Semi-Analytical Investigation into the Balanced Performance of Thick-Walled Fiber-Reinforced Flexible Pipes

The balanced performance of fiber-reinforced flexible (FRF) pipes is essential for maintaining dimensional stability and structural integrity in pipelines. However, current theoretical approaches face challenges in simultaneously incorporating end effects, geometric nonlinearity, and material nonlinearity, resulting in a persistent reliance on engineering experience when determining balanced fiber winding angles. This work proposes a semi-analytical method for evaluating the balanced performance of thick-walled FRF pipes, based on the strain energy density function, with governing equations established by integrating finite deformation theory and the principle of minimum potential energy. A displacement trial function is adopted to approximate the actual displacement field, with its coefficients determined iteratively using the Newton-Raphson method. An eight-coefficient displacement trial function demonstrates effectiveness in characterizing the pipe's deformation characteristics under the maximum working internal pressure, capturing key deformation features such as radial inward expansion with outward restraint gradient, nonlinear axial deformation, and axial end warping. The proposed method is validated against both experimental results and finite element simulations, and an analysis of the fiber winding angle's influence on balanced performance is conducted, thereby establishing a theoretical basis for the design of self-balanced thick-walled FRF pipes.