In light of the risks of tanker vessel fittings failure, how can a safe working load be ensured?

Tanker vessel equipment used for active escort towing can be subject to forces significantly greater than static bollard pull of the tug when undertaking certain indirect escort methods. Failure of ships’ fittings during active escorting can potentially lead to grounding or blockage of the navigational channel. However, there is no clear standard ensuring that such tanker vessels are designed with sufficient strength to withstand these potential forces.

‘Indirect escort’ refers to a method of towing where hydrodynamic forces generated by the water flow in relation to the aspect of the tug’s hull, generate steering and stopping forces to mitigate mechanical failures during port approach or transit through restricted port waters. Many ports in the UK and beyond have identified ‘active indirect escort towing’ as a risk mitigation for grounding and/or channel blockage, should a tanker vessel suffer a loss of power and/or steering.  

Active escorting refers to the escorting tug being connected to the vessel being escorted rather than running alongside which is referred to as passive escorting. Indirect escort methods refer to the use of hydrodynamic forces generated by the water flow in relation to the aspect of the tug’s hull to generate steering and stopping forces.

Rate of Turn (ROT) is a crucial parameter for ensuring safe and effective manoeuvring; this refers to the rate at which a vessel changes its heading or direction while navigating. The tug force (steering pull) required to provide the required ROT will depend on the size, displacement, speed and under keel clearance (UKC) of the vessel being escorted, combined with environmental factors and the geography of the port. Indirect escort towing manoeuvres are intended to either generate sufficient steering pull to induce the required vessel ROT or stopping force to safely slow the vessel and, in some cases, a combination of both.

Taking an example of a modern 100 tonne (t) Bollard Pull (BP)⁽¹⁾ escort tug, using indirect escort methods, operating at speeds of around 10 knots (kts) and an optimal incidence angle between the waterflow and the tug skeg it is possible that the escort tug could generate a steering pull up to two times the escort tug static BP (up to 200 t). The vertical angle between the tug’s towing point and the vessel’s fairlead or chock at the edge of the deck can also increase the force in the towline, particularly if the angle is acute. Environmental factors such as wind speed and sea state can also create snatch loads⁽²⁾ through the towline that can increase the forces exerted through the towline.

Modern escort tug winches (termed ‘render-recovery winches’) maintain an almost constant towline tension up to the set limits of the winch brake. This is achieved through a rapid transition between rendering (letting out the towline) and recovery (hauling in the towline) reactive to the forces generated by wind and wave action on the tug. However, the Minimum Breaking Load (MBL)⁽³⁾ of the escort tug’s towline is likely to be three to four times the static BP of the escort tug, which may mean the tug winch brake force is set relatively high. The towline forces being monitored by the escort tug do not consider geometric forces that add a Force Multiplication Factor (FMF)⁽⁴⁾ to the towline forces exerted on the fittings of the escorted vessel.

When an escort tug is positioned at a broad angle on the quarter of the vessel being escorted, a towline wrap angle is created around the fairlead or chock to the vessel towing strongpoint; this can multiply the towline force transmitted through the fairlead or chock significantly. Figure 1 shows the worst-case wrap angle of 180º, creating a FMF of 2.0.

Figure 1: 180º wrap angle creating a FMF of 2.0

Taking the example above where the escort tug (generating up to 200 t of steering pull) is operating at 70º off the centreline, and there is a 20º angle between vessel fairlead or chock to the vessel towing strongpoint, a wrap angle of 90º is created at the fairlead or chock and a FMF of 1.4. Approximately 280 t of force could be acting on the fairlead or chock. This force is very likely to be significantly more than the SWL of the fairlead or chock and could lead to failure.

Learn more about tanker escort towing in our White Paper, including:

• Industry rules and guidance for mooring and towing
• Emergency Towing arrangements for the safe and efficient towing of vessels in distress
• Responsibilities for setting towing requirements
• Determining requirements for escort towing

Read our White Paper, The Challenge of Tanker Escort Towing

^{(1) BP Tugs are rated for Bollard Pull, a tug’s towing capacity measured by the force it can exert when tethered to a bollard.}

^{(2) Snatch load is the additional increase in force due to the towline slackening then tensioning very quickly due to sea state.}

^{(3) The Minimum Breaking Load (MBL) is the minimum load that would cause a towing line to fail or break.}

^{(4) Force Multiplication Factor (FML) is the increase in force applied to a towed vessel beyond the direct pulling capacity of the tug, e.g.  through an indirect towing method.}

ABPmer’s maritime risk specialists are dedicated to helping ports and other marine operators meet the requirements of the Port Marine Safety Code, regularly developing and auditing Safety Management Systems.

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