Numerical Investigation on Wave-Induced Boundary Layer Flow over a Near-Wall Pipeline

Pipelines and power cables are critical infrastructures in coastal areas for transporting energy resources from offshore renewable installations to onshore grids. It is important to investigate the hydrodynamic forces on pipelines and cables and their surrounding flow fields, which are highly related to their on-bottom stability. The time-varying hydrodynamic forces coefficients and unsteady surrounding flows of a near-seabed pipeline subjected to a wave-induced oscillatory boundary layer flow are studied through numerical simulations. The Keulegan–Carpenter numbers of the oscillatory flow are up to 400, which are defined based on the maximum undisturbed near-bed orbital velocity, the pipeline diameter and the period of the oscillatory flow. The investigated Reynolds number is set to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>4</mn></mrow></msup></mrow></semantics></math></inline-formula>, defined based on <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>U</mi></mrow><mrow><mi>w</mi></mrow></msub></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>D</mi></mrow></semantics></math></inline-formula>. The influences of different seabed roughness ratios <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>k</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>/</mo><mi>D</mi></mrow></semantics></math></inline-formula> (where <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>k</mi></mrow><mrow><mi>s</mi></mrow></msub></mrow></semantics></math></inline-formula> is the Nikuradse equivalent sand roughness) up to 0.1 on the hydrodynamic forces and the flow fields are considered. Both a wall-mounted pipeline with no gap ratio to the bottom wall and a pipeline with different gap ratios to the wall are investigated. The correlations between the hydrodynamic forces and the surrounding flow patterns at different time steps during one wave cylinder are analyzed by using the force partitioning method and are discussed in detail. It is found that there are influences of the increasing <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>k</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>/</mo><mi>D</mi></mrow></semantics></math></inline-formula> on the force coefficients at large <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">K</mi><mi mathvariant="normal">C</mi><mo>,</mo></mrow></semantics></math></inline-formula> while for the small <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">K</mi><mi mathvariant="normal">C</mi></mrow></semantics></math></inline-formula>, the inertial effect from the oscillatory flow dominates the force coefficients with small influences from different <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>k</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>/</mo><mi>D</mi></mrow></semantics></math></inline-formula>. The FPM analysis shows that the elongated shear layers from the top of the cylinder contribute to the peak values of the drag force coefficients.