The four-legged jacket foundations standing beneath Borssele's offshore substations look calm today — repetitive, almost routine. But during load-out, transport, and upending, each jacket went through structural conditions it was never designed to see once in service.

While the Borssele wind turbines themselves stand on monopile foundations, the wind farm's two offshore high-voltage substations — Borssele Alpha and Borssele Beta — were built on jacket structures, a lattice design whose permanent-state engineering assumes a fully braced, piled, and grouted system. Getting there is a separate problem entirely.

4-leg
Jacket configuration
≈22 km
Distance offshore
14–36 m
Site water depth
2019–20
Construction window
Phase 1 — Load-out
Horizontal loads the final design doesn't address
Before the jacket ever sees water, it must be loaded out from the fabrication yard onto its transport vessel — a horizontal support condition that puts the lattice structure under bending loads its final, vertically braced design was never engineered to resist.
Phase 2 — Marine transport
Dynamic motion at sea
Once at sea, the jacket rides as cargo, exposed to the vessel's pitch, roll, and slam loading — a dynamic environment entirely different from the static, seabed-fixed condition its structural design ultimately assumes.
Phase 3 — Upending
Rotating from horizontal to vertical
Upending — rotating the jacket from its horizontal transport orientation to the vertical position required for installation — is typically performed using a crane, ballasted flotation, or a combination of both, and represents one of the highest-risk phases in the entire sequence.
Phase 4 — Pre-piling and pile driving
The structure still isn't finished
Once positioned, the jacket stands on temporary support before pin piles are driven through its legs into the seabed. Only once those piles are driven and the connection is grouted does the structure reach the fully braced condition its permanent design was actually built for.

Before piles are driven. Before grout locks the system. The jacket you see standing today spent most of its life as a completely different structure.

Borssele's substation jackets are a reminder that a structure's permanent design only describes one state among several it must survive. Load-out, transport, and upending each impose their own load cases — and getting a jacket safely through all of them, intact, is a distinct engineering problem from designing the structure it becomes once the piles are driven.

For further reading on temporary structural states, see What is Temporary Works & Erection Strategy?

Related case studies: London Array · Horns Rev 1 · Hywind Tampen

Core concepts: What is erection engineering? · What is heavy lift engineering?

A row of offshore wind turbines on the horizon at sunset, seen across a calm grey sea under an orange sky
The Borssele offshore wind farm on the horizon at sunset, April 2025. Resized from the original for web display.
Photo: Thorstentbln, via Wikimedia Commons, CC0 1.0.
Four-panel schematic of the Borssele substation jacket sequence: load-out from the yard in a horizontal support condition; marine transport as cargo exposed to pitch, roll and slamming; upending from horizontal to vertical by crane, ballasted flotation or both; standing on temporary support until pin piles are driven through the legs into the seabed in 14 to 36 m of water.
Borssele offshore substations — when the jacket isn't working as designed yet.
Diagram: EE&HL Network, 2026. Conceptual schematic — not to scale.
Tap or click the diagram to open it full size.

Sources: HSM Offshore Energy — EPCI contractor's record for the Alpha jacket and topside; Offshore Energy — Beta jacket and topside installation dates. This case study covers the two TenneT offshore high-voltage substations (Alpha and Beta), not the wind-farm turbine foundations.

Frequently Asked Questions

While the Borssele wind turbines themselves stand on monopile foundations, the wind farm's offshore high-voltage substations — Borssele Alpha and Borssele Beta — were built on four-legged jacket foundations, a lattice steel structure distinct from the single-pile monopile design used for the turbines.

During load-out, transport, upending, and pre-piling, a jacket experiences structural conditions it was never designed to see in service — horizontal transport loads, dynamic motions at sea, and the bending stresses of rotating from horizontal to vertical. Before piles are driven and grout locks the system into its final, braced configuration, the jacket must survive as a standalone structure under a completely different set of load cases than its permanent design addresses.

Upending is the controlled rotation of a jacket foundation from the horizontal orientation used for transport — typically on a barge or vessel deck — to the vertical orientation required for installation. It is usually performed using a crane, ballasted flotation, or a combination of both, and represents one of the highest-risk phases in the jacket's installation sequence.

Once upended and positioned, pin piles are driven through the jacket's legs into the seabed, and the connection is typically completed with grout, which locks the jacket into its final, fully braced structural system — the state its permanent design actually assumes.