Stress Analysis During of Offshore Pipelines Installation
Authors: Powers J.T. , Finn L.D. 
Published: October 2008

A finite-beam-element, initial-value analysis procedure determines stresses in a subsea pipeline suspended between the ocean floor and a laybarge or stinger. The basic theory, its advantages over other theories, and a comparison of results with the results of analytical procedures based on other theories are included. The finite-element theory is applicable over a wide range of marine pipelaying problems, and compares favorably with other accepted theories in the-ranges of their applicability.

INTRODUCTION

Thick-wall pipe and concrete coating are used to weight marine pipelines to insure that the pipeline will remain in place after installation. During instillation in deep water, this weight imposes high stresses in the pipe sus-pended from the laybarge-and may cause the line to fail. Deepwater repair may not be possible; as a result a failure may require the entire line to be replaced. To select the proper pipe weight and grade and to prevent costly failures, the stresses imposed in the pipe during laying must be defined and construction procedures and equipment must be accurately analyzed. This paper presents a procedure for making these analyses. The calculation technique may be used to analyze any construction method which installs the line from the surface in a continuous string. This paper describes the analytical procedure specifically in terms of the stinger-Iaybarge construction method. The basic theory used in the analysis, its advantages over other theories, and a comparison of results with results using other theories are presented.

THE MARINE PIPELINE SUSPENSION PROBLEM

The problem of predicting the suspended geometry of marine pipelines in deep water is one of nonlinear large-angle bending. This problem may be solved numerically through the use of a finite-beam element, initial-value approach that treats the total beam as a series of small beams, each of which is analyzed with linear theory. Combining all the small-angle bending solutions produces the large-angle bending marine pipeline solution. In the initial-value approach, unknown boundary conditions at the ocean floor are assumed and then systematically changed until the profile which has been built up, segment by segment, satisfies boundary conditions specified at the upper end. The appendix to this paper contains the basic equations and their derivation.

The theory accounts for applied tensions, external fluid pressures, variations in water currents With depth, pipe stiffness variations due to weakness of the weight coating at the field joints, and support buoys [if used]. The approach is quite general and is applicable to both two-and three-dimensional forces and deflections; crosscurrents and lateral barge movement can be considered.

Although the developed procedure can be used to analyze the stresses in the suspended portion of a pipeline under any support condition, it is discussed here in terms of using the stinger-laybarge construction method [Fig. 1]. The stresses calculated account for static loads only, considering ocean currents as static loads. The effects of dynamic loads and external hydrostatic pressure on the actual longitduinal stress must be considered in the selection of allowable stress levels used in conjunction with this procedure.

USE OF FINITE-ELEMENT CALCULATION PROCEDURE

The laborious and repetitive nature of the finite-element procedure makes it necessary to use a computer.