Design For Deepwater Pipelines On Steep Slopes Impacted By Mass Gravity And Turbidity Current Flows
Authors: Kumar Amitabh , McShane Brian M. , McDonald William Leith 
Published: July 2010

Deepwater pipelines and flowlines that are routed across areas of steep slopes have the potential for seabed instability. They are at risk of being impacted by geohazards in the form of mass gravity flows (e.g. mud flows, debris flows etc.) and turbidity current flows.

This paper discusses the impact of accidental environmental/geohazard loads on oil and gas pipelines routed across an area of seabed with a steep slope. The vertical gradient along the route ranges between 5° and 15° with a maximum local gradient in excess of 36°. The main design issues that needed to be resolved were the pipelines' global stability while resting on the steep seabed slope and their capacity to resist environmental loads (geohazards). Due to the nature of the problem that encompasses several unknown factors, conventional pipeline/flowline engineering was not sufficient to adequately define the response of the overall system. To address this issue, a series of fit-for-purpose full 3-Dimensional finite element models were completed to assess the global stability of the system and subsequently assess the impact of geohazard loads.

With the help of extensive 3D finite element modeling, the impact of geohazard loads on the pipelines was assessed and the feasibility of the system was qualified through application of Accidental Limit State (ALS) design.

Introduction

Routing a pipeline or flowline across a steep slope in deepwater is not considered an uncommon design practice. There are several existing pipeline systems routed across aggressive seabed features. Some of these features include steep seabed gradients, areas of seafloor faults, canyons etc. However, there are some locations where an additional design challenge is introduced due to the potential for seabed slope instability. If locations of seabed slope instability are close to the pipeline, it adds further complexity due to the risk of the geohazard potentially inducing large displacements of the pipeline system. Additionally, uncertainties in the historical and geotechnical/geophysical data also contribute to the risk associated with routing a pipeline across a steep slope.

This paper addresses design issues related to global axial stability of oil and gas pipelines that is routed across an area of steep seabed slope. In addition, the paper discusses the structural integrity of these pipelines when subjected to geohazard events such as debris flows, turbidity currents and subsequent condition of the pipelines following these events. A comparison between the results from FEA simulations and the field data obtained during as-laid and post-hydrotest survey has also been discussed.

Pipeline Routes

The oil and gas pipelines (D/t ~20, Steel Grade = X65) considered for this case study are located in the northern Gulf of Mexico and traverse the Outer Continental Shelf, an area of steep seabed slope and Upper Continental Rise (see Figure 1). As the routes ascend the slope the seabed shoals steeply with gradients generally ranging between 5° and 15°, but in some areas it is in excess of 20° to 36°. Figure 2 shows the histogram of the seabed vertical gradients along the sloped section of the pipeline route. The average gradient within the slope section lies between 6° and 8°.

The seabed slopes close to the pipeline routes were investigated to identify potential areas of instability that could impact the feasibility of the pipeline system. These areas and soil provinces were then used as input parameters for debris flow/turbidity current simulation modeling. The results of the debris flow/turbidity current modeling were subsequently used to assess their impact on the pipeline stability and integrity.