International Journal of Fatigue 25 (2003) 609–619 www.elsevier.com/locate/ijfatigue Structural integrity assessment of offshore tubular joints based on reliability analysis J. Rajasankar * , N.R. Iyer, T.V.S.R. Appa Rao Structural Engineering Research Centre, CSIR Campus, Taramani, Chennai 600 113, India Received 5 November 2001; received in revised form 3 November 2002; accepted 10 December 2002 Abstract The paper presents a reliability-based approach and the investigations conducted based on it to assess the structural integrity of offshore tubular joints. The reliability analysis is carried out using Monte Carlo simulation technique and first order reliability method (FORM). The crack in addition to the geometry, material, and loading of the tubular joint are modeled as random variables to compute the reliability of the joint. Linear elastic fracture mechanics principles are employed to evaluate the crack growth. Degradation in the reliability of the tubular member due to crack propagation is estimated. Structural integrity assessment is carried out for joints having two adjacent discrete cracks to study the influence of the relative sizes of cracks on the reliability of the member. The significance of randomness in the variables modeled and the sensitivity of the reliability index on those variables are studied using example problems. The application of the results of the present study to schedule inspections with optimal time interval is highlighted. Finally, a method based on Bayesian approach is applied to update the reliability of the member based on the outcome of in-service inspections. The paper contributes to the knowledge of the application of reliability-based techniques for integrity assessment of offshore tubular joints.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Structural integrity assessment; Linear elastic fracture mechanics; Reliability analysis; Offshore tubular joints; Inspection planning; Reliability updating 1. Introduction The principal criterion in the design of an offshore structure, as in the case of any other structure, is to ensure that the structure safely performs its intended functions during the design service life. A design based on rational approach guarantees that the structure per- forms its intended functions for the whole of design life. In certain situations, the structural design and assessment practices are based on component level whereas in cer- tain other situations, a global level approach is necessi- tated. The safety requirements of offshore structure are generally assessed at component level, following design codes. In general terms, if the safety requirements at component level are not satisfied, it implies that the * Corresponding author. Tel.: +91-44-2254-2130; fax: +91-44- 2254-1508. E-mail addresses: sankar@sercm.csir.res.in (J. Rajasankar); http://www.sercm.org (J. Rajasankar); nri@sercm.csir.res.in (N.R. Iyer); emeritusFa@sercm.csir.res.in (T.V.S.R. Appa Rao). 0142-1123/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-1123(03)00021-5 structure needs strengthening in order to meet the additional demands. However, taking into account the design procedures and the structural redundancy avail- able in following standard design practices, it can be taken that in spite of failure of few components, the structure can undergo a member load redistribution and thus avoid failure. Maintenance of structural integrity of critical compo- nents is an important issue in many fields of engineering. The objective is to ensure economical and safe operation of the facility that employs the structure. Offshore struc- tures are intended to perform in hostile/aggressive mar- ine environment. As such, these structures are highly susceptible to fatigue cracking due to high stress concen- tration, random loading and influenced by corrosive mar- ine environment. The presence of fatigue cracks intro- duces a compromise on the integrity of the structure or its components. The critical components in an offshore steel platform are the steel tubular members and the associated joints formed by welding the members. The joints are highly susceptible to fatigue cracking due to