Page 348 - Offshore Electrical Engineering Manual

CHAPTER

Floating Production, 2

Storage and Offloading

Units

INTRODUCTION
In deepwater where it is not economic to locate a platform, the floating production,
storage and offloading (FPSO) facility comes into its own. Often based on a con-
verted tanker hull, these vessels can accommodate the complete oil and gas recov-
ery process on the main deck, whilst using the existing (ex-tanker) storage until the
shuttle tanker arrives and offloads it. In order to achieve this, it is necessary to install
some extra equipment not required on fixed platforms.

1. Escape tunnel
Because of the length of the vessel, it is usually necessary to install a pres-
surised fire-protected tunnel for the length of the deck to allow personnel to
reach the end of the ship where the accommodation and lifeboats are located.
Usually, as with a tanker, this is the aft end of the vessel and the forward end is
where the offloading facility is located.
2. Offloading facility and ballast control
In order to offload the crude oil to a shuttle tanker, oil from the oil export pumps
is fed to a hose reel which can be connected to the shuttle tanker using ropes
fired by rockets. The shuttle tanker must be fitted with dynamic positioning
(DP) to allow close approach to the FPSO. There are several safety systems
forming part of the offloading facility:
a. The green line system monitors the offloading hose for leaks by comparing

the level of flow into the shuttle tanker with that flowing from the FPSO.
b. The ballast control system must keep the FPSO stable and avoid stressing

the hull structure by pumping water in to replace the oil, or vice versa, dur-
ing oil production and storage.
3. Differential Absolute and Relative Positioning Sensor
The Differential Absolute and Relative Positioning Sensor (DARPS) radar
system is designed to detect possible collision threats whilst offloading is tak-
ing place. DARPS is a DP position reference system made for offshore loading
operations. It combines sensors for reliable and accurate absolute and relative
positioning of two vessels such as a shuttle tanker and an FPSO.
4. Inert gas system
As with a tanker, it is necessary to maintain a blanket of deoxygenated air or
nitrogen in the ‘air’ space above the crude in the hull storage tanks. This is usu-
ally generated with a main and standby nitrogen generator designed to combust
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