Ship shape05 February 2020

From the advanced polar research vessel RRS Sir David Attenborough to the new £6.2 billion Queen Elizabeth Class carriers, on-board water systems are playing a vital role at sea

RRS Sir David Attenborough (SDA), one of the most advanced polar research vessels in the world, is about to transform the way British Antarctic Survey (BAS) conducts its science. Representing the pinnacle of marine research technology, the £200 million vessel, under construction at Cammell Laird Shipyard, Birkenhead, is scheduled to undergo sea trials later this year. With a crew of 30, and accommodation for 60 scientists and support staff, SDA will be able to navigate the oceans for up to two months at a time.

Why is this vessel so important? Because, through long-term shipborne research investigations, BAS scientists will be able to advance their study of the combined impacts of global climate-driven change and commercial fishing on polar marine ecosystems. This vital work is informing the development of conservation and ecosystem-based management strategies, while helping to shape government policy.

With a complement of up to 90 people on board at any one time, there is enormous reliance on the performance of the vessel’s on-board water systems to provide for a multitude of needs – and dynamic positioning (DP) sits at the heart of this. DP is a key factor on many vessels at sea, with an automated heading and position control system ensuring the safety of personnel and equipment. SDA will actually operate at a higher level, being a DP2 vessel, which means the ship and its systems have been designed in such a way that there is complete redundancy, boasting a number of duplicate systems, including two operating consoles and two DP monitors. In effect, the port and starboard sides are mirror images of one other.

“The seawater cooling system, for example, consists of a high sea suction either side and a low sea suction/ice chest in mid-ships,” says second engineering officer Carrie-Anne Harris. “In normal operation, the cross connection would be closed, with each side operating independently. However, in an emergency, the cross connection can be opened to supply seawater from one side to the other. The high sea suction is used in shallow waters, such as in port, while the low sea suction/ice chest is used when the vessel is underway.”

Bearing in mind the SDA’s primary operating locations, with polar temperatures and the risk of icing up of the sea chests, a low sea suction/ice chest has been designed with a weir in the middle to prevent any ice taken in entering the pipework. Should ice ingress become an issue, the chest has been fitted with 30 bar compressed air blowing to agitate and break up the ice, averting blockage.

“For low temperatures and in emergency conditions, there is a sea bay on either side of the vessel,” adds Harris. “These double-bottom tanks, filled ready for use, allow heat recirculation in the seawater while operating in very low temperatures, as the ‘heated’ water will return to these tanks, rather than go straight overboard as is normal practice. In an emergency, such as the sea suction becoming blocked by an ingress of ice, the sea bays can be used for cooling the main engine’s jacket water, ballasting the vessel, firefighting via hydrants and hoses, and also as a source for freshwater production.”

There are two independent freshwater production systems on board: one is a reverse osmosis plant and the other a freshwater generator plant. The latter uses the circulating hot water to boil off the seawater. This is achieved at around 85°C by creating a vacuum in the generator unit by means of an ejector. “The seawater enters the bottom of the unit [where it reaches boiling point] and then the vapour rises to the upper section of the unit,” Harris explains. “There, the vapour is cooled [by seawater] and condenses into distilled water, which is then pumped to the freshwater tanks. The saline/brine solution remaining is then pumped back into the sea. This unit has the capacity to produce around 30m3/24h – sufficient to keep up with consumption on board with a full ship of 90 persons and have some left over to refill the tanks.” Maintenance on both plants is carried out on a condition monitoring basis, or time basis, whichever is sooner.

Should the pressure differential increase across the reverse osmosis plant, or the production quantity drop, it is likely the membrane has become fouled by mineral scale or biological matter. If caught quickly, the membrane can be cleaned in-situ, reducing downtime, with an alkaline cleaning solution circulated through each stage individually. The freshwater generator can also be cleaned internally in-situ; again, reduced production will be an indicator that the unit has become fouled.

