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Designing for oil rigs: harsh environment design considerations for measurement-while-drilling electronics.

The continually increasing global demand for oil has required down-hole oil rigs to become much more technologically advanced, featuring an array of electronics designed to enable more efficient exploration and provide better information to the rig. The electronics in measurement while drilling (MWD) equipment must be able to withstand extremely harsh environmental conditions, including depths greater than seven miles, internal module temperatures greater than 200[degrees]C, and high vibration. As such, many component manufacturers have developed parts ideally suited for use in high vibration applications that experience temperatures up to 250[degrees]C.

Component Background

Ceramic and tantalum capacitor technology, in particular, are especially well suited to meeting the needs of harsh environment applications. Ceramic capacitors can be stacked on top of each other and attached to a lead frame, which provides improved capacitance, reduced parasitic resistance, and reduced parasitic inductance without increasing the amount of space required on a PCB. They can also be made with leads attached to the chips, either in radial or axial configurations, and then encapsulated in a plastic case and sealed with epoxy, improving their ability to reject dust and moisture. Leads and lead frames also provide critical vibration resistance for ceramic capacitors by reducing the tensile forces applied to the naturally brittle material. Alternately, the vibration experienced by non-leaded, surface mount ceramic capacitors can be reduced through the use of some variety of flexible termination material.

Vibration is far from the only hazard capacitors can face in down-hole environments. Basic X7R and NPO ceramic capacitors are only rated for use up to 125[degrees]C, and are thus unsuitable for down-hole applications.

Some vendors have addressed this issue through the development of ceramic materials that extend operating temperature ranges up to 250[degrees]C. These materials can reflect the temperature characteristics of basic NPO and X7R dielectrics, but have been modified to reduce the steepness of the capacitance roll-off above 125[degrees]C. However, tantalum capacitor solutions are often employed when both high vibration and high temperatures must be overcome while maintaining higher capacitance than what a ceramic capacitor can provide in smaller case sizes.

Tantalum capacitors, which are widely available as chipstyle surface mount devices, radial leaded conformally coated devices, or axial leaded wet tantalum parts, can typically withstand temperatures up to 125[degrees]C.

One leading manufacturer has even released tantalum capacitors available in a hermetically sealed ceramic case, which provides resistance to dust, moisture, radiation, and temperatures up to 230[degrees]C. Surface mount tantalum capacitors come in a variety of case styles, each of which offers varying degrees of vibration resistance and other advantages. Basic J-lead tantalum capacitors have metal leads attached to the sides of the capacitor body and wrapped under the capacitor, which provides complete mechanical decoupling from the PCB, and enables extremely high vibration resistance. Undertab and conformal body styles can provide higher capacitance in a similar case size as J-lead styles, but do not provide the mechanical performance seen in J-lead capacitors.

If vibration is of critical importance, however, a leaded tantalum capacitor, such as a conformally coated radial or axial-leaded wet tantalum, will provide even greater mechanical performance than J-lead styles, due to the fact that the lead length can be modified to allow the capacitor to move as much as needed for a given application.

MWD Applications: Sonic Logging Tools

Sonic logging is frequently used in down-hole oil and mineral exploration to measure the porosity of the materials surrounding the well hole. Sonic logging modules generally contain an acoustic module consisting of piezoelectric sensors driven by voltage regulators, amplifiers, filters, and shaping networks. These components transmit and receive acoustic signals that are crucial for determining the correct depth and direction of the drill bit, and must be capable of withstanding the extreme vibrations and high temperatures that comprise normal operating conditions within wells.

The sensors used in sonic logging modules and downhole operations typically operate at low frequencies due to the fact that lower frequency operation tends to penetrate dense materials more effectively. As such, these sensors often employ tantalum capacitors, which--in addition to effectively withstanding high temperatures and vibrations--operate beautifully at low frequencies. Since tantalum capacitors typically exhibit high capacitance, they can provide fairly low attenuation in low frequency ranges, while also rejecting higher frequency noise. This enhanced signal-to-noise ratio improves the logged data by rejecting erroneous information, and only sending accurate signals concerning the surrounding environment to the surface.

MWD Applications: Gamma Ray Logging Tools

Gamma ray logging tools are primarily used to determine the amount of shale surrounding a well. The specialized sensors in these tools measure the naturally-occurring radiation of various isotopes of potassium, thorium, and uranium, and convert that radiation into an electrical signal that is useful for both processing and evaluation. The initial signal is fairly low voltage, though, so it must be amplified significantly by the time it reaches the surface. This is typically achieved using high voltage regulators, which are made of amplifiers with decoupling ceramic capacitors between the stages, and are used to transmit a signal from logging tools to the surface.

Generally mounted inside the drill bit, gamma ray sensors are subject to extremely harsh conditions, including temperatures exceeding 200[degrees]C and severe vibrations. As such, it is vital that the components within these sensors can also withstand very high operating temperatures and exhibit optimal mechanical shock and vibration characteristics.

Stacked ceramic capacitors with temperature ratings of 200[degrees]C not only satisfy those application demands, but also provide high capacitance values and, due to the performance benefits imparted by the lead frames, generally provide low parasitic inductance, making them ideally suited for decoupling applications in gamma ray logging tools. Stacked ceramic capacitors can also help improve overall performance, since Class I dielectrics (such as NPO) can withstand high frequency requirements and voltages up to 500 V, and require little to no capacitance de-rating as a result of temperature or DC bias.

MWD Applications: Resistivity Logging Tools

Resistivity logging tools measure the resistivity of the materials surrounding the borehole, and relay that information back to the surface, where engineers can use it to gauge material qualities. For example, low resistivity readings can indicate the presence of salt water, and high resistivity readings can indicate the presence of hydrocarbons. Temperature has a significant effect on the resistivity of materials. These tools have to employ temperature compensation networks to maintain stability in down-hole environments and ensure accurate resistivity readings.

Temperature compensation networks tend to be comprised of a temperature-current transducer, such as a thermistor, and a voltage-to-frequency converter (VFC) used as feedback to control the output of the power supply. The VFC uses the thermistor's current to deliver a signal that can control the power supply's operation, and a timing capacitor to set the frequency.

For the VFC operation to be a success, the RC network must have the correct time constant. So, the use of a robust, high temperature capacitor with NPO dielectric is recommended to ensure stable operation.

Summary

The capacitors employed in harsh environment applications like down-hole oil exploration must be both mechanically robust enough to continually withstand extremely high temperatures and harsh vibrations for long operational lifetimes, as well technologically advanced enough to reliably perform an array of precise, demanding functions. As such, a number of leading electronic component manufacturers now offer capacitors in various dielectrics, case sizes, and styles especially designed to resist vibration and temperature, while providing reliable operation in harsh, high voltage, high frequency environments. Although 125[degrees]C was the longtime upper limit for ceramic capacitors, several ceramic and tantalum capacitors are now available with temperature ratings as high as 250[degrees]C to help satisfy the ever-evolving demands of harsh environment applications, including: sonic, gamma ray, and resistivity logging equipment.

By Patrick D. German, Field Applications Engineer, AVX Corporation
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Title Annotation:ISSUE FOCUS: HARSH ENVIRONMENTS
Author:German, Patrick D.
Publication:ECN-Electronic Component News
Date:Mar 1, 2016
Words:1305
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