Substructure Overview
Fig. 94 Three different substructures (in yellow) connected to the tower bottom (grey).
Optionally, a substructure definition can be added to the structural model of a wind turbine in QBlade. The substructure definition can fulfill several different roles. Below a few typical applications of substructure definitions are given:
Lattice towers: The substructure can be used to model the turbine tower with greater freedom as the standard tower file, which can only model tubular, straight towers, for instance by explicitly defining a lattice tower structure in the substructure file.
Floaters and moorings: The substructure can be used to define the floater of a floating offshore wind turbine (FOWT). This requires that the floater is modeled both hydrodynamically (Morison or Potential flow) and structurally /flexible or rigid) within the substructure definition. The substructure definition is also used to define the mooring system which keeps the floater place.
Soil modeling: In the substructure file a p-y curve can be defined to model nonlinear soil dynamics, both onshore or offshore.
Multi-rotor assemblies: The substructure definition is also used to define the substructure of a multi-rotor assembly, with an arbitrary number of rotors.
To add a substructure to a turbine definition you need to add the filename of the substructure definition followed by the keyword SUBFILE anywhere within the main structural input file.
It is possible to connect the substructure either to the tower bottom, torquetube bottom or directly to the rotor nacelle assembly (RNA) of a turbine definition. If the main structural input file contains a TWRFILE the transition piece of the substructure (TP_INTERFACE_POS) is automatically connected to the tower bottom. If the substructure contains no TWRFILE keyword the transition piece is connected to the rotor nacelle assembly (RNA).
Modeling Options for an Offshore Substructure
When it comes to modeling the hydrodynamics and structural dynamics of an offshore substructure, there are several options to consider. This section is meant to give an overview on how different aspects can be modeled:
Substructure Mass and Inertia
The substructure is typically composed of interconnected members and joints (see Substructure Topology). These members can be modeled as either flexible (with associated stiffness, mass, etc.) or rigid (with or without mass). If the members have nonzero mass properties, the substructure’s mass and inertia are explicitly included. Alternatively, the complete substructure’s mass can be represented using a 6x6 mass matrix, known as lumped mass (see Lumped Mass, Inertia and Hydrodynamic Forces).
Substructure Buoyancy
Similar to mass and inertia, substructure buoyancy can be modeled explicitly or in a lumped linearized manner. Explicit modeling involves enabling the buoyancy calculation for each member based on its submerged part in the SUBMEMBERS table (see Substructure Members). Explicity buoyancy applies buoyancy forces locally to each member. In the lumped approach, a 6x6 hydrodynamic stiffness matrix is defined to represent the substructure’s buoyancy restoring forces and moments (see Lumped Mass, Inertia and Hydrodynamic Forces). This linearized hydrodynamic stiffness matrix is valid for small rotations and translations of the substructure.
Substructure Hydrodynamics
The hydrodynamic forces acting on a substructure can be evaluated by means of linear potential flow theory or the Morison equation. When the Morison Equation is used (see Morison Equation (Strip Theory) Modelling) Morison coefficients are assigned to each substructure member and the resulting forces (based on local wave and floater kinematics) are applied locally to each member. Alternatively, a linear potential flow solver (such as WAMIT, NEMOH, or ANSYS AQWA) can generate databases representing lumped hydrodynamic forces acting on the substructure (see Linear Potential Flow Modelling). Furthermore, it is also possible to assign lumped 6x6 hydrodynamic linear or quadratic drag and added mass matrices (see Lumped Mass, Inertia and Hydrodynamic Forces).
Different Scenarios
It is possible to combine different approaches mentioned above. A common mix involves using linear potential flow hydrodynamics along with the Morison equation to account for viscous hydrodynamic drag. If obtaining distributed loads for substructure components is important, hydrodynamic forces should be distributed as well (Morison equation with explicit buoyancy modeling), and the substructure members should be modeled as flexible elements. By carefully selecting and combining these modeling options, a comprehensive understanding of the hydrodynamics and structural dynamics of an offshore substructure can be achieved.
Keywords and Tables
As with the other structural definition files, the substructure is defined by a series of keywords that are recognized by QBlade when creating the turbine. The format is the same as with the other structural file definitions:
A parameter is defined by its value followed by the parameter Keyword:
<Value>
KEYWORD, for parameters defined by a single values.
Value KEYWORD
A table is identified by its Keyword and the row and column count of the subsequent ASCII values, which need to separated by space(s) or tab(s). An example of a table with two rows and tree columns is shown below.
KEYWORD<new line> <Header> <new line> <Values> for parameters defined by a table. The <Header> <new line> part is only optional and can be omitted.
KEYWORD
Header1 Header2 Header3 ...
Value(1,1) Value(1,2) Value(1,3) ...
Value(2,1) Value(2,2) Value(2,3) ...
... ... ... ...
There is no particular order in which these keywords and the associated data tables should be placed. The only exception is when defining tables. When a table is defined by a keyword, it should be immediately followed by the table header (optional) and the table content.
Miscellaneous Substructure Parameters
The following keywords can be used to define different properties and modeling options for the substructure.
ISFLOATINGA flag that determines if the substructure is floating of bottom-fixed. If the structure is bottom-fixed the joint coordinates (see
SUBJOINTSbelow) are assigned in a coordinate system with its origin placed at the seabed. For floaters, the origin is placed at the mean see level (MSL) and marks the floaters’s neutral point (NP)WATERDEPTHSets the design water depth of the substructure, this value is only used for visualization of the turbine and the identification of flooded members during turbine setup. Note that this water depth is only for the turbine setup and is not used during the simulations. During the simulation the water depth is obtained from the simulation settings.
WATERDENSITYSets the water density to calculate the mass of the flooded members. Note that this water density is only for the turbine setup and is not used during simulations. During simulations the water density is obtained from the simulation settings.
SEABEDDISCSets the sub-discretization length for mooring lines in contact with the seabed, in [m]. A value of 1 means that when a mooring line element is in contact with the seabed the mooring element will be discretized into elements of 1m length for which the contact forces will be evaluated. The default value is 2.
CONSTRAINEDFLOATERA flag that if set to true constrains the floater. A constrained floater can be subjected to a prescribed motion via a Prescribed Motion File (see Turbine Events and Operation).
BUOYANCYTUNERA multiplication factor that affects the calculation of the explicit buoyancy forces. Buoyancy caused by the linear hydrodynamic stiffness matrix is not affected by this factor.
ADVANCEDBUOYANCYAn option to use an advanced discretization technique to calculate the explicit buoyancy of partially submerged members, especially useful if non-vertical substructure members are located close to the mean sea level. Each partially submerged member will be discretized into the user defined number of elements. The value used must be a square integer number (a value of 100 is suggested).
STATICBUOYANCYAn optional flag that controls for which sea level the explicit buoyancy is calculated in QBlade. If set to true, the buoyancy is considering only the mean sea level. If set to false (default), the local wave elevation at each member is used to calculate the buoyancy. When evaluating the hydrodynamics using potential a potential flow theory excitation database (
USE_EXCITATION) it is recommended to enable theSTATICBUOYANCYoption since the hydrodynamic forces due to a change in wave elevation are already accounted for by the excitation forces. Using the the instantaneous sea level for the evaluation of buoyancy in such a case would cause this part of the buoyancy force to be double-accounted for.MARINEGROWTHA table that allows the user to define different types of marine growth that is present in the members. In QBlade, marine growth is simulated as an additional thickness that affects the diameter of the cylindrical or rectangular element. An entry is defined by its ID number, the thickness of the growth (added to the cylinder radius) and the density of the growth.
MARINEGROWTH ID Thickn Density 1 0.1 1100
TRANSITIONBLOCKAdds a rectangle between the substructure and the tower base. It is used just for visualization purposes.
TRANSITIONBLOCK WIDTH LENGTH HEIGHT 12 12 4
TRANSITIONCYLINDERAdds a cylinder between the substructure and the tower base. It is used just for visualization purposes.
TRANSITIONCYLINDER HEIGHT DIAMETER 0.5 6.5
RGBCOLORDefines the color of the complete substructure. It is used just for visualization purposes.
RGBCOLOR Red Green Blue 255 200 15
Substructure Topology
In general, a substructure consists of Members that are defined between Joints. A Member is a cylindrical or rectangular element that connects two Nodes. A Member is oriented along the vector that connects the two Joints, this vector also defines the Members length. A Member can either be defined as rigid or as flexible. When several members are defined between the same joint(s), these members are rigidly connected through the common joints (see Fig. 95).
Fig. 95 Three cylindrical members defined between four joints of a substructure.
Substructure Joints
Joints are defined via the SUBJOINTS table. A joint is defined by its position and optionally by its orientation. In most cases it is sufficient to define only the position of the joint, however when imposing constraints along certain degrees of freedom of a joint the joint orientation becomes important. Joints generally dont have a mass, but can be assigned a mass using the ADDMASS_<JntID> keyword.
Fig. 96 Three subjoints and their coordinate system (x-red, y-blue, z-green).
SUBJOINTSDefines a table that is used to place spatial joints that help define the members of the substructure. Each row of the table defines one joint and has four entries: the first gives the id number of the joint and the other three the Cartesian coordinates of the joint (in m). The origin is the seabed if
ISFLOATINGis false and the MSL ifISFLOATINGis true.The table is structured as follows:
SUBJOINTS JntID JntX[m] JntY[m] JntZ[m] X1 Y1 Z1 X2 Y2 Z2 1 0.00000 0.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 2 0.00000 0.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 3 14.43376 25.00000 -14.00000 1.00 0.00 0.00 0.00 1.00 0.00 4 14.43376 25.00000 12.00000 1.00 0.00 0.00 0.00 1.00 0.00 5 -28.86751 0.00000 -14.00000 1.00 0.00 0.00 0.00 1.00 0.00 6 -28.86751 0.00000 12.00000 1.00 0.00 0.00 0.00 1.00 0.00 7 14.43376 -25.00000 -14.00000 1.00 0.00 0.00 0.00 1.00 0.00 8 14.43376 -25.00000 12.00000 1.00 0.00 0.00 0.00 1.00 0.00 9 14.43375 25.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 10 -28.86750 0.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 11 14.43375 -25.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 12 9.23760 22.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 13 -23.67130 3.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 14 -23.67130 -3.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 15 9.23760 -22.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 16 14.43375 -19.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 17 14.43375 19.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 18 4.04145 19.00000 -17.00000 1.00 0.00 0.00 0.00 1.00 0.00 19 -18.47520 6.00000 -17.00000 1.00 0.00 0.00 0.00 1.00 0.00 20 -18.47520 -6.00000 -17.00000 1.00 0.00 0.00 0.00 1.00 0.00
SUBJOINTS(orientation defined by y- and y-axes)Defines a table that is used to place spatial joints that help define the members of the substructure. Each row of the table defines one joint and has four entries: the first gives the id number of the joint and the other three the Cartesian coordinates of the joint (in m). The origin is the seabed if
ISFLOATINGis false and the MSL ifISFLOATINGis true.The values X1, Y1, Z1, X2, Y2 and Z2 are optional and can be used to define the local coordinate axes of the joint. X1, Y1 and Z1 are defining the vector of the joints local X-Axis (in global coordinates). X2, Y2 and Z2 define the joints Y-Axis (in global coordinates). The Z-Axis is then constructed to define a right-hand coordinate system. The standard joint orientation is X1, Y1, Z1 = (1,0,0) and X2, Y2, Z2 = (0,1,0). If the user wants to define joint orientations they have to be defined for each joint in the table.
