In Depth: MULTRATUG 32

Following successful sea trials, Multratug 32 was recently delivered to her owners Novatug, which will lease her to Multraship. Mutratug 32 is the first tug to be designed around the Carrousel system, taking full advantage of its dynamic towing point. Both Novatug and Multraship are divisions of the Muller Maritime Group.

Few ship types have such diverse technical solutions as harbor tugs. Even the slightest shift in focus on efficiency, safety (redundancy) or power can lead to a completely different design, ranging from the hull shape to the propulsion installation and the towing arrangement.

The latest concept to see the light is the CRT, short for CARROUSEL RAVE TUG. Novatug, a sister company to towage operator Multraship, developed the CRT concept together with world-renowned tug designer Robert Allan Limited in Vancouver, Canada and propulsion specialist Voith in Heidenheim, Germany.

Winch manufacturer Luyt Groep from Den Oever as well as engine supplier ABC from Ghent, Belgium, were also involved as partners in the project from the start. After the design was finalized, including extensive CFD and model tests, a tender followed to select a shipyard. Damen Shipyards, through its German subsidiary VanderVelden Barkemeyer, was ultimately selected to build the first two CRT’s. They contracted the hulls to be built by the Buschmann shipyard in Hamburg, Germany and the final outfitting took place at Damen’s Maaskant Shipyards in Stellendam.

Infrastructure

Julian Oggel, managing director of Novatug, explains:

“Since the start of this century, the scale in shipping has seen unprecedented growth. Container ships for instance have more than doubled in size over this period, from about 8,000 TEU’s in the year 2000 to over 22,000 TEU’s today. In the same time span, the capacity of the infrastructure has not doubled. We see that these larger vessels challenge the limits of such infrastructure and in doing so they are increasing the operational risks for ports as a whole.

“Where before the consequences of any technical failure on a visiting ship would be limited to that ship and its cargo, now any large cargo vessel can potentially cause a serious domino effect by blocking the entire port, completely messing up the slot times for many other vessels and this even at different ports where containers are already waiting for these ships to arrive at their scheduled slot time. Seaports are very similar to airports in this respect.”

“The CRT is specifically developed to deal with this issue and mitigate those risks, by providing effective steering and braking assistance for ships while they are moving. At the base of those possibilities is simple physics, which dictates that large ships need to maintain sufficient speed to maintain sufficient steering capability of their own. Their rudders are simply not effective at low speeds.

“But when these ships would make their approach through rivers at a higher speed where they are able to steer, the speed is too high for normal harbor tugs to provide effective assistance. The simple mass of these large vessels also makes it hazardous. Stopping and steering such a vessel requires tugs that are able to generate dynamic forces, but in general with harbor tugs as used today these same dynamic forces can capsize a tugboat which can lead to a terrible accidents including loss of life among tug crews.”

Carrousel

Multraship has been experimenting with an inherently safe towing system on their harbor tug Multratug 12 since 2002, providing thousands of assists with the system. The Carrousel system was invented by Marcus van der Laan in 1999 and is a patented concept.

In the Carrousel system, the towing winch is mounted on a freely rotating ring, which revolves around the superstructure. This leads to a ‘dynamic towing point’, shifting the point of pull on the tug towards the towing line. When pulling sideways on a conventional towing system fixed on deck along the centerline of the tug, this will normally heel the boat towards the towing line, ultimately leading to capsize if the towing force remains stronger than the tug’s righting moment for long enough.

With the dynamic towing point, the towing cable in that condition actually also pulls the side of the tug out of the water, making capsize due to towing force impossible. In 2002, Novatug won the Maritime Innovation Award with the Carrousel system.

Design freedom

Bureau Veritas’ new rules for tugboats’ stability make an exemption for inherently safe towing systems, such as the Carrousel system. This exemption gave the designers of the Novatug concept more freedom to design the vessel, for example by using a more slender hull, which is better for the tug’s own speed performing in transit and can generate dynamic towing forces more efficiently.

The potential to design a more slender hull in this way is enhanced by the in-line propulsion configuration of the RAVE concept, which was developed independently from Novatug by Voith and Robert Allan earlier. The CRT combines the RAVE concept with the Carrousel system to form an optimal combination.

This is the basis of the new Novatug concept: rather than generating pulling forces by brute engine power, it relies on dynamic towing forces, helped by the fact that it is safe to do so because of the Carrousel system. It took a long time to go from the retrofit application of a Carrousel system on Multratug 12 to the new vessel, which was designed from the bottom up, taking full advantage of the system in all aspects.

An earlier prototype of a carrousel tug concept got stranded mid-build in South-East Asia, partly because of the choice of shipyard and partly because it was designed to do everything, from harbor towage to deep sea towage. For Multratug 32, the focus was purely on escort-type towage for harbors and their approaches.

