Safety Assessment on Bracing Node Structure of Semi-submersible Platform for Sub-arctic Based on Fracture Characteristics

Abstract

A semi-submersible offshore platform will be one of the possibilities for the development of natural resources in the Subarctic under harsh environmental conditions.Among the structural components composing a semi-submersible structure, the connection part of horizontal brace to the column is considered to be a key part essential to the overall safety from a structural strength point of view. In order to reduce the stress concentration there, several structural arrangements have been designed. This is one way to provide continuous external and internal brackets penetrating the brace shell and to grind off the bracket ends smoothly to reduce the stress concentration. In this case, the full penetration weld groove at the external bracket end is complicated. Though careful nondestructive testings are carried out, there is still a possibility of weld defects remaining in this area. Therefore, it is worthwhile to confirm the safety, with respect to the brittle fracture, by assuming that a weld defect exists in this area.In this study, a full size local structural model including a bracket end is manufactured, and strain distribution, fatigue crack propagation and brittle fracture initiation characteristics are examined using a 8, 000 ton test rig.In addition, finite element analysis and compact tests such as CTOD tests and fatigue crack propagation tests using 1 TCT specimens are carried out, and it is confirmed that the local model test results of fatigue crack propagation and brittle fracture can be reasonably assessed from these compact test results.Based on these results, safey assessment on brace node of an existing semisubmersible platform is carried out. First, three dimensional elastic stress analysis of the brace-column connection is carried out to get the stress concentration factor at the bracket end and to measure the effectiveness of grinding off the bracket end. Secondly, the long-term statistical distribution of stress responses during 20 years and the maximum expected stress amplitude in 100 year waves at the brace end are estimated. Also, the stress at the bracejend due to the load from ice floe is approximately estimated by beam theory. Thirdly, using these stresses, allowable defect sizes for brittle fracture are calculated by the equation of WES 2805 and the CTOD criterion based on the linear fracture mechanics. Finally, through the fatigue crack propagation analysis over 20 years based on the Paris' law, allowable initial defect sizes are obtained.The result shows that these allowable initial defect sizes are large enough to be detected by nondestructive testings at the fabrication stage.In this way, it is confirmed that the brace node of the semi-submersible platform under consideration has enough toughness to resist fractures.