Contralled Rolled Steels for Offshore Pipelines
Authors: Mihelich J.L. 
Published: October 2008

Controlled rolled X60, X65 and X70 steels, originally developed to meet the stringent requirements of Arctic pipelining, have been used in offshore oil and gas pipelines. The technology and application of these acicular ferrite and pearlite reduced steels, which have been used successfully in several North Sea submarine pipelines, are discussed. Fracture toughness and field weldability of these low carbon steels are highlighted.

INTRODUCTION

The last ten years have seen a flurry of alloy design and development activity, worldwide, by manufacturers of large diameter line pipe. This effort was stimulated by the discovery of large deposits of crude oil and natural gas in hostile environments. The North Sea, as well as the Arctic regions of North America and the Soviet Union, require high strength, tough and weldable steels, up to X70 quality, to meet severe installation and operating Conditions.

In the early sixties, pipe manufacturers commonly offered large diameter line pipe with only 52 ksi specified minimum yield strength (SMYS). Wall thicknesses were usually no greater than .50 inches. The new trend in the industry is, however, to higher strength, heavy wall thickness pipe. Large pipe, ranging in size from 30 inch to 48 inch diameter and, in the Soviet Union up to 56 inch diameter, is being utilized. High operating pressure, up to 2000 psig and above, and pipelaying in water to depths well below 400 feet, emphasize the need for heavier wall thickness product. Pipe with wall thicknesses of.625 to .750 inch is already being used and 1 inch thick pipe will be needed for future projects.

While pipe with yield strength of 60 ksi (X60) is now in common use, products with higher strength, X65 and X70 grades, have already been placed into crude and natural gas transmission service. Several major projects planned for construction in the near future including the Canadian Arctic. Gas Pipeline-in North America and the Statfjord Project in the North Sea will make use of these higher strength grades for technical and economic reasons.

Installation and operation of oil or gas pipelines in cold, remote regions have led to an increase in toughness requirements by the pipeline designer. Battelle Drop Weight Tear Test (BDWTT) criteria are used as a measure to ensure that the steel will behave in a ductile manner.l The operating temperature of the pipe- line must be above the ductile-brittle fracture appearance transition temperature (FATT). Extensive testing is carried out to insure that the pipeline ?operates above the BDWTT-FATT of the steel.

To guard against fast running ductile fracture, Charpy V-notch requirements, established by correlation with full scale burst tests which simulate actual pipeline operating conditions, are oftentimes specified. Recently, Battelle,2 in a program sponsored by the American Gas Association (AGA), had established the following empirical equation:(Mathematical Equation)(Available in full paper) where Cv is the minimum, full size, Charpy energy in foot-pounds that will produce fracture arrest, ?H the operating stress in kai, R the pipe radius and t the pipe wall thickness also expressed in inches.