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New LNG tug opens a market

Wärtsilä recently signed a contract related to the first LNG Tug to operate in the Middle East. This represents a milestone for two major reasons: the very demanding performance requirements of this vessel, and the opening of a new geographical area for the utilization of liquefied natural gas (LNG) as fuel.


LNG as a marine fuel is not a new topic and neither are tugs. However, the combination of these two elements makes the recently signed contract between Drydocks World (DDW) and Wärtsilä a remarkable milestone in the marine industry.

The background of this development lays in the ‘green’ initiative launched by the Dubai government. This is intended to set an example for promoting environmental sustainability throughout the region.

Consequently, and in alignment with the same initiative, in May 2014 Drydocks World announced the signing of a Memorandum of Understanding (MOU) with Wärtsilä. The target was to put into practice the Dubai Government’s wish and develop the marine market in the United Arabs Emirates (UAE) accordingly. The vessels identified as the most promising for being the first capable of operating with a drastically reduced environmental footprint were tugs.

In November 2014, the technical and economic details between the two parties were settled and the contract for the first in a series of nine harbour tugs was signed and announced to the market.

When delivered later in 2015, this vessel will be the first to be fuelled by LNG in the Middle East region. LNG was already present and available in the market, but had always been used as a commodity and, up to now, never as marine fuel. The milestone represents, therefore, not only a technical and environmental achievement, but it opens new opportunities for different applications to be developed in the area. The vicious circle commonly related to the lack of an available LNG fuel supply slowing developments in LNG-powered vessels, and vice versa, has thus been dissolved by this first contract signed. It is now more likely that other vessel types will be designed to operate on LNG in the Persian Gulf. Hypothetically, even bigger liners calling at ports in the area could possibly bunker LNG for use on longer routes, or for when operating in Emission Controlled Areas (ECA).

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Fig. 1 - An artistic impression of the 55 tbp LNG-powered tug scheduled to become operational in 2015.

The technical challenge

Tugs are vessels specifically intended to be capable of the highest performance in assisting, towing or re-positioning a vessel. The commonly accepted measure for characterising the effectiveness of such vessels is the “tons of bollard pull” (tbp), the force that the vessel is able to exert on the towing line at zero speed. This is, in fact, tested on a shore-mounted bollard.

This force is achieved from the effective performances of the propulsion components and their integration. For the specific project presented in this article, a level of 55 tbp was specified. Two Wärtsilä 9L20DF (nine cylinders in-line 20DF dual-fuel) engines, combined with WST-18 Wärtsilä Steerable Thrusters (WST) were selected as being the best choice for achieving the required performance.

A typical characteristic of the operational profile of tugs is their very low utilization rate. Tugs often have an extremely low number of running hours during a year. In indicative terms, a harbour tug could be running for some 2000 hours a year, which is very little, especially if compared to liners or Ro-Pax vessels that usually operate on a 24/7 basis.

While assisting a vessel, the majority of the time is spent waiting on stand-by with the engines idling or operating at extremely low power. This new tug has been designed to be capable of handling all operational requirements while continuously running on gas. The selected Wärtsilä 20DF dual-fuel engines match perfectly this requirement, since they are capable of being started, running without load, and operating continuously at any engine loadconstantly in gas mode.

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Fig. 2 - Tug operating profile and operational time distribution.
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If it is true that engines installed on a tug will be operated for the majority of their lifetime at extremely low loads, the transition time needed to reach their maximum output is usually extremely short. The operator commonly demands full power almost instantly from the tug’s thrusters. This requirement stems from the need to reduce the operational time to a minimum, and to be able to handle possible emergency situations with the most time-efficient response. Again, Wärtsilä dual-fuel technology was selected as the most suited for achieving maximum vessel performance. The engines themselves have outstanding load taking capabilities, being able to adjust to the tug operators’ requirements. In addition, the dual-fuel technology ensures that, even in a possible combined situation of high load demand and a failure in the gas supply system, the engine will be able to continue increasing its load by simply switching to diesel. The operator would, therefore, not even notice the difference in vessel response to his operational requirements and could complete the manoeuvre in the most safe and reliable way. Once the possible failure in the gas supply system is fixed, the engines could again be switched back to gas operation without any loss in power or speed. (Figure 2)

Performance is not the only criterion. Tugs are commonly extremely compact and agile vessels, where the equipment installed onboard should occupy the minimum space required without compromising the reliability and safety of the vessel.

