OrcaFlex Manual - Orcina

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System Modelling: Data and Results, 3D Buoys 412 w Set the Drag Force Coefficient based on values given in the literature. For short simple cylinders fully immersed there are standard values given in the literature (see Barltrop & Adams, 1991, Hoerner,1965 and DNV-RP-C205). However, the standard book values do not include energy absorption by wave-making at the free surface. Strictly, this is a linear term (forces directly proportional to velocity), but in OrcaFlex this must be done by adjusting the drag coefficients of one or more cylinders. The Unit Damping Force data can be set to zero. If you later find that the buoy shows persistent small amplitude oscillations then you may wish to set a non-zero value to damp this out. Set the Drag Area Moments, Drag Moment Coefficients and Unit Damping Moment data. For the normal direction these data items can usually all be left as zero, providing you have subdivided the buoy into short enough cylinders (since these terms involve a high power of L, the cylinder length). For the axial direction these data items model the yaw drag and damping effects, so if this is important to you then set them to model the two main sources, namely skin friction on the cylinder surface and form drag on any protuberances on the buoy. Having set up this drag and damping data, it is well worth now running simulations of heave and pitch oscillations and checking that their rate of decay is reasonable and consistent with any real data you have available. Discus and CALM Buoys These types of buoy require different treatment since they have little axial extension. Instead it is their radial extension that most affects the buoy's pitch properties. As a result the axial discretisation of the buoy into cylinders does not capture the important effects. For example the pitch damping is often mostly due to radiation damping, i.e. surface wave generation; this is especially important for a CALM buoy with a skirt. To deal with this OrcaFlex offers the rotational drag and damping data, but there is little information in the literature to help in setting up this data. We therefore strongly recommend that you set the data up by calibration against real test results from model or full scale tests. The easiest information to work with are time history graphs of the buoy heave and pitch in still water, starting from a displaced position. This will give the heave and pitch natural periods and the rates of decay and you can adjust the buoy's drag and damping data until you get a good match with this measured behaviour. Here is the approach we use: � For the normal direction, set the Drag Area, Drag Force Coefficient and Unit Damping Force as described for Spar buoys above. � Then set the axial Unit Damping Force to zero and run a simulation that matches the conditions that existed in the real heave time history results, i.e. with the same initial Z displacement. � Then adjust the axial Drag Area and Drag Force Coefficients until the OrcaFlex buoy's Z time history matches the real time history. These two data items are simply multiplied together when they are used to calculate the drag force, so you can give one of the two data items a fixed positive value (e.g. 1) and then adjust the other. � The match will probably be poor in the later parts of the time history, where the heave amplitude has decayed to small values. This is because the square law drag term is insignificant at small amplitude and instead the damping force takes over. Therefore we now adjust the axial Unit Damping Force to further improve the match where the amplitude is small. You may find that this disturbs the match in the large amplitude part, in which case you might need to readjust the drag data. � For the axial direction, set the Drag Area Moment, Drag Moment Coefficient and Unit Damping Moment as described for Spar buoys above. � Then set the normal Drag Area Moment, Drag Moment Coefficient and Unit Damping Moment to best match the real pitch time history, in a similar way to that used above to match the heave time history. 6.10 3D BUOYS OrcaFlex 3D Buoys are simplified point elements with only 3 degrees of freedom: X, Y and Z. They do not rotate, but remain aligned with the global axes. They therefore do not have rotational properties and moments on the buoy are ignored. They should therefore be used only where these limitations are unimportant. 3D Buoys are able to float part-submerged at the surface, and may also be used independently, with no lines attached. Although they are much less sophisticated than 6D Buoys, 3D Buoys are easier to use and are convenient for modelling buoys at line junctions etc.
w height/2 height/2 Figure: 3D Buoy 6.10.1 Data Name Used to refer to the 3D Buoy. Included in Static Analysis z B y x 413 Buoy Axes always aligned with Global Axes System Modelling: Data and Results, 3D Buoys Determines whether the equilibrium position of the buoy is calculated by the static analysis. See Buoy Degrees of Freedom Included in Static Analysis. Initial Position Specifies the initial position for the buoy origin as coordinates relative to the global axes. If the buoy is not included in the static analysis then this initial position is taken to be the static position of the buoy. If the buoy is included in the static analysis, then this initial position is used as an initial estimate of the buoy position and the statics calculation will move the buoy from this position iteratively until an equilibrium position is found. See Buoy Degrees of Freedom Included in Static Analysis. Mass Mass or weight in air. Volume Used to calculate buoyancy and added mass. Bulk Modulus Specifies the compressibility of the buoy. If the buoy is not significantly compressible, then the Bulk Modulus can be set to Infinity, which means "incompressible". See Buoyancy Variation. Height Used to model floating buoys correctly, where the buoyancy, drag etc. vary according to the depth of immersion. It also determines the height used to draw the buoy. The Height is the vertical distance over which the fluid-related forces change from zero to full force as the buoy pierces the surface. It is taken to be symmetrical about the buoy's origin.
