A probabilistic approach to the lengths of free pipeline spans

Abstract

Free spans of submarine pipelines may occur as the result of seabed unevennesses. The length of such a free span is crucial for the structural behavior and safety of a pipeline. Until measurements of actual span lengths become available during the operation of a pipeline, the length of a free pipeline span will be an uncertain quantity owing to uncertainties associated with the topography of the seabed and the laying of the pipeline. This paper presents a model for the length of a free pipeline span over a long-troughed seabed depression. The model is based on the equation of a chain and expresses the span length as a function of seabed topography, water depth, laying vessel motions, etc. The model is applied to a reliability analysis of the event that the span length exceeds a critical threshold during the laying of a pipeline. Important uncertainty sources are identified, and a sensitivity study is included. The model is also applied to establish the entire probability distribution for the span length. Finally, the implications of reliability analyses based on the span length model within a general span analysis approach for pipeline planning and design are outlined.