Part D | 33.2
748 Part D Offshore Technologies
Magnus
Cognac
(Gulf of Mexico)
Bullwinkle
(492 m)
CN Tower
Toronto
(553 m)
Empire
State
Bldg.
Sears
Tower
(443 m)
Eifel
Tower
(300 m)
(North Sea)
Forties
(4 platforms)
106–
128 m
186 m
312 m
412 m
70 m
80 m
150 m
Fig. 33.3 Fixed platforms compared
with known landmarks (after [33.12])
to a bare minimum by maximizing onshore prefabrication and modular assembly.
33.2.1 Fixed and Compliant
Offshore Platforms
As stated earlier, this group of platforms is governed
by fixity to the ocean bed. Depending on the fixity (or
bottom supported) conditions the platforms can be classified as:
i) Fixed (rigidly or elastically)
ii) Bottom supported (with or without skirts and piles,
providing transverse resistance)
iii) Compliantly restrained.
Use of these platforms is governed mainly by the
maximum seawater depths at which they are to be
located. Figure 33.3 [33.12] gives a typical comparison of various fixed platforms used in the offshore
contexts, comparing them with some noted onshore
landmarks. When water depths become deeper than 450
to 500 m, the fixed jacket and bottom supported gravity
platform costs become exorbitant; hence the compliantly restrained platforms (at sea bottom) are needed
to increase the depths (up to 1300 m), at which these
platforms can be located economically. Even though
the tension leg platforms and spars are moored to the
ocean bottom through elastically restraining tension
legs, mooring cables, and marine risers (similar to some
floaters like semi-submersibles and FPSOs), they are
still listed under fixed or compliant platforms, since
their transverse motion is limited by the depths under
which the marine riser systems can function properly
due to:
i) Large bending stresses exerted on the conductors
(from loads transmitted from the riser system)
ii) The need for uninterrupted hydrocarbon flow
through the risers due to limited rotation (2
ı ) permitted at its bottom.
In some of the recent TLP and spar systems, these
restrictions have been removed (by using new tension
riser systems) and they are now listed under floaters
where there is no limit on the transverse surge motions [33.13]. These floaters can operate in up to 3000 m
water depths.
Fixed platforms have a number of advantages over
compliant and floating systems. These platforms can
support very large deck loads (with facilities for refining the extracted oil and gas), can be prefabricated (in
Topsides
Shafts
Cells
Skirts
Fig. 33.4 Components of a gravity platform (after [33.14])
748 Part D Offshore Technologies
Magnus
Cognac
(Gulf of Mexico)
Bullwinkle
(492 m)
CN Tower
Toronto
(553 m)
Empire
State
Bldg.
Sears
Tower
(443 m)
Eifel
Tower
(300 m)
(North Sea)
Forties
(4 platforms)
106–
128 m
186 m
312 m
412 m
70 m
80 m
150 m
Fig. 33.3 Fixed platforms compared
with known landmarks (after [33.12])
to a bare minimum by maximizing onshore prefabrication and modular assembly.
33.2.1 Fixed and Compliant
Offshore Platforms
As stated earlier, this group of platforms is governed
by fixity to the ocean bed. Depending on the fixity (or
bottom supported) conditions the platforms can be classified as:
i) Fixed (rigidly or elastically)
ii) Bottom supported (with or without skirts and piles,
providing transverse resistance)
iii) Compliantly restrained.
Use of these platforms is governed mainly by the
maximum seawater depths at which they are to be
located. Figure 33.3 [33.12] gives a typical comparison of various fixed platforms used in the offshore
contexts, comparing them with some noted onshore
landmarks. When water depths become deeper than 450
to 500 m, the fixed jacket and bottom supported gravity
platform costs become exorbitant; hence the compliantly restrained platforms (at sea bottom) are needed
to increase the depths (up to 1300 m), at which these
platforms can be located economically. Even though
the tension leg platforms and spars are moored to the
ocean bottom through elastically restraining tension
legs, mooring cables, and marine risers (similar to some
floaters like semi-submersibles and FPSOs), they are
still listed under fixed or compliant platforms, since
their transverse motion is limited by the depths under
which the marine riser systems can function properly
due to:
i) Large bending stresses exerted on the conductors
(from loads transmitted from the riser system)
ii) The need for uninterrupted hydrocarbon flow
through the risers due to limited rotation (2
ı ) permitted at its bottom.
In some of the recent TLP and spar systems, these
restrictions have been removed (by using new tension
riser systems) and they are now listed under floaters
where there is no limit on the transverse surge motions [33.13]. These floaters can operate in up to 3000 m
water depths.
Fixed platforms have a number of advantages over
compliant and floating systems. These platforms can
support very large deck loads (with facilities for refining the extracted oil and gas), can be prefabricated (in
Topsides
Shafts
Cells
Skirts
Fig. 33.4 Components of a gravity platform (after [33.14])