Voith Schneider Propellers

For propulsion, Novatug recognized the uniquely suitable properties of Voith Schneider Propellers (here in the size-type VSP32). This is remarkable, as Voith has seen its market share in tugs dwindle due to the arrival of powerful and lower-cost azimuthing thrusters.

Voith Schneider Propellers (VSP) consist of a number of vertical propeller blades, which are fixed at the top to a spinning disk with an ingenious system to vary the pitch during rotation. The control mechanism can also vary all angles of attack of each blade in an instance, providing extremely fast response and thrust in any direction. The unique characteristics of the VSP are possible because the propeller blades rotate around the vertical axes instead of around the horizontal axes with other propulsion systems.

The main reason to opt for VSP’s in this case, is the nature of the application on the CRT. While a thruster with ducted propeller performs excellently when water has a straight inflow on the nozzle and on the propeller disc, this is rarely the case in real life for tugs.

The inflow can be obstructed by the vessel to be pushed/pulled, disturbed by the towed ship’s propeller wake or simply because the orientation of the required thrust is not parallel to the direction in which the thruster is moving.

The latter effect is for instance demonstrated by the fact that tunnel bow thrusters quickly lose their effectiveness with increasing ship speed. Novatug carried out model tests at the towing tank in Vienna, and the system proved very robust and controllable. In the most extreme test case, at a speed of 16 knots, a significant wave height of 3 meters and with the towing line attached, the VSP’s still provided a very predictable thrust force that was fully available to change the tug’s heading relative to the towing line.

Engine room

The slender hull form is made possible by the fact that on the CRT the VSP’s are not mounted side-by-side but fore and aft, each with a wing-shaped plate on the bottom to protect the blades in case of grounding and to increase thrust. This in-line configuration also ensures optimal control of the tug’s ability to pivot, which in the case of the CRT is the mechanism to in- or decrease the force on the towing line when it is attached to the carrousel.

In the engine room, the engines from ABC Diesel driving these VSP’s are mounted next to each other at an angle to the ship’s centerline, as there is not enough ship length to mount them both on the centerline. Each engine is coupled to a VSP through a fluid coupling, allowing slipping for load-free starting of the engine, vibration damping and overload protection.

The VSPs are in fact controllable pitch propellers, and the vessel can therefore use the combinator mode, allowing for the control of both the “pitch” and the engine speed for the most fuel-efficient operating point. Electric power is generated in three small 70 kW gensets, which can run in parallel. This ensures that any running generator is always sufficiently loaded.

This is regulated by a power management system from Maritime Elektric International, the electrical contractor for this project. The system also includes a seamless transfer from shore power to generator power and vice versa, without black-out. This will lead to shore power being used more often, with obvious benefits for the emissions in port areas.

The alarm and monitoring system is from Praxis. The high involvement of German suppliers and subcontractors ensured that it was possible to use the excellent German export credit insurance facilities to finance the project.

Closed coolers

The cooling water for the main engines is a closed loop, which is cooled in rack coolers. These are closed seachests, into which a large quantity of water is pumped from near the keel. This ensures that there is always sufficient cooling, even when the ship has a high heeling angle for a prolonged period of time, something which is not uncommon during escorts.

Heating of the tug is done with a diesel-fired boiler, which also takes care of the pre-heating of the main engines. The central heating system of the vessel also uses heat recovery from the cooling water of both main engines and one of the gensets.

Bollard pull

While their solutions differ widely, all tug designers seem to agree on one fact. The single most defining characteristic of a tug boat is the static bollard pull – as this is what can be measured easily and is therefore also what ports specify – and unfortunately this characteristic falls very short of defining what a tug is really capable of.

The bollard-pull test is a test in which the tug has no forward speed and needs to pull as strongly as possible on a dynamometer under specific “laboratory” conditions (a.o. free inflow, free outflow, deep water, long line etc.). In normal daily operations for tugs, such conditions are very rare.

The bollard-pull condition happens for example when a ship is being pulled away from the quayside, an activity corresponding with about 2 per cent of a typical operational profile of tugs. Almost half of the time, the tug is free-sailing and the other half it is assisting ships in some sort of dynamic conditions. While this is done in a straight line on an oceangoing tug, a harbor tug is mostly used as an additional bow thruster, rudder or even the brakes of a large ship.

On Multratug 32, the static bollard pull is 70 tons at the maximum engine output of some 5.300 kW, but in dynamic conditions at 10 knots, the vessel can easily generate a maximum steering force of 160 tons, with only about 2.800 kW of engine power. To be able to deal with such forces, the towing winch on Multratug 32 was developed by Luyt Group and is an innovation in itself.

It uses a drum with cylindrical section to store the cable, but during heavy-load towage operations, the cable’s last windings are on a smaller diameter shaft on the right hand side, around an co-centric section of the drum meant to act as a lever on the holding force of the winch. In this way, the towing winch manages a holding power of 260 tons whilst keeping the weight of the winch well below 10,000 kg.

