A Test Program for the Evaluation of Oilfield Thread Protectors

Abstract

Summary Transportation, handling, and corrosion damage to the threads of oilfield tubulars cost the industry millions of dollars each year. This problem has intensified with the increased use of proprietary connections, which represent a significant percentage of the total tubular cost. The thread protector is the primary means of protecting the threads from impact and corrosion damage. A test program has been designed and a test facility built. To evaluate the performance of oilfield protectors, a wide variety of thread protectors were tested, including metal, plastic, and composite (metal-elastomer) designs. This paper discusses this testing and outlines a guide for the selection of oilfield thread protectors for various service conditions. Introduction Oilfield tubular goods are exposed to a variety of hazards during transport, handling, and storage from the time they leave the steel mill until they arrive at the drilling rig site. These hazards can damage the pipe and cost the industry millions of dollars each year. Sources of these problems includetransfer between vessels (e.g., ships or trains to trucks),improper strapping of pipe during transit,handling by inspection companies, andcorrosion during storage. One way to minimize these problems is to cover the pipe ends with a good thread protector. A well-designed and constructed thread protector can provide a great amount of protection against these problems, since much of the most severe damage occurs at the pipe ends. Tubulars represent a significant percentage of an oil company's operating expenses. It is not unusual for a major oil company to spend several hundred million dollars in purchasing new casing and tubing and to spend several million dollars more to inspect those tubulars in a year's time. In almost every case, the selection of thread protectors is left to the steel mills, who attempt to follow the very brief guidelines stated in API Specs. 5A, 5AC, and 5AX. Neither API, protector manufacturers, nor steel mills have published any detailed specifications or test procedures for the selection of thread protectors. Most of the handling damage to tubulars occurs in the threads and connection seal faces. The API connections depend on thread interference for a seal, while most proprietary connections depend on a metal shoulder interference for a seal. In either case, if the critical sealing area is damaged by impact or corrosion, the connection will no longer be effective against internal or external pressure. The tubular's pin end is especially susceptible because of its exposed external threads and the lack of protective coating against corrosion (except for some proprietary connections). Zinc, tin, or phosphate coatings generally are used only on the coupling or box end. When a connection is damaged, the only recourse is to rethread the pipe. Rethreading costs for tubulars are quite high. For an API 95/8-in. [24.5-cm], N–80, 8 round (rd) long thread and coupling (LTC) set, today's cost is about $120 (but may vary on the basis of several factors). The costs to rethread proprietary connections are several times greater-often by a factor of 5 to 10. Thus, there is an even greater incentive to protect proprietary connections. To protect the pipe adequately, a well-constructed thread protector must have various attributes. It must provide adequate axial and lateral impact absorption at a wide range of temperatures (–50 to 150F [–45 to 65C]). The design and construction should cover the critical sealing areas and protect them from moisture. The protector should be vibration resistant and capable of resisting axial tension, since the protector is sometimes used to lift or drag the pipe. A good protector also should not be as hard as the pipe to prevent metal galling or seizing of the thread elements. Some other desirable features include ease of makeup or breakout, chemical resistance to cleaning solvents and sunlight, and an economical price. The price of thread protectors varies significantly depending on composition and manufacture and should be considered in relation to the cost of the tubular and the likelihood of thread damage. This paper describes the test program and generalized results of an evaluation of oilfield thread protectors. Six tests are described, including:axial and lateral impacts,vibration,thread stripping,low temperature impact,corrosion, andsolvent resistance. The purpose of each test was to simulate a real-world condition to which a protector may be subjected. Some of the tests are for relative comparisons among protectors, while other tests are based on specific criteria. Table 1 outlines the test program followed. Types of Thread Protectors In this paper, several different terms are used to identify the various thread protectors. JPT P. 306^