Effect of Ovality Length on Collapse Strength of Imperfect Sandwich Pipes Due to Local Buckling

The effect of ovality length on imperfect sandwich pipes is investigated using the finite element method in the scenario of local buckling under external pressure. First, the finite element model of the imperfect sandwich pipelines is established in ANSYS and is validated by comparing the results from numerical simulation with those from experiments. Then, the effect of ovality features on the collapse strength of the sandwich pipes is studied. At last, based on the calculation results from 1200 cases, a prediction equation is proposed to represent the relationship between collapse strength and ovality length of imperfect sandwich pipes. Good agreement is achieved between the proposed equation and the calculation results, leading to the conclusion that the proposed simplified model can be an efficient tool in the evaluation of the local collapse strength of subsea sandwich pipes under external pressure.

Investigation on the Depth of Slip Hanger Teeth Bite into Casing and the Mechanical Properties of Casing under Different Suspension Loads in Ultra-Deep Wells

There is great difficulty in controlling the setting load of the large-size slip casing hanger in the Northwest Oilfield in China, and a reasonable setting load is of great significance. This paper studied the relationship between slip hanger bite depth and suspension load in the Ф 273 mm WE-type slip hanger in the Northwest Oilfield in China through experiment, theoretical computation, and finite element analysis. The accuracy of the finite element model was proved by comparing the finite element simulation results with the experimental bite marks on the casing surface. The study results show that the bite mark of the slip insert in the casing is deeper in the lower part of the sitting position. When the hanging load increases from 1000 kN to 6000 kN, the maximum bite depth of the slips in the casing gradually increases with the suspension load. The residual collapse strength of the casing decreases correspondingly. When the residual collapse strength decreases to a certain value, the maximum suspension force corresponding to the bite depth of the slip insert can be obtained. Based on the finite element research results and theoretical equations, the stress distribution on the casing wall where the slips bite the deepest is obtained by derivation. The suggestions on improving the material structure of the casing under this stress were proposed. The limit of the setting load of the large-size casing wellhead for avoiding casing collapse was obtained, which is of great significance for guiding field-casing setting.

Casing Collapse Strength Analysis under Nonuniform Loading Using Experimental and Numerical Approach

Multistage fracturing is the main means of shale gas development, and casing deformation frequently occurs during fracturing of shale gas horizontal wells. Fracturing fluid entering the formation will change in situ stress nearby the wellbore. The changes of in situ stress are mainly reflected in the following two aspects: one is the increase of in situ stress and the other is the nonuniformity of in situ stress along the wellbore. And it is for this reason that the production casing is more likely to collapse under the nonuniform in situ stress load. According to the service conditions of production casing in shale gas reservoir, this paper studied the casing deformation and the collapsing strength subjected to the nonuniform loading by the experimental and numerical simulation method. The results show that under the condition of nonuniform loading, (1) the diameter variation rate of the casing reduces with the increase in the ratio of sample to tooling length. When the ratio is less than 3, the casing collapse strength will be significantly reduced. And when the ratio is greater than 6, the impact of sample length on casing collapse strength can be ignored. (2) The increase in the applied loading angle will decrease the diameter variation rate. When the loading angle increases from 0° to 90°, the critical load value increases from 1600 kN to 4000 kN. (3) The increase in load unevenness coefficient will rapidly decrease the casing collapse strength. When the load unevenness coefficient n is 0.8, the casing collapse strength reduces to 60%, and when the load unevenness coefficient n is 0, the casing collapse strength reduces to 28%. The findings of this study can help for better understanding of casing damage mechanism in volume fracturing of shale gas horizontal well and guide the selection of multistage fracturing casing type and fracturing interval design.