Nonlinear Time Domain Seismic Analysis for a Coupled Mooring Jacket-Yoke-FSO System

A mooring facility for a Floating Storage and Offloading (FSO) system, due to site conditions such as shallow water, often uses a fixed mooring tower for mooring of the FSO. When a fixed mooring tower in the form of a jacket structure is used, the turntable is mounted on the top of the jacket so that the FSO can weathervane due to actions of wind, wave and current forces. Product swivels are also located on this structure for uninterrupted flow of the product to the FSO when it rotates. The connection of the FSO to the turntable is through a rigid yoke. The yoke consists of two yoke arms meeting at a point hinged at the turntable, one large diameter cylinder for providing the stabilizing ballast load and two pendants supporting the ballast. The jacket has to be designed for the mooring loads in addition to the wind, wave and current loads on itself. The rigid yoke system is designed so that the varying draft conditions of the FSO as well as its motions can be suitably handled and absorbed. Complications may arise when the jacket is located in a seismically active site. When a site is prone to very strong ground motions, seismic response of the jacket in conjunction with the moored FSO has to be studied. The additional requirement is that any vibration of the jacket is suitably absorbed by the yoke system or a suitable isolation device is designed between the link or the yoke structure and the FSO. The weight of the suspended mass is a key design variable which affects this behavior. A structural dynamic model of the coupled jacket-yoke-frame-FSO system is analyzed using nonlinear time domain analysis technique. The calibrated El Centro ground accelerations are used for this analysis as a representative seismic excitation. A comparison of the results for jacket alone and the coupled system enables us to determine the effect of the yoke-frame-FSO on the dynamic response. The requirement, if any, of vibration isolation device for the nonlinear link (yoke) structure is decided from the dynamic analysis results. The dynamic analysis of the coupled system is complex. The complexities in the model arise due to: • The nonlinearity of the soil-pile system; • Nonlinearity of the yoke mechanism; • The fact that the FSO is a floating structure and it is free from the base excitation; • The FSO involves a large mass and is essentially free floating in water. The dynamic analyses are performed in several stages in view of the above complexities. Initially, the mode shapes and frequencies of the jacket alone are evaluated. Then the jacket is analyzed using the response spectrum approach with the design seismic spectrum. Subsequently time domain analysis of the jacket alone is performed using the calibrated El Centro seismic time history. Finally, the coupled system is analyzed for the time history of ground motion. Since the seismic event represents the design Strength Level Earthquake (SLE) condition, which is a rare event, only the FSO is coupled to the jacket, the offtake tanker is not assumed to be present during this extreme event. The nonlinear time domain analysis includes the nonlinear link (yoke) which is a mechanism by virtue of the hinges present. Therefore, the analysis requires geometric nonlinearity of the link to be considered to simulate the large displacements and the large rotations of the link, in addition to the nonlinearities of the pile-soil system. From the results of the analyses conclusions are drawn about effectiveness of vibration isolation by comparing the results of the jacket-yoke-FSO system to those of the jacket alone.