Controlled Weld Toe Profiles for Fatigue Life Extension of T-Butt Joints: An Application to FSOs & FPSOs

Abstract

Application of controlled weld toe profiles can be considered an option to extend the fatigue life of welded connections when ongoing tankers are converted in dry docks to serve like offshore ships (FPSOs and FSOs). Very slim chances to implement such fatigue improvement will arise when these vessels are in service, since a converted ship is designed to be inspected, maintained, and repaired in situ and not in dry dock as it is uneconomical to interrupt production. Codes recognize fatigue life extension by means of a controlled weld toe profile (2004, NORSOK Standard N-004 Rev. 2 October). Application of a controlled weld toe profile during conversion in selected areas previously identified by stress analysis of the hull structure can lead to extend the converted vessel fatigue life to comply with an expected field life. The American Bureau of Shipping S-N curves allow a credit of 2.2 on fatigue life when suitable toe grinding and NDE are provided. A controlled weld toe profile can be applied during dry dock ship conversion to FSO or FPSO to welds in a noncracked condition but that were identified prone to fatigue cracking in a stress assessment analysis under new service conditions. Credit on fatigue life in various codes and results from experimental data obtained from fatigue tested specimens with a controlled weld toe profile are given. Comments on the design of a controlled weld toe profiles and recommendations based on experimental experience for the implementation of equipment to perform a controlled weld toe profile are also given. A fracture mechanics approach for the assessment of controlled weld toe profiles for fatigue life extension purposes is described. Initially, a comparison of stress concentration factors for a typical T-butt ship hull plate connection with and without weld toe profile control determined by finite element analysis (FEA) is presented. Results obtained from the FEA connection such as through plate stress distribution are used in a fracture mechanics analysis to compare the fatigue crack growth curve in as-welded condition to that with controlled weld toe profile. It is demonstrated that weld toe profile control is a feasible method to be implemented to improve fatigue life in the order of 2 of T-butt welded connections of ships, which are under conversion to serve as FPSOs or FSOs. This fatigue life extension factor should not be considered at the design stage.