Elastomer life estimation protocol for the energy sector. (Tech Service).Elastomeric or rubber sealing elements have been used extensively in the energy sector to provide gas and liquid tight seals between mating metal components. Recovery of oil and gas resources from greater depths have placed greater emphasis on the long term performance of elastomeric sealing elements at higher temperatures and more severe chemical environments such as subsea Subsea is a general term frequently used to refer to equipment, technology, and methods employed to explore, drill, and develop oil and gas fields that exist below the ocean floors. This may be in "shallow" or "deepwater". production equipment. Without a technique to verify long-term performance of elastomeric material used in sealing elements, the end user is left with a gamble as to the performance of the elastomeric sealing element. This, in some cases, has led to the use of metallic sealing elements that tend to be very costly and susceptible to installation damage. Elastomeric sealing elements tend to be less costly than metal to metal scaling elements and less susceptible to installation damage. Thus, development of a testing protocol which would better evaluate the long-term effects of a thermo-chemical environment on the elastomers used in long life sealing elements was called for. Traditional elastomeric material evaluation methods to determine compatibility with the service environment do not address the long-term effects of a chemical and thermal environment. These methods involve standard short term American Society for Testing and Materials (ASTM ASTM abbr. American Society for Testing and Materials ) immersion testing for retained physical properties. Short-term test methods such as these deal mainly with tensile tensile, adj having a degree of elasticity; having the ability to be extended or stretched. properties that are finally tested at room temperature. Some of these methods are inconclusive INCONCLUSIVE. What does not put an end to a thing. Inconclusive presumptions are those which may be overcome by opposing proof; for example, the law presumes that he who possesses personal property is the owner of it, but evidence is allowed to contradict this presumption, and show who is in relation to elastomeric seal failures in service. Also, when using ASTM type testing methods as a tool for material selection, exotic elastomeric compounds have shown great promise when using ASTM tests methods, but do not perform as expected in service. The severe environments encountered when drilling for and producing crude oil and natural gas vary widely from producing zone to producing zone. From light crude oil, which is high in aromatics, to sour crude oil Sour crude oil contains the impurities hydrogen sulfide (H2S) and carbon dioxide, or mercaptans. All crude oil contains some impurities. When the total sulfide level in the oil is > 1 % the oil is called "sour". containing dissolved hydrogen sulfide hydrogen sulfide, chemical compound, H2S, a colorless, extremely poisonous gas that has a very disagreeable odor, much like that of rotten eggs. It is slightly soluble in water and is soluble in carbon disulfide. ([H.sub.2]S), and from produced natural gas which is mostly methane (C[H.sub.4]) and can contain [H.sub.2]S and carbon dioxide carbon dioxide, chemical compound, CO2, a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. (C[O.sub.2]). Most wells produce a mixture of chide oil, water and natural gas. It is evident that no one elastomer elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. compound can successfully be used in all production wells. The problem then becomes, how do you evaluate the long-term potential of an elastomer compound in such a varied environment? Traditional methods of evaluating elastomeric materials Elastomeric material evaluation for typical oilfield service generally involves free state immersion of the elastomer sample in the specific oilfield crude oil or gas environment for a short period of time (one, three, seven, 14 days or longer). Applicable ASTM specifications for immersion testing are shown in reference 1. After immersion, the samples (ASTM dumbbell Dumbbell An investment strategy, used mainly for bonds, where holdings are heavily concentrated in both very short and long term maturities. Notes: This is also known as a barbell, charting on a timeline gives the appearance of a barbell or dumbbell. specimens or o-rings) are subjected to stress/strain (tensile) testing, and measurements of volumetric volumetric /vol·u·met·ric/ (vol?u-met´rik) pertaining to or accompanied by measurement in volumes. vol·u·met·ric adj. Of or relating to measurement by volume. swell, weight changes and hardness changes. In free state immersion testing, 100% of the sample's surface area is exposed to the environment. The testing does not relate to long-term changes in the elastomer-sealing characteristic caused by the chemical environment. Sometimes compression set or stress relaxation Stress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion.