This arrangement produces a buffer zone providing protection to the storage tanks from external penetration resulting from any collision event. A typical cross-section of the vessel shows the transverse tank sections (Fig.5).
At the time of the initial design there was no requirement to meet the Marine Pollution Prevention requirements applied to Oil Tankers. However, as a matter of policy the double bottom height and the wing ballast tank breadths were sized to comply with MARPOL (the International Convention on Marine Pollution from Ships). In conjunction with the ice-strengthened skin, this ensures that damage to the outer skin will not lead to hull penetration or oil being released.
Deck Payload. The 'topside' payload is approximately 12,500 tonnes with a combined mooring and turret load of about 5,000 tonnes. In iced up conditions there is an additional ice load of 2,000 tonnes. The basic PV150 vessel design has the capacity to increase the payload further by using double bottom ballast tanks for fields where cargo waxing is not a consideration and there are no restrictions on the use of adjacent ballast tanks and crude storage tanks.
Focsle and poop height. The extreme weather conditions, exacerbated by the comparatively shallow water of the Grand Banks gives rise to steep breaking storm waves which in the 100 year storm can be up to 30 metres high. It is of great importance to keep water off the main deck and prevent wave crests from breaking over the focsle and poop, (Fig.6 and 7).
Length of the poop and focsle were chosen to prevent storm wave crests breaking aboard and checked using computer simulation and model tests. Bluff bows can steepen storm waves as they reach the ship, causing them to break. The Terra Nova bow profile, sections and waterlines have been shaped to minimise this effect and to divert wave crests as far as possible away from the main deck.
Bilge keels. The vessel is fitted with 1.05 metres wide bilge keels over 50% of its length, predominantly over the parallel mid body. They are broken into a series of panels with the panels tapering into the hull at the hull construction joints. The keels have been sized to keep roll angles to a maximum of about 7 degrees in all anticipated weather conditions. The reduced motions benefit the vessel structurally and improve working conditions for the crew, while increasing the production of oil and reducing the loads on the topsides equipment and structure.
Turret Location. The location of the turret compartment is a compromise between natural weathervaning of the vessel, hull bending loads and the wave induced motions at the turret. It had been intended that the vessel should weathervane naturally in conditions up to the 1 year storm. However, early studies showed that a turret position which allows full natural weathervaning would give unacceptable clashes between the mooring chains and the forward azimuthing thrusters.
The final turret position adopted is approximately 27% of the ship's length aft of the bow. This keeps the moorings clear of the bow thrusters and gains benefits in hull strength and thruster power.
Structural layout and hull proportions. The vessel is longitudinally framed throughout except in way of the additional ice strengthening in the bow. A double bottom extends the full length of the cargo block and machinery spaces. The stability characteristics of the hull have been exploited to increase the depth of the hull girder, reducing stresses and deflections. A moderate Block Coefficient and reduced waterline beam have, in turn, reduced the wave bending moments on the hull. Cargo and ballast spaces have been used to minimise still water bending moments and relative hull deflections during the loading/offloading cycle.
Stability. Stability requirements, in both intact and damage cases, are more demanding than for many FPSOs due to the need to operate both on and off mooring and in an area of heavy icing. These considerations result in extra weight being carried, high in the vessel, with a corresponding reduction in stability. The vessel stability meets the requirements both of the IMO Code for Mobile Offshore Units (usually applied to offshore vessels, i.e. UK North Sea Vessels) and applicable damaged stability requirements of MARPOL (usually applied to trading Oil Tankers).
Minimising Environmental Loads. Hull induced loads (wave, hydrodynamic, wind, etc) have been minimised by the use of a slender shape, restricting the vessel beam and using amoderate Block Coefficient.
Additional loads due to pitching have been reduced by avoiding blunt endings and abrupt changes of shape in the bow.
At normal angles to the wind, longitudinal wind loads have been reduced by the screening effect of the forward superstructure and the focsle.
Ice Strengthening. The FPSO hull has been designed to withstand the loads imposed on it by icebergs of up to 100,000 tonnes and pack-ice of 0.3 metres thickness in 5/10 coverage.
The extent of the ice strengthening is as follows;
Vessel Description: Extent of Ice Strengthening
Vessel Bow: Bottom turn of bilge to lower focsle
Vessel Midbody & Stern: Bottom turn of bilge up to 24.0 m
For the purposes of ice strengthening the extent of the bow section is defined as "forward of the forward bulkhead of the turret compartment, frame 48" with the combined midbody/stern section defined as "aft of the forward bulkhead of the turret compartment".
