Qualification of Weldable Modified 13% Cr Stainless Steel Linepipe for Reelship Installation
Authors: Duthie Stephen A. 
Published: October 2008

The application of weldable modified 13%Cr linepipe is considered a cost effective alternative to solid duplex stainless steels and bi-metallics currently used by the Oil & Gas industry for subsea Corrosion Resistant Alloy (CRA) pipelines. Installation of pipelines by the reelship method results in the pipe material undergoing plastic deformation during several stages. The selected line pipe and girth welds which make-up the pipeline must therefore exhibit reeling capability such that the material uniformity is not affected and material properties still comply to the design limits set for conventional installation. Coflexip Stena Offshore (CSO) have undertaken a test programme to establish the suitability of 13%Cr line pipe for installation by the reelship method. The test programme evaluates several material types and includes trials to select optimized welding processes and consumables followed by simulated reeling trials of 14 m lengths of pipe material which contain girth welds. These being subject to dimensional monitoring, non-destructive, destructive and corrosion testing. The linepipe material has to exhibit good weldability, mechanical and corrosion properties after being subject to plastic strains from bending. Welding processes and consumables have to offer good weld productivity plus achieve satisfactory weld properties in the 'as welded' condition after straining. Conclusions from the test programme highlighted that the linepipes tested exhibited good weldability and material properties after simulated reeling. Similarly, using duplex and super duplex welding consumables in conjunction with the mechanised Pulsed Gas Metal Arc Welding (PGMAW) and gas Tungsten Arc Welding (GTAW) processes, desirable girth weld mechanical and corrosion properties were achieved.

Introduction

The use of martensitic 13%Cr stainless steels offer excellent resistance to corrosion in CO2 containing fluids and consequently have been extensively used as downhole tubular products for the oil and gas industry. More recently, the development of super 13%Cr steels has improved the general corrosion resistance of these materials and offers limited resistance to SSCC in environments containing small amounts of H2S. In the past, 13%Cr stainless steels have been precluded from use as linepipe due to their poor weldability. As these materials are considerably cheaper than duplex stainless steels and offer comparable corrosion resistance in CO2 environments containing small amounts of H2S, there has been considerable interest in recent years for the development of a weldable 13%Cr. On the basis of this, linepipe manufacturers have undertaken extended development programmes to produce a weldable 13%Cr linepipe. This was claimed to be achieved by principally lowering of carbon and nitrogen levels. Prompted by the interest of the oil and gas industry in the development of weldable super 13%Cr linepipe, it was decided to undertake a testing programme to evaluate these materials for installation by the reelship method. This would give the opportunity, to gain information about these materials prior to acceptance of a contract award where they may be applied. Joining a Group Sponsored Project (GSP) was considered in '95, however, there was concern that this would not yield results before an operator had already opted to select and procure 13%Cr linepipe.