[1] testing is conducted under the influence of the chemical environment (gas and/or liquid). However, this technique will not estimate how long the elastomeric material will successfully function as a sealing element under the stress of the chemical environment, even if elevated temperature is utilized to accelerate the process. Life estimation method Quantitative service life estimation of the elastomer used to make sealing elements has become an important part of many subsea production equipment contracts. This is because the cost of premature equipment recovery from the sea floor (400 to 3,000 feet) due to seal failure is prohibitive. These sealing applications involve severe thermal, chemical and mechanical stress conditions. The life estimation testing protocol we have developed, refined and regularly utilize is based on the Arrhenius equation The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of a chemical reaction rate, more correctly, of a rate coefficient, as this coefficient includes all magnitudes that affect reaction rate except for concentration. which describes first order reactions, such as a chemical reaction. In this technique, we expose the candidate elastomeric material in the form of an o-ring to the specified chemical environment and utilize elevated temperature to accelerate thermo-chemical degradation. The o-ring of the candidate elastomeric material is used to seal an autoclave autoclave Vessel, usually of steel, able to withstand high temperatures and pressures. The chemical industry uses various types of autoclaves in manufacturing dyes and in other chemical reactions requiring high pressures. , which is pressurized pres·sur·ize tr.v. pres·sur·ized, pres·sur·iz·ing, pres·sur·iz·es 1. To maintain normal air pressure in (an enclosure, as an aircraft or submarine). 2. . The aging technique described by the Arrhenius equation is based on the observation that for every 10[degrees]C (18[degrees]F) increase in temperature, the first order chemical reaction rate doubles. Conversely, for every 10[degrees]C decrease in temperature, the first order chemical reaction rate halves. Therefore, if we conduct our material evaluations at an elevated temperature in a specified chemical environment until the o-ring fails to hold pressure, we are able to extrapolate extrapolate - extrapolation to a lower service temperature and estimate seal life at that temperature. Therefore, short-term testing (in the order of weeks or months) of an elastomeric sealing element at elevated temperature in a specific chemical environment can be used to approximate long-term effects at lower temperatures. References 2, 3, 4 and 6 offer more detailed discussion of the theory and practices of life estimation testing. Cameron Elastomer Technology (CET CET abbr. Central European Time CET Central European Time CET n abbr (= Central European Time) → hora de Europa central CET abbr ) has successfully utilized and refined the technique described above for over 15 years in estimating the service life of elastomer compounds used in sealing elements for subsea oil and gas production equipment. The testing described in our original work presented in 1998 was conducted to estimate the performance potential of elastomeric compound used in the fabrication fabrication (fab´rikā´sh  n),n the construction or making of a restoration. of high pressure sealing elements planned for use in a North Sea gas injection project. The customer had requested our testing confirm the elastomer in the sealing elements to have an estimated life of 20 years at 300[degrees]F in the specified service environment. An ongoing research project at CET was developing an improved family of elevated temperature oilfield elastomers. To verify improved performance over an existing family of elevated temperature elastomer compounds, we decided to expose one of the improved elastomer compounds to the same environment we had previously tested compound M1-20. The experimental compound we evaluated was EXP. Statement of theory and definitions As stated above, life estimation testing as used by CET is based on the Arrhenius equation which describes a first order reaction, such as a chemical reaction. We use the equation to describe the effect of temperature on a first order chemical reaction. In general, for every 10[degrees]C (18[degrees]F) increase in temperature, the first order chemical reaction rate doubles. Conversely, for every 10[degrees]C (18[degrees]F) decrease in temperature, the reaction rate reduces by one half (1/2). A brief discussion of the Arrhenius equation and its application to accelerated thermo-chemical aging has the basic form: k = [Ae.sup.-Ea/RT] Where: k = the rate of chemical reaction; A = a proportionality constant related to collision frequency
