Final report for WP4.3: Enhancement of design methods ... - Upwind

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UPWIND WP4: Offshore Support Structures and Foundations also allows the creation of movie sequences so that the dynamic behaviour of a mooring can be visualised. Mooring configurations can be plotted, as shown in Figure 8.36, and the components are also listed in detail in the MATLAB console window. Figure 8.35: MDD’s main GUI form. The user can also enter current-depth profiles (i.e. a set of depth/speed/direction values), and wind profiles (speed and direction). The software uses this information to calculate the drag forces on the various elements of the mooring, in order to derive a steady-state solution for the mooring shape. Figure 8.36: Typical mooring configuration (design view) in MDD. MDD takes an iterative approach; it starts from a vertical configuration for the mooring and iteratively “moves” the mooring horizontally until a steady-state solution is reached. It can deal with time-varying inputs, i.e. a timeseries of current-depth profiles can be provided as an input. When handling these time-series, it is assumed that the mooring has time to reach an equilibrium state between time-steps. In other words, the variation in the current is slow compared to the response time of the mooring. There are currently no detailed plans to extend MDD to include the action of wave forces. 122
UPWIND WP4: Offshore Support Structures and Foundations MDD can produce any of the following outputs: • Graphical plots of the steady-state shape of the mooring; • Movie sequences showing the changing shape of the mooring in response to a time-varying current; • Tension in each segment of the mooring; • Final steady-state position of each component (horizontal and vertical); • Final angle of each component to the vertical; • Anchor mass required; • Adequacy of buoyancy elements in maintaining vertical position of the mooring. Key findings - MDD MDD is designed as a stand-alone, self-contained mooring design package. It is not designed to interact dynamically with other software. In principle, as the MDD m-files are freely available, it would be possible to modify it so that it could be used as a mooring-force calculation module for a wind turbine package, but this would not be straightforward. Also it only produces what are effectively steady-state solutions, which are suitable for slowly-varying forces like those from wind or currents, but would not be appropriate for wave-induced forces. MDD as it currently exists is not suitable as a mooring analysis module for interfacing with wind turbine codes. Its most serious drawback is its inability to carry out a true dynamic analysis of moorings subjected to rapidlyvarying (wave-induced) loads, which would require a considerable extension. The principles of the approach are similar and some components (e.g. drag force calculation algorithms) and some of the post-processing capabilities may be useful (in both the frequency and time-domain). ROMEO ROMEO is a mooring and riser analysis package produced by GL Noble Denton. It can perform both static and dynamic (frequency-domain) analysis in response to wind, wave and current loadings. Systems of up to 16 mooring lines are supported. Riser calculations are ignored here. ROMEO carries out three types of analysis on a mooring system: static analysis, dynamic analysis (both wavefrequency and low-frequency) and then a quasi-static analysis which is a combination of the first two. The inputs for all analyses include the details of the moored device or ship, “no-load” mooring geometry, and environmental conditions. Static analysis The environmental inputs for static analysis are steady wind and current forces. It is possible to enter current as a depth-dependent velocity profile. The algorithm uses Morison’s equation to calculate the drag on the mooring lines. This equation gives the inline force (i.e. force in the direction of the flow) on the mooring line element as 1 = ρVu + ρCaV ( u& − v& ) + ρC ( u − v) u − v 2 & [8-13] F d (i.e. the sum of Froude-Krylov force, hydrodynamic mass force, and drag force). It also calculates wind and current forces on the ship (or the WEC) using a simple drag equation as a function of cross-flow area, height coefficients and shape coefficients for the body in question. The coefficients are loaded in as part of the vessel definition file, so they have to be obtained externally. They are typically obtained either by manual estimation or, if higher accuracy is needed and there is no published data available for similar-shaped objects, they would be scaled-up from model tests. The static analysis outputs consist of: • steady-state shapes of mooring catenaries. • steady-state (mean) position of the moored device. Dynamic and quasi-static analysis Dynamic analysis is carried out in the frequency-domain only. The following parameters are used to characterise the wave environment: • significant wave height. • zero up-crossing period. • spectral shape factor (peak enhancement factor, γ). 123
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Project UpWind Contract No.: 019945
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Acknowledgement The presented work
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Summary The overall aim of the UpWi
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Table of Contents Acknowledgement..
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Task 4.1 Integration of support str
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Sequential coupling The sequential
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analysis. In the new multibody code
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Figure 2.4: 2nd global bending mode
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3.2 Comparison of deterministic loa
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Figure 3.5: Time series of axial fo
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• Time domain simulation in ADCoS
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• A supplementary load vector for
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The following results are found: Fi
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Figure 4.5: Output positions in the
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It is directly visible that there i
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ated eigenfrequency is shifted into
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Water levels For the calculation of
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guidelines for the inclusion of cli
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damaging if the frequency of the ex
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uncertain parameters carefully. Thr
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Table 5.3 shows annual reliability
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Steel substructure for offshore win
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Table 5.16 shows the required FDF v
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A Fracture Mechanical (FM) modellin
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Figure 5.4: Annual reliability indi
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Further, the effect of possible ins
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Rainflow evaluation The load cases
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For DLC 7.2 it has to be considered
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Guideline, DLC 1.5). Furthermore, t
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Table 5.28: Output locations for da
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From these results it can be clearl
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Figure 5.11: Normalised DELs with v
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Figure 5.14 presents normalized DEL
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Figure 5.16: Output locations for e
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UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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