1 © 2023 by ASME Proceedings of the ASME 2023 42nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2023 June 11-16, 2023, Melbourne, Australia OMAE2023-108068 EFFECT OF NONLINEARITY ON TUBULAR JOINT OF JACKET STRUCTURE AND ITS RESPONSE UNDER DYNAMICS LOADING Ram Kumar 1 , Deepak Kumar 1 1 Indian Institute of Technology Madras Chennai, India-600036 ABSTRACT The jacket structure is one of the most important offshore structures. It's a relatively stiff structure because it base is fixed to the foundation. Due to the fixidity at the bottom its natural period of vibration is much small in comparison to other floating structures. A jacket structure is framed structure that is generally made of tubular members and each member is connected rigidly. Under a huge load of platform and ocean waves, the member’s connection/joint undergoes rotation and deformation. Due to rotational and deformational responses at joints, the nonlinearity develops, and depending on structure properties and load it can be significant enough to affect the responses of the overall jacket structure. In this study, the joint of the jacket structure is remodeled using an analytical technique available in the literature, and the effect of the angle between the members is considered one of the factors for the nonlinearity in its flexibility matrix. The present study shows that considering the changing angle between the members shows a higher degree of nonlinearity in the flexibility matrix. Further, the same joint is modeled in Abaqus software for understanding the analytical results obtained previously. Further, buckling of the tubular member at the joint level due to external load is also considered in Abaqus for obtaining the relationship between load and displacement. This helps in obtaining the nonlinear stiffness matrix in the concerned degrees of freedom under the consideration of varying angles between members and buckling at the joint level of the tubular member. Further, the obtained nonlinear stiffness matrix is utilized to do the dynamic analysis of the joint in its concerned degree of freedom. Study shows that the nonlinear effect of joint is considerable enough and its effect is visible on dynamic response. Keywords: Nonlinear dynamics, Jacket structure, Offshore structure, Joints, Dynamics of offshore structure. NOMENCLATURE α alpha β beta τ tau γ gamma IPB In-Plane Bending FE Finite Element LJF Local Joint Flexibility θ theta 1. INTRODUCTION Offshore oil and gas extraction accounts for close to 30% of all the energy used by humans. Scientists and engineers are trying hard to keep human needs for energy interacts with economic issues regarding projects to be cost-effective. Small hydrocarbon discoveries in deep water may be more cost- effective due to the significant upfront investment in production platforms. However, fixed offshore platforms, like jacket structure, are used to extract oil and gas from marginal fields in shallow or fairly deep water. So, the jacket structure is one of the important offshore structures. A huge amount of money is involved in its design and installation. So, the design and analysis of jacket structure in an ocean environment at various stages of installation require a very carefull analysis. Generally, the tubular sections are used as members of the jacket structure. The use of tubular sections is, mainly, due to their excellent properties with regard to resisting compression, tension, bending, and torsion forces. Also, circular hollow sections have relatively low drag coefficients, so they are the most favored choice for elements subjected to wind and wave loading. The most common connection type for tubular structures consists of one or more bracing members (i.e. struts or branches) that are welded directly to a main structural member, or chord. The design of these connections ensures that the connecting struts buckle before joint failure. This type of joint in jacket structure also shows nonlinearity and its effect on structure response and reliability of