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weathervane by using a spread mooring with slack aft moorings,
giving the vessel the options of some limited weat hervaning
Oil production is through either flexible risers or riser
towers with flexible jumpers. The motions of the FPSOs generally
prohibit the attachment of rigid vertical risers or steel catenary
risers.
The FPSOs have a large area for setting a deck on the top
of the hull. However, many FPSO hulls are conversions and the
deck structure may not be designed to carry a Floating Offshore
Platfornz Design processing facility. This needs to be checked
carefully before committing to a surplus tanker for conversion.
The converted FPSOs often offer the shortest and the
cheapest path to initiating production. Their main limitations
include a lack of ability to operate dry trees, and technical
feasibility of mooring in very deep water in harsh environments.
The turret assembly can become very complicated and difficult to
integrate with the hull.
4.2.1 FPSO Hull Design
There are four principal requirements that drive the size of
a typical FPSO:
1. Provision of oil storage capacity compatible with the
production rate and offloading arrangements, i.e. shuttle tanker
turnaround time
2. Provision of topsides space for a safe layout of the
process plant, accommodation and utilities.
3. Provision of displacement and ballast capacity to reduce
the effects of motions on process plant and riser systems.
4. Provision of space for the production turret (bow, stern
or internal). and the amount of hull storage capacity lost as a
consequence (new-build or conversion).
As water depths increase, the sensitivity of the FPSO
mooring and riser systems to wave frequency motions increases
rapidly. The FPSO hull form can be optimised to counter this by
varying primarily the length, width, depth, draft and mass
distribution.
The hull length can be adjusted with respect to the expected
wave lengths to ensure that the extreme environmental conditions
do not generate wave lengths located in the range of the peak
heave and/or pitching responses, thus reducing the turret vertical
motions and associated dynamic loads. The hull form can also be
adjusted in order to control rolling and pitching behaviour thus
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