Tug designers meet the special demands of LNG shipping
The LNG transportation market puts new demands on the tug and towage industry; Jack Gaston reports on how tug design is evolving to meet these specialist needs.
Tug owners, designers and specialist shipbuilders have, throughout the history of the towage industry, met a wide variety of challenges to ensure the vessels they operate or construct are fit for purpose. Tug operators in the shiphandling business in particular are forced to meet the demands of their clients when new generations of ships enter service and require special services. One of the most recent challenges stems from the massive growth in the trade and shipment of Liquefied Natural Gas (LNG) between exporting and importing facilities.
The handling of LNG carriers in coastal waters and the ports they frequent is a sensitive subject due to the safety implications inherent in that particular cargo. Towage requirements generally require a tug escort between the seaward limits of the port and the loading or discharging terminal. Very specific shiphandling and mooring procedures are required, using tugs of a predetermined bollard pull and handling characteristics. Attendant tugs must generally be equipped for fire fighting to at least FiFi standard class notation. Whilst the ship is on the berth tugs may be required to standby, within visual range, and if necessary be prepared to monitor and enforce a predetermined ‘exclusion zone’.
The ships themselves, although not unduly large by present day standards, are often, high, slab-sided and have there particular handling characteristics. Detailed ‘passage plans’ are required by most port authorities and computer simulation often is used to validate the berthing procedures and train both tugmasters and pilots.
Tugs performing escort and shiphandling duties at LNG terminals are invariably built to a very high specification regarding performance, reliability, and fire fighting capability. Bollard pull figures of between 65 and 95 tonnes are common. In the case of escort tugs the towline forces exerted while operating in the indirect towing mode are frequently twice that figure when working with a ship underway at 10 knots.
A designated escort tug may be required to cover a route that involves long distances at sea, in areas with predominantly poor weather conditions for much of the year. Many new LNG terminals are located in remote and difficult locations, and - increasingly - offshore. Good seakeeping characteristics, inherent stability, and a high free running speed are also necessary to deal with the rendezvous, and making a towing connection, with modern tankers and LNG carriers.
Among the many recent examples of tugs designed with LNG traffic as a first priority, the RAstar azimuthing stern drive (ASD) class spawned in the design office of Robert Allan Ltd (RAL), naval architects of Vancouver, BC, is proving to be a popular choice.
A batch of six RAstar class tugs entered service in the Port of Milford Haven, UK, during 2009 forming part of the most powerful tanker handling fleet in the world. Multinational towage operator Svitzer is responsible for supplying efficient towage, escort and marine services, to new terminals, operated by South Hook LNG at Milford and Dragon LNG upriver at Waterston, in addition to the existing oil berths. The new importation and re-gasification facilities have a combined capacity to supply up to 25% of the UK's gas requirement. Svitzer has built nine new tugs and six line handling boats specifically for the new fleet, working from its base at Pembroke Dock.
Six of the tugs are high performance RAstar class terminal/escort tugs built by Freire Construcciones Navales of Vigo, Spain. The six vessels were built to two distinct RAstar designs, five 34m RAstar 3400 Class and one 39m RAstar 3900.
The RAstar class represents a truly unique and significant development in offshore tug design. The hull form incorporates a significant outward flare (or sponson) on the upper hull sides; a feature developed by RAL and unique to tugs designed by the company. When the tug is heeled over under influence of the towline during an escort operation, the ‘weather’ sponson is submerged and a large righting force is generated to improve the stability, thus increasing the towline force. In addition, the hull has a large foil-shaped skeg, also designed to provide increased indirect towing forces. This design approach is claimed to be far superior to simply making a tug ‘fatter’ on the waterline, and results in better fuel economy in all aspects of operation.
The hull form also provides dramatic reductions in roll amplitude and roll accelerations compared to old-style, wall-sided hull forms, and thus provides a much safer and more comfortable platform for the crew. This aspect of performance was one of the stated objectives of the international SAFETUG joint industry project carried out in The Netherlands by the MARIN Research Institute to study the design and operation of tugs in exposed locations. The study was carried out with the involvement of participants from the petrochemical industry, shipowners, tug owners, designers and specialist shipbuilders. RAL is an active participant in SAFETUG and the RAstar hull form demonstrated a performance superior to any other concept evaluated during the project. Extensive private model tests indicate that the roll accelerations of a RAstar hull are some 60% lower than in a typical hull form of the same overall dimensions and displacement.
