Evaluation of Offshore Breakwater Stability Under Wave Action
Authors: Fischer Joseph A. , Lu Bill T.D. 
Published: October 2008

Attacking storm waves affect the overall design of any offshore breakwater used as a protective structure. This paper described the methods and considerations required to evaluate the stability of an offshore breakwater under design storm wave action. The following various storm wave effects are of concern:The possibility of wave forces physically causing damage to the breakwater embankment materials,The stability of, the breakwater against horizontal sliding,The stability of the breakwater embankment against slope failure, andThe effect of continuing storm waves on the strength and stability of the foundation soils.

The wave force effects upon the breakwater material can be studied in model tests, which are briefly described herein. A thorough field investigation must be performed to obtain the subsurface information. In addition, an extensive laboratory testing program should be conducted to determine the material properties and strengths of the foundation soils. The characteristics of wave and breakwater configuration must be considered in determining the design magnitudes of breaking wave force and hydrostatic pressure acting on the breakwater. In addition, the rapid drawdown condition of the water level as the wave retreats outside the breakwater may be critical and should be considered in the analyses. Both wedge and conventional circular failure types of stability analyses should be utilized in analyzing the slope stability of an offshore breakwater.

The procedures described in this paper are part of the necessary stability considerations in the design of a breakwater to protect a floating nuclear generating plant (FNP). Although developed for a floating nuclear power plant, the concepts presented in this paper are considered to be applicable to the stability evaluation of any major offshore breakwater-foundation soil system.

The subject of breakwater stability under storm wave actions has been, in most cases in the past, limited to some minimal engineering consideration. Due to the ever increasing potential of development for offshore floating nuclear power plants, drilling structures, deepwater parts, etc. simple analyses to evaluate stability against overturning and sliding are no longer considered to be sufficient for the design of a critical offshore gravity structure under storm wave action. The detailed evaluations and considerations described in this paper are applicable to the design of any important offshore breakwater or gravity structure. They also represent an advance in the state-off heart of stability analyses for offshore structures.

INTRODUCTION

A breakwater system is usually constructed as part of the protective structures to a floating nuclear generating plant or other important facilities. The breakwater must be designed to resist various types of environmental loadings. One of the design considerations is the stability of the breakwater-foundation soil system under storm wave action. This paper describes the various stability analyses which are essential to the satisfactory performance of an offshore breakwater.

Marine soil mechanics is not a new subject in the engineering literature. The basic principles are practically the same as those developed by Terzaghi many years ago (1). However, to resist the forces of wave and wind has-not been easy for man's works.