Reliability-based Assessment of Deteriorating Ship-shaped Offshore Structures

Abstract

Floating production, storage and offloading ships, referred to as FPSOs, are hybrid structures in the sense that these vessels are ships being operated as offshore facilities. Production ships or FPSOs constitute an efficient solution for remote oil field locations due to their storage capacity of crude oil. Production ships are either tanker conversions or purpose-built vessels. Most of the applications of production ships are converted ocean-going oil tankers, of different ages, to operate in relatively benign environmental areas.

Keeping in mind that merchant ships are usually dry-docked every fifth year for inspection, maintenance and repair (IMR) tasks, they are designed according to a more relaxed safety criteria compared to those normally applied for permanent offshore structures, in which the required IMR activities need to be carried out in-situ as they cannot be easily dry-docked. Moreover, ocean on-going ships may in principle avoid heavy weather conditions, whereas FPSOs are moored to a fixed location and could be exposed to harsher extreme conditions. Additionally, FPSOs are exposed to continuously varying still-water load effects, due to the permanent loading-offloading operations. These fundamental differences are even more important to recognize as most of the existing FPSOs around the globe are built based on converted hulls of oil tankers that have already operated during some years. This issue is particularly relevant for vessels whose service life need to be extended, as the deteriorating agents such as fatigue cracking and corrosion become more important.

During the initial design of ship structures, the effects of fatigue and corrosion are accounted for separately. However, as the structure ages, the interaction between these two degrading agents increases, and thus such an interaction needs to be accounted for. Moreover, as the number of cracks in a hull structure increases with time, the likelihood of fracture occurrence in main structural components rises accordingly. Therefore, it is indispensable for FPSO operators to assess the safety of existing vessels with proper consideration of the uncertainties involved.

The aim of this dissertation is to discuss the safety assessment of an existing shipshaped offshore structure subjected to deterioration, where the interaction among different deteriorating phenomena such as fatigue, corrosion and fracture, are explicitly accounted for in a systematic and consistent manner. The format of this thesis report consists of an extended summary intended to emphasize the main contributions achieved and the relevant issues dealt with during this research work, which resulted in the production of three articles that have already been published, annexed at the end of the report.

The first article, referred to as Article 1 throughout this report, was published in the International Journal of Fatigue (2007). This article deals with the treatment of uncertainties related to the fatigue crack growth of surface cracks on plated connections and compares different reliability-based limit state formulations including a bi-linear crack growth law that is recommended by the British Standard BS-7910 (1999). Calibration of the bi-linear fracture mechanics formulation is performed with respect to design SN curves considering the parameters with largest uncertainties.

The second article, Article 2, published in Reliability Engineering and System Safety (2008) describes a procedure based on reliability techniques to assess the safety level on a welded connection of an aging FPSO with respect fatigue failure, taking into account in the crack growth estimation the effect of the vessel being exposed to various climate conditions throughout the service life. This means that the fatigue damage accumulated under previous operational conditions of the vessel, e.g. as tanker before conversion, is explicitly accounted for in the failure function. Thus, the uncertainties are also explicitly considered for each environmental condition the vessel has been or is being exposed to. In this study, the combined effect of corrosion and fatigue is used to evaluate the failure probability i.e. explicit interaction between fatigue and corrosion is accounted for. Moreover, reliability updating with inspection results is carried out by defining relevant inspection events, emphasizing the implications of inspection quality. The procedure can be used to assess service life extension.

Finally, Article 3 of this dissertation, which was published in the Journal of Offshore Mechanics and Arctic Engineering (2007), deals with the assessment of fracture likelihood of a cracked stiffened panel on an FPSO hull girder when exposed to extreme environmental conditions. The proposed methodology includes a procedure to quantify the random propagation of a long crack in a stiffened panel, i.e. through-thickness cracks. Further, the procedure takes into account the mean stress effects induced by the continuously varying still-water loading, which relates to the loading-offloading operation of the offshore vessel. Moreover, an efficient timevariant reliability based procedure is established to estimate the probability of fragile fracture failure of a stiffened panel with an abnormal transverse crack, detected by leak at the bottom plating/deck structure of an FPSO. The master curve approach is utilized to determine the decreasing fracture resistance threshold, induced by the long crack random propagation in the panel, in order to perform the up-crossing rate estimation of the loading process. The failure probability is determined by means of Monte Carlo simulation based on a time-invariant procedure proposed by Marley and Moan (1994).