The subsea oil and gas industry in ultra-deep waters requires very long risers, flexible or rigid, which may clash due to their proximity in some field configurations. Clashing should be avoided whenever possible. However, when no mitigation is effective to avoid clashing in a chosen layout or configuration, it is critical to ensure that no damage will occur in any one of the riser sections.

The key to avoiding damage is to correctly evaluate the clashing energy, which is a function of the relative speed between the risers at the moment when clashing occurs, as well as the clashing stiffness and damping of each riser section. The correct evaluation of the clashing stiffness grows in importance when the possibility exists for impact between risers with a high relative velocity.

In order to evaluate the energy effectively absorbed by the riser sections, it is necessary to perform parametric analyses involving different riser types and diameters, riser configurations (single catenary, lazy wave), field layouts, water depths, and associating global clashing analysis with local analysis of the clashing sections using finite element software.

When high velocity impacts occur, part of the kinetic energy is transferred as strain energy at the contact point. Part of this strain energy occurs at the clashing section which will be deformed (local strain), while the other part is transferred to the surrounding area of the risers as axial and bending strains (global strain). This makes it extremely difficult to correctly evaluate the clashing stiffness of each riser. In order to obtain the local stiffness in the contact section, it is necessary to identify how much of the kinetic energy is locally transmitted by the sections.

The aim of this paper is to present a discussion about alternatives on how to evaluate the clashing energy based on the clashing stiffness of each riser. A methodology for the energy clashing calculation is presented in a way where it is possible to identify and separate the local strain from the global strain in a specific contact section based on the kinetic energy transmitted by each part.

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