Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

On the drag coefficient of artificial mattress under waves
Zhanjie Chen1, Liang Cheng2, Yunfei Teng 1, Bin Wang3,4,Jiacheng Wang1, Guoqiang Tang 1*
1, State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, Liaoning, China
2 School of Marine Science and Engineering, South China University of Technology, Guangzhou, Guangdong,China
3 Key Laboratory of Far-shore Wind Power Technology of Zhejiang Province, Hangzhou, Zhejiang, China
4 Powerchina Huadong Engineering Corporation Limited, Hangzhou, Zhejiang, China

ABSTRACTgenerate a certain blocking effect on the fluid when the fluid flows
through them, performing the hydrodynamic drag on the obstacle

Artificial mattresses made of interlinking concrete blocks (by ropes)

imposed by the fluid. When the hydrodynamic drag force on theare often used for local scour protection of offshore foundations.

obstacle becomes larger than its friction force, generated between theStability of the mattress under storm conditions is a requirement of theobstacle and the seabed, the protective obstacle will lose its stability.

design. There are no design guidelines on this so far. The present study

Hence the accurate evaluation of hydrodynamic drag could provide anaims at developing empirical formulae to estimate hydrodynamic dragessential scientific basis for the on-bottom stability design and security

on the mattress through physical experiments. The geometry of theassessment of scour protection measures.

bottom is prisms with a triangular cross-section. Six bed morphologieswere formed by changing the arrangement spacing ∆L of the prism. InAt present, the hydrodynamic drag of bed surface can be obtained bythis experiment, total force on the bottom was measured by a force

direct measurement and indirect calculation. Indirect methods are moresensor. The experimental results indicate that the total drag force on

widely used. The shear stress is generally inferred from velocity

the scattered arranging blocks can be estimated by the mean bed shear

measurements via three methods: the log-profile fitting method, thestress, considering that contribution of the form drag on the total dragmomentum integral method and the Reynolds stress method. Itsforce is ignorable.

macroscopic law is usually: the log-profile fitting method > momentum
integral method > the Reynolds stress method. The accuracy of theKEY WORDS: Hydrodynamic drag, shear stress, friction factor,

three methods largely depends on the measurement precision of theroughness

velocity profile, but the error of the experimental measurement isdifficult to control over, hence, the reliability of the shear stress

INTRODUCTION

obtained by the indirect measurement method needs to be furtherverified. In order to overcome the shortcomings mentioned above, a

There are many basic energy sources, such as oil and natural gas undermethod was developed to measure the total drag force on the seabed

the sea, which are usually transported by pipeline. The marine

roughness elements directly instead of measuring the velocity profiles,

environment of the pipeline is very complex, which is directly affected

and then the measured drag force is transformed into the shear stress.

by the wave and tide, leading to serious local scour and in-situ

Based on this method, the present study aims at developing empiricalinstability failure around the submarine pipeline. Artificial mattresses

formulae to estimate hydrodynamic drag on the mattress throughmade of interlinking concrete blocks (by ropes) are often used for local

physical experiments.

scour protection of offshore foundations. The protective obstacle will

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