The influence of ride control systems on the motion and load response of a hydroelastic segmented catamaran model

The operation of high-speed catamarans in large waves can produce significant vessel motions that can lead to passenger discomfort as well as extreme loadings during full bow immersion and wave slam impact. This not only can generate large bending loads on the hull structure but also has significant effects on the fatigue life of the vessel. These large loads and motions can be effectively reduced through the implementation of a Ride Control System (RCS) that can significantly reduce the extreme loads sustained by the hull girder and reduce the incidence of motion sickness for passengers on-board the vessel. Although ride control systems have been implemented on full-scale catamaran vessels, the parameters that influence the response have not been previously quantified partly due to the limitations of testing at full-scale. To accurately investigate these parameters it is needed to undergo testing in controlled conditions and to this end a 2.5 m scaled catamaran model has been developed with an active ride control system based on the 112 m INCAT catamaran to investigate the system parameters influencing the response of the vessel. The 2.5 m hydroelastic segmented catamaran model was set-up with an active T-Foil and stern tab ride control system specifically for towing tank testing at the Australian Maritime College (AMC) to determine the motions and loads response in headseas. Step and frequency responses of the ride control system were first investigated by calm water open-loop tests to determine the control gains needed for closed-loop active control tests. Appropriate combinations of the control surface deflections were then determined to produce pure heave and pure pitch response forming the basis for setting the gains of the ride control system to implement different control algorithms in terms of the heave and pitch motions in encountered waves. Two hydrostatic methods were applied to predict the T-Foil and stern tab responses based on a static load experiment and a hydrostatic prediction and there was close agreement between the two outcomes. This was extended by a dynamic prediction of the response of the moving model based on a two degree of freedom rigid body analysis using strip theory. A series of model tests in head seas at different wave heights and frequencies was then undertaken at the AMC towing tank for different ride control conditions including without RCS, passive RCS and active RCS to measure the heave and pitch motions as well as the centre bow slam force and the demihull slam induced bending moments. Three different ride control algorithms of heave control, local control and pitch control were developed to activate the model scale ride control surfaces in a closed loop control system configuration. The response amplitude operators (RAOs) as well as the response phase operators (RPOs) of the model were evaluated from the heave and pitch data. In addition to the RAOs and RPOs, the amplitude and phase of control surfaces were analysed in order to present the range of control surface deflection as well as the phase lag between the control surfaces deflection and the model motions. Comparing the results of the model without RCS with the results of the model with a passive RCS it was found that the deployment of the T-Foil to a fixed position and acting as a passive control surface reduces the peak heave and pitch motions. As expected heave and pitch were more strongly reduced by their respective control algorithms. This was more evident in the pitch control mode where it significantly reduced the pitch RAO. The centre bow slam force and the demihull slam induced bending moments were also significantly reduced by implementing the pitch control mode. The motion and load results obtained through this unique and comprehensive investigation of a catamaran model fitted with the model scale ride control system have clearly demonstrated the positive effects of using improved ride control algorithms on the vessel motions and loads response. This consequently has a direct impact on the design of future catamaran vessels allowing ship designers to optimise the structural design of the vessel whilst improving passenger comfort and reducing the incidence of motion sickness in particular when operating in larger waves.