Damage To Unburied Flowlines In The Gulf Of Mexico During Hurricane Andrew
Authors: Collins J. Ian 
Published: October 2008

The US Minerals Management Service reported that 454 sub-sea pipelines were damaged during Hurricane Andrew. Previously, image to pipelines and flowlines has been reported in a series of papers by Blumberg (1964). These earlier papers present very little quantitative analysis of loads and stresses but lateral displacements of several thousands of feet were documented. In the present paper a formulation of the hydrodynamic loads acting on a flowline which is lying on the seafloor in the presence of waves and currents is summarized.

In general, the line dynamics can be represented by a fourth order differential equation with nonlinear forcing but a method is presented which assumes that the complete response can be broken down into distinct phases from "early motion" to "taut line" to 'yield and breaking", The selection of appropriate force coefficients and boundary layer interactions is discussed. The initial stages of pipeline migration across the seafloor is shown to be followed by loading of the flowline once all of the "slack" is taken up. The loads are shown to be a function of the current orientation and the maximum tension is approximately proportional to the square of the distance between risers.

Comparisons of predicted and observed damages confirm that "effective" boundary layer thicknesses at the seafloor and typical drag and lift coefficient ts which were selected based on available Literature are consistent with observed flowline damage. The results of sidescan sonar mosaics of pipeline migrations and reported damage are consistent with the loads predicted by relatively simple hydrodynamic/structural models.

The material presented in this paper permits the evaluation of the risk of damage to unburied flowlines using relatively simple tools. Guidelines to the assumptions of force coefficients and flowline responses are provided.

INTRODUCTION

During the passage of Hurricane Andrew through the Gulf of Mexico numerous offshore structures were damaged. Pipeline damage was observed and reported in many tracts. The US Minerals Management Service report (MMS, 1993) stated that 454 sub-sea pipelines were damaged during the storm. The report included preliminary estimates of the proximate causes of the damages but included no computations or detailed analysis of the various loads which were placed on the affected pipeline segments. The locations of damaged pipelines are illustrated on Figure 1. It is seen i n Figure 1 that heavy pipeline damages occurred i n the South Timbalier, South Pelto and Ship Shoal Areas.

OVERVIEW OF PIPELINE RESPONSES TO WAVES AND CURRENTS

An exposed pipeline on the seafloor is potentially subjected to loads from waves and currents. Numerous references in the ocean engineering literature have summarized the basic forces to be addressed when assessing the stability of subsea pipelines for both buried and unburied pipelines.

For unburied pipelines environmental loads such as those due to gravity, hydrostatics, hydrodynamic and soil forces have to be considered. Figure 2 (from API, RP 17A) presents a schematic of the loads. The following subsections present a summary of the procedures used for the computation of environmental loads during a storm.