Simulator

Towage operator Multraship disposes of a tug simulator, allowing to run simulations with all the tugs in their fleet and compare them to the new concept, of which the characteristics were known from the CFD and model tests undertaken. Because of the improved control at a higher speed, it is expected that the CRT’s will increase the capacity of limited waterways such as the River Scheldt or even the Panama Canal, where often vessels are waiting at an anchorage for their turn to be escorted through.

Navigational limitations and/or waiting times are a loss both for the shipping companies and for the ports, which share the common goal of having ships underway at sea for the maximum amount of time possible. Due to the usage of more dynamic forces rather than brute engine power, it is further expected that the CRT’s annual fuel consumption will be significantly lower than that of conventional harbor tugs.

Safety

A black-out is always possible even with the most reliable engines. Because of the Carrousel system, the tug will remain at an angle with the towing line even when propulsion is lost. It then still acts as a brake for the ship, without having to release the towing cable to avoid the risk of capsizing. The tug simply becomes a very large and effective sea anchor.

On the risk posed by the rotating towing hook on deck, Julian Oggel is very clear: “During coupling actions the towing cable is guided through the hydraulically retractable locking pins on either the bow or the stern. As soon as actual towage starts, the deck is no place for crew on any tug, due to the risk posed by a snapping towing line. The rotating ring also has a remote-controlled brake, so it can be held in position when people have to go on deck.“

Multratug 32 has a gas detection system allowing her to operate near chemical tanker, gas tankers, etc. At the exits of the deckhouse, Emergency Escape Breathing Devices are provided, to allow the crew to escape through a toxic environment.

The tug also has an external sprinkler system, so it can be protected when working close to a fire. With a firefighting capacity of two times 1.450 m3/hour, the tug has Bureau Veritas’ highest class notation Fire Fighting Ship 1. A foam tank and two remotely controlled fire monitors are installed.

The engine room is protected by a Novec firefighting system. Construction After signing the contract at Europort Maritime in November 2015 with Damen Shipyards, hull construction started in Germany at Theodor Buschmann GmbH. A challenge posed by the Carrousel system is that the superstructure has a round shape, which makes it difficult to design a workable interior.

This was however successfully done, providing excellent accommodation for the crew of up to six. The deckhouse is slightly offset towards the bow, in order to always be able to decrease the force on the towline even at the highest forces. The wheelhouse has excellent views all around and features two steering positions, one facing forward and one facing aft. Retractable towing pins on bow and stern were supplied by Smits-Berger (aft) and Karmoy (forward). Palfinger supplied a provisioning crane.

Dynamic towing point

Other systems exist for a dynamic towing point, such as the Oval Towing Point, also developed by Marcus van der Laan. The advantage of the Carrousel system is that the load is spread over the entire 11 metres diameter of the ring much like is the case on a ball-bearing, while a moving car on an oval ring creates a point load. Furthermore the outer ring has a variable thickness, using the material where it is most needed for strength and stiffness.

To help balance the weight of the towing winch on the ring, the hydraulic powerpack, which includes redundant diesel engines and fuel tank, is mounted on the opposite side of the ring. Because the power supply is mounted on the ring, there is no other connection between the inner and outer ring than the wheels it turns on, allowing for an unlimited freedom of rotation in either direction.

The winch is controlled by radio control from a portable console that is normally located in the wheelhouse. For versatility, the vessel also has a fixed towing point on the aft deck, equipped with a Mampaey quick-release towing hook.

Leasing

In addition to a technical innovation, Novatug also introduces a financial innovation. Much like we see the ownership model moving to a usage model in other sectors, such as airplanes, cars, houses, etc., Novatug believes that tug operators are not primarily interested in being ship owners.

Clients prefer to have “tug availability” for a period of time rather than to own a tug, although this is also possible. Novatug then takes care of the financing, build contract and supervision. There is even an option to have fixed-cost maintenance for the given period, allowing the tug operator to focus on servicing their clients, having only to deal with running costs such as crew and fuel. Clients can choose between buying, financial lease or full operational lease.

Prospects

Asked whether more CRT’s are on order, Novatug says a second tug is currently in build, which will be leased by Multraship as well. Other potential clients have indicated they wish to see the tug in operation before making a decision. There are about 4,000 tugs in operation worldwide in the relevant size range, which means about 200 newbuilds per year, just for replacement.

In addition, Julian Oggel believes that new markets for tugs will open up. He believes that the CRT can increase the capacity of existing infrastructure to such an extent, that the tugs can be considered a viable alternative for heavy investments in fixed infrastructure.

Bruno Bouckaert, Maritime Holland
Image Courtesy: Van der Kloet Foto and Videoproducties

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