Tugs are rated in terms of their 'bollard pull'. This is the maximum load that the tug can apply to a static point, where the force is generated by her engines driving the propellers or thrusters. The acid test of this rating is when she is required by Charterers to put up a wire onto a strong bollard and, meeting all of the related environmental and technical requirements, to run her plant at 100% power for a defined period of time, where the sustained maximum pull is measured. It should be similar to the original trial performed when the new tug is delivered by her builders, as proof of design, performance and build quality.
As tugs age and modifications to plant may take place, it is not uncommon for Charterers to request a recent Bollard Pull Certificate.
OMS was contracted last week, to carry out a bollard pull trial on a small but relatively powerful terminal tug, calling at Cape Town. Verification of both her pull capability in ahead and astern mode were required.
We arranged for the optimal berth and test bollard, allowing the tug the widest scope to manoeuvre away from the berth, once the pull trial was underway.
Our Technical Team assembled the bollard 'jewelry' and then commenced with the tug's forward towing winch. The process was simple and satisfactory. With the tow wire requiring to be hand laid onto this drum, under tension, the recovery of the extended wire, after the pull, was a slow business.
The tug nosed up to the quay and our riggers disconnected her forward wire. She then turned around and presented her stern to the quay, in order that our riggers could connect her main aft towing winch wire, 52 mm diameter, to our load cell. A mobile crane was provided by OMS to pass the wires from the tug up onto the quay, where our riggers manhandled the wire and joining shackles.
We have conducted similar bollard pull trials of up to 150 tonnes, where the gear to be handled is substantial and include wires and pennants of 77 mm diameter, 200 tonne SWL shackles and a beast of a load cell. This terminal tug required a notably smaller quay arrangement.
Where the expected loads exceed the safe holding capacity of available quay bollards, we then proceed to sea and conduct the pull in the sheltered waters of Table Bay, having also carried out at-sea pull trials in both Mossel Bay and Walvis Bay.
On this trial in Cape Town, both forward and aft full power trials were successfully completed and accepted by Class, who were in attendance and thereafter the tug could recover her main tow wire. This trial presented the tug with a rare opportunity to re-lay all the wraps of wire on the towing winch drum.
Owners were fully satisfied with the procedures, conduct and efficiency of these two trials and have decided to route another sister vessel to Cape Town to also conduct a bollard pull trial on that vessel.