API standards are developed to ensure that products and processes in the oil and gas industry meet legal and environmental requirements. These standards provide a reference framework for various products in the industry, including OCTG (Oil Country Tubular Goods) products. API 5L and API 5CT are two of the most important standards. Although they both involve steel pipes, their uses and requirements are different. This article will compare the main differences between API 5L and API 5CT in detail, covering different aspects.
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API 5CT standard is for oil casing pipeand oilfield drilling steel pipe. Oil casing pipe is used in oil wells to stabilize the well wall and prevent the well wall from collapsing. Drilling pipe is used to transport oil and gas from the well to the ground. API 5CT specifies the size, weight, technical requirements, test methods, inspection rules, marking, packaging, transportation and storage of these pipes. The standard is designed to ensure that oil casing and drilling pipe can withstand high pressure, corrosive liquids and extreme temperatures in harsh downhole environments. API 5CT steel pipe materials include J55, K55, N80, L80, C90, T95, P110, Q125, etc. Each material is designed for different downhole conditions.
API 5L: API 5L: This standard is specifically for pipeline steel pipes, which are mainly used in long-distance pipeline systems for transporting fluids such as oil and natural gas. It covers two types of steel pipes: seamless steel pipes and welded steel pipes. The API 5L standard is applicable to both ground and underground pipelines, covering application scenarios from medium pressure to high pressure.
API 5L: API 5L pipes are manufactured using a microalloying heat treatment process to improve their compressive strength and ductility. Especially in the production of X80 and above grade steel pipes, there are strict restrictions on carbon equivalent to ensure the welding performance and durability of the pipes. In addition, the API 5L standard also requires pressure testing and welding process evaluation to ensure the safety of the pipeline system.
API 5L: API 5L pipes are mainly used for surface and underground pipelines, responsible for transporting oil and gas from production sites to refineries or other processing facilities. Its application environment is relatively simple, mainly involving long-distance transportation of gas and liquids, and the working pressure and temperature are manually controlled.
API 5L: The working environment of API 5L pipes is relatively mild. The transport medium has been initially purified and no longer contains impurities such as drilling fluids and sand. Therefore, the pipeline is mainly in contact with pure media such as oil, gas and water, and the working pressure and temperature are also manually adjusted and controlled.
American Petroleum Institute (API) 5CT high strength steels are extensively used for casing strings in wells subjected to high cyclic hydraulic fracturing loads. While non-sour grades of API steel such as P110 casing strings have been used satisfactorily for well construction, standard API P110 connections have seen higher rates of failures than pipe body failures in shale wells that require hydraulic fracturing. P110 pipe body and connection failures have also been experienced in cases where poor manufacturing practices have been employed to produce P110 steel casings and connections used in wells subjected to these high hydraulic fracturing loads.
The following material properties are important when understanding the limitations of API 5CT high strength steels: yield strength (YS), Charpy V-notch (CVN) impact energy, hardness and tempering.
API Specification 5CT/ISO defines the YS by drawing a vertical line at an elongation of 0.5% to 0.65% depending on the material grade (0.6% for P110) and where it intersects the stress-strain curve in a tensile test is the YS. The ASTM method draws a line parallel to the linear portion of the stress strain curve starting at 0.2% elongation and where it intersects the stress-strain curve on the stress axis is defined as the 0.2% offset YS. Both methods generate very similar results. This value obtained from tensile testing is used for determining if a material meets the API specified minimum yield strength (SMYS) requirement, which is critical for load case considerations and determining design safety factors.
CVN Impact Energy or “toughness” is the energy required to initiate and propagate a crack in the material. Toughness is a valuable property as it enables a material to arrest a crack rather than allowing the crack to grow until pipe failure. Toughness has a strong correlation to temperature and can be very high at bottomhole temperatures; however, some high strength steel materials will become more brittle at cooler, surface temperatures.
