Analysis of Bimetallic Pipe for Sour Service

Abstract

Summary. This paper presents the results of laboratory investigations conducted to better our definition of the serviceability of bi-metallic pipe manufactured by the thermohydraulic (tight-fit) method for Mobile Bay pipe manufactured by the thermohydraulic (tight-fit) method for Mobile Bay service. Both Alloy 625/API X-65 and Alloy 825/API X-65 tight-fit pipe (TFP) were evaluated under conditions of (1) standard corrosion test environments to evaluate the metallurgical conditions of the corrosion-resistant-alloy (CRA) liner tubes, (2) long-term, full-scale TFP exposure under a simulated Mobile Bay production environment containing high levels of H2S and CO2, and (3) hydrogen-permeation experiments designed to examine potential effects of CRA liner collapse from hydrogen produced by corrosion on the ID and/or cathodic protection on the OD. produced by corrosion on the ID and/or cathodic protection on the OD. Results indicate that bimetallic TFP exhibited an acceptable metallurgical condition of the CRA liner materials. Under the simulated Mobile Bay production environment, TFP exhibited good resistance to general corrosion, production environment, TFP exhibited good resistance to general corrosion, stress-corrosion cracking (SCC), and liner collapse. Hydrogen-permeation tests indicate that very conservative estimates of service-life liner collapse from interfacial hydrogen pressure range from 150 to more than 800 years, depending on conditions. For all practical purposes, liner collapse from hydrogen is not a limiting factor for TFP flowline applications. Introduction Application of CRA's in offshore oilfield production environments has been a topic of significant interest, involving both performance and economic questions. For example, one must consider the service performance (corrosion, pitting, and SCC) of the materials. In addition, their cost-effectiveness relative to the more conventional methods of corrosion mitigation, such as the use of inhibitors and coatings, must be evaluated. CRA materials are commonly used in solid (monolithic) CRA components and in bimetallic or clad CRA components. In many applications, economic or engineering limitations dictate that solid CRA components are not always feasible. Examples include clad wellhead components and process vessels, which use CRA's on the ID installed by either overlay welding or hot-isostatic-pressing (HIP) cladding. The purpose of this study is to evaluate the serviceability of bi-metallic tubular components manufactured by a thermohydraulic (TFP and tight fit tube) manufacturing process for severe sour service in Mobile Bay. This method involves assembling a CRA liner tube inside a finished steel outer tube or pipe. The finished tubular thereby has the load-carrying capability of the steel pipe and the internal corrosion resistance of a CRA pipe, but has a lower overall cost than solid CRA. An experimental program was conducted to evaluate two key aspects of TFP performance as they relate to application of this material system for severe sour service in Mobile Bay offshore flowlines. They are the resistance of TFP to the sour production environment in terms of corrosion and SCC and the resistance of TFP to collapse of the CRA liner as a result of hydrogen buildup at the interface. This hydrogen is the result of corrosion on the ID and cathodic protection of the steel outer pipe in seawater. Background Thermohydraulic Fabrication Method. As Fig. 1 shows, the thermohydraulic method of fabrication used in TFP involves the hydraulic expansion of a CRA liner tube into a heated steel outer pipe. As a result of this expansion and subsequent cooling to room temperature, a compressive residual stress is induced in the liner tube. This compressive residual stress puts the CRA liner and steel outer pipe into intimate contact, creating a tight fit or mechanical bond. This process is contrasted with a metallurgical bonding process for bimetallic construction, where the two materials must be fused by welding, extrusion, or diffusion bonding by HIP methods. For metallurgical bonding of CRA materials and steels, the thermal treatments necessary for the steel outer pipe can result in a nonoptimum condition of the stainless or nickel alloy liner tube. Such conditions can promote localized corrosion and SCC of the CRA materials. Similarly, the promote localized corrosion and SCC of the CRA materials. Similarly, the thermal heat treatments commonly used for stainless and nickel alloys can result in inferior mechanical properties in the steel outer pipe. For TFP, however, the thermomechanical bonding process delivers both the CRA liner tube and the steel outer pipe in the final optimized condition, which should yield superior performance. To complete the TFP fabrication process, the CRA liner and outer pipe must be seal-welded at the ends of the pipe. Furthermore, for flowline applications, TFP can be joined by conventional field welding techniques. Therefore, one critical aspect of TFP performance is the corrosion behavior in this welded region in the severe conditions commonly found in Mobile Bay produced fluids, which may contain high levels of H2S, CO2, and brine. produced fluids, which may contain high levels of H2S, CO2, and brine. Potential Liner Collapse Problems. The potential collapse of CRA liners in Potential Liner Collapse Problems. The potential collapse of CRA liners in bimetallic tubulars is a topic of debate. Early experience with downhole bimetallic tubulars revealed that liner collapse could be a potential failure mode in sour-gas wells. This experience was, for the most part, obtained with loose-liner bimetallic tubes. In these cases, atomic hydrogen produced on the back of the tubing by sulfide corrosion may diffuse into the material and accumulate at the CRA/steel interface. Continued accumulation has been shown to result in liner blistering and collapse of some bimetallic tubular systems in laboratory tests, even when metallurgical bonding techniques were used. In Colwell et al.'s study, however, one of the materials that did not exhibit liner collapse from hydrogen accumulation even under severe conditions of hydrogen charging was pipe manufactured by the thermohydraulic method. Even though these tests demonstrated the superiority of TFP over other available methods, its long term behavior in terms of liner collapse is still being debated. To understand the phenomenon of liner collapse from hydrogen accumulation more fully, we must examine the stresses that act on the liner tube. Liner collapse should occur when the collapse stress, is equal to the stress caused by hydrogen pressure at the CRA/steel interface, plus the fit-in-stress in the liner resulting from thermohydraulic processing, which may be written as (1) The problem is then defined as determining the hydrogen pressure, pH: pH: (2) where d1,= outer-pipe ID and h1= thickness of the CRA liner tube. Combining Eqs. 1 and 2 yields (3) August 1991 P. 291