TUMSAS: A New Solution for Shallow Water FPSO Mooring
Authors: Zeng Hengyi , Li Xinzhong , Chen Jiecheng , Chen Minhao , Tan Jiaxiang , Jing Yong , Shi Shan , Li Zhigang , Li Xin 
Published: April 2012

Shallow water mooring for an FPSO system is a challenge for offshore engineers.TUMSAS - Turret Mooring with Self Adjusting Stiffness system is invented toprovide a new and effective solution. The work presented here includes thedevelopment of TUMSAS and supporting model tests results.

FPSOs at 20m to 30m water depth range require nonlinear mooring systems whichcan only provide a soft initial restoring stiffness at small vessel offsets andlarge stiffness at relatively large offsets. A conventional catenary turretmooring system is suitable for mid to deep water applications due to itscatenary configuration only can be maintained at a small vessel offset forshallow water. Submerged soft yoke turret mooring systems are economicaldesigns for shallow water marginal field applications, but they cannot providelong enough FPSO offset. Over stress and even total damage has been reportedfrom applications in the China production field. TUMSAS is developed with helpfrom various offshore engineering companies and research institutions toprovide an effective and economical solution for FPSO shallow watermooring.

TUMSAS has advantages taken from both conventional catenary turret mooring andsubmerged yoke systems. It consists of a turret, a hanging weight module andcatenary moorings connecting to the lower edge of the weight module. As a keycomponent of the TUMSAS design, the weight module combined with catenaryprovides self adjusting restoring stiffness required for shallow water mooringwhich has been designed by one of the authors and successfully used for thepermanent mooring of a retired aircraft carrier for more than ten years.

TUMSAS system has been developed and ready for FPSO shallow water mooringapplications. Satisfactory results are obtained from model tests of a TUMSASsystem designed for the China Bohai BZ3-2 field. The tests are conductedrecently at the ocean engineering basin of the State Key Laboratory of OceanEngineering in Shanghai Jiao Tong University.

Introduction

Development of a shallow water FPSO mooring concept called Turret Mooring withSelf Adjusting Stiffness [TUMSAS] system is introduced in this paper. The workpresented here starts with discussion of shallow water mooring philosophy andthe motivation for the development of such a system. A TUMSAS system design forChina Bohai BZ3-2 field application is introduced with detailed sizinginformation, material properties, and a mooring/riser configuration. Theworking mechanism of the TUMSA system is described and comparisons toconventional soft-yoke FPSO single point mooring systems presented.

The numerical wave hydrodynamic and coupled analysis model, the analysisprocedure and the load matrix are described in the document. The analyses areconfirmed by the model tests performed at the State Key Laboratory of OceanEngineering in Shanghai Jiao Tong University. The economics and economicfeasibility of the TUMSAS system for shallow water mooring of offshoreproduction and offloading FPSOs are also studied in this paper.