Collision frequency is defined in chemical kinetics, in the background of theoretical kinetics, as the average number of collisions between reacting molecules per unit of time. Its symbol is Z. and orientation of molecules; e = base for natural logarithms Natural logarithm Logarithm to the base e (approximately 2.7183). ; Ea = activation energy activation energy, in chemistry, minimum energy needed to cause a chemical reaction. A chemical reaction between two substances occurs only when an atom, ion, or molecule of one collides with an atom, ion, or molecule of the other. ; R = gas constant: and T = absolute temperature. Rewriting the equation using natural logs gives: In k = -Ea/RT + ln A If we let ln k = y; ln A = b; -Ea/R = m; l/T = x It can be seen that the equation represents a straight line y = mx + b, where -Ea/R is the slope. If times to failure for various temperatures are converted to common logs, the experimental data can be plotted on semi-log graph paper. A regression analysis In statistics, a mathematical method of modeling the relationships among three or more variables. It is used to predict the value of one variable given the values of the others. For example, a model might estimate sales based on age and gender. of the data points will give the best straight line fit through the experimentally determined data points. If the correlation coefficient Correlation Coefficient A measure that determines the degree to which two variable's movements are associated. The correlation coefficient is calculated as: is at or near 1.0, the line can be extended, mad estimated time to failure for other temperatures can be determined. In life estimation testing, a critical material parameter is tested to failure. We use elevated temperature to accelerate the thermo-chemical reaction, which ultimately causes failure of a sealing element under test. An elastomeric sealing element failure occurs when the sealing element loses the ability to maintain a seal in Verb 1. seal in - close with or as if with a tight seal; "This vacuum pack locks in the flavor!" lock in confine - prevent from leaving or from being removed the specified chemical environment. For a compression type sealing element such as an o-ring to function long term, it must be able to resist the effects of the environment to which it is exposed and not be susceptible to excessive stress relaxation/compression set and chemical degradation. To approximate (estimate) the long term life of an elastomer used in a sealing element, a life prediction plot must be developed. The plot is constructed by collecting test data (time to failure at a specific temperature) and plotting the logarithm logarithm (lŏg`ərĭthəm) [Gr.,=relation number], number associated with a positive number, being the power to which a third number, called the base, must be raised in order to obtain the given positive number. of time to failure (in hours) versus the inverse of absolute temperature (in degrees Kelvin kelvin, abbr. K, official name in the International System of Units (SI) for the degree of temperature as measured on the Kelvin temperature scale. A unit of measurement of temperature. ). Figure 1 is a representation of a typical life prediction plot. [FIGURE 1 OMITTED] To generate the required test data to construct the life prediction plot, short term tests of the candidate elastomer are conducted at several elevated temperatures, significantly higher than the service temperature. Elevated temperatures are utilized to accelerate the thermo-chemical degradation of the candidate elastomer in the simulated service environment. The elevated temperatures at which the test runs are conducted have been selected to force failures in a minimum number of test cycles. Time to failure test data are collected at three or four different elevated temperatures. At each selected elevated test temperature, several test runs are conducted to assure repeatability of results. After sufficient failure data have been collected at each temperature, the median value Noun 1. median value - the value below which 50% of the cases fall median statistics - a branch of applied mathematics concerned with the collection and interpretation of quantitative data and the use of probability theory to estimate population at each temperature is calculated and used in a least squares regression analysis to determine the slope of the best fit line representing the data points. The best fit line of reaction rates is extrapolated to the desired service temperature and an estimation of service