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Mechanical integrity

Casing wear

3D casing wear process

Casing wear evaluation requires a good realistic calculation of the forces applied by the structure onto the casing along with a robust 3D integration of a adapted wear law model.

See our video about casing wear

The evaluation of orces is based on our 3D rigid model which was optimized to run a large number of calculculation if a minimum amount of time.

The wear model law implemented uses latest publication along with a large test campaign conducted with our partners. Nonlinear effects du pressure, multiple diameters and locations are being investigated using a detailled 3D meshing of the casing.

Fatigue

Cumulative fatigue on drill pipe

Cumulative fatigue on drill pipe

Faced with the complexity of the oil and gas wells drilled today, the fatigue phenomenon is the most significant cause of drill string failure. Different sources of cyclic loadings acting on drill string can cause the fatigue phenomenon during well operations: rotation of equipment in a curved portion of the well, rotation of a buckled structure, and vibrations in the drill string…

We developed a methodology and numerical models to evaluate the fatigue of equipment during drilling operations of wells with complex trajectories. A step-by-step algorithm, coupled with a 3D mechanical behavior model of a structure in well, is used to implement the fatigue approach in each calculation increment.

The first model is based on the cumulative fatigue concept using S-N curves of drill-pipes and a fatigue accumulation process by applying the Miner damage law. This model requires the knowledge of S-N curve of equipment which can be determined from full-size drill-pipe tests which are very costly, or by using the empirical method based on Moore’s machine fatigue test (small scale).

The second model is based on the crack propagation theory. This model requires the existence of an initial crack in the equipment, and the coefficients of the crack propagation law in the material which can be determined from standardized tests on small samples.

Casing Resistance

Classical approach

Casing design involves the selection of various aspect of casing so that is is able to resist loads for the entire life of the well. The resistance of the casing against these loads can be determined through various methods, either in the elastic material region or the plastic material region. The resistance of the casing in the elastic material region is defined as Working Stress Design while the resistance of the casing in the plastic region is called Limit State Design.

Working stress design involves using historical API equations, one to determine the burst resistance of the casing called API minimum internal yield pressure and another to determine the collapse resistance of the casing.

Limit state design extends this notion to the plastic material limit using Tomano collapse which takes into account potential ovalisation of the casing.

Reliability based approach

Reliability based design

Reliability based design

In probabilistic methods, every variable (yield strength, wall thickness, diameter, etc.) and loads against the casing are considered a random variable, with a specified distribution. The distribution therefore must be quantified in terms of uncertainty and variability.

For a given strength equation, the parameters in the strength equation are treated as random variables. An uncertainty model is then developed for each of these variables, using actual measurements and statistical modelling of the data if possible. The various individual distributions are then used in the strength equation and the distribution of the strength function is determined using either Monte Carlo or other analytical statistical methods (first-order or second-order uncertainty propagation methods).

We obtain a strength R and a load S according to a distribution (here normal). When the load is upper to the resistance that is when G = (R-S) < 0 there is failure.

PDC wear

PDC cutters with wear flat

PDC cutters are composed of a syntetic diamond layer fixed to a tungstene carbide cylinder. Wear process is the result of the following phenomenons :

  • temperature increase that modifies the cutter hardness
  • small grooves done by hard mineral (quartz)
  • cracks propagation due to thermal stress and/or interaction forces fluctuation and chocs
Wear tests bench

Wear tests bench

Wear is a complexe process to model ; we adress it with macro-laws that we adjust to experimental observations.