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CT110 is ultra-high-strength coiled tubing (CT). It has been applied in the drilling, completion, fracturing and stimulation process of deep wells, ultra-deep wells and large displacement horizontal wells. Under complex load, the CT will crack and eventually failure. Fracture toughness of CT materials can be measured using Crack Tip Opening Displacement (CTOD). In this paper, a three-point bending finite element model (FEM) of CT110 material was established, and its critical CTOD value is obtained by calculation as 0.732 mm. The FEM of three-dimensional CT with initial crack was established and its feasibility was verified. The ultimate bearing failure analysis of CT with initial crack defects shows that under the same working condition, internal pressure accelerates the crack initiation and propagation speed, so that the crack of tubing body reaches the critical value in advance, and the maximum value at 130 MPa is close to the critical value. Under the combined loads, the maximum CTOD value of the CT with a crack increases with the increase in the crack depth and length, and the deeper the crack depth, the greater the influence of the crack length on its maximum value. This study on crack initiation and propagation of CT with defects provides a basis for predicting the fatigue life and revealing the failure mechanism of CT.

Coiled tubing (CT) is known to be susceptible to corrosion due to the complexity of wellbore environment and limited material selection. Since the weld seam (WS) and the base metal (BM) of CT are different in composition and microstructure, WS material usually shows very different (lower) corrosion resistance compared with BM. To better understand the corrosion-related behaviors of CT materials and the mechanism behind them, the WS and BM samples were directly processed from a finished product of CT, and a series of electrochemical experiments were conducted in laboratory conditions to study the corrosion behaviors of both WS and BM materials. The results showed that the corrosion potentials of BM and WS under the condition of 60 °C and saturated CO2 increased by 57.1% and 41.6%, respectively, comparing with the condition of no CO2 presence. Also the corrosion current of the WS was found to be 14 times of the BM’s. A corrosion model was established assuming the principles of galvanic couple corrosion applied on the working conditions of CT. The surface morphologies of the samples were analyzed by the multi-physical field coupling method. The results showed a very good consistence between the corrosion model and the experimental data. Meanwhile, this model was also used to evaluate the impacts of the welding reinforcement, the area ratio of WS and BM, and the material defects on their corrosion rates. The results indicated that the area ratio of WS and BM is in direct proportional to the corrosion rate, and the other two parameters showed very little effect.

Hydrate formation and wax deposition in submarine pipelines hinder the transportation of oil and gas. Pigging operation is an important means to ensure pipeline flow assurance. In this paper, the i...

Coiled tubing (CT) endures high pressure during operation, thus causing high possibility of fatigue failure. Firstly CT's failure under high pressure and effects of pressure on CT fatigue life were investigated. Based on classical elastic mechanics theory, the CT ultimate internal pressure theoretical model was established. Based on classical plasticity stress computational formulas, the CT fatigue life theoretical model was established. Experimental research of internal pressure's effect on CT fatigue life was carried out. The theoretical calculation result is proven to be in good agreement with experimental data of fatigue cycle time under different pressure. The results show that internal pressure is one of the main influential factors on CT's bearing capacity, and that CT is exceedingly easy to have fatigue failure under internal pressure. The result validates the feasibility of CT ultimate internal pressure theoretical model and fatigue life model. The fatigue life of CT decreases with the increase of CT internal pressure. It is suggested to use CT with high strength and low inner-to-outer diameter ratio when working with high internal pressure. Study of internal pressure effects on CT bearing capacity and fatigue life can be used as a reference for the pressure control and safe operation of CT on field.

Buckling deformation of pipe in actual bore hole will limit the maximum running depth and is likely to further damage the drilling pipe, causing huge losses. Therefore, the buckling analysis of the...

Influence study on ultimate bearing capacity of coiled tubing (CT) under the multi-group load can effectively avoid early failure and improve the service life. Based on the classical mechanics model, the mathematical model of CT under the multi-group load is established. The feasibility of the finite element (FE) model and the mathematical model are verified by the experimental result. Under tensile load, bending moment load, internal pressure and collapse load, the ultimate bearing capacity of 2 3 8 ″ CT is comparatively researched. The computed result shows the result of axial stress is correlated to the tensile load and bending moment load, the results of hoop stress and radial stress are linked to internal pressure and collapse load, respectively. According to the calculation results, the theory result is less than the FE result, and the error is acceptable in engineering. And by comparison, the maximum influence on ultimate bearing capacity of CT is internal pressure and collapse load.

Coiled tubing (CT) technology is widely used in oil and gas industry. In the operation process, low cycle fatigue failure is easy to occur. Therefore it is necessary to predict the fatigue life of CT. In this paper, a new theoretical model of low cycle fatigue life for CT is established, the feasibility of the theoretical model is verified by experimental result and Finite Element (FE) value. This method effectively solves the problem that the result of the previous model is not more accurate at some conditions. Using the theoretical model, the effect of CT steel grade, wall thickness, outer diameter (OD), reel radius, guider radius and internal pressure on the fatigue life are studied by using the theoretical model. The result shows that the reel radius, CT OD and internal pressure are the sensitive parameters that affect the fatigue life of CT.

Based on the central composite rotatable design (CCD), clad beads of Inconel 625 were deposited on the surface of AISI 4130 plates using hot wire pulsed TIG (tungsten inert gas arc welding) technique. The response surface methodology (RSM) was used to establish models between process parameters and geometrical characteristics of the clad beads. Then, multiple-track two-layer weld overlay was deposited using the optimized process parameters. The microstructure of the weld overlay is primarily composed of columnar dendrites, and there are also a few planar crystals and cellular dendrites near the fusion zone. Meanwhile, equiaxed grains and steering dendrites are mainly distributed in the upper portion of the weld overlay. Potentiodynamic polarization tests were used to evaluate the corrosion resistance of the weld overlay and the substrate. The results show that adding clad layers can enhance the corrosion resistance, which degrades with the increase in Fe dilution. Moreover, the corrosion resistance of the second layer surface is close to that of wrought Inconel 625.

Coiled tubing (CT) technology is widely used in oil and gas development. In operation process, defects are common and accelerate the fatigue failure of CT. Therefore, it is necessary to predict the fatigue life of CT with defects. In this paper, the combining method of finite element analysis and experimental analysis is used to analyze the parameters of affecting the fatigue life of CT, such as the depth, length, width and distribution of the defects. Experimental results show that the most important defect parameter is the defect depth. Considering the effect of these significant parameters, based on the elastic-plastic mechanics and Brown-Miller fatigue life theoretical model, a theoretical model of the fatigue life of CT with pit defects was established, and it was modified according to the experimental results for obtaining more accurate theoretical model of the fatigue life of CT. The theoretical model is suitable for CT with ellipsoidal defects, and the defects are volume reduction defects caused by mechanical damage.

According to the conventional well cementing method, the cementing sliding sleeve, as a key part of the casing, needs to be opened by switch tool step by step to conduct fracture with coiled tubing. However, it usually needs switch tool to locate accurately, which is indeed a challenge. This article proposes a set of fracturing sliding sleeve with six-opening-degree and hydraulic switch tool working with coiled tubing–enabled fracturing sliding sleeve. The three-dimensional models, finite element analysis, and flow analysis of the tool are also adopted, which can validate the rationality of the tool’s structure. The experimental tests show that the sliding sleeve can be opened or closed with multi-opening degree easily and overcome the gas coning, which can fully meet the requirements of down-hole operation. In addition, the switch tool can open or close the sliding sleeve under lower pressure, and it has a high success rate to open or close the sliding sleeve by coiled tubing without precision location.