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Oil tubing Proprietary Connections
- May 31, 2018 -

Proprietary connections

Special connections are used to achieve gas-tight sealing reliability and 100% connection efficiency (joint efficiency is defined as a ratio of joint tensile strength to pipe body tensile strength) under more severe well conditions. Severe conditions include:

  • High pressure (typically > 5,000 psi)

  • High temperature (typically > 250°F)

  • A sour environment

  • Gas production

  • High-pressure gas lift

  • A steam well

  • A large dogleg (horizontal well)

Also, efficiency in flush joint, integral joint or other special clearance applications improves connections. A large diameter (> 16 in.) pipe improves the stab-in and makeup characteristics; galling should be reduced (particularly in CRA applications and tubing strings that will be re-used); and connection failure under high torsional loads (e.g., while rotating pipe) should be prevented.

The improved performance of many proprietary connections results from one or more of these features not found in API connections:

  • More complex thread forms

  • Resilient seals

  • Torque shoulders

  • Metal-to-metal seals

The “premium” performance of most proprietary connections comes at a “premium” cost. Increased performance should always be weighed against the increased cost for a particular application. As a general rule, it is recommended to use proprietary connections only when the application requires them. “Premium” performance may also be achieved using API connections if certain conditions are met. Those conditions are:

  • Tighter dimensional tolerance

  • Plating applied to coupling

  • Use of appropriate thread compound

  • Performance verified with qualification testing

The performance of a proprietary connection can be reliably verified by performing three steps:

  • Audit the manufacturer’s performance test data (sealability and tensile load capacity under combined loading)

  • Audit the manufacturer’s field history data

  • Require additional performance testing for the most critical applications

When requesting tensile performance data, make sure that the manufacturer indicates whether quoted tensile capacities are based on the ultimate tensile strength (i.e., the load at which the connection will fracture, commonly called the “parting load”) or the yield strength (commonly called the “joint elastic limit”). If possible, it is recommended to use the joint elastic limit values in the design so that consistent design factors for both pipe-body and connection analysis are maintained. If only parting load capacities are available, a higher design factor should be used for connection axial design.

Connection failures

Most casing failures occur at connections. These failures can be attributed to:

  • Improper design or exposure to loads exceeding the rated capacity

  • Failure to comply with makeup requirements

  • Failure to meet manufacturing tolerances

  • Damage during storage and handling

  • Damage during production operations (corrosion, wear, etc.)

Connection failure can be classified broadly as:

  • Leakage

  • Structural failure

  • Galling during makeup

  • Yielding because of internal pressure

  • Jump-out under tensile load

  • Fracture under tensile load

  • Failure because of excessive torque during makeup or subsequent operations

Avoiding connection failure is not only dependent upon selection of the correct connection, but is strongly influenced by other factors, which include:

  • Manufacturing tolerances

  • Storage (storage thread compound and thread protector)

  • Transportation (thread protector and handling procedures)

  • Running procedures (selection of thread compound, application of thread compound, and adherence to correct makeup specifications and procedures)

The overall mechanical integrity of a correctly designed casing string is dependent upon a quality assurance program that ensures damaged connections are not used and that operations personnel adhere to the appropriate running procedures.

Connection design limits

The design limits of a connection are not only dependent upon its geometry and material properties, but are influenced by:

  • Surface treatment

  • Phosphating

  • Metal plating (copper, tin, or zinc)

  • Bead blasting

  • Thread compound

  • Makeup torque

  • Use of a resilient seal ring (many companies do not recommend this practice)

  • Fluid to which connection is exposed (mud, clear brine, or gas)

  • Temperature and pressure cycling

  • Large doglegs (e.g., medium- or short-radius horizontal wells)

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