Inspection and Remaining Life Assessment of Subsea Pipelines Exposed to Various Forms of Internal Corrosion Degradation Mechanisms

Abstract

Subsea pipelines are exposed to many threats and DNV RP-116 separates the threats into time dependent and independent threats. The time dependent threats include internal and external corrosion, hydrogen embrittlement & stress corrosion cracking and fatigue crack growth whereas the time independent threats apart from manufacturing and construction defects include third party interference and outside forces. For remaining life and risk assessment and inspection planning the total risk associated with all the mentioned threats should be covered by computation of combined risk of the individual threat probabilities and the associated consequences from a safety, company reputation and business perspectives. The paper discusses three cases of subsea pipeline remaining life assessments involving dry gas export lines, gas and condensate in field lines and flow lines, where different internal corrosion mechanisms such as CO2, Top of the Line (TOL) and micro biological corrosion (MIC) are posing the main challenges to the estimation of the remaining life, selection of inspection methods, validation of the assessment results and the overall risk assessment. The paper shows how the different operating conditions lead to less severe, more severe and extremely severe metal loss due to mainly three different types of corrosion mechanisms. In the first case it is shown how the remaining life can easily meet the future operation and life extension target with benign operation conditions, whereas two other cases are shown not to allow life extension meeting the future needs without section or complete pipeline replacement.