life of the elastomer compound is made. Figure 1 is a representation of a typical plot. We have standardized standardized pertaining to data that have been submitted to standardization procedures. standardized morbidity rate see morbidity rate. standardized mortality rate see mortality rate. on a three-day (72 hour) test cycle to allow the candidate elastomer to equilibrate e·quil·i·brate v. e·quil·i·brat·ed, e·quil·i·brat·ing, e·quil·i·brates v.intr. To be in or bring about equilibrium. v.tr. To maintain in or bring into equilibrium. in the simulated service environment. In the standard three-day test cycle, the autoclave is sealed with the candidate material in the form of an o-ring. Figure 2 is a representation of the specialized autoclave used to collect failure data for life estimation. The simulated service environment (liquid, gas or a combination of liquid and gas) is added to the autoclave. The autoclave is pressurized to no more than 2,000 psi to avoid thermo-mechanical (extrusion) failures at elevated temperatures. Candidate seal material failure is indicated when one of the seals in the autoclave no longer holds pressure. The simulated service environment is replaced prior to starting every three day test cycle to assure any chemical reaction previously started is allowed to continue, [FIGURE 2 OMITTED] Description and application of test equipment The life prediction test fixture
Test fixture refers to the fixed state used as a baseline for running tests in software testing. shown in figures 2 and 3 is a high temperature and pressure autoclave used to test o-rings (AS568-214 size) made from the candidate material that are used to seal the 100 cubic centimeter cu·bic centimeter n. Abbr. cc A unit of volume equal to one thousandth (10-3) of a liter or to one milliliter. chamber which contains the specified service environment. Test ports monitor the test o-rings for leakage. Heat is applied to the fixture using electric heater bands. The fixture is constructed of nickel based alloy 718 for corrosion resistance and is rated for 10,000 psi and 700[degrees]F. Depending on the service environment required for the test program, the fixtures are rack mounted Built into a cabinet that has a standard width of 19" or 24". All types of electronics and computing devices come in rack-mounted packages, including servers, test instruments, telecommunications components, tape drives and audio and video equipment. in the test room or rack mounted under a vent hood connected to a fume fume Occupational medicine A solid suspension resulting from condensation of the products of combustion. See Inhalant Vox populi verbTo be in the midst of a mental mini-meltdown. scrubber. Many of the tests we have conducted for energy sector clients involve [H.sub.2]S gas which is toxic. This type of testing requires that we conduct the test under the fume hood A fume hood or fume cupboard is a large piece of scientific equipment common to chemistry laboratories designed to limit a person's exposure to hazardous fumes. Fume hoods were originally manufactured from timber, but now epoxy coated mild steel is the main construction . The test temperature in the fixtures is controlled by a type J thermocouple using a LabView (ref. 1) computer program. The fixtures are monitored by pressure transducers Pressure transducer An instrument component which detects a fluid pressure and produces an electrical, mechanical, or pneumatic signal related to the pressure. , the output of which is recorded by LabView. This computer program controls and records important test variables such as test fixture temperature and test cycle duration, and tabulates pressure readings during each test cycle. Once sufficient test data have been collected at several elevated temperatures, the data can be plotted on semi-log graph paper as shown in figure 1. Further analysis of data is done using the Excel (ref. 2) program to conduct a least squares regression analysis of the test data. The regression analysis method is part of the statistical package included with Excel and is used to determine the correlation factor The ratio of a ground dose rate reading to a reading taken at approximately the same time at survey height over the same point on the ground. [r.sup.2] for the best fit curve and its linear equation. [FIGURE 3 OMITTED] Example of a life estimation test program We will illustrate the application of the CET life estimation technique by reviewing two test programs recently conducted, one for a major oil company and the other conducted under the same conditions on an improved elevated temperature oilfield elastomer. The oil company wanted assurance that a high performance elastomer compound proposed for use in elastomeric sealing elements for a major North Sea oilfield project would