Tugowners have more choice than ever before
Jack Gaston reviews the latest tug designs, looking in particular at their efficient and environmentally-friendly propulsion arrangements.
Any review of recent deliveries to the towage industry will reveal a broad spectrum of tug designs and propulsion technology, indicating that tugowners now have more choice than ever before. In its long 180-year history the towage industry, and the shiphandling sector in particular, has probably seen more development in propulsion systems and related hull design than any other branch of shipping.
As ships have become progressively larger, and, in many cases, less manoeuvrable, the demand from shipowners and harbour authorities for more powerful and efficient tugs has increased. Shiphandling with tugs has become a more refined process and high bollard pull is no longer the only consideration. The result has been a rapid acceleration in the development of new technology and vessel design, led by a number of naval architects, specialist shipbuilders and in some instances enthusiastic and forward looking tugowners.
A quick backward glance may be useful at this point. Initially, demands for increased bollard pull were met by building bigger tugs with more powerful engines driving large, deeply immersed, propellers. Hull design closely followed traditional shipbuilding practice and did not change substantially until new propulsion systems evolved. The spark that ignited a whole train of tug development was the introduction of the Voith Schneider tractor tug in Germany in the mid 1950s. Not only did the Voith Schneider cycloidal propeller produce ‘omni-directional’ thrust, it enabled the Voith tractor to apply its bollard pull during shiphandling operations exactly where it is needed. That ability stemmed from the tug’s unprecedented manoeuvrability, allowing it to reposition with ease. It was the impetus to replicate and improve on those characteristics that was responsible for the development of the fully azimuthing propulsion unit, or ‘Z drive’. Fully steerable through 360°, the unit invariably incorporates a conventional screw propeller rotating in a thrust enhancing nozzle.
The azimuthing drive has proved to be a versatile device capable of being employed in a wide range of vessel designs, many far removed from the original conventional tug hullform. That alone has enabled the designer to deviate dramatically from the constraints of the conventional propeller/stern gear configuration. Of the unique concepts that have emerged from considerable research and development using the Z-drive ,four have now become well established in the industry and each addresses the matter of ‘applying the right amount of force in exactly right place’ to handle the very largest ships, safely and efficiently. These are the azimuthing stern drive (ASD), the tractor tug with Z-drive propulsion, the ‘Rotor Tug’ and the ‘Z-Tech’.
The modern ASD
Currently the vessel of choice for the vast majority of tugowners is the Azimuthing Stern Drive (ASD) tug, having two Z-drive units located beneath the stern. Each propulsion unit is normally capable of being controlled independently, enabling thrust from each unit to be directed at any angle in relation to the hull. The ASD tug can be manoeuvred ahead, astern, sideways and turn in its own length with great precision in the hands of an experienced tug master. A high degree of agility, a relatively high bollard pull performance and no significant increase in the draught of the vessel has made the stern drive tug a viable alternative to tractor tugs in many applications. Such vessels invariably carry out shiphandling duties using a winch on the foredeck and substantial fendering on the bow to work in the ‘push-pull’ mode, using their agility to quickly reposition when needed. An optional second winch may be fitted aft to tow over the stern if required for other work. A typical hull form will be designed with a chamfered stern to give a good free running speed astern while retaining adequate manoeuvrability. A long box keel or skeg will enable the tug to operate in the ‘indirect’ towing mode to achieve dynamic towing forces substantially higher than the vessel’s static bollard pull.
Damen Shipyards has delivered large numbers of ASD tugs from its ‘standard’ product portfolio of seven designs over the past year. The range includes small ‘compact’ shiphandling tugs through to powerful escort tugs intended for work in exposed locations. One of the most popular designs, designated the ASD Tug 2810, has been delivered to a number of clients worldwide, including Smit Harbour Towage which now has over 20 in its fleet. A recent and interesting completion is the Evgeniy Yakovtsev, one of a pair of Ice Class vessels ordered by the State Enterprise Sea, Port of Yuzhny in the Ukraine. Typical of the class, the tug is 28.67m in length with a beam of 10.43m and draught (aft) of 4.95m. Two Caterpillar 3516B TA/B main engines generate a total of 3,132kW (4,200bhp) to power a pair of Rolls Royce US 255 Z-Drive units to achieve a bollard pull of 50.9 tonnes towing ahead and 48.3 tonnes astern. A maximum free running speed of 12.8 knots ahead and 11.9 knots astern was attained on trials in a tug with very good manoeuvrability.
