Seamless or ERW for Sour Gas Service?

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract The shortage of seamless API standard 5L Grade B pipe, a preferred choice for sour gas service, is forcing consideration of ERW (electric resistance welded) pipe as a replacement. ERW welds, unless they are adequately post weld heat treated, can contain hard zones post weld heat treated, can contain hard zones that are susceptible to sulfide cracking. The use of metallurgical cross-sections and microhardness surveys as quality control tests are discussed as they apply to the ERW heat affected zone. The conclusion reached is that ERW pipe is satisfactory for sour gas service provided adequate quality control is maintained. provided adequate quality control is maintained Introduction Sour gas wellhead equipment and related gathering lines as found in East Texas, Mississippi, western Canada, and other locations are subject to failure by sulfide stress cracking mechanisms. The potential release of toxic hydrogen-sulfide gas to the atmosphere due to leaks is a major concern. Hydrogen sulfide can be fatal after a few breaths in concentrations as low as 0.1 percent. The safety record of the industry has been good, largely due to the prudent selection and quality control of both materials and fabrication techniques. Cities Service Co. has been conservative in this regard and currently specifies that only seamless API 5L Grade B pipe be used for gathering lines. Recently, seamless pipe has become more difficult to acquire with suitable delivery schedules; therefore, consideration was given to the use of electric resistance welded (ERW) pipe for sour service. pipe for sour service. Tuttle, in a paper describing selection of material for use in a sour gas environment, proposes a sound philosophy, "During engineering proposes a sound philosophy, "During engineering design, materials selection, quality control, and construction, the key issue should always be, is this item suitable for sour service? Economic considerations can be included only if this question can be answered 'Yes' on all items." Mr. Tuttle goes on to state, "Before new material is chosen for sour gas service, it should be fully tested in the laboratory and the hardness determined." The preceding considerations become extremely pertinent, since ERW pipe is more economically attractive than seamless. Furthermore, the thinner walls available in ERW pipe permit a weight savings of steel and longer pipe permit a weight savings of steel and longer mill lengths, and require fewer field girth welds as compared with seamless. These two latter factors further increase the desire to use ERW pipe. The criterion is therefore one of suitability of ERW pipe for sour gas service, particularly as related to hardness level. particularly as related to hardness level.