During offshore installations in harsh sea conditions, the involved gangway system must satisfy rigorous requirements in terms of safety and accuracy. The forces resulting from the vessel motion have an extensive effect on the overall gangway structure and its lifetime. Moreover, vessel motion handicaps the operator during fine positioning of the gangway during connection to a platform. Hence, an active motion compensation system for the gangway is used. An impression of the gangway structure can be seen in Figure 1 below. In this thesis, a 2D-model for the gangway system is developed in order to simulate the Active Motion Compensation (AMC) required to compensate wave induced vessel motions. The model can be used to estimate the power required by the system, needed to compensate the motions in order to minimize the movements of the connection point. The model is developed using software Matlab Simscape.
The gangway is capable of performing single stage telescoping motion, activated by a winch that pulls the telescoping section back and forth, luffing motion by activating of the hydraulic cylinders and slewing motions by means of a slewing bearing. In order to develop an as accurate as possible model, the whole gangway system is divided in three sub-systems: the mechanical, the hydraulic and the control sub-system. The main outputs of the mechanical sub-system are the movements, velocities and accelerations of the gangway system relative to its mass and center of gravity and based on the RAO’s of the vessel. For the hydraulic luffing system, all relevant hydraulic components are modeled. A directional proportional (control) valve in the hydraulic sub-system is used to regulate the amount of flow through the system. In order to compensate the gangway tip, a feedback on the position and a feedforward on the velocity of the compensation point is applied. The inputs from the MRU, which are the imposed ship motions (heave, sway and roll), are converted to the motion of the tip of the gangway by a PLC controller. These converted signals are used to determine the control set points for the luffing and telescoping actuators. A basic proportional controller is used for the Active Motion Compensation system of the gangway.
In order to validate the numerical model, the simulation results are compared with experimental results from quayside tests. During these tests, different sinusoidal signals are applied for the 3 possible motions: translation in vertical direction which mimics the heave motion, translation in the horizontal direction which mimics the sway motion and a rotation around the main hinge which mimics the roll motion of the vessel. When applying only the roll motion or the sway motion, the luffing function and the telescoping function respectively could be assessed separately. Besides these three signals, combinations of this motions are also provided to assess the total performance of the system. Several variables are logged during the tests of which the compensation error, the measured set points, and pressures in the hydraulic cylinder are analyzed thoroughly and compared with the simulation results. It is observed that the simulation results show important similarities in the dynamic response of the gangway on the imposed ship motions.
The current model provides a good insight in the behavior of the gangway system and can be used to simulate offshore tests in different virtual weather windows before testing the real gangway offshore. This is the main advantage compared to the case before this thesis is conducted. It is recommended to extend the 2D simulation into a 3D simulation by adding the slewing function. It’s also recommended to further study the controller design in order to improve the AMC performance.