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Scrubbers ready for installation on board Norwegian Escape at Meyer Werft. These are the biggest inline scrubbers worldwide.
Exhaust gas cleaning systems are a key technology for ensuring compliance with emissions regulations. DNV GL experts explain when it makes sense to invest in scrubbers, what type of system to choose, and how to avoid common operational problems.
Scrubber Installations
SCRUBBER INSTALLATIONS - More than half of scrubbers are or will be installed on cruise, ferry and ro-ro ships.

With stricter sulphur (SOx) and nitrous oxide (NOx) limits in emission control areas (ECAs) in Europe and the Americas, and new control areas being established in ports in China, international organizations are slowly closing in on pollution from ships. By the time the global 0.5 per cent sulphur cap is introduced in 2020 or 2025, up to 70,000 ships may be affected by the regulation according to IMO estimates. A growing number of shipowners are beginning to weigh their options for ensuring compliance. They face a choice of switching from heavy fuel oil (HFO) to marine gas oil (MGO), burning ultra-low-sulphur HFO, retrofitting vessels to use alternative fuels such as LNG or installing scrubber systems which allow them to continue operating on regular HFO. 

More than 320 scrubber projects

“So far, most operators have opted for the fuel switch from HFO to MGO, but our experience has shown that this is not the best option for every ship type,” says Markus Osterkamp, DNV GL Deputy Head of Section Environmental Certification. Out of the current total of more than 320 confirmed scrubber projects (see diagram below), more than half are or will be installed on cruise ships, passenger ferries or ro-ro ships, and a much smaller percentage on general cargo vessels, bulk carriers, tankers and gas carriers (see graphic on the right-hand side). Most of the current projects are being implemented with support from DNV GL. 

“Scrubber technology is a very popular solution for cruise ship and ferry operators because many of their vessels have a fixed trade route so they can plan for future operation in ECAs. They also have a higher fuel consumption than comparable cargo ships, as they need to produce additional electricity for hotel loads,” adds Océane King, Head of Section Maritime Advisory Division Miami at DNV GL. In these cases, scrubbers could lead to fuel savings of approximately 45 per cent compared to switching from HFO to MGO in ECAs at current fuel prices (IFO380: USD/metric tonne 172.50, April 2016).

The right choice for each vessel

Océane King specializes in cruise ship projects, which includes advising customers on scrubber technology. The best scrubber type for a given ship depends on the operational profile and the routes of the vessel, she says. For example, open-loop systems use seawater to neutralize the sulphuric acid formed during the exhaust gas cleaning process. The resulting waste water must meet MARPOL requirements before being discharged. “Seawater is a natural alkaline, so this works well in most cases. However, if a vessel occasionally sails through waters with a slightly more acidic pH-level, such as rivers or brackish waters, we would advise the customer to choose a hybrid solution. Another drawback of open-loop-only systems is that several ports in Europe and the port of New Haven in the United States now prohibit the release of open-loop water. In addition, some cruise companies have a zero-discharge internal policy close to ports,” King explains. Hybrid solutions can switch from open to closed mode, eliminating the risk of compliance violations. Closed-loop systems use fresh water mixed with chemicals such as caustic soda to boost the alkalinity of the wash water, which is then recirculated through the system and partially purged. 

Currently, hybrid scrubbers are the most popular solution, followed by open-loop systems. Closed-loop scrubbers are installed on ships sailing mainly in fresh water or low-alkalinity areas such as the Great Lakes in the United States. Most cruise ship operators originally opted for open-loop scrubbers because they offer lower system complexity. However, a number of owners in the cruise segment have chosen hybrid systems recently. Overall, more than 100 cruise ships have, or are in the process of installing scrubbers – more than 80 of these being retrofits. “The main challenge owners face in this scenario is to choose a system that adds the least possible weight to the vessel and has the smallest space requirement while still performing at optimum capacity and complying with both the exhaust quality and wash water regulations,” explains Océane King. 


The number of scrubber towers needed can differ, depending on the exhaust gas flow created on the regular itinerary. DNV GL supports customers in choosing an exhaust gas cleaning system that fits their vessels’ requirements. In the next step, the DNV GL Technology Qualification Process (TQP) helps identify and mitigate any potential design risks, such as corrosion, soot on deck or increased back-pressure to the engine. Back-pressure is caused by exhaust impedance and can restrict engine performance. “In the case of ships with a limited back-pressure margin, the installation of exhaust fans can alleviate the problem while ensuring that the exhaust air exits the boundary layer of the ship and does not linger around the funnel,” Océane King explains. 

DNV GL has performed partial or full TQPs for six of the largest scrubber suppliers. Before and during the installation phase, DNV GL uses its risk assessment expertise to identify risks that could compromise the safety, schedule, costs or performance of the system and its installation. Follow-up actions are identified to ensure the risks are successfully mitigated. By combining all of these measures, the DNV GL EasyRisk software provides a platform which makes it easier to monitor statistics and trends of the total and detailed risk exposure within a project.

DNV GL is also the only class society to offer hardware-in-the-loop testing on scrubber automation systems through its Marine Cybernetics team. These tests are carried out in a virtual environment, enabling experts to check whether a scrubber control system is robust enough to withstand the expected stresses. By the time an automation system goes into operation at sea, its performance has been fully verified down to the individual line of software code. In addition, DNV GL works with scrubber system manufacturers to certify their products, and the DNV GL class notation Scrubber Ready helps shipowners make room for potential installation of a scrubber in a newly designed vessel at a later date. 

A scrubber being hoisted onto an cruise vessel owned by Royal Caribbean Cruises.

Future trends

“Looking into the future, we expect scrubbers to get smaller as manufacturers try to make them a more viable solution for vessels with little capacity for additional equipment on board,” Océane King explains. As the industry gains more experience with the installation of scrubbers, most cruise operators have turned to installing scrubber technology for use during operation at sea in order to avoid downtime for extended dry docking. “At the moment, passenger and ro-ro vessels stand to benefit the most from this technology, as the potential fuel savings are significant and the return on investment is very good in these sectors.”