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Offshore Platforms
Introduction The offshore oil and gas industry celebrated its 50th birthday in 1997. The
first offshore platform was installed off the coast of Louisiana in 1947 in
just 6m of water. Today there are over 7,000 offshore platforms around
the world in water depths up to 1,850m.
Offshore platforms can be categorised according to whether they are rigid
structures that extend all the way from above the water surface to the
seabed (fixed-bottom platforms), or whether they float near the water
surface.
Platforms can be further categorised according to way they are connected
to the seabed.
Type of Platform Connection to Seabed Example
Fixed-bottom Piles Steel jacket
Gravity Concrete Gravity Structure (CGS)
Floating Tethers Tension Leg Platform (TLP)
Catenary mooring lines Semi-submersible, Spar
Single Point Mooring (SPM) Ship-shaped vessel (FPSO)
Fixed Bottom Jackets
Platforms Approximately 95% of offshore
platforms around the world are
supported by steel framed structures
(for example, Goodwyn A and North
Rankin A), called jackets because they North Rankin A
provide a protective layer around the
pipes which pass from the seabed up to
the topsides. Jackets are pinned to the
seabed by long steel tubes called piles,
which act like giant tent pegs.
Concrete Gravity Structures
Some fixed-bottom structures are made from
concrete and are so heavy that they remain in
place on the seabed without the need for
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place on the seabed without the need for
piles. These are called Concrete Gravity
Structures (CGSs). Wandoo B is the only
CGS built in Western Australia to date.
Two other CGSs, Bream B and West Tuna, Wandoo B
have been built in Port Kembla and installed
in Bass Strait off the coast of Victoria.
West Tuna
The design and construction of offshore
Size platforms holds a number of unique
Comparison challenges. One challenge is the sheer size of
Chart
the structures. The Goodwyn A platform is
significantly taller than the tallest building in
the southern hemisphere.
Bullwinkle, located off the USA coast, is
currently the world's tallest platform, with a
total height of nearly 0.5km. Soon this record
will be beaten by the new Petronius platform,
which is 30% taller than Bullwinkle and taller
Troll A
than the tallest onshore structures anywhere.
The tallest concrete offshore platform is Troll
A, located off Norway - it is 470m tall.
Jacket Construction and Installation
Jackets can be built in many different ways
depending on their size and shape. Tall
jackets are built on their side. Typically steel
frames are welded together on the ground and
then rotated to the vertical to build up the
three-dimensional structure at the quayside. Frame roll-up
The finished structure is then skidded on to
the transportation barge.
Many jackets are installed at the field site by
launching them off the transportation barge.
The jacket is pushed along low-friction
j skidways down the barge until it picks up
momentum and dives nose-first into the
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momentum and dives nose-first into the
ocean. The launch process takes about a Jacket launch
minute and is a very impressive spectacle.
"Smaller" jackets can be installed by lifting
them off the barge using a floating vessel
with one or two cranes installed on it. By
lowering one crane hook faster than the other,
the jacket can be upended to vertical and then
set on the seabed. Jackets weighing up to Lift installed
10,000 tonnes have been installed in this way.
CGS Construction and Installation
Concrete platforms are built and installed in a
very different way from steel jackets.
Construction commences in a dry dock
adjacent to the sea. The structure is built
vertically, from the bottom up, a little like an
onshore building. At a certain point, the dock West Tuna
Construction
is flooded, the partially completed structure
floats and is then towed out to deeper,
sheltered water. Further construction, to bring
the structure up to its final height, takes place
while it is floating. It is then towed to the
field site, and filled with sea water to make it
heavy enough to sink down to its final
position on the seabed. The huge Troll
platform weighed over 1 million tonnes
during its tow to site.
Floating Some floating oil and gas production
facilities resemble large ships but others have
Platforms quite different hull shapes. The first floating
facility commenced oil production in 1975 on
the Argyll field off the Scottish coast. It is the
rapid development of floating platform
technology over the past decade that has
enabled oil and gas engineers to produce
petroleum from reservoirs in very deep water.
Floating Production, Storage and
Offloading
Ship-shape platforms are generally called
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Floating Production, Storage and Offloading
(FPSO) facilities. A major advantage of
FPSOs is that they can store the oil they
produce inside their hull: this avoids a long
and expensive pipeline to shore. Every few Laminaria
days an oil tanker comes along and links to
the FPSO, the oil is transferred and then
taken by the tanker to a refinery. Many
concrete platforms also have storage
capability.
FPSOs are held in position over the reservoir
at a Single Point Mooring (SPM). The vessel
is able to weathervane around the mooring
point so that it always faces into the
prevailing weather: this ensures that its
motions are minimised. Australia is a world
leader in FPSO technology and there are
currently 6 FPSOs on our North and North
West coasts.
Tension Leg Platforms
Tension Leg Platforms (TLPs) are floating
facilities that are tied down to the seabed by
vertical steel tubes. There are currently 8
TLPs around the world. Although there are
none yet offshore Australia, the East Spar
buoy on the North West Shelf resembles a
tiny TLP.
Heidrun
Semi-submersibles
Semi-submersibles look quite similar to
TLPs, but are held in place by anchors
connected to a catenary mooring system.
Troll Olje
Spars
Spar platforms also have catenary moorings.
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These are a very new concept: the first spar
platform, Neptune, was installed off the USA
coast in 1997.
The choice of the best platform type for a particular oil or gas field is
dependent on many factors. The water depth is a very important consideration,
but other factors include topsides requirements, field life, environmental
conditions and the infrastructure available locally to build and operate the
platform. Offshore engineers study the various options and determine the
safest and most economic solution for each field.
Click here for details of other projects and for
more pictures
Click here for video sequences
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