Patent Application
20170097233
The present invention relates to a method for qualification testing of tubular connectors. The connectors typically are threaded connections of oil country tubular goods (OCTG), such as casing, drill pipe sections, and similar pipe sections. A multitude of such pipe sections may be connected to make up tubular strings for application in a wellbore, such as, for instance, casing strings or drill string.
OCTG tubular connectors of this type are generally used for applications such as casing, liner and production tubing and may generally be referred to as OCTG connections, or just connectors or connections.
Connectors for OCTG typically include at least a first connector member and a second connector member. The first connector member may be referred to as pin section, and the second connector member may be referred to as box section. The pin member may have a threaded outer surface and the box member may have a corresponding threaded inner surface allowing the pin member and box member to be connected.
Together, the pin and box sections can be made up to connect respective pipe sections. The connector sections have contact surfaces of varying quality, providing correspondingly different levels of sealing ability but also cost. Thus, a wide range of different quality connectors can be made, suitable for a wide range of downhole conditions and applications.
API (American Petroleum Institute) threaded connectors provide a range of commonly used types of connector which perform relatively well at relatively low cost. API connectors however may not always provide reliable gas-tight sealing. In view thereof the API threaded connectors are generally primarily used for liquid service applications, while more expensive premium connections are used for applications where gas-tight sealing is required. Gas-tight sealing may for instance be required for production tubing of gas wells, for high-temperature high-pressure wells (wherein downhole conditions may exceed for instance a temperature of 150 degree C. and/or a pressure of 300 bars), and/or for deep water wells (in water depths exceeding for instance 1 km).
In addition, for many oilfield applications it may be required that the connectors provide liquid-tight, or even gas-tight sealing, while also providing sufficient mechanical integrity to withstand internal and external loads that may be exerted on the connector during its lifetime. Premium connectors may be required to provide gas-tight sealing. Premium connectors typically comprise a metal-to-metal seal area to achieve the required gas-tight sealing capability.
In order to objectively determine whether new connector designs meet the required qualifications, the oil and gas industry has developed qualification protocols. For example ISO-13679 is a commonly used industry standard to assess the performance and quality of OCTG connectors.
Different applications may require (slightly) different connections, for instance due to varying wall thickness, threading, steel grade of the pipe material, pressure rating, required collapse or burst strength, etc. Before application in the field, every new connection must first pass one of these qualification protocols. An operator may require a multitude of new connections each year, which each have to be tested before they can be applied in the field. The applicant for instance may require a number, for instance in the order of 20 to 40, new connections per year for its operations, which each must pass a test to prove the connector qualifies the protocol. Due to the many different connectors, the relatively high cost per test procedure, and the possibility that connectors may fail the test requiring even more connectors to be tested, the total cost for connector testing per year may be, and typically is, considerable.
To qualify a proposed connector for a particular tube diameter, material and wall thickness, a number of test specimens—i.e. samples—will be prepared for each proposed connector. For instance about two to ten samples are made per connector design, for instance about five specimens. Respective specimens usually differ by geometry. Their geometries are for instance machined at various extreme combinations of machining tolerances in order to obtain test specimens with minimum and maximum interference fit of the metal-to-metal seal.
During the test procedure, each respective specimen is typically subjected to repeated make-up and break-out (assembly and disassembly) of the connector to investigate whether damage to the thread or to the metal-metal seal area occurs as a result of make-up or break-out. Thereafter, specimens may be subjected to axial load, internal or external pressure, and/or temperature cycling to investigate the effect on sealing performance. Limit load testing may also be part of the qualification procedure.
Typically, a connector design is used for a range of different tube sizes, wall thicknesses and materials. However, in order to qualify one connector design for such different tubes, it is often required to repeat the qualification test for a number of different combinations of wall thickness, diameter and material. A full qualification test is usually a time consuming and expensive process. Consequently, having to repeat the test procedure for each specific combination of dimensions and differing characteristics, in addition to the testing of multiple connector designs, is a considerable cost burden for drilling operations.
It is an object of the invention to provide a method to improve efficiency of selecting connectors for qualification testing.
The invention provides a A method for qualification testing of a tubular connector for a hydrocarbon fluid production application, the tubular connector comprising a sealing surface, the method comprising the steps of:
The method of the invention reduces the chance that connector designs will fail a qualification test, and thus limits testing costs and time and improves the efficiency of qualification testing. The method allows executing the qualification test of OCTG connectors according to approved industry standards, while reducing the required time and costs.
