A vessel was sailing in its assigned standby location during heavy weather when the VSAT satellite antenna dome mounted on the vessel’s mast fell off and landed in the monkey island above the wheelhouse. The fallen equipment was secured and the vessel master informed. The event was duly reported to shore side management. There were no injuries.
What went wrong?
Preliminary investigation revealed corrosion as the cause of the failure. The corrosion was discovered in the bolts and the mounts and was due in part to the use of galvanically-incompatible materials.
Deeper investigation looked at weather conditions, the securing arrangements of the VSAT satellite antenna dome, maintenance of the mast and its attachments, activities associated with the mast and the area above the wheelhouse. The investigation team found to be unsuitable, the arrangements by which the pedestal was attached to the mast:
- The bolt sizes were deemed insufficient for the static and dynamic loads that would be imposed on a VSAT antenna assembly while in service, particularly at the mast location, particularly given that corrosion is a long-term concern;
- The materials used for securing the pedestal to the mast were galvanically incompatible for the environment, i.e. mild carbon steel bolts used in combination with stainless steel nuts.
What were the causes?
The use of dissimilar metals in the nuts and bolts anchoring the pedestal of the VSAT satellite antenna dome to the mast allowed gradual galvanic corrosion of the mild steel bolts and eventual failure. The location of the dome on the mast would have exacerbated the problem, as the dome would be exposed to both windage effects and tangential g-forces as the vessel responded to wave movement.
- The securing arrangement was very difficult to inspect properly, as the nuts and bolts were almost invisible;
- Welding the dome pedestal to the vessel structure should be of benefit;
- Where nuts and bolts are used, careful consideration should be given to ensuring the nuts and bolts are both the correct material for the environment in which they are used, and that they are the correct size.
What actions were taken?
- Inspections and maintenance – revise the vessel planned maintenance system to provide:
- specific guidance as to what equipment to check when performing a visual inspection, and
- clarity as to what common concern(s) to look out for;
- Improvements to the design process:
- consider the use of a welded solution where practical
- where a nut and bolt arrangement is practicable then the mounting material applied should be commonly used (and suitable for) in the marine environment, such as galvanized steel bolts and nuts. Mounting materials of different composition should be avoided to minimize corrosion in a marine environment.