Evaluation of using new materials in offshore jacket structures and risers

Generally, comparing the conditions faced by land-based structures, marine structures have to face more difficulties. As a matter of fact, the analysis and design of marine structures that are correspondent with the marine conditions, could be the most demanding and creative task. Offshore jackets are principal substructures widely used in offshore oil and gas industry and offshore wind industry. Furthermore, risers are significant constituents of jackets in offshore interventions. Conventional steel is usually used to make offshore jacket structures and risers. A careful evaluation in finding a favorable alternative to conventional steel could be really productive to simplify the construction procedures and to increase the fatigue life and consequently serviceability life of the entire structure. To this aim, as a new material, high-specific-strength steel has been considered for the manufacture of jacket structure. Moreover, thermoplastic composite pipes has been considered as a researchable alternative to steel risers. Through ANSYS software, dynamic analysis in the time domain has been executed on two assumed three-legged and four-legged jackets and these structures have been utilized respectively as a substructure for an offshore platform and wind turbine. The jackets have been assumed to be situated in the Persian Gulf region. It has been considered that the modeled jackets are made with conventional steel and high-specific-strength steel. The effects of freak wave load and vessel impact load have been included for the analysis. Furthermore, the time domain analysis has been performed on a gas export riser and oil export riser made with conventional steel and thermoplastic composite pipe made with new thermoplastic composite material. The risers have been assumed to be situated in a jacket type offshore platform located in the North Sea. The effect of freak wave load has been exerted on the mentioned risers. Two well-known wave theories which are the 1st and 5th order wave theories are utilized for the analysis of risers. Regarding the three-legged and four legged jacket structures, with comparing axial stresses, maximum displacements, reaction forces and lateral moments of the structures, it has been shown that structural performances of the high-specific-strength steel jackets affected by the mentioned loads, are better in comparison to the conventional steel jackets. Regarding the risers, with comparing the displacements and reaction forces of the mentioned risers, it has been shown that for gas export riser and oil export riser, the 5th order wave exerts less force on the offshore platform in comparison to the 1st order wave. In addition, thermoplastic composite pipe's reaction forces to freak wave has been demonstrated to be less than gas export and oil export risers.