Naval Architect March 07 

This year water ingress monitors start to become compulsory on small single hold cargo ships affecting many thousands of ships worldwide.

 WATER ingress monitoring systems have been mandatory in bulk carriers since 2005 and with effect from 1 January 2007 it also becomes compulsory on smaller new and existing single hold dry cargo ships under a change in the SOLAS regulation.

Who will be affected

This new requirement comes as a change to Regulation II-1/23-3 introduced in May 2005. It requires the fitting of a specially IACS type approved system that has undergone homologation testing specific to this application standard.

The new rule applies to existing cargo ships of less than 100m in length built before 1998 and all new and existing cargo ships less than 80m in length built after that date. Systems applied to existing ships must be installed during the first intermediate or full survey after that date.

Operators will note the classification now covers single hold “cargo” vessels therefore widening the application from the previous “bulk carrier” definition of the larger ships and will mean the inclusion of types such as container vessels, general cargo and timber carriers

 Water Ingress in small single-hold general cargo ships is often the cause of tragic incidents with casualty rates highest in this group of vessels.  The new legislation will undoubtedly save lives and prevent some of the many losses.

When the initial legislation came into force in 2005, there was little time for owners and operators to gain compliance resulting in a large number of suppliers bringing WIS systems to the market attracted by the sheer number of vessels to be retrofitted.

Simple systems, particularly those using simple float operated switches as a primary source of water detection, were initially attractive on equipment price. However, the cost of installation easily outweighed the cost of the equipment and the bottom line saving was not only negligible but worse was to be realised. Simplicity does not always mean reliability especially when related to such an arduous environment and many owners found their systems failed after a very short time or only one voyage.

Comments from the larger bulk carrier sector suggest that some of the systems fitted to those ships have proved less robust than might have been expected. Common faults reported include sticking float stems, clogging of filters by dusty cargoes, physical damage to sensors and cables even when installed in pipes and false alarms generated by condensation or wet loading.  The regulation requires the system to be fully effective at all times and in many ports Port Safety inspection will require to witness equipment tests before allowing the ship to depart and will sometimes prevent the ship from leaving until it is rectified. That could entail removing the cargo because many systems are only accessible in that state.

 

The water ingress system is a primary safety device intended to prevent such disasters as befell the M.S “Derbyshire” that sank in only a few minutes during a typhoon. The contention surely must be that with only minutes to act the alarm system must have a 100% reliability. If the primary sensor is a passive device there can be little confidence that it will operate when it needs to. This syndrome is not limited to float switches but encompasses any passive detector. Even the simplest float switches for example might be tested by immersion in a bucket of water before loading but that is hardly practical once the hold is full and the switch located near the bottom of the hold. Some manufacturers offer some mechanism to simulate this even after loading but if it fails immediately after test you would not know.  Some float switches are also liable to corrosion and this can be a big problem.  The contention here must surely be that such a system may actually become a liability. Take a situation where the crew had observed an unusual condition that might be due to water entering the hold. Their confidence would be misplaced if they felt reassured that the WIS alarm showed a contrary normal condition. Even the most conscientious crew might waste valuable time carrying out a viability check on the alarm system first. 

 

System healthy status

When PSM Instrumentation considered the application they drew on their 30 years experience of marine level measurement and selected one of their existing and type approved analogue level sensors as the initial detector for the system. As the water depth increases so does the pressure on the sensor’s membrane increasing the current as it does so. PSM claims that the sensor is accurate enough to show the gradual rise in water depth in a flooded hold allowing the situation to be better monitored than those systems that just have low and high level alarms.

 Geoff Taylor, Managing Director of PSM explains that the 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, PSM have devised a simple and practical method of mechanically testing the entire system at any time and any condition of hold loading. By applying a negative pressure to rear of the sensor’s detection membrane the sensor behaves in correct operative manner.

 

Reliability does not always come at a price

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.  However, PSM claim their system is fully IACS type approved using only one sensor.  Again Geoff Taylor explains “ we were able to demonstrate to class that a system with continuous health self-monitoring would negate the need for a second sensor by affording both a pre alarm and main alarm derived from a single reliable sensor. This not only reduces 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

PSM sees 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.  PSM claim that their “check from the Deck” test after load will reduce the turnaround time by affording a single and swift proving test for surveyors. PSM also say they are very confident in their selection of sensor since this series has seen extensive service in similar applications for over twenty years. Nevertheless in the unlikely event of a sensor failure their system can allow for the replacement of a sensor directly from the deck, even when the hold is full. 

PSM has a range of sensor fittings and styles and installation is therefore, not limited to one fixed method or position. This should provide added benefit in substantially reducing costs for smaller ships since it should be practical to install the sensor on the aft bulkhead, either inside the hold or external abutment with cables passing through into the engine room behind. With conventional bulkers it is most usually achieved by installation inside a pipe opening to the deck.