BASICS OF FLEXIBLE PIPE STRUCTURAL DESIGN

The use of low-stiffness flexible pipes has increased dramatically in the past few years. The flexibility of these pipes is such that the usual buried pipe models must be used with caution. Several flexible pipe models are reviewed; external loads, pressure rerounding, longitudinal effects, instability (buckling), etc., are discussed, and the importance of some properties of pipe materials and soil is emphasized. The following principles, verified by experience, are proposed: (1) Soil design should supersede deflection in design calculations; (2) deflection should be limited in the long run to generally 5%, strains permitting; (3) the ratio w/S (total external pressure w on the pipe to long-term S(E'/D3) pipe stiffness, should not exceed 40 (the long-term stiffness S should exceed 1,500 N/m2, or PS should exceed 12 psi, for frequent loadings); (4) the ratio E'/S (the bulk soil reaction modulus E' to pipe stiffness S) should be less than a few thousand (i.e. a fairly rigid pipe in stiff soil). The pipe would thereby maintain an elliptic shape, and strains could be contained; (5) in the strain formula, the maximum permitted deformation, and, for low-stiffness pipe, strain factors D(f) between 3 and 8, should be used; (6) no rerounding as a result of internal pressure should be taken into account; and (7) all loadings, including pressure, should be superimposed, with the exception of those for pipe made of materials not sensitive to strain (i.e. steel and most plastics).