One of most powerful, and quite recent, additions to the Damen range,is the ASD Tug 3213, designed with terminal operation in mind and embodying a number of features from the MARIN Joint Industry Safe Tug project – a study into tugs for operation in exposed locations. Seven tugs have been completed for Smit, Multraship and Lamnalco. Known colloquially as the ‘Big Cat Class’ the 32.14m long tug has a beam of 13.29m and draught of 6.43m aft. In addition to the features mentioned earlier, the hull design incorporates an increased sheer and higher stern, to enable the tug to operate in a swell and remain remarkable dry on deck. Propulsion power is provided by two Caterpillar C280-8/MC diesels developing a total of 7,268bhp at 1,000rpm MCR, coupled to Rolls Royce US 285 Z-Drives, incorporating controllable pitch propellers. On trials the original Smit ‘Big Cats’ produced a bollard pull of 94.7 tonnes ahead and 88.9 tonnes astern, along with a free running speed of approximately 14 knots in either direction.
A large portfolio of ASD tug designs, including the RAmpart and RAstar classes, are marketed by Robert Allan Ltd, naval architects of Vancouver. Detailed design packages are provided for construction to a high standard by large and small shipyards for worldwide operation.
The ‘Z-Tech’ is a unique development intended to incorporate the best handling and operational characteristics of both ASD tug and azimuthing tractor. This increasingly popular design was developed by Robert Allan Ltd, working in conjuction with Hong Kong based Cheoy Lee Shipyards, expressly for PSA Marine of Singapore and is now available worldwide. The result is a distinctive design with an almost uniform performance in either direction and which is claimed to offer significant improvements in performance and safety.
A ‘Z-Tech’ tug is a true 'double-ended' vessel with a unique hull form based on an ASD configuration with a very deep box keel (or skeg) beneath the bow. When ship-handling the heavily fendered bow, with its low mounted towing winch and fairlead, becomes the 'business end' of the vessel operating in much the same way as any other ASD tug. With the wheelhouse located aft of amidships this affords good overhead clearance and very stable towing characteristics. The tug's stern incorporates a distinctive flare, enabling it to operate at sea in the same manner as an azimuthing tractor tug. In this configuration the tug tows from the same winch and has an equally good performance whilst towing or running free in this direction.
The first tug, designated a 'Z-Tech 2800', entered service in 2004. Since that date large numbers have been built for the original client PSA, the Panama Canal Company, the US Navy and others.
Cheoy Lee Shipyards currently has under construction a second batch of 13 ‘Z-Tech’ tugs for the Panama Canal Authority. This latest order is for a more powerful Z-Tech 6500 series measuring 27.40m long overall, with a 11.50m beam and 5.83m draught. Power from two GE 12V228 engines, producing a total of 5,930bhp, drive two Schottel SRP1515FP fully azimuthing propulsion units, with 2.6m diameter four-bladed fixed pitch propellers. This arrangement gives the tugs a bollard pull of 65 tonnes and a maximum speed of 12.5 knots both ahead and astern. Two separate MacGregor Plimsoll single-drum towing winches are installed; one on either side of the towing deck, in a configuration designed specifically to suit the working practices on the Panama Canal.
Three vessels were completed in 2010, with a further nine due by the end of 2011 and one in 2012
The ‘Rotor Tug’
The ‘Rotor tug’ was the ‘brainchild’ of Ton Kooren, president of the company Kotug. This radical design was the result a personal quest for a powerful, versatile, tug capable of ship-handling, coastal towing, and a wide range of other duties. The main requirement was to produce a extremely agile vessel capable of moving and exerting all of its maximum bollard pull in any direction.
‘Rotor Tug’ uses three fully azimuthing propulsion units, with two located forward in normal tractor configuration and one aft replacing the normal skeg. The hull form is of a hard chine design, almost flat bottomed to achieve the best possible water-flow to all three units. An important feature of the ‘Rotor Tug’ is the control system for all three propulsion units. Each propulsion unit has its own manual controller, giving the tugmaster individual control of thrust direction and propeller speed. A fourth 'Masterpilot' controller enables any combination of units to be controlled in unison. A suitable combination of controls is chosen for the operation in hand.
The first four ‘Rotor Tugs’ were introduced early in 1999 in Dutch and German ports served by Kotug fleets. This first batch measured 31.43m in length and each was fitted with three engines producing a total of 6,300bhp, and a bollard pull of 75 tonnes. The agility of the ‘Rotor Tug’ is spectacular, with a high rate of turn, the ability to move rapidly sideways at up to 6 knots, and exert its entire bollard pull in any direction. All four were employed successfully under a wide range of operating conditions, in harbour, during coastal and short sea tows, and even on salvage operations. A particular strength of the ‘Rotor Tug’ is its ability to handle large ship in locks and confined spaces.
