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G-Octopus: Adding value to projects with pile driving monitoring

What’s covered in our webinar?

In this webinar, presented by Emilio Nicolini, Technical Director of G-Octopus, we’ll introduce you to the business, our expertise and our global reach followed by an in depth look at pile driving monitoring (PDM). We’ll cover an overview of what PDM entails, it’s benefits and why it shouldn’t be overlooked as well as the value it can add to your next project. Then, to help bring it all to life, there’s three great case studies followed by an excellent Q&A session with great questions from the audience.

You can see the questions and answers below:

Q: What are the advantages of your PDM system compared to the system of any competitors?

There are actually several. Two key advantages are: how we install the sensors and the quality of those sensors. We use high quality instrumentation, cables and sensors, at the origin and over the years we’ve developed our own way of carrying out the installation. In many cases, electrical problems occurring during driving actually reduce the quality of the signals and reduce the precision of the test. We’re dedicated to providing a precise and well planned service, we make sure the connection is guaranteed and the instruments are performing at their theoretical (laboratory proven) maximum and we also take care of the regular certification of the instruments.

Q: Is there any limitations for pile diameter (L:D) to use PDM?

No there aren’t any limitations, PDM is a very flexible methodology and can be adapted to your specific project. We can carry out PDM on all piles regardless of their diameter. For example, traditionally offshore monopiles piles have a very large diameter of up to 8m.
Also, by instrumenting your piles at different levels you can measure how the energy is distributed along the pile to see if any energy is reflected before it reaches the soil.

Q: How accurate are the bearing capacity predictions?

The bearing capacity predictions can be between 90-100% accurate however it depends on how accurately the methodology is applied. As experts in PDM we will always work with our clients to make sure all instrumentation is applied precisely, allowing them to achieve the most accurate results. A long, public track record is available to prove this.

Q: Is PDM recommended in standards for offshore wind?

PDM is not strictly recommended by standards, but it is very often required by the certification authorities which require you to prove the pile is designed, as required, for the lifetime of the structure.
Currently, we believe that only the BSH in Germany requires pile testing for jacket or tripod piles (normally not for monopiles, at least for Wind Tower Generators). If dynamic tests are undertaken in Germany, they should be ideally calibrated against static tests in similar conditions, otherwise the uncertainty factor will be very high. It seems that Poland is following Germany’s route (i.e. design approach following Eurocode), therefore, pile testing may also become a requirement in this country as well.

Q: Do you know if marine warranty surveyors request PDM for some pile installation in challenging soil conditions?

The marine warranty surveyor (MWS) only deals with installation aspects not design aspects. Therefore, it’s unlikely that any MWS would asks for pile tests however certifiers may ask if they consider that the design is not safe enough.

Q: Based on your experience on offshore wind, how many piles do you instrument on an offshore wind farm?

Depending on which standard you use, you’d usually instrument 5-10% of piles in an offshore wind farm. Eurocode specifies an absolute number of tests (i.e. not a proportion of piles to be tested). The lowest uncertainty factors correspond to more than 20 tests per soil province. Dynamic tests on 20 piles is obviously a lot. Therefore, the approach that is generally followed is to indicate that pile driving data (blowcount and hammer energy) are available for all piles and selected piles are dynamically tested in each soil province (say 3 Wind Tower Generator positions).

Q: Roughly what percentage of offshore pile driving projects use PDM?

The percentage of offshore pile driving projects varies by country and depends on engineering traditions and they types of piles used. In some cases, PDM / Pile Dynamic Testing is not welcome due to “bad reputation” of this technique or bad experience had by a client. When solicited by asking our clients to share the results which caused this, we always discover that the method was not applied properly and carried out by non professionals. Although we cannot share these as case histories due to confidentiality, we were able to change clients’ opinions to prove they cant rely on this method.

Q: Do you find that the pile OEM or Client insist on PDM measurements, or is this a ‘nice-to-have’ reassurance for the installation contractor?

Often the developer specifies the need for PDM, to assure the installation process and for future piece of mind. Of course, PDM also monitors the actual energy imparted to the pile so it can also control the risk of pile damage, which the contractor is often responsible for.
As discussed in the Webinar, we believe that PDM/PDT is no just a “nice-to-have” reassurance, but provides an economical benefit, even if in some cases, you’ll benefit in terms of future improvements or risk mitigation.

Q: Is PDM applicable to onshore projects and to concrete driven piles?

Yes, it definitely is. Pile Driving Monitoring can be used on projects in any location – onshore, nearshore, offshore and underwater. It is also applicable to concrete driven piles and behaves very well in precast concrete piles and in custom site drilled concrete piles. Onshore, it can be also used on drilled, cast in place piles; but during design it should be checked whether the pile can withstand the dynamic stresses and if some additional rebars should be added to allow for this.

Q:Would you recommend PDM for piles socketed in rock?

