Implementation of a propeller pitch and engine load control system.
During November 2009, Wartsila, an international supplier of marine engines and marine engine management systems, was approached by Sea Harvest to upgrade the engine management system on the ocean-going trawler, Miriam Makebe. The old system had become obsolete and no spares were available. As the engine was still in good order, only the control system needed upgrading.
Designing a propeller pitch control system goes hand in hand with understanding and implementing engine load control. Both systems must be integrated to obtain an efficient closed loop system. There are other variables that also need to be taken into account in designing such a system, the first being whether to consider either a purpose-built dedicated system, or an open architecture alternative, such as a PLC with HMI/scada interface.
Having decided on the latter, the first thing to take into consideration was the environment in which the PLC and HMI would have to operate and the maintenance free periods the system would have to endure. The second was the ability of the PLC to execute complicated mathematics and lastly, the affordability of large scale data acquisition using a PLC in conjunction with an HMI. The LS Industrial Systems XGI PLC was chosen.
For the data-logging and advanced graphics, the InfoU scada package, also from LS, was chosen for quick graphical user interface development and installed on a Nagasaki industrial touch screen PC. The GUI was set up to allow the operator to view real-time control values and then compare them with calibrated values stored in the PLC. If any re-calibration becomes necessary during the lifetime of the system, it can be done at the touch of a button on-screen.
The load of the engine is monitored continuously by plotting the real-time throttle position against the 10 point pre-charted maximum power curve of the engine. The calculated load is then used to work out the rate of change of propeller pitch and automatic load reduction. If the real-time load exceeds 100% of the maximum available load, the system automatically reduces the pitch on the propeller and warns the operator of imminent engine overload. The system also establishes whether or not the vessel is in rough seas and reduces the pitch accordingly. The start and stop sequence and automatic shutdown of the main engine are done by the PLC and clearly visible on the GUI, almost completely eliminating operator intervention.
All of the necessary exhaust temperatures, water and fuel pressures and temperature sensors are included for complete engine monitoring. Set points are monitored by the PLC and alarms are displayed on every page through all levels of the scada screens. Alarm events are logged, so that when the ship returns to port, maintenance personnel are immediately aware of any ‘event’ that occurred, resulting in fast and efficient repair. After the event has been cleared a supervisor resets the flag under password control.
The system underwent successful sea trials during January 2010. A further series of heavy weather trials are planned to take place during the coming winter months.
Information supplied by Juan Bester, Wartsila South Africa.
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