The table is structured as follows:
SUBJOINTS JntID JntX[m] JntY[m] JntZ[m] X1 Y1 Z1 X2 Y2 Z2 1 0.00000 0.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 2 0.00000 0.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 3 14.43376 25.00000 -14.00000 1.00 0.00 0.00 0.00 1.00 0.00 4 14.43376 25.00000 12.00000 1.00 0.00 0.00 0.00 1.00 0.00 5 -28.86751 0.00000 -14.00000 1.00 0.00 0.00 0.00 1.00 0.00 6 -28.86751 0.00000 12.00000 1.00 0.00 0.00 0.00 1.00 0.00 7 14.43376 -25.00000 -14.00000 1.00 0.00 0.00 0.00 1.00 0.00 8 14.43376 -25.00000 12.00000 1.00 0.00 0.00 0.00 1.00 0.00 9 14.43375 25.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 10 -28.86750 0.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 11 14.43375 -25.00000 -20.00000 1.00 0.00 0.00 0.00 1.00 0.00 12 9.23760 22.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 13 -23.67130 3.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 14 -23.67130 -3.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 15 9.23760 -22.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 16 14.43375 -19.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 17 14.43375 19.00000 10.00000 1.00 0.00 0.00 0.00 1.00 0.00 18 4.04145 19.00000 -17.00000 1.00 0.00 0.00 0.00 1.00 0.00 19 -18.47520 6.00000 -17.00000 1.00 0.00 0.00 0.00 1.00 0.00 20 -18.47520 -6.00000 -17.00000 1.00 0.00 0.00 0.00 1.00 0.00
SUBJOINTS(orientation defined by Euler angles))An alternative way to define the orientation of the substructure joints is to define the orientation of each joint by means of three consecutive Euler rotations around the global coordinate system. The first rotation is performed around the global X-axis, the second rotation around the global Y-axis and the third rotation around the global Z-axis. The last three columns are also optional, if not defined the orientation of each joint is the same as the global coordinate system.
SUBJOINTS JntID JntX[m] JntY[m] JntZ[m] RotX[deg] RotY[deg] RotZ[deg] 1 0.00000 0.00000 -20.00000 0.00 0.00 0.00 2 0.00000 0.00000 10.00000 0.00 0.00 0.00 3 14.43376 25.00000 -14.00000 0.00 0.00 0.00 4 14.43376 25.00000 12.00000 0.00 0.00 0.00 5 -28.86751 0.00000 -14.00000 0.00 0.00 0.00 6 -28.86751 0.00000 12.00000 0.00 0.00 0.00 7 14.43376 -25.00000 -14.00000 0.00 0.00 0.00 8 14.43376 -25.00000 12.00000 0.00 0.00 0.00 9 14.43375 25.00000 -20.00000 0.00 0.00 0.00 10 -28.86750 0.00000 -20.00000 0.00 0.00 0.00 11 14.43375 -25.00000 -20.00000 0.00 0.00 0.00 12 9.23760 22.00000 10.00000 0.00 0.00 0.00 13 -23.67130 3.00000 10.00000 0.00 0.00 0.00 14 -23.67130 -3.00000 10.00000 0.00 0.00 0.00 15 9.23760 -22.00000 10.00000 0.00 0.00 0.00 16 14.43375 -19.00000 10.00000 0.00 0.00 0.00 17 14.43375 19.00000 10.00000 0.00 0.00 0.00 18 4.04145 19.00000 -17.00000 0.00 0.00 0.00 19 -18.47520 6.00000 -17.00000 0.00 0.00 0.00 20 -18.47520 -6.00000 -17.00000 0.00 0.00 0.00
JOINTOFFSETDefines a table that can be used to apply a global offset to the positions of all
SUBJOINTS. Note that the offset is only applied to the joints and not the mass and hydro reference points defined in Linear Potential Flow Modelling.The table is structured as follows:
JOINTOFFSET XOFF YOFF ZOFF 10 0 0
ADDMASS_<JntID>can be used to add a mass at a joint <JntID>.
ADDMASS_<JntID>can be followed by up to 7 numeric values (at least one) to assign mass and rotational inertia properties. For example:ADDMASS_5 10 1 2 3 4 5 6adds a mass of 10kg at the joint with ID 5. The following numbers assign the rotational inertia in local joint coordinates: Ixx = 1, Iyy = 2, Izz = 3, Ixy = 4, Ixz = 5, Iyz = 6.ADDMASS_5 10 1 2 3 4 5 6ADDFORCE_<JntID>can be used to add a constant force, defined in the global coordinate system, to a joint <JntID>.
ADDFORCE_<JntID>can be followed by up to 6 numeric values (at least one) to assign forces and moments. For example:ADDFORCE_5 10 1 2 3 4 5applies a force vector of (10,1,2)N and a torque vector of (3,4,5)Nm to joint ID 5. All forces and moments are defined in the global coordinate system.ADDFORCE__5 10 1 2 3 4 5 6ADDFORCELOC_<JntID>can be used to add a constant force, defined in the local joint coordinate system, to a joint <JntID>.
ADDFORCELOC_<JntID>can be followed by up to 6 numeric values (at least one) to assign forces and moments. For example:ADDFORCELOC_5 10 1 2 3 4 5applies a force vector of (10,1,2)N and a torque vector of (3,4,5)Nm to joint ID 5. All forces and moments are defined in the local joint coordinate system.ADDFORCELOC_5 10 1 2 3 4 5 6
Substructure Elements
Four different types of element exits that can be used to construct the substructure geometry in the SUBMEMBERS table. Each element definition, identified by a unique Element ID, can be used to generate multiple members. The available element types are: cylidrical flexible elements, cylindrical rigid elements, rectangular flexible elements and rectangular rigid elements.
Fig. 97 A cylindrical element, geometry defined by its end-joints and the diameter.
SUBELEMENTSDefines a table that defines flexible cylindrical elements that can be used for the substructure definition. Each row represents one (cylindrical) element, which is defined by its structural parameters. When setting up the substructure, one
SUBELEMENTdefinition can be used for severalSUBMEMBERS(see below). Each row has 20 entries. These define the structural parameters of the element. The entry placement is very similar to the blade and tower structural element table (see Blade, Strut and Tower Structural Data Files). However, there two important differences:The first entry is used to indicate the ID number of the element (ElemID).
The last (20th) entry is used to indicate the Rayleigh damping of the element.
SUBELEMENTS ElemID MASS_[kg/m] Eix_[N.m^2] Eiy_[N.m^2] EA_[N] GJ_[N.m^2] GA_[N] STRPIT_[deg] KSX_[-] KSY_[-] RGX_[-] RGY_[-] XCM_[-] YCM_[-] XCE_[-] YCE_[-] XCS_[-] YCS_[-] DIA_[m] DAMP[-] 1 4.7868E+03 6.7007E+13 6.7007E+13 1.2805E+13 5.0380E+13 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 2.7735E-01 2.7735E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 6.5000E+00 1.0000E-02 2 1.7668E+04 8.4228E+14 8.4228E+14 4.7263E+13 4.7260E+13 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 2.0412E-01 2.0412E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 1.2000E+01 1.0000E-02 3 3.5424E+04 6.7890E+15 6.7890E+15 9.4764E+13 5.1050E+15 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 1.4434E-01 1.4434E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 2.4000E+01 1.0000E-02 4 6.8297E+02 5.7201E+11 5.7201E+11 1.8271E+12 4.3010E+11 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 5.5902E-01 5.5902E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 1.6000E+00 1.0000E-02
SUBELEMENTSRIGIDDefines a table that defines rigid elements that will be used for the substructure definition. Each row represents one (cylindrical) element, which is defined by two attributes: its mass density and its diameter. When setting up the substructure, one
SUBELEMENTRIGIDdefinition can be used for severalSUBMEMBERS(see below). An exemplary table is shown below.SUBELEMENTSRIGID ElemID BMASSD DIAMETER 1 1 6.5 2 1 12 3 1 24 4 1 1.6
Fig. 98 A rectangular element, geometry defined by its end-joints and its x and y dimension.
SUBELEMENTS_RECTDefines a table that defines rectangular flexible elements that will be used for the substructure definition. Each row represents one (rectangular) element, which is defined by its structural parameters. The only difference between the
SUBELEMENTS_RECTand theSUBELEMENTStables is that the element dimensions along its local x-axis (XDIM column 19) and its local y-axis (YDIM column 20) need to be specified, instead of the cylindrical diameter. Thus, two additional values are required and the Rayleigh damping coefficient is shifted to column 22 accordingly. The diameter is in this case only used as a hydrodynamic equivalent diameter for the calculation of Morison forces at the end faces of a member (if aHYDROJOINTCOEFFis defined for one of the members end nodes).SUBELEMENTS_RECT ElemID MASS_[kg/m] Eix_[N.m^2] Eiy_[N.m^2] EA_[N] GJ_[N.m^2] GA_[N] STRPIT_[deg] KSX_[-] KSY_[-] RGX_[-] RGY_[-] XCM_[-] YCM_[-] XCE_[-] YCE_[-] XCS_[-] YCS_[-] XDIM_[m] YDIM_[m] DIA_[m] DAMP[-] 1 4.7868E+03 6.7007E+13 6.7007E+13 1.2805E+13 5.0380E+13 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 2.7735E-01 2.7735E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 6.5000E+00 6.5000E+00 6.5000E+00 1.0000E-02 2 1.7668E+04 8.4228E+14 8.4228E+14 4.7263E+13 4.7260E+13 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 2.0412E-01 2.0412E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 1.2000E+01 1.2000E+01 1.2000E+01 1.0000E-02 3 3.5424E+04 6.7890E+15 6.7890E+15 9.4764E+13 5.1050E+15 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 1.4434E-01 1.4434E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 2.4000E+01 2.4000E+01 2.4000E+01 1.0000E-02 4 6.8297E+02 5.7201E+11 5.7201E+11 1.8271E+12 4.3010E+11 0.0000E+00 0.0000E+00 5.0000E-01 5.0000E-01 5.5902E-01 5.5902E-01 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 1.6000E+00 1.6000E+00 1.6000E+00 1.0000E-02
SUBELEMENTSRIGID_RECTDefines a table that defines rectangular rigid elements that will be used for the substructure definition. Each row represents one (rectangular) element, which is defined by four attributes: its mass density, its dimensions along the local x- and y-axis and an equivalent hydrodynamic diameter which is used to evaluate hydrodynamic forces at the members end faces. When setting up the substructure, one
SUBELEMENTRIGID_RECTdefinition can be used for severalSUBMEMBERS(see below). An exemplary table is shown below.SUBELEMENTSRIGID_RECT ElemID BMASSD XDIM YDIM DIA 1 1 2 6 1 2 1 3 1 1 3 1 5 5 1 4 1 4 2 1
STIFFTUNERA multiplication factor that affects the stiffness of the flexible elements defined in
SUBELEMENTS.MASSTUNERA multiplication factor that affects the mass density of ALL elements defined in
SUBELEMENTS.
