Page 468 - Offshore Electrical Engineering Manual

APPENDIX

Guide to Offshore A

Installations

A.1 TYPES OF INSTALLATION
SEMISUBMERSIBLES

These are by far the most common type of installation and are used as mobile drill-
ing platforms, accommodation and 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 buoyancy chambers placed well below the
zone of wave action, and with a draft of up to 90 ft to 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.

JACK-UP OR SELF-ELEVATING PLATFORMS
A jack-up rig is a barge-like vessel with steel legs each having racking gear which
allows it to be racked down to the seabed until the entire vessel is lifted out of the
water. These vessels are obviously limited by the length of the legs to water depths
of around 300 ft. They are inherently a temporary facility, more suited to drilling and
maintenance activities than production.

FIXED PLATFORMS

A fixed platform may be described as consisting of two main components:

1. The substructure
This consists of either a steel tubular jacket or a prestressed concrete structure.
2. Steel structures
Steel jacket structures normally consist of tubular legs held together by welded
tubular bracing, the whole unit being securely piled to the seabed through
tubes attached to the bottom of the legs. Also in the structure are various verti-
cal steel tubes required for obtaining seawater for platform utilities (stilling
tubes), for protecting subsea electrical cabling (J-tubes) and for guiding for
well risers. The jacket when first installed without superstructure may be more
than 500 ft high.

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