0045-7949(93)EOO40-U zyxwvutsrqponmlkjihgfedcbaZYXWVUT compum & Srrllclures Vol. 51. No. 5, pp. 571-578. 1994 Copyright Q 1994 Elsevier Science Ltd Printedin Great Britain. All rights reserved 004s7949/94 57.00 + 0.00 MODELING OF THE INTERACTION OF WATER WAVES WITH NONLINEAR POROUS SEABED R. SIDDHARTHAN and M. EL GAMAL Department of Civil Engineering, University of Nevada, Reno, NV 89557, U.S.A. (Received 15 June 1992) Abstract-Currently, the evaluation of the seafloor response to wave loading is accomplished by uncoupling the water-soil interaction and treating the issue as two independent problems. The assump- tions used in the solution techniques adopted within these independent problems are often inconsistent. This paper presents a coupled water-seabed interaction approach to study the response of a layered seafloor. The seafloor is treated as a nonlinear effective stress-dependent two-phase medium. The study shows that, for a given wave, the thickness and the stiffness properties of the soil deposit govern the extent of the water-oil interaction. During wave loading in deep water, the wavelength and sea bottom pressure can be either higher or lower than the corresponding values given by the much simpler uncoupled approach. For a seafloor comprised of sands, however, the coupled approach is appropriate to evaluate the seafloor response. It appears that the coupled approach becomes important only for seabeds with soft sediments. such as those found in the Gulf of Mexico. INTRODUCTION should be noted that the linear wave theory assumes Large ocean waves can produce substantially high the presence of a rigid and impermeable bottom stresses and movements in the underlying soil. Waves boundary (i.e., rigid seafloor). Then, this pressure generated by storms have resulted in severe damage loading is used as input to study the seafloor re- to a number of offshore facilities. Bea 111 gave agood sponse, treating the seafloor as a deformable medium. summary of the damage to offshore facilities located The study reported here investigates the effects of this in the Gulf of Mexico from storm activity. For inconsistency associated with the assumptions above example, the wave-induced seafloor instability that on the response prediction of the seafloor. It outlines occured during the passage of Camile in 1969 in the a coupled wave-seabed interaction model developed Mississippi River Delta resulted in the failure of three to undertake this study. The approach treats the of the eight platforms in the area. The influence of seafloor as a nonlinear, stress-dependent, two-phase seafloor behavior on offshore installations, such as medium. The influence of factors, such as shallow pipelines [2,3], gravity platforms [4, 51 and jacket- versus deep water conditions and unsaturated type structures [6,7], has interested researchers for seafloor conditions, on the seafloor response is also presented. many years. One of the major design considerations for an offshore facility is an evaluation of the seafloor EXISTING METHODS OF ANALYSIS response, ignoring the presence of the offshore facility When water waves propagates over a porous bed, (free-field response). One of the first analytical e.g., a sand bed, water flows into the porous medium methods to predict seafloor instability was developed which results in stresses and deformations in the bed. by Henkel[8]. The study reported here is restricted to The response of the seafloor is a result of the inter- a level sandy seafloor (porous medium), which means action between the fluid and the seafloor soil. In that a nonlinear effective stress based approach is various approaches proposed by past investigators required to realistically model the seafloor behavior. the seafloor has been treated as either a one-phase The papers by Bea et al. [7] and Siddharthan [9] (impermeable) or a two-phase medium. The re- provide a good summary of current methods to searcher must assume a two-phase medium because evaluate seafloor response. the sandy soil behavior is governed by effective The investigation of seafloor response often starts stresses. Recently, Siddharthan [9] proposed a model with the specification of the largest design wave, incorporating a nonlinear stress-dependent constitu- specified in terms of wave height and wave period. tive relationship for soil to study the wave-induced Almost all of the investigators uncouple the problem response of a sandy seabed. However, he used linear and treat it as two independent problems. First, the wave theory to determine the wave pressure at the linear wave theory is used to compute the pressure top of the seafloor. Furthermore, his original study loading at the surface of the seabed (sea bottom). It assumed fully saturated soil conditions (i.e., 571