Back to the programme printer.gif Print




Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Advanced operation & maintenance' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Gert De Sitter OWI-lab / VUB, Belgium
Co-authors:
Gert De Sitter (1) F P Wout Weijtjens (1) Yves Van Ingelgem (2) Daan De Wilde (2) Kristof Verlinden (3) Stefan Milis (4) Christof Devriendt (1)
(1) OWI-lab / VUB, Brussels, Belgium (2) Zensor, Brussels, Belgium (3) Parkwind NV, Brussels, Belgium (4) OWI-lab / Sirris, Brussels, Belgium

Printer friendly version: printer.gif Print

Abstract

Foundation monitoring systems: analysis of 2 years of monitoring at the North Sea

Introduction

When it comes to offshore foundations, scouring, corrosion and reduction in grout and foundation integrity over time can be problematic. Therefore, OWI-lab installed foundation-monitoring systems at several turbines. A first monitoring system, which is installed on a monopile foundation of a 3 MW Vestas turbine at the Belwind wind farm, has now been running for almost two years. Recently two additional monitoring systems were installed at the Northwind wind farm. The motivation is gaining the insights that are necessary to minimize the installations and O&M costs of the future planned wind turbines at the Belwind 2 concession.

Approach

The purpose of the R&D projects at the Belwind farm and the Northwind farm is to get a precise view of how the structure of the foundation of an offshore foundation is changing over time and what causes the changing behavior. It has been decided that the following parameters should be monitored:

• Loads, displacements, strains and local temperatures in the grouted connection: Those parameters are necessary to see if the grouted connection is deteriorating. In one turbine the displacements and loads are measured. In two additional turbines also the strains in the grouted connection are monitored. The temperature distribution is needed to correct the stain measurements.
• vibration and strain levels on tower and foundation: Those parameters are needed to calculate the resonance frequencies and the damping values of the offshore structure. Additionally they give some information about the correlations between scour and the vibration levels.
• resonance frequencies, damping values and mode shapes of the offshore structure: With this information it is possible to track potential scour. This information also can be used to update the remaining life time estimates by applying a modal expansion method and calculating the stresses at the critical locations.
• oxygen levels and corrosion rates inside the monopile: Corrosion will have an influence on the remaining life time of the foundation. Therefore both the cause (oxygen levels) and the effect (corrosion rate) are monitored.
• water ingress in the grouted connection: This parameter can be linked with potential damage of the grouted connection. Therefore also the water ingress is monitored.
• …


Main body of abstract

The presentation will briefly discuss the following ongoing monitoring cases with their state-of-the art analysis techniques, the challenges encountered and their main findings.

Grout monitoring: Two grout monitoring systems are presented. The first system is installed on a turbine of the Belwind wind farm. It consists of the continuous monitoring of the loads taken by the grout connection and the relative displacement between the monopile and the transition piece. The most important observation made by the monitoring system was the confirmation that the transition piece was indeed slipping downwards on the monopile foundation as a result of the failing grout-connection. Retrofit installed brackets and bearings now prevent the transition piece to completely slip downwards. The monitoring system however continues to measure the slippage of the transition piece and the loads taken by the installed brackets and bearings. The presentation will discuss the main findings after one year of monitoring. With the lessons learned from the grout monitoring system of the Belwind farm, the design of the grouted connection of the Northwind wind farm has been adapted (e.g. shear keys were included). Also an improved grout monitoring system has been proposed. In two turbines of the Northwind wind farm, this improved grout monitoring system will be installed. The main improvement is that now also the strains inside the grout will be measured. This will be done by using optical strain sensors.

Dynamic monitoring: consists of the continuous monitoring of the vibration levels, resonant frequencies and damping values of the wind turbine structure.

It will been shown that the dynamic monitoring solution is able to detect daily variations in the vibration levels due to changing ambient conditions e.g. wave periods, wind speed and wave heights. Also small changes in resonant frequencies due to e.g. tidal level are detectable. It will be discussed how we will exploit these parameters to access structural integrity of the offshore wind turbine and detect changes in the dynamic behavior due to e.g. scour. While until now only results have been shown of parked wind turbines, in this presentation, it will be shown for the first time that also the resonance frequencies, damping values and mode shapes of rotating turbines can be tracked. Also for the first time, the evolution of the resonance frequencies during a period of 2 years will be presented. Finally it will be shown how we will use these measurements combined with numerical models to estimate the remaining fatigue life-time of the foundation.

Corrosion monitoring: consists of the continuous monitoring of the corrosion rates, corrosion potential and oxygen concentration inside the monopile foundation.

Corrosion management inside the monopiles has been based on the assumption that no oxygen is present within the confined space of the monopile. The monitoring-setup at Belwind has however been able to detect that the level of oxygen inside stays high. Based on installed electrodes that are lowered inside the monopile structure measurements of corrosion rates at multiple depths are conducted. These measurements confirmed that active corrosion is present inside the monopile. It will be discussed how the corrosion management for these type of structures needs to be adapted and how the presented monitoring system can play a crucial role within this. Also the lessons learned from the Belwind farm and its implications on the monitoring system of the Northwind farm will be presented.

Water ingress in the grouted connection: consists of the continuous monitoring of the water ingress on the grouted connection. The monitoring of this parameter has only recently started at the Belwind wind farm. The first results of this campaign will be presented.


Conclusion

This presentation gives an overview of all the running R&D projects at the foundations of one turbine at the Belwind wind farm and two turbines at the Northwind wind farm. First, the results of 2 years of monitoring at a turbine at the Belwind wind farm will be presented. This includes the results of a dynamic monitoring system, the results of a grout monitoring system, the results of a corrosion monitoring system and the results of a recently installed water ingress monitoring system of the grouted connection. A new analysis method of the dynamic monitoring system will be presented. It will be shown that it is possible to monitor the resonance frequencies, damping values and mode shapes of rotating turbines, which also makes it possible to show the long term monitoring results.

The lessons learned from the Belwind monitoring campaign will be discussed. Then, the improved monitoring systems and the first results of the new systems, which are installed between September and November 2013 at 2 turbines of the Northwind wind farm, will be presented. For the first time a grout monitoring system will be presented that also monitors the strains inside the grouted connection. How this is achieved, which sensors are used and how the data is treated, will be presented. Finally, it will be shown how the dynamic monitoring system, together with the monitoring system of the grouted connection, the corrosion monitoring system and a new modal expansion approach, will make it possible to accurately estimate the remaining life time of an offshore wind turbine.



Learning objectives
This presentation will show how foundation-monitoring systems can help operators to gain some insight into the evolution of the foundation. It focuses on those parameters that influence the lifetime prediction of a turbine foundation.