Client: Seaways 16
Date: 19 October 2016 | Project duration: 6 hours
Vessels are required to conduct bollard pull load testing to ensure that the vessel’s towing capacity is still within specifications and according to what the vessel is designed for. A lot of fine planning is involved, getting the necessary approvals from the port authority and ensuring that it is safe to carry out the operation.
A bollard pull is a conventional measure of the pulling (or towing) power of a watercraft. It is defined as the force (in tons or kiloNewtons (kN)) exerted by a vessel under full power, on a shore-mounted bollard through a tow-line, commonly measured in a practical test (but sometimes simulated) under test conditions that include calm water, no tide, level trim, and sufficient depth and side clearance for a free propeller stream. Like the horsepower or mileage rating of a car, it is a convenient but idealised number that must be adjusted for operating conditions that differ from the test. The bollard pull of a vessel may be reported as two numbers: the static or maximum bollard pull; the highest force measured and the steady or continuous bollard pull; the average of measurements over an interval of, for example, 10 minutes. An equivalent measurement on land is known as drawbar pull, or tractive force, which is used to measure the total horizontal force generated by a locomotive, a piece of heavy machinery such as a tractor, or a truck, (specifically a ballast tractor), which is utilised to move a load.
A bollard pull is primarily (but not only) used for measuring the strength of tugboats, with the largest commercial harbour tugboats in the 2000-2010s having around 60-65 tons of bollard pull, which is described as 15 tons above "normal" tugboats.
For the subject vessel, the anchor handler Seaways 16, we needed to do a 120MT bollard pull, whereby normally vessels range from 60-80 metric tons only. A normal bollard pull is conducted by connecting a large enough sling around a bollard. Steel towing wires are connected onto the sling for the specific tonnage required. It is then connected to a shackle with a load cell connected directly on it, in order to measure the pull in metric tons. The shackle is connected to the vessels towing wire, which in turn is connected to the wire drum on the vessel used to pull the whole arrangement.
A normal bollard pull requires the vessel to pull straight in one direction at all times. This is not always possible due to a range of factors e.g. wind, current etc., which can make the operation dangerous as the force exerted on the slings and bollards are not evenly distributed. Should the vessel pull in another direction, this can lead to bollards breaking off and wires snapping as the force might be more than what the sling and bollards are designed for.
Metrotech Calibration Services was the company conducting the test and as such was responsible that all safety measures were in place. For this operation, due to the fact that a 120 metric ton pull was required, we needed to prevent uneven distribution of the load to make the operation safer. In order to distribute an even force and to achieve a bigger pull, a special sheave block was inserted, which ensured that the load being pulled was distributed evenly across the two slings connected to the bollards. In addition, the sheave block acted as a swivel point for the steel wires which were connected onto the slings.
The sheave block was not locally available and therefore had to be imported into Namibia from South Africa. Currently, Logistics Support Services and Metrotech Calibration Services are the only companies in Walvis Bay who own a sheave block for such operations, and thus they are the only companies able to carry out such a specialised operation. In fact, Namport (Namibian Ports Authority) has issued new guidelines instructing all future bollard pulls should utilise this new method, and that they will no longer approve normal bollard pull operations due to the risk of injury and the possibility of damage to the infrastructure of the port.