Marine control systems otherwise known as Marine cybernetics is defined to be the science about techniques and methods for analysis, monitoring and control of marine systems.
The main application fields for marine control systems are the three big marine industries: Sea transportation (shipping), offshore oil and gas exploration and exploitation and fisheries and aquaculture. So far most of the examples are collected from the mature industries like shipping and offshore oil and gas exploration and exploitation. It is believed that these industries will be even more technology demanding with focus on safe and cost effective solutions in the years to come. More of the offshore activities is assumed to take place in deeper water based on floating solutions in combination with sub-sea installations. The need for conducting all-year marine operations in the ocean space with varying complexity will increase and motivate to the development of underwater robotics with increased autonomity. Due to increased industrialization within the fisheries and aquaculture, the industrial content and thereby the introduction of advanced technology are expected to increase. It is foreseen that there is a huge potential for technology transfer among all the marine industries.
Concerning marine control systems it is suggested to divide the control structure into two main areas: real-time control and monitoring and operational and business enterprise management. We will here in particular focus on the various aspects related to the design of real-time control and systems. The integration of real-time systems with operational man- agement and business transactional systems is by the automation industry denoted as Industrial IT.
The real-time control structure is as shown in figure below divided into low-level actuator control, high-level plant control and local optimization. We will in the text use demonstrating examples from the offshore oil and gas industry. In particular, examples with dynamically positioned (DP) offshore vessels will be used. A DP vessel maintains its position (fixed location or predetermined track) exclusively by means of active thrusters. Position keeping means maintaining a desired position in the horizontal-plane within the normal excursions from the desired position and heading. The real-time control structure for a DP system may then consist of:
- Actuator control : The actuators for DP systems are normally thrusters, propellers, and Local control of propellers and thrusters may be done by controlling e.g. the speed (rpm), pitch, torque, and power or combinations of these. Dependent on the actuators are mechanically, hydraulically and/or electrically driven controllers with different properties will be used.
- Plant control: In station keeping operations the DP system is supposed to counteract the disturbances like wave (mean and slowly varying), wind and currents loads acting on the v The plant controller calculates the commanded surge and sway forces and yaw moment needed to compensate the disturbances. A multivariable output controller often of PID type using linear observers e.g. Kalman filter or nonlinear passive observers may be used. Setpoints to the thrusters are provided by the thruster allocation scheme.
- Local optimization: Depending of the actual marine operation the DP vessel is involved in optimization of desired setpoint in conjunction with appropriate reference models for e.g. drilling operations, weather vaning, pipe laying, tracking operations, are used. Notice that in the guidance, navigation and control literature local optimization corresponds to the guidance block.
As the controllers rely on proper measurements to work on, signal processing is of vital importance for the stability and robustness of the control system. This will be the first topic of the text.