Adhesion Effect on the Ultimate Strength of Sandwich Pipes

Sandwich pipes composed of two steel layers separated by a polypropylene annular can be used for deepwater oil&gas transportation. They combine high structural strength to resist external pressure with thermal insulation to prevent blockage by paraffin and hydrate. In this work, experimental tests and numerical models were employed to verify the influence of the inter-layer adhesion on the ultimate strength under external pressure and longitudinal bending of a sandwich pipe prototype. The maximum shear stress obtained from sandwich pipe specimens bonded with a specific adhesive indicated the adhesion levels to be adopted in the numerical simulations. Two contact models were employed to simulate the bonding and slipping conditions between layers, one adopting a friction model and the other including non-linear springs between metal and polymer nodes. The latter is an adapted solution to simulate both tension and shear loads. As expected for a sandwich structure, the structural strength is strongly dependent on the interface stickiness. The analyzed geometry is able to withstand a water depth up to 3,000 meters with a bonding strength corresponding to only 10% of the idealized perfect adhesion condition.