Dutch proposal for innovative escort tug

Designated the ET 1541 Escort Tug, the design comes from Dutch concerns Stouco Consultance BV and Ubitec BV working under the name STUBITEC. The main feature of the tug is where it is built around the dedicated winch which to the eye is not visible.

Designers and builders are constantly improving established designs for shiphandling and escort tugs and the rationale behind this latest project is centred on some forthright observations presented by STUBITEC and based on MARIN Research Institute’s 2005-2007 joint industry ‘Safe Tug’ project. The consultancy states that one finding of the project was that the winch was the weak point and that there was in fact no suitable winch on the market for heavy duty escort tasks.

As well as the central aspect of the design (the winch location) there are also interesting thoughts on the method of power generation for the tug. Key to the design however is where the winch is hidden in the deckhouse, protected from the elements. It is electrically driven, the drive motors used as brakes allowing excess power to be fed back to the net. STUBITEC state the spooling device is frequently a problem on winches where due to high side forces (up to 500 tonnes) the device can break down. The ET 1541 has no spooling device as the winch is located well aft and the fleet-angle is around 3’. The cable leaves the winch drum on top of the winch and passes beneath the deckhouse to a swivelling fairlead with large sheaves at the very bow of the tug.

The winch brake during towing is carried out by two electric motors with no magnetic or other brake required other than two hydraulically-operated brake bands for holding power. Maximum rendering/heaving speed is 120m/min with a nominal pull of 100 tonnes at approximately 10m/min. The winch is mounted on two shock absorbers permitting a certain longitudinal movement avoiding cable failure during snatch loads. The winch skid is positioned on four pens with strain gauges for load measurement, the foundation consequently not welded to the deck itself thus avoiding distortion of the winch frame which could lead to bearing and gearbox problems. There is also a standard towing winch aft of the escort winch.

The power plant arrangement is interesting and takes into account the now well recognised fact that escort tugs use maximum bollard pull for only a small percentage of the time. Based on this, four 1MW diesel generators are installed feeding a DC-bus system with a battery bank at each end, total available power for maximum of 20 minutes is around 7MW. Renderings of the tug indicate an ASD configuration with azimuth thrusters and the anticipated bollard pull is 100 tonnes.

The battery banks will act as booster power and take care of power peaks with excess energy from propellers or winch fed back into the net. It is acknowledged that electric propulsion can give a sluggish response from both drives and winch, often caused by too little ‘spinning reserve’ as it takes time for the diesel to react to an increased demand for power. STUBITEC claim this problem will not be present with the ET 1541 as battery boosters will always supply sufficient power on top of diesel-generated power when required.

Another interesting aspect is the selection of LNG as main fuel source. STUBITEC state that many ports around the world have concerns about emissions, some ports raising pilot and harbour fees for what they describe as “polluters” while others ban them all together. Certainly the debate about what route tug owners take to develop the ‘green tug’ principle, be it hybrid, diesel electric, or LNG to mention three options will continue but LNG as a fuel is appearing now in a number of newbuild tugs.

Marine gas oil/diesel will also be available as back-up and for ignition with the tug. The inclusion of conventional fuel is also to allow a more convenient arrangement for passages where LNG bunkering facilities may not be available. Two main engine options are proposed; calculations for fuel consumption based on a typical 36 hour period with Wärtsilä 6L20 engines indicate about 20m3 of LNG and 0.4m3 MDO/MGO or about 12m3 of MDO/MGO. The same comparison utilising ABC 6DZD dual fuel gas engines indicates about 21m3 of LNG and 1.0m3 MDO/MGO or about 12m3 of MDO/MGO.

Safety is arguably the most important consideration with the operation of ship-assist towing operations and thankfully, serious accidents are relatively few when judged against the many thousands of ship-handling acts which take place daily in ports around the world. Nonetheless, mishaps are expensive both financially and sometimes personally and the designers are clearly aware of the importance (and value) of paying due regard to this aspect of the proposal.

They list the reasons why in their words the Escort Tug 1541 is ‘the ultimate safe tug’: High freeboard forward, difficult to pull under water; towing point forward, as far forward as possible; winch well protected; long lifetime of steel cable (or other type of cable) due to large drum diameters and large sheave diameter; in case of gas release in the vicinity of the tug, all openings will close and it will continue operations and move to a safe environment on batteries.

By Peter Barker