Effects of Different Design Parameters on the Vortex Induced Vibration of FRP Composite Risers Using Grey Relational Analysis

Risers are indispensable components of offshore platform systems that connect the wellhead at the sea bottom and to the platform at the sea surface and are normally made of high grade steel. Nowadays, fiber reinforced polymer (FRP) composite has been recognized as an attractive alternative riser material. Similarly to steel risers, FRP composite risers are also vulnerable to vortex induced vibration (VIV), and the effects of the composite makeup of these risers on VIV are the subject of the present investigation. Three risers (the tailored design composite riser, the composite riser with orthogonal reinforcements and the steel riser), three current velocities (0.36 m/s, 1.22 m/s and 2.13 m/s) and three water depths (12.5 m, 25 m and 37.5 m) are considered. In total, 9 study cases using orthogonal array (OA) sampling are investigated to study the risers&#8217; VIV characteristics. The computational fluid dynamics (CFD) simulations with coupled fluid&#8722;structure interaction (FSI) are used to obtain the risers&#8217; natural frequencies, global displacements, global stresses and the stress distributions in each composite lamina. The effect of 5 parameters (E_tension, E_bending, L/D_outer, tension force and current velocity) on the VIV amplitude in a cross flow direction of the risers is analyzed using Grey relational analysis (GRA) and the Grey relational grade of these parameters are: <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>r</mi> <mrow> <mn>05</mn> <mrow> <mo>(</mo> <mrow> <mi>Velocity</mi> </mrow> <mo>)</mo> </mrow> </mrow> </msub> <mo>&gt;</mo> <msub> <mi>r</mi> <mrow> <mn>03</mn> <mrow> <mo>(</mo> <mrow> <mi mathvariant="normal">L</mi> <mo>/</mo> <msub> <mi mathvariant="normal">D</mi> <mrow> <mi>outer</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </msub> <mo>&gt;</mo> <msub> <mi>r</mi> <mrow> <mn>04</mn> <mrow> <mo>(</mo> <mrow> <mrow> <mi>Tension</mi> <mtext>&nbsp;</mtext> <mi>Force</mi> </mrow> </mrow> <mo>)</mo> </mrow> </mrow> </msub> <mo>&gt;</mo> <msub> <mi>r</mi> <mrow> <mn>01</mn> <mrow> <mo>(</mo> <mrow> <msub> <mi mathvariant="normal">E</mi> <mrow> <mi>t</mi> <mi>e</mi> <mi>n</mi> <mi>s</mi> <mi>i</mi> <mi>o</mi> <mi>n</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </msub> <mo>&gt;</mo> <msub> <mi>r</mi> <mrow> <mn>02</mn> <mrow> <mo>(</mo> <mrow> <msub> <mi mathvariant="normal">E</mi> <mrow> <mi>b</mi> <mi>e</mi> <mi>n</mi> <mi>d</mi> <mi>i</mi> <mi>n</mi> <mi>g</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </msub> </mrow> </semantics> </math> </inline-formula>.