There is no question that LNG is presently emerging as the most viable alternative fuel to conventional diesel for many classes of ships, and in the tug market this is certainly also the case. The principal attraction of LNG is the ability to achieve low emission standards without the costly and bulky after-treatment components required on diesel engines, which also result in much larger casings and exhaust trunks, thus impeding operational visibility. The downside in a small vessel such as a tug however is the large volume required to accommodate LNG storage tanks and their associated control systems. Ultimately the range and endurance of a tug with LNG is severely compromised in comparison to a diesel-powered tug of the same dimensions.
LNG tug concepts promoted to date all take the traditional tugboat configuration and squeeze in the storage tanks, most typically severely impacting the space available for storage and the crew accommodation. After studying many options for LNG-fueled tugs, the design team at Robert Allan Ltd decided to take a completely fresh approach to an LNG tug design and not be constrained by conventional tug layouts. Starting with the essential basics of tug design and operations, we first looked carefully at the primary working deck layout and ensured that was not compromised. The next priority was to examine the LNG storage and distribution requirements with the associated engines etc., and determine where in the tug that was most efficiently located. Then we worked to fit the rest of the design requirements (accommodations, control rooms, stores etc) into available spaces in a logical and sensible manner.
One primary target for review was the aft deck. On many tugs today, especially those dedicated to terminal support or escort towing, the aft deck is essentially redundant. A well-designed modern terminal tug should be able to tow and manoeuvre equally well going in either direction, hence has no need for an aft winch. The aft deck space on many tugs is thus often just a large empty area. We considered this to be prime real estate for locating the accommodation facilities displaced by the LNG tanks.
After numerous iterations and concept exploration studies a completely fresh idea for a truly modern LNG powered tug design was born; the RANGLer class. (Robert Allan’s Natural Gas (Liquefied) [tug]!) Departing from “traditional” diesel tug designs, the RANGLer deckhouse is biased aft to provide excellent visibility from the wheelhouse and an efficient working deck forward The spacious crew accommodations are located within a stern “castle” replacing the below deck accommodations of conventional tug designs. The space forward of the engine room is used for maximum LNG storage capacity, and is configured to allow easy installation and removal of the entire LNG tank system as an “LNG Fuel Module”.
The LNG Fuel Module includes not only the IMO Type C LNG storage tank and gas processing equipment, but also the bunkering station, engine gas regulation units, controls, gas-related ventilation fans, enclosures and access ways. By integrating all the key LNG-related equipment into a single module that can be tested and approved ahead of time, the final installation of the LNG fuel system is made a much more straightforward and time-efficient and less risky process than if equipment is installed separately in a more piecemeal way.
The first of this new series, the RANGLer 3600 design, illustrated on the accompanying drawings is a twin Z-drive terminal support and tanker escort tug, designed specifically to maximize the benefits of natural gas as fuel.
The RANGLer Series also embodies the now very well-proven sponsoned hull shape of the popular Robert Allan Ltd. RAstar Series, providing truly enhanced indirect escort towing performance and highly effective motion damping in a seaway.
Working with closely with both Bureau Veritas (BV) and American Bureau of Shipping (ABS) on the LNG safety aspects of the design, Robert Allan Ltd is pleased to announce that the RANGLer 3600 Class concept has received Approval in Principle from both classification societies for either single gas fuel or dual fuel engines.
The particulars of this new LNG tug design are as follows:
- Length, Overall: 36.5 m
- Beam, Moulded: 15.4 m
- Depth, Least Moulded: 7.1 m
- Draft, Navigational: 5.8 m (above bottom of drives)
- Installed Power: 2 x 2430 kW
- Complement: up to 10 crew
- LNG Capacity: 80 m³ (gross)
The predicted performance of the RANGLer 3600 is as follows:
- Vessel Speed, ahead: 14 knots, approx.
- Bollard Pull, ahead: 80 tonnes, approx.
The use of LNG in smaller vessels such as tugboats is an extreme challenge. Simply adapting existing design configurations to this new fuel does not appear to offer very effective working arrangements on the tug. Fresh, innovative thinking as illustrated by the RANGLer design concept is required to make LNG a truly viable fuel option in working vessels of this type.