Among the most important measurements that are taken on vessels are those needed for calculating ship trim and draught, to ensure vessel safety and stability.
The new Servowatch Systems solution ServoTrim is designed to control a balanced draught for a vessel with trim conditions that have been set manually, providing automatic follow-on control after manual trim has been set. It is particularly suitable for small waterplane area twin hull (SWATH) vessels but can also be applied to other vessels, including inland waterway barges and heavy-lift platforms.
While the system is not a fully automatic dynamic stabilisation system, it can be reset to automatically recognise the optimum draught and trim required under different operational conditions, such as loading or unloading fuel, supplies and personnel.
The use of high accuracy hydrostatic level transmitters means that the measurement of ballast tank levels and multi-point draught indication can be achieved concurrently. These instruments have an encapsulated stainless steel housing to Ingress Protection class IP68 and IP69K. This is essential in order to protect the measuring cell and electronics from external moisture.
The crew manually set the datum point when the vessel is at rest and the loading of fuel, commodities and personnel is complete and stable. They are responsible for determining the trim correction that is required. This is undertaken by manually operating the ballast tanks’ fill and empty control system until the correct trim is obtained. This correct trim datum point is then entered into the control system memory by selecting the automatic control function.
The ServoTrim system then takes a reference signal from the four external draught sensors, which measure the height of the sea water at each corner of the vessel with reference to the fixed height of the side of the vessel.
The draught sensors are mounted within a vented pipe inside the vessel. The bottom of the pipe is open to the sea through the vessel hull. Each ballast tank is equipped with a level sensor to determine the depth of the water in it.
When the automatic mode control is selected, the ballast tank sensor data and the draught sensor data are fed into the control system to form a datum point for each ballast tank. The ServoTrim system then automatically controls the water level in each ballast tank to maintain the vessel’s selected datum point within the dead bands of the control system.
The system will self check the datum point every 10 minutes by comparing the actual draught level sensor signals with the draught level sensor signals registered following the initial trim and draught manual setting. Any correction will be within the control system limits set by the filling and emptying of the ballast tanks.