Deepwater environment brings unbonded flexible pipelines with collapse risk of interlocked armours, such as pressure armour and carcass. An effective and accurate prediction for collapse is required in design process.

This paper presents a theoretical approach for estimating collapse of the interlocked armours based on strain energy equivalence principle of representative volume method. The strain energy with Dirichlet boundary is calculated using FEA software for the armours, which usually have complex cross-section. Meanwhile, an equivalent ring model is established as the representative volume, of which strain energy is acquired analytically. Equivalent thickness can be obtained by equivalence of both the two energys. And then the resistance to collapse of the flexible pipe can be carried out directly.

Theoretical results from the approach presented in the paper are compared with numerical results proved by experimental ones from reference, which verifies the equivalence approach. This approach provides a new angle to process parametric design for critical collapse of the flexible pipe.

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