed program. • The responses under only ISW and combined excitation of ISW and RW are calculated. • Comparisons have been made between two kinds of riser configurations in the coupled system: SCR and LWR. • The mechanism of tension, shear force and bending moment in both riser configurations have been discussed in detail. [ABSTRACT FROM AUTHOR]

The presented work studied the structural response of the hydraulic pneumatic tensioner (HPT) in a TLP-TTR system with failure of the tensioner cylinder. A fully coupled hull-tendon-TTR-tensioner model was established in AQWA to simulate the failure numerically. A specific HPT was modeled by considering 4 cylinders and the real-time stroke of each piston. A set of formulas was proposed to calculate the real-time tension including different components, e.g. Stribeck friction, in the tensioner. A riser array including 6 independent production TTRs and their tensioners was also modeled. The production TTR model was stacked up by different specific riser joints. The hydrodynamic force acting on the hull was obtained by using the 3D potential flow theory. The real-time tensions on different tensioner cylinders were obtained by using an in-house-developed program. Different environmental conditions, including a calm sea, regular waves, and extreme sea states, were considered in the simulations. In the results, the behaviors of different cylinders of the failed tensioner were presented. The results show that 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. • A fully coupled TLP-TTR system is established to simulate the accidental local failure of a hydraulic pneumatic tensioner. • A specific HPT is modeled by considering 4 cylinders and each piston stroke. A riser array including 6 TTRs is also modeled. • An in-house developed FORTRAN subroutine is applied to simulate the multi-cylinder tensioner and its local failure. • A residual response is found to propagate several cycles through the failed tensioner and riser. • It is proved that an accidental local failure of the HPT can hardly develop into a progressive failure of other cylinders. [ABSTRACT FROM AUTHOR]