All five RAstar 3400 tugs have an overall length of 34m, a moulded beam of 14.50m and a maximum operating draught of 6.55m. The tugs have a raised foredeck, a very large, clear, afterdeck and excellent fendering, particularly on the bow and forward shoulders where protection is required during push-pull operations.
Svitzer Lindsway, Svitzer Waterston and Svitzer Haven have two GE 7FDM 16 main engines, generating a total of 7,880 bhp (2 x 2,900kW) at 1,000 rev/min (MCR). Power is transmitted to Schottel SRP 3030 CP fully steerable propulsion units incorporating controllable pitch propellers. This installation gives the tugs a bollard pull of well over 100 tonnes pulling ahead and an almost identical figure pulling astern (exceeding the contractural 92 tonnes by about 10%), with a maximum free running speed of 13.7 knots at 85% MCR.
Svitzer Caldey and Svitzer Ramsey each have a Niigata propulsion package, including main engines developing a total of 5,978 bhp (2 x 2,200kW) driving a pair of ‘Z-Peller’ propulsion units with fixed pitch propellers, via Omega slipping clutches. The result is a bollard pull of 82 tonnes and a free running speed of 13.5 knots.
The largest tug of the entire fleet is the Svitzer Kilroom measuring 39.10m in length overall, with a moulded beam of 14.70m and a maximum operating draught of 6.55m. This RAstar 3900 variant is also powered by two General Electric GE 7FDM 16 diesels but each rated at the higher figure of 3,050 kW at 1,050 rpm at 100% MCR (total approximately 8,288 bhp). The tug has the same Schottel SRP 3030 CP propulsion units as the Svitzer Haven, Svitzer Lindsway and Svitzer Waterston. During trials Svitzer Kilroom delivered a performance beyond all expectations achieving a maximum bollard pull ahead of 117 tonnes and113 tonnes at MCR and a maximum bollard pull astern of 113 and 107 continuous. Speed trials revealed a maximum performance ahead of 15.7 knots.
All six RAstar class tugs are fitted with a Rolls-Royce Marine towing winch, combined with an anchor windlass located on the fore deck. A single drum carries 250m of 76mm diameter high performance fibre towline. The three-speed winch incorporates the render-and-recover facilities, essential for escort work, and can be controlled from the wheelhouse or on deck. A touch-screen display enables towline tension and length to be preset by the tugmaster. Towing bitts and a quick release tow hook are fitted on the afterdeck but no winch.
A fire-fighting outfit, meeting the FiFi 1 standard, is installed on all six tugs. A pair of main-engine driven pumps, each rated at 1,400 m3/h, supply a pair of Kvaerner remotely controlled monitors and a self-protection waterspray system.
Svitzer Lindsway, Svitzer Waterston, Svitzer Haven and Svitzer Kilroom are all capable of carrying out full escort duties at Milford Haven. The operation involves meeting incoming vessels at sea and providing a ‘tethered’ escort through the complex approach channel and on to berth in the Haven.
A seventh RAstar 3900 class tug, also ordered from the Freire yard, was delivered to Svitzer in August of this year. Named Svitzer Pembroke and almost identical to the Svitzer Kilroom, this vessel will be used in various Svitzer fleets as required.
Two new tugs designed by RAL for the Boston Towing and Transportation Company in the USA, are faced with quite a different operation. The pair’s primary role is to berth and stand-by LNG carriers using an offshore mooring buoy, located in a fully exposed deep-water location some 15-20 miles offshore at the Neptune LNG LLC, Deep Water Port, in Massachusetts Bay.
The tugs are of two quite different designs. Independence, the larger of the pair was built by Derecktor Shipyards in Bridgeport, Connecticut, to the Robert Allan ASD 39/70 design. This ASD designation describes a design comparable to the RAstar Class but with certain unique features. The tug is configured for harbour towage, shiphandling, fire-fighting, coastal towing, salvage and rescue operations.
Principal dimensions of the ASD 39/70 are; 39.10m in length overall, with a beam (moulded) of 13.50m, and a maximum draft of 5.75m. The design features a long, high, forecastle to provide a very high standard of accommodation, the ability to work in quite extreme sea-states, and the best possible sea-keeping capability for operation in very exposed locations.
