On-Bottom Pipe line Stability In Steady Water Currents

Abstract

ABSTRACT A recommended practical is established for the on-bottom stability design of a pipeline subjected to steady water currents. The analysis leading to the recommended design procedure recognizes that, the water velocity decreases as the bottom is approached due to the boundary layer effect and that the magnitude of the hydrodynamic forces on the pipe will be a function of the shape of this velocity profile. The analysis also assumes that the lateral resistance to movement provided by the bottom soil is proportional to the net vertical resultant force acting on the bottom. Equilibrium, or stability, occurs when the lateral resistance is just equal to the forces exerted on the pipeline by the steady current. The resulting design equation (or the required pipeline weight contains several parameters whose numerical value depends on the pipe, seawater, and soil properties. A nominal see of these values is chosen and a parametric study is made of their effect on the required pipeline weight. Design curves are generated based on the chosen parameter values, and procedures for their user are recommended. The parametric study demonstrates that the minimum specific gravity required for equilibrium is relatively insensitive to large variations in the value of most of the parameters in the design equation. It also demonstrates that the selection of the design current velocity is the most significant factor in the stability analysis. Hence, further research on stability can most profitably be aimed at developing better methods of determining the design velocity for a particular location in the ocean. INTRODUCTION The design of an offshore pipeline system must give due consideration to each of the following factors. The operating requirements internal pressure, throughput, etc. have a large influence on the design. At the same time the design must provide for the safety and integrity of the line in the face of external and environmental factors over the expected lifetime of the system. Factors influencing the selection, design, and efficiency of the construction system must also be included in the overall consideration of the design. The specifications for the line pipe and the pipe coating system must not exceed those required to ensure adequate operating performance and long-term safety of the line. Unnecessary pipe weight and too stringent pipe handling criteria impose undue operating limits on present construction systems even in moderate water depths. In deeper water they may impose impractical requirements on the construction system. The selection of the pipe and specifications for the coating system are directly affected by the criteria established for the maximum environmental conditions expected over the life of the pipeline. Therefore, the environment of the operating pipeline is the basic starting point for the design. A complete discussion of the environment near the seawater soil interface is beyond the scope of this paper.