Part 8 - Wall Thickness Design

Wall Thickness Analysis

The selected wall thickness shall be satisfying each of the following requirements:
· Internal Pressure Containment;
· Hydrostatic Collapse;
· Local Buckling
· Buckle Initiation
· Propagation Buckling.

a) Internal Pressure Containment
For pipelines and risers the tensile hoop stress due to the difference between internal and external pressures.

b) Hydrostatic Collapse

During pipeline installation, the pipe is required to sustain the net external pressure without yielding or collapse. This is of particularly of significance in deep water where the external hydrostatic pressure alone may cause the pipe to collapse in plastic mode. Collapse pressure is usually defined as the pressure required causing a local collapse due to external water pressure, pipe imperfections, bending and tension.

c) Local Buckling

The local buckling calculation method is outlined in DnV 1981. Conservatively, the minimum pipeline radius of curvature used in the local buckling analysis is calculated.

d) Buckle Initiation

For a pipe in water depth where the net external pressure is less than that of propagation, any damage to the pipe will remain local. Even if the external pressure is higher than the propagation pressure, damage to the pipe still requires a pressure high enough to trigger unstable collapse for subsequent propagation of the damaged section. Evidently, such a dynamic collapse will stop once the propagation front encounters a physical obstruction, such as an adequate increase in wall thickness (buckle arrestor), or lowering of pressure to below the characteristics propagation value (shallow water).

Unlike buckle propagation which is, in essence, a unique property of a particular pipe section, buckle initiation is not only a function of pipe geometry and material properties but is also dependent upon initial imperfection levels and impact or deformation energy. Initial imperfections may be in the form of geometrical distortions, material defects and/or load induced deformation.

For this reason, prediction of buckle initiation relies heavily upon information on the level and form of any initial imperfection. Impact or deformation energy can result in buckles during installation or significant dropped object.

e) Propagation Buckling

Buckle propagation is related to a situation where a transverse dent, caused by excessive bending, or any other causes, changes its configuration into a longitudinal buckle and propagates along the pipe, causing collapse of the pipe along its traveling length. The driving energy, which causes a buckle to propagate, is the external hydrostatic pressure.

The nature of a propagation buckle is that a greater pressure level is required to initiate a propagation buckle, called buckle initiation pressure, than the pressure required to maintain propagation of the buckle. As a consequence of this, a buckle initiated in an offshore pipeline propagates and collapses the line until the external pressure becomes equal to or less than the propagation pressure.

Many theoretical and experimental investigations had been made by various organisations to study buckle propagation and to determine the propagation pressure for offshore pipelines. These studies have resulted in similar but rather simple expressions for calculating the propagation pressure, Ppr.