First of an innovative ro-ro quartet from FSG

Tom Todd writes: She is of an innovative design developed specifically for a series of four ships being delivered up to next June. Among other things the new concept embodies the shipyard’s own new stability standard ISEI (insufficient stability event index), with the help of which the sea-going behaviour of these ships could be optimally adapted to the special conditions found in the Irish Sea.

Seatruck Progress is the biggest and most efficient ro-ro cargo ship capable of operating into and out of the UK port of Heysham. By mid-2012, three further ships will enter service on the route between Great Britain and Ireland.

Seatruck Ferries is part of the Bahamas-based Clipper Group, and operates four services across the Irish Sea. From Heysham, Warrenpoint, Larne and Dublin are served while Dublin is also served from Liverpool.

Planning and Design

The challenge with this type of ship was optimal combination of existing and fixed length and draught restrictions with maximum cargo capacity. The ship’s length of 142m and draught of 5.2m were predetermined by local conditions in the port of Heysham. FSG was able to realise a 20% greater cargo volume and at the same time a fuel saving of 10% compared with the ships regarded until now as the biggest capable of serving Heysham.

The 3,830dwt 25m wide Seatruck Progress was designed as a ro-ro freight ferry with four loading decks. Trucks, trailers and ro-ro cassettes can be stowed on a total of 2,166 lane metres. The loading bays are 5m high on the tank top and 5.3m high on the main and upper decks. A layer of high-cube containers on ro-ro cassettes can be stowed on the entire main deck. All lanes are 3m wide and equipped with the Speed Lash system. A range of hazardous cargo can also be transported, particularly on the uppermost deck.

The ferry has been built to DNV classification standards and is classed DNV + 1A1, General Cargo Carrier RoRo, E0, TMON, DG-P. Finite element analysis and the targeted use of high-strength steels have contributed greatly to minimising weight. By means of operational strength analyses at an early stage along with global and local studies of vibration, decks and bulwarks could be further optimised. As a result, this ship type has the best possible speed-power performance ratio.

Cargo loading

The 17.6m wide stern ramp offers direct access not only to the main deck but also to the upper deck. The access to the main deck has been made wide enough to allow simultaneous loading and unloading at any time. From the main deck the tank top can be accessed via a 4.4m wide fixed ramp which can be sealed water-tight. The upper deck is reached using the port side section of the stern ramp via a 6.5m wide fixed ramp. From here a 4.4m wide ramp leads to the weather deck. All the ramps have 7° gradients.

Both closed loading bays (main deck and tank top) have artificial ventilation. During loading and unloading operation the fans change the air up to 20 times – 10 times during sea operation. The upper deck is ventilated naturally via generously sized openings in the external hull.

Engineering Equipment

Main propulsion is provided by two MAN 7L48/60-CR medium speed Diesels, each with an output of 8,000kW operating through gears on to four-bladed high-skew variable pitch propellers. The propellers have a diameter of 4.3 m and are designed for 150rpm. Provision is made on the trailing edge of the propeller hub for a cone-shaped shaft deflection cap. In conjunction with a Costa propulsion bulb which has been optimised hydrodynamically, this cap increases the propulsion efficiency of the ship and at the same time helps forestall cavitation damage to the rudder.

Using this propulsion configuration, the ships attain a service speed of 21.0 knots (90% MCR, 10% sea margin, 2% gear losses) on a draught of 5.2m. Fuel consumption is 63.59t/day. Two Diesel gensets each with an engine/generator output of 840kW/1000kVA come into play as auxiliaries. In addition, the necessary power requirements can be produced by two wave generators each of 1,500kVA. The emergency Diesel plant can, when required, feed 400kVA into the on-board grid. An exhaust boiler, backed up by an oil-fired boiler, is installed to utilise heat generated by main engine exhaust gases. The maneuverability of the twin screw vessel is achieved by using a twist-flow rudder (of FSG design) turned by a rotary vane steering gear from Rolls Royce. In addition, two Wärtsilä transverse thrusters fitted with controllable pitch propellers can be used during manoeuvering. The two bow thrusters each provide 1,000kW. The entire machinery plant is monitored by more than 700 sensors and control elements, all part of the MCS 2200 ship automation system developed by SAM Electronics and which can be operated as required from either the bridge or from the ECR.

Bridge and Deck House

The deck house is on three levels with the integrated bridge forming the uppermost level. Special attention was paid here to creating all-round vision and ergonomic styling suitable for one-man watch operation. Bridge extension arms, for example, are fully covered, glazed to floor level and equipped with extension consoles complete with screens for radar and ECDIS. During manoeuvering operations from the bridge extension, the floor level windows permit an unrestricted view of the ship’s sides. Extension arm consoles are arranged in a central hub which includes not only radar, ECDIS and autopilot, but also all controls and displays. With ergonomic considerations in mind, all sectors including navigation/sea space monitoring, communications, safety and route planning are accessible at all times.

Directly below the bridge is an accommodation deck, with cabins for the two senior officers and five further officers, plus 10 cabins for crew members. The ship’s office and the converter room are located midships. On the lower deck’s forward area are the mess facilities and common rooms.

On the starboard side, separated from the other areas, are four more crew cabins and the ship’s hospital as well as six double-bed cabins for drivers, a mess and a common room.

Hub of the area is the kitchen with provisions stores, an air-conditioned room and numerous stores and operations rooms. The ship accommodates 34 people, all of whom can be carried by the free-fall lifeboat located in the stern.

