Study Shows Installation Method Strongly Influences Fatigue Damage of Foundations

ECN (Energy research Centre of the Netherlands) has released results of a test campaign that took place in June 2014 in the Bremerhaven where IHC IQIP, together with other industrial parties, measured the behaviour of monopile foundations under different driving methods in onshore conditions.

Three piles were driven in the soil by hydraulic hammers of IHC IQIP. A novel approach of pile driving has been developed named ‘HiLo’. The principles are simple. Instead of aiming for a small number of blows with high impact energy, the HiLo approach is High frequency driving with Lower energy per blow. This reduces noise level and leads to a reduction of loads in the pile with a limited penalty on the installation time, according to IHC IQIP.

The Energy Research Center of the Netherlands (ECN) has conducted a study to investigate the development of fatigue damage in the pile during the driving operation. Correlations have been found between the measured structural response of the pile and the operational hammer parameters. The gathered insights of the loads during impact driving resulted to a better estimation of the consumed fatigue lifetime of an impact driven pile.

Three different driving strategies have been considered:

• Industry standard procedure: 25 blows / 25 cm penetration.
• IHC recommended driving strategy: 40 blows / 25 cm
• IHC’s new HiLo procedure: 80-120 blows / 25 cm.

The fatigue damage was determined for three tests, comparable to the generic procedures (respectively 22 blows / 25 cm, 40 blows / 25 cm and 95 blows / 25 cm). Compared to the industrial standard procedure, the HiLo installation shows a fatigue damage reduction of up to 89 percent. Compared to the IHC standard recommended driving strategy, the reduction still is 74 percent.

The fatigue damage due to the mean stresses in the pile was not very high in any of the experiments, according to Koen Hermans (ECN), researcher in the project. However, fatigue is much more of a concern when stress concentrations due to local defects, weld beads or other structural details are taken into account. This leads to an increase of up to 40 times the fatigue damage for a stress concentration factor of 3.3; in such a case, application of the HiLo strategy will limit fatigue damage considerably. This leads to a longer lifetime for the installed pile and potentially allows reduction of material used in the pile.

Source: ECN