Tug (Escort Tug)

100 Tonnes Bollard Pull Escort Tug ET1541

1 Introduction
The design of the ET1541 Escort Tug is based on the results of the MARIN Research Institute results of the joint industry project “Safe Tug” that was carried out in 2005 -2007. One of the findings of the project was that the winch was the weak point. There was in fact no suitable winch on the market for heavy duty escort tasks. Various Escort Tug projects have been launched since the time of the study but they all have a “conventional” winch forward and not really suitable for the task. The escort winch forward is always an “add on” and not part of the escort tug design. Stouco Consultance B.V. ( john.stouten@stouco.com ) and Ubitec B.V. ( bvu@ubitec.nl ), under the name of STUBITEC has launched a project where a dedicated winch for an escort tug has been developed and the tug is designed around the winch. The winch is most important piece of equipment on board and that is where the tug operator is earning his money.

2 Escort Tug Duties
The definition of a typical escort tug is according to Lloyd’s Register:
Tug Intendent for Escort Operations
• Escort Operation is when the tug closely follows the assisted ship
• Providing Control by Steering and Braking, as necessary
• Steering and Braking forces are generated by a combination of propulsive and hydrodynamic forces generated by the Escort Tug.
The typical escort tug is generating lift with its “keel” or skeg when towed sideways. The azimuthing thrusters ¬provide bollard pull, maneuverability and stopping force.

3 The Winch
The winch is hidden in the deckhouse, well protected from the elements and is electrically driven for the following reasons:
• No hydraulic oil spill!
• Drive motors are used as brakes and excessive power is fed back to the net.
A frequent problem is the spooling device on a winch. Due to the very high side forces at times (500 Tonnes) the spooling device breaks down. Then the tug is not able to continue the operation. The ET 1541 has no spooling device as the winch is located well back and the fleet-angle is about 3°. The cable is leaving the winch drum on top of the winch and passing below the deckhouse to a swiveling fairlead with large sheaves at the very bow of the Escort Tug.
The brake during towing are the two electrical motors and no magnetic brake or other brake is required. Two hydraulically operate band brakes will also be fitted for holding power. The winch will be very quick in rendering and heaving in the
cable. Maximum speed is 120 m/min. Nominal pull is 100 Tonnes at about 10 m/min. The winch is also mounted on two shock absorbers which will permit a certain longitudinal movement.
This in order to avoid a cable failure during snatch loads. The winch skid is positioned on four pens with strain gauges for load measurement. The winch foundation is consequently not welded to the deck, which avoids distortion of the winch frame which may result in bearing and gearbox problems. There is also a “normal” towing winch aft of the Escort winch.

4 The Power Plant
An Escort Tug very rarely uses the maximum bollard pull. Certain sources indicate the following:
Based on the above about 4 x 1 MW will be installed in the form of diesel generator power feeding a DC-Bus system with a battery bank at each end. The total available power for a maximum of 20 minutes is about 7 MW.
The battery banks will act as booster power and will take care of the power peaks. Excess energy from the propellers or from the winch will be fed back into the net where it will be absorbed without problems.
The normal complaint about electric propulsion is that it gives a “sluggish” reaction of the winch and of the propellers. This is often caused by a too small “spinning reserve” as it takes time for the diesel to react on an increased power demand.
A gas fueled engine also have the same problem. This problem is not present with the ET 1541 as the battery boosters will always supply sufficient power on top of the diesel generated power when required.
Studies have showed that fuel savings for a diesel electric support vessel compared to a diesel direct support vessel can be up to 15 %. Peak shaving with batteries can account for another 10 % of fuel saving.

5 Fuel
Many ports around the world are concerned about emissions. Certain ports are raising the pilot and harbor fees for “polluters” and others ban them all together.
Therefore we have selected LNG as main fuel with MDO/MGO as back up. There will be four dual fuel gas engines installed that will run on LNG with a little bit of diesel oil for ignition. The engines can also operate on diesel oil only if equired.
It is of course possible to have pure gas engines installed, but for the eventual a longer passage it may be convenient to operate on MDO/MGO. Based on the Wärtsilä 6L20 the fuel consumption will be, based on the operational profile mentioned above, but based on a 36 hour period:

• LNG; about 20 m3 and 0.4 m3 MDO/MGO
• MDO/MGO; about 12 m3

ABC 6DZD dual fuel gas engines are an alternative to the Wärtsilä engines and the fuel consumption will be:
• LNG; about 21 m3 and 1.0 m3 MDO/MGO
• MDO/MGO; about 12 m3

6 Safety
The Escort Tug 1541 is the ultimate safe tug for the following reasons:
• High freeboard forward, difficult to pull under water
• Towing point forward, as far forward as possible
• Winch well protected
• Long life time 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 Escort Tug, all openings will close and the Escort Tug will continue operations on batteries and move to a safe environment on batteries

See us on the APM stand F-B38 in Singapore on 16th - 18th march.