Garden Banks 388 Deepwater Pipeline Span Avoidance
Authors: Davis S.W. , Sawyer M.A. , Kenney T.D. 
Published: October 2008

This paper will describe the span avoidance measures taken for the installation of the Garden Banks 388 deepwater oil and gas gathering pipeiines. The two 12' pipelines connect a shallow water facility in Ei-315 to a deep water subsea template in GB-388. These pipelines run across the irregular continental slope typicality found in moderate to deep water in the Guif of Mexico. To minimize pipeiine spans, steps were taken during design, survey, and installation phases of the project. During each phase, as additional information became available, analyses and resulting recommended approaches were refined. This continuity, seldom easily obtained, proved beneficial in transiting design work into field results.

INTRODUCTION

In the past, pipelines in the Gulf of Mexico were installed on the relatively flat continental shelf. Post pipelay surveys found few pipeline spans. Spans that were found were rectified with divers using sand bags or jetting out the high spots under the pipeline.

Development of deepwater oil and gas reserves is increasing in the Gulf of Mexico. In order to produce the resulting hydrocarbons, pipelines will increasingly cross the irregular seabed of the continental slope. Rectification of spans in deep water can be expensive. Span avoidance will be increasingly critical in minimizing costs of the pipelines and making deepwater projects feasible.

This paper will discuss the analysis techniques, survey methods, and installation steps that may be employed to reduce the number of spans that will occur when laying pipelines over an irregular seafloor. Various approaches used for the Garden Banks 388 project will be addressed. These will include span analysis during survey to allow adjustment and re-routing in the field and targeted application of extra weight to increase pipeline conformance to the seafloor. Experience gained implementing these approaches will be presented and possible improvements suggested.

Careful planning and use of the suggested prevention techniques, will reduce the number of spans along a pipeline route. Careful engineering, planning, and span prevention is very inexpensive compared to the price of correcting a deepwater span after pipeline installation.

PIPELINE ALLOWABLE SPAN DETERMINATION

The first step of a spanning analysis is to determine maximum allowable scan length. Factors affecting the allowable span length 'include 'pipe wall thickness, grade, product to be carried, residual tension, weight coating, and currents. Allowable span lengths are governed by code Imitations regarding maximum allowable stresses and by vortex shedding criteria. The allowable pipe unsupported span lengths can be determined from ANS1/ASME code allowable combined stresses and limiting the lengths of the unsupported spans to less than those which might cause resonant in-line or cross-flow vibrations to occur. In-1ine oscillations often induce stresses so small that fatigue damage will not occur. Thus, in-line oscillations if considered in a fatigue analysis, often times can be allowed to occur.

In summary, analysis is required to determine the maximum allowable spans for the following criteria:equivalent static bending stresses due to vortex-inducedin-line oscillations for fatigue consideration,prevention of vortex induced cross-flow oscillations,longitudinal and shear stresses.