DP-Stability During Heavy Lift Operations Using a Modified Kalman Filter

Abstract

During heavy lift operations, staying on position using a Dynamic Positioning (DP) system offers many advantages compared with a mooring system. However, when the vessel is connected to another fixed or floating object during the lifting operation through its hoist wires it may experience instabilities in the DP-system. These DP-instabilities are caused by the inability of the DP system to handle the relatively stiff external spring of the hoist wire correctly. This phenomenon is well known and mitigating measures such as Heavy Lift Mode have been developed over the years that work well for stationary vessels. However, when two vessels are lifting a single object together (e.g. QUAD lift), existing solutions to prevent this DP-instability are insufficient, as the nature of such lift requires a synchronous move on DP. During studies to the fundamental behavior of a DP system during heavy lift operations it is found that modifications to the Kalman filter can prevent these DP-instabilities. Heerema Marine Contractors presented the DP-stability challenges to Kongsberg Maritime, and a joint effort resulted in an implementation of a modified Kalman filter in the Kongsberg Maritime DP system. Also a dedicated engineering analysis to predict risk of DP-instabilities for specific lift configurations has been developed. The modified DP-system is tested in large number of simulations (both desktop and a full mission simulator) to test the ability of the updated DP-system to deal with a wide range of specific heavy lift conditions. Results were evaluated between Heerema office, Kongsberg and offshore personnel for developing the optimum Kalman filter parameters. Finally, the system is tested during a dedicated DP-trial program onboard Thialf. As the results of all these tests were very successful, the new High Kalman filter was made available onboard Thialf as a permanent option next to the original functionalities. The paper addresses the steps followed to define the new Kalman filter settings, the simulations performed to test the new filter as well as to show results of the offshore tests that were done to validate the numerical analysis.