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Offshore Hydromechanics J.M.J Journée

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When geological predictions based on seismic surveys have been established that a particu-
lar o¤shore area o¤ers promising prospects for …nding oil, a well is drilled to examine these
predictions. These drilling operations are carried out from barges, ships, semi-submersibles
or jack-up rigs; see …gure 1.5.
Figure 1.5: Ship and Semi-Submersible Type of Drilling Vessel
For ‡oating drilling units it is important that they can work in high sea conditions. Gen-
erally, semi-submersibles will have the least vertical motions and hence the highest worka-
bility. In order to enhance further the workability of drilling vessels, the vertical motions
of the vessel at the location of the drill string are compensated for by a so-called heave
compensator. This device is essentially a soft spring by means of which the drill string
is suspended from the drilling tower on the vessel. In this way, the drill string can be
maintained under the required average tension, while the vertical motions of the vessel -
and hence of the suspension point of the drill string in the drilling tower - do not result in
unduly high drill string tension variations or vertical movement of the drill bit. Also the
marine riser is kept under constant tension by means of a riser tensioning system.
Horizontal motions are another factor of importance for drilling vessels. As a rule of thumb,
the upper end of the drill string may not move more to one side of the center of the drill
than approximately 5 % of the water depth. This places considerable demands on the
mooring system of the vessels. Semi-submersible drilling vessels are generally moored by
means of 8 to 12 catenary anchor legs. Drill ships can be moored by means of a spread
mooring system which allows a limited relocation of the vessel’s heading or a by means
of a dynamic positioning (DP) system. The latter system of mooring is also used for
semi-submersible drilling vessels for drilling in water exceeding 200 m depth.
Summarized, aspects of importance or interest of drilling vessels are:
- the wave frequency vertical motions at the drill ‡oor,
- the horizontal motions at the drill ‡oor,
- the deck clearance of the work deck of a semi-submersible above a wave crest (air gap),
- the vertical relative motions of the water in the moonpool of drill ships,
- the damage stability of a drilling semi-submersible,
- the forces in the mooring lines,
- the mean and low frequency horizontal environmental forces on DP ships and
- the thruster e¤ectivity in high waves and currents.
As an example, some limiting criteria for the ship motions - used for the design of the 137
m length drilling ship ’Pelican’ in 1972 - were:
- ship has to sustain wind gusts up to 100 km/hour,
- maximum heel angle: 3 degrees,
- roll: 10 degrees in 10 seconds,
- pitch: 4 degrees in 10 seconds and
- heave: 3.6 meter in 8 seconds.
1.2.4 Oil Production and Storage Units
Crude oil is piped from the wells on the sea bed to the production platform where it is
separated into oil, gas, water and sand. After puri…cation, the water and sand returns to
the sea. Gas can be used for energy on the platform, ‡ared away or brought to shore by
means of a pipe line. In some cases the produced gas or water is re-injected in the reservoir,
in order to enhance the production of crude oil. Crude oil is piped to shore or pumped
temporarily into storage facilities on or near the platform.
Fixed platforms for oil and gas production are used at water depths ranging to about
150 m. In most cases they consist of a jacket, a steel frame construction piled to the
sea bed; see …gure 1.6. The jacket supports a sub-frame with production equipment and
accommodation deck on top of it.
The jacket has to be transported on a barge to its installation site at sea. During transport,
the aspects of importance or interest of a jacket are:
Figure 1.6: Jackets
- the accelerations and motions of the jacket,
- the forces exerted by the sea-fastenings on the jacket and
- the wave impacts on overhanging parts of the structure.
During the installation phase of the jacket, it has to be launched from its transportation
barge; see …gure 1.7. Then, the jacket is temporally a ‡oating structure.
Figure 1.7: Schematic View of Launching a Jacket from a Barge
During launching, the aspects of importance or interest of the jacket are:
- the motions of the jacket on the transportation barge in waves,
- the forces exerted on the jacket by the launchways and rocker arm beams,
- the maximum depth to which the jacket dives and
- the static as well as the dynamic stability of the jacket.
Then, the free-‡oating jacket has to be up-ended before it can be put on the sea bed.
During this installation phase, the aspects of importance or interest of a jacket are:
- the ‡ooding sequence of the ballast tanks,
- the hydrostatic stability of the jacket,
- the current forces on the jacket and
- the internal stresses in the jacket.
When the jacket is ballasted to stand on the sea bed, in some cases it will have to be
positioned over the pre-drilled template; see …gure 1.6. The guiding pins have to enter the
receiver cans on the template. In many cases this operation is carried out with help of
a crane vessel. During this installation phase, the aspects of importance or interest of a
jacket are:
- the motions in waves,
- the current forces,
- the internal stresses in the jacket,
- the loads in the guiding pins and
- hoist wave loads in case of crane assisted positioning.
Finally, the jacket will be anchored to the sea bed by means of long steel pipes which are
lowered through the pile sleeves and hammered into the sea bed. This can be done by a
crane vessel, which also installs the top-side modules.
A jack-up is a mobile drilling unit that consists of a self-‡oating, ‡at box-type deck structure
supporting the drilling rig, drilling equipment and accommodation; see …gure 1.8. It stands
on 3 or 4 vertical legs along which the platform can be self-elevated out of the water to
a su¢cient height to remain clear of the highest waves. Drilling operations take place in
the elevated condition with the platform standing on the sea bed. This type of platform
is used for drilling operations in water depths up to about 100 m. Jack-ups spend part of
their life as ‡oating structures. This is when such platforms are towed to a new location
by means of ocean-going tugs. In this mode, the legs are lifted up and extend upwards
over the platform.
Figure 1.8: Jack-Up
Aspects of importance or interest in the towing condition of a jack-up are:
- the towing resistance,
- the course stability under tow and
- the bending stresses in the legs at the point of connection to the deck structure.
These bending stresses are adversely a¤ected by the wave induced roll and pitch motions
of the vessel. In some cases the highest sections of the legs are removed for the towing
operation in order to reduce the bending stresses to acceptable levels.
Gravity Base Structures
Gravity Base Structures (GBS) are applied to remote …elds in deep and harsh waters in
the central and northern part of the North Sea. They consist of a combination of a number
of large diameter towers, placed on top of a large area base which contains also storage
capacity. Piling to the sea bed is not required because of the large size of the base and the
mass of the structure, but the sea bed has to be leveled. The towers support a sub-frame
with a production equipment and accommodation deck on top of it. Figure 1.9 shows some
examples of large gravity base structures.
Figure 1.9: Some Gravity Base Structures
After construction inshore, the unit is mated with the top-side structure in relatively
sheltered waters. Aspects of importance