Coupling of marine riser and tensioner system

Abstract

A coupled model of a marine riser and a tensioner system is built. The riser is modeled using the multi-body dynamics program MSC Adams, and the tensioner system using the powerful controls and systems simulation tool, MSC Easy5. The hydrodynamic forces on the marine riser are calculated according to linear wave theory, and implemented in the model using a custom made subroutine. The riser is modeled using flexible beam elements according to Timoshenko beam theory.

The tensioner system is modeled using moments of fluid system and body mechanics. The system is solved using equations of equilibrium of fluid momentum, mass and energy. For the unfamiliar reader, an introduction to deep sea oil exploration and production is given. Each subsystem included in the model is explained. The mode of working of the more complex subsystems is presented, followed by a discussion of the simplifications made to include them in the model.

The hydrodynamic force module is based on linear potential theory of long waves, and Morison’s equation for oscillating structure in oscillating flow. An introduction to wave theories with derivation of the necessary equations is given, together with a thorough discussion of hydrodynamic coefficients.

An introduction to the mechanics of fluid systems is presented, to give the reader the knowledge to understand the underlying principles of the Easy5 tensioner model.

A rough guide to MSC Adams and MSC Easy5 explaining normal usage of the programs, alterations made to them and use of the 4subsea interface, is presented. Theory regarding computational methods of solving ordinary differential equations is also offered.

Finally all results are verified by analyzing similar models in the widely used and highly acknowledged global analysis program for slender marine structures RIFLEX. Excellent compliancy is found for the hydrodynamic force module and the forced vessel motion subroutine. The coupled marine riser and tensioner model shows how tension between lower marine riser package (LMRP) and blow-out preventer (BOP) becomes a critical parameter.