Global Analysis of the Terra Nova FPSO Turret Mooring System
Authors: Duggal Arun S. , Heyl Caspar N. , Vance George P. 
Published: October 2008

This paper describes the global mooring analysis conducted for the Terra Nova FPSO, located on the Grand Banks, offshore Eastern Newfoundland, Canada. The FPSO has a disconnectable internal turret mooring system that must withstand the 100-year storm environment, and be disconnectable to avoid collision with large icebergs. The global analysis of the FPSO vessel and mooring system focuses on the response of the system in storm conditions, and during offloading, disconnect and reconnect operations. The analysis was performed using sophisticated analytical and numerical techniques, integrated with a comprehensive model test program primarily for verification of the analysis. As the Terra Nova FPSO is the first such platform to operate in an iceberg region, a focus of the analysis was in studying the interaction of pack ice and icebergs with the FPSO system. The methodology used to analyze the FPSO system is outlined in this paper and some key results are presented.

Introduction

The Terra Nova FPSO will be located in approximately 95 meters water depth on the Grand Banks off the East Coast of Newfoundland, Canada. The region has a harsh environment, much like the Northern North Sea, with intense storms occurring frequently in winter. In addition, there is sea ice excursion into the region, including pack ice and icebergs. The FPSO system consists of a new-build FPSO vessel and a disconnectable turret with a thruster-assisted, 9-leg mooring system. The turret supports 14 risers and 5 umbilicals servicing wellheads in four or more glory holes. The internal turret mooring system has been designed to maintain station in the 100-year storm environment, and to be disconnectable to avoid an approaching iceberg on a collision course. Once the FPSO disconnects, the mooring and riser system is supported by a spider buoy that has an equilibrium depth of 35 meters below sea level.

The global analysis of an FPSO system requires detailed knowledge of the environment, vessel characteristics, and the mooring system design. The analysis of the Terra Nova system has further complications due to the use of an automatically controlled thruster system to assist the mooring, and the disconnect and reconnect operations. The following sections briefly describe the methodology used to study the combined FPSO vessel and mooring system response, and present a few results to illustrate the performance of the system, and the accuracy of the analysis.

Stationkeeping Design Basis

The mooring system has been designed to meet or exceed the requirements of Lloyd's Register (References 1 and 2) for a thruster-assisted, permanent mooring system. The design basis for the FPSO global analysis is summarized below:The Terra Nova FPSO must be moored on station in a 100-year storm, requiring the turret mooring system to be designed for the 100-year environment. The mooring system can depend on the full capability of the thruster system (25 MW).In seastates up to the 1-year storm the mooring system can only utilize the heading control mode of the thruster system, with a maximum peak power demand of 10 MW.