Impact Of Water Depth On Shallow Water Effect And Seabed-Touching For SYM FPSO

For some Soft Yoke Mooring FPSOs working in shallow water, the shallow water effect may greatly increase the second order wave drift forces while the touching sea bed problems may lead to severe movment of the FPSO, both can greatly increase mooring forces on the Soft Yoke Mooring Systems. Eight loading conditions of three different tonnages' FPSO were utilized to investigate the /LWD T(Critical ratio of water depth and draft) in this paper. For each loading condition, dynamic response capability of the soft yoke mooring FPSO under different WD/ T(Ratio of water depth and draft)were analyzed based on multi-body dynamic method. The second order wave drift force were considered by Newman approximation and Pinkster approximation method separatly, and the mooring forces under different WD/ T were calculated afterward.After comparation of the results get from Newman approximation and Pinkster approximation, a method to calculate the /LWD T of FPSO was carried out. In that method, we calculated the /LWD T for the 8 loading conditions metioned above, the laws of mooring forces varies with WD/ T and FPSO tonages were found. Based on these laws, the database of /LWD T for different kinds FPSO?different tonages and different draft was proposed. Further more, the /LWD T for Seabed-Touching was also calculated for the three full loading conditions in frequency and time domain in this paper.Some main conclusions are as follows:(1)The QTF(Quadratic Transfer Function) matrix decreases versus the increase of WD(Water depth), under shallow water, the QTF matrix calculated by Pinkster approximation is far more bigger than the results calculated by Newman approximation.(2)When calculated with Newman approximation, the mooring force of the SYMS increase first and decrease afterward versus the increase of WD/ T. When calculated with Pinkster approximation, the mooring force continually decrease versus the increase of WD/ T, the mooring force is far bigger in shallow water than the force in deep water.(3)Under shallow water, the mooring force calculated by Pinkster approximation is bigger than the results calculated by Newman approximation, while under deep water, the results calculated by Newman approximation becomes bigger. Thus, there must be an intersection between the curves of mooring force versus WD/ T calculated by Pinkster approximation and Newman approximation, the WD/ T of the intersection point could be defined as the /LWD T for FPSO to consider the shallow water effect.(4)For the same tonage, the /LWD T for FPSO to consider the shallow water effect increase versus the decrease of FPSO draft. For the same nondimensionalized T(Draft) of the FPSO, the /LWD T for FPSO to consider the shallow water effect increase versus the decrease of FPSO's tonnage.(5)Considering the parameters such as type of FPSO, tonnages and loading condition, we proposed the concept of a data bank containing /LWD T for FPSO to consider the shallow water effect data. Through the FPSO design parameters, we can find the /LWD T through the data bank instead of complex calculation or experiments.(6)The analysis method for sedbed-touching in time and frequency domain was given in this paper, through time domain analysis on full-loading condition of the 3 FPSOs, the /LWD T of the 30 WT FPSO is 1.04, the /LWD T of the 16 WT FPSO is 1.05, the /LWD T of the 5WT FPSO is 1.34.