Page 354 - Offshore Electrical Engineering Manual

CHAPTER
5
Tension Leg Platforms

INTRODUCTION
The tension leg system consists of a hull-like structure having positive buoyancy
which is held to a deeper draught by means of tension legs at each corner.
The legs consist of flexible metal tubes anchored in foundations in the seabed
and tensioned to hundreds of tons in the platform. A computer-controlled ballasting
system prevents load movements on the platform deck causing large differences in
tension and hence avoids dangerous stressing of the hull structure or the legs them-
selves. The tension leg mooring system allows for horizontal movement with wave
disturbances, but does not permit vertical, or bobbing, movement, which makes ten-
sion leg platforms a popular choice for stability, such as in the hurricane-prone Gulf
of Mexico. In a simpler form, it is also a useful concept for wind turbine structures,
in locations where deepwater makes the usual tower design uneconomic.

TENSION LEG PLATFORM IN OIL AND GAS INSTALLATIONS
The tension leg monitoring, tensioning and ballasting system is the only major dif-
ference from the normal jacket or concrete platform installation. The safety facilities,
such as hazardous area certification, emergency shutdown, fire and gas and process
control, are similar, and in UK waters, a Safety Case would need to be prepared in the
same way. As the tension leg monitoring, tensioning and ballasting system is vital to
the safety of the platform, it would definitely be regarded as safety critical.

TENSION LEG PLATFORM AS A WIND TURBINE STRUCTURE
This would be a much simpler normally unmanned installation, and the flammable
material inventory is very small.
(Note: As part of a wind farm, no oil-filled transformer should be required if the
wind turbine generates at the export voltage (in the region of 20 kV). The transmis-
sion step-up transformer would normally be located on a separate substation plat-
form, see PART 6 Chapter 6).

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