Objective
Accurate prediction of wave and current impacts is essential in any offshore development project. Extreme wave and current scenarios must be known during the design in order to establish survival conditions, and frequently occurring severe situations must be known for assessing fatigue problems. The present project aims at developing new knowledge and methods in the following fields :
- Knowledge about water particle velocities above mean water
level in storm waves.
- Three-dimensional numerical modelling of currents impacting offshore structures.
- Methods for assessing joint probabilities of wave and current loads.
- Time-domain modelling of the full non-linear interaction
between waves/currents and floating structures.
Anticipated results are given under item 2 above. Description of the final projects will be available during 1989.
PROJECT No 1 : THE WAVE PROJECT
Designing of fixed offshore structures, e. g. jackets is often performed on the basis of loads from one extreme wave. In the North Sea this wave may typically have a height of 20 m, out of which approx. 13 m will be above the mean water level. It is evident that this part of the wave is of tremendous importance to the total wave force and the overturning moment. It is therefore imperative to have knowledge about the velocities in the wave crest.
The scope of work will comprise initial pilot test and prototype development of an instrumentation system for laboratory measurements of wave crest kinematics, and planning of measurements in nature on an existing platform.
PROJECT No 2 : THE CURRENT PROJECT
DHI has since 1980 carried out various feasibility studies aiming at the development of a general three-dimensional flow model.
The plan is now to develop this model within the next 3 years.
The scope of work consists of three parallel activities :
a) general three-dimensional flow modelling. An implicit finite difference approximation to the generalized Navier-Stokes equation will be employed
b) modelling of turbulence
c) modelling of structural response from flexible cylindrical members.
PROJECT No 3 : THE COMBINED WAVE/CURRENT PROJECT
DHI has carried out pilot tests on the effect of following a more precise design approach based on an extrapolation of the critical key load parameter, e. g. the overturning moment for a jacket structure. These tests have led to quite encouraging (less conservative) results. The scope of work for the present Project No 3 consists of a more thorough and extensive analysis of this advanced methodology, aiming at the development of a well documented practical calculation procedure.
PROJECT No 4 : THE STRUCTURAL RESPONSE/INTERACTION PROJECT
The calculation of forces and movements of floating offshore structures will be solved in a two-step procedure.
Step one consists of the description of the acting wave field on basis of actual measurements or mathematical model simulations. Step two consists of the calculation of the resulting movements in a separate model.
The numerical model to be used in step one will be a further development of an existing non-linear, time-domain wave model developed earlier by DHI (SYSTEM 21 MK8). A (coarse) description of the floating body will be introduced in this model in order to resolve the wave/structure interaction in step two. Furthermore, a boundary element time domain solution will be used in step two to describe the wave field in the immediate viscinity of the floating structure and the associated structural motions. This model can be used independently in case of deep water and simple wave fields, and will be used as a "near-field" description, coupled to the SYSTEM 21 model, in the case of complicated shallow water wave fields.
Programme(s)
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DEM - Demonstration contractsCoordinator
2970 Hoersholm
Denmark