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Abstract The existing API equation for internal leak predicts the internal pressure to overcome the pin-box contact pressure generated from the makeup interference plus the energizing effect of internal pressure which enhances the seal. For threaded connections, internal and external pressures close the connection and increase the leak resistance, whereas axial loads open the connection and decrease the leak resistance. These competing effects must be included to accurately assess the connection leak resistance under any combination of loads at any point in any string. Following the same approach used by API for internal leak, this paper obtains similar results for external leak. For API connections, the effects of combined axial force and backup pressure are then incorporated into the internal/external leak equations using results from the Mitchell and Goodman (2018) paper presented at the 2018 SPE-IADC Drilling Conference. Sensitivities of leak ratings to combined loads for API connections are presented for both tubing and casing sizes. An example design case shows the importance of considering combined loads.
Abstract The API equation for internal leak of API connections is uniaxial since it ignores axial force and external backup pressure. ISO 13679 for qualification of premium connections is biaxial at best. It includes tension/compression but ignores backup pressure for both internal and external leak tests. For tubular design, this paper introduces a new fully triaxial safety factor for threaded connections with dependence on thread shear and hydrostatic pressure. Hydrostatic behavior is modelled with the Mean Normal Stress, and thread shear behavior is modelled with the shear component of the von Mises Stress. A Leak Line for use like the pipe body ellipse is proposed for quick leak assessment. Leak ratings are presented for an example case of 7-in. 35-ppf N80 LTC. The new triaxial safety factor with two connection constants applies to all types of threaded connections, including tubing, casing, and drill pipe, so long as the two constants are evaluated with appropriate but simple physical tests.
Summary Connector joint strength and leak resistance depend on internal and external pressures. Axial tension or compression opens (dilates) the connector, while the pressures close the connector. This paper presents a "toy connector" model that incorporates actual connector elements, but in a simplified form useful for analysis, so equations of joint strength and leak resistance can be determined as functions of axial load and internal/external pressures. The model and example case in this paper confirm that dilatancy occurs and hydrostatic pressure plays a critical role in connector failure (leak, pull-out, or thread shear). Von Mises stress alone cannot model dilatancy and the hydrostatic effect.
Abstract With the increased use of coiled tubing in high-pressure wells, the collapse of coiled tubing between the injector and the stripper has received much attention recently. In high-pressure wells, the failure of this tubing section (typically less than 2 ft in length) usually occurs under the combined loadings of axial compression and internal pressure. Previous analytical models to predict failure under such loading conditions have focused mostly on the buckling behavior of the short coiled-tubing section with minor modification of yield strength to account for the effect of internal pressure. Such approaches underestimate the effect of internal pressure on the collapse failure, especially for higher internal pressure. In this paper, a new analytical model is developed to predict the collapse of short coiled tubing under the combined loadings of axial compression and internal pressure. The analytical model first analyzes the buckling load of the short coiled-tubing section under axial compression only, and the burst pressure of the coiled tubing under internal pressure only, respectively. Then an interaction failure criterion is used to model the failure locus of the short coiled tubing under the combined loadings of axial compression and internal pressure. Experimental data are used to validate this new model. Introduction With the increased use of coiled tubing in high-pressure wells, the collapse of coiled tubing between the injector and the stripper has received much attention recently. To snub the coiled tubing into a high-pressure well, the injector has to exert a significant amount of compression to overcome the resistance from the wellhead pressure. Given the fact that the concerned section of coiled tubing between the injector and the stripper has an unsupported length of about 6 to 18 in., with each end supported by the injector chains and the stripper, respectively, the compression exerted by the injector could become high enough to cause catastrophic failure on this coiled-tubing section. In fact, field experiences indicate that such failures indeed happen. However, failure of this kind is not addressed in API Recommended Practice RP5C7, which was released almost eight years ago. To mitigate the risk of this kind of collapse failure, many high-pressure coiled-tubing units have an anti-buckling guide installed above the stripper to shorten the length of the unsupported coiled tubing between the injector chain and the stripper. However, it is still desirable to develop a model to predict the failure envelope for the concerned tubing section under the combined loadings of axial compression and internal pressure. Considerable effort has been devoted to predicting the failure envelope of coiled tubing under the combined loading of axial force and pressure (both internal and external). Ref. 4 presented an approach to calculating the failure envelope for coiled tubing under the combined loadings of axial force and pressures. For coiled tubing under compression, the approach is more suitable for long coiled tubing inside a wellbore where helical buckling occurs prior to tubing failure. Ref. 5 presented a model to calculate the buckling load of the coiled tubing between the injector and the stripper. In this model, a short column buckling theory based on the Gordon-Rankine formula is used to calculate the buckling load. The model considers the effect of internal pressure by modifying the yield stress. Such a treatment obscures the interaction between the compression failure and burst failure modes and may not adequately account for the effect of internal pressure on failure when the pressure is high. Recent test data indicate that high internal pressure does have a significant effect on the collapse of a short coiled-tubing section, which prompted the development of this work. In this paper, an interaction failure criterion is developed to predict the failure of short coiled tubing under the combined loadings of axial compression and internal pressure. Experimental data from existing publications and from new testing are used to validate the model. In the following sections, a recent experiment on short coiled tubing under combined loading of axial compression and internal pressure is first presented, followed by the development and validation of an interaction model for collapse failure under such loading conditions.
This paper discusses different finite element models to assess the structural integrity of pipeline bundles installed on uneven seabeds. A pipeline bundle consists of an outer carrier pipe and several internal flowlines. This concept offers many advantages compared to conventional pipelines such as fabrication and pressure testing onshore, simplified installation, thermal insulation and mechanical protection of the internal flowlines. The high bending stiffness of the bundle cross section implies that longer free spans can be accepted compared to the case where the flowlines are installed separately. However, large axial compression forces may develop in the bundle due to high operating temperatures and pressures in the internal flowlines. These forces may give raise to significant additional bending in free spans due to second order effects. Some of the compression force is released at the bundle ends due to axial expansion. Restraints at the ends and soil friction acting along the carrier pipe may, however, give raise to high compression forces, particularly for long bundles. The high bending stiffness in addition to the lateral soil resistance normally prevent lateral buckling when the bundle rests on a flat seabed. However, the second order bending effects caused by the compression forces have to be considered when the bundle is to be installed on an uneven seabed. In this paper the second order bending effects arising from the compression forces in bundles are addressed. Different finite element analysis models to assess the structural integrity are described. These models are applied to a real life case and the results are discussed. INTRODUCTION Often several interfield flowlines are needed to connect the subsea wells and the production and exportation facilities in an offshore oil/gas field. If several flowlines run in parallel it may be beneficial to bundle them into an outer carrier pipe. Then the internal flowlines are kept in place by spacers at regular intervals