Evaluation of Brittle Fracture Strength of Tubular T-Joints of Offshore Structures under Tensile Loading (1st Report)

Abstract

As offshore structures are used in rougher and colder seas, the rigorous safety has been demanded. Nowadays, it becomes more common to conduct CTOD tests for weldments as a steel prequalification test and/or a welding procedure test. However, there have been arguments that the CTOD criterion give a much conservative assessment.In these circumstances, it is essential to establish the precise evaluation method for the structural integrity from fatigue crack growth to brittle fracture in offshore structures. For offshore structures, the most important structural members are tubular joints, where high stress concentration takes place.From this viewpoint, the strain distribution and the fatigue crack growth characteristics in the hot spot area have been investigated with tubular T-joints model specimens under tensile loading, and the brittle fracture strength of the model tests has been evaluated based on the CTOD criterion.The results obtained are summarized as follows.(1) The elastic stress distribution around the connection of the tubular joint is well evaluated by the FEM stress analysis, and the elastic stress concentration factor at the hot spot agrees with the predicted value.(2) The plastic strain concentration factor can be estimated using Eq. (1) as a function of the elastic stress concentration factor and the nominal applied stress. In case where residual stress exists, Eqs. (2) to (6) can be used to analyse the effect of residual stress.(3) The fatigue crack propagation behaviour at tubular joints can be evaluated reasonably using the linear elastic fracture mechanics.(4) The critical CTOD obtained from the tubular joint model test agrees well with that from 3-point CTOD test. Therefore, the CTOD criterion can be applied to the fracture of structural component with 3-dimensional stress concentration such as tubular joints.(5) The brittle fracture strength of the tubular joint models can be evaluated using the estimated hot spot strain and the proposed CTOD design curve by taking into account the welding residual stresses.