Abstract
In deepwater test condition, the riser–test pipe (tubing string) system (RTS) is subject to the vortex-induced effect on riser, flow-induced effect on test pipe and longitudinal/transverse coupling effect, which is prone to buckling deformation, fatigue fracture and friction perforation. To resolve this, the three-dimensional (3D) nonlinear vibration model of deepwater RTS is established using the micro-finite method, energy method and Hamilton variational principle. Based on the elastic–plastic contact collision theory, the nonlinear contact load calculation method between riser and test pipe is proposed. Compared with experimental measurement results, calculation results using the proposed vibration model in this study and the single tubing vibration model in our recent work, the correctness and effectiveness of the proposed vibration model of the deepwater RTS are verified. Meanwhile, the cumulative damage theory is used to establish the fatigue life prediction method of test pipe. Based on that, the influences of outflow velocity, internal flow velocity, significant wave height, as well as top tension coefficient on the fatigue life of test pipe are systematically analyzed. The results demonstrate that, first, with the increase in outflow velocity, the maximum alternating stress and the annual fatigue damage rate increased. The location where fatigue failure of the test pipe is easy to occur at the upper “one third” and the bottom of test pipe are easy to occur fatigue failure. Second, with the increase in internal flow velocity, the “one third damage effect” of the test pipe will decrease, and the “bottom damage effect” of the test pipe increased that needs the attention of field operators. Third, during field operation, it is necessary to properly configure the top tension coefficient so that there can be a certain relaxation between the riser and the test pipe, so as to cause transverse vibration and consume some axial energy and load. The study led to a theoretical method for safety evaluation and a practical approach for effectively improving the fatigue life of deepwater test pipe.
Highlights
With the increasing demand for oil&gas resources in the world, the exploitation trend of offshore oil&gas resources gradually develops from shallow water (Water depth is less than 500m) to deep water (Water depth is between 500m and 1500m)
Compared with conventional water depth testing conditions, the riser-test pipe system (RTS) is subjected to greater risks in deep-water test conditions; these risks are mainly caused by severe non-periodic vibrations of the riser and test pipe (RTS) induced by the vortex induced effect on riser, flow induced effect on test pipe, nonlinear contact/collision of the tube in tube and longitudinal/transverse coupling effect, thereby making the RTS more susceptible to buckling deformation (Fig. 1(a)), fatigue fracture(Fig. 1(b)) and friction perforation (Fig. 1(c)) [1]
The cumulative damage theory is used to establish the fatigue life prediction method of test pipe combined with the stress response which was determined by the proposed model and the S-N curve of the pipe material (13Cr-L80) which was measured by fatigue test
Summary
With the increasing demand for oil&gas resources in the world, the exploitation trend of offshore oil&gas resources gradually develops from shallow water (Water depth is less than 500m) to deep water (Water depth is between 500m and 1500m). The deep-water test pipe is the name of the tubular structure under the test condition, and the tubing string is the name of the tubular structure under the production condition Their vibration characteristics have the same trend, which are nonlinear vibration induced by inside flow. In view of the fatigue failure of the risers caused by VIV, the researchers have established a fatigue life prediction method for risers in deep-water [44, 45] In their method, the acquisition of alternating stress ignores the contact and collision factors between riser and test pipe, which makes it impossible to accurately simulate the fatigue life of riser under severe working conditions. The influences of outflow velocity, internal flow velocity, significant wave height, as well as top tension coefficient on the fatigue life of test pipe are systematically analyzed
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