Yes, indeed. Using PDM and PDT for piles socketed in rock can be done and has been done, however it requires attention regarding how the testing is carried out on site and the behaviour of the rock. Simple and straight forward PDM makes the assumption that soil doesn’t lose its resistance during driving, i.e. its not fragile, and if it does lose resistance it can be recovered. In the case of having a pile socketed in rock, the pile needs to be drilled and grouted and it’s important to be careful not to break the rock or the grout when setting up the PDM testing.
By adapting the PDM and PDT technique, although you will not be able to test your pile to the ultimate capacity because that would require you going towards the failure, you can proof test the capacity of your pile.

Q: What period do you recommend between end of driving (EOD) and restrike?

We’d assess this on a case by case basis – the more time available to wait after the end of driving, the more precise the results will be, however this will add cost. We would determine it by understanding how far you are from capacity at the end of driving and if you can safely reach the ultimate capacity by taking the lower bound of the data.

Q: Can you discuss fiber optic technology on instrumentation or steel in R&D?

Fiber optics is a very promising technology and is being used more and more frequently. One substantial advantage of fiber optic technology is that there is no electricity so it reduces a lot of problems when working in water. Another advantage is the size of the sensors , they are very small, making them easier to install and simpler to protect. Overall the instruments perform quite well, although currently not quite at the same level as electric instruments (accelerometers in particular) so we usually need to use a mix of fiber optic and electrical instruments.
In terms of R&D, we are developing the use of fiber optic technology in particular for pile tip buckling.

Q: What percentage of your projects use wireless sensors?

While we do use wireless sensors, and they are certainly a very interesting technology, at the moment we tend to keep their use to a minimum. This is because our clients want us to provide 100% reliability and at the moment, using cables allows us to deliver more accurate results as there is no interference with the steel and it is easier and more precise to install sensors using cables.

Q: Do you prefer to apply bolted or glued method?

The standard, bolted method provides the simplest solution and performs the best overall, so where possible, we would choose to use this method. The glued sensors are very delicate and require detailed planning and design before installation, so at the moment, we would use them if the other option was investigated and was found not to be feasible.

Q: Do sensors have to be glued on uncoated areas?

Yes, glued sensors do have to be applied to uncoated areas to make sure they can grip onto the steel.

Q: How many orientations around the pile circumference would you apply sensors?

Normally you would apply the sensors in pairs. The number we’d apply depends on the diameter and the type of pile. For offshore piles, we would usually apply a minimum of two positions (2 gauges and 1 accelerometer) and then scale up in pairs.

Q: Can you elaborate on the use of the glued PDM sensors? Does it require any special surface preparation and who carries it out?

Glued PDM sensors are very delicate so it requires detailed planning and design beforehand. It does also require special preparation of the surface because it is glue and will not work if it’s not secure. We prefer to prepare the surface ourselves to guarantee the procedure is applied correctly however it is possible for us to arrange this with the team on site, if the client prefers.

Q: Is it possible with the system to detect the exact position of a large boulder at pile tip during pile driving?

PDM can help you to understand if you’ve hit something during driving because the signal might change, however it depends on the size of the boulder as well as how the boulder is hit. Still today this is a subject of G-Octopus research and there is still not a straightforward method to do this. For example if the pile is reacting by buckling due to this large boulder, the pile wouldn’t easily show this at the pile head. Depending on how the pile is designed, in some case we can do something and this is something we’re studying.

Q: How does PDM help to deal with the pile tip buckling issue during the piling process?

Pile tip bucking is really a tricky phenomena. We know, because we’ve been studying this problem for a long time and we’ve set up complex numerical modelling to simulate this. What we see is that buckling can happen in many ways, most of which are completely unnoticed at pile head. We’re making progresses every week, but there is still a long way to go.
However, at least at the beginning, one of the way buckling occurs is by overstressing of the base of the pile. In this case, by using PDM, we can understand if there is an overstressing of the base or if there is unusual activity happening there because this will change the signature of the signal and, in most cases we can also detect if there are hard reflections from the base.
Currently there is no real solution to pile tip buckling, even PDM cannot really observe what happens at the pile base however we are undergoing research on this topic and starting to see some results.

Q: Do you have any examples comparing previous drivability studies and real PDA for vibro hammer?

We have examples of the comparison of driveability studies for PDA and vibro hammers regarding the impact of hammers but the data we have is the property of our client so it’s not something we’re able to share at the moment.

Q: How would instrumenting vibrated piles work?

Instrumenting a vibrating pile during installation, with the vibrator working, has proven to be a very good source of data to understand what really happened during driving. We have good experience in vibrated piles so we can confirm this technique can be profitably used. By instrumenting the pile it’s possible to get a lot of information about how the vibration works and how much you can reduce the vibro driving resistance. There is still no agreed and straight procedures about how to interpret the data, but we worked on this with what we consider a really satisfactory conclusion. Thus, we recommend instrumenting vibrated piles.
As well, PDT can be used to test vibro installed piles after installation is completed; this would really provide a benefit to a project, to verify if the required capacity is available.

For more information on pile driving monitoring or anything discussed during our webinar, please get in touch and team will be happy to help. Or sign up to our mailing list and be the first to know about our next webinar!