November 2, 2012
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№ 10 (October 2012)

New Model of Coiled Tubing (CT) Fatigue Life Prediction

   Safety and risk exclusion during coiled tubing operations is the main task arising before coiled tubing (CT) manufacturers and customers. Along with consumables and equipment quality improvement, high results are provided by methods of CT fatigue life prediction and remaining fatigue life control.

By S.Gouskov, A.Brylkin, K.Kolesnikov, V.Buksbaum, M.Usova (JSC "Uraltrubmash", Chelyabinsk), V.Poroshin (South Ural State University, Chelyabinsk)

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Model’s basic principles

   Suggested method of coiled tubing fatigue life estimation is based on calculation of such parameter as damage which characterizes the degree of defectiveness (or damage) of CT material at the current moment of time. The damage is linearly summarized over all round-trips into the well and back (working cycles), where each one of them consists of 3 bending cycles: 1 cycle at the reel and 2 cycles at the guiding arch. When reaching the critic value of damage (the moment of through-thickness crack occurrence), CT is considered to be failed. Cyclic properties of the material (cyclic curve parameters) can be obtained from specific tests or, as for actual model estimation, by means of calculations using for instance Manson’s equations. These data along with consideration of strain-stress state allow predicting the ultimate number of bending cycles (as well as working cycles) to failure.

   At any point on the surface of tubing while it is being bent around a curve the full stain is determined only by geometrical characteristics: CT diameter and bending radius. In order to determine the contribution of non-elastic strain the full strain is separated into elastic and plastic components with using the relation between strain and stress approximated by exponential curve of static loading. During the following loadings according to the Masing’s principle the loading curves coincide with “doubled” curve of initial loading.

   The calculation model also takes into account the effect of internal pressure with considering the radial stresses induced by the pressure to be negligible.
Ultimate value of fatigue damage obtained by using the hypothesis of linear summarizing will be as follows:
, where  – plastic strain amplitude; С, α – cyclic curve parameters;  – fatigue damage per cycle.

Verification of the model

    Comparison of the results obtained by model calculations for tubing with minimum mechanical properties of grade CT-80 and experimental results obtained with laboratory fatigue testing machine of JSC “Uraltrubmash” for real tubing with the similar mechanical properties shows good consistency especially at high pressures that confirms correctness of the theoretical approach: 340 (model) and 230 (experiment) cycles without internal pressure; 95 cycles (both, model and experiment) with internal pressure 25 MPa. The model calculation can predict the number of working cycles at the well-site to be in the range from 254 to 170 within the working pressure range of 10 – 25 MPa.

   Numbers of cycles performed in laboratory testing and during the operation at the well-site will obviously differ, as CT stressed state under conditions of alternating (fatigue test machine) and constant (well-site) sign deformation are distinguished. The obtained data allow determining a relation between these two values as following function , which is properly approximated as follows: . This function provides estimation and prediction of real CT fatigue life at the well-site on laboratory testing basis.

Prospects

   Automatization of the process of analytical estimation and designing the CT fatigue life calculator based on the proposed model will make it possible in the future to supply the software application for CT life prediction together with the tubing itself that can essentially increase the opportunity of trouble-free CT operations. Inclusion of tubing geometry variations and deformation due to the dead-weight into the model is the main objective of further development of the CT fatigue life estimation scheme. Together with real-time data acquisition systems the CT fatigue life calculator will submit the information about CT damage level and remaining fatigue life of any tubing section at any moment of time.

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