Welding And Construction Requirements For X80 Offshore Pipelines
Authors: Price J.C. 
Published: October 2008

The trend for increased demand of natural gas has prompted the development of huge gas fields, which will require large diameter pipelines at higher operating pressures. The ability to decrease wall thicknesses through the use of higher yield strength pipe can dramatically effect fabrication costs and make projects more commercially viable. It is therefore anticipated that API 5LX80 pipelines will become commonplace by the end of the decade; provided welding technology can be developed to match mechanical properties without affecting productivity, as compared to X65 and lower pipe grades.

/t is questionable whether manual welding techniques, such as traditional stick-electrode welding, will consistently achieve the required weld properties. As a result of this, more emphasis will be placed on the utilization of mechanized pulsed-gas-metal-arc (PGMA) welding equipment, such as that developed by CRC-Evans Automatic Welding.

This paper discusses the welding procedure development work that has taken place in order to match the mechanical properties of X80, such as toughness, and how to maximize productivity with various filler wire alloy systems shielding gases and PGMA welding parameters. The effects of steel metallurgy on properties and the economics of using X80 steels are also discussed.

INTRODUCTION

The recent completion of several large diameter long distance offshore pipelines in the North Sea and the anticipation of many more by the end of the decade, particularly in the Far East, has prompted the development of API 5LX80 linepipe by means of plate rolling technology by Thermo-Mechanical Controlled Processing (TMCP). Future pipelines could be 84 inches in diameter and operate at 100 bar. This increase in yield strength, provided by X80 steels over present X65 steels, will enable lower wall thicknesses (25% reduction over 42" X60) and possible lower construction costs. The micro alloy design and compositional control of such steels is critical in order to achieve the desired properties, such as satisfactory weldment toughness and weldability, to avoid problems such as heat affected zone cracking. In addition, maximum resistance to hydrogen induced cracking (HIC), otherwise known as stepwise cracking (SWC) or hydrogen pressure cracking (HPC), sulphide stress corrosion cracking (SSCC) or stress orientated hydrogen induced cracking (SOHIC) is also of prime consideration for sour gas service. The typical composition of an X80 steel is shown in Table 1.

The following sections detail all aspects of welding procedure development work to date.

WELDING PROCESS SELECTION

Gas Metal Arc Welding (GMAW)

Mechanized gas metal arc welding has become the standard for welding long distance pipelines greater than 20 miles in length throughout the world. The CRC-Evans Automatic Welding System has been used by many contractors, with great success, and with over 18,000 miles laid since 1969. It consists of (1) a pipe facing machine used to produce the unique, multi-faceted bevel, (2) a combination lineup clamp internal welder to deposit the root pass on the inside of the pipe, and (3) the external mechanized welders used to deposit all subsequent passes