Study On Nonlinear Coupled Motion Response Of Truss Spar

Classic spar platform is a kind of deep draft cylinder which is used for deep water oil/gas exploitation, production and storage. Under normal sea condition, the motion response of the platform is small. Therefore, dry tree and rigid steel production risers can be used. Unfortunately, the platform may lead to heave-pitch coupled motion when the incident wave period is long enough to excite heave resonance. Truss spar is the second generation spar platform, the truss structure combined with heave plates is used for the midsection instead of closed cylinder which leads to different motion response compared with classic one.The main purpose of this paper is studying the coupled motion response and stability of the truss spar under regular wave. Then the parameters which may influent the motion are discussed. At last a new type of deep water platform is invented based on the comprehensive study of spar platform and semi-submersible platform.First the linear uncoupled motion equation and nonlinear coupled motion equation are established separately while the parameters of the equations are got from SESAM software which is based on diffraction and Morison theory. Then the equations are solved numerically by using MATLAB software. Later, the approximate analytical results are got and the inter resonance stability are studied. The influence of center of gravity location, wave parameters and mooring system are discussed based on the above study. At last, a new type of deep water platform is presented and the motion response of the new platform is compared with truss spar and semi-submersible.The results show that 1) the linear motion equation could be used in normal sea condition, but at the heave resonant wave period, nonlinear coupled motion should be used especially when the periods of heave and pitch are close to 1:2, since the linear equation will produce large errors in that condition; 2)the motion response of truss spar is better than the classic one at the heave resonant wave periods; 3)the stability diagram and analytical results are got which offer the theoretical basis for the study on parameter analysis;4) The center of gravity location, wave parameters and mooring system have a great influence on the motion and they need to be carefully checked during the design progress;5) the new platform shows a great advantage in motion response compared with spar and semi-submersible.