Massive heavy-duty crane ships that can carry thousands of tons navigate the rough waves and high winds off the coast to build wind turbines and oil fields in the ocean. An international team of researchers has developed a new modeling system to improve the control and ultimately the safety of such ships. They published their approach in the April issue of the IEEE / CAA Journal of Automatica Sinica.
“Dynamic positioning allows the ship to stay in a specific location by acting on the engine,” said author Simone Baldi, professor at the School of Mathematics and School of Cyber Science and Engineering at Southeast University in China and Guest at the Delft Center for System and Control, Delft University of Technology in the Netherlands.
This positioning process acts as a counterweight. The engine exerts the same force in the opposite direction of the wind or wave trying to pull the ship away.
“But it sometimes happens that the dynamic positioning cannot cope with such changes and causes the ship to oscillate instead of staying in one place,” said Baldi. “Our approach enables robust dynamic positioning in difficult sea conditions with large waves.”
So that the ships can work safely under harsh conditions, Baldi and his team have added a digital observer to the dynamic positioning model system, who can convert wind or wave disturbances into specific measurements that reflect position and speed.
Baldi noted that other proposed models also include observers, but these observer designs typically depend on ships responding quickly to measured disturbances – which is often impossible due to the sheer size of the engines and propellers.
To address this challenge, the researchers incorporated known variables such as the strength of the lines and the thrust that keep the ship in place, as well as the worst-case scenario areas for unknown variables such as wind and waves. The researchers then used an observer-controller network that converts motion into measurements to provide operational instructions while allowing the ship to respond in a timely manner. The design is based on key performance indicators, taking into account the worst-case uncertainty scenarios.
“We have currently tested our method in a realistic simulation, this is only a first step,” said Baldi.
Next, Baldi said, the researchers hope to test the proposed solution on a small ship under controlled conditions before moving on to tests on heavy-duty crane ships at sea.
J. Ye, S. Roy, M. Godjevac, V. Reppa and S. Baldi, “Robustifying dynamic positioning of crane ships for heavy lifting operations”, IEEE / CAA J. Autom. Sinica, Vol. 8, no. 4, pp. 753-765, Apr. 2021.
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