Hardness Testing measures the ability of a material to resist deformation from a known compressive force applied to an indenter. The higher the strength of the steel, the less deformation will occur; hence, the higher the hardness value obtained. Since there is a correlation between hardness and tensile strength, the hardness test can be used as a non-destructive small scale quality assurance test to determine the approximate strength level of the steel. Another important associated consideration is that materials with higher hardness and yield strength are more vulnerable to environmentally assisted cracking.
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Tempering is a thermal process where steel is heated to a predetermined temperature for a certain period of time after being hardened by quenching from a relatively high temperature. As quenched, carbon steel is very hard and behaves in a brittle fashion; however, the tempering process lowers the hardness/yield strength andrestores the toughness of the steel back to a more acceptable level, based on the time and temperature used during this process.
API Specification 5CT requires P110 grade steel to have a yield strength range between 110 ksi and 140 ksi. Due to the high strength of this material, it will inherently have a very high hardness, making it vulnerable to environmentally assisted cracking, especially as it gets closer to 140 ksi. Many high strength steel materials have a maximum hardness requirement to address this issue; however, P110 steel lacks this requirement.
Furthermore, all high strength sour grade oil country tubular goods (OCTG) steels have a minimum tempering temperature requirement to ensure hardness is reduced, and sulfide stress cracking (SSC) resistance increased along with toughness. However, P110 is required to be quenched and tempered, but lacks a minimum tempering temperature requirement. Some basic P110 specification requirements are shown in Table1.
The advancements of today’s standards in steel making and processing technologies enables the steel industry to produce cleaner and tougher steels; however, this is not reflected in the current edition of the API 5CT specification. Additionally, P110 pipe body material does not have a defined toughness testing requirement unless specifically invoked by the end user. Even when toughness testing is required, the minimum CVN impact value requirements in API 5CT (for pipe body and coupling stock) are too low.
Due to these low CVN toughness value requirements and/or the lack of toughness testing requirements, the end user must acknowledge if there is a need for higher-than-standard toughness material to be used in their well designs.
While P110 steel tubulars perform adequately in many wells, it is more susceptible to failure in wells requiring high cyclic fracturing loads. Most of the documented failures occur at the connections where the fracturing cyclic stress levels are the highest due to make-up stresses overlaid on the high applied internal pressures. Additionally, even the presence of very low concentrations of H2S not considered sour by NACE and/or frack fluids may generate enough hydrogen to embrittle high strength steels such as P110 and cause it to fail when subjected to these high frac loads. In other cases, failures have also occurred due to manufacturing defects in the electric resistance welding (ERW) seam welds and/or due to seamless pipe defects such as seams, laps, scabs, and/or cracks.
When evaluating an onshore well design requiring high cyclic fracking loads, three important parameters must be considered: (1) stress level, (2) environment and (3) material susceptibility to cracking due to the environment and/or high cyclic loads.
Since the highest stresses are always expected to be at the connections, the stress levels can be reduced by using a connection with low make-up stresses. Depending on the economics of the well(s), a standard API connection with high make-up stresses may be substituted by a connection with lower make-up stresses such as a low thread interference connection having a metal seal near the end of pin (i.e. metal to metal seal proprietary connection) and/or any other type of connection having lower stresses upon make up (i.e. modified API connections).
Since non-producing environments during fracking on some of these offshore wells can be unpredictable, it has become standard practice by some operators to specify P110 casing material with some restrictions on the maximum allowable yield strength. Depending on the manufacturer, restricted P110 material may have different designations such as restricted yield (RY), or mild sour (MS), just to name a few. The typical maximum allowable yield strength for this RY P110 material is 125 ksi. Reducing the maximum allowable yield strength decreases the susceptibility to environmental cracking and, at the same time, increases the toughness of the material since the tempering times and temperatures utilized produces a softer, tougher material.
Another important aspect in mitigating casing failures in onshore wells is having a robust QA/QC program and pipe purchasing process. Figure 1 shows a generic recommended pipe purchasing process schematic that can help avoid failures due to pipe manufacturing issues. Customized QA/QC programs can then be developed depending of the operator’s needs.
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