Monopile vs Jacket Foundations: How Water Depth Decides

The simplest option: a monopile

A monopile is exactly what it sounds like — one large steel tube, typically 6-10 meters in diameter, driven directly into the seabed like a giant nail. It's the simplest, cheapest foundation type to fabricate and install, which is why it dominates shallow and mid-depth sites. Sif and other European fabricators now build monopiles over 100 meters long for the largest turbines.

Why depth breaks that simplicity

A monopile resists the enormous overturning force of wind and waves purely through its own stiffness and how deep it's embedded in the seabed. Past roughly 35-40 meters of water, that approach starts to demand an impractically thick, heavy tube — the structure becomes its own worst enemy, since more steel means more weight, which means more force to resist in the first place.

The alternative: a jacket

A jacket foundation is a lattice of smaller steel tubes — similar in principle to an oil and gas platform or an electricity pylon — braced diagonally and anchored to the seabed with multiple smaller piles at its base rather than one giant one. By distributing load across a wider base and multiple structural members instead of one solid column, it stays viable at depths where a monopile would need to be absurdly oversized.

The real tradeoff

Jackets cost more to fabricate (more welds, more components, more precision in assembly) and take longer to install, but they scale to depth far better than monopiles do. Developers choose based on a straightforward calculation: site depth, seabed soil conditions, and turbine size all factor in, but depth is usually the deciding variable. Most of today's shallow North Sea projects use monopiles; deeper sites and larger structures (including some offshore substations) increasingly use jackets.

The takeaway

Neither foundation type is "better" in the abstract — they're optimized for different physical conditions. As offshore wind pushes into deeper water and uses heavier turbines, expect jackets (and eventually floating foundations, for the deepest sites) to take a growing share of what's currently monopile territory.