David Short is an Engineer at Empire Engineering. Here, David shares his insight into sand berm storage of monopiles.
As offshore wind turbines continue to grow in size and capacity, so too do their foundations. The monopiles supporting these giants are becoming increasingly massive, presenting new challenges that the industry must address head-on. One area that deserves particular scrutiny is how we store these XXL monopiles during the critical period between fabrication and installation.
Sand berm storage has emerged as a popular solution, offering apparent advantages in terms of cost, construction speed, and operational flexibility. However, my recent analysis suggests we may need to rethink this approach as monopile dimensions continue to expand.
The appeal of sand berms
On the surface, sand berm storage appears to tick all the right boxes. The construction is relatively straightforward, costs are kept low, and the system offers the flexibility that project schedules demand. For project managers juggling tight timelines and budgets, it’s an attractive proposition that seems to solve multiple problems simultaneously.
But as any experienced Engineer knows, what appears simple on the surface often conceals complex underlying challenges. This is certainly the case with sand berm storage for large monopiles.
Uncovering the hidden risks
Through detailed finite element analysis (FEA), it is possible to identify several concerning phenomena that occur when XXL monopiles are stored on sand berms. The results reveal stress concentrations and deformation patterns that should give the industry pause for thought.
High stress regions under self-weight are observed due to local punching / bending action directly at the small contact areas between the monopile and sand berms as well as due to large ovalisation seen in the monopile cans. This ovalisation is far greater than that seen in alternative storage methods using stiff cradles that provide constraint against can ovalisation.
Ovalisation stresses in monopile cans are observed as high tensile stress areas at the top and bottom and high compressive stress areas at the sides as can be seen for the toe can of the monopile shown below.
Does sand berm design hold the key?
When faced with such high stresses, the question becomes: can the sand berms be optimised to eradicate any structural integrity issues?
Investigation into the sensitivity of monopile stresses to sand berm indent geometry and material properties reveals a few key findings:
- Increasing berm indent height by 40% leads to roughly a 10% reduction in maximum stress.
- Increasing berm indent length by 40% leads to roughly a 10% reduction in maximum stress.
- Approaching 0 cohesion increases ovalisation stresses observed in the toe can significantly.
- Reducing sand berm stiffness reduces punching stresses observed at the can sections but increases ovalisation stresses in the toe can.
So, sand berm design can help. But unfortunately in the real world there are hard constraints on sand berm geometry due to additional operation requirements at the yards such as clearances for the Self-Propelled Modular Transporters (SPMTs) used to lift the monopiles from the berms.
When applying reasonable real-world geometrical constraints significant stress reduction is often not possible through sand berm design alone.
The uncertainty problem
All FE analysis contains a degree of idealisation and modelling uncertainty but when geotechnical material such as sand enters the picture these uncertainties become especially significant.
Sand berms present a complex geotechnical environment where material properties can vary significantly across the storage area. Full system FEA typically requires a single idealisation of the sand berm material to avoid unfeasible analysis time meaning accuracy must be compromised. Additionally nonlinear geomechanical constitutive models are required to simulate the real-world behaviour of the sand material and as is so often the case for analysts a balance must be struck between accuracy and stability. More often than not stability must be prioritised to achieve a converged solution at the expense of accuracy.
All the uncertainty above compounds the structural risks. While we can model expected behavior with reasonable accuracy, the range of possible outcomes includes scenarios where structural integrity may be compromised.
Looking toward the future
In the work we have completed on current industry projects, yes stresses are surprisingly high but still not outside the recognised standards acceptance criteria but will that be the case for future monopiles?
To test this out I analysed the mother of all future monopiles, following the widely publicised maximum capacity of the newly built CWHI Qinzhou monopile fabrication facility. They plan to produce monopiles with a maximum diameter of 15m and a maximum weight of 4500 tonnes.
When creating the model I stuck to a maximum diameter / thickness ratio of 140 and applied load factors defined by DNV, including an ultimate limit state factor of 1.2 and a material uncertainty factor of 1.15.
As you can imagine the XXXL monopile didn’t fare well. The bottom section of the monopile experienced plastic collapse and the simulation diverged at 88% of the load application as shown below.
This points to a couple of opposing conclusions, namely that storing giant monopiles on sand berms will at some point become unfeasible without additional anti-ovalisation tools deployed or that the current DNV standards are overly conservative and will need to be reassessed.
One thing however is very clear and that is that we should stop thinking of sand berms as an inherently low risk storage option that only requires a quick back-of-the-envelope style assessment. Fundamentally we should be taking as much care as possible when storing such key equipment to avoid any permanent plastic strains. Allowing even a small amount of plastic strain should not be accepted due to the risk of plastic strain accumulation during additional storage, transport and installation operations leading to potentially disastrous consequences such as triggering buckling and fracture during pile driving.
Reach out to David on LinkedIn or read more about Empire Engineering’s offshore wind expertise.