According to another aspect, the invention provides a method for qualification testing of a tubular connector for a hydrocarbon fluid production application, the method comprising the steps of:
According to yet another aspect, the invention provides a method of selecting a tubular connector for qualification testing for a hydrocarbon fluid production application, the connector including a first connector member and a second connector member being adapted to be engaged to each other whereby a first contact surface and a second contact surface of the first connector member and the second connector member respectively are in sealing contact with each other, the method comprising the steps of:
In an embodiment, the surface parameter is derived from a three-dimensional surface topology measurement of the test surface. The three-dimensional topology measurement of the test surface may comprise providing a replica of the test surface by applying a body of deformable material to the test surface whereby the body of deformable material becomes an imprint of the test surface.
The method may comprise determining a material ratio curve of the test surface from the three-dimensional roughness measurement. Material ratio curves are for example used in the automotive industry to quantify the surface roughness of cylinder bores at various stages of running in. In such application, said at least one parameter may be selected from parameters Spk, Svk, Sk, Vmp, Vmc, Vvc and Vvv of the test surface; wherein Spk is a reduced peak height, Svk is a reduced valley depth, Sk is a core roughness depth, Vmp is the material volume of the peak section, Vvc is the material volume of the core section, Vvc is the void volume of the core section, and Vvv is the void volume of the reduced valley section.
Suitably said at least one parameter may comprise a friction factor for friction between the test surface and the counter surface. The variation of each parameter may comprise an increase of the friction factor indicative of galling between the test surface and the counter surface.
In another embodiment of the method, the representative test surface is provided by a sample that exhibits comparable material properties and/or surface topology as the selected surface on the connector. For example, the sample can be a flat metal strip of similar material as the connector material and machined with similar machining procedures as the metal-metal seal of the connector.
In another embodiment of the method, the load is applied to the test surface using a dedicated device or test set-up, which is specifically selected or designed to apply loads similar to the loads that occur on the connector surface during qualification testing.
In another embodiment, the test surface may be coated and/or lubricated during loading with substances suitable for OCTG connections. Examples of such substances are API compliant thread compound and Zinc-Phosphate coating.
The invention will be described hereinafter in more detail and by way of example, with reference to the accompanying drawings, in which:
Tubulars used in the oil and gas industry for the construction of oil and gas wells may be referred to as casing, liner or tubing, depending on the application. Such tubulars are typically semi-permanently installed in the wellbore for a number of reasons, such as:
i) Prevent collapse of the wall of the wellbore;
ii) Prevent formation fluids from entering the wellbore, at least from other zones than a hydrocarbon production zone;
iii) Prevent drilling fluid in the wellbore from entering the formation; and/or
iv) Provide sufficient collapse or burst strength to enable adequate well control operations, such as cap and contain operations.
The tubulars are designed to have a predetermined strength to be able to withstand a pressure difference between outside and inside the tubular without leakage of fluid or gas. The predetermined collapse and burst strength of the tubular may be based on pressure differences to which the tubular may be exposed. Said pressures may depend on the downhole conditions and loads that are expected during the lifetime of the well. The required strength of the tubular may be determined during a well design process.
Casing strings and liner strings typically have a length ranging from a few hundred feet to several thousands of feet (from about 100 m to several km). In order to construct casings of such length, pipe sections of, typically, approximately 30 to 40 ft (about 10 to 13 m) in length are joined by threaded connections. The connectors are made up on the drilling rig. An additional section of tubing is connected to the string which is already in the wellbore, and subsequently the tubular string, including the new section, is lowered into the wellbore until yet another pipe section can be added.
The connections to connect adjacent tube sections preferably meet the same strength and leak tightness requirements as the body of the pipe section. The requirements have been pre-determined during the well design process.
In order to objectively determine the strength and leak tightness of a connection, the oil industry has defined qualification test standards. During a qualification test, a set number of specially prepared test specimens—i.e. samples—of the respective connection are subjected to, for instance, a load, a sequence of loads, or particular combinations of loads.
The load may selected from external pressure, internal pressure, axial compression, axial tension, and/or bending up to a predetermined percentage of the design strength of the connector. The predetermined percentage is for instance in the range of 90% to 98%, for instance about 95%.
Testing may also include load cycling and/or temperature cycling. Cycling herein implies repeated application of, for instance, load or temperature, between two or more extremes. A lower temperature extreme may be room temperature or below. An upper temperature extreme may typically be about or exceed 180 degrees C. The load cycling may include repeated make-up and break-out of the connector, for instance at least 3 times make-up and break-out. Repeated make-up may expose a tendency of galling or thread damage.
An example of an industry standard for connector testing is ISO 13679.
