The Terra Nova FPSO has been ice strengthened as follows;
The midbody and stern sections have been ice strengthened to satisfy the requirements of Lloyd's Register Baltic Ice Class Notation 1A (appropriate to ships intended to navigate continuously with a speed of at least 5 knots in first year ice conditions equivalent to unbroken level ice with a thickness of 0.8 metres, with a 0.3 metres snow covering.) with an extended ice belt.
The bow section is ice strengthened in accordance with the requirements of Lloyd's Register for multi year ice with an ice pressure in accordance with a project specific ice pressure area relationship, developed from a study completed by C-CORE on the Grappling Island tests.
The vessel hull strength has been verified for iceberg impact based on energy distribution methods, through an independent iceberg impact analysis carried out by Westmar Consultants Inc of Vancouver and the pack-ice and iceberg tests at the IMD test basin in St John's.
Maximising Propulsive Efficiency. The primary concern was to allow the thrusters to operate efficiently while supporting the moorings and providing dynamic positioning. Good flow to the thrusters was also important to ensure propulsive efficiency and avoid vibration when self propelled. Low resistance as such though, was secondary to the stability and seakeeping characteristics of the design.
The aft body avoids separation of the flow into the thrusters, with a maximum buttock angle of 12 degrees and both the turn of bilge in way of the thrusters and the transition from the flat of bottom to the buttocks being generously radiused. The forebody in way of the forward thrusters has a U section with a large bilge radius. The thrusters themselves have been configured to minimise interference between units. The aft thrusters operate predominantly in surge and are arranged across the beam with the wing thrusters forward of the centre unit in normal triple-screw format. The bow units tend to operate predominantly in yaw and are in line on the centreline. Interference between the bow and stem sets of thrusters is reduced by keeping the stem units above the baseline, the bow units being below it.
Summary. The Terra Nova hull form has been designed to meet the overall requirements set out in the Introduction. It uses a hull of moderate Block Coefficient, minimum beam and maximum depth to produce an easily built and structurally efficient hull form. A reduced midship section coefficient is used to optimise the motion characteristics in both head and beam seas whilst giving a long parallel body for cargo storage and simplicity of construction. It also allows an increase in depth, reducing hull stresses and deflections while maintaining stability and topside load capacity. A ship shaped bow, minimum beam and adequate freeboard together with a forward superstructure reduce environmental loading on the mooring and DP system and minimise water on deck. The hull form and the thruster layout are arranged to maximise the efficiency of the individual thruster units while minimising the interference between them.
Performance
The performance of the vessel has been assessed and refined by applying the best analytical and testing techniques available.
Analysis. The seakeeping performance of the vessel has been analysed extensively during the original design and correlated with model test results. The motions of the vessel and the sea surface were calculated by diffraction analysis.
Animations of the ship and sea surface motions have been used to check the potential incidence of green water over the bow or on the main deck.
The pressure loads and motions provide the starting point for the design and analysis of the hull structure, mooring, turret and topside equipment. Independent motion calculations have been carried out using different software packages to ensure the validity of the results.
Model Tests. An extensive model test programme has been carried out to verify the design of the vessel hull and mooring system and to assist in the design of the thruster system.
Testing was carried out in Denmark, Norway and St John's, Newfoundland, covering five distinct phases.
1. Wind tunnel testing at the Danish Maritime Institute (DMI), Lyngby, were performed on a 1:200 scale model to determine the wind and current loads on the vessel, evaluate the airflow around the helideck and to evaluate the funnel performance.
2. Vessel motion and mooring response tests at Marintek, Trondheim on a 1:60 scale model. These tests also included preliminary green water measurements and thruster system tests on heading control and dynamic positioning.
3. Resistance and propulsion and pack ice performance tests at the National Research Council's Institute for Marine Dynamics (IMD), St John's, were performed on a 1:27.65 scale model.
While low vessel resistance was not a main criterion in the hull design, resistance and propulsion tests have shown the propulsive performance to be up to average tanker standard, allowing a speed of approximately 10 knots to be scheduled for the delivery voyage and giving ample performance for iceberg avoidance when fully laden in-field.
4. Mooring disconnection and detailed seakeeping tests at IMD were performed on a 1:44 scale model. Green water incidence and slamming were checked for a wide range of "on mooring" and "off mooring" conditions, as were the roll responses in beam seas following disconnection.
5. Iceberg impact tests were also carried out at IMD to verify the ice impact analysis carried out by Westmar.
The test results have been good throughout, the vessel exceeding its design requirements comfortably without incurring build cost penalties, (Figs. 8, 9, 10, and 11).
