MS8200 Multibeam Echo Sounder Playes Big Role in Wind Farm Pile Foundation Inspection

The combination of MS8200 and POS25 has unique advantages for offshore wind power pile foundation scouring detection project. First of all, its data is accurate. The good signal tracking performance and unique muddy water algorithm of MS8200 multibeam echo sounder can effectively solve various complex problems caused by big surge and quick velocity in the wind farm. They can exactly show the subtle changes of pile foundation scouring situation. Then in post processing software, we can use different model to calculate the size, scouring range & volume and comparative change of pile foundation damage.

In May 2021, we participated in a project of "Offshore Wind Power Operation and Maintenance Pile Foundation Scouring Detection" in the sea near Lianyungang City. The task had planned to use the multibeam echo sounder to examine the scouring and silting situation within 100m around the pile foundation of 48 wind turbine in order to screen and investigate the complete hidden dangers of the underwater part.

The wind farm consists of 48 wind turbines scattering as nearly square area as shown in following fiture, which is 6500m width from north to South and 8000m length from east to west. It is gradually deepen from southwest to northeast. The water depth in shallow area is about 3m and it can reach 12m in deep water. 

There are 4 difficulties in surveying area investigation and assessment.

1) Flow speed is very quick

The wind energy farm is located on the North Bank of Guanhe River Entance to the sea. There are several rivers of Xinyi River, Guanhe River and Wuguan River intersecting flow into the sea at this place. The water flow situation is relatively complex here. The flowing velocity is quick and the sand concentration of seawater is high.

2) High Surge

The seabed is affected by river flows and sea currents and showing a "gourd shape" from northeast to southwest. In addition, the surge environment is complicated. The surveying ship is small medium-sized vessel, which is not wind and wave resistance enough, so swing serously.

3) Water is Shallow

Except the two drainage areas near the northeast is deep, the water depth of other areas is 3-6M or even shallower during low tide.

4) Little Experience

As the pile foundation measurement is different with traditional full coverage measurement, it only needs to survey the circle area with 100m diameter    centering pile foundation. How to determine the measurement scheme after total considering the flow velocity, flow direction, wind direction, water depth and other factors? We also need to ensure the data quality and the safety of the measurement process. Further more, working efficiency should be improved twoo. All combine to a new topic.

This survey to wind turbine pile foundation was conducted in the Yellow Sea area. The local water quality is relatively turbid   and the average swell is about 0.4m. The following solution is adopted :

Product Solution:

1) Ms8200 multibeam echo sounder @ 200kHz has extraordinary performance in muddy water bathymetry.

2) With POS-25 attitude instrument, which integrates POSMV SurfMaster of Applanix and adopts TrueHeave processing algorithm, the heave accuracy reaching up to 2cm is reliable. It support Fugro MarineStar™ GNSS service and ensure the realtime positioning accuracy is better than decimeter level at any place. Therefore, it is more suitable for the complicated measurement environment of this special wind farm.

Surveying Implementation

According to the water depth distribution and sound velocity changing situation, the surveying area is divided into five parts unevenly. Then sound velocity profile data collection, area division and surveying line layout are carried out before, during and after the regional survey.

1) According to the water depth of each wind turbine, the surveying lines are arranged in four directions showing in Figure below.

 2）According to the changing of sound velocity，divide into 5 surverying areas display in following Figure.

According to the schedule, the engineers start the measurement of the whole project.

1）Install the multibeam echo sounder.

2）Bathymetry in surveying area

3）Operation of multibeam echo sounder

After field survey and data collection, we need check the data of multibeam echo sounder, sound velocity and tide level, then post processing all of these datas. Until output the final result, did we finish the wind power survey.

 Analysis the pile foundation measurement results

1) According to the analysis of pile foundation damage and scouring range, through the following scouring model, it can be   seen that there is scouring around the pile foundation and there is worn on the surface of the pile foundation, as shown in  Figure below.

Affected by the flow velocity and direction, the scouring pit is obvious in the west and south of pile foundation. Red circle 1 in above Figure is the place where the scouring pit is more serious. It is 29m long from north to South and 16.5m wide from   east to west.

Below picture is enlarged to see irregular area of the scouring pit is around 602m², which is 0.8m deeper than the surface of the pile foundation. The average scouring depth is 0.6m and the total volumn of the scouring hole is 363m³. Along with the service life increase, the scouring area will become more and more obvious, which is a big potential safety hazard.

There are also varying degrees of wearing changes on the pile foundation surface. The shallowest sounding value of the pile foundation surface is 6.4m, the sounding value in most areas is 6.5m, and the sounding value in some areas reaches 6.8m. Through the combination of the pile foundation model and sounding data, it can be judged that in most area scouring and erosion on the pile foundation surface is 0.1m and the erosion in some places (Shown in blue area below) is 0.46m.

2）Compared with the measuring data in July 2020, it can also better reflect the changes of underwater terrain around pile foundation.

The following Figure shows the multibeam echo sounding data in July 2020. Comparing with the result above surveyed in May 2021, it can be clearly seen that there was a scouring and silting pit in Jul. 2020, which is diameter 18m & 0.4m depth and left by the pile leg of the installation ship. Now it can be observed to be filled by water scouring and silting in our latest measurement.

Adding two results overlap, we can get pile foundataion terrain changing after 10 months scouring. From the modeling figure below, we can learn:

1）Grey area changed less than ±0.2m.

2）Green area is 0.5m higher than laster year.

3）Blue area is 0.8m lower than last year.

Through the quantitative comparison with the measurement results of the previous year, the change of underwater terrain around pile foundation can be accurately adjusted, and the harm degree of defects can be clearly calculated according to the changes.

After measuring verification and data results analysis, the combination of MS8200 and POS25 has unique advantages for   offshore wind power pile foundation scouring detection project.

1) High data quality. The good signal tracking performance and unique muddy water algorithm of MS8200 multibeam echo sounder, combining with the stable and unique performance of POS25, can effectively solve various complex problems caused   by big surge and quick velocity in this area. They have the micro terrain scanning ability required by this project and accurately   show the subtle changes of pile foundation.

2) Intuitive display of rich results. Mature application model is easy to use. The superposition and comparison of historical    results can accurately and intuitively show the size, distribution range, pit volume and comparative change of pile foundation   damage.

We firmly believe that with the continuous upgrade and improvement of MS8200 multibeam echo sounder and POS25, it will   make more contributions to China's offshore wind power industry.