Gravity base foundations
A large concrete structure that stays put because of its own weight, with nothing driven into the seabed at all.
Last updated 2026-09-07
What is Gravity base foundations?
A gravity base foundation holds a turbine upright by being heavy. It is a large concrete structure, typically a wide base slab with a conical or cylindrical shaft rising to the tower flange, that is set down on a prepared seabed and ballasted until its weight and its footprint resist everything the wind and the sea can apply. Nothing is driven, nothing is drilled and nothing is pumped into the ground. That single fact is the whole appeal: installation is essentially silent underwater, which removes the marine mammal noise constraint that dominates the programme of every driven foundation and can open working seasons that driving cannot reach.
The trade is that the structure has to be enormous, and it has to be made somewhere. A gravity base for a commercial turbine is a substantial civil engineering job in its own right, cast either in a dry dock, in a basin that can be flooded, or on a quayside with a heavy lift to the water. It then has to be floated or carried out to position, which needs either buoyancy designed into the structure or one of the small number of vessels able to lift it. Ports capable of casting, storing and launching a run of these structures are rare, and where they do not exist the project has to build them, which changes the economics entirely and is often what decides against the option.
The ground decides the rest. A gravity base transfers its load straight into the seabed over a wide area, so it needs competent material at or near the surface, and it needs that material to be there consistently across the array. Soft or variable seabed either rules the option out or forces expensive preparation. Every position is dredged, levelled and given a prepared bed, usually of graded gravel, to a flatness the designer specifies, and that preparation is a marine operation with its own vessels and its own tolerance regime. Scour is the through-life issue, because a structure resting on the seabed relies on the seabed staying where it is, and scour protection around a wide base is a significant part of the installed cost.
How does Gravity base foundations work, step by step?
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Step 1: Confirm the ground can take a surface-bearing structure
The site investigation has to show competent material at or close to the seabed across every position, with enough consistency that a repeated structure makes sense. Bearing capacity, sliding resistance, settlement and the risk of scour are all assessed, and the designer sets the base diameter, the ballast requirement and the bed preparation from that assessment. Where the ground is soft, layered or highly variable the option is usually abandoned early, because the preparation cost to make it work overtakes the saving from not driving.
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Step 2: Secure the casting and launch facility
Gravity bases need somewhere to be built. That means a dry dock, a flooding basin, a barge-mounted casting facility or a quayside with the crane capacity to lift a completed unit into the water, plus laydown to store finished units while the next ones cure. Water depth at the quay and along the departure route has to suit a structure with a deep draught. Securing or building that facility is a project decision taken years ahead, and on many projects the absence of a suitable port is the reason gravity bases are not used.
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Step 3: Cast the structures
Each unit is built as a heavily reinforced and usually post-tensioned concrete structure, cast in a controlled sequence with slipformed or jumpformed shafts and a large base slab. Concrete for marine exposure is specified for durability as much as for strength, and cover, curing and crack control are the quality battleground because the structure sits in seawater for its whole life with no realistic prospect of major repair. Embedded items - the tower flange assembly, the cable entries, the ballast and grouting penetrations, the access arrangements - are set precisely, because getting them wrong is far more expensive here than on a steel structure.
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Step 4: Prepare and level the seabed
At each position the seabed is dredged to remove unsuitable material, and a prepared bed, commonly graded gravel, is placed and levelled to the flatness the designer requires. This is a precise marine operation carried out with dredgers, fall-pipe or placement vessels and survey-guided screeding gear, and it is verified by survey before anything is set down. The tolerance is tight because a wide, rigid, extremely heavy structure landing on an uneven bed concentrates load where it was not designed to go. Dredged material disposal is itself a consented activity with its own conditions.
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Step 5: Transport the unit to position
Depending on the design, the structure is either floated out under its own buoyancy and towed, or lifted onto and off a heavy transport vessel. Floating units are stability-critical during tow, and the tow is planned as a marine operation with defined limits and independent review. Draught along the route, tidal windows and the availability of a sheltered waiting position all shape the plan. Because the units are so large, only a small number move in any one weather window, and the campaign rate is set by the transport far more than by the placement.
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Step 6: Set down and ballast
Over the prepared bed, the structure is lowered or de-ballasted onto its footprint under close survey control, with position, orientation and level monitored as it lands. Once down, the compartments are filled with ballast - typically sand, aggregate or rock - to bring the structure to its design weight. Ballasting is done in a controlled sequence so that the load comes on evenly, and level is checked as it proceeds. Where a design uses a grouted skirt or a levelling layer beneath the base, that is completed as a separate controlled operation.
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Step 7: Place scour protection and complete outfitting
A wide structure resting on the seabed disturbs the flow around it, so scour protection is placed to the design extent, usually as a graded rock layer with a filter beneath it, and surveyed to confirm coverage and thickness. Cable entries and J-tubes are made ready for the array cable campaign, boat landing and access arrangements are proved, and navigation aids are commissioned. The tower flange is cleaned, protected and surveyed for level, because it is the interface the turbine will land on.
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Step 8: Monitor settlement and scour through life
A gravity base is monitored for exactly the things that could undo it: settlement and tilt of the structure, and movement of the seabed and the scour protection around it. Instruments installed at construction give the designer real data to compare against the assumptions, and repeat bathymetric survey tracks the seabed. Because there is no pile holding the structure down, evidence that the bed is behaving is the assurance, and remedial rock placement is a normal maintenance activity rather than a failure.
What are the benefits of Gravity base foundations?
- No driving at all, so underwater noise during installation is very low and marine mammal constraints largely fall away
- Can open working seasons and locations that a driven foundation could not be consented for
- Uses concrete and local labour rather than large-diameter steel, which can suit regional supply chains
- Very long design life with a robust, damage-tolerant structure once in place
- Straightforward decommissioning in principle, since the structure is de-ballasted and lifted or floated rather than cut below the seabed
- Avoids the fatigue-critical driven pile connection details entirely
What are the limitations of Gravity base foundations?
- Needs competent, consistent ground at or near the seabed, which many sites simply do not have
- Requires a dry dock, basin or heavy-lift quay for casting and launch, and few ports can do it
- Every position needs dredging, bed preparation and levelling to a tight tolerance before anything is placed
- Very large and very heavy units limit how many can be transported and placed in a weather window
- Scour protection around a wide base is a major cost and a through-life maintenance commitment
- Deep draught during float-out restricts the ports and routes that can be used at all
What is Gravity base foundations best suited for?
What plant does Gravity base foundations need?
- Dry dock, flooding basin or heavy-lift quay with laydown for completed units
- Slipform or jumpform equipment, batching plant and post-tensioning gear for casting
- Dredgers and fall-pipe or placement vessels for bed preparation and scour protection
- Survey-guided levelling and screeding equipment for the prepared bed
- Tugs and towage spread, or a heavy transport vessel, for float-out and delivery
- Ballasting plant and remotely operated vehicles for placement monitoring and as-built survey
How is Gravity base foundations quality-checked?
- Concrete durability control - mix, cover, curing and crack width - recorded against the marine exposure specification
- Post-tensioning records for every tendon, with duct grouting verified
- Dimensional survey of embedded items, especially the tower flange assembly, before the unit leaves the yard
- Prepared bed level and flatness survey signed off before set-down at every position
- Set-down position, orientation and level recorded under survey control, and re-checked after ballasting
- Scour protection coverage and thickness surveyed, with a bathymetric baseline for through-life monitoring