Reliability and Risk-Based Assessment of Offshore Jacket Structures in the Niger Delta, Nigeria.
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos.
In Nigeria, a large number of jacket platforms installed in the Niger Delta have been operating beyond design life of 25 years due to the exorbitant cost of replacement with new ones. These Jacket structures often undergo corrosion and fatigue damages due to hostile offshore conditions and accidental release of corrosive agents from crude oil production activities. Therefore, there is a growing need to closely monitor the structures to protect unexpected failures. Applications of appropriate assessment methods for corrosion and fatigue hazard mitigation measures can assist to check mate or prevent the jacket structure premature failures. The available basic design techniques and the standards in the petroleum industry standards for the new structures are somehow inappropriate for the assessment of the existing jacket structures. The ambiguity in the characteristics of corrosion and fatigue hazard has made the deterministic approach unsuitable for the risk-based assessment of jacket structures. The above mentioned reasons have made it imperative to search for a new structural assessment technique for the jacket structures. This study established the appropriate relationship between chloride accumulation and diffusion process within the offshore jacket structures. Marine steel structure corrosion damage model was developed for corrosion damage monitoring and mitigation. The study also evaluated the existing jacket platform integrity in the Niger Delta with special reference to the jacket structure system reliability and operational safety. Relevant engineering standards for offshore structures, new design and assessment with special reference to API RP 2A WSD which are widely used in the petroleum industry were appraised. Several jacket component damage scenarios were evaluated and simulated with due consideration to corrosion and fatigue hazard. Reliability method for the assessment of corroded jacket structures was developed. A ratio between reliability of an intact and a corrosion damaged jacket structure known as reliability factor (RF) was also derived to establish when the jacket platform with associated corroded jacket structure would be due for abandonment. The study revealed that jacket structure reliability (Rsj) and RF for three jacket structures investigated in the study are 85.8% and 1.166 respectively. It demonstrated that components with localized corrosion and fatigue damage exhibited an unacceptable risk level that urgently required revamp works