Global Analysis of the Terra Nova FPSO Turret Mooring System

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. • The turret mooring system is required to be disconnectable to avoid collisions with icebergs greater than 100,000 MT. There are two conditions placed on the disconnectability of the system. A controlled disconnect must be accomplished in seastates up to the 1-year ice season storm (7.4 meters significant) in less than 4 hours, with all risers flushed and de-pressurized. The system must also provide an emergency disconnect that must be accomplished in 15 minutes or less. • The FPSO is required to reconnect to the mooring and risers in seastates up to 2.1 meters significant with no external assistance. • The design life of the system is 25 years. The design basis has resulted in the design of the first disconnectable turret mooring system that is required to stay connected in a severe 100-year storm environment, and be OTC 11914 Global Analysis of the Terra Nova FPSO Turret Mooring System Arun S. Duggal and Caspar N. Heyl, FMC SOFEC, and George P. Vance, ExxonMobil 2 A. S. DUGGAL, C. N. HEYL, G. P. VANCE OTC 11914 disconnectable to avoid possible collisions with icebergs only. This has resulted in a disconnectable turret system that can withstand the large loads transmitted through the turret in a 100-year environment, requiring the development of several specialized components to provide the high load carrying capacity and safety. This is in contrast to disconnectable systems that have been employed in the South China Sea that disconnect for typhoons, resulting in reduced environmental criteria and the resulting loads for the turret mooring design. The stringent disconnect and reconnect criteria have also resulted in the design of a very sophisticated and automated disconnectable turret mooring system. Design Environmental Criteria The environment at the Terra Nova site is one of the harshest in the world with a 100-year significant wave height of 16 meters, and 1-hour mean wind speeds of 40 m/s. The site is situated in “iceberg alley” where large icebergs from Greenland and Ellesmore Island drift south with the Labrador current. Surveys have shown the presence of iceberg scour marks on the seabed, and statistics indicate that the site could see as many as 66 large icebergs in a single season (April – July). Table 1 provides a summary of the design storm conditions for both the 1-year and 100-year return intervals. Compared to the Gulf of Mexico and the North Sea, the amount of environmental data available for the Grand Banks is limited, with very little knowledge on wave direction. This has led to a lot of uncertainty in the development of the design environmental criteria and has required the robust design of the FPSO mooring system to account for unforeseen environmental conditions. FPSO Vessel and Turret Mooring System FPSO Vessel. The new build vessel is specifically designed for the harsh conditions on the Grand Banks. The 960,000 bbl storage vessel has an LBP of 277 meters, a beam of 45.5 meters, and a depth of 28.2 meters. At the full load condition the vessel has an average draft of 18.6 meters, with a displacement of 193,000 MT. The hull is ice-strengthened to site specific criteria, and the bow and freeboard elevations have been designed to minimize any greenwater on the deck. The vessel has five azimuthing thrusters rated at 5 MW each, with two mounted at the bow and three at the stern. The thrusters are used to assist the mooring system when the vessel is on station, to control and maneuver the vessel during the disconnect operation, and to position the vessel during the reconnect operation. Details on the vessel design and construction are provided in Reference 3. The thruster system controls the FPSO position by providing surge, sway and yaw (heading) control by introducing “stiffness” and “damping” in each mode. The control system, based on a Kalman filter algorithm, uses two redundant position-monitoring systems to estimate turret position with respect to FPSO center, and obtains vessel heading information from gyro compasses. The automatic control system then uses the thruster system to provide the desired control, based on input from the operator. The thruster system with its automatic control system has been designed to meet Lloyd’s Register class PM(T3) (References 1 and 2). Turret Mooring System. The disconnectable internal turret mooring system is located 74 meters aft of the forward perpendicular of the vessel. The turret diameter varies from 12 meters at the main deck to 21.5 meters at the keel, with a total height of approximately 60 meters. The turret has two bearing systems: a lower bearing located near the vessel keel that reacts the horizontal loads imparted by the mooring, and an upper bearing located at the main deck elevation to react all vertical loads, moments, and the residual horizontal loads. The turret has a disconnectable spider buoy that supports the allchain mooring system, and the nineteen risers and umbilicals. Figure 1 provides a general arrangement of the FPSO vessel and mooring system. The spider buoy is connected to the lower turret using a single pre-tensioned connector that can be released hydraulically, allowing the spider buoy to free-fall to the desired subsurface position. The spider buoy has a diameter of 17 meters, and net buoyancy of 450 MT. The risers are connected to flanges at the bottom of the buoy, and quick-connect/quick-disconnect valves are provided at the spider buoy-lower turret interface. A schematic of the turret system is provided in Figure 2. The 9 anchor legs are arranged in three groups of 3 anchor legs each, with each group 120 degrees apart. Each anchor leg consists of studless Grade R4 chain (146 mm) terminating in an anchor pile. The mooring system is designed with a heavier section of chain (“excursion limiter”) at the touchdown point that is optimized to reduce vessel offsets and mooring loads. As described in the design basis the mooring system requires assistance from the thruster system on the vessel in the storm environments. The 14 risers and 5 umbilicals are arranged at the turret in three groups in between the anchor leg groups, using a pliant wave configuration. The risers range from 10-inch production and water injection risers to 5-inch gas lift risers. Global Analysis Methodology The shallow water depth, coupled with the harsh environmental conditions, make the global analysis of a highly non-linear system like Terra Nova very challenging. Accounting for the dynamics of a thruster-assisted mooring system further complicates the global analysis. The FPSO system has been analyzed by integrating a detailed analytical and numerical study of the system response with a comprehensive model test program. The simulation of the FPSO vessel and mooring system used both frequency-domain and time-domain methods to develop the system response, mooring system performance,