Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 362

Jacket CoG

Metacenter of
the tow

Area of
potential
impact

Tie-downs

Tow
CoG

FIGURE 5.38 Center of gravity for a barge launching a jacket.

According to Gerwick (2007) , when towing out in the open sea, boats
lengthen their towlines to offset the wide range of loads in the lines caused
by the waves and swells.
If the towed structure is considered a deep-draft vessel (as, for example,
some of the offshore platforms in the North Sea have drawn from 110 to 120 m),
the structure will be towed by a deep-draft vessel, and the towline, if attached
to the structure belowwater near the center of rotation, can have a steep inclination,
which may pull the stern of the boat down into the water.
Required channel widths in sheltered areas are usually twice the beam, but
this must be considered in relation to the environmental conditions and naviga-
tional accuracy. According to Gerwick (2007) , in exposed areas the channel
width depends on currents and navigational accuracy and thus may vary from
about 600

1500 m in the relatively short distance of 12 km. Unlike a towed ship
or barge having a draft of around 8 to 10 m and a width from 30 to 40 m, an
offshore structure like a deep-water caisson may have a draft of over 100 m
and a width of 100

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150 m. Therefore, it is not enough to plot only the position
of the area of crossing; the extremities must also be considered. For a restricted
area, the current speed and direction should be surveyed on the surface and at a
reasonable depth.
If a structure is being towed by two boats between two islands, the location
of the electronic and visual stations to achieve a safe crossing will be set on the
islands.
Towed vessels and shallow-draft structures may have an actual draft greater
than their mean draft. This may be due to trim, squat, list or wind heel, or it may
be due to the lower density of fresh water discharging from a river into the adja-
cent sea.

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