Abstract

An analytical method to directly calculate fatigue lives for free spanning pipes has been derived from the works of Hobbs (1986) by adding fatigue calculations to screen the effect of Soil-Structure Interactions. Lives will depend on many parameters, especially sea current velocities at spans, the choice of fatigue SN-curves, pipe bending stiffness and in particular soil-structure interaction at top of freespan shoulders. Instead of using simplified pinned-pinned or clamped-clamped support conditions, formulations for Beams on Elastic Foundations (BEF) are shown to be useful as a more accurate screening tool for allowable free span lengths and fatigue from Vortex Induced Vibrations (VIV). A formula is developed to directly calculate the fatigue life in a pipe span under different boundary conditions. Both maximum and minimum span lengths are identified. Results are checked by FE modal analyses.

INTRODUCTION

Flexible pipelines and umbilicals are often used for in-field transportation of oil, gas, water and signals between template locations, platform or land terminals. Traditionally a detailed bathymetry description of the pipe route and the seabed bottom roughness is investigated to find freespans and line curvatures from the terrain geometry. Eigenfrequencies and mode shapes are then derived for each span type, with current velocities and variables that define span vibration characteristics and fatigue.

With the presented method, only estimates of soil conditions and environmental conditions at locations along a line are used to find allowable span lengths, or rather identify span lengths that should be avoided, prior to detailed knowledge of survey results.

Fatigue estimates are found by considering freespan responses from the works of Hobbs (1986), where soil characteristics at span shoulders are introduced. By analyzing a range of physical free spans and current velocities, look-up tables for parametric interpolations may be created.

During an actual lay, if pipe relocation is possible, a routine to give instant advice on span corrections based on observations of soil type and created freespans can be made, resembling advice given from machine learning procedures, if automated. A common type of freespans is considered in this study, as illustrated and idealized below.

This content is only available via PDF.
You can access this article if you purchase or spend a download.