Abstract

Suction foundations (that is, foundations using piles, skirted compartments, or caisson units installed by or with the aid of suction pressure) are finding increased use in offshore foundations for deep water. Finite element analysis provides a powerful solution technique for solving complex mechanics problems from a fundamental approach without need for many simplifying assumptions. This paper explains the role of finite element analysis in the analysis and design of suction foundations for offshore production and exploration systems.

Suction foundation units have markedly different dimensions and relative geometries than traditional offshore foundation units. Accordingly many of the "ad hoc" assumptions employed in customary offshore foundation design and analysis methods may not be applicable or productive, and more sophisticated design analysis methods can be beneficial. The finite element method both enables freedom from undesired assumptions and provides guidance and justification for proper formulation of simplifying assumptions.

Among the aspects of suction foundation performance that have been investigated via the finite element approach include:

  • The significance of interaction between vertical and horizontal components of capacity,

  • Effects of separation of the soil from the trailing side of the pile or caisson,

  • Sensitivity of foundation performance to skin friction conditions,

  • Sensitivity of foundation performance to load point location.

Proper attention to details such as these via finite element analysis of suction foundation designs can help to avoid costly overdesign without incurring undue risk.

General

The finite element method is experiencing increased use in offshore foundation engineering. This increase is mostly in connection with suction foundations for deep water applications. The most common applications of the method have been to provide insight into foundation design parameter selection through elucidating performance phenomena (e.g., Sparrevik (2002)), to perform parametric studies (e.g., Zdravkovic, L et. al. (2001)) and to provide benchmarks for results of either other calculation methods (e.g., Randolph and House (2002)) or centrifuge testing (e.g., Clukey and Morrison (1993)). Through applications like these, finite element analysis can help to reduce uncertainty in the design process by providing highly accurate solutions to complex foundation problems.

The finite element method is a technique for solution of mathematical problems governed by systems of partial differential equations. It can produce close approximate solutions to problems with highly complex geometries, material behaviors and boundaries which would result in highly complex feildwise variations in the solution variables. The method accomplishes this by subdividing the solution space into many pieces (the finite elements) sufficiently small that the variations in the solution variables can be well approximated within each by very simple functions. Implementation of the method numerically on modern digital computers enables highly accurate solutions with extremely large numbers of small elements. All of the governing equations are then solved on all of the elements, and the elemental solutions are assembled into the solution for the whole, subject to compatibility and continuity requirements.

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