Caterpillar has produced its first hydro-mechanical hybrid propulsion and MAN introduced its integrated system with batteries to a Robert Allan design
Original engine manufacturers have created integrated propulsion with auxiliary drives for harbour tugs designed by Robert Allan.
Caterpillar developed a hydro-mechanical hybrid propulsion system for a new generation of tugs Sanmar is building for harbour operations. The first tug with a Cat advanced variable drive (AVD) was installed on Boğaçay XXXVIII, which entered Sanmar’s fleet supporting ships in Izmit Bay, Turkey, at the beginning of this year.
This tug was built to a Robert Allan RAmparts 2400-SX design and ABS class. The Caterpillar integrated propulsion system includes main and auxiliary engines and thrusters. Including the hydro-mechanical hybrid component and auxiliary engine allowed Sanmar to install smaller main engines on Boğaçay XXXVIII without impacting available power and performance. This tug has a pair of 12-cylinder Cat 3512 main engines, instead of Cat 3516 engines, which would normally be used in a tug of this power. A Cat 3512 develops 1,765 kW of power at 1,800 rpm.
AVD uses this power and can accept 870 kW from two 435-kW hydraulic motors powered by a single CAT C32 auxiliary engine, rated 1,081 kW at 2,100 rpm.
This power is delivered to two CAT MTA 627 Z-drive units with 2.7 m diameter fixed pitch propellers. These generate up to 70 tonnes of bollard pull for Boğaçay XXXVIII. The C32 engine also drives the tug’s fire-fighting pump.
Caterpillar Marine product definition engineer Nathan Kelly says the AVD enables propeller speed to be independent of engine speed. “So optimal engine efficiency can be achieved leading to fuel savings of 15-20%,” he explains.
“Basically, all the benefits of a variable speed diesel-electric propulsion (DEP) system at a fraction of the cost and size.”
Cat AVD consists of a pair of dual-input and continuously variable transmissions, located in the shaftlines between the main engines and the Z-drives. They can modulate propeller speed to zero revolutions, similar to a slipping clutch.
They can also spin the propeller faster than would be possible if the engine was directly coupled to the drive. This enables engines to operate in their peak efficiency zone instead of operating along the propeller demand curve at higher specific fuel oil consumption, reducing fuel costs.
With a fixed-pitch propeller pitched for bollard condition at zero knots, this feature enables full power to be taken from the engine when free running, much like a controlled-pitch or DEP system.
Mr Kelly says other advantages include improved response and acceleration and lower overall maintenance costs due to reduced operating hours on the main engines.
There are four operating modes: Mode 1 is transit, covering 78% of tug operating time. In this mode, the auxiliary engine transmits power through the AVD to the Z-drives.
Mode 2 covers 19% of the tug’s work time, with main engines and auxiliary on and loads variably controlled by power demand. Mode 3 is a boost for maximum bollard pull with both main and auxiliary engines at full power. When Boğaçay XXXVIII is fire fighting, mode 4 is required with propulsion from the main engines and the auxiliary powering the FiFi pump. MAN Energy Solutions has developed an integrated hybrid propulsion system for a harbour tug designed by Robert Allan. This combines MAN’s new high-speed diesel engine with an energy storage system, electric motors and azimuthing thrusters.
This combination was added to a RAstar 3000H tug design, says MAN sales manager for tug and workboats Mark Watson.
“Our MAN 175D engines come with a selective catalytic reduction (SCR) system for IMO Tier III requirements,” says Mr Watson.
“We also include thrusters that are optimised for these engines and have electric motors,” he explains. “We include auxiliary generator sets, variable speed drives, a hybrid control and batteries in an energy storage system.”
Robert Allan’s 30-m harbour tug has 80 tonnes of bollard pull. With MAN’s hybrid system this is achieved with lower rated main engines. “With hybrid we can reduce the size of the engines from 16 cylinder to 12 cylinders, creating more space in the engineroom,” says Mr Watson. “The batteries are located where we can control the temperature to maximise efficiency and lifetime of the energy storage.”
Batteries enable this RAstar tug to run up to 9 knots fully electric. “There would be no use of the engines for transits,” says Mr Watson, “which reduces the running hours and engine maintenance.”
There are other savings in the construction costs. “Using batteries reduces operating costs and installing 12-cylinder engines would reduce capital expenditure and maintenance costs for operators,” says Mr Watson. “This reduces the total cost of ownership versus tugs with conventional propulsion.”
In this design, Robert Allan combined the MAN engines with a pair of Schottel Rudderpropellers of SRP 490 type.
Rolls-Royce Power Systems worked with Robert Allan to design a green shallow-water push-boat. This incorporates two 8V MTU 4000 series M55RN natural gas engines into a RApide 2800-Z2 design.
This vessel is designed for inland waterways, including the River Amazon where similar diesel-powered push-boats operate. Two MTU engines each generate 746 kW from gas fuel. Rolls-Royce provides the entire LNG supply system including storage tanks. This design is compliant with IMO Tier III emissions control and has achieved approval in principle from classification society DNV GL.
This certifies RApide 2800-Z2 complies with rules for classification of maritime vessels and the international safety regulations for vessels using gas or other low-flashpoint fuels.
New engine series
Mitsubishi unveiled its SU series of medium-speed marine engines in Q4 2019. This series generates power from 1,007 kW to 3,580 kW. They have a robust, rigid structure and low fuel consumption ratio, specifically designed for tugs. An intermediate shelf is provided in the cylinder head to enhance rigidity and cool the combustion area.
The exhaust valve is made of heat-resistant alloys and its seat area is reinforced cobalt-based alloy to prevent temperature corrosion. Forced cooling improves the durability of the piston at high outputs and temperature control optimises combustion.
SU series’ high-pressure fuel injection is 1,500 kg/cm2 and fuel consumption at rated output is around 200 g/kWhr. Mitsubishi reduced NOx emissions and particulates by improving the integration between piston combustion chamber, compression ratio and fuel injection timing.
All pumps, oil coolers and filters in the cooling water, lubrication and fuel systems are compactly installed on the engine.
For harbour tugboat propulsion with a four-stroke, water-cooled, diesel engine, turbocharged with air cooler and for an engine speed of 1,150 rpm, engines outputs at 45° are:
Caterpillar has redesigned its Cat C4.4 Acert marine generator set to reduce costs and deliver better performance.
It features a common rail fuel system that uses less fuel and produces fewer emissions. C4.4 Acert can be installed as a retrofit due to its single-circuit cooling system. It can run 500 hours without an oil change.
C4.4 Acert has a self-priming fuel system, minimising the need to bleed the system and anode-free operations, meaning no zinc anodes are required. It has gear-driven water pumps to prevent overheating due to a failed belt and hydraulic valve lash adjusters eliminating the need for manual adjustments. All this leads to less downtime for maintenance and higher reliability. It runs at frequencies of 50-60 Hz with minimum rating of 51 ekW and maximum of 105 ekW.