ABSTRACT

An efficient computer program for predicting the dynamic response (movement and stress) of submarine pipelines to storm waves and current has been developed. The pipeline is assumed initially straight, is moving on a horizontal plane (bottom) and is exposed to loading from waves and current. The structural response is based on small displacement theory. The increase in the axial force in the pipe due to bending deformation is included.

New models based on recent experimental results for the hydrodynamic force and the soil resistance force are available in the response model. Among the hydrodynamic models included, the wake model has been shown to yield the best description of the horizontal and lift forces and their correlation. Either deterministic or random wave excitation can be used. In the random wave case, time series for fluid velocity and acceleration along the pipe are generated from directional wave spectra using Fast Fourier Transform and linear wave theory. The history dependent soil models for both sand and clay reflect increased penetration and multiple large amplitude cycling.

Simulations of the dynamic response are presented in computer plots. Typical results are given for both single element (pipeline section) systems and long pipes, simple and history dependent soil models for sand and clay, and regular and irregular sea states.

INTRODUCTION

An accurate prediction of response is crucial to the assessment of the safety of submarine pipelines. Accurate response prediction requires a well adapted structural formulation and a realistic description of both the external loading and the interaction between the response and the loading. The response of the pipeline is highly nonlinear because of the nonlinear pipe-soil interaction and the hydrodynamic loading. Large pipe deformations may also require nonlinear description of pipe geometry since the structural behaviour of a typical pipeline is that of a slender beam.

During the last 5 years several computer programs have been developed to predict the structural response of submarine pipelines subjected to wave and current loading. Small deflection beam theory with tension is employed in References [1], [2] and [3]. A nonlinear geometric description with finite beam elements is employed in [4], [5] and [6].

The pipe-soil interaction models in [2] and [3] are hysteretic and elasto-plastic, respectively, involving the lateral direction only. An elasto-plastic soil resistance model both in the lateral and axial direction with no coupling between the directions is used in [4] and [5]. In [6] the coupling of lateral-axial soil resistances is included based on an elliptical yielding surface of the elastoplastic soil model. The hydrodynamic forces are based on the simple Morison's force equation in [1] - [6].

In the present work, the main goal has been to develop an efficient special purpose computer program which could be used to perform parametric pipeline response studies for long storm durations with sufficient accuracy for small to moderate pipe displacements. Consequently, a small displacement formulation is used.

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