Page 340 - Offshore Electrical Engineering Manual

CHAPTER

Notes on Dynamic 1

Positioning and Ballast

Control for Floating

Installations

DYNAMIC POSITIONING
Apart from the obvious advantages of docking a vessel more easily in crowded ports,
dynamic positioning (DP) becomes vital to avoid collisions when it is necessary
to hold position close to an offshore installation or maintain position over subsea
equipment.
Examples of this are

1. a supply boat offloading containers to the laydown area on a platform,
2. a floating production, storage and offloading (FPSO) unit offloading crude to a
dynamically positioned tanker,
3. a pipelaying vessel offloading pipe along a strictly contoured seabed route,
4. a diving vessel with divers deployed.

There are a number of defined grades or classes of DP depending on

1. the weather conditions that the ship can keep position in using its full propul-
sion capabilities,
2. the degree of redundancy in its machinery (e.g., standby generation),
3. the degree of redundancy in its position reference systems (e.g., GPS),
4. whether the back-up equipment is physically separated to avoid common-mode
failure,
5. whether the DP system has an independent joystick control which the DP opera-
tor can use if the automated control system fails.

These classes have been developed by IMO MSC/Circ.645 and are listed in
Table 6.1.1.
As can be seen from Table 6.1.1, for a dynamically positioned ship to hold
position, there are a number of ship systems that are required to support the
propulsion.
Class 2 and 3 vessels are normally utilised in offshore operations, with class 3
vessels reserved for the most onerous duties, for example, operation with divers
deployed.

Offshore Electrical Engineering Manual. https://doi.org/10.1016/B978-0-12-385499-5.00034-0 327
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