West of England P&I -Loss Prevention Bulletin
West of England P&I Club has published a Loss Prevention Bulletin regarding the Risk of of Tugs Capsizing due to Girting.
Towing operations, by their very nature, can be dangerous if not safely managed and executed. One particular hazard is “girting” which can rapidly lead to the tug or towing vessel capsizing and lives being lost.
Girting
Girting may also be referred to as girthing, tripping or girding.A towline under tension will exert a heeling moment on the tug if the line is secured around amidships and is leading off towards the beam. As with any vessel which heels over due to an external force, a righting lever is formed as the centre of buoyancy moves towards the centre of the tug’s underwater volume, countering the heeling moment and pushing the tugback upright.
However, if the force in the towline is sufficiently powerful, it may overcome the tug’s righting lever and cause it to capsize or “girt”. Girting can occur very rapidly and incidents have occurred where crew members have not been able to escape in time. Moreover, it should not be assumed that the winch or winch brake will render or that the towlinewill break before a potential girting situation occurs as lessforce may be required to capsize the tug.
Tug stability, design and manoeuvring considerations
On a vessel’s curve of statical stability (GZ curve), the point of contraflexure represents the point of deck edge immersion. From this angle of heel onwards, the rate of increase in the righting lever diminishes. Tugs generally have a small freeboard; therefore deck edge immersion will generally occur at a relatively small angle when heeled over. This means that tugs generally have less residual stability when compared withlarger vessels or other ship types. The immersion of a tug’s deck edge leads to a reduction in its ability to overcome the external transverse heeling force caused by the towline.
Potentially hazardous towing operations
If a conventional tug is acting as a bow tug and is overtaken by its tow at speed, manoeuvring control may be lost due to the drag force of the water flow on the tug’s underwater hull. Coupled with the heeling effect of the towline, this may lead to a situation where girting is possible.
Minimising the risk of girting
Some of the measures used to lead a towline away from amidships and nearer the stern are considered below. These need to be effected prior to commencing towing operations as to do so once the towline is under tension or has been made fast would be hazardous.
- Fixed gog (or gob)
- Running gog (or gob)
- Towing pods
- Karmoy Roller
- Towing pins
- Norman pins or stop pins
- Tow Stops
A fixed gog arrangement
(Image Credit: West of England P&I)
Additionally, towline emergency quick release mechanisms should be considered as well asadditional considerations and recommendations provided in the West of England P&I Club Loss Prevention Bulletin.
For further information, view the Loss Prevention Bulletin The Risk of Tugs Capsizing due to Girting.
Source: West of England P&I