perform for a specified time at a given temperature. For the sake of this article, the elastomeric compound will be identified as M1-20. The specified service environment for this test program was as follows: Liquid phase--25% of autoclave internal volume, consisting of: 94% #2 diesel fuel (to simulate crude oil); 5% water; 1% amine amine (əmēn`, ăm`ēn): see under amino group. amine Any of a class of nitrogen-containing organic compounds derived, either in principle or in practice, from ammonia (NH3). type corrosion inhibitor A corrosion inhibitor is a chemical compound that, when added in small concentration, stops or slows down corrosion of metals and alloys. A typical good corrosion inhibitor will give 95% inhibition at concentration of 80 ppm, and 90% at 40 ppm. Gas Phase--to overpressure overpressure, n excessive pressure applied at the end of a physiologic joint range to confirm the severity of pain, thus helping determine the manual treatments. liquid phase, consisting of; 10% [H.sub.2]S; 5% C[O.sub.2]; 85% C[H.sub.4] (methane) Gas test pressure utilized throughout this program was 2,000 psi. Pressure was limited to prevent thermo-mechanical failure of the test o-rings. On compound M1-20, initial testing was conducted at 486[degrees]F (252[degrees]C) and terminated without failure after 28 three-day test cycles or 2,016 hours. Testing was stopped because initially selected test temperature did not result in failure in a reasonable time. After installing new compound M1-20 o-ring seals, testing was resumed at 262[degrees]C (504[degrees]F) and at 272[degrees]C (522[degrees]F). O-ring seal failures occurred after 20 cycles (1,440 hours) and nine cycles (648 hours), respectively. The tests were terminated because pressure could not be maintained in the fixtures to failure. Subsequent testing was conducted at 282[degrees]C (540[degrees]F) and 292[degrees]C (558[degrees]F) with times to failure and o-ring seal conditions shown in table 1. Analysis of time to failure data at test temperatures is shown in tables 2 and 3. To update our original life estimation study conducted on compound M1-20, we conducted a similar study on an experimental oilfield elastomer compound EXP utilizing the same chemical environment. This compound was designed to have low compression set at elevated temperature. The EXP compound was exposed to the same severe service environment as compound M1-20, except at temperatures of 292[degrees]C, 302[degrees]C and 312[degrees]C. The data for this testing are shown in table 1 and plotted along with the original test data for M1-20 in figure 4. [FIGURE 4 OMITTED] Conclusion A review of figure 4, life estimation plot, indicates the approximate life of elastomer compound M1-20 in the specified service environment to be greater than 20 years at 300[degrees]F. We believe it is important to be conservative in our life estimations and, as a general rule, limit our estimation to 20 years even though the experimental data indicates a longer life. The life estimation is based on the elastomer being used as a static sealing element such as an o-ring or other compression type sealing element confined con·fine v. con·fined, con·fin·ing, con·fines v.tr. 1. To keep within bounds; restrict: Please confine your remarks to the issues at hand. See Synonyms at limit. to a seal gland gland, organ that manufactures chemical substances. A gland may vary from a single cell to a complex system of tubes that unite and open onto a surface through a duct. The endocrine glands, e.g. with limited surface exposure to the service environment. Observing the fife estimation curve, the EXP compound could be expected to provide a longer service life at a higher temperature. However. the same constraint of limiting our estimation in 20 years would apply to compound EXP. Based on the results of this series of life estimation tests, the end user of the subsea equipment utilizing M1-20 elastomer or EXP elastomer would be able to obtain confirmation of anticipated long-term performance of selected elastomeric seals in the specified service environment. We believe the life estimation technique discussed above will permit increased utilization of elastomeric sealing elements in subsea projects where long and trouble-free life is essential to the economic success of the project. A recently published API (Application Programming Interface) A language and message format used by an application program to communicate with the operating system or some other control program such as a database management system (DBMS) or communications protocol. document, API TR6JI, reference 6. outlines the procedures utilized in this article.
Table 1 - experimental raw data for M120
Test data used for service life prediction
T Cycles T 1/T ([degrees]k) t
([degrees]C) ([degrees]K) (hrs.)
