Improved focusing for thick-wall pipeline girth weld inspections using phased arrays

Abstract

Since Engineering Critical Assessment (ECA) became a standard procedure for assessing pipeline girth weld defects, sizing has become a major issue. Despite claims and requests for better sizing, most pipeline trials have shown that sizing accuracies are only within about I mm at best in the vertical (through-wall) direction, and typically worse in the horizontal (circumferential) plane due to beam spread. Defect sizing requirements are becoming more stringent as new high-strength pipes and new applications are being introduced. This applies especially to thick-walled offshore pipelines, particularly tendons and risers, where fatigue life is critical. This project has developed a curved 1.5D matrix array, primarily for improving focusing in the horizontal direction on thick-walled risers and tendons. Initially, the focusing was modelled for a variety of arrays, including the standard flat array. Modelling showed the optimum array to be 120 elements long, with three rows, with a slight curvature for initial focusing. Detailed focusing can be performed using the electronic capability of phased arrays. Normal unfocused Pipe WIZARD phased arrays use 60-element arrays on either side of the weld; in contrast, the new array uses 360 elements for one-side only inspection. The results from this array were compared with the standard unfocused 1D array on a typical calibration block; significant improvements were obtained.