SOx scrubber overboard pipe failure (revised Feb 2020)

Scrubbers are a relevant compliance option for many ships to meet the IMO Global Sulphur Cap 2020. DNV GL has for many years worked with owners and operators on scrubber installation and operation. This casualty information focuses on the importance of selecting the correct design and appropriate materials for the scrubber overboard spool piece, the need for good workmanship as well as the need for regular inspections in order to avoid similar incidents occurring.

Relevant for ship owners and managers, yards and design offices.

Course of events

A hole in the spool piece, the piece of piping between the SOx scrubber overboard valve and the ship’s hull (Figure 1), 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 (Figure 2).

Lessons learned and recommendations

Despite the spool piece being made to an approved design, namely steel piping 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. Either which is very difficult to spot during installation.

In the recent past, there have been several other, less dramatic occurrences of leaking SOx scrubber overboard piping. Therefore, we would like to emphasize the need to both select a suitable and durable material as well as a robust design.

Even though SOx scrubbers have been available in the landbased industry for quite some time now, the maritime industry is still on a learning curve, encountering problems very specific to the use in the marine environment. Until the industry converges on reliable design(s) and material(s) for the overboard piece, 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 (see also ref. 1). 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.

Recommendations

With the current state of technology, DNV GL recommends that the spool piece is designed with a bolted or otherwise non-welded stainless sleeve, suitable for non-stagnant seawater per class guidance. 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.

The appendix on page two of the PDF version illustrates the pros and cons of the most common overboard pipe solutions.

References

Contact

• Email us at Hull and Machinary or
• use our office locator to find the nearest DNV GL office.