NEWSLETTER

The RV Triton has recently commenced sea trials for a joint UK-USA assessment of the trimaran concept. A significant part of the trials is the measurement of wave- induced strains in the hull structure. These measurements are made continuously whilst the vessel is at sea to assess the loads arising in typical ship operations. However, because of the novel configuration it is necessary to establish confidence in the structure at an early stage and there is also to be a dedicated rough weather trial to examine the structural response in extreme conditions.

The ship is fitted with the 'Trials Instrumentation System' (TIS)* supplied by the US DOD as the primary means of handling strain gauge data. This is assembled from COTS components and in its present configuration can handle up to 323 analogue and 100 digital channels. There are 200 electrical resistance strain gauges connected to the TIS and further inputs from bow mounted wave-measuring radar, from GPS and from ship operating data. The data is recorded continuously at 200 Hz scanning rate and produces up to 10 gigabytes per day; there is an option to calculate the statistical parameters for each channel at five-minute intervals. Slamming events, i.e. excitation of the hull at natural vibration frequencies, are detected automatically and scanned at 2000Hz for a five-second period.

The system is monitored and controlled from a dedicated compartment, the TIS office. Individual channels can be interrogated in real time to check their status, compare strain levels in different parts of the structure and verify the validity of the data.

Triton is also fitted with three DERA fatigue strain recorders**. Each of these records data from two opto-mechanical strain gauges at a scanning rate of 72 Hz and processes the data to derive rainflow counts for fatigue analysis. There is also the option of collecting continuous strain time histories for the gauges; this allows direct comparison with the TIS output. Optionally, there is continuous output of current strain level at each gauge. This equipment is also controlled and monitored from the TIS office.

In addition to the two continuous recording systems, there are twenty-three DERA mechanical recording strain gauges. These are less accurate and have much lower discrimination than the electrical or opto-mechanical gauges. They merely record the total strain range observed in a four-hour period, but they are entirely self contained and have the advantage of being unaffected by electromagnetic interference and power supply instabilities.

The vessel was dry-docked before the start of the sea trials and the strain gauges calibrated by applying static loads to the structure. Because of the novel form of the trimaran the strains in the cross deck induced by loads in the side hulls are of particular interest and the calibration loads included this case.

The early stages of the trials revealed problems with the power supply to the instrumentation. Voltage stability was poor, leading to automatic shutdown of the strain recording systems. This was resolved by providing a dedicated power supply, entirely separate from the ship's generators.

The first phase of the structural trials are scheduled to complete in May 2001.

The instrumentation will remain in place for subsequent trials, which concentrate on handling, propulsion and equipment rather than structural aspects. A strain history will thus be built up over a long period from operations in diverse operating conditions.