Hybrid Rotortug – the Well Cultivated Green Tug Revolution

On 26 November 2014 Elisabeth Ltd of Malta took delivery of the latest addition to their fleet of (Rotor-) tugs, which happens to be the first new ART 80-32 Hybrid Rotortug.

This tug, named RT Evolution, was built by Damen Shipyards of Hardinxveld-Giessendam, the Netherlands and designed by an alliance of Rotortug in the Netherlands and Robert Allan Ltd in Canada. The project was overseen, co-designed and commissioned by Kotug, who will operate the tug on behalf of the owners in the Rotterdam harbour area.

The Rotortug distinguishes itself from conventional ASD, Tractor or Voith Schneider designs by using a triangular propulsion configuration. The RT Evolution, to be known as ‘E-Kotug’-class, is the first of a pair of next-generation Rotortugs presently being built by Damen Shipyards.

The Robert Allan-designed Hybrid Rotortug is characterised by improved fuel economy and reduced harmful exhaust emissions. The hybrid propulsion configuration is a continued development of the RT Adriaan and features combined e-motors and alternators on a common shaft line, complemented by a battery system matched with an intelligent power management system. The building of the second vessel of the pair, to be called RT Emotion, is well on its way in an advantage stage of outfitting and delivery is expected in the first quarter of 2015.

The designation ART 80-32 Hybrid Rotortug in combination with its name ‘RT Evolution’, says it all. With the redesign evolution of the RotorTug (RT), the type notation went from RT to ART: Advanced RotorTug. The numbers 80 and 32 respectively stand for the bollard pulling force and the length of the vessel. The hybrid propulsion configuration of RT Evolution is an additional option.

Why a Rotortug?

A tug is a vessel used to assist in the manoeuvring of large seagoing vessels in confined spaces. In addition they also support these vessels by providing escort duties, because at slow speed a large vessel’s steering capacity is reduced. This second duty is a procedure fraught with danger (imagine a large bow with bulb under water coming at you at ten knots). Using these requirements Rotortug set out to design a new tug concept, providing improved visibility and ergonomics, including deck and bridge layout and the movement of crew around the vessel during normal working activities.

The resulting design concept benefits from improved versatility, flexibility, and the ability to respond very quickly to changes in manoeuvring requirements. The end result is a user-friendly, highly responsive vessel, with the accent on safety.

As can be seen below, the improved propulsion layout and superior hull design result in increased leverage and thus a larger steering moment when compared to other builders’ tug designs. This provides the tug with exceptional steering capabilities in all directions allowing for easier position changing and towing.

When tethered and changing position, towing lines can remain constantly under tension, whilst at the same time keeping vector and thrust force responses highly controllable in almost any desirable direction.

These operational advantages mean that two such tugs can be employed for towing duties that would previously require four traditional vessels. This fact alone provides the Rotortug with a head start over the competition.

What makes a Rotortug?

The very first Rotortug found its origin in the Tractor Tug design with two Azimuth propulsion units installed in the forward hull of the tug (forward of the towing point). This ‘traditional’ tug configuration features a skeg aft, for course stability and for generating indirect towing forces, with a docking stool forward.

Rotortug replaced the ‘static’ skeg in this design with a ‘dynamic’ or ‘active’ skeg, so, in fact, this tug comprises three azimuthing thrusters arranged in an isosceles triangle lay-out. As such, it follows the Tractor Tug with the two forward units, but with the addition of a unit aft. The latter is placed directly under the towing point. This allows the tug to perform pushing actions in the direction of the towing line, as well as the more common pulling activities. By omitting the traditional box keel or large skeg, the manoeuvrability and the capabilities of the tug were greatly improved. At the same time, this increased the vessel’s (dynamic) stability and makes it safer. Furthermore, no transverse (bow) thruster is required.

The second striking deviation from the traditional design is the removal of the docking stool. The docking stool as well as the aft skeg, originally also had two other purposes (as well as course stability): the first being protection of the thrusters and secondly for keeping the tug upright to support it when dry docking. The docking stool has been removed, because experience has shown that it was not really required and removing it reduced the drag and improved the fuel economy of the vessel.

