Thrusters have become a fundamental technology to aid low-emissions dynamic positioning operations
Coming in at a height of 4.5 m on the Norwegian continental shelf, the waves are hitting the shuttle tanker at an angle of 110°, normally impossible conditions in which to load fuel. But AET’s two latest tankers can hold station with sufficient accuracy in even these hostile conditions for loading hoses to be connected through the latest generation bow-loading system.
Thanks to the latest generation of transverse thrusters, these vessels are setting a new standard in what are often harsh conditions.
Contracted to Norwegian energy company Equinor for action on the North Sea, Norwegian Sea, southern Barents Sea, and the UK continental shelf, where steep waves are common, these LNG-powered dual-fuelled shuttle tankers rely on a brand-new computer controlled system for dynamic positioning (DP).
Combined with LNG fuelling and a method for capturing emissions of volatile organic compounds – the residues that escape into the air from crude oil cargos during loading and sailing – the technology should save up to 3,000 tonnes of fuel a year compared with conventional DP shuttle tankers, according to classification society DNV GL.
Thrusters have become an important element in the pursuit of low to zero emissions. But lately the pressures have become even greater. As AET global director John Baptist explains, the shuttle tankers’ areas of operation “not only requires compliance with the global sulphur cap but also with the new Gothenburg Protocol [that] requires emissions of volatile organic compounds to be capped at 131,000 tonnes a year.”
First launched in 2012, the Gothenburg Protocol is an international clean-air mandate that gets tougher in 2020 because of a series of amendments. The latest fine print sets ambitious international targets for the reduction of damaging air pollutants such as sulphur and nitrogen oxides, ammonia, non-methane volatile organic compounds and fine particulate matter.
And AET’s latest ships are undoubtedly making a contribution. Built by Samsung Heavy Industries and project-managed by MISC’s Eaglestar and Norway’s OSM Maritime, AET’s tankers – Eagle Blane and Eagle Balder – are twin-skeg, 123,100 dwt sister vessels that will be active in these waters for many years.
Lying at the heart of DP operations, thrusters have migrated from OSVs, where they started in the offshore oil and gas industry, to become essential for all kinds of ships.
“[Thrusters] have become a popular technology for a wide range of vessels, from cruise ships and flotels through to cable-laying ships, cranes and supply vessels of all types,” ABB’s Carlo Collotta of the global product management team explains in a technical paper, How smarter power systems are making dynamic-positioning ships more reliable. “The systems take data from a host of environmental sensors to determine the magnitude and direction of environmental forces affecting the ships position,” says Mr Collotta.
The management of thrusters has made giant strides too. “The DP systems create a real-time model of the vessel in the water, taking account the wind and currents acting on the vessel and the location and effect of the thrusters,” further explains ABB. “This and other data gathered from instruments such as the gyrocompass allow the computer to calculate the required steering angle and thruster output for each thruster to position the vessel.”
Kongsberg’s K-Pos system, for instance, is so sophisticated that it can calculate the precise forces that the thrusters must produce in order to control the vessel’s motion in the horizonal plane. Even more impressively, thrusters in a sense discipline the vessel’s surge, sway and yaw within a tolerance of 3°. For crews slithering around an open deck in high winds and seas in supply ships, drilling rigs, cable-layers, rock-dumping vessels, shuttle tankers and crane ships, to name a few, this is a considerable boon.
This is why platform supply vessels, for example, can be equipped with up to five thrusters, drill ships with up to six, and mighty floating platforms with up to eight.
But DP demands a lot of power. As Kongsberg explains, the propulsion systems required to keep a modern drilling rig on station generate the power equivalent of one of Norway’s medium-sized hydropower plants, roughly equivalent to the consumption demands of about 15,000 people.
Therefore, as thrusters have become more powerful and sophisticated, control systems have evolved to match. Kongsberg’s latest system, K-Thrust 720, ties everything together – electrically-driven thrusters, main propulsion and steering gear – in a neat and versatile package. Combined with K-Pos and other proprietary systems, it becomes a complete ship-handling technology. Crew can steer by a variety of ways according to the conditions – manually, by auto pilot, by joystick with an automatic heading option, or by automatic station-keeping, whereby the system pretty much takes over.
And it can all be done from the engine control room, the bridge or any other suitable location.
Kongsberg’s system can be run at several levels: in high-precision mode, the vessel stays on the spot in any weather condition; however, there is a price to pay in terms of power consumed as well as wear and tear on the equipment.
In ‘relaxed’ mode, suitable for calmer conditions, the thrusters are not put under as much pressure because they are operated more smoothly and predictably. However, in this mode the vessel may stray from its defined operational area.
And finally, in recognition of the low-emission era, Kongsberg’s ‘green’ mode reduces fuel consumption by up to 20%, all the while holding the vessel within a specified area known as its ‘operational boundary’.
