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Recorder function of PDR used for offshore PDA-test

 

In september 2015, Allnamics was asked to monitor stresses in the flange and on the inside of a monopile for an offshore windfarm, during installation of the pile with an impact hammer. Special challenge was that all the instrumentation had to be mounted on the inside of the pile, with limited space AFoto01available for placing sensors and equipment. On top of that, drilling and welding were not allowed and the test had to be completed within 3 weeks after the first enquiry. The recorder mode of the Allnamics PDR, ingenuity and improvisation made it possible!! AFoto02

 

The instrumentation consisted of 2 acceleration sensors placed vertically, 2 horizontal strain gauges on the flange and 4 vertical strain gauges on the inside of the pile wall; this setup also incorporated a “normal” PDA (Pile Driving Analysis) setup. 2 PDR-systems were used for data acquisition of these 8 channels, with the acceleration sensors used for triggering.

 

Challenges

Main challenge for this project was that the instrumentation had to be installed on the inside of the pile, because of the hammer sleeve. Obviously this would make it impossible to establish a wireless connection to a computer on the vessel. But a cabled connection was also not possible, because the pile did not contain a cable outlet hole or snorkel hole near the pile head and below the hammer sleeve.Measuring on the inside of a pile without any communication from the outside was solved by using the Allnamics-PDR in recorder mode. All monitoring data were saved and stored in the PDR-systems during pile installation and transferred to a computer after they were retrieved from the pile.

 

Figure 1 : Configuration D: PDR operating in recorder mode

Figure 1 : Configuration D: PDR operating in recorder mode

 

The required battery life time of the PDR needed to be approximately 12 hours. The installation cycle was expected to be around 3000 blows By using an external battery the monitoring time could be extended to 12 hours. The standard memory is already able to record 10.000 blows.

 

Positioning of sensors

Next challenge was the restrictions imposed by the space that was needed for the grippers of the upending tool and an airtight platform that had to be mounted inside the pile, before removal of the equipment. This required mounting of the instrumentation above the platform, less than 1 m below the pilehead. All in all very little space for positioning the sensors and recorders was left.

 

Photo 1 : The gripping tool just before pile upending

Photo 1 : The gripping tool just before pile upending

 

 

Photo 2 : airtight platform in mobilization port

Photo 2 : airtight platform in mobilization port

 

Photo 3 Finding space to place sensors and monitoring equipment (pile in horizontal position)

Photo 3 Finding space to place sensors and monitoring equipment (pile in horizontal position)

 

Mounting of sensors

The next challenge was the way to mount the acceleration sensors and the PDR-systems without drilling holes in the pile or welding on the pile. For the strain gauges (and their tension relief) this was solved by glueing them on the pile. For the acceleration sensors this was solved by adapting their housing, so they could be bolted to the earthing stubs that were already inside the pile.

 

hoto 4 : Earthing stub used for mounting the acceleration sensor and the strain gauge glued directly on the pile

Photo 4 : Earthing stub used for mounting the acceleration sensor and the strain gauge glued directly on the pile

 

For suspension of the PDR-systems this was solved by placing wooden stub between the flange and the platform ring. The stubs were kept in position by using the bolt holes in the flange and the stiffener plates of the platform mounting ring. Each PDR was suspended between 2 stubs, with elastic bands.

 

Photo 5 : One of the 2 PDR’s mounted between 2 wooden stubs (pile in horizontal position)

Photo 5 : One of the 2 PDR’s mounted between 2 wooden stubs (pile in horizontal position)

 

Execution of the project

Next challenge was to prepare and monitor the project within 3 weeks from the first enquiry. The mounting of the sensors had to be done outdoors, in the mobilization port prior to pile installation on site. Conditions were quite hostile that day, cold and very windy!

 

 

 

Photo 6 Installing the sensors from a cherry picker: 2 acceleration gauges & 6 strain gauges

Photo 6 Installing the sensors from a cherry picker: 2 acceleration gauges & 6 strain gauges

 

But al sensor mounting and glueing went smooth and successful. After all these challenges were solved, there was one more left: Allnamics had no monitoring specialist available in the planned time frame for the offshore testing campaign….  Luckily our German partner company Fichtner could provide us with one of their specialists to do the job offshore!  The PDR’s were successfully retrieved and had recorded all blows, giving the client valuable information of the pile during driving without disturbing the production process.

 

Photo 7 : The piles on deck of the installation vessel

Photo 7 : The piles on deck of the installation vessel

 

Photo 8 : Bringing the pile in position

Photo 8 : Bringing the pile in position

 

Photo 9 : Hammering down the pile down

Photo 9 : Hammering the pile down

 

Photo 10 : Exciting moment: just after lifting the hammer

Photo 10 : Exciting moment: just after lifting the hammer

 

Photo 11 : The equipment after pile installation and installation of the airtight platform.

Photo 11 : The equipment after pile installation and installation of the airtight platform.

 

In case you are interested in more information on Pile Driving Analysis or the PDR, please feel free to contact us.

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