Escort tugs require specific designs, incorporating the correct winch and powerful propulsion systems, to aid LNG carriers coming into terminals
Escort tug propulsion systems and the associated winch tension are used to slow oncoming LNG carriers down and then control their speed upon arrival at LNG terminals. Classification societies have ‘escort’ notations for these tugs, defining the winch components and towing apparatus.
Winches used for escort tugs must have a fast rendering system in case of emergency and the tension to which the towline is subjected should be monitored. During their operation, all towing must be carried out over the winch; the drum brakes are not used, according to industry guidance.
An escort tug works on the stern of the LNG carrier to generate retaining forces and steering operations, reducing speed to a constant of around six knots.
The operational mode of the escort tug will follow the following phases: rendering and recovering the towline; controlling tension; and releasing it in an emergency. The distance between tug and ship and the towline tension should be maintained to pre-determined values.
There are two methods of connecting a towline to an LNG carrier, depending on the ship operator and terminal owner: one involves using equipment and bollards on the ship deck; the other uses the shell, or sunken, bitts on the LNG carrier’s hull.
NYK LNG Shipmanagement deputy marine and HSEQ manager Capt Ed Narraway says that his company prefers tugs to use the shell bitts on carrier hulls for escort and towage because of better control compared to using deck bollards. LNG carriers also have strengthened tug points on the hull sides for pushing the vessel to the terminal jetty.
“Tugs are essential to LNG terminal operations as carriers cannot go alongside without them,” Capt Narraway says. “There is always a standby tug in case there is need for an emergency departure.”
Terminals are often exposed to harsh weather, sea swell and tidal or current forces that cause ship motion during loading and unloading operations.
These forces also affect LNG carrier towage during arrival and departure. “Escort tugs need to be more powerful and manoeuvrable and use their escort winches,” says Capt Narraway. Azimuth stern drive (ASD) tugs are manoeuvrable and have fast reaction winches, while other types available for ship handling include tractor, Rotortug and Voith Schneider tugs.
Escort tugs should have a bollard pull of around 70-100 tonnes to control and steer LNG carriers. They need a stronger righting torque to counteract the heeling torque created by the towline. In direct mode towage, force on the towline is produced by the tug’s propulsion, reducing the carrier’s speed and assisting during steering by either keeping the tug parallel to the vessel’s centreline or open to the corresponding ship side.
In indirect mode, the tug’s propulsion is used to find an appropriate position to create lifting forces, which will be transmitted through the towline, while hydrodynamic forces will be created by the vessel’s hull. These forces will be used to determine a high degree of arrest and steering forces.
Elengy requires terminal tugs with bollard pulls of at least 70 tonnes at its LNG import facilities in France – Fos Tonkin and Fos Cavaou in Marseille and Montoir-de-Bretagne terminal on the Atlantic coast.
Terminal tugs are operated by Boluda France, which added two 73-tonne bollard pull terminal and emergency response tugs to the fleet in the port of Marseille-Fos in December 2019.
Elengy LNG marine expert and marine manager Pierre-Antoine Chevé says up to four tugs are sometimes required for berthing LNG carriers at its Mediterranean terminals because of environmental factors. “Wind speed and direction and sea conditions have an impact at Fos Cavaou and Fos Tonkin,” he says.
“Sometimes sea currents and winds are too strong for using mooring vessels and then we may need to postpone berthing operations. At Montoir this is less of an issue as it is sheltered, but there are still currents to consider,” Mr Chevé says.
Boluda France deployed VB Surprise and VB Acheron at Marseille-Fos in December. These 30.3-m tugs were built in Piriou Shipyards in Vietnam and feature fire prevention and fire-fighting systems.
New escort tug design
Royal IHC has developed a new escort tug design specifically for towing and manoeuvring large LNG carriers with a lower environmental footprint. Its Delta Escort Tug T100-17 is azimuthing with electric propulsion that uses LNG as a fuel, backed up by batteries.
At the centre of this escort tug is a fibre rope winch that IHC director for offshore oil and gas Henk Cornegé says influenced the vessel’s design and its propulsion system.
“The winch is at the heart of the tug,” he says. IHC researched methods to minimise the risk of fibre line breakage and then designed a tug around these results.
It used simulators to mimic how the winch and vessel would behave in different escorting operations and environments.
“We designed a fully electrical, high-speed render/recover winch that would minimise the risk of line breakage,” says Mr Cornegé. “Behind an LNG carrier, the behaviour of the tug fluctuates,” he explains. The tug needs to react to steering forces, peak loads and snatch loads on fibre rope.
“Loads can go from zero to 150 bollard pull in seconds, and the winch needs to respond,” says Mr Cornegé, noting, “We designed the winch with electric motors so it can provide torque and speed of response.”
The electric winch is self spooling and quick reacting, with render and recovery at 120 m/min. It has two shock absorbers that allow limited longitudinal motion and can absorb 500-tonne snatch loads; they help provide Delta T100-17 with bollard pull of 100 tonnes.
On the escort winch, the torque of the drive motors provides braking power. It has a nominal pull of 130 tonnes at 15 m/min.
During tug design, IHC removed the staple on the tug bow and led the towline through a sheaved fairlead to prevent bending of the rope, which induces internal friction to lower risk from wear and tear.
It designed the 40-m long hull with a breadth of 17 m and a large skeg for high stability. “We designed the hullform by optimising skeg and the position of the sheaved fairlead,” says Mr Cornegé.
This tug has a design draught of 5 m, the depth of the main deck is 6.7 m and there is accommodation for seven people.
Its primary propulsion is from dual-fuel generator sets of 5,340 ekW in total. This is combined with a redundant energy storage system rated at 1,560 kW/hr. Propulsion delivers 6,700 kW full power and gives Delta T100-17 a top speed of 13 knots.
LNG fuel could be sourced from the terminal that the tug operates within.
“The biggest issues in our industry are protecting the environment and reducing emissions, especially particulate matter, SOx, NOx and CO2,” explains Mr Cornegé.
“The beauty of our Delta design is the batteries for winch response times and for topping up the dual-fuel gensets.”
A redundant DC-bus can provide total available power of 7 MW for up to 20 minutes.