Sea-Fastening Analysis and Design of a Horizontally Transported Large Jacket

Due to advances in the maritime industry and the availability of a wide range of transportation barges, jackets are now being fabricated far from installation sites. They are commonly towed from fabrication yards to installation sites. This paper focuses on the innovative techniques and methodologies used for the seafastening design during such transportations. During transportation, the jacket will experience inertial loads due to barge motions. The intensity of these loads depends on various environmental factors (wave height, wind speed, for example). Additionally, the barge will experience hogging and sagging that results in large concentrated loads at the tie-down locations. Therefore, in order to eliminate unnecessary conservatism, it is essential to also include the structural behavior of the jacket during the tow and design the seafastening members accordingly. The most significant finding is that the hogging and sagging deflections are significantly decreased by considering the jacket stiffness in the transportation analysis. This results in a more practical & optimized seafastening design. A detailed comparison of the numerical model with and without the jacket stiffness is provided to illustrate this stiffening effect. In addition, for launch jackets, the actual value of the timber stiffness is applied between the jacket launch rail and the barge skid beam. Furthermore, appropriate load dispersion is assumed vertically through this arrangement. These features are critical when estimating the actual loads that will be transmitted to the barge transverse frames and bulkheads. It is concluded that the stiffness and load path play an essential role in the seafastening design and barge strength checks. Therefore, this paper discusses its importance by considering an actual jacket transportation case study. The analysis methodology provides practical recommendations to evaluate the actual stiffness of the entire system for the jacket transportation. Conservatisms in the seafastening design are minimized and this results in a more pragmatic design approach.