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Monday, November 18, 2019 

DNV GL has drawn to the attention of ship owners and managers, yards and design offices, potential problems associated with overboard pipes for scrubber installations.

A recent incident involved a hole in the spool piece, the piece of piping between the SOx scrubber overboard valve and the ship’s hull (see picture), which led to a large quantity of seawater entering the engine room. Only with considerable difficulty did the crew manage to stop the water ingress and limit the damage to the engine room. During the damage survey, severe corrosion of the spool piece was found. In addition, the diffusor necessary to dilute the acidic outflux of the scrubber wash water and ensure compliance with the emission regulations was found heavily corroded.

Despite the spool piece being made to an approved design, with epoxy coating, the acidic wash water managed to come into contact with the steel pipe and react. Most likely this was due to a flaw in the application of the epoxy coating or damage to the coating incurred during final installation. DNV GL adds that there have been several, less dramatic, occurrences of leaking SOx scrubber overboard piping. There is a need to select a suitable and durable material as well as a robust design.

Despite long experience with land-based SOx scrubbers, the maritime industry is still on a learning curve, encountering problems very specific to the use in the marine environment. DNV GL is of the opinion that a yearly inspection of the spool piece is necessary to prevent further serious incidents. Subsequently, class requires an annual inspection, either through Ultrasonic Thickness Measurements (UTM) or visual inspection by diver. Particular attention should be paid to the bottom part of the pipe, closest to the valve flange, since recent incidents indicate that this is the area most affected by corrosion.

DNV GL recommends that the spool piece is designed with a Super duplex (254 SMO or equivalent Stainless Steel (SS) with a high Pitting Resistance Equivalent Number (PREN)) bolted liner in a steel outer pipe. Experience collected so far gives reason to believe this is the most robust solution. Since this design does not allow for UTM, annual inspections should be done by divers or, more conveniently, a leakage indicator (tell-tale) should be fitted. If a tell-tale is fitted, annual inspection by class may be waived.

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