262 20 535 0.00187 1,440
272 9 545 0.00183 648
282 6 555 0.00180 432
282 6 555 0.00180 432
282 12 555 0.00180 864
282 4 555 0.00180 288
282 2 555 0.00180 144
292 3 565 0.00177 216
292 6 565 0.00177 432
292 10 565 0.00177 720
292 3 565 0.00177 216
292 4 565 0.00177 288
292 3 565 0.00177 216
292 2 565 0.00177 144
T Cycles log t Avg.
([degrees]C) (hrs.) log t
262 20 3.158
272 9 2.812 3.1584
282 6 2.635 2.8116
282 6 2.635 2.5650
282 12 2.937
282 4 2.459
282 2 2.158
292 3 2.334
292 6 2.635 2.4448
292 10 2.857
292 3 2.334
292 4 2.459
292 3 2.334
292 2 2.158
Table 2 - mean of test result for M1-20
T ([degrees]C) T ([degrees]F) # of cycles Time (hrs.)
262 504 20 1,440
272 522 9.0 648
282 540 5.1 363
292 558 3.86 281
T ([degrees]C) Condition of o-rings
262 Hardened,
compression set
272 "
282 "
292 "
Table 3 - test data anaylysis for M1-20
T ([degrees]C) T ([degrees]K) 1/T ([degrees]K Time (hrs.)
262 535 0.00187 1,440
272 545 0.00183 648
282 555 0.00180 363
292 565 0.00177 281
T ([degrees]C) Log t (hours)
262 3.158
272 2.812
282 2.565
292 2.445
References (1.) ASTM test procedures applicable to elastomers: ASTM D395 - compression set; ASTM D412 - tensile (stress-strain)properties; ASTM D471 - mass and volume changes: ASTM D1414 - o-ring testing; ASTM D2290 - durometer hardness. (2.) J.C. Vicic, "Testing of polymers for oil and gas applications," Energy Rubber Group of the American Chemical Society The American Chemical Society (ACS) is a learned society (professional association) based in the United States that supports scientific inquiry in the field of chemistry. Founded in 1876 at New York University, the ACS currently has over 160,000 members at all degree-levels and in , Houston, TX, September 1984. (3.) B.G. Parker and J.C. Vicic, "Selection of high performance elastomers for oil and gas drilling and production," Offshore Engineering Committee, Plastics and Rubber Institute, London, England, June 1986. (4.) B.G. Parker. "Status report on life prediction tests covering high performance elastomers in severe environments," Energy Rubber Group of the American Chemical Society, Houston, TX, September 1988. (5.) P.I. Abrams, K.J. Kennelly and D.V. Johnson, "A user's approach to qualification of dynamic seals for sour gas Sour gas is natural gas or any other gas mixture which contains significant amounts of hydrogen sulfide (H2S). According to this reference [1], natural gas is usually considered sour if there are more than 5. environments," Rubber Division, American Chemical Society, Cincinnati, OH, October 1998. (6.) American Petroleum Institute The American Petroleum Institute, commonly referred to as API, is the main U.S. trade association for the oil and natural gas industry, representing about 400 corporations involved in production, refinement, distribution, and many other aspects of the industry. , TR 6JI, Elastomer Life Estimation Testing Procedure, 1999; 1 - National Instruments National Instruments, or NI (NASDAQ: NATI), is an American company with over 4,000 employees and direct operations in 41 countries founded in 1976 by Dr. James Truchard, Bill Nowlin and Jeff Kodosky. Corp.: 2 - Microsoft Corp.; 3 - Cameron Elastomer Technology Research and Development. Bruce Parker Bruce Parker is a British journalist and television presenter whose career spanned the middle 1960s to 2003, when he retired. Strongly committed to regional broadcasting, he was responsible in the mid-1960s for a pilot local radio station in the Channel Islands, which eventually is the technical marketing manager for Cameron Elastomer Technology. He has been with the company for 17 years. Jogesh Arora is the manager of Research and Development for the company. He has been in the rubber industry for 23 years. Hamid A. Salem is an analytical engineer for Cameron. He joined the company in 1997. |
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