Substructure Members
The members of the substructure are defines within the SUBMEMBERS table. Each line in the table generated one element that is defined by an element definition, identified by its Element ID and two joints, defined by their Joint ID. In addition the SUBMEMBERS table assigns the member rotation (for rectangular elements) hydrodynamic coefficients, marine growth, flooded area and the discretization for each member.
SUBMEMBERSDefines a table that contains the members that make up the turbine substructure. A member, with the ID MemID, is defined between two entries of the
SUBJOINTStable (Jnt1ID and Jnt2ID) and one entry from an element table (ElmID) (SUBELEMENTS,SUBELEMENTSRIGID,SUBELEMENTS_RECT,SUBELEMENTSRIGID_RECT). The column ElmRot can be used to rotate the member around its principal axis. Rotations are entered in degree. Additionally, it can have one Morison force coefficients group (HyCoID) defined in theHYDROMEMBERCOEFFtable and a marine growth entry (MaGrID) from theMARINEGROWTHtable. Also, this table allows the member to be flooded via a flooded cross sectional area entry in [m^2] (FldArea). The member can be subdivided into smaller elements for a more accurate structural and hydrodynamic evaluation. This is done in the MemDisc column; it gives the maximum allowed length of a discrete structural element of the member in [m]. Also, this table has the option to enable the buoyancy forces (IsBuoy) for the individual members (0 = False, 1 = True). Finally, the member can be optionally named for easier recognition in the output tables (Name). The last three optional columns can be used to assign a unique color, specified by its RBG components (Red, Green, Blue), to the member.The keyword table has the following format:
SUBMEMBERS MemID Jnt1ID Jnt2ID ElmID ElmRot HyCoID IsBuoy MaGrID FldArea MemDisc Name Red Green Blue 1 1 2 1 0 3 1 0 0 2 Main_Column 100 200 100 2 45 4 2 0 4 1 0 0 2 Upper_Column_1 100 200 100 3 46 6 2 0 4 1 0 0 2 Upper_Column_2 100 200 100 4 47 8 2 0 4 1 0 0 2 Upper_Column_3 100 200 100 29 3 45 2 0 4 1 0 0 2 Upper_Column_flooded_1 100 200 100 30 5 46 2 0 4 1 0 0 2 Upper_Column_flooded_2 100 200 100 31 7 47 2 0 4 1 0 0 2 Upper_Column_flooded_3 100 200 100 5 48 3 3 0 5 1 0 0 2 Base_Column_1 100 200 100 6 49 5 3 0 5 1 0 0 2 Base_Column_2 100 200 100 7 50 7 3 0 5 1 0 0 2 Base_Column_3 100 200 100 26 42 48 3 0 5 1 0 0 2 Base_column_flooded_1 100 200 100 27 43 49 3 0 5 1 0 0 2 Base_column_flooded_2 100 200 100 28 44 50 3 0 5 1 0 0 2 Base_column_flooded_3 100 200 100
Substructure Constraints
When multiple members are connected to the same joint these members are rigidly constrained through this common joint. By using the SUBCONSTRAINTS table it is possible to constrain arbitrary joints, and thereby also the element connected to those joints. The SUBCONSTRAINTS table also allows to fix a joint or to connect a joint with the transition piece, which connects to the turbine structure. The joints can be constrained along any of their degrees of freedom (DoF). Furthermore, it is possible to constrain joints with a spring or damper, instead of a rigid constraint.
SUBCONSTRAINTSDefines the table that defines the constraints between two joints that are not already connected by members, constraints of joints to the ground or to one
TP_INTERFACE_POStransition piece point.Each row of the table has 12 entries. The first entry defines the constraint ID number (CstID). The next entry define the joint which shall be constrained (JntID). The joint can now be constrained to a second joint, by inserting the corresponding JntID into the JntCon column, to a
TP_INTERFACE_POSby inserting its number into the 4th column (TpCon) or to the ground by setting the GrdCon column to 1. A joint can either be constrained to a second joint (JntCon), to the transition piece (TpCon) point or to the the ground (GrdCon), so only one of these three columns should be used at the same time.The sixth entry specifies that the constraint is realized as a non-linear spring-damper element (defined via an the spring ID number). If no spring or damper element is selected the constraint is realized as a stiff connection. The last 6 entries specify which degrees of freedom are constrained (either stiff or with a spring damper element): the three translational and three rotational degrees of freedom. For these entries 0 is interpreted as unconstrained (free) and 1 is interpreted as constrained. A spring-damper element is always acting along all the constrained degrees of freedom at the same time.
The coordinate system for these constraints is defined by the type that JointID1 is connected to. If Joint1ID is connected to Joint2ID, or the transition piece, the coordinate system in which this constrained is carried out is that of Joint1ID. If Joint1ID is connected to the ground, the constraint is realized in the global world coordinate system.
An exemplary
SUBCONSTRAINTStable is shown below. In this example all joints in the table are connected directly to the transition piece.SUBCONSTRAINTS CstID JntID JntCon TpCon GrdCon Spring DoF_X DoF_Y DoF_Z DoF_rX DoF_rY DoF_rZ 1 2 0 1 0 0 1 1 1 1 1 1 2 24 0 1 0 0 1 1 1 1 1 1 3 26 0 1 0 0 1 1 1 1 1 1 4 28 0 1 0 0 1 1 1 1 1 1 8 30 0 1 0 0 1 1 1 1 1 1 9 32 0 1 0 0 1 1 1 1 1 1 10 34 0 1 0 0 1 1 1 1 1 1 14 12 0 1 0 0 1 1 1 1 1 1 15 14 0 1 0 0 1 1 1 1 1 1 16 16 0 1 0 0 1 1 1 1 1 1
Note that at least one joint of the substructure members
SUBMEMBERSshould be constrained to the transition piece (defined byTP_INTERFACE_POS), to connect the member to the tower bottom of the wind turbine.- Connections to a Second Transition Piece
A joint can be connected to any created transition piece by entering number of the
TP_INTERFACE_POS_<X>into the TpCon column.- Connections to the Torquetube
When building a floater for a vertical axis wind turbine (VAWT) the user also has the option to connect a joint to the bottom of a rotating torquetube. This is done by inserting a negative number into the TpCon column. So to connect to the torquetube of the 1st turbine, the user would insert -1 into column TpCon. To connect to the torquetube bottom of the second turbine insert -2.
It is also possible to connect a joint to the top of the torquetube of any turbine, to do this subtract 100 from the value inserted in the TpCon column. As an example: to connect to the torquetube top of the second turbine (located at
TP_INTERFACE_POS_2) insert -102 in the TpCon column.- Connections to the Tower Top
Connections to the tower top are realized in a similar way as connections to the torquetube top. By adding 100 into column TpCon. So to connect to the tower top of turbine 1 insert 101 in column TpCon.
Changes to in SUBCONSTRAINTS QBlade 2.0.7
In QBlade versions prior to 2.0.7 the constraint degrees of freedom were always defined in the coordinate system of the object to which Jnt1ID was constrained, e.g. Joint2ID, the transition piece or the ground. From QBlade 2.0.7 on the coordinate system for the constraint is now that of Jnt1ID, except when Jnt1ID is connected to the ground. In the latter case the coordinate system that is used is the global coordinate system.
The Transition Piece
Fig. 99 The transition piece
The transition piece is the reference position in the substructure definition that defines the interface between the turbine definition and the substructure. It is possible to connect the substructure either to the tower bottom, to the torquetube bottom or directly to the rotor nacelle assembly (RNA) of a turbine definition. If the main structural input file contains a TWRFILE the transition piece of the substructure (TP_INTERFACE_POS) is automatically connected to the tower bottom. If the substructure contains no TWRFILE keyword the transition piece is connected to the rotor nacelle assembly (RNA). Through the SUBCONSTRAINTS table joints (and their connected members) can be connected to the transition piece.
TP_INTERFACE_POS_<X>Defines the (x,y,z) coordinates (in m) of the position of the transition piece location of the substructure. It is defined as the point where the substructure is connected to the tower base of the wind turbine.
For floating substructures it is defined in (x,y,z) [m] from the MSL = (0,0,0).
For bottom fixed substructures, it is defined from the seabed.
Note that the inertia and hydrodynamic reference points (
REF_COG_POSandREF_HYDRO_POS) are always automatically constrained to this point (see Linear Potential Flow Modelling). There can be several transition piece points. Further points are then defined by adding additional keywords where an underscore and a number is added to the keyword (e.g.TP_INTERFACE_POS_2). This allows the user to define additional inertia and hydrodynamic reference points (see Linear Potential Flow Modelling). If a multi-rotor wind turbine is simulated the TP_INTERFACE_POS_1 would automatically connect to the tower bottom of turbine 1, TP_INTERFACE_POS_2 would automatically connect to the tower bottom of turbine 2 and so on.All transition piece points can be constrained to a joint of the substructure in theSUBCONSTRAINTStable. The structure of the table is:TP_INTERFACE_POS X[m] Y[m] Z[m] 0 0 10
Note: for the 1st
TP_INTERFACE_POS_<X>the numbering _1 can be omitted, so TP1 can be defined by the keywordTP_INTERFACE_POS. This is also true for the definitions of all following reference points.TP_ORIENTATION_<X>(orientation defined by x- and y -axes)Defines the orientation of the tower base or RNA coordinate system which is connected to the
TP_INTERFACE_POS_<X>by defining its \(X_t\)- and \(Y_t\)-Axis in the global coordinate system. The first row defines the X-axis (\(X_{tp}\)) orientation and the second row defines the Y-axis (\(Y_{tp}\)) orientation of the transition piece coordinate system. IfTP_ORIENTATION_<X>is not specified the default values are \(X_{tp}=(1,0,0)\) and \(Y_{tp}=(0,1,0)\), so the tower base coordinate system is aligned with the global coordinate system. The \(Z_t\)-Axis is evaluated from the cross-product of \(X_t\) and \(Y_t\).TP_ORIENTATION X[m] Y[m] Z[m] 1 0 0 0 1 0
TP_ORIENTATION_<X>(orientation defined by Euler angles)An alternative way of defining the orientation of the tower base or RNA is to specify the orientation by means of three Euler angles. Starting from the global coordinate system three consecutive Euler rotations are performed, first around the global X, second around the global Y and third around the global Z axis.
