Methods to predict stresses in flexible pipe under both combined loading and damaging loads on flexible pipes exist, these analytical tools are generally proprietary to the manufacturers and usually incorporate empirical factors linked to test results from specific pipe types This poses a major problem in relation to incorporating the use of these tools into general pipe design guidelines.

All flexible pipes are essentially layered structures incorporating elastomer or thermoplastic layers to contain pressure and using spiralled armor to take loads. The ability of the reinforcing spirals to relocate under bending gives the pipe its bending flexibility. Two main types of pipe exist: bonded pipes, where all armoring is embedded in and bonded to an elastomer compound, and non-bonded, where the helices can slide against each other and adjoining layers. The difference in behavior of bonded and non-bonded flexible pipe is discussed. A general theoretical formulation of the stiffness relations for layered flexible pipe is presented, and it is shown how to model a pipe in a way which is consistent for both bonded and non-bonded pipes

This theory has been implemented in the computer program FLEXPIPE which has been verified against test results and calculations made by the manufacturers. Some comparisons with test results are presented which indicate that using a non-bonded load transfer model for a bonded case does not produce the required level of accuracy, and that the bonded model as implemented in FLEXPIPE is required.

The aim should be to bring flexible pipes to a documented safety level equivalent to other parts of the overall production system. Present safety factors are typically based on experience, industry practice and testing. A revised design approach should be adopted which takes into account combined loading effects, accuracy of the analytical prediction and spread in test data. A such design approach requires use of a well documented and available computer program. A case study is included to show the advantages of using a consistent design approach for optimization of a deep water pipe, layer by layer.


The term flexible pipe can cover a lot of different products, from garden hose to corrugated steel pipe. The purpose of this theory is however to describe the behavior of a pipe which provides leak-proofness, flexibility and strength through a combination of polymer elastic material and helically wound reinforcement. The spiral reinforcement takes both hoop and axial tension forces, the angle and location of reinforcement layers determining the distribution of forces.

A common special case is that in which all reinforcement is laid with the same lay angle (although cross-laid). As the ratio between hoop force and axial tension is constant under pure internal or external pressure, there is an angle (54.7 degrees) at which the forces in the pipe are balanced under such loading, avoiding excessive elongation or radial expansion. This angle is called the neutral angle.

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