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w Orcina Ltd. Daltongate Ulverston
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Contents 4 w 3.4 Libraries 40 3.4.1
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Contents 6 w 5.6 Dynamic Analysis 1
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Contents 8 w 6.5.23 Wave Scatter Co
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Contents 10 w 8.12 Results 444 8.13
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Introduction, Installing OrcaFlex
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Introduction, Parallel Processing M
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Introduction, References and Links
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Introduction, References and Links
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w 2 TUTORIAL 2.1 GETTING STARTED 21
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w 23 Tutorial, Dynamic Analysis sha
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w 3 USER INTERFACE 3.1 INTRODUCTION
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w Simulation Paused 27 User Interfa
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w Keys on Main Window New model Ope
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w Rotate viewpoint left (decrement
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w General: StaticsMethod: Whole Sys
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w A text data file saved by OrcaFle
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w 37 User Interface, Model Browser
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w Cut/Copy Cut or Copy the selected
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w 3.4.1 Using Libraries 41 User Int
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w 43 User Interface, Libraries Once
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w 3.5.1 File Menu New Deletes all o
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w New Vessel New Line New 6D Buoy N
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w 3.5.5 View Menu Change Graphics M
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w Preferences 51 User Interface, Me
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w 53 User Interface, 3D Views 3D Vi
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w 55 User Interface, 3D Views � D
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w 57 User Interface, 3D Views a lin
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w 59 User Interface, 3D Views � F
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w Replay Period 61 User Interface,
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w Frame interval in real time 63 Us
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w Numeric 65 User Interface, Data F
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w � For lines you must specify th
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w 69 User Interface, Results this i
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w 71 User Interface, Results Let th
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w 73 User Interface, Results remain
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w Results 75 User Interface, Result
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w Replays 77 User Interface, Graphs
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w Axes 79 User Interface, Spreadshe
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w Command Line Parameters 81 User I
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w Wire Frame Graphics Codec 83 User
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Automation, Batch Processing Multi-
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Automation, Batch Processing 88 w N
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Automation, Batch Processing Assign
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Automation, Batch Processing Line T
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Automation, Batch Processing SHEAR7
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Automation, Batch Processing PolarT
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Automation, Batch Processing Figure
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Automation, Text Data Files Data in
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Automation, Text Data Files Selecte
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Automation, Text Data Files SHEAR7L
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Automation, Text Data Files 4.3.2 A
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Automation, Post-processing 108 w F
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Automation, Post-processing 110 w N
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Automation, Post-processing Referen
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Automation, Post-processing Command
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Automation, Post-processing Figure:
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w 5 THEORY 5.1 COORDINATE SYSTEMS 1
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w 125 Theory, Object Connections Di
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w 127 Theory, Static Analysis Final
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w 129 Theory, Static Analysis metho
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w 131 Theory, Dynamic Analysis for
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w Static Starting Position Simulati
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w 135 Theory, Friction Theory The n
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w where μ = magnitude of the vecto
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w 139 Theory, Extreme Value Statist
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w where σ = GPD scale parameter ξ
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w 143 Theory, Environment Theory Th
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w where Pu(z) = Nc(z/D)su(z)D Pu-su
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w 147 Theory, Environment Theory
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w Repenetration Offset After Uplift
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w 151 Theory, Environment Theory sp
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w Extrapolation Stretching 153 Theo
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w 155 Theory, Environment Theory st
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w 157 Theory, Vessel Theory individ
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w 159 Theory, Vessel Theory RAOs ca
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w 161 Theory, Vessel Theory Notes:
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w 163 Theory, Vessel Theory ω is t
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w surge/sway/heave M M.L roll/pitch
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w 167 Theory, Vessel Theory that co
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w 169 Theory, Vessel Theory separat
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w 171 Theory, Vessel Theory At this
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w Segment 1 Segment 2 Segment 3 Act
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w 175 Theory, Line Theory � If to
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w where component of M2 in the Sx2
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w The hysteresis model is described
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w � (90,90,90) sets Ex along -GX,
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w = v1 + p' + ω×p Therefore its a
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w y End A Stress ID z Stress OD O S
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w 5.12.16 Hydrodynamic and Aerodyna
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w 189 Theory, Line Theory Another w
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w Drag Chain Seabed Interaction 191
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w 193 Theory, Line Theory OrcaFlex
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w 195 Theory, 6D Buoy Theory clashi
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w 197 Theory, 6D Buoy Theory For Lu
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w 199 Theory, 6D Buoy Theory (due t
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w 201 Theory, 6D Buoy Theory Note:
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w α is the angle between the relat
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w 5.13.6 Contact Forces Contact For
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w Dynamic Analysis 207 Theory, Winc
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w 209 Theory, Shape Theory Finally,
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System Modelling: Data and Results,
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Page 362 and 363: System Modelling: Data and Results,
Page 432 and 433: Modal Analysis, Theory 432 w For si
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Page 436 and 437: Fatigue Analysis, Commands 436 w Fo
Page 438 and 439: Fatigue Analysis, Load Cases Data f
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OrcaFlex Manual - Orcina

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