Tackling offshore wind foundation challenges in Australia’s carbonate soils

The industry’s preferred choice of conventional foundation methods is monopile foundations. Monopiles, large steel tubes driven into the seabed to support turbines, are cost-efficient foundations typically applied in shallow waters. As the water deepens, jacket structures become more cost-effective. These lattice frameworks are fixed to the seabed using driven piles or, in some cases, suction bucket foundations, which create a vacuum to secure the foundation. However, Australia requires innovative approaches to address its unique soil conditions..

In the Gippsland Declared Area (DA) off Victoria's south coast of Australia, progress is underway where twelve feasibility licences have been awarded for offshore wind farms. The Australian Federal Government has announced five additional DAs, opening new opportunities for offshore wind companies to apply for feasibility licenses. These licenses empower developers to conduct crucial metocean, environmental, geophysical, and geotechnical surveys needed to determine the feasibility of offshore wind farms.

Why are carbonate soils problematic?

Australia’s first offshore wind farm will take place in the Bass Strait, where water depths would typically favour monopiles or jackets with driven pile foundations. However, the region’s presence of carbonate soils demands greater scrutiny in the design and installation of these traditionally reliable foundations.

The problematic nature of the soil in the Bass Strait was first highlighted in the late ‘60s during early development of Australia’s offshore oil and gas infrastructure. Engineers noticed during installation that driven piles penetrated the soil far too easily.

Why? Because the carbonate soil was lightly cemented but highly compressible. This led to expensive remedial works to strengthen the first-generation platforms. These lessons led to a change toward drilled and grouted piles for future platforms. However, this approach also faced challenges. In the early 1990s, while installing another offshore oil and gas platform, it was discovered that the primary piles (used as casings through overlying weak sediment) had been extensively damaged, with the pile tips almost completely closed in most cases. Once again this required a significant and costly engineering remediation effort.

Foundation technology has come on a very long way since then, but offshore Australia’s carbonate soil continues to present big challenges for foundation designers.

Why is local expertise crucial?

In 1994, Advanced Geomechanics (AG), a specialist geotechnical consultancy alongside leading academics, embarked on a mission to investigate the tricky nature of offshore Australia’s carbonate soils. Their mission was to transform these findings into practical foundation engineering solutions.

Our local knowledge sets us apart. Since acquiring AG in 2013, we have combined global capabilities with deep-rooted local expertise. This combination has significantly enhanced our understanding of Australia's distinctive carbonate soils and elevated our geotechnical consulting services. From our Perth office, we deliver offshore foundation design services that utilise our knowledge of local soil conditions. This is knowledge that can't be replicated without years of hands-on experience in these challenging conditions.

With nearly 30 years of detailed foundation design work for the offshore industry in Australia, we've gained invaluable insights you won't find elsewhere. The lessons learned over decades must now guide Australia's emerging offshore wind farm industry. Without this local expertise, developers risk repeating expensive mistakes that we've already solved through decades of local knowledge.

What are the key lessons learned?

To support Australia’s offshore wind farm industry, here are key lessons we’ve learned:

• Pile free-fall: Driven piles often can't support their own weight due to low skin friction. Arrestor devices, which use vented top caps to control fall speeds, are typically used to mitigate the risk of free fall. However, they only work for fully submerged piles. This means they won't work for most offshore wind farm piles, and new solutions are required.

• Pile deformation: Hard cemented layers can damage pile tips during driving, even in uncemented sediments. Drill bits used for pile installation are only slightly smaller than the inside diameter of the primary pile, so even minor tip deformation can cause the drill bit to get stuck. Larger wind farm pile diameters increase this risk, requiring robust installation methods.

• Open-hole hazards: Australia's offshore carbonates typically have hard upper layers covering unstable lower materials, creating drilling risks. Top-down reverse circulation drilling can help but is extremely challenging and costly for wind farm applications. Proper site investigation, specialised testing, and advanced analysis are essential to assess these hazards.

• Soil weakening: Pile foundations experience substantial weakening under monotonic loading and severe degradation during cyclic loading. Specialised cyclic CNS tests are needed to track soil response under load and displacement cycles. Advanced analysis tools calibrated to this test data help design piles that can withstand storm-induced degradation.

• Get into the groove: Some calcarenites are very strong, which reduces costs by allowing shorter pile lengths and drilling depths. However, specialised under-reaming tools may be needed to cut regular ‘grooves’ in the hole wall and make it rough enough to sustain the design loads.

• Check the lateral pile response: Lateral pile behaviour varies significantly between soil types. In calcarenites, piles can be strong and stiff but require consideration of brittle fracture mechanics. In uncemented carbonate soils, piles may be softer due to high compressibility, temporarily constrained drainage, and weakening under repeated loading. These factors require careful evaluation with calibrated site-specific soil models.

Why must we exercise caution when using jack-ups?

Around the world, jack-ups are a widely used for safe, efficient and cost-effective offshore wind installations, and they are expected to play a key role in Australia. However, this depends on the presence of thick layers of sand, calcarenite or cemented material are found at or near to the seabed.

If this is not the case, then based on our unique and extensive experience in the geotechnical assessment of jack-ups in Australian carbonates, we strongly recommend taking extreme care and undergoing extensive planning and preparation. Here’s why:

• Surficial carbonate soils often have high lateral and vertical variability and tend to be silty (with different soil drainage conditions at different loading rates). This can lead to unexpected and sometimes significant penetrations, instability during storms and deep spudcan penetrations that require prolonged extraction operations.

• The jack-ups that support offshore wind farm construction activities tend to impose far greater bearing pressures on the soil than the typical jack-ups used by the offshore oil and gas industry. An offshore wind farm jack-up may require significant modification to minimise risk of excessive penetration, excessive extraction times and to manage punch-through risks.

• During stormy weather, cyclic loading may cause the jack-up’s foundation bearing capacity to drop significantly below a previously applied static preload. This may cause the spudcan to penetrate many metres deeper into the seabed. Assessing and addressing this risk requires a detailed and careful analysis that accounts for cyclic loads, soil consolidation behaviour and soil strength.

In conclusion

Foundation design and construction in offshore Australia’s carbonate soils can be very challenging and can trap the unprepared. Success will require high quality site investigation data and analysis, comprehensive testing and expert interpretation to inform engineering design.

With unrivalled experience in Australia’s offshore carbonates, our Geo-data experts have a proven track record of creating innovative solutions ensuring success where others have failed.