November 2, 2012
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№ 10 (October 2012)

Downhole Instrument Upgrading to Improve Vibration Stability and Operational Characteristics

   The article describes the results of optimization of the electronic module design on the basis of tests under severe downhole vibrations with the aim of developing a unified telemetry system having high performance characteristics.

By Vladislav Sinitsa

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   At present, development of downhole instrumentation for logging while drilling, acquisition of technical parameters of rock destruction and real-time transmission of this information to the surface is of decisive importance for enhancement of well construction and hydrocarbon production [1].

   Significant difference in the operating conditions of the instruments included in the telemetry system installed in the bottomhole assembly (BHA) and their analogs intended for open hole logging is mostly related to the incommensurable level of vibration and impact load caused by the rock cutting tool in the course of drilling. The second key distinctive factor is the requirement on minimization of the outer dimensions (diameter in the first place) of instruments to reduce hydraulic resistance to the drilling mud flow and preservation of strength of the drill string in which the instrumentation is placed.
In a number of cases, utilization of additional shock absorbers in the BHA appeared to be a justified action against vibration [2]; however installation of these shock absorbers results in larger distance between the telemetry system sensors and the drill bit, and lower rigidity of the shock absorber causes lateral movement of the drill bit and its deviation from the designed trajectory of the wellbore.

   Analysis of existing designs of bottomhole instruments and an in-depth patent search has found no implementation of any novel solutions for this problem during the last two decades: telemetry system designers give main consideration to the shock-absorbing hangers of the printed-circuit board chassis, without touching the schematic layout diagram [3]. We believe that this approach is not optimal.

   That is why, when designing a unified downhole instrument for the extended line of telemetry system standard sizes (conventional outer diameter of case elements ranging from 89 mm to 240 mm) we focused our effort on upgrading of the printed-circuit board chassis in order to improve their shock-absorbing capabilities and reduce the outer diameter with maximal use of the internal volume for placement of electronic components.

   Our work resulted in a technical solution shown in Fig.1; invention application for this solution was submitted and registered under #2012127553.
Concept of the invention of an electronic module for the downhole telemetry system is schematically presented in the drawings where the following is shown:
Fig.1. – an axonometric view of the assembled electronic module, which, for better visualization, does not show the protective outer cover (a tubular detail), printed-circuit board with electronic components 3 is partially cut-off and the right shock absorber 5 is also not shown in order to exhibit flats on the leg of the centering element 4 (profile asymmetry);
Fig. 2 – cross-section of the electronic module along the axis of fixture elements 6 to exhibit orifices in the butt end of the centering element 4, configurations of holding plates for printed-circuit boards 2 with the minimal bending radii and gaps for resilient tension in the assembled unit with the aim of improvement of shock-absorbing properties.

   Structural Elements and Functional Characteristics of the Module
The electronic module of the downhole telemetry system includes chassis 1, made of flat parent sheet shaped as U-section. Sites 2 are spotted on chassis 1 for mounting the printed-circuit boards with electronic components 3, centering elements 4, shock absorbers 5, cables (not shown in the Figure) and other necessary components and details. The spacing between these mounting sites 2 along chassis 1 can vary for wider vibration suppression range.

   The above-listed details are mounted on sites 2 with the help of fastening elements 6, which could be rivets or a set “screw-nut-washer- splint pin” or other common fastener assemblies.

   In case of one-sided mounting of electronic components, electronic Printed-circuit boards 3 installed on the mounting sites 2 of chassis 1, divide the internal volume of the electronic module into “top” and “bottom” parts. We will consider the side of electronic components to be the “top” part, and soldering spots – the “bottom” part. In this case it will be more technically efficient to lay the cables in the bottom part, in an additional plastic cable channel.

   In case of double-sided mounting of electronic components on printed-circuit boards 3, cables can be laid on any side based on the assembly requirements.
To reduce vibrations and fasten cables between printed-circuit boards 3, it is possible to use an elastomeric damper 7 installed on the legs of the centering element 4. If necessary, shock absorbers 5 and elastomeric damper 7 can be made as one detail.

   Larger details, capacitor banks and accumulator batteries 8 can be fastened using adhesive sealant in the U-shaped location plate of chassis 1 between the mounting sites 2, а and cable connection can be made in the form of a loop rounding these areas as a semi-cylinder.

   Mounting sites 2 for printed-circuit boards 3 can be stamped with the help of the “restricted bending” method to get the minimal bending radii (Fig. 2) and enhance rigidity of the bending areas.

   In addition, mounting sites 2 can be made with a gap to the ends of the walls of the U-shaped location plates of chassis 1 in order to get resilient preload as a result of the package tightening by fastening elements 6.
A similar gap can be used in the connection “centering element 4 – shock absorber 5” to get a resilient preload when assembling an electronic module with a protective cover.

   In the curvilinear ends of mounting sites 2, it is possible to make orifices for cable laying through the whole electronic module; they can be supplied with standard clips to prevent chafing against sharp edges.

   Cable through-holes and fasteners are also provided for in centering elements 4 both across the butt end and along the axis parallel to legs for bringing cables from the bottom side of the module to the top one or for fastening of additional elements.

   Beside the adhesive sealant, larger details, accumulator batteries and capacitor banks 8 can be additionally fastened to chassis 1 and centering elements 4 using standard fastening elements, for example, dowelled joints or screw connections, for which additional threaded pads can be located on centering elements 4.

   Operation of the electronic module of the telemetry system implies acquisition, processing, storage and transmission of the borehole information to the surface. This electronic module is also used for conversion, storage and distribution of electric power among the consumers. All these functions are to be performed under tough conditions of vibration and shock loads caused by the rock cutting tool. Additional problems are conditioned by the requirement of minimization of the outer diameter of downhole tools for their possible location in slim hole drill pipes. Other conditions being equal, the diameter reduction results in an increase of the electronic module length in order to install a similar amount of electronic components in this module.

   The patent-pending technical solution under discussion differs essentially from the opposable analogs and eliminates or neutralizes most of their demerits by fabrication of chassis 1as a U-shaped one-piece construction of a flat parent sheet using a highly productive method of rolling.

   An aluminum alloy strip 0.5...0.7 mm thick and of the required length (1.5...3 meters) can be used as a blank for chassis 1. Aluminum alloys are easily deformed; they are non-magnetic and have high thermal conductivity.

   Fabrication of chassis 1 in the claimed technical solution from a flat parent sheet enables the following:
Reduce the total mass of the structure, its cost and vibration load, and also requirements to the fabrication precision due to flexibility of the open section;
Move walls of location plates in chassis 1 away from the areas of soldering of the electronic components to plates 3;
Make maximum use of the internal volume of the electronic module for placement of downhole instruments;
Use electronic printed-circuit boards 3 with double-sided assembly of components;
Place any through-line cables in one-piece chassis 1 prior to fastening of plates 3; thus the number of connections is reduced, low-current circuits of plates 3 are unloaded and their heat load is decreased;
Reduce the diameter of the electronic module to extend the operational capabilities.

Conclusion
Thus, the declared technical solution differs significantly from the analogs, has significant advantages in the assembly arrangement, cost and dimension parameters, operational functionality and repairability.

References:
Optimization of Layout Diagrams of Telemetry Systems for Measurements while Drilling / V.V.Sinitsa // Engineering Practice, 2012, No1
Experience of Application of Above-bit Dampers for roller-bit drilling / A.G. Shlykov [et al] // Mining Journal. 2009, #5.
Electronic downhole instrument of the telemetry system / RF Patent No2371574

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