In response to the introduction of the IMO’s global sulphur cap in 2020, scrubber fitment was seen by some owners as an attractive compliance option which allowed them to continue to burn ‘high-sulphur’ heavy fuel oil (HSFO).
Perhaps the primary driver at the time concerned the projected cost difference between compliant ‘very low sulphur’ fuel oil (VLSFO) and the HSFO. A further influencing factor for shipowners to opt for scrubbers – more properly known as exhaust gas cleaning systems (EGCS) – was the concern at that time on the availability of compliant fuel, especially when trading away from the major bunker fuel hubs.
The lower cost of HSFO allowed owners to offer their vessels to charterers with the ability to burn the cheaper fuel when local regulations allowed. Many went on to make profit sharing agreements with charterers, sharing the savings made by burning HSFO, and offsetting the CAPEX costs associated with procuring and installing the scrubbers.
Analysis shows that over 5,000 scrubber installations have been carried out, and 80% of these are of the open loop type. This type of system was not only cheaper to buy and install, but also cheaper to run and maintain, due to it being less complicated and not needing an alkali solution to neutralise the acidic washwater as required in the closed loop systems.
With open loop type, all the washwater, which is the water used to effectively wash or ‘scrub’ the exhaust gases from the vessel’s engines, is discharged directly overboard without any treatment. This removes the sulphur compounds from the exhaust gases to an equivalent of what would be emitted if a compliant fuel was burned. The sulphur in the exhaust is converted to sulphuric acid by the scrubbing action, which when released as washwater, is neutralised by the seawater's natural alkalinity.
Under IMO Guidelines, the washwater pH discharge limit at the overboard monitoring position, is the value that will ensure a pH no lower than 6.5 at a distance of 4 metres from the overboard discharge point with the vessel stationary. This means that in reality scrubber washwater will be acidic right up to the point prior to being diluted by seawater. These harsh and acidic conditions increase the risk of structural corrosion to the discharge pipework.
Some of the early scrubber installations suffered from corrosion to the internal sections of the scrubber tower and exhaust pipework where there were lower exhaust temperatures, caused by acidic vapour in the exhaust gases condensing. Later units used more corrosion-resistant materials and were normally found in good condition during subsequent inspections.
The weakest link
Andy has extensive experience of scrubber installations and their operation, gained during his roles as a fleet manager and technical superintendent. As such, he found first hand that the weak link of the system was the section of overboard discharge pipe located between the hull shell plating and the ship side valve. The rest of the washwater discharge pipework in the scrubber system was typically manufactured from GRE (Glass Reinforced Epoxy), which is resistant to the highly acidic conditions. However, the materials used for the manufacture of ship side valves and overboard pipes is governed by class rules, which does not allow the use of the corrosion-resistant GRE.
Various solutions have been attempted to solve the issue, most of them focusing on the use of epoxy coatings to protect the exposed metal. However, the smallest imperfection in the coating will allow severe corrosion to occur. Some newer systems have diffusers fitted inside the overboard section of pipework which are designed to help distribute the washwater when discharged into the sea. These diffusers are vulnerable to corrosion attacks, but perhaps more critically, the points at where they are affixed to the pipework can present vulnerabilities in the corrosion protection coatings.
Latterly, for systems without this diffuser piece, a ‘pipe in pipe’ solution has been developed where the original overboard pipe remains in place, with a new GRE pipe is inserted within it. No metal cutting or welding is necessary. A sleeve manufactured from ‘SMO’ (a type of stainless steel) is inserted between the new GRE pipe and the existing overboard pipe. The GRE pipe is then bonded in place using adhesives.
The classification society DNV has introduced the requirement for the scrubber washwater discharge pipes to undergo thickness measurements at each annual survey to monitor any corrosion.
Hull coating damage
Hull inspections of vessels fitted with open loop scrubbers found erosion of the hull coatings in way of the scrubber overboard. In some cases, the coatings had fully eroded. This has been attributed to the hot acidic washwater being discharged from the scrubber. The risk of hull coating erosion had already been considered, with most installations using the same epoxy as they used in the overboard pipe to coat one metre around the discharge pipe, however it was found to be worse than initially expected. Hull coatings that use glass flake technology, such as ‘SUBSEA Industries Ecospeed’, are proving reliable at reducing the risk of corrosion in this area.
Sensitive sensors
Sensors play a critical role in the compliant and efficient operation of a scrubber system. Particulate matter in both the exhaust and the washwater can cause issues with sensors. Filters fitted within the sample lines often block and cause failures in the sensor systems.
Any failures lasting more than an hour need to be reported to Flag State, which allows time for cleaning and any minor maintenance. In certain circumstance Flag State may approve the continued use of the scrubber, providing all parameters are monitored and remain within limits before repair at the next port, or a change to compliant fuel.
Increasing open loop bans
Public concerns surrounding the potential for contaminants in scrubber washwater to be transferred into local waters has resulted in many bans or restrictions, the latest being OSPAR’s decision to restrict their use in the North Atlantic ports and inland waters of the member states, projected to start in 2027.
NorthStandard P&I is an excellent resource for finding out the latest restrictions on scrubber use around the world, which can be found here.
The Clean Shipping Alliance provide a useful map that illustrates the restrictions in place on the use of open-loop systems, which can be found here.
Find out more
For advice on assessments, conversions or disputes concerning scrubber issues, please contact Andy McManus, Marine Engineer Consultant here.