TP_ORIENTATION Rot_X[deg] Rot_Y[deg] Rot_Z[deg] 30 0 0
Lumped Mass, Inertia and Hydrodynamic Forces
Fig. 100 Main reference points for the substructure. The inertia reference point REF_COG_POS and the hydrodynamic reference point REF_HYDRO_POS are constrained to the transition piece point TP_INTERFACE_POS.
For each transition piece multiple reference position exist, which are rigidly constrained with the transition piece. These reference points can be used to assign lumped masses, lumped hydrodynamic forces (such as linear stiffness or damping) or lumped hydrodynamic added mass. The REF_HYDRO_POS_<X> reference point also acts as the position at which the forces from linear potential flow data are applied to the substructure (see Linear Potential Flow Modelling).
REF_COG_POS_<X>defines the (x,y,z) position (in m) of a inertia point of the system (i.e. the center of gravity). It is in this position that the
SUB_MASSmatrix is evaluated. This point is automatically constrained to the transition piece, defined byTP_INTERFACE_POS. It has the following format:REF_COG_POS X[m] Y[m] Z[m] 0 0 -13.46
SUB_MASS_<X>defines a complete 6 by 6 mass and rotational inertia matrix that is placed in the location defined by the
REF_COG_POS_<X>keyword. The units are kg for the mass and kg m^2 for the inertia. An example of this matrix is shown below:SUB_MASS 1.34730e+07 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 1.34730e+07 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 1.34730e+07 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 6.82700e+09 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 6.82700e+09 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 1.22600e+10
REF_HYDRO_POS_<X>defines the (x,y,z) position (in m) of a hydrodynamic evaluation point of the system (i.e. where the lumped hydrodynamic forces are applied). It is in this position that the hydrodynamic matrices (e.g.
SUB_HYDROSTIFFNESS_<X>,SUB_HYDRODAMPING_<X>,SUB_HYDROADDEDMASS_<X>, etc.) and the radiation and excitation forces are applied. This point is directly constrained to theTP_INTERFACE_POS_<X>point, so no additional constraints are necessary to attach this point to the substructure. It has the following format:REF_HYDRO_POS_1 X[m] Y[m] Z[m] 0 0 -10.00
SUB_HYDROSTIFFNESS_<X>defines a complete 6 by 6 stiffness matrix that is evaluated in the location defined by the
REF_HYDRO_POS_<X>keyword. The units are N/m, N/rad, Nm/m, Nm/rad, depending on the entry. The general form of this matrix is shown below:SUB_HYDROSTIFFNESS_1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3.32941e+05 0 0 0 0 0 0 -4.99918e+09 0 0 0 0 0 0 -4.99918e+09 0 0 0 0 0 0 9.834e+07
SUB_HYDRODAMPING_<X>defines a complete 6 by 6 damping matrix that is evaluated in the location defined by the
REF_HYDRO_POS_<X>keyword. The units are N/(m/s), N/(rad/s), Nm/(m/s) or Nm/(rad/s), depending on the entry. This matrix has the same form as theSUB_HYDROSTIFFNESS_<X>matrix.SUB_HYDROQUADDAMPING_<X>defines a complete 6 by 6 quadratic damping matrix that is evaluated in the location defined by the
REF_HYDRO_POS_<X>keyword. The units are N/(m/s)^2, N/(rad/s)^2, Nm/(m/s)^2, Nm/(rad/s)^2, depending on the entry. This matrix has the same form as theSUB_HYDROSTIFFNESS_<X>matrix.SUB_HYDROADDEDMASS_<X>defines a complete 6 by 6 added mass matrix that is evaluated in the location defined by the
REF_HYDRO_POS_<X>keyword. The units are kg. This matrix has the same form as theSUB_HYDROSTIFFNESS_<X>matrix.SUB_CONSTFORCE_<X>applies a constant force (and/or torque) to the
REF_HYDRO_POS_<X>point. It can be used to e.g. model the constant buoyancy force acting on the floater in its equilibrium position. The units are N or Nm, depending on the entry.SUB_HYDROCONSTFORCE_1 //the constant hydrodynamic buoyancy (and other forces,moments) 0 0 8.07081e+07 0 0 0
SUB_DISPLACEDVOLUME_<X>applies a constant force in the global z-direction to the
REF_HYDRO_POS_<X>point that is calculated based on the displaced water volume given by the user. It can be used to e.g. model the constant buoyancy force acting on the floater in its equilibrium position in a simple way without evaluating the force directly. This force is added to theSUB_CONSTFORCE_<X>entries, but can be used without specifyingSUB_CONSTFORCE_<X>.
Mooring Elements and Ground-Constraints
Fig. 101 Mooring lines connected to a floating wind turbine for ground fixing.
The connection to the ground is handled differently for floating and fixed-bottom substructures. For floating substructures, the anchoring is done via the mooring lines defined with the MOORELEMENTS and
MOORMEMBERS keywords. These keywords can also be used to define flexible cable elements of the substructure. For bottom-fixed substructures, the connection the ground is defined in the SUBCONSTRAINTS table.
It can be either a rigid connection or a connection via a system of non-linear springs and dampers. These latter elements are defined with the keywords NLSPRINGDAMPERS and optionally SPRINGDAMPK.
MOORELEMENTSis a table that contains the structural parameters of the flexible cable elements of the substructure such as mooring lines. Each row defines one set of parameters and has 6 values. These are the mooring element ID number, the mass per length [kg/m], bending stiffness around y or x in [Nm^2], the axial stiffness in [N], a structural (longitudinal) damping coefficient and a hydrodynamic diameter in [m], which is used during buoyancy and Morison force evaluations.
MOORELEMENTS MooID MASS_[kg/m] EIy_[N.m^2] EA_[N] DAMP_[-] DIA_[m] 1 1.086306E+02 6.148892E+08 7.536117E+08 0.001 0.077 2 2.013616E+02 4.234759E+08 8.513517E+08 0.001 0.137
Optionally, a nonlinear axial stiffness for a MOORELEMENT may be defined by a nonlinear data table. In this case, the data table identifier, preceded by a hashtag (#) replaces the EA value of the MOORELEMENTS table.
MOORELEMENTS MooID MASS_[kg/m] EIy_[N.m^2] EA_[N] DAMP_[-] DIA_[m] 1 1.086306E+02 6.148892E+08 #NLDATA1 0.001 0.077 2 2.013616E+02 4.234759E+08 #NLDATA2 0.001 0.137
CABDAMPIn some cases, if the alpha damping coefficient of a mooring line (or cable) element is too large, a simulation can become unstable. Therefore, be default the damping coefficient of the mooring lines is not applied. If the user wishes to activate the axial damping of mooring lines and guy cables, the keyword
CABDAMPmust be set to true.true CABDAMPMOORMEMBERSis a table that contains the information of the cable members (such as the mooring lines). Each row defines one cable member and has 10 entries. The first entry is the ID number of the cable member. The next two entries are the connection points of the cable member. There are several ways of defining the connection points. These are:
With the keyword
JNT_<ID>, where <ID> represents the ID of the joint. This way, the cable is connected directly to a existing joint.With the keyword
FLT_<XPos>_<YPos>_<ZPos>, where <XPos>_<YPos>_<ZPos> represent the global (x,y,z) coordinates of the connection point (in m). Here, QBlade creates a constraint between this point and the floater to attach the cable.With the keyword
GRD_<XPos>_<YPos>, where <XPos>_<YPos> represent the global (x,y) (in m) coordinates of an anchor point which is located at the z-position of the seabed.
The fourth entry is the length of the cable (in m). The fifth entry is the ID number of the cable element defined in
MOORELEMENTS. The sixth entry is the ID number of the hydrodynamic coefficient group defined inHYDROMEMBERCOEFF. The seventh entry specifies if the cable is buoyant (= 1) or not (= 0). The eighth entry specifies the ID number of the marine growth element used for this cable (seeMARINEGROWTH). The ninth entry is the number of discretization nodes used to discretize the cable and the tenth entry is the name of the cable element.MOORMEMBERS ID CONN_1 CONN_2 Len.[m] MoorID HyCoID IsBuoy MaGrID ElmDsc Name 1 FLT_-40.868_0.0_-14.0 GRD_-837.6_0 835.5 1 1 1 0 30 Mooring1 2 FLT_20.434_35.393_-14.0 GRD_418.8_725.4 835.5 1 1 1 0 30 Mooring2 3 FLT_20.434_-35.393_-14.0 GRD_418.8_-725.4 835.5 1 1 1 0 30 Mooring3
Mooring Element Lineloads
MOORLOADSis a table that allows to add buoyancy loads or additional weight to a cable member defined in the
MOORMEMBERStable. The first column is the cable member ID, the second column the starting position of the load, the third column is the end position of the load and the fourth column the load itself, defined in [N/m]. The loads only act along the global Z-Axis. A positive load is pointing upwards and a negative load is pointing downwards.
Fig. 102 A buoyancy load acting on a power cable.
MOORLOADS ID Start[m] End [m] Force [N/m] 1 150 180 2000 3 520 550 2000
Nonlinear Spring and Damper Constraints
NLSPRINGDAMPERSis a table that defines one or more non-linear spring-damper systems for connecting the substructure to the ground, or for the interconnection of two joints in the constraints table. A usual application would be to model the soil dynamics using nonlinear (p-y curves) springs. Another application would be to define compliant connections between substructure members or joints. Furthermore, in the
SUBCONSTRAINTStable the nonlinear springs, or dampers may be assigned to constrain any or all degrees of freedom of choice.Each row in the
NLSPRINGDAMPERStable represents a spring-damper system and has 2N + 2 entries, where N is the number of points on the definition table of the non-linear spring/damper. The first entry represents the ID number of the system (to be used in theSUBCONSTRAINTStable). The second entry defines the type of connection that is being modeled. There are two options: ‘spring’ and ‘damp’. This affects the way the coefficients in the following entries are interpreted.If ‘spring’ is selected, then QBlade expects the definition table to consists of displacement or rotation (in m or rad) and stiffness (in N/m or Nm/rad) entries.
If ‘damp’ is selected, then QBlade expects the definition table to consist of velocity (in m/s or rad/s) and damping (in N(m/s) or Nm/(rad/s)) entries.
When a spring or damper is used to constrain two joints its nonlinear definition always acts as a rotational spring or damper along the rotational DOF’s and as a translational spring or damper along the translational DOF’s. Thus, usually a spring is either defined as a rotational spring and then assigned to constrain rotational DOF’s or as a translational spring to constrain translational DOF’s.
The following 2N entries represent the additional lookup table entries for the non-linear spring/damper system. The order is \(x_1/v_1\), \(K/D(x_1/v_1)\); \(x_2/v_2\), \(K/D(x_2/v_2)\) and so on. Each spring/damper force displacement relationship is assumed to go through the origin at \(x_0/v_0 = 0\) and \(K/D(x_0/v_0) = 0\).
