WATER ingress monitoring systems (W.I.A.S.) have been mandatory in bulk carriers since 2005 and under a change in the SOLAS regulation with effect from 1 January 2007 it also became compulsory for smaller, new and existing ships of a single hold construction and classified as dry cargo ships.

Although the requirement for WIAS became a regulation as a result of investigations into the loss of the Derbyshire more than 10 years previously, it provided little time for owners to comply with the new requirements, less than a year in some cases. Doubtless the case for further safety devices was clear since the bulk carrier group of vessels suffer the highest incidents with 96 vessels lost in the 10 year period from 2005.

Test or Trial

The operator’s experience of the earlier systems fitted to larger vessels over the past 2 years should guide the industry to a best practice for reliability of WIAS devices. These are primary safety devices but there appears to be no recorded history of how often these have helped or saved in a situation. On the other hand there is plenty of anecdotal evidence from the crews to suggest that devices are not always reliable. False alarms and constant sensor replacements are common complaints. The question must be asked. “what price an unreliable primary safety device?” Worse still must be the scenario where its presence  provides comfort for the crew but in actuality they may not be aware the device is faulty. Should it fail to alert in a situation of looming disaster crews might easily overlook other warning signs because the WIAS was giving them a false sense of security.

Active or Passive 

As swiftly as the regulation was introduced so a plethora of suppliers of WIAS alarm systems rushed to fill the sudden and large demand and offering products using a variety of sensing technologies and devices. IACS laid down a test requirement based upon the spectrum of bulk products that might be carried. The WIAS system, therefore, had to suit all products and all vessels. Since the requirement was previously entirely unknown it is reasonable to assume that none of the suppliers had field experience of providing sensors in that environment nor time to carry out trials beforehand. It is undoubtedly the worst and most hostile environment for any instrument to be subjected. This is one application where simplest is definitely not always best. A passive device such as an on-off switch sensor can only be manually and periodically checked with difficulty and little on-going certainty. With ship’s operational pressures this is likely to take place only once before the voyage (and possibly even before loading) to satisfy port PSC inspectors. The rigours of loading, dust and vibration inevitably must be the peak time for sensor breakdown. A passive device gives no warning of this and in that situation the vessel would sail with no safety alarm on that hold. If this were a single hold ship – none at all.

System healthy status

When PSM Instrumentation considered the application it drew on 30 years experience of marine level measurement and selected a time proven analogue level sensor as the initial detector for the system. The sensor is extensively used in similar and navy applications, tough but accurate enough to show the gradual rise in water depth in a flooding hold allowing the situation to be continuously monitored by the alarm station.

 Our rationale behind the use of an analogue sensor is that it is always measuring an empirical level whether that level is present or not.  In this way there is always a “sensor healthy” signal monitored by the alarm station. Any deviation of this signal would activate an alarm. The alarm monitoring station also detects the difference between a sensor failure and an active alarm status and annunciates accordingly.  Furthermore, we devised a simple and practical method of mechanically testing the entire system at any time and any condition of hold. I.e. loaded or unloaded and during a voyage. By applying a negative pressure to the sensor’s detection membrane the sensor behaves in correct operative manner providing a 100% live test.

Reliability but low cost

The regulation requires that, in general, 2 alarm points are required in each hold. For most manufacturers that means installing two detectors at different heights above the hold floor. Depending upon the installation, that means two sets of cables, protection pipes, deck penetrations etc.  We were able to demonstrate to class that a system with continuous health self-monitoring would negate the need for a second sensor thereby reducing the cost of the system but more importantly,  the cost of installation materials and time.  Demonstrably, this is the larger cost of a WIS system.

Real Benefits for single hold ships

We see the market for single hold cargo ships as even more price sensitive in view of the small operating margins of these vessels. Many are owner operated and can ill afford any downtime if PSC inspectors delay vessel departures because of an inoperative WIS system.  Our “check from the Deck” test after load will reduce the turnaround time by affording a single and swift proving test for surveyors.

Installing sensors on the aft bulkhead even from the engine room side will substantially reduce costs for smaller ships making cabling and installation very much easier by completely removing any need for hold and deck pipework and cabling.  With conventional float switches and systems only approved with two sensors this would be difficult or impossible to achieve.

Contact us sales@psm-sensors.co.uk or call +44 (0) 1444 410040 and talk to Mark, Nigel or Sean in our technical sales team for a quotation for your vessel