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reducing riser bend-stiffener design requirements, particularly for
transverse conditions. This optimisation also reduces the need for
special topsides process equipment design requirements such as
baffles for separators.
The hull form can also be optimised to reduce the wave and
current actions by varying the length, width, draft and shape of
bow and stern.
This reduces the vessel loading with respect to incoming
waves under transverse conditions, reducing mooring loads and
vessel roll. The aforementioned hull form optimisation can have a
beneficial contribution to the long term operational performance of
the FPSO, but this should be carefully assessed against the
fabrication cost and Life of Field (LOF) costs.
In the more recent FPSO developments, there have been
cases where the cargo carrying capacity of the FPSO is not sized
against the initial production flow rate, but one further into the
production life where a reduced and more economical storage can
be achieved. The initial storage requirements are met by
supplementing the FPSO with a Floating Storage Unit (FSU) based
on a cheap second hand tanker.
The above highlights that a full LOF approach should be
taken in deciding both the vessel design and the field
configuration. The following sections discuss an example FPSO
design for 800,000 BBL storage in deep water.
4.2.2 Hull Structure
There is a marked difference in the hull arrangement
between the new purpose-built FPSO hulls and the converted
tanker hulls. The overall configuration for tankers is driven by the
need to transport large cargo volumes at a low cost. Tankers have
consequently evolved to a length to breadth ratio of about 6.1,
which gives a good compromise between the enclosed volume and
the resistance to forward motion. The FPSOs are not required to
move forward; consequently resistance is not an issue. However,
in a weathervaning mode the hull slenderness ratio (length to beam
ratio) serves to present a low frontal area to the prevailing
environment and assists in the natural weathervaning motion. A
low slenderness ratio results in more favorable motions and
mooring behaviour over a shorter and more bulky hull. A shorter
hull would however offer savings in steel weight and possible cost
reductions. Hull breadth to depth ratio comparisons are a different
prospect since FPSOs, unlike tankers, are not constrained by
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