The science behind this is called ‘non-linear model predictive control’ –rather than responding to existing conditions such as waves, wind and current, this mode works by forecasting the vessel’s motion. The system blocks – or filters out – small and short-term disturbances that are not likely to shift the ship from where the crew want it to be.
A product of Kongsberg’s Green Ship strategy, this low-gear mode extends the life of the equipment. “It allows for very smooth control, dramatically lowering peak loads and significantly reducing wear and tear on thrusters,” the company explains.
Thruster technology tends to improve incrementally, rather than through breakthroughs. But one of the most in-demand versions, the azimuth permanent magnet thruster (az-pm), which combines an electric motor, propeller and nozzle in a neat package, can do just about anything.
Building on the permanent magnet tunnel thrusters, the az-pm boasts an unusually high motor efficiency of 97% at nominal speed, as well as high efficiency over the entire load range. No extra cooling equipment is required because the motor is submerged, which saves space and cuts installation costs. And no oil circulation pump is required for the underwater unit.
Given the strains imposed on thrusters by DP, they have been made increasingly robust. Kongsberg’s L-drive model, for example, is specifically designed for pretty much constant operation, usually far offshore where the rigs and drill ships are located. And the permanent magnet model, Kongsberg’s latest, has been developed for the most demanding operations. Among other features the permanent-magnet technology provides rapid response in the form of full power when needed.
Simultaneously with their versatility, the power of thrusters continues to increase. Some models designed primarily for the US market can produce a bollard pull ranging from 11 to in excess of 170 tonnes.
And although they are not designed exclusively for OSVs, tunnel thrusters are a whole new genre. Their job is to provide side force to a vessel whether it is in port or keeping station under DP. Here again, tunnel thrusters are under constant development, like Kongsberg’s ‘super silent’ range for cruise ships that features a modified hydraulic power pack for low noise. Double walls throughout the length of the tunnel, allied to a flexibly mounted inner tunnel, dampen the noise still further. According to Kongsberg, noise levels can fall by up to 25 decibels.
Still, a system of thruster management is no better than its electric supply. “A dynamic positioning system is only effective as long as it is online,” ABB notes. “To calculate the actions needed to maintain and correct a vessel’s position, and apply the necessary forces through the thrusters, a robust and dependable electrical system is required.”
To keep things simple, IMO categorises DP systems into three classes, depending on their robustness. A Class 1 vessel has no built-in redundancy, meaning a single fault can knock out the system and throw the ship out of its operational area. Class 2 vessels can maintain their position even if something major like a generator, thruster or switchboard fails. Top of the robustness stakes are Class 3 vessels, which boast a fully redundant system. If a cable or pipe disconnects, the ship still stays where it is, even in the event of a fire or flood in a single compartment.
But, this being the era of low emissions, thruster management systems must also save fuel. Here, ABB’s intelligent electrical system with ‘closed bus ties’ – a redundancy solution – comes into the frame because, says the company, a ship can burn up to 5% less fuel and cut maintenance costs by up to 30%. ABB says its closed bus ties also have a much simpler cabling system that cuts commissioning time by about 60%.
Unlike a DP vessel, maritime propulsion systems never stand still.
The golden thruster
Thruster specialist Veth Propulsion in the Netherlands hit the 4,000 mark in mid-2019 when it delivered a unit for Greek ferry Protoporos 15, owned by long-time customers, the Tsokos family. Like the other 14 vessels in the fleet, the Protoporos will be equipped with four Veth thrusters ranging from VZ-400 to VZ-550 drives. To commemorate the occasion, the thruster was a special-edition, a golden model. Since the Papendrecht-based manufacturer started turning out thrusters in 1986, it has made 125 of them a year at a rate of 2.5 a week, including the increasingly popular Veth hybrid drive.
The Greek ferries are equipped with Z-drives suitable for low-draft vessels needing 360-degree manoeuvrability. According to Veth, Z-drives are slightly more efficient than a conventional propeller, deliver 360° full thrust, and are ideally suited for DP.
Veth’s latest model is a product of the swing towards electric propulsion. The integrated L-drive is a diesel-electric system where the electric motor is installed inside the ship rather than underwater, reducing its vulnerability to wear and tear. The L-drive is also light, compact and quiet.
“Interest in diesel-electric and LNG-electric propulsion has grown immensely lately, with a view to fuel economy and reduced emissions,” the company reports. “These are mostly ships with …multiple generator sets onboard which together provide the full electric propulsion power supply. When full power is not needed a few generators can be switched off to reduce fuel consumption.” However, Veth notes, diesel- and LNG-powered electric propulsion come with drawbacks, notably their higher cost relatives to diesel-direct propulsion.
The way things are heading, electric thrusters could be the way of the future, particularly for cruise and passenger ships, offshore and cargo vessels.