New Developments in Duplex' Stainless Steel Offshore Pipeline Welding
Authors: Belloni A. , Celant M. , Dallaglio D. 
Published: October 2008

Description of the paper

Weldability of duplex stainless steel was significantly improved in the recent period because of a higher and homogeneous base material quality, and because of the welding technology improvements (welding process and equipment).

Nevertheless, more work must be done to overcomecompletely some residual problems and, most of all, to define and prevent very consistently any serious risk of contamination.

The present paper deals with the advances made by Saipem in the development of a welding technique for laying duplex stainless steel offshore pipelines. In the paper the gradual progresses since the first field application (semiautomatic GMAW process) up to the latest technique (mechanized GTAW +mechanized GMAW processes) are presented.

Application

Duplex stainless steels have been often considered and at times applied in the recent past to develop aggressive oil and gas fields in hostile or aggressive environments whenever other materials were not considered safe or economical or process facilities were impractical.

The increasing number of duplex stainless steel industrial applications are a consequence of its high corrosion resistance, especially in Chloride/ CO2 media, and its high mechanical properties (equivalent to Grade X65), which make it suitable to withstand high stresses during installation (offshore laying) and service (high pressure).

Of course, the overall reliability is the final consequence Of any steps mainly: properly selected material, highest compliance with material specifications and suitable welding procedures within a complete operational cycle fail-safe in terms of contamination hazards.

Results, Observations, Conclusions and Significance

Test results, adopted welding procedure and relevant equipment are discussed in the paper in view of assessing the merits of the latest procedures in order to consistently meet the most recent (stricter) requirements.

The welding procedure was found to be the most critical one, as the weld metal, and its heat affected zone are generally the weakest point in the overall structure.

In addition, if duplex stainless steel is not properly welded, localized corrosion attacks might occur in the weld area. The rate of this attack generally depends on the applied stresses, on the weld metal and heat affected zone metallurgy, on the presence of defects in welds and on the environment.

This kind of attack is generally very severe and could lead to through wall perforation very quickly: consequently, welding procedures must be developed in order to avoid completely the occurrence of any type of localized corrosion.

The suitability of welding procedures has been evaluated both in terms of corrosion resistance and mechanical properties with conclusive good results.

It is therefore appropriate to conclude on the reliability and on the efficiency of proven welding methods. Now, whenever an offshore pipeline is required or preferable in duplex stainless steel, relevant time and cost implications are reliably predetermined always within a good overall quality.

Certainly, the relevant productivity is not yet comparable to the corresponding productivity'in welding a regular carbon steel line.

It is expected and desirable that these improvements occur on the same pattern as per carbon steel, with an additional improvement of the base material, resulting in better weldability compatible with higher welding productivity.