NLSPRINGDAMPERS ElemID Type Coefficient & Displacement/Velocity Sets (for NL springs, dampers) 1 spring 1.000 1.160E+06 2 spring 1.000 9.000E+06 3 spring 1.000 2.090E+07 4 spring 1.000 3.560E+07
Optionally, the force/displacement or force/velocity relationship can also be specified though a Nonlinear Data Tables. In this case, the data table ID, preceded by a hashtag (#) replaces the 2N lookup table entries in the NLSPRINGDAMPERS table.
NLSPRINGDAMPERS ElemID Type Coefficient & Displacement/Velocity Sets (for NL springs, dampers) 1 spring #NLDATA1 2 spring #NLDATA2 3 spring #NLDATA3 4 spring #NLDATA4
SPRINGDAMPKis an optional proportionality constant to add a damping value to the spring elements. If this keyword is used, then all of the spring elements defined in
NLSPRINGDAMPERSare treated as spring-damping systems. The additional damping coefficients are calculated using the following approach: \(D_i\) =SPRINGDAMPK\(\cdot K_i\). This keyword does not affect the ‘damp’ elements defined inNLSPRINGDAMPERS.
Changes to in NLSPRINGDAMPERS QBlade 2.0.7
From QBlade 2.0.7, the NLSPINGDAMPERS table has been slightly modified. In Previous versions the coefficient for \(x=0\) was specified by the user in column 3 of the table. In the new table format column 3 is now removed and the force/dsplacement, or force/velocity, relationship is always going through the origin at \(x_0/v_0 = 0\) and \(K/D(x_0/v_0) = 0\). This implies that in order to continue to use a NLSPINGDAMPERS table from a QBlade version prior 2.0.7 the 3rd column has to be removed. The old table format is shown below for reference:
NLSPRINGDAMPERS ElemID Type Coefficient (for x = 0) Coefficient & Displacement/Velocity Sets (for NL springs, dampers) 1 spring 0.000E+00 1.000 1.160E+06 2 spring 0.000E+00 1.000 9.000E+06 3 spring 0.000E+00 1.000 2.090E+07 4 spring 0.000E+00 1.000 3.560E+07
Nonlinear Data Tables
NLDATA<X>is a table that defines a nonlinear data distribution. The type as which the data is interpreted, depends on where the data table is used. Currently, nonlinear data tables can be used to define nonlinear stress/strain relationships for the axial (EA) stiffness of mooring lines (see Mooring Elements and Ground-Constraints) or to define the force/displacement relationships of spring (or damper) elements (see Nonlinear Spring and Damper Constraints).
Each nonlinear data table needs a unique identifier, such as NLDATA1, NLDATA2, NLDATA3, up to NLDATA100. The first column in each data table represents the x-value (such as displacement, velocity or strain) and the second column represents the y-axis (such as force or stress. A nonlinear data table is assumed to always go through the origin at x=0 and y=0 and it is not requires to input the 0,0 entry into the table. Other values should be input is ascending order of x.
NLDATA1 x_val y_val 0.001 1.0e9 0.002 2.5e9 0.005 5.0e9
Hydrodynamic Modeling of a Substructure
Two options are available in QBlade to model the hydrodynamic forces acting on an offshore substructure: The Morison equation and the linear potential flow theory.
When modeling the hydrodynamics using the Morison equation the user can distribute hydrodynamic coefficients that act in the normal direction of a substructure member. Furthermore, coefficients can be added to substructure joints so that the Morison equation is applied to the end faces of members. Thus, when modeling the hydrodynamics using the Morison equation the hydrodynamic forces are distributed over the substructure model.
The second option is to model the hydrodynamic forces using a linear potential flow theory generated database. At present, QBlade can interpret hydrodynamic input data in the WAMIT and NEMOH formats. When modeling the hydrodynamic with potential flow theory the lumped hydrodynamic forces are always applied at the hydrodynamic reference point (REF_HYDRO_POS_<X>). So in most cases a substructure modeled with potential flow theory should be modeled using rigid elements.
QBlade allows the user to combine elements from the Linear Potential Flow Theory and Morison Equation hydrodynamic models freely. The user should be careful when setting up the substructure in QBlade so that the model remains consistent.
A typical mix between the Morison equation and potential flow theory is to have all hydrodynamic forces be evaluated by a linear potential flow database and use the Morison equation to compute the hydrodynamic drag force, which is missing from the potential flow theory due to its assumption of inviscid flow.
Morison Equation (Strip Theory) Modelling
Hydrodynamic coefficients can be assigned to substructure members and joints. Hydrodynamic member coefficients (HYDROMEMBERCOEFF) act in the direction normal to the center-line of the substructure member. Hydrodynamic joint coefficients act in the direction normal to the end face of a member (see Fig. 103).
Fig. 103 Hydrodynamic coefficients acting on a substructure member.
HYDROMEMBERCOEFFdefines a table that contains the hydrodynamic normal coefficients that are used for the cylindrical members of the substructure. Each row contains one group of coefficients that can be used by one or more cylindrical members. The table contains a minimum of five entries, with an optional sixth entry. These are the ID number of the group, the normal drag coefficient, the normal added mass coefficient, the normal dynamic pressure coefficient, a flag that enables the MacCamy-Fuchs correction (MCFC) and an optional entry for a longitudinal drag coefficient CdL, which is only active when the HYDROMEMBERCOEFF is assigned to a mooring line.
HYDROMEMBERCOEFF CoeffID CdN CaN CpN MCFC CdL (optional) 1 2.0 0.8 1.0 1 0 2 0.63 0.0 0.0 1 0 3 0.56 0.0 0.0 0 0 4 0.61 0.0 0.0 0 0 5 0.68 0.0 0.0 0 0
HYDROMEMBERCOEFF_RECTdefines a table that contains the hydrodynamic normal coefficients that are used for the rectangular members of the substructure. Each row contains one group of coefficients that can be used by one or more rectangular members. The table contains eight entries. These are the ID number of the group, the normal drag coefficient along the members x-direction, the normal added mass coefficient along the members x-direction, the normal dynamic pressure coefficient along the members x-direction, the normal drag coefficient along the members y-direction, the normal added mass coefficient along the members y-direction, the normal dynamic pressure coefficient along the members y-direction and a flag that enables the MacCamy-Fuchs correction (MCFC).
HYDROMEMBERCOEFF_RECT CoeffID CdNx CaNx CpNx CdNy CaNy CpNy MCFC 1 2.0 0.8 1.0 2.0 0.8 1.0 1 2 0.63 0.0 0.0 0.63 0.0 0.0 1 3 0.56 0.0 0.0 0.56 0.0 0.0 0 4 0.61 0.0 0.0 0.61 0.0 0.0 0 5 0.68 0.0 0.0 0.68 0.0 0.0 0
HYDROJOINTCOEFFis a table that defines hydrodynamic axial coefficients that can be placed at specific joints (defined by their ID number) of the substructure that are located at the ends of cylindrical members. QBlade assumes a spherical end of the element when calculating the hydrodynamic axial Morison forces (e.g. \(F_a^{ax} = \frac{2\pi}{3}(\frac{d}{2})^3\cdot C_a^{ax}\)). The table contains the axial drag, added mass and dynamic pressure axial coefficients and is structured as shown below. The hydrodynamic reference volume for a member end face is assumed to be a semi-spheroid with the member diameter (the equivalent diameter for rectangular members). If two substructure members are connected to the same node the member face reference areas and reference volumes are subtracted from another so that just the area and reference volumes that is exposed to the fluid is considered when evaluating the Morison forces.
HYDROJOINTCOEFF CoeffID JointID CdA CaA CpA 1 9 4.8 0.0 0.0 2 10 4.8 0.0 0.0 3 11 4.8 0.0 0.0 4 1 0.0 0.0 0.0 5 3 0.0 0.0 0.0 6 5 0.0 0.0 0.0 7 7 0.0 0.0 0.0
In addition to the basic functionalities of the HYDROJOINTCOEFF table additional entries in the table can activate a more advanced modeling of member end Morison forces, to improve the prediction of low frequency responses of floating structures. Two advanced modeling techniques (see Wang et al. 1 and Behrens de Luna et al. 2), named One-Sided Morison Drag and High Pass Filtered Morison Drag can be activated by populating additional columns in the HYDROJOINTCOEFF table.
- One-Sided Morison Drag
The One Sided Morison Drag method modifies the axial drag model used in QBlade by applying the axial drag force solely when the flow normal to the surface is in the direction of the end faces normal vector (\(v \cdot n > 0\)), representing a state of flow separation. This modification is suggested because the potential-flow model already includes the increased stagnation pressure when the flow is the opposite direction of the end face normal vector. This modification more accurately reflects the physical conditions where viscous effects cause a pressure reduction on the flow-facing side, which diverges from the ideal pressure recovery assumed in potential-flow models. This modification modifies the evaluation of the end face drag force in the following way:
\(F_D = \frac{1}{2} C_{d_A} \left| v \cdot n \right| \max(v \cdot n, 0)\)
The one-sided Morison drag evaluation is activated by setting the sixth column of the HYDROJOINTCOEFF table to 1. Setting this column to 0 deactivates this correction:
HYDROJOINTCOEFF CoeffID JointID CdA CaA CpA OSD 1 9 4.8 0.0 0.0 1 2 10 4.8 0.0 0.0 1 3 11 4.8 0.0 0.0 0 4 1 0.0 0.0 0.0 0 5 3 0.0 0.0 0.0 0 6 5 0.0 0.0 0.0 1 7 7 0.0 0.0 0.0 1
- High-Pass Filtered Morison Drag
The High-Pass Filtered Morison Drag modification is another method to improve, or fine-tune the low-frequency response of a floating structure. To optimize the simulation of freely floating structures in irregular waves, QBlade incorporates an optional high-pass velocity filter for evaluation of the axial drag force on heave plates. This model allows to implement varying drag force across different wave frequency bands. This method uses a first-order high pass filter, which is applied to the relative normal velocity at a members end-face. After Wang et al. 1, the filtered normal velocity (\(\tilde{v}_{i}\)) in discrete time is evaluated as:
\(\tilde{v}_{i} = C \cdot \tilde{v}_{i-1} + C \cdot (v_{i} - v_{i-1})\),
with
\(C = exp(-2\pi f_c \Delta t)\).
The applied filtered drag force \(F_{D,f}\) is then evaluated as:
\(F_{D,f}=\alpha F_D + (1-\alpha) \tilde{F}_D\),
where \(\alpha\) is a scaling factor between 0 and 1, \(\tilde{F}_D\) is the drag force evaluated with the filtered normal velocity \(\tilde{v}_i\) and \(F_D\) the drag force evaluated with the unfiltered normal velocity \(v_i\).
