Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 360

tug to be used on that attachment. In case of emergency, for towing ahead, one
spare attachment point should be available with the required fitting.
A second principle of towing is that the towing force must be able to be
resisted through a significant range of horizontal and vertical angles, thus
imparting shear and bending, as well as tension, on the towing attachment.
A typical arrangement of a single boat towing with a bridle is shown in
Figure 5.36 . Often, a high-impact overload happens due to high-tension
force, so the weak pendant, as shown in the figure, will be break. In this
case, the shackle is pulled back on deck by a fiber rope pendant, a new pendant
is fitted, and the towline is reconnected. While passing through restricted waters
and during final positioning, the towline may be shortened to permit better con-
trol. In some cases, due to the configuration of a structure such as a caisson or
TLP structure, movement of the structure will be accomplished by a boat that
pushes the structure, using a fender in the boat and attached to the structure.
The moment of inertia of a towed structure, especially a large one such as an
offshore caisson, is tremendous. It tends to keep moving ahead long after
pull has ceased. A constant concern for boats towing in congested traffic con-
ditions or in ice is that if the boat is stopped, the towed vessel or structure may
overrun it.
Furthermore, the inertia of the towed structure makes it difficult to slow
down or change direction. In a narrow channel, therefore, additional boats
may be used alongside and astern.
Figure 5.37 presents the types of motion that affect floating structures.
In order to minimize the associated risks and to secure safe transport from
the fabrication yard to the platform site, it is important to plan the operation
carefully, following the recommendations in API RP2A (2007) , including con-
sideration of:

Previous experience along the tow route

●

Exposure time and reliability of predicted “ weather windows ” during

●

transportation

Pendant
(weak link)

Bridle

Towboat

Bow Barge Stern

Towline

Recovery
pendant

Plan

FIGURE 5.36 Typical towing arrangement for a barge.