Learning Forum

[nhanani4]

Hi,

The information of tension leg are diameter 1.2m, thickness 0.04m, length 70m, steel tension leg E =2.1x10^11 N/m2.

Water depth is 80m, wave height 2m, period 6s.

My question is :

1) what formula to calculated axial stiffness at tether boundary condition at fixed point and tether boundary condition on structure?

2) The value of axial stiffness key in at the two (at fixed point and on structure) are can key in same data?

Thank.

 

 

[Shuangxing Du]

(1) The total axial stiffness (K) of a tether is

1/K = 1/K_f + 1/K_s + 1/K_t

where K_f is the axial stiffness at the fixed point (bounday condition at the fixed point), K_s is the axial stiffness at the structure attachment point, K_t is the tether axial stiffness of EA/L.

(2) The axial stiffness values at two boundaries could be the same.

[nhanani4]

How to calculate the K_f and K_s? What the formula? 

[Shuangxing Du]

They are your inputs

[nhanani4]

What the formula to calculate area of tether mooring? Is it a circle area formula = 3.142 x radius^2?

[Shuangxing Du]

A=pi/4[D^2-(D-2*theta)^2]=pi*[D-theta]*theta, where theta is the thickness, D is diameter.

[nhanani4]

The axial stiffness of tether boundary condition at fixed point and on structure that should be key in.

1)What the formula of axial stiffness of tether boundary condition at fixed point it is right EA/L?

2) What the formula axial stiffness of tether boundary condition  on structure it is right EA/L?

3) or the I should calculate total axial stiffness (1/K = 1/K_f + 1/K_s + 1/K_t) and key in data the  axial stiffness of tether boundary condition at fixed point and on structure ? 

4) The value  axial stiffness of tether boundary condition at fixed point and on structure will be same because the data mooring line is same? (tension leg are diameter 1.2m, thickness 0.04m, length 70m, steel tension leg E =2.1x10^11 N/m2)

 

[Shuangxing Du]

Please see Aqwa Theory Manual, Section 10.2,

For an installed tether, between the vessel end of the tether and its corresponding vessel attachment point, three spring stiffnesses (, , and ) are defined as the inline stiffness and two rotational stiffnesses about the TEA y- and z-axes respectively. These are used to determine the relative translational displacement in the TEA x-direction and rotations about the TEA y- and z-axes between the vessel end of the tether and the vessel attachment point. The relative translational displacements in the TEA y- and z-axial directions are set to be zero at the vessel attachment point.

Similar to the vessel end of an installed tether, three spring stiffnesses ( , , and ), between the anchor end of the tether and the corresponding fixed point on the sea bed, are defined as the inline stiffness and two rotational stiffnesses about the TEA y- and z-axes. The relative translational displacements in the TEA y- and z-directions are set to be zero at the anchor point.

And Aqwa reference manual, Section 19.19.2.

For installed tethers, the spring stiffnesses are the inline/vertical stiffness and the two rotational stiffnesses (about the local tether Y- and Z-axes) at the ends of the tether. Default values of 1.0E15 are used, if this data record is omitted. A default value of 1.0E15 for the inline stiffness is used if the 1st field is left blank or a negative or zero values is input. For the rotational fields, any value may be entered (except negative values which will be set to zero).

[nhanani4]

can u  show the picture again becauseI can not open it,the image can not be load. sorry .

[Shuangxing Du]

See below for Aqwa theory manual Section 10.2

And Aqwa Reference mannual Section 19.19.2