The High-Pass Filtered Morison Drag evaluation is activated by setting the seventh and eight column of the HYDROJOINTCOEFF tables to the cut-off frequency \(f_c\) and the scaling factor \(\alpha\):
HYDROJOINTCOEFF CoeffID JointID CdA CaA CpA OSD f_c alpha 1 9 4.8 0.0 0.0 1 0.07 0.5 2 10 4.8 0.0 0.0 1 0.07 0.5 3 11 4.8 0.0 0.0 0 0.07 0.5 4 1 0.0 0.0 0.0 0 0.07 0.5 5 3 0.0 0.0 0.0 0 0.07 0.5 6 5 0.0 0.0 0.0 1 0.07 0.5 7 7 0.0 0.0 0.0 1 0.07 0.5
An application of this filtered drag evaluation can be found in the works of Wang et al. 1 and Behrens de Luna et al. 2.
WAVEKINEVAL_MORis an optional flag that controls how the local wave kinematics are used to calculate the Morison forces (see Modeling Considerations for Morison Elements). The available options are:
0: local evaluation of wave kinematics (this is the default value if not specified)
1: evaluation at the fixed, undisplaced/unrotated initial reference position
2: evaluation at a lagged position (controlled by
WAVEKINTAU).
WAVEKINEVAL_POTis an optional flag that control how the local wave kinematics are used to calculate the diffraction and second order forces at potential flow bodies. The available options are:
0: local evaluation of wave kinematics
1: evaluation at the fixed, undisplaced/unrotated initial reference position (this is the default value if not specified)
2: evaluation at a lagged position (controlled by
WAVEKINTAU).
WAVEKINTAUis an optional time constant for the first order low-pass filter used to determine lagged position of the Morison/Potential Flow elements (when
WAVEKINEVAL_MORorWAVEKINEVAL_POTis set to 2). The default value is 30s.
Linear Potential Flow Modelling
QBlade supports any number of linear potential flow bodies as part of a substructure definition, where each potential flow body is related to a transition piece TP_INTERFACE_POS_<X>. In order to include multiple bodies, each body has to have its own set of keywords. With the exception of the first body, additional bodies are defined by adding an underscore and a number after the keyword. So, for example, if a substructure has two bodies that use the linear potential flow theory, the second body would be defined by adding a second transition piece point TP_INTERFACE_POS_2 with its corresponding hydrodynamic reference point REF_HYDRO_POS_2, the inertia point REF_COG_POS_2, a mass matrix denoted as SUB_MASS_2 and so on (see the section: Lumped Mass, Inertia and Hydrodynamic Forces).
The keywords that are used to read in the linear potential flow databases for radiation, excitation, difference and sum frequency loads, or hydrodynamic stiffness are detailed below:
Defining a Potential Flow Body
REF_HYDRO_POS_<X>defines the position at which the potential flow loads (hydro stiffness, radiation, excitation, sum frequency and difference frequency) are applied. This is also the position at which wave elevations, positions are accelerations are obtained. As detailed in the section Lumped Mass, Inertia and Hydrodynamic Forces, this reference position is automatically constrained with the transition piece`:code:TP_INTERFACE_<X>.
REF_HYDRO_POS_1 X[m] Y[m] Z[m] 0 0 -10.00
POT_HST_FILE_<X>defines the file where the hydrodynamic stiffness matrix is stored from a WAMIT calculation. If this keyword is used then the hydrodynamic stiffness matrix
SUB_HYDROSTIFFNESS_<X>is populated with data from the file, overwriting any user-defined stiffness matrix.POT_RAD_FILE_<X>defines the file where the radiation coefficients for the linear potential flow model are located. The file ending must be included. This determines the format of the file. QBlade currently supports radiation files in the WAMIT, NEMOH and BEMUse formats.
POT_EXC_FILE_<X>defines the file where the excitation coefficients for the linear potential flow model are located. The file ending must be included. This determines the format of the file. QBlade currently supports excitation files in the WAMIT, NEMOH and BEMUse formats.
POT_DIFF_FILE_<X>defines the file where the second-order difference-frequency wave force coefficients are located. The file ending must be included. This determines the format of the file. QBlade currently supports difference-frequency files only in the WAMIT format.
DIFF_CUTOFF_L_<X>Specifies the low cut-off frequency for the difference QTFs. Intended to speed up the QTF evaluations.
DIFF_CUTOFF_H_<X>Specifies the high cut-off frequency for the difference QTFs. Intended to speed up the QTF evaluations.
DIFF_INACTIVE_DOF_<X>this keyword can be used to define a list of DOFs for which the difference QTF will not be evaluated. Intended to speed up the QTF evaluations.
Listing 70 : The DIFF_INACTIVE_DOF keyword to deactivate the DOFs 1 and 4 from the difference QTF evaluation1 4 DIFF_INACTIVE_DOFPOT_SUM_FILE_<X>defines the file where the second-order sum-frequency wave force coefficients are located. The file ending must be included. This determines the format of the file. QBlade currently supports sum-frequency files only in the WAMIT format.
SUM_CUTOFF_L_<X>Specifies the low cut-off frequency for the sum QTFs. Intended to speed up the QTF evaluations.
SUM_CUTOFF_H_<X>Specifies the high cut-off frequency for the sum QTFs. Intended to speed up the QTF evaluations.
SUM_INACTIVE_DOF_<X>this keyword can be used to define a list of DOFs for which the sum QTF will not be evaluated. Intended to speed up the QTF evaluations.
Listing 71 : The SUM_INACTIVE_DOF keyword to deactivate the DOFs 1 and 4 from the sum QTF evaluation1 4 SUM_INACTIVE_DOF
NOBODY > 1 Feature
Fig. 104 NBODY=3, 6 DOF’s are assigned to each of the three cylinders.
QBlade includes the capability to include multiple bodies (NBODY>1 feature of WAMIT) that interact hydrodynamically and mechanically. Each body can oscillate independently with up to six degrees of freedom. This extension increases the maximum number of degrees of freedom from 6 for a single body to 6N for N bodies.
POT_NBODY_<X>specifies the number of hydrodynamic bodies. Each body is associated with 6 DOF’s. By default, NBODY is equal to 1. To use NBODY>1, the potential flow files must contain the data for the additional DOF. If NBODY=3, then the data for 18 DOF’s is required.
3 POT_NBODY
By default, the hydrodynamic loads are applied at the REF_HYDRO_POS. When using multiple hydrodynamic bodies, such \(N = 3\), the hydrodynamic forces should be applied to different parts of the substructure. For this purpose the hydrodynamic loads of a particluar body can be assigned to a substructure joint:
POT_NBODY_NODES_<X>specifies the list of joint id’s at which the hydrodynamic loads are applied. This keyword can also be used to apply the hydrodynamic loads of a single NBODY to a custom substructure joint (by default the hydrodynamic loads are applied to the
REF_HYDRO_POS_<X>).Listing 73 : The POT_NBODY_NODES keyword to assign the hydrodynamic loads of three bodies to joint id’s 2, 4 and 62 4 6 POT_NBODY_NODES
Common Potential Flow Keywords
USE_RADIATIONis a flag that enables the calculation of the radiation loads on all potential flow bodies. (true or false)
USE_RAD_ADDMASSwhen this flag is set to true the hydrodynamic added mass matrix is automatically extracted from the potential flow radiation file (if such a file is defined). Using this flag will overwrite the user defined SUB_HYDROADDEDMASS definition. This is an optional flag and the default value is false.
DELTA_FREQ_RADis the discretization of the frequencies used for the calculation of the radiation forces (in Hz).
TRUNC_TIME_RADis the truncation time for the wave radiation kernel calculations (in s).
USE_EXCITATIONis a flag that enables the calculation of the excitation loads on all potential flow bodies. (true or false)
DELTA_FREQ_EXCis the discretization of the frequencies used for the calculation of the excitation forces (in Hz).
DELTA_DIR_EXCis the discretization of the directions used for the calculation of multi-directional excitation forces (in degree). The default value is 0.5 degree.
TRUNC_TIME_EXCis the truncation time for the wave excitation kernel calculations (in s).
DIFF_EVAL_TYPEis a flag that controls how the 2nd order difference-frequency loads on all potential flow bodies are evaluated:
0 - no difference forces are evaluated
1 - difference-frequency loads are evaluated explicitly (full field QTF, high computational demand)
2 - the computationally efficient Newman approximation is used for the calculation of difference frequency forces
3 - only the mean drift forces are considered
USE_SUM_FREQSis a flag that enables the (full field QTF) calculation of the sum-frequency loads on all potential flow bodies. (true or false)
UNITLENGTH_WAMITEnables to specify a WAMIT unit length different than 1.0, if not specified 1.0 is the default value.
Sensor Locations and Definitions
The locations at which data is recorded for the substructure is also controlled by keywords. QBlade can generate output for the members defined in the SUBMEMBERS and in the MOORMEMBERS tables.
The logic of defining an output is as follows:
SUB_<MemID>_<RelPos>is the keyword used for setting an output of a member from the the
SUBMEMBERStable with the ID number = <MemID> and a relative postion = <RelPos>. The relative position goes from 0 (= the position of Joint1ID) to 1 (= the postion of Joint2ID). When an output sensor is placed at a member hydrodynamic loads are displayed in the Hydrodynamic Time Graph and internal structural loads are displayed in the Structural Time Graph.MOO_<MMemID>_<RelPos>is the keyword used for placing a sensor on the cable member with the ID number = <MMemID> and a relative postion = <RelPos>. The relative position goes from 0 (= the position of Conn1) to 1 (= the postion of Conn2). When an output sensor is placed at a mooring line hydrodynamic loads are displayed in the Hydrodynamic Time Graph and internal structural loads are displayed in the Structural Time Graph.
CST_<CstID>is the keyword used for placing a sensor at the constraint from the
SUBCONSTRAINTStable with the ID number = <CstID>. The internal constraint loads (force and torque) in the constraint coordinate system and in the global coordinate system are then displayed in the Structural Time Graph. The constraint coordinate system originates at the position of JointID1 and has the axes orientation of the joint (JntCon), transition piece (TpCon) or ground (GrdCon) that JointID1 is connected to (see Substructure Constraints).JNT_<JntID>is the keyword used for placing a sensor on a joint from the
SUBJOINTStable with the ID number = <JntID>. The position and rotation of the joint in absolute coordinates are displayed in the Structural Time Graph.
Exemplary Substructure File
An exemplary substructure file for the OC4 Semi-Submersible floater is shown below. This floater is modeled with rigid cylindrical elements. The hydrodynamics are evaluated based on linear potential flow theory. The buoyancy is evaluated explicitly from the members, who also contribute to the total hydrodynamic forces with Morison based drag forces. In this example the members are defined as mass-less (0.0001kg/m) and the total mass is assigned through the 6x6 mass matrix. The total floater hydrodynamic added mass is also included via a 6x6 matrix.