Experimental testing of representative specimens for qualification is generally considered necessary. Theoretical predictions of sealing performance of the metal-to-metal seal in connections are typically insufficiently accurate to provide a reliable indication of the sealing performance.
A qualification of a connector is a costly and time consuming process. It is not uncommon that a qualification test for a new connector costs in the order of USD 500,000. In addition, the test may take up to a lot of time, for instance about 6 months of testing time, often in a dedicated laboratory. Consequently, the testing procedure may delay exploration and production of hydrocarbon resources, with associated loss of value.
Connection qualification testing is a continuous effort because:
The present invention aims to improve the efficiency of a qualification test programme. The method generally is based on:
In an embodiment, the connector 1 may comprise a box member 6. The bow member may be incorporated in a sleeve type element, the element having respective box members at opposite ends, as shown in
The inner surface 19 of the box member may have annular recessed portions 24, 26 for receiving respective end sections 12, 16 of the pin members 8, 10 when the connector is assembled. Reference sign 27 indicates an axis of symmetry of the connector.
In a first step, a list 100 is provided comprising a number of proposed connector designs C1, C2, . . . , C7, which have to be qualified.
In a second step 102, the connector designs C1 to C7 on the list 100 are ranked based on likelihood that each connector design will pass the qualification test. The connector designs are ranked in decreasing order of their likelihood to pass the test, providing a ranked list 104. A first ranked connector design C3 which is most likely to pass the qualification test is ranked first. The other connector designs are ranked in order of decreasing likelihood of passing the qualification test. I.e., a second connector design C2 which is second most likely to pass the test is ranked second, etc. The ranking can occur based on a method as described herein below.
Subsequently, the connector designs are tested, in the order of the ranked list 104. I.e., the first ranked design C3 which is most likely to pass is selected to be tested first.
In a next step 106, a number of test specimens S1 to S5 of the first ranked connector design C3 are produced. For instance two to ten samples are made, for instance about five samples. The samples S1 to S5 are produced at different combinations of extremes of machine tolerances. I.e., the samples will provide an overview of extremes of characteristics of the respective connector design, the characteristics including, for instance, leak-tightness, collapse strength, burst strength, etc.
In a first step 108 of testing a particular connector design, the specimens for said design are ranked based on their likelihood to fail the test protocol. A first ranked sample S2 which is most likely to fail the test will be tested first. A last ranked sample, which is most likely to pass the test, will be tested last. The other samples will be tested in between, in order of increasing likelihood that the respective sample will pass the test protocol.
Once the specimens S1 to S5 are ranked as described above, the first ranked specimen S2 will be subjected to the complete test in step 110. If the first ranked specimen S2 fails the test, the test will be finished, see step 112. The respective connector design will be removed from the test since one specimen did not pass. This will obviate the testing of the remaining, typically four, samples, also obviating the associated costs and time.
If the first ranked specimen S2 passes, the other samples S1 and S3 to S5 will be subjected to the same test sequence. Since the remaining samples are less likely to fail than the first sample, there is an increasing likelihood that these samples will also pass the qualification test, with the associated likelihood that the respective connector design will pass the test.
According to the method of the present invention, ranking of connector designs on the provided list and pre-selection can be performed by investigation of a seal surface, such as a metal-to-metal seal surface 28, 30 (
i) the surface roughness of the seal surface before the representative sample of the connector is assembled;
ii) measurement of the roughness of the seal surface after the representative sample of the connector has been assembled; and/or
iii) subjecting the representative sample of the connector to load conditions that are representative for a load that the seal surface will experience during qualification testing, and subsequent measurement of roughness of the seal surface.
As prescribed in second step 122, the test specimens are machined at various extreme combinations of, for instance, machining tolerances, interference of threads and/or seal surface, and taper of the pin member and the box member.
In step 124, the specimens S1 to S5 are subsequently subjected to repeated make-up and break out. Some specimens are made-up and broken out about three times, others are made-up and broken out two times. Make-up and break-out may be combined with prescribed extreme conditions, for instance minimum or maximum amounts of thread compound (e.g. lubricant), make-up torque, etc.
After each break-out, it is possible to perform non-destructive measurements of the test surface without interfering with the test protocol. In this manner, the variation, i.e. change, of one or more measured parameter values can be derived.
In step 126, a selected number of specimens, for instance specimens S1 to S4, may subsequently be subjected to application of load, to pressure cycling, and/or to temperature cycling to determine sealing performance before and/or after the test. The variation of difference between the measurements before and after the application of load provides an indication of the impact on the sealing performance. One of the specimens, for instance specimen S5, may only be subjected to limit load testing. Selection of the specimen that is most likely to fail the qualification test may be based on the value of a parameter describing the roughness of one or more of the seal surfaces.
