Assessment Of Pipeline Reliability Under The Existence Of Scour-Induced Free Spans

ABSTRACT A probabilistic model for reliability analysis of steel pipelines with random defect and material properties and exposed to static and dynamic loading has been developed. One application of the probabilistic method is for assessment of pipeline reliability with scour induced free spans. This problem is well suited for a probabilistic analysis because the nature of material properties, the number and size of welding defects and the loads are highly random. The main conclusion is that the probabilistic method provides a powerful tool for analysis of pipelines with scour induced free spans. By using the probabilistic method overly conservative assumptions which are often the outcome of deterministic analyses can be avoided. INTRODUCTION Typical for scour induced free spans is that the overall condition changes from year to year. New spans are created while others disappear. During the life time of a free span the pipeline is subject to fluctuating loads from waves and current. A typical type of damage introduced to the pipe is fatigue crack growth starting from a weld defect and leading to pipeline rupture under a high load. Due to the complicated load history and because the material properties are randomly distributed it is a complicated task to assess the reliability of a pipeline with scour induced free spans. A reliability analysis of pipelines with scour induced free spans is not a standard type of analysis which is well covered by present deterministic codes. If present codes are used one therefore tends to select input parameters over-conservatively. By using a probabilistic approach it is possible to model the distribution of the parameters directly. The analysis gives a more detailed picture of the effect of the defects and loads and shows which parameter uncertainties are of most importance. Further, over-conservative results can be avoided which again can reduce the extent of costly remedial actions which could have been the result if conclusions from a purely deterministic analysis had been followed. This paper details a probabilistic model applied on a pipeline with scour induced free spans. Additionally, some results from a practical example for a 30" pipeline are shown. MODELING OF LOAD EFFECTS The additional stresses introduced in the pipeline due to the presence of a free span can be divided into three categories: static axial and bending stresses due to span geometry, dynamic stresses due to wave loads and dynamic stresses due to vortex shedding. A mathematical model has been established to calculate the static free-span stresses, Naess Larsen et al (1983). The model utilizes that the span acts as a beam for short spans and gradually activates axial stiffness for increasing span lengths. Further, the model utilizes that the effective axial force in the span is connected with the midspan deflection.