Understanding the operational envelope of motion compensated access bridge systems
René Lindeboom, Edouard Schinkel
MARIN, Wageningen, The Netherlands
Motion compensated access bridge systems gain popularity in the offshore wind maintenance operations. These systems allow for an increase in weather windows to perform maintenance operations in progressively more extreme offshore environments. Having knowledge of the operational limits of such access systems will enable operators to even further decrease the lifecycle cost of energy of offshore wind projects. To understand the limitations of such systems model tests have been performed, where the motion response of the access bridge in disconnected mode (position control) and transfer mode (landed) has been assessed. For the model test campaign a typical offshore supply vessel with an overall length of 62.28m, a width of 13.5m and a draft of 3.75m has been constructed. During the model tests the DP system has been modeled by a softmooring system (4 horizontal springs) that mimic the stiffness of a typical DP system. In addition the access bridge has been modeled with full control in the pitch motion and telescoping motion. The performance of the system in intact and failure mode for different wave conditions reveal the operating limits of the system. For the intact cases position accuracy of the bridge and connection characteristics have been identified. Since safety is the most important aspect in offshore operations it is important to gain insight in typical parameters that define the safety levels of the system. In this model test campaign failures have been simulated for which the associated accelerations have been used to define safety in case of failures.
The method used for assessing the performance of motion compensated access bridge systems is based upon model test standards as used for the offshore industry. Firstly, an aNySIM time domain simulation model has been setup that includes the hydrodynamics, the hydraulics and the control system of the access bridge system. For optimum performance the control system has been tested and optimized in the numerical model prior to the model test campaign. Measurements that define the performance of the system have been identified and comprise, but are not limited to the motion and acceleration measurements of vessel and subcomponents of the access system, power consumption, loadings on deck and landing platform.
For disconnected mode position accuracy of the access bridge system was tested in beam on conditions for significant wave heights up to 1.5m close to the roll period of the vessel. The bridge was oriented 90 degrees sideways achieving full-scale position accuracy in height and in the horizontal plane of approximately 15cm and 6cm respectively. For certain severe roll induced conditions the telescope length was insufficient which caused inaccurate position control. Connection of the system to the landing platform is friction based, which in turn is dependent on the normal pressure applied by the bridge. During the tests maximum normal loads on the platform were exceeded. In addition safety in failure mode has been identified and appeared to be within specifications for safe operations.
In the model test campaign clearly the limitations of the access bridge system are revealed as a function of the prevailing wave conditions. It must be noted that on purpose extreme rolling behavior has been simulated to gain insight in the full operating envelope of the system. Thereby limitations at sea states around 1.5m indicate that the operation with the access bridge in sideways orientation has a limited position accuracy in disconnected mode and exceeds the allowable loading on the landing platform in connected mode. Reorientation of the access bridge would likely increase the position accuracy and reduce the loading. Hereby clearly the model tests are a means to identify and optimize the operating philosophy of such systems.
The objective of sharing our acquired knowledge on access bridge systems is to make designers, operators and developers aware of the possibility to optimize the design of the system and the operation of access bridge systems by performing model tests. By assessing the performance of the access bridge system-vessel combination in different metocean conditions a better understanding of operation windows and safety in operations will be achieved. The knowledge gained in the tests enables e.g. the identification of favourable positions of the system on the deck of the vessel and the development of good operating practices for access bridge systems.