Fully Coupled Nonlinear Dynamic Response of Spar Platform under Random Loads

摘要

Spar platform, a compliant floating structure, is well suited for deeprnwater applications like drilling, production, processing, storage andrnoff-loading of ocean deposits. It’s popularity is attributed to it’srneconomical performance and distinguished sea keeping characteristicsrnbesides various other merits. However, the analysis of Spar-mooringrnsystem poses tedious computational problems, primarily because ofrnthe uncertainties associated with the environmental loads, structuralrnconfiguration and resulting nonlinearities. Spar is modeled as rigidrncylinder, provided stability and stiffness by catenary cables attachedrnto the cylinder at fairleads and spread out horizontally. Moorings,rnpartly lying on the sea floor, hinged to the sea bed at the far end. Theirrncontact with sea bed is properly modeled. The mooring lines in deeprnwater conditions contribute towards significant inertia and dampingrndue to their longer lengths, larger sizes and heavier weights. Coupledrnanalysis, presently adopted, employs a fully integrated spar mooringrnsystem. A finite element model consists of a rigid cylinder linked andrnsupported by tensioned mooring lines at fairleads. Hybrid beamrnelements are used to model the mooring lines experiencing largerndeformations and tension fluctuations of random nature. Their linkagernwith spar cylinder is suitably modeled through connector elementrnprovided in ABAQUS. It ensures integrated coupling and structuralrncontinuity. Spar hull is modeled as an assemblage of rigid beamrnelements connecting its centre of gravity, riser reaction point andrnmooring line fairleads. Monte carlo simulation is adopted to modelrnthe long crested random sea. The implicit solver employed is based onrna step-by-step iterative time domain algorithm. The solver adjusts thernsize of time step to achieve accuracy and stability of the solution.rnHence, the convergence of the solution is computationally intensive.rnPower spectra for various hull motions and maximum mooringrntension are plotted and analyzed. Statistical characteristics of randomrnresponse histories are also obtained. The integrated coupling shows arndistinguished influence on the response behavior. The power spectrarnshows the participation of low frequency and wave frequencyrnresponse behavior. The presence of current, however, influences thernPSD’s in a significant way. The mooring tension response underrnmoderate sea state shows the interaction of higher modes near wavernfrequency.