Port of Liverpool - Towage Information

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With the towline at a large angle to the tug's centre line, indirect towing is a potentially dangerous manoeuvre. Indirect towing requires a highly skilled tug master to achieve the high towline forces without girting and capsizing the tug. The advent of the purpose-built escort tug, designed for exerting such high loads, has made this operation much more controllable and therefore much safer. Tug Types and Methods of Operation The table below summarises the suitability of the three distinct tug types in relation to the basic ship-handling methods as earlier discussed. It also gives a very clear reason why Marine Service Providers favour ASD and tractor tug types over conventional tugs for ship handling. Given the fact that, where possible, Push-Pull is the preferred method of operation, it could be said that to undertake the equivalent operation employing conventional tugs only, double the number of conventional tugs could be required when compared to a similar operation employing either ASD or tractor type tugs. Table of Tug Types and Methods of Operation TUG TYPE METHOD REMARKS Conventional Tug On the line Poor manoeuvrability at large sheering angles Push-Pull Only pushing or pulling Indirect Very difficult due to lack of directional control of the tug Azimuth Stern Drive Tug (ASD) (Z-Peller) Tractor Tug (VST or ATT) On the line Good performance when working over the stern, but risk of girting exists at higher speeds. Working over the bow reduces girting risk. Push-pull Very good performance working over the bow Indirect Good performance working over the bow On the line Good performance with reduced risk of girting. Push-Pull Good performance Indirect Good performance with reduced risk of capsizing Page 7 of 8
Conclusions Regarding Tug Types Assuming normal port operations with maximum ship speeds of six to seven knots, it can be concluded – with some reservations – that the suitability of different tug types can broadly be ranked as follows: As forward tug towing on a line: • ASD-tugs (operating from stern winch) • Combi-tugs • Conventional tugs • Tractor Tugs and ASD tugs (operating from bow winch) As stern tug towing on a line: • Tractor Tugs and ASD tugs (operating from bow winch) • Combi tugs • Conventional tugs When operating at a ship’s side: • ASD tugs and Tractor tugs • Combi-tugs • Conventional tugs At speeds of over 7 knots as stern tug: • Escort tugs Ship handling tugs operating in the Port of Liverpool are categorised as follows: Tractor Tugs Azimuthing Stern Drive Tugs Smit Collingwood (VTT) Ashgarth Smit Sandon (VTT) Oakgarth Smit Waterloo (VTT) Smit Barbados Svitzer Bidston (ATT) Smit Donau Svitzer Sussex (VTT) Svitzer Maltby Svitzer Nari Escort Tug Svitzer Stanlow (VTT Skeg First) Specification sheets for these tugs are available in the appendices. Page 8 of 8
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Port of Liverpool - Towage Information

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