Interpretation and comparison of Results


Analysis of Results

The result can be analysed to obtain some conclusions. The maximum tensile and compressive stress can be compared with the ultimate tensile and compressive strength of the grout material. By doing this we can check if there would be material failure due to tensile or compressive strains.

The next table presents the maximum tensile and compressive values from the maximum values of the different principal stresses and the ultimate strength values of the grout material.


GROUT MATERIAL FE Plain Grout Joint Ultimate Strength Grout Material Failure
Tensile stress (Max)
6.05 MPa
5 MPa
YES
Compressive stress (Max)
21.12 MPa
80 MPa
NO









  As can be observed in the table the grout material is far from failure due to compression according to Finite Element structural analysis.


  The grout material will crack in some parts of the joint according to the obtained results. Cracks in the grout are something that could be expected but it has to be considered that these cracks under dynamic loading (as it is the real situation for this joints working with OWTs) will likely get worse due to material fatigue.




The grout material failure due to tension and compression has been checked. However, the most interesting results are those related to the failure of the bonds between the grout and the steel. The most important mechanisms of failure in the grouted connection are the sliding mode failure and the grout shear mode failure. In order to check these failures in our industrial case study, we have the maximum shear stress obtained in the bond surface between the pile and the grout.

The result obtained in the Finite Element analysis can be compared with the experimental results obtained by the BMU-research project 'GROW-Grouted Structures for Offshore Wind Turbines'. Furthermore, EN ISO 19902 or DNV-OS-J101 empirical formulae can be used and the results compared in order to see how their results deviate from the experimental data obtained by GROW research project and from the Finite Element analysis.

The next graph shows experimental results obtained in the research project from different tests. The graph presents the ultimate measured bond strength (fb) in a grout joint against the unconfined compressive strength (fc) of the grout used in the joint. Some tests have been done without shear keys in the joint and others with different shear key arrangement characterised by the ratio h/s (shear key height/shear key spacing). The graph includes the shear and sliding mode failure curves given by ISO 19902 for specific grout joint.

Axial force capacity tests


The grout joint of our case study has shear keys and its parameters the following values:

fc = 80 MPa       h/s = 12 mm/300 mm = 0.04




Looking at the graph there are no exact results for our case, but interpolating graphically the available results we can expect approximately a value of 10 to 12 MPa as ultimate measured bond strength (*).

For the tests carried out on joints with no shear keys, none of them reaches a value of ultimate measured bond strength of 5 MPa.

The next equations show the calculated strength for shear and sliding failure fg,shear and fg,sliding according to EN ISO 19902:

fg.shear =[ 0.75 - 1.4( h/s) ]f c0.5


fg.sliding = Cp[ 2 + 140( h/s) 0.8 ]K0.6. fc0.3












Where Cp is the scale factor for the diameter of the steel pile and K is the radial stiffness factor of the connection given by the following expression:

K =[( Dp / tp) +( Ds / ts)] -1 ≤ 0.02






Where Eg and Es are the Young's moduli of grout and steel, and D and t are the diameter and thickness of pile, sleeve and grout respectively. The values for our case study are the following:

Es / Eg = 18            Dp / tp = 21            Ds / ts = 30            Dg / Dg = 5.333




These values give a result for the radial stiffness factor K = 0.030 which is higher than the maximum considered in the formula. This is an example of geometric dimensions deviations found when existing standards formulae are applied to grouted connections for OWT jackets. Hence, the value considered to apply the formulae is K = 0.02.

The formulae for sliding strength and shear strength have been applied for the case study's joint, considering that the joint has shear keys and that the joint is plain. For the joint without shear keys h/s = 0.

The maximum shear stress obtained in the Finite Element model, data obtained from the graph showed before and the results using ISO 19902 formulae are summarised in the next table.


BOND FE Grout Joint Model Experimental Results BMU-research project ISO 19902 FAILURE
Shear stress (Max) Ultimate Bond strength (fb) Sliding Strength (fg.sliding) Shear Strength (fg.shear)
FE Plain Grout Joint
9.61 MPa
< 5 MPa
0.91 MPa
6.7 MPa
YES
FE Shear Keys Joint
(*)10-12 MPa
5.8MPa
6.2 MPa
NO












In the light of this table some comments can be made:

  If the maximum shear stress obtained in the Finite Element model is compared with the ultimate bond strength for the plain grout joint (Schaumann, et al., 2010), we can see that the first one is well above the bond strength which indicates that the bond would fail.



  If the maximum shear stress obtained in the Finite Element model is compared with the ultimate (*)expected bond strength for the joint with shear keys (Schaumann, et al., 2010), it can be said that the first is below the bond strength so the bond will not fail. However, the value of the shear stress is close to the ultimate expected bond strength and considering the uncertainty of the value of the expected bond strength, the possibility of bond failure cannot be dismissed. In addition, shear failure is likely to happen.






  The ultimate bond strength of a grout joint increases dramatically if shear keys are deployed according to experimental data. The ISO 19902 results show that the sliding strength increases too when using shear keys.



  ISO 19902 gives results for sliding and shear strength that deviates from the experimental results (Schaumann, et al., 2010). This shows that the standard is not a valid design recommendation for this type of joints. ISO 19902 underestimates the bond strength of grout joint using high performance grout and shear keys.






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