The challenge for the utilization of LNG as marine fuel comes, consequently, from the usually considerable space needed for the LNG storage tank(s) vs. the restricted onboard space availability. The answer to that is twofold: Wärtsilä dual-fuel engines are intrinsically redundant products, thanks to their capabilities to instantaneously and seamlessly switch from gas to diesel in whichever operative condition. Since this redundancy is already inherently built into the engines, no additional redundancy is required from the fuel gas system, thus the Wärtsilä LNGPacTM fuel storage and supply system was designed with the smallest onboard footprint. A single Wärtsilä LNGPac (having a capacity of approximately 25 m3) and a single tank connection space (commonly referred as the “cold-box”, the space where the majority of the LNG process equipment and safety devices are installed) matched the project requirements with high levels of redundancy and safety and were, consequently, selected. (Figure 3)

The economic impacts

As outlined earlier, the annual running hours for a tug vessel are often fairly low. This aspect clearly has a direct impact on the cost structure for a tug owner or operator. Where in other shipping applications fuel costs are almost always among the two highest factors influencing economic performance, tug owners and operators typically see costs related to Investment and Depreciation having a greater effect than the cost of fuel.

Fuel consumption is, nevertheless, a very high priority because of both cost consciousness and environmental impact. However, the initial investment cost of a tug is one of the most important criteria for owners when choosing a new vessel. From this perspective, the LNG-as-fuel alternative could appear to be quite challenging, due to the implied increase in the initial vessel Capital Expenditures (CapEx).

DDW and Wärtsilä were able to meet the requirements of the owner and operator (consequently making this project economically sound) also from this perspective, thanks mainly to two items: again the specificity of the dual-fuel solution, meaning that a single LNG tank combined with a single “cold-box” could be selected, thereby keeping the CapEx level to a minimum, and the strong integration work done between the different elements included in the project.

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Fig. 4 -Indicative total cost of ownership structure.Source - ITS 2014 Damen Shipyards Gorinchem (Dirk Degroote/Robert van Koperen).
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Fig. 5 - Proportion of the total tug fleet operating in or passing through ECAs.

By integrating the ship design, engines, propulsion equipment, LNG storage and onboard process system(extended to include also bunkering stations), and the automation, the overall CapEx of the vessel was within market standards, making this project also a good base for possible further fleet expansions. (Figure 4)

The tug market has other peculiarities as well. Vessels are often intended as flexible assets, capable of being utilized in different ports or conditions despite having been originally intended to operate in a specific location. This makes the second hand market for tugs particularly active and transparent. The dual-fuel solution ensures that the second hand value of the owner’s asset will remain high throughout the entire lifetime of the vessel. In fact, the tug will not be limited to operating only in harbours where gas is available, but could be utilized wherever its technical specifications are suited to the requirements of the job. Therefore, during its lifetime, the dual-fuel-powered tug could be used for different periods of operation in different fuel modes (gas or diesel, independently) without the need to change or modify the installed equipment, thus avoiding possible related expenditures.

The same consideration applies to another fairly frequent activity in the tug market, which is the need to re-locate the vessels to different harbours for certain operational periods. During the voyage between harbours the dual-fuel technology allows the owner or operator to sail the vessel in complete autonomy, regardless of the transfer distance, voyage duration or fuel availability.

The tug could easily utilize diesel as an alternative fuel while re-locating, and switch back to gas when reaching the port of destination. The same applies to delivery of the tug from the shipyard to the point of operation, or when going to and coming from a repair yard.

The dual-fuel capability also enables the use of a smaller LNG tank, since missions requiring longer sailing time can be made partly in diesel mode. LNG storage is often cited as being one of the most costly items related to LNG powered vessels.

The DDW-Wärtsilä LNG Tug project is a breakthrough thanks to its technical performance, equipment optimization and integration, and its sound economics. With about 75% of the total worldwide fleet of tugs currently operating in or passing through ECAs, the LNG solution seems to have gained market acceptance. This project, therefore, opens the path to new market expansions towards operational flexibility combined with environmental sustainability. (Figure 5)


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