Prevention of Flow-Induced Pulsations in Unbonded Flexible Pipes Using a Carcass Insert Technology

Flow-induced pulsations (FLIP) originated from flexible pipes, when used in deepwater applications, can cause large vibrations and dynamic stress levels in the attached topside or subsea pipe systems. The pulsations are characterized by high frequencies that can cause fatigue failures within a short time, leading to loss of containment. To prevent the appearance of FLIP, a new technology has been developed that involves manufacturing a T-shaped insert to cover the inner spiral cavity of the carcass, making the pipe bore relatively smooth. Smoothing the pipe bore with this insert delays and prevents the onset of flow-induced pulsations during dry gas transportation by minimizing the strength of pressure pulsations caused by local vortex flow and created in the spiral cavity. The use of this insert allows higher gas flow rates to be achieved in flexible pipes without introducing FLIP, thereby eliminating all FLIP-related structural integrity issues of the systems attached to the flexible. The smoothing of the pipe bore also reduces friction and improves flow assurance of pipe, reducing the pressure drop. This paper describes the merits of new insert technology using the FLIP prevention mechanism including the theoretical work related to analyzing flow and pressure pulsation. An acoustic energy balance framework is used to determine the onset velocity of pulsations originated at residual cavities left by the implementation of the insert. A shallow cavity is formed between two adjacent windings of the spiral insert. An aerodynamic analysis of these shallow cavities is described to determine whether self-sustained acoustic oscillations are possible. Other risks are introduced by the absence of the insert near end-fittings. Finally, in-situ conditions leading to non-uniformity such as bending of the flexible and pipe ovality are addressed. FLIP risks can be fully eliminated with the new insert technology in its nominal design and within its manufacturing tolerances. Copyright © 2024 by ASME.