Full Scale Investigation on Strain Capacity of High Grade Large Diameter Pipes

SUMMARY: The increasing gas demand needs reliable and economic transportation options to gather huge natural gas quantity from remote regions to the final market. High pressure transportation coupled with the use of high strength steel line pipe has been proved as the most viable way to combine technical feasibility, cost effectiveness and reliability over the whole pipeline lifetime span. Crossing harsh and very often "unknown or un-surveyed" areas involves complex ground hazards to be faced. As a matter of fact, in such areas, the integrity of pipelines does not depend any more on pressure containment only, as additional external loading must be considered in the design stage and faced during operation and maintenance (O&M). In order to be able to describe the pipe limit capacity under combined loading, accurate models are required. EPRG (European Pipeline Research Group) companies have launched investigations to study a large variety of modelling approaches, specifically dedicated to combined loading (internal pressure, bending and, possibly, axial load action) [1]. The need to study in depth the strain behaviour under bending and pressure pushed the development of an improved full-scale bend testing facility as pre-requisite for undertaking realistic validation tests. Details of this new full-scale four-point-bend testing equipment, specifically designed by CSM to test large diameter gas line pipe in the plastic regime, are presented. The full-scale testing approach is accompanied by an engineering approach and computational models to forecast pipeline buckling behaviour. Materials selected for full-scale bending tests reported within this paper are the following: API X80 UOE pipe (LSAW), 48" OD x 22.9 mm WT, produced by Europipe; API X80 spiral welded pipe (HSAW), 48" OD x 18.9 mm WT, produced by Salzgitter Mannesmann Großrohr GmbH.