ABSTRACT:

Since many years Bosch Rexroth develops and builds linear and rotational active heave compensation systems (AHC) and passive heave compensation systems (PHC systems) such as riser tensioners and line tensioners. Those systems are designed for offshore vessels to compensate for vessel motion during the (sub sea) handling of loads. AHC systems actively control the absolute position of a load by means of a MRU setpoint. PHC systems passively maintain tension on a wire line or riser by means of a large volume of compressed gas. The accuracy of a passive (PHC) system is largely dependent on the behaviour of the gas volume, friction and inertia in the system. Recently an increasing demand is seen for more accurate cylinder based PHC systems. To improve this accuracy Bosch Rexroth is developing an active controller that can control the wire line or riser tension in a similar way as Constant Tension (CT) winches. Next to the passive tensioner, an active cylinder is placed to control this tension. This new development results in Active Tension Control for Linear Heave Compensation systems. To test this new development in reality, a 700 Tons linear AHC system delivered to ENI Saipem - Saipem3000 was equipped with Active Tension Control (ATC) and during the sea trials this new feature was tested in reality. The ATC function on the existing AHC is achieved by adapting the control algorithm of this heave compensator, such that the tension in the cable can be controlled accurately. Mainly three objectives are set: Increased tension control accuracy, compensation of friction and gas pressure variations and smoother transfer between position control (AHC) and tension control (ATC). This paper describes the objectives of such a system function and the simulations done to come to a control concept. It also compares the actual sea trials with the simulations resulting in a conclusion about Active Tension Control for Heave Compensation Systems

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