12
STRUCTURES IN THE OFFSHORE ENVIRONMENT
1. Laybarge method. Pipe sections, which sometimes hâve been coated previously with concrète for protection, are welded together on the deck of a barge
and deployed on rollers over the stem. Near the stern, the pipeline passes
over pontoons called stingers that relieve excess bending in the pipeline as it is
deployed to the seabed. See Figure 1.9.
2. Reel barge method. Small to medium diameter pipe sections (up to 41 cm
in diameter) are prewelded and coiled onto a reel mounted to the deployment
vessel. As the pipeline is unreeled at sea, it passes through straightening rollers
and then deployed as in the laybarge method.
3. Bottom pull method. Pipe sections are assembled on shore, and the pipe
string is towed into the sea by a barge. During the launch to its place on
the seabed, pontoons are often used under the string to avoid excess pipeline
bending.
The following two methods of pipeline deployment hâve been studied extensively but hâve yet to be used.
1. J-lay method. A dynamically positioned vessel such as a drill ship, a
converted pipelaying vessel, or a semisubmersible may be used in this operation.
On board the vessel a derrick is used to hold the pipeline vertically as it is
lowered, and the pipeline forms a J-shape between the vessel and the seafloor.
An efficient single station pipe welding procedure has to be developed before
this method can be adapted to conunon practice.
2. Floating string method. According to this concept, the pipeline is floated
on pontoons at the water surface. Then the pontoons are released successively
so that the pipe string gradually sinks to the sea floor.
Vertical Pipelines
One type of vertical offshore pipeline is the marine riser, which is shown
for several of the structures in Figure 1.1. Although marine risers make up
a fraction of the 80,000 km network, they are nonetheless key components of
offshore structures and serve a variety of functions. For example, a riser may
contain a bundle of smaller pipelines connecting a wellhead to its platform, or
it may transport oil directly from its platform to a sea floor pipeline. A riser
may act as a drilling sleeve, or it may contain electric power lines for operating
seafloor mining vehicles or other subsea facilities. A typical drilling riser, with a
bail joint at the bottom and a telescoping joint at the top, is maintained under
tension to ensure stability and is kept to within 8 degrees of the vertical by
computer-controlled positioning of its parent drilling vessel. In the mid-1980s,
the longest riser was operating off the east coast of the United States, where
depths of 2100 m were reached. At présent, risers for depths approaching 3000
m are becoming a reality.
Vertical pipelines are used in deep-sea dredging operations. A most challenging engineering problem, which began to receive serious attention in the
late 1960s, was that of designing dredge pipes to suck up and transport manganèse nodules from depths of 3000 to 5500 m to the ship. See Lecourt and
Williams (1971).These nodules, about the size of a man’s fist and found in a
STRUCTURES IN THE OFFSHORE ENVIRONMENT
1. Laybarge method. Pipe sections, which sometimes hâve been coated previously with concrète for protection, are welded together on the deck of a barge
and deployed on rollers over the stem. Near the stern, the pipeline passes
over pontoons called stingers that relieve excess bending in the pipeline as it is
deployed to the seabed. See Figure 1.9.
2. Reel barge method. Small to medium diameter pipe sections (up to 41 cm
in diameter) are prewelded and coiled onto a reel mounted to the deployment
vessel. As the pipeline is unreeled at sea, it passes through straightening rollers
and then deployed as in the laybarge method.
3. Bottom pull method. Pipe sections are assembled on shore, and the pipe
string is towed into the sea by a barge. During the launch to its place on
the seabed, pontoons are often used under the string to avoid excess pipeline
bending.
The following two methods of pipeline deployment hâve been studied extensively but hâve yet to be used.
1. J-lay method. A dynamically positioned vessel such as a drill ship, a
converted pipelaying vessel, or a semisubmersible may be used in this operation.
On board the vessel a derrick is used to hold the pipeline vertically as it is
lowered, and the pipeline forms a J-shape between the vessel and the seafloor.
An efficient single station pipe welding procedure has to be developed before
this method can be adapted to conunon practice.
2. Floating string method. According to this concept, the pipeline is floated
on pontoons at the water surface. Then the pontoons are released successively
so that the pipe string gradually sinks to the sea floor.
Vertical Pipelines
One type of vertical offshore pipeline is the marine riser, which is shown
for several of the structures in Figure 1.1. Although marine risers make up
a fraction of the 80,000 km network, they are nonetheless key components of
offshore structures and serve a variety of functions. For example, a riser may
contain a bundle of smaller pipelines connecting a wellhead to its platform, or
it may transport oil directly from its platform to a sea floor pipeline. A riser
may act as a drilling sleeve, or it may contain electric power lines for operating
seafloor mining vehicles or other subsea facilities. A typical drilling riser, with a
bail joint at the bottom and a telescoping joint at the top, is maintained under
tension to ensure stability and is kept to within 8 degrees of the vertical by
computer-controlled positioning of its parent drilling vessel. In the mid-1980s,
the longest riser was operating off the east coast of the United States, where
depths of 2100 m were reached. At présent, risers for depths approaching 3000
m are becoming a reality.
Vertical pipelines are used in deep-sea dredging operations. A most challenging engineering problem, which began to receive serious attention in the
late 1960s, was that of designing dredge pipes to suck up and transport manganèse nodules from depths of 3000 to 5500 m to the ship. See Lecourt and
Williams (1971).These nodules, about the size of a man’s fist and found in a