Pre-selection and specimen selection can also be based on a relative change of the one or more seal surfaces before or after the surfaces have been subjected to a load.
Measurement of surface roughness and other relevant parameters can be carried-out using commercially available measurement devices. For example, the surface topology can be measured in three dimensions using interferometry. Optic microscopy can be used to inspect the surface and visually determine changes in roughness. Electron microscopy can be used to determine changes in roughness and measure material transfer from one surface to another surface. Friction can be measured using a tribometer.
To subject samples to a representative load, commercially available tribometers can be used.
The test apparatus 33 may comprise a pair of hardened anvil elements 38, 40 arranged opposite each other and movable towards each other. The test apparatus may for instance comprise a first actuator (not shown) for compressing the strip 34 between the anvil elements 38, 40. The actuator may be adapted to vary the magnitude of the compressive force between the anvil elements and the flat strip 34. The compressive force is indicated by reference signs 42A and 42B.
Furthermore, the apparatus 33 may be provided with a second actuator for moving the strip 34 with respect to the anvil elements 38, 40, wherein arrow 46 indicates a direction of movement. The movement may include reciprocating movement. The anvil elements 38, 40 may be made of, or comprise, for example, low carbon steel or stainless steel.
During use, the strip 34 is positioned between the anvil elements 38, 40. The test surface 36 faces anvil element 38. The first actuator compresses the strip 34 between the anvil elements. Simultaneously, the second actuator may be operated to move the strip with respect to the anvil elements.
Compressive stress between the strip 34 and the respective anvil elements may be gradually increased to a maximum compressive stress, corresponding to a compressive stress between the contact surfaces 28, 30 of the connector 1 (
Several methods allow to determine changes or variation of material properties of the surface 36 of the strip 34. For instance, as a start, the surface 36 may be visually inspected for signs of galling. Herein, galling refers to cold welding between small portions of surface 36 and anvil element 38 due to compressive stresses, which may locally exceed a threshold for welding, followed by rupturing of the welded portions due to continued sliding of the surface 36 along the anvil element 38. Galling would cause the surface to become increasingly irregularly shaped.
In accordance with an embodiment of the method of the present invention, the qualification test procedure of the respective connector design will be ceased if the galling threshold level P1 is equal to or lower than a maximum contact pressure which is expected during either the test procedure and/or during operational use in hydrocarbon production applications. If P1 exceeds the expected said maximum expected contact pressure, the test procedure will be continued.
The horizontal axis of
Prior to establishing the material ratio curve, a certain percentage of the peak points (referred to as the Peak Offset) and valley points (referred to as the Valley Offset) may be ignored to minimize the effects of outliers.
Typically the Peak Offset and Valley Offset may be set to about 1%. The core section Sk is determined from the secant of the material ratio curve in the 40% section of the horizontal axis with the lowest gradient.
In the example of
A change of any of the parameters Spk/Sk, Svk/Sk, Vvc or Vvv after subjecting the test surface to the load may indicate an increased risk of fluid leaking past the metal-to-metal contact surfaces of the connector. Namely, an increase of Spk/Sk or Svk/Sk may indicate the occurrence of galling between a coated layer on test surface 36 and anvil element 38. An increase of Vvc or Vvv may indicate a volume increase of fluid channels in the test surface 36.
In an embodiment of the method of the invention, the parameters Spk/Sk, Svk/Sk, Vvc and/or Vvv are determined from the material ratio curve of the test surface. These parameters are optionally measured or determined both before and after subjecting the test surface 36 to the load. The qualification test procedure of the connector 1 may be ceased if at least one of ratio Spk/Sk, ratio Svk/Sk, Vvc and Vvv has changed unfavourably after subjecting the test surface to the load. Conversely, if there is no significant change of each of Spk/Sk, Svk/Sk, Vvc and Vvv after subjecting the test surface to the load, the qualification test procedure of the connector will be continued. Herein, the qualification of “significant change” of each of Spk/Sk, Svk/Sk, Vvc and Vvv may be determined relative to a pre-determined threshold or safe level.
The method of the present inventions improves the efficiency of a connection test programme. This will result, for instance, in the following advantages:
The present invention is not limited to the embodiments as described above, wherein various modifications are conceivable within the scope of the appended claims. Features of different embodiments may for instance be combined.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/060285 | 5/20/2014 | WO | 00 |
