NEWSLETTER

Ship structural monitoring
Richard May, BMTSeaTech, Southampton

Structural monitoring on ships was given a big push when the International Maritime Organization recommended that large bulk carriers should be fitted with a hull stress monitoring system. This was a response to the large number of bulk carriers lost through structural failure in 1990/91. These systems are now fitted on approximately 80 bulk carriers worldwide and their use is spreading, mainly to tankers but there are a small number of applications on other vessels ranging from naval frigates to container vessels. A lot of the latter are research projects.

The basic commercial system consists of four long-base strain gauges fitted on the main deck of the vessel (figure 1), with a vertical accelerometer near the bow, all sending data back to a computer in the Bridge or control centre.

Figure 1: Long base strain gauge on the deck of a bulk carrier with protective cover removed.

The stresses and accelerations are displayed continuously, with alarms if pre-set limits are exceeded (figure 2).

Figure 2: Bridge display of the BMT hull stress monitoring system.

The data is also processed and a number of statistical measures are recorded, such as maxima, mean, standard deviation and a bin count of stress ranges determined by a rainflow counting algorithm. The main use of these systems is to warn of high stresses so that the crew can take immediate action. Bulk carriers in particular are vulnerable to overstressing during cargo loading and unloading. Modern terminals can load cargo at rates exceeding 16000 tonnes/hr, with the possibility of excessive bending and shear force if the loading sequence is not carefully supervised.

High stresses can also be experienced at sea and the system will also warn of these, allowing the crew to reduce the risk of failure by altering course or speed. The accelerometer is used to warn when slamming occurs at the bow. On a large modern ship it can be impossible for the crew to tell when the forefoot slams into a wave, and considerable structural damage can occur. One customer of BMT SeaTech, who operated bulk carriers across the stormy North Atlantic, found that the system easily recovered its cost over one season through the reduction of repair bills for slamming damage.

At present very few operators make much use of the recorded data. Although the systems are useful in warning of peak stresses, most structural problems occur through a combination of fatigue and corrosion. These are both localised phenomena and the sensors are not typically fitted in the worst affected areas. In addition, ship structures are subject to an inspection and maintenance regime determined by the Classification Societies, such as Lloyd's Register. At present there is no agreed way of using the stress monitoring data to influence this regime. Although an owner can carry out additional maintenance if he wishes, the data from an HSM system cannot (yet) be used to support a variation from the "standard" system. Lastly, although interesting to a naval architect, the presentation of the data is not generally in a form which is useful to a busy ship's superintendent. BMT have recently addressed these problems through the development of a shore-based module for the processing and interpretation of the data.

The immediate challenges in extending the use of SIM aboard ships are to extract useful information from the present systems and to encourage the owners to install systems with a local monitoring capability so that they provide information on those areas which are known to be critical. Once this is achieved it should be easier to convince the classification societies that it is justifiable to use the data as part of the inspection and maintenance system for a ship. There are a number of research projects aimed at just these areas: two current EU funded ones are OPTIMISE (www.bmtech.co.uk/optimise/) and HullMon+, managed by the Norwegian company Marintek.