Duktilitetsgrenser for rørkutepunkt
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- Institutt for marin teknikk 
The nonlinear computer program USFOS is used extensively by oil and engineering companies world wide to evaluate the ultimate limit strength and accidental limit state behaviour of offshore structures, notably in conjunction with reassessment of existing platforms. In this context it is often necessary to take into account strength reserves on both components and connections (joints). Generally the nonlinear behaviour of components in the form of buckling or large deflection, plastic bending is well known, while the behaviour of tubular joints during extreme plastic deformations is more uncertain. To large degree one has to rely on relatively few experimental data. MSL in UK has developed joint strength formulas expressed as nonlinear P-d curves. Such curves have been implemented in USFOS, but they give sometimes strange results, e.g.- the ductility limit is reached before ultimate strength. Ductility limits are also only given for axial forces and not bending moments. An alternative to physical testing is to perform virtual experiments by means of nonlinear finite element analysis. Provided that simulations are verified against available experimental data, parametric studies of various geometrical configurations and load conditions may expand the data basis. The objective of the work is to perform nonlinear analysis with ABAQUS of various joints and contribute to the development of the data basis. The thesis is a continuation of the specialization project done in 9th semester. Simulation of joints with abaqus is performed to verify the simulation procedure with respect to force-deformation behaviour and strain development. Single joints and the same joints as a part of a frame system plane frame system have been simulated. In this paper, non-linear analysis with ABAQUS of X-joints is performed and the simulation results are verified against existed data and studies. Conclusions and further recommendations are given.The results show that behavior of the joint is different when analyzed independently from when in frame system. The reason is that when a single joint is analyzed, the force doesn?t change direction. While in a frame system, the braces has a significant influence to the joint, as the braces can buckle, rotate, etc. which changes the direction of the force acting on the joints. When the through member is in tension, the other two braces will compress it to a very large extent, which leads to a large strain development. That can also explain why the frame system is more stable when the joint is rotated by 90 degree. It is the most critical condition when the separate braces are in compression, which should be avoided in reality.