Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 375

Jacket detaches completely and comes to its floating equilibrium position

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Jacket is upended by a combination of controlled flooding and simultaneous
lifting by a derrick barge.

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The loads induced, static as well as dynamic, can be evaluated by appropri-
ate analyses, which also consider the action of wind, waves and currents
expected during the operation.
To start the launch, the barge must be ballasted to an appropriate draft and trim
angle and subsequently the jacket must be pulled toward the stern by a crane.
Sliding of the jacket starts as soon as the downward force from gravity and the
crane pull exceeds the friction force. As the jacket slides, its weight is supported
on the two legs that are part of the launch trusses. The support length keeps decreas-
ing and reaches a minimum, equal to the length of the rocker beams, when rotation
starts. It is generally at this instant that the most severe launching forces develop in
reaction to the weight of the jacket. During the last two stages, variable hydrostatic
forces arise that have to be considered at all members affected. The engineering
firm should have done buoyancy calculations for every stage of the operation to
ensure fully controlled, stable motion. Computer programs are available commer-
cially to perform the stress analyses required for launching and upending and also
to present the whole operation graphically.
The typical launch barge is very large and strongly built, is long and wide, and
is subdivided internally into numerous ballast compartments since it must support a
progressively moving jacket weighing thousands of tons. Heavy runner beams or
skid beams extend the length of the barge, as shown in Figure 5.45 . These girders
distribute the jacket

s load to the barge structure. The stern end of the barge, over
which the jacket will rotate and slide into the water, requires special construction.
First, for a short period of time the stern will have to support the full weight
of the jacket.
Second, since this reaction force has to be transmitted into the jacket, it must dis-
tribute the reaction over as long a length as feasible to avoid a point reaction. The
jacket will be sliding on its specially reinforced runners shown in Figure 5.45 ;
even so, they need a distributed rather than a point reaction. Hence, the stern of
the barge is fitted with a rocker section that rotates with the jacket as it slides off.
For load-out of the jacket at the fabrication yard, the usual method is to
ground the launch barge at the appropriate depth so that the barge deck is at
the same elevation as the yard. Therefore, the jacket can be skidded out onto
the barge with no change in relative elevations. When the load-out is performed
with the barge afloat, then ballast must be rapidly adjusted to maintain the rela-
tive elevation at the barge deck as the load of the jacket comes on. Step-by-step
adjustments under computer control are used to adjust deck elevation and trim.
A launch barge is also fitted with heavy cranes on the bow to pull the jacket onto
the barge and later, by re-rigging through sheaves on the stern, to pull the jacket
off the barge during the launching process. The base of a deep-water jacket may
reach to 60 m wide, so it will overhang the sides of the barge significantly.

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