Civil Engineering Reference
In-Depth Information
When heavy loads are skidded on or off a barge, they punish the deck edge
and side because of the concentrated loading. Skid beams are often arranged to
partially distribute the load to interior bulkheads.
Thus, sea fastenings are designed to resist the static and dynamic forces
developed under any combination of the six fundamental barge motions (roll,
pitch, heave, yaw, sway or surge). The dynamic component is due to the inertial
forces that develop due to acceleration as the direction of motion changes.
Roll accelerations are directly proportional to the transverse stiffness of the
barge, which is measured by its metacentric height (GM) (see
Figure 5.39
).
Since a barge typically has a large GM, roll accelerations are severe.
Conversely, if high cargo, such as the topsides or jacket, cause the GM to be
low, the period and amplitude of roll and the static force resulting from the load
are greater, but the dynamic component may be less.
The loads that apply to the fastenings are mainly from waves, which are cyc-
lic, so sea fastenings tend to work loose as the wire rope stretches and wedges
and blocking fall out. Under repeated loads, fatigue may occur, especially at
welds. Welds made at sea may be especially vulnerable because the surfaces
may be wet or cold. Using low-hydrogen electrodes in welding will help in
this case. Chains are the preferred method for securing the transportation of
jacket and topside in the sea, since chain does not stretch.
The effect of the accelerations is to increase the lateral loading exerted by
the cargo due to the inclination of the barge by a factor of two or more. Flexing
of the barge can also have a significant effect on support forces and the sea fas-
tenings. Therefore, deeper, and hence stiffer, barges will experience a smaller
range of loads than shallow, less stiff barges.
For decks or jackets, which are valuable cargo, sufficient freeboard should
be provided to ensure stability, even if one side compartment or end compart-
ment of the barge has been flooded, which, in most cases, means the submer-
gence of the hull to the deck line, plus an arbitrary load of 3 m of water on deck.
Proposals are often made to build a structure on a barge and then to sub-
merge the barge by ballasting and to float the new structure off.
5.10.8 Crane Barges
A crane barge is an offshore barge that has a sheer-legs crane or fully rotating
crane. A sheer-legs crane can pick loads and luff but not swing. The sheer legs
consist of an A-frame made up of two heavy, tubular members or trussed col-
umns held back by heavy stays to the bow.
A sheer-legs barge is maneuvered by deck engines, tugs, or mounted out-
board engine propellers. The crane barge positions its stern at the side of the
material barge, picks the load, and then moves as necessary to set the load in
exact position. Modern torque-converter deck engines and propellers with vari-
able pitch allow a high degree of accuracy in positioning, on the order of 50 mm.
One of the advantages of a sheer-legs crane barge over a fully revolving derrick
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