Controlled Rolled Steels for Offshore Pipeline
Authors: Mihelich J.L. 
Published: December 2007

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. This paper discusses the technology and application of these acicular-ferrite and pearlite-reduced steels, which have been used successfully in several North Sea submarine pipelines.

Introduction

The last 10 years have seen a worldwide flurry of alloy design and development activity 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, weldable steels, up to X70 quality, to meet severe installation and operating conditions.

In the early 1960's, pipe manufacturers commonly offered large-diameter line pipe with only 52-ksi specified minimum yield strength. Wall thicknesses were usually no greater than 0.50 in. However, the new trend in the industry is to higher-strength, thicker-wall pipe. Large pipe, ranging from 30 to 48 in. in diameter and, in the Soviet Union, up to 56 in. in diameter, is being used. High operating pressures of 2,000 psig and above and pipelaying in water to depths well below 400 ft emphasize pipelaying in water to depths well below 400 ft emphasize the need for products with thicker walls. Pipe with wall thicknesses of 0.625.to 0.750 in. is already being used and 1-in.-thick pipe will be needed for future projects. projects. While pipe with 60-ksi yield strength (X60) is now in common use, products with higher strength (X65 and X70 grade) already have been placed in crude and natural gas transmission service. Several major projects planned for construction, including the Canadian Arctic Gas Pipeline in North America and the Stratfjord project in Pipeline in North America and the Stratfjord project in the North Sea, will use 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 Research Laboratorie's drop-weight tear test (BDWTT) criteria are used to be sure that the steel will behave in a ductile manner. The operating temperature of the pipeline must be above the ductile-brittle fracture-appearance transition temperature (FATT). Extensive testing is carried out to assure 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 that simulate actual pipeline operating conditions, are often specified. Recently, in a program sponsored by the AGA, Battelle established the following empirical equation:

Cv = 0.0108 (H)2 (Rt)1/3,......................(1)

where Cv is the minimum, full-size Charpy energy in foot-pounds that will produce fracture arrest, H is the operating stress in ksi, and R is the pipe radius and t is the pipe-wall thickness, both expressed in inches. The specified minimum yield strength of the pipe multiplied by a suitable "safety factor" is equal to the operating stress, H.

Eq. 1, using a safety factor of 0.72, which applies to natural gas transmission pipelines in the U.S., is shown graphically in Fig. 1.

JPT

P. 730