Hydrodynamic Performance Of S-Spar Platform And Its Hydrodynamic Analysis Under Internal Wave

Nowadays, the global economic growth increasingly depends on the petroleumand natural gas. Ocean reservoirs are focused on by the oil company with decreasingreserve on the land. After tens of years’ development of offshore oil industry, itsproduction may not support the requirement of economic growth. Then deep waterreservoir becomes more attractive to the industry. Spar platform is in service since1998, and gains acceptances by the offshore oil industry. When there are not enoughdesign parameters taken into account, the offshore platforms will be damaged underthe environment loads due to the severe ocean environment. As the offshore platformis regarded as high-technology, high-investment and high-risk one, how to determinethe environment load, strength analysis, motion response and fatigue prediction arebecoming the hot zone of offshore research.S-Spar is a new concept of spar platform, South China Sea is the location whereS-Spar installed at and internal wave is taken into consideration during the designprocess. S-Spar is a concept that combines the advantages of Classic Spar and TrussSpar. The midsection of S-Spar is different from Classic Spar and Truss Spar is asmall diameter cylinder. Then the environment load becomes smaller especial the loadcaused by internal wave. The heave motion is suppressed by the three heave platesfixed on the midsection which can get larger additional mass and additional damping.Taut mooring system is introduced as the S-Spar mooring system in order to controlthe motion response of the platform during the operating condition.Three-dimension hydrodynamic analysis finite element model is built.Hydrodynamic performance and motion response in frequency-domain is calculatedwith the potential theory. The frequency-domain results contain the additional mass,additional damping, wave exiting force under first order and response amplitudeoperators (RAO). Then frequency-domain conclusion is applied to predict short termmotion response for motion performance of S-Spar under the non-regular wavecondition. The results indicated that the S-Spar platform has excellent hydrodynamicbehaviors.The excursion force relationship of mooring line is determined with the catenaryequation. Cables and chains which are hanged with their own weight can be solved with catenary equation. The nonlinear relationship between excursion and horizontalforce is solved with iteration algorithm with the catenary equation. Assume that themooring line motions only in the plane, the excursion force relationship of themooring system can be determined with single mooring line’s performance.Internal wave is common in the ocean. Internal wave distinguished into internaltide and internal solitary wave. Internal solitary wave is a large amplitude nonlinearwave, and propagates in single direction with the velocity and wave shape remains thesame. High current is induced by the internal solitary wave and will increase the loadacting on the offshore structures. The internal wave is described with the KdVequation. Then the velocity distribution of water particle can be solved with KdVequation. The wave force that act on S-Spar is solved with the Morison equation andthe motion response during the internal wave propagation is determined intime-domain.The consequence of this thesis are mainly as following:1. A new concept of Spar platform called S-Spar is introduced. The mainhydrodynamic performance parameters such as additional mass, additional damping,excited wave force under first order and response amplitude operator are determinedwith3-D potential theory. At last short term motion response prediction for motionperformance of S-Spar under the non-regular wave condition is calculated.2. The excursion force relationship of S-Spar taut mooring line is determinedwith the catenary equation. The S-Spar mooring system’s relationship betweenexcursion and force is calculated due to the mooring system distribution.3. Wave shape sketch is drawn with KdV equation. The velocities in bothhorizontal direction and vertical direction of water particle are calculated with KdVequation.4. With the mooring stiffness and the distribution of flow domain, wave forces inboth horizontal and vertical directions that act on S-Spar are determined with Morisonequation. At last the moment acted on S-Spar and pitch motion response arecalculated.