The Role Of Duplex Stainless Steels For Downhole Tubulars

Abstract

ABSTRACT In sour conditions there is an increasing trend to turn to corrosion resistant alloys for downhole tubulars. The most commonly used CRA tubular is 13Cr, and there are thousands of feet in service. However, there are limits to the use of 13Cr, ie., the risk of sulphide stress corrosion cracking at high HZS levels, and the possibility of pitting or high corrosion rates in waters with high chloride contents. Where the service conditions are felt to be too severe for 13Cr alloys it has been traditional to switch to nickel base alloys such as alloys 825 and C-276 (UNS N08825 and N10276). The alloys are much more expensive than 13Cr, and in recent years the duplex stainless steels have been selected as alloys with superior corrosion and SSCC resistance compared with 13Cr, and having lower cost than nickel alloys. Originally the 22Cr duplex alloy (UNS 31803) was used, but more recently the 25Cr super duplex alloys (UNS S32760 and S32750) have become more available. The present paper reviews the data available for 13Cr and the limits of applicability. Data is also presented for laboratory tests for both the 22Cr and 25Cr super duplex alloys. There is extensive service experience with both 22Cr and 25Cr super duplex in the North Sea, covering both downhole tubulars, manifold and post wellhead equipment. Data is presented showing some of the sour condition being experienced in the North Sea by super duplex alloys. These results show that there is a substantial gap between the limits of use for 13Cr and the 25Cr super duplex stainless steel alloys. This means that in many sour environments super duplex stainless steel provides a cost effective alternative to nickel-base alloys. INTRODUCTION Oilfield downhole tubulars (OCTG'S) are usually high strength carbon manganese steels. However, under severe operating conditions, such as high C02 or H2S, excessive corrosion occurs. Under such conditions it is necessary to consider the use of a corrosion resistant alloy (CRA). These range from 13Cr martensitic steels through duplex stainless steels to nickel base alloys. The aim of the present report is to review the available data on the corrosion resistance of CRA's from both laboratory tests and service experience, and suggest limits of use. MATERIALS The alloys being considered in this review are listed in Table 1 with their compositions. The 13Cr martensitic alloys are widely used by the oil and gas industry. Alloy 410 is low carbon and hence it is not possible to obtain such high strength as alloy 420 and L80. However, because of its lower carbon content alloy 410 can be welded, and it has been widely used for wellhead and Christmas trees. It is easier to achieve a uniform martensitic and delta-ferrite-free microstructure with alloy 420. Delta-ferrite is detrimental to both impact and environmental cracking resistance.