Are ships’ deck fittings up to the task of towing?
With the increasing use of escort towing involving ever more powerful tugs, has the old design principle of the towline being the ‘weak-link’ been replaced by the ‘Achilles-heel’ of adequate ships’ deck fittings?
At ITS 2018 in Marseille, organised by The ABR Company Ltd and attended by MJ, Captain Arie Nygh from SeaWays Consultants, Australia presented a paper examining instances of catastrophic failures of shipboard towline securing bitts, fairleads and their foundations along with the misunderstandings surrounding safe working load definitions relating to towing operations. Together with other industry professionals the author set up a research programme examining the subject including measures to reduce the risk of failure.
Ships’ deck fittings are primarily provided to secure the ship to a quay and not for towage operations, their strength determined by the breaking strain of mooring lines. When used for mooring, a figure-of-eight style of securing lines to bitts is adopted, the SWL based on collapsing forces of the component’s vertical levers near the top of the bitt’s legs. If the towing line is placed at the base of one of the bitts however the SWL can in effect be doubled. This misunderstanding can result in tugs not employing the maximum power they can offer.
A fairlead redirects a line from the quay or tug to the ship’s bitts, the inboard line angle determined by the deck fittings’ layout. Outboard, the angle to the quay or tug is variable depending of the ship’s freeboard and positions of shore bollards or tug respectively with worst-case scenarios of this so-called ‘wrap angle’ being 180’ in effect doubling the maximum load on the fairlead.
Rules specify the mooring line strength but not the forces on the towing line other than that fittings should meet the tug’s static bollard pull, the assumption made that the towline is secured by an eye passing over a single post of the ship’s bitts. A tug’s static bollard pull is determined in controlled conditions, but in reality, can be multiplied many times over (as much as 600 per cent in worst-case scenarios) by combinations of towline angles and the tug’s operating mode including: direct, indirect, combination arrest and transverse arrest.
Here, the loadings are more dynamic than with just a static pull particularly with escort towing where the hydrodynamic effect of the tug’s underwater profile can easily double the load on the towline and ship’s fittings. This has led to relatively new additional measures of a tug’s pulling power, steering force and braking force, figures notably higher than traditional static bollard pull figures. Downward force on the fitting’s foundations if a short tow line is employed increases the load further resulting in significant increases in both towline force and load on the ship’s fittings.
All this is against the background that many ships were built years ago when no one envisaged harbour tugs of 70 or 80tbp or the higher loads created by high-performance escort tugs. The paper examined the relationship between the bitts, considered as a single box mounted to the deck and the associated foundation beneath, an integral part of the ship’s design.
Conclusions included the finding that rules for design and construction of ship’s deck fittings from one industry entity to another “to be not only confusing, but in some cases contradictory.” The paper examined in detail related effects of active escort towing, describing that the issue “goes to another level” when taken into consideration.
Examining what to do about the problem, the long-term fix of course is for ship owners, designers and classification societies to ensure newbuilds have deck fittings capable of withstanding these previously unforeseen forces. An interim fix however suggests deploying two escort tugs connected to either side of the vessel’s transom. The obvious consequence of doubling the steering force was not the motivation, more creating the required force without the risk of failure of ship deck fittings by spreading the load over two sets of such fittings. The two-tug provision also provides the pilot with numerous initial response possibilities along with redundancy in the case of towline or tug failure. SeaWays has produced a single-sheet diagram guide describing the towline forces on ship’s deck fittings and can be downloaded from SeaWays Consultants website.
By Peter Barker
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