Multi-Member Tower

Introduction

The multi-member option allows users to define the tower and support structure as a series of nodes, joined by a series of members. This allows relatively complex geometrical structures to be modelled. For example, fixed offshore jacket and floating support structures. The members that interconnect are modelled as beams that can be rigid or flexible. At support structure nodes it is also possible to define local joint flexibility.

Defining the Tower and Support Structure

Click the Add Node button to add new nodes. The height, and x and y positions of the node should be specified. The x-axis initially points south, but can be rotated by entering a non-zero value in the X-axis clockwise from south box. Point masses can be defined at any of the nodes. The node at which the nacelle joins the tower must be selected from the Nacelle node drop-down menu. The selected node must be the highest node on the structure.

Clicking Add Member will allow members to be defined to join the nodes. The grid will display two rows per member, one for each end of the member referred to as 'End 1' and 'End 2'. Most parameters can be different at each end, but some must be the same, for example the material properties and whether or not the member is flooded. Selecting the Allow tapered wall thickness option will enable the wall thickness to vary along the member. Clicking on the materials column will bring up a drop down list of all the materials defined. Select the appropriate material for that member. If a diameter, wall thickness and material have been defined, the various mass, stiffness and inertia properties will be calculated automatically. These can be overwritten if desired. The diameter is assumed to be the outside diameter for the purposes of mass, stiffness, inertia and drag calculations.

Marine growth (fouling): For an offshore turbine, enter a value for the thickness in millimetres of marine growth (fouling) at each end of a member, if required. The thickness is assumed to vary linearly along the member. The increase in member diameter due to the marine growth affects drag, mass and buoyancy of tower member.

Rigid Members

Using Project Info it is possible to list a set of members that will be made rigid during a simulation. This is useful in cases where users want to define members that have very high stiffness explicitly as rigid. This can prevent numerical issues during modal analysis and simulation run time by avoiding an ill-conditioned stiffness matrix of the structure caused by large variations in the stiffness values. A rigid member may also be used as a rigid offset away from the main physical structure. This may be valuable for connecting multiple mooring lines to the shell surface of a member. Alternatively a user may want to offset the location at which loads are applied or kinematics reported away from the physical structure.

MSTART EXTRA
NRM 6 * number of rigid members
RIGID 36 37 38 39 40 41 * member numbers that are rigid
MEND

Pre-Tensioned Elements

Pre-tension can be added to support structure beam or bar elements in Bladed. If an element represents a wire, it must also be assigned as a bar element (no bending, shear or torsional stiffness).

If two or more pre-tensioned beam elements are included end to end in the model, the pre-tension must only be applied to one of the elements (otherwise it will be double counted).

Cables should be modelled using a single bar element.

The option is enabled using Project Info and an example is given below for two wires, each consisting of one element:

MSTART EXTRA
NumBarElements 2
BarElements 40,41
NumPreTensionEls 2
PreTensionEls 40,41
PreTension 5000, 5000
MEND

Member Coordinate System

When the multi-member tower option is selected, each member is defined with its own local coordinate system. The local x-axis is defined along the member axis, in the direction from End 1 to End 2. Thus Fx is the member axial force and Mx the torsional bending moment. Clicking on the Member axes button will bring up the direction cosines for the member z-axis. These will have defaulted to an orientation in line with the convention for other offshore engineering codes. The default orientation in Bladed is to set the local member y-axis in the horizontal plane, with the local member z-axis making up a right-handed coordinate system. For a vertical member the default orientation is for the local y-axis to be in the global x-direction, with the local z-axis in the global y-direction. Thus for a vertical member Mz describes the principal overturning moment and My the side-side overturning moment. These orientations can be changed by the user if required.

Display Structure

Clicking on the Display Structure button will bring up a 3D drawing of the support structure. It is possible to rotate, pan and zoom using the appropriate toolbar options. Clicking on the green “N” toolbar option will label the nodes in the display, and the “M” will label the members. If members are selected in the Member grid whilst the plot is open, the selected members will be highlighted in red and the local coordinate systems displayed.

Member End Offsets

Bladed allows the user to define a member end offset at each end of each member of a multi-member tower. In the Tower window, member end offsets can be specified by entering values in the Offset X, Offset Y and/or Offset Z columns. These offsets are applied to the corresponding member end in the tower proximal frame, which is aligned with global coordinates for a fixed structure with no earthquake.

Each member end with any non-zero offsets will generate an additional structural node that is displaced from the original node (as defined in the nodes grid) by the offset, and connected by a rigid link with no mass, buoyancy or drag.

Last updated 15-11-2024