Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 359

FIGURE 5.35 Tow boat.

Bollard pull is the force exerted by the towboat running full ahead while
secured by a long line to a stationary bollard; that is, the boat is making no
headway through the water. The relationship between indicated horse power
(IHP) and bollard pull is such that a boat with around 10,000 IHP can exert
100 140 tons of static bollard pull. The effective bollard pull falls off as the

speed through the water increases. The largest tugs have static bollard pulls
of over 300 tons.
For safe and efficient operation, towboat length should be equal to or more
than 11 times the expected maximum significant wave height. In major storms,
the boat may have to cut loose and subsequently recover its tow after the storm
has passed.

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5.10.4 Towing
The first principle of towing is that the attachments to the structure or barge
must always be sufficiently strong that they do not fail or damage the structure
under a force that breaks the towline. In selecting the tow wire, consider that
the actual breaking strength of wire rope is typically 10% - 15% greater than

the guaranteed minimum breaking strength.
Note that actual breakage usually occurs under a dynamic load rather than a
static load. It is important that, during overload, the structure or vessel being
towed remains undamaged. A usual requirement is that the ultimate capacity
of any towline attachment to the unit be at least four times the static bollard
pull and at least 1.25 times the breaking strength of the towline from the largest