Jacket constructions for offshore structures

Jacket constructions belong to the dissolved load-bearing structures in the offshore industry. They are already used for many years for the establishment of oil and gas platforms. There are now but also in the field of wind energy first prototypes, which were set up, both onshore and offshore of REpower Systems AG together with industrial partners.

A Jacket consists usually of four corner posts and tubular crossing diagonals, which are assembled at the factory with the help of welded joints (see figure at left). Further, a transition node is required in order to safely transfer forces and moments from the first tower section with a circular geometry of the jacket with a rectangular geometry can. This transition node is one of the most important components in Jackets for offshore wind turbines (see figure at right).

Jacket design for a wind energy plant
Transition node between Jacket and 1. tower section

OC4-Jacket

For validation of offshore tools the steel jacket is used by the international benchmark OC4 (Offshore Code Comparison Collaboration continuation). This uniform bases for geometry, materials and loads are based on a 5MW reference turbine was set by NREL. Accompanying the IWES is also a reference structure for the Jacket.    

The aim of the offshore initiative is the coupling between aerodynamic and hydrodynamic simulation for a 5MW offshore wind turbines including the supporting structure. Through this co-simulation interactions between the turbine, tower and jacket are to be shown, whereby savings can be uncovered.  

       

Detailed calculations for the nodes of a tube Jackets

In addition to the gateway node and the double-K-nodes are subject to the corner posts (see left figure) and the X-node high between diagonals dynamic stresses. They must be dimensioned not only for the extreme loads, but also to fatigue loads due to wind and waves. In particular, the welds must be checked with respect to their fatigue strength. Often there for local fatigue concepts, such as the structural stress approach, the application because not all nodes geometries and structural details are covered by the notch case catalog of the nominal stress approach.

Alternatively, there is also the version with cast node to using larger fillet radii to reduce stress concentrations (see figure at right) compared to a welded version.

Double-K-node in a Jacket
Stress concentration at the cross section junctions