REPORT EXAMINES TOWLINE PENNANT SNAP

The UK’s Marine Accident Investigation Branch has published its report following the breaking of a tug’s towline pennant while assisting a container ship, including recommendations to the tug’s classification society and owner.

The ASD tug was being taken over by new owners, and during the initial three-month handover period was being operated by the former owner’s agency crew supplemented by the new crew, who provided local knowledge and received familiarisation training from the outgoing crew.

On the day the tug and two others were tasked to assist an ultra-large container ship to its berth at a UK container terminal. It was allocated the role of stern tug and after meeting the vessel, connected its towline in the usual way, the pennant secured to the ship’s bitts with up to 40m of towline run out.

As the tug commenced manoeuvring the ship at the berth, the pilot requested first 50% then 100% astern thrust with the tug’s winch brake slipping twice and a further several metres of towline being released.

Soon after, the towline pennant parted close to the ship’s deck and snapped back towards the tug. Part of the line shattered the starboard forward wheelhouse window, also fracturing the inner pane of the centre window, and two of the inner panes of the aft-facing windows were also cracked.

Five crew in the wheelhouse were struck by flying glass fragments, suffering multiple minor facial, arm and upper body lacerations. Fortunately the crew were all wearing some form of eyewear and avoided any eye injuries.

Investigations

The investigation included close examination of the failed towline pennant, comprising three-strand polyester load-bearing core and braided protection jacket. The pennant had a minimum breaking load (MBL) of 229.5t, which was three times greater than the tug’s full astern bollard pull.

An outside organisation contracted to examine the failed pennant identified two damaged areas in its load-bearing core, including short wavelength Z-kinks associated with axial compression fatigue in the yarns – damage that would have been difficult for the tug’s crew to detect.

Much of the MAIB report analyses the pennant in detail, the analysis concluding the load at failure was likely to have been equal to or less than the 119t load recorded during a destructive tensile load test, or 52% of its original MBL, and that it was “much more likely” it parted when exposed to higher levels of shock loading following the brake slippages.

The analysis adds that as pennants are a sacrificial element, they should be frequently replaced, certainly after damage or high load occurrences. Also examined was whether the particular pennant used was recommended by the rope’s manufacturer.

Class standards for shipboard glass windows involve withstanding a hydrostatic pressure head equating to ‘green sea’ loading from waves, and was another area examined in detail. Snapback speed can be anything up to 380mph, with little time for reaction by anyone in the vicinity, and the fact that fragments from the shattered forward window also cracked two aft-facing windows demonstrates the potential for harm following such a heavy impact load.

In 2016 and following increased reports of wheelhouse glass failures, the shipyard that built the tug involved researched safety glass options that exceeded Class standards. Following this research and an in-house testing programme, it developed its own high-impact resistance glazing, which met recognised security glass standards, incorporating a polyvinyl butyral laminate, a combination of toughened and laminated glass panes withstanding manual, ballistic or blast attacks.

Following the incident the tug’s owner commissioned a detailed examination of the failed pennant with a number of actions taken forward.

It also introduced a database of its tow gear equipment to provide towing arrangement data across it global tug fleet. The MAIB recommended the Classification Society takes the investigation findings to the International Association of Classification Societies, including the wheelhouse glazing failure. The tug’s owner was recommended to undertake a fleetwide risk assessment to determine the level of risk associated with towline failure and snapback, and where appropriate employ laminated glass or other defences to mitigate the risk from flying glass.

The above is a brief summary of what is as usual with the MAIB a very detailed forensic examination, its safety recommendations in no case creating a presumption of blame and liability. The report can be viewed in full on its website.