Model building - Tower & support structure

Articles on this page:

• Anchor points for mooring lines
• Coordinate system used in the foundation
• Beam element model
• Modelling guyed towers
• How do direction cosines of member z-axis work?

Problem

Are the coordinates for anchor points for mooring lines global coordinates or locally based in the node to which the line is connected?

Solution

The anchor points are in global coordinates.

Keywords

Anchor points; Global coordinates

Problem

Which coordinate system is used in defining stiffness, damping and mass matrices? Global coordinates or local?

Solution

The coordinate system used in the foundation definition is the global coordinate system.

Keywords

Foundation; Stiffness; Damping; Mass; Coordinates

Problem

Is it possible to choose the kind of beam model to use for the support structure members (Euler-Bernouilli type versus Timoshenko type)?

Solution

No, the only beam element available is Timoshenko beams.

Keywords

Beam; Model; Thimoshenko; Euler; Bernoulli

Problem

How would I model a tower supported by taut cables (guys)?

Solution

It is possible to model towers supported by guy cables using the ‘Multi-member’ tower module (offshore module).

Each guyed cable is modelled as an additional support structure member. This means that the ground anchor points need to be defined as additional nodes, and the cables themselves are defined as additional members connecting the tower attachment node with the ground anchor position.

In order to configure these members as guy cables, the following Project Info text is required:

MSTART  EXTRA  
NumBarElements  6  
BarElements  27, 28, 29, 30, 31, 32  
NumPreTensionEls  6  
PreTensionEls  27, 28, 29, 30, 31, 32  
PreTension  274680, 274680, 274680, 274680, 274680, 274680  
MEND

The above example is just for illustration and would of course vary according to the model. It shows a definition for 6 cables, numbers as additional members 27 to 32, and all of which have pre-tension of 274680N. It would require the creation of six new nodes in the tower screen, all at ground level, as well as six new elements connecting the new nodes to existing tower nodes.

Keywords

Tower; Support structure; Guy / guys / guyed; Cable

Problem

In the multi-member support structure model, direction cosines are used to specify the orientation of a support structure member. They can be quite confusing! An illustrative example should help understand these.

Solution

We imagine a unit vector in the direction of the member Z axis. These three numbers (the direction cosines) are the projected lengths of that vector along each of the three global (GL) axes respectively. (i.e. they are the components of the vector in the GL frame). So the sum of the squares of all three must always equate to 1.0.

For example. The member number 35 in the 5MW Tripod project (supplied with Bladed) is a vertical member. So its x axis is vertical, i.e. in line with the GL Z axis. If you look at the "Member axes" for this member, by default it looks like this:

The cosine in the Y direction is 1, this means the angle between member Z axis and GL Y axis is zero, because cos 0 = 1. The cosine in the X direction is 0, because the member Z axis is at 90 deg to the GL X axis and cos 90 = 0. The cosine in the Z direction is also 0, because the member Z axis is at 90 deg to the GL Z axis and cos 90 = 0. If you look at the 3-D rendering for this member, it looks like this:

But if you wanted this member's Z axis to be at, say, 60 degrees to the GL X axis, you would change the direction cosines to:

This then changes the orientation of the member Y and Z axes to look like this:

You will not be able to change the value in the "Z" cell because the member Z axis must always be at 90 degrees to the GL Z axis for a vertical member.

The default values chosen by Bladed are explained in the User Manual, i.e.:

Keywords

Direction; Cosine; Member axes; Multi-member