Transient dynamics of direct acting riser tensioners

This article presents an innovative numerical approach to model the transient dynamic behavior of Direct Acting Tensioners (DATs) used in deep-water drilling risers connected to mobile offshore drilling units. First, a mathematical model describing the top-tension fluctuations is established based on fluid mechanics principles for unsteady flow in closed pipes, resulting in a system of Partial Differential Equations (PDEs) that represent pressure variations along the transmission lines, pistons, valves, and accumulators. Next, a numerical solution procedure based on the Method of Characteristics (MOC) is developed, with special attention given to the non-linear boundary conditions. The innovative aspects lie in the inclusion of transmission fluid compressibility, inertance, and hose wall elasticity – effects neglected in the current state of the art, which typically relies on steady-flow formulations where DAT components are represented by simplified models using non-linear spring and damper elements. Three case studies are performed, considering stroke amplitudes representative of both operational and accidental disconnection scenarios. The results obtained from the proposed model and solution procedure are compared with those from the classical steady-flow formulation, revealing significant differences and demonstrating the substantial influence of the aforementioned parameters, particularly in transient riser disconnection simulations. • A mathematical model that describes the transient behavior of DATs is derived. • Transmission fluid compressibility, oil inertance, hose wall elasticity are considered. • A numerical method to solve the governing equations is developed. • The method is used to assess the impact of the aforementioned parameters. • Results indicate relevance of the method in severe situations such as disconnection and recoil.