A methodology to assess the structural reliability of a pressurized subsea pipeline combing hydraulic analysis of gas-liquid flow and reliability assessment of corroding pipeline is proposed in this paper. The corrosion rate of given corroding subsea pipeline is investigated by analyzing the corrosion mechanism in gas-liquid regime. The limit state functions of the small leak and burst recommended in the general specification ASME B31G are employed to calculate the failure probabilities of the small leak and burst. Due to uncertainties of the basic parameters in the limit state functions, a probabilistic approach rather than deterministic approach is applied to evaluate the failure probabilities with the Monte Carlo method. According to the evaluation result, several suggestions about the in-line inspection and preventive maintenance are given. Furthermore, a detailed procedure for structural reliability analysis of corroding subsea pipeline is presented, and its feasibility is confirmed with a numerical example.
Subsea pipeline is one of the most efficient and safety way to transport oil and gas from offshore field and ensuring its safe and stable operation has theoretical and engineering significance. Evidently, subsea pipeline is required to resist the influence of the harsh marine environment as well as the effect of transmitting fluid which may carry corrosive elements. Among all the causes of the subsea pipeline failures, internal metal-loss corrosion is a major threat which may cause two distinctive failures, namely the small leak and burst through literature review and incident data analysis (Dong, 2015; Yang, 2016; Palmer, 2004; Achebe, 2012). To ensure the integrity of subsea pipelines and develop corrosion management programs, many researchers take efforts to estimate the remaining strength and assess the failure probability of pressurized pipelines containing multiple active corrosion defects.
The deterministic and probabilistic methods are usually adopted in remaining strength estimation and reliability assessment of corroding pipeline. Currently, it is common to employ the deterministic methods to assess the residual strength in pipeline industry (DNV, 2010; Kiefener, 1990; Klever, 1995; Leis, 1997; Netto, 2005). The deterministic methods evaluate the severity of each individual corrosion defect by comparing the operational pressure (the load parameters) with the estimated failure pressure (the resistance parameters). However, there are some drawbacks to use the deterministic approaches (Caleyo, 2002; Melchers, 2008) because the approaches ignore the uncertainties of load and resistance parameters which result from:
the inherent random nature of themselves,
the measurement uncertainty and
lack of data.
Moreover, the deterministic methods are unable to calculate the failure probability of a corroding pipeline at given time due to changes in the load and resistance during the pipeline's service life. Whereas the probabilistic approaches can assess the corroding pipeline effectively and accurately, the reliability assessment based on probabilistic approaches have been studied extensively recently (Caleyo, 2002; Melchers, 2008; Hassan, 2011; Stephens, 2006; Teixeira, 2008; Timashev, 2010; Bisaggio, 2015; Zhou, 2010; Zhang, 2012). The probabilistic approaches are developed mainly using the failure pressure models recommended in the industrial codes such as ASME B31G and the reliability algorithms such as Monte Carlo simulation to predict failure probability of corroding pipeline containing multiple active corrosion defects.
