This type of jacket uses standard pipe sections in lattice structures. The idea of these jackets is to cut the cost of steel but at the same time it is much more complicated and costly to manufacture. Hexa-base jacket - subseaworldnews.com

Advantages

  Hexa-base jacket cuts the cost of steel by up to 40%     according to weight. (WINDPOWEROFFSHORE, 2013).

Disadvantages

  It is more complicated to build than other types of     jackets and its manufacture may require more     operations, thus increasing the costs.

  It has six piles which mean six piling operations with their     respective piling noise and environmental impact.

  They use no less steel than other types of jacket.

  They have not been proven for OWT foundation.

They have the largest market share and there is a proven track record of their performance as foundations for OWT in deeper water depths. Four-legged jackets were originally installed with skirt piles and mud mats. In this way the jacket could be installed and piled in two different operations by vessels used to each operation. However, the expected flexibility of installation was not met since the installation vessels were expensive and the same vessel was used to do the installation and the piling. Therefore, the industry has moved to pre-installed pin piles which are installed before the jacket and avoids using mud mats. The connection between the pre-installed piles and the piles of the jacket legs is done by grouted connections. The drawback of this approach is that the pile installation vessel must be fitted with an expensive template to ensure that the piles are drive into the seabed correctly. (WINDPOWEROFFSHORE, 2013).

Four-legged jacket - www.bifab.co.uk

Advantages

  Four-legged jacket is a proven technology used in the wind industry and can withstand the conditions in locations with 35 m of water depth and beyond.



  They are easier to manufacture than twisted and hexa-base jackets and hence their cost of manufacture is lower if compared.



  If one leg fails it does not mean the collapse of the structure which could happen in case of a three-legged jacket.

Disadvantages

  It has an extra leg compared to three-legged jackets so it is 33% more expensive regarding set-up time and piling.


  The extra pile means an extra piling operation with its respective noise and environmental impact.

Three-legged piles have the minimum number of legs, they have fewer joints to make and fewer piles to drive into the seabed. On top of that, they hold the level while the grout sets between the jacket and the pile better than four-legged jackets. The reason is that three points of support defines a plane whereas four support points can define more than one plane. A real-life example is a wobbly table with four legs compared to a stool with three legs which never presents that problem.

However there is a concern about if the one leg fails, the three-legged jacket becomes very unstable. This fear comes from oil and gas industry where the consequences of this kind of failure were very important. It can be argued that in the wind industry the consequences would not be as important and therefore the industry should focus on finding solution to this problem and make use of the advantages of three-legged piles.

Advantages

  Three-legged jackets have fewer joints to make and they     are easier to produce so their manufacture cost is lower.

  The amount of steel used is reduced and so are the material     costs.

  They have fewer piles to drive into the seabed so that     the noise and the environmental impact associated with     piling operations is minimised.

  They hold the level better than four-legged jackets.

Disadvantages

  In case of one leg failure the jacket substructure becomes highly unstable and could bring     the structure to collapse.