Page 350 - Offshore Electrical Engineering Manual

CHAPTER

Semisubmersibles 3

and Mobile Offshore

Drilling Units

INTRODUCTION
These are by far the most common type of installation and are used as mobile
drilling platforms, accommodation, construction centres and fire tenders. They are
sometimes modified for use as oil production platforms. With a displacement often
exceeding 20,000 tons, these float on pontoons well below the zone of wave action,
and with a draft of up to 90 ft, dampen the movement caused by the waves. The
platforms are normally held in position by eight or more 15-ton anchors and like
the fixed platforms are designed to withstand extreme weather conditions. There
are obviously some differences to a fixed jacket or concrete platform which need
to be borne in mind (see later discussion), but otherwise the requirements such as
emergency shutdown, process control, fire and gas and power management are
similar.

POWER GENERATION
The normal generation outfit of around six diesel generators is sufficient for all drill-
ing operations, with some standby. However, if used for production, there may be
some gas turbine sets.

PONTOON AND LEG EQUIPMENT INSTALLATION
It is usual to have mainly tanks and seawater lift and fire pumps in the pontoons and
legs, but there is a design with diesel generators in the pontoons which makes main-
tenance difficult. In case of fire the legs can act like chimneys and so extinguishing
systems are essential, especially if there are generator sets in the pontoons.

WATERTIGHT DOOR MONITORING AND BALLAST CONTROL
SYSTEMS

Comprehensive monitoring and control of ballast tanks, fuel tanks, cement tanks,
etc. is vital for control stability. Watertight door control and status is also essential.

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