Bollard pull is the zero speed pulling capability of the tug. It is a measure of the usefulness of theship in a stranding scenario or in holding a large tanker or aircraft carrier off a lee shore. However, the bollard pull figuremust be under- stood.
Ideally, bollard pull is tested when a tug is built and certified by one of the classification societies. Bollard pull tests sometimes are performed after major engine overhauls. Tug owners whose tugs have been tested usually provide a copyof the certificate attesting to the bollard pull figure.
Bollard pull, like horsepower, is a selling point for tugs and is sometimes overstated. For instance, there are rules ofthumb for converting propeller power (SHP) to bollard pull, such as one ton pull per 100 horsepower for a conventional propeller, or 1.2 to 1.5 tons pull per 100 horsepower for a propeller fitted with a nozzle. The owner may save the cost of a bollard pull test and simply apply one of the factors to convert propeller power to bollard pull without ever knowing what the real figure is It is unlikely that this owner will ever select a conservative conversion factor.
European owners generally report bollard pull in their literature and reputable salvage tug owners generally are able to produce a certificate to document the test. American owners, and the worldwide offshore oil support industry, rarely report bollard pull. When they do, the figure may not have been validated by a test. Horsepower is probably a morereliable measure among ships of these types.
Bollard pull is not the only useful measure of the puling capability of a tug. Except in the case of a stranding, the objective of the tug is to move its tow. In this case, some of the tug’s power is expended on overcoming the hull resistance of the tug itself, and some on the hydrodynamic resistance of the towing hawser. Bollard pull can be maximized by propeller and nozzle design, but at the expense of towline pull at towing speeds. This adversely impacts free-running speed and fuel usage. Most tug designs, however, are optimized for towing.
Tugs generally are expected to operate in the 4- to 8-knot speed range. Modern tugs usually use propeller nozzles so that bollard pull still is quite high, but with a significant disadvantage in tug speed and fuel consumption. A tug optimized for rescue towing probably would not employ nozzles, being most concerned with high speed running to the casualty, and accepting some loss in efficiency of the tow itself.
The foregoing aspects of tug design and owners’ claims demonstrate that a tug should be considered as a balanced design, with some being more suitable for some tasks, others for other tasks. The balance extends to the task as wellChartering a 20,000 IHP salvage tug to tow a 200-foot barge would be just as inappropriate as sending a 5,000 HP platform supply ship, with no tow hawser or winch, on a rescue tow mission.
source: Integrated Publishing
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