Although failures and inaccurate readings are possible, they are rare, as noted by the UK MAIB in its latest Safety Digest. The report refers to a case where a breached tank was slowly filling with seawater because the positive reading went unchecked. This incident, which involved the flooding of a supply vessel, highlights the need to use other means to verify readings when in doubt.
Operating close inshore in support of a pipe‑laying barge, an offshore supply vessel(OSV) was engaged in the routine delivery
of pipes and personnel from a nearby port. Although some of these deliveries required the OSV to operate in shallow water, the master was confident that safety could be assured by using the locally supplied large-scale charts, operating at slow speed with dynamic positioning (DP) and paying close attention to the on board echo sounder.
The weather was fair; however, the low-lying land offered little protection from the strong offshore winds that had been blowing for the past week. One delivery required the OSV to cross the 5m depth contour with an under keel clearance (UKC) of around 0.5m. The master concentrated on the echo sounder and manoeuvred the OSV slowly in DP mode. All seemed to go well and the OSV made passage back to harbour for the next load. During that passage a contents sensor in one of the empty port side double bottom fuel tanks started to give a positive reading.
The chief engineer (C/E) thought that the sensor had developed a fault and made a note to change it when the OSV was on a longer break in harbour.
Almost a month later, the C/E had completed their time on board and mentioned the faulty sensor to the incoming C/E during handover. The newly arrived C/E decided to investigate the sensor problem and, on sounding the tank, found that it was full of salt water and that the sensor had been correctly reading the entire time.
The C/E and master examined the tank sensor trend data and the OSV’s track and discovered that, despite all their precautions, the vessel had hit an underwater obstruction when crossing the 5m contour several weeks earlier.
A dive survey of the hull found some scraping damage and a 1cm diameter hole in the external surface of the tank.
approach to the pipe-laying barge/ Image Credit: UK MAIB
Lessons learned
#1 Equipment
Trust your sensors. Although possible, failures and inaccurate readings are rare and in this case mthe breached tank was slowly filling with seawater because the positive reading went unchecked.
#2 Check
Use other means to verify readings when in doubt. Sounding the affected tank might have highlighted the problem sooner and confirmed whether the sensor was working correctly or had actually failed. Follow documented safety procedures before entering a confined space to undertake checks.
#3 Margin of safety
The master took decent precautions to manage the UKC for these operations. However, it was later established that local sailing directions mentioned the mean sea level could be reduced by up to 0.9m by sustained offshore winds. Sailing directions represent the amassed knowledge of mariners and UKC should reflect this. It is also worth considering your vessel’s footprint and the position of the echo sounder within it; an echo sounder located away from the hazard and the extremities of your vessel could give a false sense of security.
#4 Communicate
The company involved completed a thorough investigation with a no-blame approach and shared the lessons widely. This maximised the opportunity to learn from the case and everyone in the fleet had the chance to benefit.
