Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-01-24
2002-12-31
Hilten, John S. (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S150000
Reexamination Certificate
active
06502041
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for inspecting the dimensions of a male or female tapered thread located on the periphery of a male or female tubular element itself located at the end of a metal pipe, more particularly to a process for inspecting the dimensions of the pitch diameter of such a male or female tapered thread in a given cross-sectional plane and inspection devices for carrying out the method.
DISCUSSION OF THE BACKGROUND
The term “pitch diameter” of a thread means the diameter taken on the flank of the male thread at the thread half-height. The female thread is defined with respect to the male thread.
For a tapered thread, the nominal value of the pitch diameter must be defined in a given cross-sectional plane.
Threaded connections for metal pipes are known which are in particular used to constitute drilling, production tubing or casing pipes for hydrocarbon wells or mine wells, the connection being made between a male element and a female element located at the pipe end and each respectively provided with a male and female tapered thread located respectively on the external or internal periphery of the male or female element.
The term “pipe” as used here means any type of pipe, not simply a long pipe but also a short tubular element forming, for example, a coupling and enabling two long pipes to be associated together.
Specification API 5CT from the American Petroleum Institute (API), which constitutes a world-wide standard in the hydrocarbon extraction industry, specifies pipes coupled using such threaded connections comprising tapered threaded portions with triangular, round or trapezoidal threads.
Specification API 5B, also from the API, specifies the corresponding thread and a method for inspecting it.
Specification API 5B specifically mentions, for each pipe dimension, the value of the thread nominal pitch diameter in a cross-sectional plane located at the end of the perfect male threads on the pipe body side, male threads exceeding this plane having an incomplete height and fading out.
In the remainder of the present document, this plane is termed the “reference plane of the pitch diameter”, abbreviated to “reference plane”. The term “first thread” relates to the side of the threaded portion directed towards the free end of the corresponding male or female element; the term “last thread” refers to the side of the threaded portion oppositely directed to the free end of the corresponding element.
The last perfect male thread is thus located in the reference plane while the last male thread corresponds to the end of the threaded portion on the pipe body side.
Threads produced in accordance with API 5B must be inspected by hand tightening of gauges such as ring gauges comprising an internal thread in the case of inspecting male threads or plug gauges comprising an external thread in the case of inspecting female threads.
The relative axial position when the gauge has been screwed on is checked with respect to the inspected thread and specification API 5B defines a value and a tolerance for this relative axial position.
The inspection method specified by API 5B has advantages, namely simple and rapid global inspection of the thread, but it also has a number of economical and technical disadvantages.
Firstly, for each thread diameter to be inspected, the hard gauge inspection method requires sets of gauges for different levels, namely master gauges and secondary or working gauges, the working gauges having to be discarded when the wear on them exceeds a critical value.
This means that an extremely large number of high precision gauges have to be made and they have to be managed depending on their state of wear; thus costs are high.
That thread inspection method provides a global result which depends on a number of parameters including the pitch diameter and also the taper and ovality, which parameters interact and thus do not facilitate fine interpretation of the inspection results.
Thus when inspecting a male thread, if the taper of the male thread to be inspected is less than that of the gauge, the first roots of the male threads are in contact with the gauge threads while the last roots of the male threads have a radial clearance with respect to the corresponding threads of the gauge. In contrast, if the taper of the male thread to be inspected is higher than that of the gauge, the roots of the last male threads are in contact with the threads of the gauge but not the roots of the first threads.
In both cases, the pitch diameter of the male thread in the corresponding plane on the gauge in the reference plane is less than the nominal pitch diameter but, in addition in the second case, the pitch diameter at the first male threads is not accurately known.
Similarly, in the case of inspecting a female thread, when the taper of the female thread to be inspected is less than or greater than that of the gauge, the pitch diameter of the female thread in the corresponding plane on the gauge to the reference plane is higher than the nominal pitch diameter but, if is its lower, the pitch diameter at the first male threads is not accurately known.
The manufacturers of particular threaded connections such as the connections described in European patent EP-A-0 488 912, which are known to have service performances which are superior to those of API connections, have been forced to use inspection methods similar to those specified for API connections because of international recognition and imposition of API specifications.
The cost of implementing those methods is considerable; the manufacturer must have available complete sets of gauges for itself and for sub-contractors.
Thread inspection methods have thus been developed which do not use hard gauges but which carry out a direct determination of the pitch diameter in the reference plane or at another set position.
United States patent U.S. Pat. No. 4,524,524 describes a method and a device for direct inspection of the pitch diameter of a horizontally disposed male or female thread in which:
The device comprises an upper contact surface and a lower contact surface located in a vertical plane at an adjustable horizontal distance from a vertical bearing surface;
The vertical distance between the upper and lower contact surfaces are adjusted to a pre-determined value;
The device is placed such that the vertical bearing surface bears against the end of the element the thread of which is to be inspected and such that the two contact surfaces are in contact with the thread crests at diametrically opposed points on the thread;
The difference in distance between these two contact surfaces is measured with respect to the pre-determined value using a comparator, for example, which has been zeroed to the predetermined value.
The pre-determined value in this case corresponds to the nominal value of the diameter between the thread crests and thus to the nominal value of the pitch diameter increased or reduced by the height of one thread depending on whether a male or female thread is being inspected.
The instructions for the device sold by the proprietor of U.S. Pat. No. 4,524,524 defines:
a) the required relation for establishing the link between the tolerance in the pitch diameter (&Dgr;D) when using the inspection method using the measurement of the pitch diameter and the tolerance in the axial position (&Dgr;S) of the hard gauge as defined by specification API 5B:
&Dgr;
D=&Dgr;S·TT
nom
/100
TT
nom
being the thread nominal taper in % with respect to the diameter;
b) fine corrections for accounting for the influence of the geometry of the contact surfaces of the inspection device on the pre-determined value of the vertical distance between the contact surfaces;
c) the method for determining the pre-determined value of the vertical distance between the contact surfaces of the inspection device to account for location of the measuring plane out of the reference plane. Thus a quantity equal to: L·TT
nom
/100 is subtracted from the nominal pitch diameter;
&emsp
Assens Gilles
Noel Thierry
Vallourec Mannesmann Oil & Gas France
Washburn Douglas N
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