Innovative ro-ro bolsters Australian short-sea trade

13 Dec 2016
The LNG-fuelled 'Searoad Mersey II', built to support future growth of the Tasmanian economy.

The LNG-fuelled 'Searoad Mersey II', built to support future growth of the Tasmanian economy.

SeaRoad Shipping’s purpose-designed, gas-fuelled ro-ro signals a step change in capacity, efficiency, and technology applied to the intensely competitive Bass Strait services. David Tinsley reports.

Central to an investment of more than A$110m (US$81m) in the traffic between Tasmania and the Australian mainland, the ro-ro freight carrier Searoad Mersey II is both the country’s first ship to use LNG propulsion technology and the world’s first dry cargo vessel to employ a ro-ro solution to LNG fuel supply.

While tailored to the requirements of Tasmania-based logistics firm SeaRoad Shipping, and to the multifarious nature of the Bass Strait trade and its specific sea conditions, the new fleet addition bears distinctive hallmarks of her builder, Flensburger Schiffbau-Gesellschaft. The sleek profile, with superstructure about two-thirds aft, single funnel on the port side, and covered forward upper deck, are characteristics of the German yard’s prolific output of ro-ro trailerships.

However, the exterior similarities belie the individual nature of the design, which breaks new ground for both the shipowner and the market sector through its particular combination of energy efficiency, cargo mix, seakeeping, and environmental engineering features. The project is further testament to Flensburger’s propensity for product added-value. It is that capability, imbued by a strategy of commitment to a continuous ploughback of earnings into R&D and maintenance of a large in-house technical design department, which has underpinned the yard’s global competitiveness in core fields.         

Searoad Mersey II denotes the start of a new era on the Bass Strait. She offers a cargo intake corresponding to approximately 2,000 lane metres, nearly 50% in excess of the company’s current vessels, and a significantly higher speed of 20.5 knots, together with enhanced efficiency and maximised use of more environmentally-acceptable LNG fuel. Payload capacity is not simply much greater than hitherto, but also far-reaching as regards the range and permutations of cargo that can be handled. The ship’s speed, four and a half knots in excess of the 1991-built Searoad Mersey and Searoad Tamar, allows faster transit times and the potential for later cargo acceptance and earlier discharge.

COMPETITIVE MARKET

Scheduled to make her regular service debut in December, the vessel will become the mainstay of the route linking Melbourne with the north Tasmanian harbour of Devonport. The Bass Strait is a fiercely competitive market, with two other overnight service providers and other weekly and monthly alternatives for shippers. Commercial rivalry ensures stable freight rates, and accentuates the need for operational dependability and reliability on a stretch of water that is often inclement.

Flensburger’s expertise in the hydrodynamic development of hull lines has had a signal bearing on the anticipated seakeeping qualities of the SeaRoad ro-ro, to better ensure schedules throughout the year in the face of harsh conditions on the Strait, while also contributing to lower fuel consumption. It is reported that the values have been verified by tank tests in Denmark during the project phase in addition to manoeuvring simulations conducted in Europe and Australia. A specially designed flume stabilisation tank reduces rolling to a minimum.  

The main propulsion plant comprises two MaK 8M46DF dual-fuel engines, each with a power output of 7,200kW, driving Schottel controllable pitch propellers through Siemens GCH 1000 reduction gearboxes. Two shaft generators of 1,500kVA ensure that the ship’s electrical load is covered while under way, complemented by a pair of 3,100kVA gensets, also driven by dual-fuel engines in the shape of six-cylinder MaK M34DF models. The shipset of MaK engines was manufactured at Caterpillar Motoren’s Warnemuende factory in Germany.

Although the main and auxiliary machinery affords the flexibility for operating on diesel fuel, SeaRoad’s intention is to run almost exclusively on LNG. The rationale behind the choice of plant is grounded in both long-term economic and environmental considerations. The ship’s comparatively high, overall electrical power generating capacity is in part a reflection of the cooling demands of the reefer freight, and of the heightened load during the very hot summer months in southern Australian latitudes. The genset installation has been designed and sized to afford the ship 100% electrical power redundancy, to better protect schedules and the high value freight carried.

LNG’s much-vaunted cleaner-burning properties suit a more ecologically-responsible culture while affording an alternative to installing exhaust scrubbers or using costly, ultra low sulphur distillate in the face of future, tougher environmental legislation. At the same time, the ship’s powering arrangements enable SeaRoad to draw on domestic LNG, obtainable locally through multiple sources, rather than be dependent on imported heavy fuel oil. In the event of unavailability of LNG as primary fuel, the vessel can switch to marine diesel fuel at any time.

CHANGING OPERATING CONDITIONS

SeaRoad’s eye to the future looked both to the stability of supply, given Australia’s abundant reserves of natural gas, and to what it viewed as the strong likelihood of Australia introducing legislation at some stage that would effectively preclude use of heavy fuel oil in coastal waters. The need to re-tonnage presented a timely opportunity to invest in the most advanced, environmentally-responsible technologies suited to the type of vessel.

