Using Qualitative Slope Hazard Maps and Quantitative Probabilistic Slope Stability Models to Constrain Least-Cost Pipeline Route Optimization
Authors: Bruce Brittany , Haneberg William C , Drazba Marina C. 
Published: May 2013

Abstract
Qualitative maps of past seafloor slope failures can be combined with physics-based probabilistic simulations of potential future failures to help constrain deep-water pipeline route selection using cost surface optimization workflows. The qualitative maps are produced using standard techniques to visually identify and, where possible, estimate relative ages of past slope failures based upon geomorphological criteria. The quantitative probabilistic maps are produced using a non-iterative first-order, second-moment infinite slope approximation well suited to large bathymetric data sets comprising tens of millions of bins or rasters. The qualitative and quantitative maps are then used, optionally with additional geomorphologic or geohazard maps, to develop geohazard cost surfaces from which a least cost pipeline route can be calculated using GIS software. The final step is to manually perform slight adjustments using an in-house interactive application to ensure that no portion of the proposed route fall below the minimum acceptable radius of curvature. Application of the method is illustrated using a hypothetical pipeline route across a 3100 km2 deepwater demonstration area containing a variety of submarine slope hazards.

Introduction
Selection of a deepwater pipeline route is a process that ideally incorporates information about the locations of the pipeline termini, the material characteristics of the pipe and the fluid being transported, soil-pipe interaction, spanning potential, cultural features such as shipwrecks and unexploded ordnance, and potential and actual geohazards along the route. Tootill et al (2004) considered primary factors like pipeline terminus locations, secondary factors that included geological and biological constrainst, and cost. They further wrote, " Therefore, the challenge for any pipeline route selection team is to find the shortest route while conforming to the requirements set out by the primary and secondary factors. " The difficulty of working in deep and ultradeep water makes remediation of unfavorable situations largely impossible. Therefore, the most effective mitigation strategy is early identification and avoidance of significant problems during route selection.
In this paper, we show how two fundamentally different kinds of geohazard information—qualitative maps showing the locations and relative ages of past slope failures and quantitative maps produced by probabilistic slope stability models that predict the possible locations of future failures—can be combined to create a composite geohazard cost surface that is in turn used to calculate an optimal pipeline route that minimizes the total geohazard cost along the route. Other kinds of geohazards can be included in the optimization procedure if appropriate. For example, we included young and old erosional features, fluid expulsion features, young seafloor faults, slope angle, and pipeline spanning potential in addition to the kinds of slope stability information we describe here when recently performing a deepwater pipeline route optimization study. Likewise, cultural and biological data can also be included in the analysis if they are available and appropriate.