Turret Mooring System Design and Analysisfor Harsh Environments

Abstract

Abstract This paper discusses key factors that must be considered in the design and analysis of internal turret mooring systems for ship-shaped FPSO vessels in harsh environments. Peculiarities of this type of mooring system, such as extreme sensitivity to non-collinear environments, excessive vertical motions at the fairlead, and complicated thruster control strategies, require that design and analysis of the mooring system be closely interrelated to other subsystems such as turret and riser systems, as well as to the vessel itself. In addition, the selection of the mooring design code that will be followed can have a significant impact on the final mooring line tensions derived and it is important to understand the types of analytical approaches that can be used as well as their limitations when applied to turret moored vessels. Model testing, especially for turret moored FPSOS, can prove particularly beneficial if properly planned. With advance knowledge and understanding of the possible implications of all these factors, trade-offs in the design can be properly evaluated. Introduction In recent years Floating Production, FPSO mooring system Storage and Offloading (FPSO) systems have &en gaining popularity due to the cost effectiveness of developing marginal fields with expected production lives of less than 10 years. ?Ilk is especially true when these fields are located in remote areas lacking the support of infrastructures such as existing pipeline networks. One type of application that is becoming more prevalent is the internal turret moored FPSO tanker. As these vessels move into deeper waters with harsh environments, the design of their subsystems, including moorings, turrets, and risers, becomes critical. These three key subsystems are interrelated. The riser design will determine the vessel offset criteria for the mooring system design. The mooring line loads have to be transferred to the turret for structural strength analyses. The turret location will affect not only the weathervaning capability of the vessel, but also the dynamic loads on the mooring lines due to the vertical motions at the fairlead The turret design may place restrictions on the number of mooring lines allowed, and the layout of the mooring lines may pose interference concerns for the risers, It is important to understand how these subsystems are related, and why many aspects of the design or analysis of the mooring system for a turret moored FPSO must be carefully approached. Other important considerations for turret moored FPSOS, particularly in harsh environments, are the selection of the mooring analysis procedures and the design cede. Theenvironmental loads on the vessel, the motions of the vessel, and ultimately the mooring line tensions are all impacted by the type of analysis performed and details of the code requirements. The mooring analysis procedure generally followed today for turret moored vessels were developed for more conventional semisubmersible-type mooring systems, and the resulting limitations should be understood.