Since its inception the ‘Rotor Tug’ concept has gained slow but positive acceptance by other tugowners and at the time of writing over 30 vessels are either in service or under construction. Three basic versions are offered by Kooren Shipping & Trading (KST), a Kotug associate company. A 32m vessel designated RT80-32, has a beam of 12m, a draught of 6.1m and a bollard pull of 80 tonnes is specified, along with a maximum speed of 12.5 knots. Steering forces of over 90 tonnes can be generated towing in the indirect (escort) mode at 10 knots. This design has winches for and aft and is derived from the original prototype and incorporates changes made in the light of a decade of experience.
In all 14 RT 80-32 tugs have been built or are under construction at either the Niigatta Ship Repair yard in Japan or ASL Shipyard in Singapore. RT Rob, RT Peter, RT Adriaan, RT Champion and RT Leader were completed between 2009 & 2010 for use in Europe. RT Eduard, RT Tough, RT Force, RT Darwin and RT Tasman are the most recent deliveries and are now on charter in Australia.
ASL was also responsible for the construction of eight RT80-28 tugs, a shorter, lower profile vessel, equipped principally for shiphandling in port, towing over the bow or stern. Four were completed in 2008, three for Bugsier Reederei of Hamburg, and named Bugsier 4, 5 and 6 and one - RT Margo, for a Kotug associate. A further quartet was ordered by Unterweser Reederei of Bremen. Dorum and Midlum were delivered in 2010 and Accurat and Exact will follow in 2011.
KST has also announced the sale of a ‘design and technical support’ package to Groupe Océan of Quebec for the construction locally of the first Rotor Tug 110-37E. The RT 110-37E is the first of a new generation of high performance ‘Rotor Tugs’ suitable for escort towing and a wide range of other duties, including work offshore. Naval architects and consultants Robert Allan Ltd was commissioned by KST to produce the design and carry out tank testing. A large vessel of 37m in length, with moulded breadth of14.50m and a depth of 5.65m, the new ‘Rotor Tug will have a maximum draught of 7.25m. The hull-form embodies several features found in RAL tractor tug designs, including sponsons, a pronounced sheer and a bulbous bow. Such features will result in an extremely stable, high performance tug capable of 110 tonnes bollard pull, a free running speed of 15 knots - ahead and astern, side stepping at up to 6 knots, and with good seakeeping characteristics.
Z-drive tractor tugs
Tractor tugs employing propulsion units of the azimuthing type, using steerable screw propellers, were first built in Europe in the early 1970s as a possible alternative to the Voith Schneider system, introduced some years earlier. The units are mounted in much the same way, side by side, under the forward part of the hull. Thanks to greatly improved Voith Schneider cycloidal propellers, very few Z-drive tractors are built for the current market. The most notable examples are a series of four tractors constructed by Damen for use in the UK Naval dockyards by contractors Serco Marine Services.
Voith Schneider tractor
The Voith Schneider propeller (VSP), with its vertical blades, is still alive and well. Following an extensive product improvement programme by Voith Turbo Marine the units are now far more efficient than the early versions in terms of tonnes bollard pull/hp and offer precise control of thrust force and direction. Capital cost may be an issue, but the VSP is frequently the preferred propulsion system for agile tractors for use in confined dock systems, and many highly sophisticated escort tugs and similar towage applications where precise control is paramount.
Notable recent examples are a pair of rugged, powerful, tanker handling and escort tug built for the Shetland Islands Council by the Union Naval Valencia (UNV) shipyard in Spain in a deal valued at £14 million. Named Bonxie and Solan the tugs are used to handle tankers at the exposed Sullom Voe oil terminal. UNV was responsible for the design with input from the owner, Sener Engineering and Systems, and Voith Turbo Marine.
The identical sisters measure 40.0m in length overall, with a moulded breadth of 14.0m and maximum depth of 6.4m. The hull and superstructure design incorporates a forecastle, one complete deck in height, and a deck-house positioned well away from the bow for maximum protection in heavy weather.
Two Voith Schneider 36R5/250-2 cycloidal propulsion units, with a blade orbit of 3.6m, are installed along with an electronic, ‘fly by wire’ control system. A pair of MAN 9L27/38 diesels provide the power, each delivering 3,285kW at 800rpm at MCR, a total of approximately 8,810bhp. This propulsion system has given the tugs a bollard pull at MCR of 87 tonnes towing ahead and 81 tonnes astern (pushing) and a free running speed ahead of 14.1 knots.
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