Wake Interference |
 
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Wake Interference |
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This section describes the wave interference modelling facility provided by Flexcom. Three wake formulations are available, namely Huse’s formulation, Blevins’ formulation and a user-defined wake model for complete generality. This section introduces the range of wave interference modelling options.
In wake analysis, the wake of an upstream structure has the potential to affect the current forces experienced by a downstream structure. A typical application would be in the analysis of a pair of top tensioned risers or a pair of jumpers operating in close proximity. You model the upstream and downstream structures in the normal fashion, and also indicate to Flexcom that you want to model wake interference. Three options are provided for calculating the wake of an upstream structure, namely Huse’s formulation, Blevins’ formulation and an option for a user-defined wake model, which allows you define drag and lift coefficients for the downstream structure in terms of a grid covering the upstream wake region. VIV effects may also be considered, as Flexcom may automatically perform a modal analysis using Modes, and computed enhanced drag coefficients using Shear7, as part of the wake interference analysis process. The output from a wake interference analysis is essentially the clearance between the upstream and downstream structures.
Further information on this topic is contained in the following sections:
•Operation describes the typical sequence of steps in a wake interference analysis.
•Huses Model outlines the wake interference formulation presented by Huse.
•Blevins Model outlines the wake interference formulation proposed by Blevins.
•User-Defined Wake Model describes the general purpose wake model provided by Flexcom.
•VIV Drag explains the option to amplify drag coefficients, based on the results of a Shear7 analysis, which allows you to model the increase in drag that VIV typically causes.
•VIV Effects explains the automatic interface to Shear7, which allows you to iteratively perform a VIV analysis of the downstream structure as part of the overall wake interference computations. This produces enhanced drag coefficients may be more accurately determined based on the reduced current profile.
•Summary of Scenarios presents a sample list of possible wake interference scenarios and analysis sequences.
•Wake interference effects are only considered when constant hydrodynamic coefficients are specified for the downstream structure. If you specify Reynolds number dependent coefficients, then the effects of wake interference are ignored, and the simulation proceeds without considering any shielding effects. The wake interference model in Flexcom is designed for consistent current flow only (i.e. where the current profile does not vary over time). During the iteration process, the current velocities experienced by the downstream structure can vary due to shielding effects. Hence Reynolds number dependency is not considered as this would add another layer of complexity to the solution process, and potentially conflicting scenarios could occur during the iteration process.
•Wake interference effects are considered for static load cases only (although it is possible to perform a simulation quasi-statically if this is necessary to assist solution convergence). Flexcom is unable to simulate wake interference effects for regular wave or random sea simulations as the wake interference theories implemented in the software are only applicable to established flow regimes. From a theoretical perspective, it is much more difficult to quantify the effects of wake interference in a dynamic context so there are no immediate plans for further development of Flexcom in this area.
•*WAKE UPSTREAM is used to specify the composition of the upstream structure in terms of element sets, for use in wake interference calculations. The keyword also selects the wake interference model to be used, and defines associated data to characterise the wake field.
•*WAKE DOWNSTREAM is used to specify the composition of the downstream structure in terms of element sets for use in wake interference calculations. The keyword also facilitates the (optional) definition of lift coefficients in the case of a User-Defined wake interference model.
•*VIV DRAG is used to instruct Flexcom to read vortex-induced vibration (VIV) drag coefficient amplification factors from the results of a Shear7 analysis.
•*VIV EFFECTS is used to instruct Flexcom to continuously run Modes/Shear7 analyses of the downstream structure during the wake interference analysis.
•*CURRENT is used to specify current loading.
If you would like to see an example of how these keywords are used in practice, refer to A04 - TTR Wake Interference or G02 - Jumper WakeInterference.
Note also that the old *WAKE INTERFERENCE and *UPSTREAM STRUCTURE keywords have effectively been superseded by the new wake interference keywords which are more generic and provide additional functionality.