Searoad Mersey II incorporates a novel method for loading LNG bunkers. As there is no immediate infrastructure for refuelling with LNG at the ports of call, and as SeaRoad operates LNG-fuelled trucks as part of its Australian logistics network, LNG will be delivered by loading LNG road tankers on to the ship during normal cargo handling operations. The tank trailers will then remain aboard, parked and locked in designated bays on the aft weatherdeck, and connected to a permanent fuel manifold, thereby serving as the vessel’s LNG tank storage. Each tank trailer is secured using six twistlocks.

For a complete round-trip between Melbourne and Devonport, three LNG road trailers will be required to meet main and auxiliary dual-fuel engine consumption. While the vessel is at sea, a spare set of trailers will be filled at the local LNG plant and will be waiting at the terminal in Melbourne ready for changeover after every round-voyage. The system makes for a comparatively simple alternative to conventional LNG fuel bunkering, and also dispenses with the need for the ship to embody integral or deck-mounted tanks. The locking system for the LNG trailers was developed by the shipyard, which has built a strengthened slab into the holding area so as to withstand cryogenic cracking in the event of LNG leakage.  

Flexible, cryogenic pipes connect the trailer tanks to a manifold that feeds the LNG to a deck-mounted gas-handling room. There, waste heat from the engines is used to change the liquefied gas back into gaseous form in specially designed heat exchangers. The cryogenic pipes are of double-wall, stainless steel construction to cope with the extremely low temperatures. The piping carrying the gas from the heat exchangers is simpler, but still requires ducts that are vented and monitored for potential leaks. In the machinery spaces, the gas is supplied to the engines by way of gas valve units, which regulate the gas pressure in accordance with power demand.

DNV GL provided full classification services, including interpreting and progressing the LNG fuel handling concept in the light of prevailing international and Australian rules and regulations. All the parties involved conducted an extensive risk analysis, given the pioneering status of the LNG-powered vessel in the context of Australian-flag shipping, and with regard to the hitherto untried LNG fuel handling arrangements.

MANOEUVRING DEMANDS

Restrictions on the Tasmanian side necessitate a high degree of manoeuvrability, answered by the adoption of bow and stern Brunvoll tunnel thrusters and the responsive, variable pitch main propellers. Furthermore, the two Flensburger-designed twisted-flow rudders enhance manoeuvring performance as well as reducing drag during normal sailing.

SeaRoad’s liner service, entailing six sailings per week in each direction, plays a vital infrastructural role, and the new ship has been configured to efficiently cater for payloads of multifarious composition. The required cargo scope and mix has driven design planning, allowing for double-stacked cassettes with containers, standard road trailers, reefer units, hazardous freight, project shipments, cars and livestock, brought aboard over two stern axial ramps and distributed over three fixed decks and a hoistable car deck.

The portside stern ramp is 4.9m wide and provides direct access to the weatherdeck, while the 10m-wide starboard ramp serves movements to and from the main cargo hold, allowing for two-way flows. The tank top, lower hold is reached from the main deck by way of a fixed ramp of 4.1m width. Hydraulically-actuated watertight ramp covers are interposed between the main deck and tank top levels. The comprehensive, tailor-made ro-ro equipment package was supplied by Cargotec under the MacGregor brand.  

The weatherdeck forward of the superstructure is covered, and fitted with gas detectors and special drainage and sprinkler systems, enabling the combined transport of reefer and dangerous goods trailers. In fact, Searoad Mersey II is claimed to be the first ro-ro ferry worldwide equipped and certificated to carry reefers and hazardous cargo juxtaposed.  An area on the weatherdeck below and aft of the superstructure has been designated for livestock transport.

Total investment in the new ship, plus its unique fuelling solution and infrastructure, berth improvements and additional cargo handling equipment amounts to over A$110m. SeaRoad has indicated that it will consider similar LNG bunkering arrangements for its next newbuild. It believes that the basic principles employed by Searoad Mersey II will contribute to the future development of clean, reliable short-haul shipping.

PRINCIPAL PARTICULARS—Searoad Mersey II

Length overall

181.52m

Length b.p.

171.46m

Breadth, moulded

26.60m

Depth

17.60m

Draught, maximum

6.32m

Draught, design

5.90m

Deadweight

7,920t

Gross tonnage

25,409t

Freight capacity

1,960 lane-m

Trailers/or cassettes/or containers

80/70/455TEU

Reefer points

150

Cars

110

Main engine power

2 x 7,200kW (dual-fuel)

Main diesel generators

2 x 2,500kW (dual-fuel)

Speed

20.5 knots

Class

DNV GL

Class notations

+1A1 General cargo ship RO/RO, BIS, DG (P), E0, Gas fuelled, NAYT (AW), TMON

Flag/registry

Australia/Devonport