Control of fixed offshore jacket platform using semi-active hydraulic damper

Abstract

Semi-active control of fixed offshore jacket platforms in shallow water depth using semi-active hydraulic damper (SHD) is presented. The semi-active control uses Bang Bang control algorithm to provide the required control command to the SHDs. In the control strategy, SHDs are installed in the bracings, making it an attractive proposition for the use in jacket structures. Although the main objective of providing semi-active control devices is to control the acceleration response of the deck for smooth operation and comfort condition, the reduction in base shear and displacement are also considered as important. In particular, great reduction in foundation cost can be achieved by base shear control under extreme sea state. The effectiveness of the control scheme is investigated by varying some important parameters such as the damper capacity, the maximum damping coefficient, and the distribution of dampers along the height of the structures under various sea states. A four legged steel jacket platform in a water depth of 122 m is considered as an example problem for finding out the controlled responses. Three wave conditions are considered, namely, operating wave, extreme wave, and irregular sea state. Results of the study indicate that significant control in acceleration response can be achieved by semi-active control using SHDs. Further, considerable reduction in base shear is also possible by providing SHDs of adequate capacity at the bottom-most level of the jacket.