Behaviour of Turret Moored Tankers in Combined Extreme Metocean Parameters
Authors: Wichers Johan , Ji Chun Qun 
Published: October 2008

Abstract
At present the mooring technology of turret moored tankers is challenged as exploration activities for oil and gas move into deep and ultra-deep water and more hostile environments. The more hostile environment concerns the Atlantic Frontiers and the typhoon prone areas. For the field development in these water depths and under these weather conditions, theavailability of a cost effective mooring system can be an important factor in determining economic success. The approach for developing fields in the mentioned areas has typically relied upon the adaption of mooring technology developed for shallower water depth applications.
To improve the understanding of and the insight in the reliability and integrity in the application in the deep water mooring systems, numerical design tools can be used. As the numerical design tool a by model tests validated computer program based on the fully integrated dynamics of turret moored F(P)SO systems has been recently developed and applied to deep water. Further the computer program can be used to optimize the system and to enhance the safety of the design by applying numerous combinations of extreme weather conditions in order to discern the most severe weathercondition.
In this paper the program has been described. The results of the validation with model tests on a tanker moored with a conventional mooring system in 350 m water depth are shown. The results of the computations agree well with the results of the model tests. Finally by means of computations with the tanker moored with a conventional system with and withoutspring buoys were investigated in 400 and 1200 m water depth applications. By varying extreme metocean parameters the understanding of and the insight in the reliability and integrity in the mooring system can be improved.

Introduction
The theoretical approach to mooring problems has been advanced considerably in the last decades. In 1971 theoretical knowledge of mooring systems did hardly exist. With the establishment of the 3-D diffraction theory in 1976, see Ref. 1, computations of the transfer functions of first order tanker motions with a high reliability could be carried out. Based on this program it became also possible to compute accurately the matrices of the quadratic transfer functions of the second order wave drift forces in 1980, see Ref. 2.
In order to compute the low frequency (1.f.) tanker motions in the horizontal plane, the speed dependent wave drift forces and the required quadratic transfer function of the wave drift damping were necessary. The computations with a tanker with forward speed in head waves were camed out with areasonable degree of accuracy in 1988, see Ref. 3. Furthermore the procedure to determine experimentally the 1.f. hydrodynamic viscous reaction forces and moment in the horizontal plane were also established in 1988, see Ref. 3. The CFD code to compute the quadratic transfer function of the wave drift forces with forward speed with current and waves coming from arbitrary direction were completed in 1996, see Ref. 4.