Page 469 - Offshore Electrical Engineering Manual

456 APPENDIX A: Guide to Offshore Installations

3. Concrete structures
Two proprietary designs of concrete gravity substructure are most common in
the North Sea.

a. The ‘CG Doris’ type consisting of a large single watertight caisson sur-
rounded by a circular breakwater designed with a honeycomb of holes to act
as a breakwater. A central column sprouts from the caisson from which the
superstructure is supported.
b. The ‘Condeep’ type consisting of circular towers or legs sprouting from a

cellular caisson.
In both cases, the weight of the structure ensures stability on the seabed,
avoiding the need for piles or anchors and parts of the structure may be used
for the storage of oil or housing production equipment.
4. The superstructure
The superstructure usually consists of a deck truss assembly which supports all
the prefabricated production process facilities, drilling, utilities and accommo-
dation modules, communication facilities and cranes that make up the ‘topsides’
of the platform.
With concrete platforms, it is normal to float out the substructure with as
much of the superstructure in place as possible. This is to avoid the higher costs
associated with carrying out installation work offshore.
Again, to minimise offshore installation work, which costs in the region of
four times that of similar work onshore, all superstructure is modular, the mod-
ules being fitted out as completely as possible taking into account the loading
limits of available shipping and craneage capacity.
If it is not possible to complete the superstructure onshore then, having landed
and skidded the modules into position and secured them to the supporting deck
assembly, the services and process connections such as electrical power, cooling
water, heating, ventilation, and air-conditioning ducting, process oil, condensate
and gas may be interconnected between modules. This is known as the hook-up
phase.

TENSION LEG PLATFORMS
Where oil and gas fields are found in deeper water, the costs and technical problems
involved in producing ever taller structures have forced producers to consider more
novel methods. The tension leg system is currently in use on one installation and
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 ballast-
ing 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
themselves.