200 WATERDEPTH //design depth
1025 WATERDENSITY // design density, used for flooded member mass calcs
true ISFLOATING //if the structure is fixed the joint coordinates are assigned in a coordinate system with O(0,0,0) at the mudline, for floaters O(0,0,0) is at the MSL and marks the floaters's NP
100 ADVANCEDBUOYANCY //using an advanced discretization technique (N must be a square int number) to calculate buoyancy of partially submerged members, especially usefull if "lying" cylinders are used to generate the draft
0 WAVEKINEVALTYPE // 0 - local evaluation, 1 - eval at fixed ref pos, 2 - eval at lagged position
30 WAVEKINTAU // time constant for the lagged waveKin position evaluation
// potential flow model options, specify RADiation and EXCitation files separately (only RAD if BEMuse), don't forget the file endings, as this identifies the format!
true STATICBUOYANCY // static buoyancy, based on the MSL should be used when using Morison member buoyancy combined with potential flow diffraction forces
radiation.1 POT_RAD_FILE
true USE_RADIATION
0.05 DELTA_FREQ_RAD
60.0 TRUNC_TIME_RAD
excitation.3 POT_EXC_FILE
true USE_EXCITATION
0.05 DELTA_FREQ_EXC
0.50 DELTA_DIR_EXC
60.0 TRUNC_TIME_EXC
difference.12d POT_DIFF_FILE
2 DIFF_EVAL_TYPE (0-none,1-explicit,2-newman,3-meandrift)
sum.12s POT_SUM_FILE
false USE_SUM_FREQS
JOINTOFFSET // these global offsets are only applied to joints (not the TP or cog position)
XPOS YPOS ZPOS
0 0 0
MARINEGROWTH
ID Thickn Density
1 0.1 1100
//all following positions are defined in (x,y,z) [m]: for floaters: from the neutral point, which is located at MSL (0,0,0); for bottom fixed substructures: defined from seabed
TP_INTERFACE_POS //the interface position between substructure and tower or RNA
X[m] Y[m] Z[m]
0 0 10
REF_COG_POS //cog reference position, at which the mass matrix is evaluated
X[m] Y[m] Z[m]
0 0 -13.46
REF_HYDRO_POS //reference point for the evaluation of linearized hydrodynamic stiffness, damping, quaddamping, addedmass matrices and the constant force vector
X[m] Y[m] Z[m]
0 0 0
SUB_MASS //the floater mass matrix is defined at the REF_COG_POS
1.34730e+07 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00
0.00000e+00 1.34730e+07 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00
0.00000e+00 0.00000e+00 1.34730e+07 0.00000e+00 0.00000e+00 0.00000e+00
0.00000e+00 0.00000e+00 0.00000e+00 6.82700e+09 0.00000e+00 0.00000e+00
0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 6.82700e+09 0.00000e+00
0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 0.00000e+00 1.22600e+10
SUB_HYDROADDEDMASS //the hydrodynamic added mass is defined and applied at the REF_HYDRO_POS
6.3199481E+06 0 -5.4452131E+02 0 -8.4184511E+07 0
0 6.3199122E+06 0 8.4184511E+07 0 2.0423668E+02
-1.8215736E+02 0 1.4673705E+07 0 1.7654785E+03 0
0 8.4181805E+07 0 7.1983946E+09 0 1.0104395E+04
-8.4190835E+07 0 -8.7227367E+04 0 7.1983290E+09 0
0 6.2468769E+03 0 -3.6169083E+04 0 4.7423470E+09
SUBJOINTS //defined either from MSL (if isFLoating) or from seabed using the designDepth variable (if !isFLoating)
JointID JointX JointY JointZ
1 0.00000 0.00000 -20.00000
2 0.00000 0.00000 10.00000
3 14.43376 25.00000 -14.00000
4 14.43376 25.00000 12.00000
5 -28.86751 0.00000 -14.00000
6 -28.86751 0.00000 12.00000
7 14.43376 -25.00000 -14.00000
8 14.43376 -25.00000 12.00000
9 14.43375 25.00000 -20.00000
10 -28.86750 0.00000 -20.00000
11 14.43375 -25.00000 -20.00000
12 9.23760 22.00000 10.00000
13 -23.67130 3.00000 10.00000
14 -23.67130 -3.00000 10.00000
15 9.23760 -22.00000 10.00000
16 14.43375 -19.00000 10.00000
17 14.43375 19.00000 10.00000
18 4.04145 19.00000 -17.00000
19 -18.47520 6.00000 -17.00000
20 -18.47520 -6.00000 -17.00000
21 4.04145 -19.00000 -17.00000
22 14.43375 -13.00000 -17.00000
23 14.43375 13.00000 -17.00000
24 1.62500 2.81500 10.00000
25 11.43376 19.80385 10.00000
26 -3.25000 0.00000 10.00000
27 -22.87000 0.00000 10.00000
28 1.62500 -2.81500 10.00000
29 11.43376 -19.80385 10.00000
30 1.62500 2.81500 -17.00000
31 8.43376 14.60770 -17.00000
32 -3.25000 0.00000 -17.00000
33 -16.87000 0.00000 -17.00000
34 1.62500 -2.81500 -17.00000
35 8.43376 -14.60770 -17.00000
36 1.62500 2.81500 -16.20000
37 11.43376 19.80385 9.13000
38 -3.25000 0.00000 -16.20000
39 -22.87000 0.00000 9.13000
40 1.62500 -2.81500 -16.20000
41 11.43376 -19.80385 9.13000
42 14.43376 25.00000 -19.94000
43 -28.86751 0.00000 -19.94000
44 14.43376 -25.00000 -19.94000
45 14.43376 25.00000 -6.170000
46 -28.86751 0.00000 -6.170000
47 14.43376 -25.00000 -6.170000
48 14.43376 25.00000 -14.89000
49 -28.86751 0.00000 -14.89000
50 14.43376 -25.00000 -14.89000
1.00 STIFFTUNER
1.00 MASSTUNER
1.00 BUOYANCYTUNER
SUBELEMENTSRIGID
ElemID BMASSD DIAMETER
1 1 6.5
2 1 12
3 1 24
4 1 1.6
// Heave hydro forces of base columns
HYDROJOINTCOEFF //hydrodynamic coefficients to be assigned to joints, acting on connected members faces in axial direction, occulation of interconnected members is automatically accounted for
CoeffID JointID CdA CaA CpA
1 9 4.8 0.0 0.0 // Bottom_Base_Column_1
2 10 4.8 0.0 0.0 // Bottom_Base_Column_2
3 11 4.8 0.0 0.0 // Bottom_Base_Column_3
4 1 0.0 0.0 0.0 // Main_Column
5 3 0.0 0.0 0.0 // Top_Base_Column_1
6 5 0.0 0.0 0.0 // Top_Base_Column_2
7 7 0.0 0.0 0.0 // Top_Base_Column_3
HYDROMEMBERCOEFF //hydrodynamic coefficients to be assigned to rigid or elastic cylindrical members, defined for the normal-to-axis direction of the cylinders
CoeffID CdN CaN CpN MCFC
1 2.0 0.8 1.0 0 // Mooring_Lines
2 0.63 0.0 0.0 0 // D_1.6m
3 0.56 0.0 0.0 0 // D_6.5m
4 0.61 0.0 0.0 0 // D_12m
5 0.68 0.0 0.0 0 // D_24m
SUBCONSTRAINTS //in this version of the OC4 the member nodes are connected directly through the constraints
ID JntID Jnt2ID TrP Fixed Spring DoF_X DoF_Y DoF_Z DoF_rX DoF_rY DoF_rZ
1 2 0 1 0 0 1 1 1 1 1 1
2 24 0 1 0 0 1 1 1 1 1 1
3 26 0 1 0 0 1 1 1 1 1 1
4 28 0 1 0 0 1 1 1 1 1 1
8 30 0 1 0 0 1 1 1 1 1 1
9 32 0 1 0 0 1 1 1 1 1 1
10 34 0 1 0 0 1 1 1 1 1 1
14 12 0 1 0 0 1 1 1 1 1 1
15 14 0 1 0 0 1 1 1 1 1 1
16 16 0 1 0 0 1 1 1 1 1 1
20 18 0 1 0 0 1 1 1 1 1 1
21 20 0 1 0 0 1 1 1 1 1 1
22 22 0 1 0 0 1 1 1 1 1 1
26 36 0 1 0 0 1 1 1 1 1 1
27 38 0 1 0 0 1 1 1 1 1 1
28 40 0 1 0 0 1 1 1 1 1 1
29 9 0 1 0 0 1 1 1 1 1 1
30 10 0 1 0 0 1 1 1 1 1 1
31 11 0 1 0 0 1 1 1 1 1 1
SUBMEMBERS
MemID Jnt1ID Jnt2ID ElmID ElmRot HyCoID IsBuoy MaGrID FldArea ElmDsc Name (optional)
1 1 2 1 0 3 1 0 0 2 Main_Column
2 45 4 2 0 4 1 0 0 2 Upper_Column_1
3 46 6 2 0 4 1 0 0 2 Upper_Column_2
4 47 8 2 0 4 1 0 0 2 Upper_Column_3
29 3 45 2 0 4 1 0 0 2 Upper_Column_flooded_1
30 5 46 2 0 4 1 0 0 2 Upper_Column_flooded_2
31 7 47 2 0 4 1 0 0 2 Upper_Column_flooded_3
5 48 3 3 0 5 1 0 0 2 Base_Column_1
6 49 5 3 0 5 1 0 0 2 Base_Column_2
7 50 7 3 0 5 1 0 0 2 Base_Column_3
26 42 48 3 0 5 1 0 0 2 Base_column_flooded_1
27 43 49 3 0 5 1 0 0 2 Base_column_flooded_2
28 44 50 3 0 5 1 0 0 2 Base_column_flooded_3
23 9 42 3 0 5 1 0 0 2 Base_column_cap_1
24 10 43 3 0 5 1 0 0 2 Base_column_cap_2
25 11 44 3 0 5 1 0 0 2 Base_column_cap_3
8 12 13 4 0 2 1 0 0 10 Delta_Pontoon_Upper_1
9 14 15 4 0 2 1 0 0 10 Delta_Pontoon_Upper_2
10 16 17 4 0 2 1 0 0 10 Delta_Pontoon_Upper_3
11 18 19 4 0 2 1 0 0 10 Delta_Pontoon_Lower_1
12 20 21 4 0 2 1 0 0 10 Delta_Pontoon_Lower_2
13 22 23 4 0 2 1 0 0 10 Delta_Pontoon_Lower_3
14 24 25 4 0 2 1 0 0 10 Y_Pontoon_Upper_1
15 26 27 4 0 2 1 0 0 10 Y_Pontoon_Upper_2
16 28 29 4 0 2 1 0 0 10 Y_Pontoon_Upper_3
17 30 31 4 0 2 1 0 0 10 Y_Pontoon_Lower_1
18 32 33 4 0 2 1 0 0 10 Y_Pontoon_Lower_2
19 34 35 4 0 2 1 0 0 10 Y_Pontoon_Lower_3
20 36 37 4 0 2 1 0 0 10 Cross_Brace_1
21 38 39 4 0 2 1 0 0 10 Cross_Brace_2
22 40 41 4 0 2 1 0 0 10 Cross_Brace_3
MOORELEMENTS
MooID MASS_[kg/m] EIy_[N.m^2] EA_[N] DAMP_[-] DIA_[m]
1 1.086306E+02 6.148892E+08 7.536117E+08 0.000 0.077
MOORMEMBERS
ID CONN_1 CONN_2 Len.[m] MoorID HyCoID IsBuoy MaGrID ElmDsc Name
1 FLT_-40.868_0.0_-14.0 GRD_-837.6_0 835.5 1 1 1 0 30 Mooring1
2 FLT_20.434_35.393_-14.0 GRD_418.8_725.4 835.5 1 1 1 0 30 Mooring2
