Development of Leak Resistance in Industry Standard OCTG Connections using Finite Element Analysis and Full Scale Testing

Abstract

The original American Petroleum Institute (API) tubular specifications were established in the early 20th century with the intent to standardize pipe sizes and connections so that material from one mill/user could be assembled with material from other sources. The original API specifications for threaded connections did not focus specifically on ensuring leak resistance-only interchangeability. Today, the importance of connection integrity and reliable leak resistance is more widely recognized. Casing and tubing strings represent the primary environmental and safety barrier(s) for oil and gas containment. The threaded connections for these strings are one of the performance-limiting and most critical features for the tubular product. To address the leak resistance issue, an API work group consisting of user and manufacturer representatives developed a performance-based connection specification for LTC (Long, Threaded and Coupled)1 connections for casing and tubing. These new specifications are currently being finalized as a Supplemental Requirement to API Specification 5CT, SR22. Development of these supplemental specifications employed Finite Element Analysis (FEA), full scale physical testing, and evaluations of product specifications, inspection and gaging methods, process control, makeup and thread compound. The objective of this API initiative is to minimize costly failures, increase performance, improve delivery and decrease inventory, and minimize supplier and user life cycle cost by developing an engineered connection as an industry standard. API Supplemental Requirement (SR) 22 specifications enable manufacturers to supply a reliable fieldready product with the performance users require. The Incentive to Develop API SR22 Connections Tubulars are a fundamental component of well design. Tubular connections are one of the performance-limiting aspects of tubular products. The critical nature of connection performance is reflected in the Industry's failure experience. Typically, connection failures account for up to 90% of all tubular failures. One user's failure database, for example, cites twice as many connection failures as all other modes combined (Fig. 1). Remarkably, typical well design methodologies do not address threaded connection performance in any detailed fashion. Rather a safety factor is applied to pipe body ratings. In contrast, SR22 methods specifically address connection performance and reliability. Performance limits have been defined and specifications have been developed to ensure leak resistance of 100% API internal pressure rating with a tension design factor of 1.6. The objectives of developing an engineered performancebased LTC connection as an industry standard are:Identify and eliminate the sources of connection failures,Optimize performance and cost,Save suppliers and users time and labor, andImprove user-supplier relationships. These objectives have been pursued through an API work group representing joint industry input from both the manufacturer and operator communities. The LTC is an 8- Round thread form connection. 8-round connections are the workhorse of the industry, representing over 50% of OCTG production. 8-Round connections are supplemented with API Buttress and proprietary connections in deep or critical wells.

The Environmental Costs and Benefits of Fracking

“…Vignes & Aadnøy (63) found that leaks through steel tubing and casing accounted for most failures in offshore Norway. Schwind et al (64) observed that 90% of casing failures were attributable to faulty connections.…”

Theoretical study on the hydraulic sealing integrity of cement sheath in horizontal shale oil wells under fracturing: Crack propagation mode and mechanism

“…14–16 In addition, a weak bonding strength at the interface and poor cementing quality can cause the cement sheath to lose its barrier function. 17,18…”

On the Sealability of Metal-to-Metal Seals With Application to Premium Casing and Tubing Connections

“…Finite element analysis (FEA) has been applied for screening purposes by calculating, for the metal-tometal seal, the ratio of the average contact stress to applied pressure [4,5]. However, the success or failure of a metal-to-metal seal is governed by other parameters than only contact stress acting between a perfectly flat and a round surface.…”

“…The addition of surface topography in the contact simulation allows the introduction of the concept of permeability of the (deformed) gap between the contacting areas of the pin and box of a metal-to-metal seal [10][11][12][13][14]. This permits the definition of a leakage criterion based on permeability instead of a contact stress to pressure ratio, as used in e.g., [4,5]. In addition, it allows the computation of the actual leak rate when combined with the differential pressure and fluid properties [13].…”