The RAL ‘sponsoned’ hull form is used for lower roll amplitudes and roll accelerations, as well as safety and crew comfort. By current standards the power installed is rather small for a tug of this size but was dictated by demands to provide a high standard of comfort and sea-worthiness for the crews who will be on station in fully exposed North Atlantic sea conditions for up to two weeks at a time.
Independence has a main propulsion system comprising two MTU 16V-4000 diesel engines, each rated 2,000 kW at 1,800 rpm (total approximately 5,436bhp), driving Rolls-Royce US 255 CP azimuthing propulsion units. Controllable pitch propellers incorporated in these units are said to provide enhanced fuel economy, operational flexibility, and enable efficient operation of the main engine-driven fire-fighting system. This arrangement delivers a minimum bollard pull of 67 tonnes, and a free running speed of13.5 knots.
For shiphandling, the forward towing winch is a single drum, Markey EEPC-52 model, with a brake capacity of 264 short tons carrying 750ft of 10in circumference synthetic towline. A JonRie model 512 winch is located aft for towing astern and is equipped with a 2,500ft (760m) 2.25in steel wire rope.
The second, smaller, tug delivered to Boston Towing and Transportation, is the Justice constructed by J M Martinac Shipbuilders of Tacoma, WA. This vessel is a variant of the established Robert Allan RAmparts 3000 class tug, engaged primarily in a wide range of harbour and coastal duties, but serving as a back-up for the larger Independence in the key role of standing by tankers using the offshore mooring buoy.
Robert Allan Ltd has also been responsible for many other RAstar and RAmparts design tugs for use at LNG facilities at many locations including the Far and Middle East, Northern Europe and Australasia. These include escorting and berthing ships at terminals and a variety of offshore buoy moorings and similar facilities.
Smit Harbour Towage has recently taken delivery of four 32m ASD Tug 3211 terminal tugs, from Damen Shipyards, Gorinchem, designed very much with LNG operations in exposed locations in mind. Named Smit Panther, Smit Jaguar, Smit Cheetah and Smit Tiger, the quartet was constructed at the Song Cam shipyard, in Hai Phong, Viet Nam to meet the class requirements for Lloyd’s 100A1 Escort Tug +LMC, UMS, Fire Fighting Ship 1 with waterspray.
The tugs incorporate many established features common to the Damen ASD product range but are designed for exceptional, power, strength and stability. Measuring 32.14m in length, 13.29m beam, with a draught (aft) of 6.00m the hull form incorporates increased volume forward, a raised forecastle, deep box keel and a pronounced sheer aft, increasing freeboard at the open stern. The additional freeboard aft enhances the vessel’s performance running astern and reduces the ingress of water on deck. Fendering to a high standard enable the tug to operate effectively in the ‘push-pull’ mode without fear of damage to ship or tug in exposed locations. Smit and Damen also participate to the SAFETUG project and the ASD 3213 design has benefited from that association.
Smit Panther and its sisters are powered by pair of Caterpillar C280-8/MC main engines developing a total of 7,268 bhp (5,420 bkW) at 1,000 rev/min. The engines comply with IMO Tier 2 emission regulations and flexible mountings and couplings, and heavily insulated exhaust silencers minimise noise and vibration. Power is transmitted via cardan shafts to Rolls Royce US 285 fully azimuthing propulsion units incorporating 3000mm diameter controllable pitch propellers.
During trials in Viet Nam the tug achieved a bollard pull towing ahead of 94.7 tonnes and 88.9 tonnes pulling astern, along with free running speeds of 14.3 knots ahead and 14 knots astern.
Rolls-Royce Marine towing winches, fore and aft are configured for escort work and offshore operation. The forward winch has maximum brake holding capacity of 320 tonnes and full ‘render and recovery’ capability.
The external fire-fighting system comprises; two remotely operated Alco short barrel monitors, each capable of delivering 1,200m3/h of water/foam, self protection spray nozzles and hose connections on deck. Both monitors are mounted on the forward superstructure at bridge deck level.
On its delivery voyage to Europe Smit Panther was diverted to provide escort and berthing duties with the Smit-Adriatic-Panfido joint venture fleet serving the unique Adriatic LNG gas terminal in Italy. The terminal, located nine miles off the coast of Porto Levante, is the first offshore gravity-based structure to be used for unloading, storage and regasification of LNG. This was exactly the type of work the Damen ASD 3213 (the Smit ‘Big Cat’ class) was designed for.
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