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

Guomin Ji,Muk Chen Ong

doi:10.3390/en12193635

Abstract

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–cable–soil interaction. In the present study, the methodology for dynamic lateral stability analysis is first discussed; and the reliable hydrodynamic load model and cable–soil interaction model based on large experimental test data are described in detail. The requirement of the submerged weight of a cable w s 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 w s , r o c k > w s , c l a y > w s , s a n d 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 z 0 / D 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–soil interaction model, the friction coefficient μ and the development of penetration affect each other and the total effect of friction force F f and passive resistance F r is complicated. The effect of the friction coefficient μ 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.

Highlights

Offshore wind farms can be essential in renewable energy policy and are an important element in the battle against climate change
In a new analysis released by the International Renewable Energy Agency (IRENA)—Innovation Outlook: Offshore Wind—offshore wind power has the potential to grow from 13 GW in 2015, to 100 GW in 2030 [1]
Several pipe–soil interaction models have been developed in past decades, and the energy-based pipe–soil interaction model developed by SINTEF Ocean is still regarded as the state-of-art for soil modeling in dynamic on-bottom stability analyses [16]

Summary

Introduction

Offshore wind farms can be essential in renewable energy policy and are an important element in the battle against climate change. In the oil and gas industry, a major design issue for any pipelines or flexibles is how to ensure that the product remains on the seabed where it was installed, as opposed to moving excessively laterally. This design facet is called on-bottom stability, which involves determining a submerged weight capable of withstanding hydrodynamic loads through friction and passive soil resistance, and it has long been an important study for offshore practice [3,4,5]. The severe consequences are damage of the pipelines and considerable loss of production [6]

Methods

Results

Conclusion