Analysis of torsional instability and reliability of marine flexible pipelines
Introduction
As a conveying channel, a marine flexible pipeline is an important part of connecting a floating production platform and an underwater production system. It has less bending stiffness and high axial stiffness, meeting the requirement of axial tensile strength and resistance to bending deformation with large curvature. Resisting the external load by bending deformation, a flexible pipeline can relieve the stress level at the top and bottom and is suitable for the harsh marine environment of the deep sea.
During the installation of marine flexible pipelines, the touch down zone (TDZ) is subjected to low tension, and wave motion and ship yaw have a high torque effect on them, which results in the torsion instability phenomenon of flexible pipelines in the TDZ, forming a loop. When the tension on the marine flexible pipeline increases, the radius of the loop decreases and kinks are formed, resulting in damage or fracture of the flexible pipeline, which affects its normal operation (see Fig. 1).
Some scholars have studied the instability of flexible pipelines from the perspectives of theoretical analysis, finite-element analysis, and experimental verification. Zajac (1962) and Rosenthal (1975) attempted to solve the torsion problem using the elastic theory, treating the cable as a planar elastic ring, and analyzed the critical load and end displacement of the ring cable. Liu (1975) conducted theoretical analysis on the multi-conductor cable model, compared the theoretical analysis results with the experimental results, and derived the Greenhill formula, which can predict the critical torque caused by the loop under static conditions. Ross (1977) proved that a stable loop can be formed in the cable, when the tension is equal to half of the tensile load predicted by Liu (1975), that the loop diameter of the instability analysis is uncertain, and that the “balanced” loop formed in the cable is determined by the mechanical properties, torsion, and tension of the cable. Knapp (1979) studied spiral armored cables, ignoring the reduction of spiral diameter due to contact between spirals, assuming that the material of the spiral and core unit is distributed uniformly, isotropically, and with compression. Thus, considering the tensile and torsional loads, the spiral-armored cable stiffness matrix is deduced. Yabuta (1984) analyzed the formation of the loop of spiral cables and deduced the Greenhill formula. It was found that when the radius of the loop was reduced in proportion to the diameter of the cable, the kink was formed; this result was verified by the data obtained from an experiment with sheathing optical fiber.
From the perspective of theoretical analysis, Yabuta (1982) studied a cable forming a loop under torque, and, upon forming a kink, it was subjected to the effect of tension. Yabuta also carried out experiments with sheathing optical fiber to study the reopening of the cable. Neto and Mattos (2013) took an umbilical as a research object, considering the environmental loads such as self-weight, hydrostatic pressure, platform motion, wave, and current, ignored the self-contact phenomenon, and studied the formation of loops under the action of different torque loading rates. Neto and Martins (2013) studied the instability of cable structures under different degrees of friction, considering the bending load and torsional coupling, as well as the nonlinear unilateral constraint between the cable and seabed. Opgard (2017) built a model of an umbilical with two different parameters, and studied the torsion deformation of the umbilical and the critical torque under different installation environmental conditions by taking static load, dynamic load, platform movement, and soil properties into consideration. Yan (2018) analyzed the strength failure of umbilical cable under dangerous working conditions considering the randomness of geometry and material properties. The reliability analysis results show that the randomness of the geometry and material properties parameters has an important influence on the safety of umbilical cable.
In this paper, the loop and kink formation are analyzed by simulation with OrcaFlex software, which considers the wave and current movement, and the torsional moment, bending moment, tension, and rotation angle when the loop is formed are discussed. Moreover, the bending failure response surface function of the flexible pipeline is constructed considering the randomness of design parameters, e.g., elastic modulus, diameter, and density, and the reliability of flexible pipeline is innovatively analyzed using both the first-order second moment and Monte Carlo simulation methods.
Section snippets
Numerical model of the flexible pipeline
The coordinate system and geometric position of the flexible pipeline are shown in Fig. 2. A flexible steel pipeline with an external diameter of 16 inches is used for overall torsion analysis, and its specific structural and material parameters are given in Table 1 (Neto, 2014).
As the floating production storage and offloading (FPSO) platform moves periodically with waves, the torque of pipelines cannot be fixed, so it is difficult to study the effect of torque on the loop forming. Therefore,
Dynamic analysis of the flexible pipeline
The installation process of the flexible pipeline is dynamically simulated by OrcaFlex, and torque is applied smoothly and linearly at the end of the pipeline. As the torque is continuously applied, the flexible pipeline undergoes bending deformation and begins to slip on the seabed. At time t = 105 s, when the bending deformation reaches its maximum value, the flexible pipeline will form a loop at the TDZ, as shown in Fig. 5.
The torque is 8756 when the loop forms at t = 105 s. For the
Analysis of bending radius of flexible pipeline torsion loop under different tensions
Fig. 11 shows the variation of loop radius along the length of flexible pipeline under the action of end torque and different degrees of tension. The figure also shows that the minimum bending radius appears in the TDZ area, and it decreases with increasing tension. Under different degrees of tension, the minimum bending radius appears at different positions in the length of the flexible pipeline. As the tension increases, the position gradually moves to the right, because the flexible pipeline
Conclusions
During installation, flexible pipelines are subjected to torque and tension, which could cause loops at the TDZ. In this paper, the mechanical parameters of flexible pipeline are analyzed when the loop is formed, and based on response surface construction theory, a flexible pipeline failure criterion is constructed. In addition, the failure reliability is calculated. The conclusions drawn from the results are the following:
- (1)
The loop in the pipeline forms when it is subjected to torsion, the
CRediT authorship contribution statement
Yu Zhang: Conceptualization, Methodology, Software, Formal analysis. Zhansheng Guo: Data curation, Writing – original draft, Formal analysis. Haikun Zhao: Software, Investigation. Guoyin Ma: Writing – review & editing. Feifan Zhang: Writing – review & editing.
Declaration of competing interest
The authors declare that they do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
This research is supported by the National Natural Science Foundation of China (Grant No. 51779266) and China University of Petroleum (Beijing) National Key Research and Development Plan Found Program (Grant No. ZX20200136).
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