An Innovative Approach for Free Span Analysis of Subsea Pipelines to Optimize Free Span Corrections

Abstract

The integrity of subsea pipelines is required to be verified through external surveys and inline inspection to ensure safe operation. If free spans based on external subsea inspection exceed the design calculated allowable free span length, free span corrections are performed by placing grout bags or mechanical supports along the span. However, as free span corrections are very costly, therefore, further free span assessment should be performed prior to any subsea intervention. An alternative approach of "local free span analysis" is proposed to optimize the free span corrections. Local free span assessment can be performed for each survey span exceeding allowable span length taking into consideration the relevant input data applicable for the specific free span location rather than for a section of the subsea pipeline. It shall be noted that during design phase, allowable free span lengths are calculated by dividing the total pipeline length into segments, and for each segment the worst case input data is conservatively taken. On the contrary, local free span analysis approach considers key input factors such as effective axial force, wave and current (environmental) data, water depth and pipe-soil interaction parameters pertinent to explicit span location. Local free span analysis has been successfully applied for free span assessment of a number of operating and new subsea pipelines. This paper will present case studies where revised allowable free span lengths have been calculated prior to previous approach of direct subsea intervention and highlight the associated benefits. Local free span assessment approach will significantly reduce the number of required free span corrections especially for long length subsea pipelines. The optimization of free span corrections will result in significant cost savingsas well as schedule improvement.