These developments and improvements however were not enough for the designers and with the aid of experienced masters and other experts further developments were initiated. What was needed was a second set down the road to reducing harmful emissions, fuel consumption and maintenance, in short creating a truly green hybrid tug. The first step was the conversion of the propulsion system of RT Adriaan from conventional to hybrid as a floating test bed. Further results were achieved by improving the hull shape design and after almost two years of intensive studies the new design of a 32 metre Rotortug was ready for construction. In short RT was about to be replaced by ART.

Advanced hybrid tugnology

The development of the hybrid version of the ART 80-32 Rotortug represents Kotug’s green policy and is optimised for the operating profile of a harbour tug, suitable for port area’s worldwide.

In normal use a harbour tug only spends approximately five per cent of its time at full propulsion power, the remaining time will be at lower speed. This means that for 95per cent of the time, a single auxiliary engine needs only to provide some 85 per cent of its power for pushing the tug forward. In the ‘free-sailing’configuration at high speed the aft engine mechanically drives the aft propulsion unit; to negate their own drag the forward units are electrically driven at very low speed.

At high speeds and demanding tug operations, any combination (as described below in the ‘Engine room and propulsion’ paragraph) of the power sources can be used to drive all the propulsors, depending on the requirement.

As a result, the use of the main engines is much reduced and the vessel spends more of its time on generators. This reduces the maintenance on the main engines, whilst the generators and their much smaller diesels can produce the required power much cheaper with marginal maintenance, thus leading to considerable cost savings for owner and/or operator.

The surplus of E-output of the generator sets or main engines is continuously stored in the battery packs. This, on average, provides enough battery power to provide for the whole vessel while moored in a harbour during the night for six to eight hours. As a result, the high emission regulations of harbours like Hamburg and Bremerhaven are easily met, because no diesel engines need to run when moored. Furthermore, (almost) no shore-power is required and the crew have an undisturbed night.

In addition, the ART tug has a heat recovery system, consisting of an accumulator reservoir of approximately 350 litres. The fresh water in this ‘barrel’ is heated by the waste heat of the cooling water of the engines and other systems on a heat exchange basis. In addition, it can be heated directly by an oil-fired boiler. This additional system is used for maintaining the temperature of the water in the barrel, whilst the heat recovery circuits are used to increase the temperature. The ‘recovered heat’ from this barrel is used for heating the accommodation or the shower water and to keep the (main and auxiliary) engines at operating temperature. To keep the engines on standby they are constantly heated up to 70 degrees Celsius, so when called upon when the vessel will need to make speed, the engine’s full power is instantly available at a moment’s notice.

Engine room and propulsion

The Xeropoint (hybrid) power management system, provided by Aspin Kemp & Associates (AKA) of Canada, is customised to this vessel’s power and propulsion requirements, integrating all electrical and mechanical devices onboard to provide optimal modes of operation for power and propulsion. The hybrid system’s energy management system strives to eliminate the unnecessary idling of diesel engines by determining the most efficient configuration of the electrical and mechanical devices onboard at any given time.

The hybrid system comprises of three main diesel engines, each coupled to a propeller shaft with a dedicated inline electric motor/ alternator via ‘directional slipping’ clutch. Each diesel engine can drive a propulsion shaft independently or simultaneously with the electric motor on that shaft. With only the diesel engine providing power, the electric motor acts as a shaft-driven alternator, providing power to meet the vessels hotel loads or to charge the electrical storage. The auxiliary generators and electrical storage, consisting of two lithium polymer bank of 78 kW/hrs, provide power for the propulsion electric motor during electric or diesel-electric modes of operation.

The vessel’s operator theoretically has four modes of operation available: strictly electric (power from battery packs), diesel-electric (power from auxiliary diesel generator sets), mechanical (power directly from main engines) and mechanical-electric (power from main engines and the shaft-alternator). Using the appropriate combination of these modes greatly improve the vessel’s flexibility. Furthermore, these multiple driveline configurations provide redundancy by offering alternate sources of propulsion to the vessel. Switching between power sources is performed seamlessly as the battery packs provide ample buffer capacity during the short ‘black-out’ that appears during change.