Meanwhile, oil-fired heaters and exhaust gas economisers heat the water that is pumped to the various compartments via fan heaters, keeping the ballast, freshwater and wastewater tanks from freezing, while also providing air conditioning for accommodation and lab spaces. “Again, for redundancy, the system can be isolated down the middle, separating the port and starboard sides, should a problem arise,” states Harris. “There is an oil-fired heater on either side, which can be run independently or together. They will be used as a back-up to the exhaust gas economisers.” Each of the four main engines is fitted with an exhaust gas economiser. These utilise the wasted heat from the exhaust gas to heat the circulating hot water, removing the need for further fuel to be burned for this process.

With the SDA’s priorities being logistical support and scientific research in the polar regions, there is often a need to carry large, heavy and unusually shaped cargo, such as vehicles and scientific equipment. This can cause difficulties in loading the ship with the weight distributed evenly. This is achieved by pumping seawater into the various tanks split up around the 27 ballast tanks. But there is a downside. “Increasingly, there has been evidence of biological contamination around the world, due to ships’ ballast waters. Introduction of invasive marine microbes, plants and animals has already had devastating consequences to some local ecosystems.”

As a result, IMO (International Maritime Organization) introduced the Ballast Water Management Convention in 2017. In order to comply with the convention, there is a ballast treatment unit on board the vessel.

When pumping seawater into the ballast tanks, or when pumping the water from the tank back to the sea, it will pass through the treatment unit. This consists of a filter to remove larger particles and organisms. The water then goes to a UV reactor where a bank of ultra-high intensity UV lights kill or sterilise any remaining organisms.

“The maintenance for this unit is much more frequent,” confirms Harris. “After each ballasting operation, while still in the same ecological zone or in international waters [200 Nm from the base line], the unit can be cleaned in place. The UV reactor is filled with a biodegradable cleaner, which is circulated for around six hours.

"The unit is then flushed with freshwater and filled to prevent scaling, and algae growth, for example. This will avoid the risk of any residual untreated water being discharged in a different ecological zone.”

The ballast treatment unit can be bypassed, so that, in the event of an emergency, the vessel can be ballasted or deballasted.

Equally, life for those on board the two new Queen Elizabeth Class aircraft carriers – total cost £6.2 billion – can be challenging enough, without concerns over the plentiful availability of freshwater for those on board. Each of the 65,000-tonne carriers – HMS Queen Elizabeth and HMS Prince of Wales – provides the armed forces with a four-acre military operating base, which can travel up to 500 miles per day to be deployed anywhere around the world.

“As with any large facility, there is a requirement to maintain a habitable and safe environment for the people employed and living within,” says Martin Douglass, engineering director at the Aircraft Carrier Alliance and chief engineer of the Queen Elizabeth Class aircraft carrier programme. “The issue with any vessel, especially one designed to remain independent of port support for protracted periods, is that providing the basic needs can prove a challenge.”

With a ship’s company of around 700 and overall manning of up to 1,600 (when including air group and other embarked personnel), and deployments typically lasting nine months, the aircraft carriers’ crews require a dependable and consistent supply of pure water on demand, whenever and wherever they are across the globe.

To meet this need, a £1 million reverse osmosis system draws in seawater. High pressure is then applied to the salt water, pushing it through a semi-permeable membrane. Due to the size of the salt molecules, only smaller water molecules can pass through, transforming it into freshwater at a rate of 175m3 per day.

The highest levels of fire safety standards are also vital, as commissioning manager Gary Butterfield confirms: “During the system trials, which took place on the forward mooring deck, we were able to demonstrate to the Naval Fire Authority, Lloyds Register and Maritime Capability Trials and Assessment that the firefighting system meets the design specification for the flow and pressure of the foam. This confirms that the system produces enough foam through the pipework and nozzles, and that the system will produce the correct concentration of Aqueous Film Forming Foam and seawater.”

Operating Joint Strike Fighter Lightning II jets and several helicopter types, the QE Class carriers are used across the full spectrum of military activity, from waging war to providing humanitarian aid and disaster relief. Just as it was once stated that 'an army marches on its stomach', none of this would be possible without an adequate supply of freshwater on board for drinking, cooking, showers, washing and general ship husbandry.

Brian Wall

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