Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 361

Fenders

Caisson

Pusher tugs

Guide
boat

Notches

FIGURE 5.37 Types of motions.

Accessibility of safe havens

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Seasonal weather systems

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Appropriate return period for determining design wind, wave and current
conditions, taking into account characteristics of the tow, such as size, struc-
ture, sensitivity and cost.

●

Transportation forces are generated by the motion of the tow, the structure
and the supporting barge. Loads will result from the design winds, waves and
currents. If the structure is self-floating, the loads can be calculated directly.
According to API RP2A, towing analyses must be based on the results of
model basin tests or appropriate analytical methods and must consider
wind and wave directions parallel, perpendicular and at 45

º

to the tow
axis. Inertial loads may be computed from a rigid body analysis of the tow
and by combining roll and pitch with heave motions, when the size of the
tow, magnitude of the sea state and experience make such assumptions rea-
sonable. For open-sea conditions, the following may be considered typical
design values:

Single-amplitude roll: 20 º

Single-amplitude pitch: 10 º

Period of roll or pitch: 10 s
Heave acceleration: 0.2 g

When transporting a large jacket by barge, stability against this large size is
a primary design consideration because of the high center of gravity of the
jacket ( Figure 5.38 ). Moreover, the relative stiffness of jacket and barge may
need to be taken into account, together with the wave forces that could result
during a heavy roll motion of the tow, when structural analyses are carried
out for designing the tie-down braces and the jacket members affected by the
induced loads. There are special computer programs (or a module for the soft-
ware used for structure analysis) available to compute the transportation loads
in the structure-barge system and the resulting stresses for any specified envir-
onmental condition.