Oil and gas offshore pipelines are one of the main components of a subsea system. A major accident can have a great economic impact due to loss of revenue and the expenses involving actions to mitigate damages to the environment. Therefore, investment in accident prevention through a carefully designed inspection and maintenance plan is necessary. In this scenario, many companies have changed their procedures to ensure the structural integrity of their pipelines — from a model that incorporates empirical safety factors and periodic inspections to another, based on methods that consider concepts of structural reliability to establish risk based inspections.
The collapse pressure of pipelines containing corrosion defects is usually predicted by deterministic methods, either numerically or through empirical formulations. The severity of each individual corrosion defect can be determined by comparing the differential pressure during operation with the estimated collapse pressure.
However, loads and resistance parameters have uncertainties which define the basic reliability problem. These uncertainties are related to the geometric and material parameters of the pipe and the operational conditions. In recent years, many studies have been developed using reliability concepts in order to predict the probability of failure of a corroded pipeline at any given time. The main problem in assuring the integrity and safe operation of pipelines lies in obtaining the necessary accurate prediction of their future condition.
A simple deterministic procedure for estimating the collapse pressure of pipes with narrow and long defects has been recently proposed by Netto (2010). This formulation was based on a combined small-scale experimental program and nonlinear numerical analyses accounting for different materials and defect geometries.
Probabilistic failure analyses of pipelines considering different failure mechanisms have been performed by different authors over the last decade. Limit state functions similar to the mentioned above, coupled with reliability algorithms such as the first-order second-moment (FOSM) iterative method, the Monte Carlo integration method, and the first-order and second-order reliability methods (FORM/SORM) are generally used. The analyses take into account the natural spread of material properties, geometric and operational parameters, and the uncertainties associated with the sizing of eventual corrosion defects. In this paper, Netto’s deterministic formulation and the crude Monte Carlo method were used to obtain the reliability of corroded pipelines under external hydrostatic pressure.
This approach provides a method to predict the probability of collapse of a corroded pipeline along its operational life. It applies concepts of structural reliability to evaluate the detrimental effect of corrosion damages, giving the basis to develop a risk based maintenance strategy.