Analysis and Prediction of Monopile Oscillation During Installation
1, Richard Smith1, Jonathan Grigg2, Lizzie Morris2
1Longitude Consulting Engineers Ltd., Exeter, UK, 2Rampion Offshore Wind Ltd., Coventry, UK
Longitude Engineering - a specialist marine engineering subsidiary of the LOC Group - working closely with Rampion and its contractors, has developed an innovative analysis and modelling process to fully understand why large monopiles oscillate in relatively small wave heights; and to increase the operating window for their installation.
Monopiles and turbines are increasing in size as wind farms move farther offshore. At the Rampion Offshore Wind Farm, currently in construction off the south coast of the UK, the developer was concerned that these large structures were beginning to oscillate in relatively small wave heights. This was affecting the operational window for lowering them into position. As installation limits are generally set by operator experience, Rampion recognised that the oscillation had the potential to cause difficulties and invited Longitude to work alongside them to develop a more in-depth understanding of the problem and to assist with developing solutions and more rigorous guidelines.
Numerical analysis has been validated with scale model testing and full scale data acquisition, none of which had been previously undertaken.
The foundation phase was completed in early December 2016 ahead of schedule.
Numerical models were developed from first principles and in the analysis software OrcaFlex. These were automated to process a large array of possible environmental conditions and pile sizes.
Possible solutions were analysed to demonstrate increases in operability, these included fitting new crane boom tugger winches with an integrated control system to damp pile oscillations.
Early in the process Longitude highlighted the lack of quantitative pile oscillation data from previous wind farms- specifically relating it to the wind and wave conditions during instillation. To rectify this Motion Reference Units (MRUs) were used and results coupled with detailed MetOcean and wave radar data which has given a valuable data set of actual pile oscillations in known conditions that was previously unavailable.
The initial data set was used to build a planning tool for the developer to allow them to assess predicted uptime for various parameters against historical MetOcean data, key results from this was the identification of limiting criteria in the system and understanding how these could be improved.
The tugger winch control system was also demonstrated to be effective at reducing pile oscillation and therefore increasing the operable weather window.
The recorded data was of great value to validate the numerical modelling and give confidence in the prediction tool. The analysis clearly highlighted the sensitivity of the piles to certain wave periods and showed how quickly allowable crane limits could be exceeded.
This paper demonstrates the very real limitations on monopile installation that is only becoming critical as the size and weight of these piles increases.
Given the very high day rate of vessel charter the importance of minimising weather downtime is very clear- this analysis demonstrates that the windows of operation can be considered in high levels of detail to maximise the possible weather windows whilst ensure safety margins are maintained.
The paper also demonstrates the value of actual pile oscillation data and therefore quantifying the previously highly subjective measurement of pile oscillation.
The delegates would learn: