Figure: Natural frequencies depending on support conditions. Example of jacket foundation (Height, diameter top, diameter bot) = (40000, 14000, 7000)mm Shows above are the 1st mode shape of the two assumptions of support. The case of fixed support the bottom points are fixed. The case of finite stiff support (linear) the bottom points are vibrated up and down according to the assumed linear springs 10000kN/m translation and 10000kNm/rad rotation. This numerical test shows that the ratio of natural frequency is up to 2.018/0.393=5.1 between the cases. The following remarks can be drawn: § Soil-structure interaction needed to analysis at the points assuming to be support. The soil surrounding the pile element (here assumed point) does not keep the pile moving up and down with the same force, as assuming with linear spring kUZ=10000kN/m. It is about the shear modulus G of the soil and also the external actions on the structure.
§ Natural frequency of the full model is lower with the structural turbine and components model above the jacket foundation.
At the seminar the soil model surrounding the piles and the soil properties will be discussed. |