Power is generated by three Caterpillar ‘C3512C TA/HD+’ diesel engines of 1,765 kW at 1,800 rpm each. These engines are coupled to the Twin Disc MCD 3000-3 LD that drive a propeller shaft with an alternator. Propulsion and steering are provided by three Schottel SRP3000FP units with fixed pitch propellers of 2,300 millimetre diameter in a nozzle. These Schottel units are arranged two forward and one aft, as described above. The electric motors/alternators in the drive-line are Teco Westinghouse units of 500 kW each. As a result of this well-balanced propulsion arrangement, RT Evolution is able to provide a continu- ous bollard pull of over 83 tons, whilst a maximum speed of 13.5 knots ahead as well as astern and 7.5 knots transverse performance is achieved. Engine cooling is achieved by heat transfer between the Blokland Non Ferro heat exchangers and the sea chest.

The auxiliary generator sets are a Caterpillar C18 TA and a C9 TA producing 575 and 250 kVA (450 and 200 EkW) respectively, at 400/230 V – 50 Hz. Van der Leun was responsible for the coordination, installation and implementation of the electrical installation.

Deck and towing equipment

For tug duties, RT Evolution features an aft and forward towing winch, each with a single drum. Both winches have a brake force of 230 metric tons and have the capacity for 200 metres of 80 millimetre diameter ultra-line and 20 metres of 64 millimetre diameter Dyneema ASB. The electrically driven winches, delivered by DMT Marine Equipment, are outfitted with an external e-motor. The controls, fitted to the gearbox between the winch and the e-motor, and the break are hydraulically operated.

The vessel is provided with mooring equipment selected with respect to its size and deck arrangement. The foredeck features a HHP (High Holding Power) AC-14 bow anchor of 360 kilogrammes to portside with 165 metre stud-link anchor chain of 22 millimetre diameter. This is deployed and recovered by an anchor winch integrated in the forward towing winch. The anchors and associated equipment are provided by Damen Anchor & Chain Factory and Damen Marine Components. Furthermore, there are eight bollards: three on foredeck, three on aft deck and one on each side amidships.

Crew accommodation

Forward and below decks, separated from the engine room by the switchboard rooms and the battery storage compartment, is where we find the crew’s night accommodation. This four-person accommodation consists of two twin berths cabins with adjoining private bathrooms. Also in the forward hull compartment are the laundry, galley storage and bosun’s store.

The superstructure itself comprises of two levels: main deck and wheelhouse deck. The lower superstructure level accommodates the master mariner and chief engineer, each provided with a single berth cabin with adjoining private bathroom. All cabins and accommodation spaces have individually controlled heating and air-conditioning. Furthermore, this level comprises the galley and a combined mess room/ lounge, as well as ample storage rooms and sanitary spaces.

The bridge itself features integrated twin navigation islands, comprising two deck-mounted consoles, complemented by additional overhead consoles, separated by a corridor through which the helmsman’s seat can move on rails. These ergonomically designed ‘Alphabridge Tugboat’ consoles provide a good view of the instruments and ease of operation. The lay-out and design are developed by Alphatron Marine in collaboration with some of the leading tugboat operators in the world. The concept is based on the principle that the helmsman has to have the best achievable ergonomic work position for both forward and astern steering with all controls within reach (one-man bridge operations) and maximum visibility. Furthermore, to minimise the transition from fore to aft position the lay-out has been rationalised. A second overhead split console accommodates all the navigation and communication instruments. These are located in the corners of the bridge between the side and overhead windows. They include VHF plus critical readouts of thruster power, rpm and angle. This has allowed the shipyard to design a wheelhouse that is as small as practical with optimal visibility around the vessel.

Against the aft of the accommodation are the engine room ventilation casings and the engine exhausts. The life rafts are to be found in dedicated cradles on both sides of the bridge island. With this technically advanced ship, the Damen-knowledge and experience in tug and workboats is reflected in the low noise and vibration levels in the accommodation. On top of the superstructure is a reduced height mast with the required radar scanners and antennas; the design provides a restricted air draught to allow for free manoeuvring under towing lines, especially when towing large contain- er vessels with extremely flared bows.

Evidence provided

Through a process of ongoing development, Damen and Kotug continue to make major contributions to achieving the NINA (No Incidents, No Accidents) status for operators. Combining extensive knowledge and hard-won experience, they strive to provide the best tug for the job. As a result the RT Evolution’s manoeuvrability, strength and flexibility provide operators with a high-level of operational safety under all conditions and enhanced operational possibilities.

Tom Oomkens