Lifting of a large object from a porous seabed

Abstract

The time required to pull a large object from a sandy seabed is estimated by assuming that the seabed is porous but rigid. The phenomenon of breakout (i.e., sudden release) is shown to occur without the assumption of elasticity of the soil skeleton (Foda 1982). A new case of wedge-shaped gap is also studied, and compared to a uniform gap, Laboratory experiments are shown to support the theory.

Numerical modeling of breakout process of objects lying on the seabed surface

“…Mei et al [3] proposed an analytical theory to simulate the with-gap stage of the long plate problem, in which the seabed was assumed to be porous but rigid. The model has been extended to the axisymmetric disk problem by Zhou [9].…”

“…movement of the disk increases sharply when breakout is approached, which may lead to different definitions of breakout but this would only lead to minor differences in view of the long time elapsed [3]. In addition, because the disk is not in contact with the subsoil in the with-gap stage, F 0 = F À G 0 can be employed as an independent parameter to replace F and G 0 in Eq.…”

“…Comparison of results from numerical model and those from extension theory of Mei et al [3] Since in the with-gap stage, c w Rt l is not sensitive to E c w R and m, here we arbitrary let E c w R ¼ 10 5 and m = 0.3 in the numerical model. H/R is assumed to be 10 to ensure the permeable soil layer is deep enough such that the bottom boundary does not affect the breakout process.…”

“…Usually the force needed to extract an object from the sea bottom would be greater than the submerged weight of the object itself and the time needed for the extraction process is related to this force. From field experience, it is known that this extraction is a very slow process until a critical time when the object is broken loose suddenly [1][2][3].…”

“…Despite the many approximations adopted to simplify the problem, Foda's theory is still rather complex. Mei et al [3] proposed an alternative theory to simulate the second stage of the breakout process, in which the seabed 0266-352X/$ -see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compgeo.2007.…”