On-Bottom Stability of Umbilicals and Power Cables for Offshore Wind Applications

• Main Authors: Guomin Ji, Muk Chen Ong
• Format: Article
• Language: English
• Published: MDPI AG 2019-09-01
• Series: Energies
• Subjects: on-bottom stability; umbilical; power cable; finite element method; soil type; reduction factors; initial embedment; friction coefficient; undrained shear strength; submerged weight
• Online Access: https://www.mdpi.com/1996-1073/12/19/3635

With the increase in offshore wind farms, the demands for umbilicals and power cables have increased. The on-bottom stability of umbilicals and power cables under the combined wave and current loading is the most challenging design issue, due to their light weight and the complex fluid&#8722;cable&#8722;soil interaction. In the present study, the methodology for dynamic lateral stability analysis is first discussed; and the reliable hydrodynamic load model and cable&#8722;soil interaction model based on large experimental test data are described in detail. The requirement of the submerged weight of a cable <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>w</mi> <mi>s</mi> </msub> </mrow> </semantics> </math> </inline-formula> to obtain on-bottom stability is investigated for three types of soil (clay, sand and rock), using the finite element program PONDUS, and the results are <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>w</mi> <mrow> <mi>s</mi> <mo>,</mo> <mi>r</mi> <mi>o</mi> <mi>c</mi> <mi>k</mi> </mrow> </msub> <mo>&gt;</mo> <msub> <mi>w</mi> <mrow> <mi>s</mi> <mo>,</mo> <mi>c</mi> <mi>l</mi> <mi>a</mi> <mi>y</mi> </mrow> </msub> <mo>&gt;</mo> <msub> <mi>w</mi> <mrow> <mi>s</mi> <mo>,</mo> <mi>s</mi> <mi>a</mi> <mi>n</mi> <mi>d</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula> under the same load conditions. Several different aspects related to optimization design of the on-bottom stability are explored and addressed. There is a significant benefit for the on-bottom stability analysis to consider the reduction factors, due to penetration for clay and sand soil. The on-bottom stability is very sensitive to the relative initial embedment <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>z</mi> <mn>0</mn> </msub> <mo>/</mo> <mi>D</mi> </mrow> </semantics> </math> </inline-formula> for clay and sand soil, due to the small diameter of the cables, and therefore, reliable prediction of initial embedment is required. In the energy-based cable&#8722;soil interaction model, the friction coefficient <inline-formula> <math display="inline"> <semantics> <mi>&#956;</mi> </semantics> </math> </inline-formula> and the development of penetration affect each other and the total effect of friction force <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>F</mi> <mi>f</mi> </msub> </mrow> </semantics> </math> </inline-formula> and passive resistance <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>F</mi> <mi>r</mi> </msub> </mrow> </semantics> </math> </inline-formula> is complicated. The effect of the friction coefficient <inline-formula> <math display="inline"> <semantics> <mi>&#956;</mi> </semantics> </math> </inline-formula> on the on-bottom stability is different from engineering judgement based on the Coulomb friction model. The undrained shear strength of clay is an important parameter for the on-bottom stability of umbilicals and cables. The higher the undrained shear strength of the clay, the larger the lateral displacement. Meanwhile, the submerged weight of sand has a minor effect on the lateral displacement of cables. The method used in the present study significantly improves the reliability of the on-bottom stability analysis of umbilicals and power cables for offshore wind application.