The Investigation of Non-Linear Effect for a Circular Cylinder Shaped FPSO Available to Purchase

Nowadays, a circular cylinder shaped FPSO appears as a new type FPSO because it does not need the expensive turret system. A circular cylinder shaped FPSO has an excellent motion performance because pitch and roll natural periods are longer compared to traditional FPSO and heave natural period is also longer due to its small water plane area. Samsung heavy industries has developed a circular cylinder shaped FPSO called as S-Line. The feature of S-line has a shape cutting a groove around draft to reduce the water plane area. Through reducing water plane area, the heave natural frequency moves to lower frequency which is far from ocean waves. According to the results of a linear calculation by WAMIT, S-Line showed reduced vertical motion of heave, roll and pitch compared to conventional FPSO. But, S-Line showed unsuspected large pitch response in 1^st model test which was carried out with horizontal mooring system by spring in SSMB (Samsung Ship Model Basin). It is conjectured that this large pitch response is caused by 2^nd nonlinearity of the wave and/or parametric pitch phenomena. To verify the mooring load and improve the 2^nd order pitch motion, model tests were carried out in SSMB and offshore basin of MOERI (Maritime & Ocean Engineering Research Institute) with two kinds of bilge box. This paper describes the nonlinear effect of 2^nd order for pitch motion of a circular cylinder shaped FPSO based on model test and numerical simulation. As a result, S-Line developed by Samsung was also verified being able to use SCR (Steel Catenary Riser) due to its improved heave motion performance.