3 FLT_20.434_-35.393_-14.0 GRD_418.8_-725.4 835.5 1 1 1 0 30 Mooring3
TRANSITIONBLOCK // just for visualization
WIDTH LENGTH HEIGHT
0 0 0
TRANSITIONCYLINDER // just for visualization
HEIGHT DIAMETER
0.5 6.5
RGBCOLOR
R G B
255 200 15
MOO_1_0.0
MOO_1_1.0
MOO_2_0.0
MOO_2_1.0
MOO_3_0.0
MOO_3_1.0
Substructure File Format Changes from QBlade v2.06b
The section describes the changes that have been made to different parts of the substructure file format from QBlade 2.06b onward. In most cases compatibility is still ensured and older formats are automatically detected, however you are strongly advised to update the substructure files that you are working with to this new format. All in all the changes are very little and can be implemented in a few minutes per file. See a summary of the changes below:
SUBMEMBERS Table
In QBlade versions prior to 2.06b the SUBMEMBERS table looked like this:
SUBMEMBERS MemID Jnt1ID Jnt2ID ElmID RElmID HyCoID IsBuoy MaGrID FldArea ElmDsc Name (optional) 1 1 2 0 1 3 1 0 0 2 Main_Column 2 45 4 0 2 4 1 0 0 2 Upper_Column_1 3 46 6 0 2 4 1 0 0 2 Upper_Column_2 4 47 8 0 2 4 1 0 0 2 Upper_Column_3 29 3 45 0 2 4 1 0 0 2 Upper_Column_flooded_1 30 5 46 0 2 4 1 0 0 2 Upper_Column_flooded_2 31 7 47 0 2 4 1 0 0 2 Upper_Column_flooded_3 5 48 3 0 3 5 1 0 0 2 Base_Column_1 6 49 5 0 3 5 1 0 0 2 Base_Column_2 7 50 7 0 3 5 1 0 0 2 Base_Column_3 26 42 48 0 3 5 1 0 0 2 Base_column_flooded_1 27 43 49 0 3 5 1 0 0 2 Base_column_flooded_2 28 44 50 0 3 5 1 0 0 2 Base_column_flooded_3 23 9 42 0 3 5 1 0 0 2 Base_column_cap_1 24 10 43 0 3 5 1 0 0 2 Base_column_cap_2 25 11 44 0 3 5 1 0 0 2 Base_column_cap_3
Pay attention to the columns 4 and 5. In the old version column 4 specified a flexible element ID ElmID and column 5 a rigid element ID RElmID. In QBlade v2.06b we have added more available element types (rectangular, rectangular rigid) that can be used for the construction of the substructure.
Because we did not want to add an extra column for each element type to the SUBMEMBERS table all element types can now be assigned in column 4. However, this change also requires that an element ID is unique across all different element types. Whereas previously you could have a rigid element with ID 1 and a flexible element with ID 1 this is not possible anymore since each element requires a unique ID.
Furthermore, for rectangular elements their orientation becomes important (vs cylindrical elements which are unidirectional), so we are using column 5 of the SUBMEMBERS table to define the rotation of an element (in degrees).
To sum up the changes:
Column 4 is now used to define all different element types
All elements (across different types) need a unique element ID
Column 5 is now used to assign the element rotation
Based on this we can easily convert the old SUBMEMBERS table to the new format that is shown below:
SUBMEMBERS MemID Jnt1ID Jnt2ID ElmID ElmRot HyCoID IsBuoy MaGrID FldArea ElmDsc Name (optional) 1 1 2 1 0 3 1 0 0 2 Main_Column 2 45 4 2 0 4 1 0 0 2 Upper_Column_1 3 46 6 2 0 4 1 0 0 2 Upper_Column_2 4 47 8 2 0 4 1 0 0 2 Upper_Column_3 29 3 45 2 0 4 1 0 0 2 Upper_Column_flooded_1 30 5 46 2 0 4 1 0 0 2 Upper_Column_flooded_2 31 7 47 2 0 4 1 0 0 2 Upper_Column_flooded_3 5 48 3 3 0 5 1 0 0 2 Base_Column_1 6 49 5 3 0 5 1 0 0 2 Base_Column_2 7 50 7 3 0 5 1 0 0 2 Base_Column_3 26 42 48 3 0 5 1 0 0 2 Base_column_flooded_1 27 43 49 3 0 5 1 0 0 2 Base_column_flooded_2 28 44 50 3 0 5 1 0 0 2 Base_column_flooded_3 23 9 42 3 0 5 1 0 0 2 Base_column_cap_1 24 10 43 3 0 5 1 0 0 2 Base_column_cap_2 25 11 44 3 0 5 1 0 0 2 Base_column_cap_3
As you can see in the above table we have moved all entries from column 5 of the old format (RElmID) to column 4 of the new format (ElmID). Since the rotation for cylindrical elements has no effect we simply added 0 for all members into column 5 (ElmRot).
SUBELEMENTS Tables
Each subelement that is defined now requires a unique element ID (the first column of each table) across all element types (SUBELEMENTS, SUBELEMENTSRIGID, SUBELEMENTS_RECT, SUBELEMENTSRIGID_RECT). In contrast in previous versions of QBlade the element ID only needed to unique within each table.
As an example, such an element definition was possible in older versions of the substructure format:
SUBELEMENTSRIGID ElemID BMASSD DIAMETER 1 1 6.5 2 1 12 3 1 24 4 1 1.6 SUBELEMENTS ElemID MASS_[kg/m] Eix_[N.m^2] Eiy_[N.m^2] EA_[N] GJ_[N.m^2] GA_[N] STRPIT_[deg] KSX_[-] KSY_[-] RGX_[-] RGY_[-] XCM_[-] YCM_[-] XCE_[-] YCE_[-] XCS_[-] YCS_[-] DIA_[m] DAMP[-] 1 1.106E+04 2.515E+12 2.515E+12 2.959E+11 1.940E+12 1.141E+11 0.000E+00 5.000E-01 5.000E-01 3.512E-01 3.512E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 8.300E+00 1.000E-02 2 2.817E+03 1.819E+11 1.819E+11 7.537E+10 1.403E+11 2.907E+10 0.000E+00 5.000E-01 5.000E-01 3.515E-01 3.515E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 4.420E+00 1.000E-02 3 1.481E+03 1.922E+10 1.922E+10 3.963E+10 1.483E+10 1.529E+10 0.000E+00 5.000E-01 5.000E-01 3.482E-01 3.482E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 2.000E+00 1.000E-02 4 1.107E+03 1.447E+10 1.447E+10 2.961E+10 1.117E+10 1.142E+10 0.000E+00 5.000E-01 5.000E-01 3.496E-01 3.496E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 2.000E+00 1.000E-02
This now needs to be change so that every element ID is unique across all element types, in this example the only change is to change the ElmID in column 1 of the SUBELEMENTS table:
SUBELEMENTSRIGID ElemID BMASSD DIAMETER 1 1 6.5 2 1 12 3 1 24 4 1 1.6 SUBELEMENTS ElemID MASS_[kg/m] Eix_[N.m^2] Eiy_[N.m^2] EA_[N] GJ_[N.m^2] GA_[N] STRPIT_[deg] KSX_[-] KSY_[-] RGX_[-] RGY_[-] XCM_[-] YCM_[-] XCE_[-] YCE_[-] XCS_[-] YCS_[-] DIA_[m] DAMP[-] 5 1.106E+04 2.515E+12 2.515E+12 2.959E+11 1.940E+12 1.141E+11 0.000E+00 5.000E-01 5.000E-01 3.512E-01 3.512E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 8.300E+00 1.000E-02 6 2.817E+03 1.819E+11 1.819E+11 7.537E+10 1.403E+11 2.907E+10 0.000E+00 5.000E-01 5.000E-01 3.515E-01 3.515E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 4.420E+00 1.000E-02 7 1.481E+03 1.922E+10 1.922E+10 3.963E+10 1.483E+10 1.529E+10 0.000E+00 5.000E-01 5.000E-01 3.482E-01 3.482E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 2.000E+00 1.000E-02 8 1.107E+03 1.447E+10 1.447E+10 2.961E+10 1.117E+10 1.142E+10 0.000E+00 5.000E-01 5.000E-01 3.496E-01 3.496E-01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 2.000E+00 1.000E-02
MOORELEMENTS Table
The format of the MOORELEMENTS table has also been updated to align it more with the other element table formats. Older MOORELEMENTS formats are still accepted and recognized, however only the new format is documented from now on and should be preferred.
The old MOORELEMENTS format shown below:
MOORELEMENTS ID Dens.[kg/m^3] Area[m^2] Iyy[m^4] EMod[N/m^4] RDp.[-] Dia[m] 1 2.35723E+04 4.6084E-03 3.7601E-03 1.6353E+11 0.015 0.0766
has now been updated to:
MOORELEMENTS MooID MASS_[kg/m] EIy_[N.m^2] EA_[N] DAMP_[-] DIA_[m] 1 1.086306E+02 6.148892E+08 7.536117E+08 0.015 0.0766
As you can see the new format requires one less column and specifies the stiffness and mass per length directly, in the same way as the other element tables.
- 1(1,2,3)
Lu Wang, Amy Robertson, Jason Jonkman, and Yi-Hsiang Yu. Oc6 phase i: improvements to the openfast predictions of nonlinear, low-frequency responses of a floating offshore wind turbine platform. Renewable Energy, 187:282–301, 2022. URL: https://www.sciencedirect.com/science/article/pii/S0960148122000635, doi:https://doi.org/10.1016/j.renene.2022.01.053.
- 2(1,2)
R. Behrens de Luna, S. Perez-Becker, J. Saverin, D. Marten, F. Papi, M.-L. Ducasse, F. Bonnefoy, A. Bianchini, and C.-O. Paschereit. Quantifying the impact of modeling fidelity on different substructure concepts for floating offshore wind turbines – part 1: validation of the hydrodynamic module qblade-ocean. Wind Energy Science, 9(3):623–649, 2024. URL: https://wes.copernicus.org/articles/9/623/2024/, doi:10.5194/wes-9-623-2024.