Dynamic Response of Flexible Risers due to Unsteady Slug Flow
Doctoral thesis
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Date
2015Metadata
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- Institutt for marin teknikk [3389]
Abstract
In this work the influence of unsteady slug flow on the dynamic response of flexible
risers is analysed using a computational tool consisting of one program for prediction of
slug flow development, and another program for calculation of dynamic response of
the riser. Both programs apply a time integration method, and since slug flow leads to
dynamic response of the riser, and riser dynamics influences slug flow behaviour, the
two codes exchange feedback of information during the integration process. Information
exchange is established by building a computer federation of programs based on High
Level Architecture (HLA). The federation is composed of two standalone in-house
computer codes. The first one is a two-phase flow code which simulates unsteady slug
flow through pipes and risers using a Lagrangian tracking model. The second one is a
program for static and dynamic global riser analysis based on a two dimensional nonlinear
finite element formulation. Using the HLA rules, a communication interface was
built, so the two programs are able to perform synchronized time integration with
mutual feedback of information for each time step.
The slug flow – riser dynamics interaction is a complex problem. The riser response
depends on the slug flow behaviour, and the slug flow development is influenced by the
riser configuration. Displacement response of the riser and the influence of them on the
slug flow development have been studied for different flexible riser configurations
carrying different liquid and gas flow rates. Results have shown the importance of using
complete mathematical models for each phenomena and the importance of a strong
interface of communication in order to predict a realistic behaviour of the slug flow -
riser dynamics interaction. Analysis on the influence of the slug flow on the effective
tension and bending moment of a flexible riser was studied showing increment of axial
and bending stresses. Moreover, the simulations have demonstrated the importance to
use a full non-linear finite element model for the riser dynamics, because the riser
response influence strongly on the slug flow behaviour. Finally, the numerical results
were validated against experimental data showing a reasonable agreement.