Neptune Thelisis: Leaking thermal oil led to an engine room fire is investigation report finding

leaking thermal oil which led to an engine room fire onboard Neptune Thelisis.
leaking thermal oil which led to an engine room fire onboard Neptune Thelisis.

Transport Malta’s Marine Safety Investigation Unit has published an investigation report on leaking thermal oil which led to an engine room fire onboard Neptune Thelisis.

A fire was discovered by the engine room crew members on board the Maltese registered ship Neptune Thelisis shortly after 16.51 (UTC+1) on 17th January 2022. The crew members gathered at the designated muster station and commenced fire-fighting operations. The fire was controlled and extinguished after the fixed fire-fighting installation was activated. The safety investigation established that the immediate cause of the fire was the failure of a pipe on the thermal oil system. Leaking thermal oil came in contact with a hot surface on the boiler’s economiser.

Metallurgical analysis

A section of the air vent pipe, was shipped to the MSIU for analysis in a metallurgical laboratory. The pipe section had a pipe nominal diameter of 21.3 mm, while the wall thickness was measured at 2.5 mm.

Section preparation

In preparation for standard metallographic tests, the pipe was cleaned using ethanol and a soft brush to allow inspection and photography. The sample’s wider end was then ground and polished to a mirror finish following which, microscopical imaging was carried out using an Optiphot-100 optical microscope and a Nikon SMZ-2T stereomicroscope equipped with a Leica DFC 290HD fast-acquisition digital camera. Finally, the sample was sectioned along its principal axis. The section was mounted in cold setting resin and once again this section was ground and polished. The section was also etched in Nital (2% Nitric acid) to allow for microstructural evaluation. Microhardness tests were carried out on the same sample using a Mitutoyo MVK-H2 microhardness testing machine equipped with a Vickers indenter at loads of 500 gf and for a dwell time of 10 seconds.

Findings of the tests

Examination of the pipe clearly showed a relatively large amount of weld material. Cracking was very apparent around the transition between the weld and the pipe material and the polished cross section of the sample showed very large internal pores/blow holes inside the weld material build up.


– Leaking thermal oil came in contact with a hot surface on the economiser
– The examination of the sample showed an array of welding defects ranging from gross and excessive weld material build-up (necessarily generating large thermal stresses), distributed unevenly around the pipe circumference, and creating visible fillet distortion, to large undercutting at the pipe toe end of the fillet.
– The cross section of the failed pipe also showed that the poor tie-in into the base metal caused a local reduction in the pipe wall, further weaking the vent tube.
– The undercut served as a highly effective stress intensity site for crack initiation, with the pipe bending into the undercut region and allowing progression of the crack front into the inside of the tube.
– Mechanical stress, either through vibration of the assembly, or by pressure variation inside the tube and / or residual thermal stresses, could have caused any number of these features, leading to premature failure.
– Previous damage to the pipe may have been repaired, using inadequate welding techniques.
– While specifically undercutting is a defect typically associated with improper welding parameters, the range and extent of defects present in this small section of weld may have been compounded on site, where access to proper surface preparation (before and in between runs) and positioning around the pipe was limited, as was the case for any subsequent weld inspection.

Ensuing safety actions

Following the fire onboard Neptune Thelisis, the Company has taken the following preventive measures:
– Aadditional supports have been fitted in the new air vent pipe to minimise vibrations and possible cracks, to the satisfaction to the attending class surveyor;
– The Company’s internal investigation report has been circulated to all vessels in the fleet.


Neptune Lines Shipping & Managing Enterprises S.A. is recommended to:
– 01/2023_R1 Communicate the findings of this safety investigation report to the fleet under its management, to highlight the issues related to poor welding techniques.
– 01/2023_R2 Highlight to the fleet under its management the different design concepts of a BA set and an EEBD.

Download the full report: Neptune Thelisis Transport Malta investigation report

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