Design of Pipelines in Mudslide Areas

Abstract

ABSTRACT A design strategy is presented for routing and configuring pipelines in mudslide areas. This strategy is illustrated with a case study of a pipeline in the Mississippi River Delta. The case study focuses on the geotechnical aspects of pipeline settlement, flotation, and analysis of mudslide forces and stresses. INTRODUCTION It is impossible to prevent pipeline failures in an active mudslide area (5,12,18). Key strategies to design pipelines that have acceptable cost and reliability in these areas include:Minimum exposure to existing and potential locations of mudslides.Minimum lateral soil forces.Weighting to minimize penetration into the sea floor.Analysis to establish soil loadings, restraints, flexibility, and ultimate strength of the pipeline.Design of terminals to incorporate flexibility, reparability and control of escape of products. DESIGN STRATEGY The primary objective of the pipeline design process is to design a system that will reliably transport products during its "lifetime at the lowest total cost. The pipeline design process (Figure 1) must consider the constraints posed by environment, construction, operations and design. These constraints are discussed next. Constraints Environmental constraints include definition of the waves, currents, mudslides, fault movements, soils profile and bathymetry that can influence-the pipeline during its lifetime. Construction constraints include the equipment used for fabrication and installation; pipeline steels, welding and quality controls, and pipeline bedding, backfill and armoring. Operational constraints include desired tie-in points; volumes, pressures, temperature and corrosives of fluids to be transported; pipeline maintenance, pipeline repair, fluid escape control measures, and acceptable failure incidence. Design constraints include analysis methods to be used, routing guidelines, regulatory requirements and codes, allowable stresses and factors-of-safety. Design constraints also include economic and impact considerations. Economic considerations include costs of construction, operation, failure and repair. Impact considerations include potential effects of the pipeline on other operations and facilities, impacts of the pipeline on the environment, and social and political effects of failures. In view of the above constraints, the pipeline designer has to gather the data and information needed to define the constraints at the outset of the engineering process. The design process then focuses on a logical balancing of these constraints to result in an optimum pipeline design. Of particular importance is the use of hazard mitigation techniques in the pipeline design process. Backflow valves to prevent escape of fluids; pumping or compression shutdown systems; breakaway couplings to control the points of failure; use of pipe coatings to minimize forces; and incorporation of failure detection and repair systems are examples of such techniques. Perhaps the most important hazard mitigation technique is perceptive sitting and routing of the pipeline to avoid or minimize exposure to present and future hazards (12), which are discussed next.