welding & Laying experience of pipelines in CM materials
Authors: Belloni A. 
Published: October 2008

Over the last ten years there has been a gradual increase in the use of CRA materials for offshore installation. Since the first relevant offshore pipeline project laid in 1985 for Shell (tnde project/North sea : 12"-20" O.D. in Duplex), the major operator involved in the oil and gas industry has exerted a continuous and progressive actions directed to improve the overall reliability of CRA pipelines mainly : the linepipe manufacturing methods, the girth welding and NDT procedure and the acceptance criteria for pipeline corrosion resistance.

Information on a number of projects laid in Duplex and Clad material are reported, showing the progress in the adopted welding techniques andthe chained benefits.

On the basis of positive result recently obtained, pipeline project in CR-A material maybe planned for installation with more safe and economical approach.

Furthermore, the same may apply on advance solution like : deepwater subsea completion and direct evacuation lines (multiphase flow), extending the variety of fluids to be transported without need of process and treatment facilities.

The major obstacles for CRA pipeline installation grow, are the high pipe manufacturing and installation costs, As regards of pipeline installation, costs are directly depending on the lay-barge productivity rate which is influenced by the simplicity and the speed of the adopted welding procedure, Saipem is involved in a research and development work, partially funded by E.E, C. to increase the speed of CRA (Duplex, Superduplex and Clod) pipeline welding.

The use of the Hat wire GTAW process (Roboweld system) and the application of the GMAW process or the root pass (Passo system) are considered the novelty of the program in that they will generate a faster welding procedure over the present (GTAW+GMW) normally used in production, The increase in productivity is expected to be at least 20%. All the relevant information are reported together with the recommendation for field application.

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CONSOLIDATED RESULTS

The analysis of the laying data acquired through the construction of four previous projects in CRA material consent to outline the following considerationsOffshore pipeline projects in less tolerant than those made CRA material are in carbon steel in term of pipe handling and overall pipelaying operation, but the present consolidated precautions adopted to treat the pipeline during offshore work, are safe enough to guarantee the pipelaying operation with high reliability.The welding procedures developed to perform the girth weld are now well established and such to satisfy the present requirements in term of mechanical properties and corrosion resistance.The repair rate, based on the acceptance criteria provided by the most qualified international. Standards, is not higher than 5% and therefore similar to that obtainable on carbon steel pipe lines by the conventional GMAW process (Fig. 1).The pipelaying productivity is at present not yet attractive as that obtainable on a carbon steel pipeline. Further research work is therefore required on this subject even a substantial progress has been made since the beginning(Fig. 2).