An Application of Computed Tomography Inspection on Butt Weld of Super Duplex Umbilical Tube

摘要

Super duplex stainless steel (SDSS) tube is a main component in steel tube umbilicals to convey hydraulicrnfluids, chemicals or to control the flow of gas into subsea wells. Butt welds are used in the umbilical tubernmanufacturing for all tube joints and their quality directly impacts the in service performance of the steelrntube umbilical.rnThe butt welds are 100% inspected by X-Ray radiography. The obtained images are 2D shadowgraphsrnof a 3D object and, as such, they are limited in providing the exact depth of the defects inside the weldrnand part of geometry information of the weld is easily lost. To address this radiography limitation, thernacceptance criteria of NDT are therefore very strict, and generally no porosity is allowed within the buttrnwelds. As result, high rejection rate can be observed for thick wall tube (WT > 2.5mm). This limitationrncan be overcome by replacing X-ray radiography with X-ray Computed Tomography (CT). This techniquernprovides a full 3D picture of the butt weld and allows enhancing the weld quality and improvesrnproductivity by reducing rejection rate caused by lack of information on the flaw depth through the wall.rnThe technology can constitute a step change in the Non Destructive Testing (NDT) technology for thernproduction of umbilical tubes.rnThis paper presents the results of the X-ray CT inspection of umbilical tube butt weld and the conceptrndesign of a computed tomography machine for on line butt weld inspection. The machine is based on arncomputer controlled rotating gantry on which an X-ray source and a digital detector are installed. Thisrngantry can perform a 190° rotation, during which several hundred radiographic projections of therninspected weld can be acquired under different view angles. A purpose developed image reconstructionrnsoftware processes the acquired data to provide the full 3D representation of the weld.rnThe article describes the selected X-ray chain, the reconstruction algorithm and the effect of thernreduction of the number of acquired projections on the resolution of the 3D image of the weld. The studyrndemonstrates that CT is a very pertinent technique for the inspection of butt welds in an industrialrnproduction environment, with superior accuracy compared to film or digital radiography. The combinationrnof a dedicated CT machine and state of the art reconstruction algorithm can provide volume informationrnof the weld flaw, for example gas pores, as small as 5 % of the tube’s nominal wall thickness (150 ^mrnfor a 2.7 mm WT tube), showing the ease of interpretation of CT images with respect to classicalrnradiographies.