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deck, over, but independent from the earth bound turret top part.
The gantry supports a traveling hoist crane suitable for
maintenance activities in the turret area, and can handle the swivel
stack modules, either in the maintenance mode or as part of a field
expansion development.
Swivel Stack Assembly
The swivel stack assembly consists of an inner non-rotating
ring and an outer rotating ring that encloses a toroidal shaped
chamber. The individual swivel rings are stacked atop of each
other to provide multiple independent flow paths. The inner and
outer ring concentricity is maintained by triple race roller bearings.
Seals around the periphery of the interface between the inner and
the outer rings prevent leakage of the working fluid.
The swivel design is based upon the following guiding
principles:
Flow re-routing as back mode of operation
Swivel redundancy
Ease of maintenance with minimum intervention
Maximum use of field proven designs
The driving arrangement for the swivel stack is an integral
part of the overhead framework structure. An individual driving
ring is supported at two opposite sides by a fork arrangement.
Each swivel unit is therefore driven on both sides of the outer ring
simultaneously resulting in a pure driving torque with no net shear
being applied on the piping flanges.
The swivel sealing system incorporates double seal sets in
each module. A leak detection system and recuperation system is
built-in between the seals which collects any product fluid and
returns it to the production flow.
The gas swivel sealing principle is based upon an over-
pressurised back-up fluid that serves as a barrier between the gas
to be sealed and the environment. This is achieved by using three
seals on each side of the gas chamber. In order to assure the
absolute gas tightness, the pressure on the back-up fluid is always
maintained higher than the gas pressure. This is achieved through a
simple accumulator.
4.2.5 Marine Systems
Inert Gas and Tank Venting System
To avoid the presence of an explosive mixture in the cargo
tanks above the oil an inert gas is installed to provide a slight

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