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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.

6.1.2 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
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