B.2.2 Jacket

Function

The primary function of a jacket is to support the static and dynamic loads of the wind turbine by anchoring it firmly to the sea bed using a set of pin piles or suction buckets. Secondary functions include supporting the wave loads on the jacket itself and enabling cable entry.

A jacket foundation does not have a separate transition piece. The upper part of the jacket performs many of the functions of the transition piece, which are described in B.2.3 and its sub-sections.

What it costs*

About £476 million for the jackets for a 1 GW wind farm at 50 m water depth. This cost includes the pin piles and the upper part of the jacket, which performs many of the functions of the transition piece. Suction buckets are typically more expensive than piles and ground conditions are the main driver of selection.

Who supplies them

Jacket suppliers include Cedeco, Eni, Lamprell, Navantia Windar, SK Oceanplant, Smulders and W3G Marine.

Key facts

Jacket foundations make up approximately 15% of current installed capacity and are typically used at depths greater than 55 m.

There are several different versions of jacket structures, three legged are the most widely used. Others include, four legged, “twisted” and “true X-braced”. Jackets are steel structures with small struts welded to larger legs creating a latticed structure. Most jackets are secured to the sea bed using pin piles, with suction buckets providing another option. Both are typically installed at the base of each leg.

Pin piles are essentially mini monopiles which are driven through apertures in the base of the jacket’s legs into the sea bed. These can be designed for most soil conditions by varying pin dimensions such as length and diameter. For hard ground conditions drilling and grouting can be used as an alternative to driving. 

Suction buckets are inverted buckets connected directly to the jacket which are secured to the seabed using suction. This force is created by pumping water out of the bucket as the structure is lowered into the seabed resulting in a vacuum which resists upward force acting on the suction bucket. Suction buckets have the advantage of little or no piling noise. They have the disadvantage of only being able to be installed in medium sand or clay. Sites with very shallow bedrock or the presence of boulders in clay soils are not suited to suction buckets.

Suction buckets are typically wide and short whereas pin piles are narrow and long. For comparable conditions a suction bucket may be 11 m diameter by 10 meters in length, whereas a pin pile may be 3 m by 45 m in length. 

For commercial scale projects multiple fabricators tend to be required for jacket, pin pile and suction bucket supply.

For a 15 MW turbine in 55 m water depth the approximate dimensions of a jackets will be: 40 m footprint, 85 m height, with jacket mass of 1,500 t and total pin pile mass of 600-700 t.

The main reason for utilising jackets is water depth. There are a number of other reasons which include:

• Ground conditions which are too hard for monopiles, as it is easier to drive small piles compared to large monopile foundations into the sea bed.
• Ground conditions which are too soft for monopiles as the load can be more effectively transferred to the ground by a jacket due to it having a wider footprint and spreading of the load across at least three piles or suction buckets.
• Lower noise emissions, driving smaller diameter pin piles creates less noise than driving a single large monopile.
• For sites where moderately shallow bedrock means that monopiles are not a viable solution, jackets can be used in conjunction with drilled and grouted piles or suction buckets.

Manufacturing savings can be achieved because jackets use less steel than monopile for an equivalent turbine. This improvement is partially offset by the increased complexity of welded nodes, the joints between pin piles and jacket base and the transition at the top between the lattice structure and the tubular tower base.

Jackets may make up a higher percentage of future installed capacity if: 

• The cost of steel increases, 
• Further automation can reduce the cost of fabrication, 
• There is an increase in the proportion of sites with more than 55 m water depth, and/or
• The combination of load sets at a particular site favours them as a design solution.

What’s in it

• Steel lattice
• Struts
• Nodes
• Pin piles
• Protective coating