Safe drilling operations in the oil and gas industry require high attention and greatest care for any aspect from design to completion. Cyclic loads while drilling cause the initiation and growth of cracks in oil tubulars. This phenomenon, known as fatigue, results in permanent reduction of the failure-free service envelope of a certain tubular. As its aftermath, sudden rupture of material below its static service envelope will be encountered. Several well-accepted approaches in the industry are utilized for analyzing the tubular's material behavior under cyclic loads in the laboratory.

There have been so far three most prevalent standard fatigue testing machines in the industry to carry out the analysis of response of tubulars under cyclic loads:

  1. Rotating Bending,

  2. Resonance Bending,

  3. Multiaxial Fatigue Setups including bending only in one plane.

The name of each type speaks quite straight forward for itself in defining the loading conditions on tubulars. The purpose of these test rigs is to study the behavior of a certain geometry and material after going under a given loading condition. One of the usual challenges in designing the fatigue rigs has been compounding the torsional loads simultaneously to the other loads, such as axial, bending etc.. The simultaneous application of torsional loads to the tubulars has been achieved in multi axial fatigue testing rigs, however only in one plane, better to say only two fibers of the tubular are subjected to the full loading sequence.

A new concept for multiaxial fatigue rigs provides a useful tool to predict the fatigue life and integrity of the drill (or casing) string under the cyclic loads present during the drilling process.

The rotating bending rig is capable of applying bending and axial load, while twisting of tubular plays no role in reconstructing the loading conditions. In conventional multi-axial fatigue rigs torsion and bending are applied only in one plane to two fibers of tubulars.

In resonant fatigue rigs, where the sample is excited to one of its natural frequencies, it is not possible to apply twisting or axial tension. Additionally, the distribution of bending moment looks like a parabola, a non-uniform loading on the tubular.

The new concept of multi axial fatigue testing rig excludes the drawbacks of legacy fatigue testing rigs. The bending is applied uniformly on all the planes over the circumference of the specimen. This multi-axial fatigue testing rig enables imposing high loads as axial, bending, torque and loads due to internal pressure, all simultaneously.

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