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Dynamics of Flexible Riser Systems

Soltanahmadi, Abbasali (1988). Dynamics of Flexible Riser Systems. (Unpublished Doctoral thesis, City, University of London)

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Abstract

This thesis is concerned with the static and dynamic analyses and the model testing of deep sea flexible riser systems which are the key components associated with semi-submersible oil platforms. A numerical method based on explicit integration of Newton's second law is developed to predict the 3-dimensional dynamic behaviour of the riser due to the hydrodynamic loadings induced by wave and current motion. In this analysis the effects of waves and currents from separate directions, vessel movements, vortex-shedding and structural damping are included. The material damping for the riser is modelled by a single Kelvin system and the hydrodynamic loadings are assessed from the modified Morison equation. The effect of vortex-shedding on the riser is modelled by considering the interaction of drag-inertia and lift forces due to wave and current motion. The drag coefficient being modified ~ the vortex-shedding effects which are predicted by calculating the maximum response of the flexible riser in the lift force direction.

The formfinding and static analysis of the riser when subject to structural self-weight and other static loadings is carried out by the method of Dynamic Relaxation using kinetic damping. The method is well suited to computer aided design procedures in which various shapes for the riser catenary have to be investigated together with the effects of boundary support conditions and alternative arrangements of mooring buoys.

The theories described above are implemented into three computer programs. The first program deals with the formfinding of the riser and the second investigates the dynamic behaviour of the riser due to non-linear current and wave loadings. The third program is concerned with the static solution of the riser due to current loading. The latter is employed when the hydrodynamic force consists of current loading only and therefore the dynamic solution is not desired. The iterative use of the first and third programs allows potential designs to be quickly investigated.

The results predicted by the numerical analyses are compared with those obtained from two series of model tests in wave flumes. The tests were scaled from prototype situations using Froud number criteria. The first set of tests used a small scale flexible chain model with negligible material damping and structural bending stiffness and no induced vortex shedding; the aim of these tests being only to validate the assessment of the hydrodynamic forces on the riser. '!he second set investigated the response of a larger scale model which induced vortex-shedding and in which the riser system had significant structural damping. The comparison of experimental and computed results showed close agreement.

The developed computer programs were also validated numerically by comparing the predicted results with those obtained from the well known riser program "FLEXRISER".

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering
School of Mathematics, Computer Science & Engineering
School of Mathematics, Computer Science & Engineering > Engineering
School of Mathematics, Computer Science & Engineering > Engineering > Civil Engineering
URI: http://openaccess.city.ac.uk/id/eprint/20155

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