Innovative Ship Type

Seatruck Progress and the following ro-roships with hull numbers 747, 751 and 752 are based on FSG’s innovative concept nd tailor-made for the specific route to be served. One of the crucial pre-conditions of the ships’ design was access in the Port of Heysham on the Irish Sea. Restrictions apply in this port not only with regard to ship length but also draught. The challenge for FSG was to significantly increase ship transport capacity despite these port restrictions. Increasing lane metres for trucks by nearly 20% compared to similar ships was the result of extensive hydro-dynamic, ship-theory and relevant safety aspect studies.

Stability standard ISEI

In order to achieve the required performance parameters, CFD and simulation technology were used in the design process, based on the premise that optimisation of often contradictory design demands becomes possible when different methods are closely integrated. For example, resistance optimisation alone to achieve required shallow water resistance, quickly reaches its limit because of the very high demands placed on stability afloat.

In a particularly challenging region like the Irish Sea, where heavy seas can be expected regularly, hull forms must be tested not only statically but also dynamically for possible failures such as capsizing.

In the case of the 746 ship series FSG’s stability standard ISEI, determined from simulated sea conditions, was utilised for the first time. This developed a hull which claims significantly better behaviour in heavy seas than conventional designs and which at the same time demonstrates considerably better shallow water resistance characteristics over similar ships.

Also new is the design of the shaft bossings. With the help of CFD, a completely new shape could be developed here which demonstrated less resistance and at the same time significantly improved the water flow to the propeller.

FSG’s stability standard index has been developed in co-operation with the Hamburg University of Technology, and is based on direct calculation of ship response to irregular, short-crested waves. With the help of the Blume Criterion a maximum wave height can be determined above which limit the ship can generally be regarded as unsafe. The boundary values used in the Index were obtained from accident investigations.

Economic efficiency

The design aims for superior efficiency compared to similar ships. At the same service speed and load capacity increased by 20%, fuel consumption of the FSG design is 10% lower. FSG also expects lower compensation costs for cargo damage, because the sea-keeping has been optimised to conditions in the Irish Sea.

The risk of erosive cavitation damage to the rudder on the FSG 746 series has been significantly reduced by optimising the propeller hub and rudder bulb. By modeling the hub and bulb into a single water flow unit (with as small a gap as possible) the hub vortex is eliminated. The configuration of propeller hub and rudder is individually optimised for each FSG ship type, taking into account the operational profile of the vessel, and propulsion efficiency is optimised in all relevant operational modes (such as speed and rudder angle).

Model tests in the hydrodynamic cavitation tunnel (HyKaT) at the Hamburg Ship Model Basin (HSVA) have confirmed the results.

PTO optimisation

Compared to fast-running auxiliary Diesels, shaft generators can produce cheaper electricity and save fuel by using the mechanical energy of the main engine system. This benefit is normally limited because the complete propulsion system has to be operated at full revolutions, even if only part load is required at the propeller, which causes loss of efficiency. Additionally, operating a propeller at high rpm and low pitch increases cavitation risk.

In the FSG design, the arrangement of the gearing and PTO generators permits PTO operation at reduced drive system revolutions. As a result, overall ship propulsion efficiency increases during part-load, and cavitation on the propeller blades is reduced.

The gear ratio is chosen so that the PTO generator could generate the required power for the on-board network when operating at 95% of rated rpm. With lower revolutions during constant speed mode, the overall efficiency of both propulsion and power generation for part-load operation is significantly increased. During operation at near full-load, the PTO generator is switched off to avoid excessive revolutions, but this operational situation is seldom required, and then only for short periods. Lower main engine plant revolutions during constant speed mode is marked by a noticeable reduction in fuel costs and by improved part-load propulsion efficiency because of the use of the PTO generator.

Low-load operation

The planned itinerary of the ship demands sailing regularly and slowly over relatively long distances. Using only one of the two propulsion lines in this situation provides overall efficiency gains. An innovative package of measures was developed for both the drive and shaft plant and for the propeller control sector to make operations as wear-free and efficient as possible.

During ship operation when only one engine system is used, several measures are employed to minimise resistance and thus reduce fuel consumption and wear and tear and avoid damage to the unused engine system.

A switch coupling installed in the gearbox separates the propeller line and the drive motor. In this way it is possible to allow the unused propeller to turn at blade pitch in the wake field of the slowly moving ship, which causes lower resistance than a blocked propeller. The (relatively low) additional resistance of the uncoupled engine system is offset by better fuel consumption.

Operated in this way the propeller is driven by the wake field of the ship as a turbine. To avoid wear on the bearings, a minimum speed is required to enable hydro-dynamic lubrication of the bearings. By ensuring that the propeller turns fast enough so that the lube oil pump driven by the gearbox can lubricate the bearings, this alone saves around 20kW of power needed to run electric pump drives.

The rpm of the propeller depends on the relationship between the regulated propeller pitch and the water inflow speed. A remote control device specially developed to regulate the propeller on the switched-off engine line uses propeller pitch to ensure that propeller speed is held within acceptable levels. This level is just above the minimum rpm to minimise additional resistance. As long as ship speed is sufficient, this control can prevent damage through overspeed, while maintaining minimum revs on the towed drive system. If the rpm of the towed system drops below the minimum, an automatic device ensures that the propeller pitch reduces to zero at the same time and that the plant is stopped by a hydraulic braking system.

The ability to use only one of the two drive systems contributes to cutting operating costs, reducing wear and increasing the value of the ship.

Principal particulars: Seatruck Progress

Loa  142.00m

Draught  5.20m

Trailer lane metres 2,166m

Deadweight   3,830dwt

Gross tonnage 18,920gt

Propulsion 2 x MAN 7L48/60-CR

Service speed 21 knots