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 connections (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). 

Transition node between Jacket and 1. tower section
Double-K-node in a Jacket
Stress concentration at the cross section junctions
OWT - Jacket

In addition to the transition node, the double-K nodes on the corner posts (see figure on the left) and the X nodes between the diagonals are also subject to high dynamic stresses. They must be designed not only for the extreme load but also for the fatigue loads due to wind and waves. In particular, the weld seams must be checked with regard to their fatigue strength. Often this is done by local fatigue concepts, such as the Structural stress concept, because not all node geometries or notch details are available via the nominal stress approach .

Alternatively, there is also the version with Cast nodesin order to be able to reduce the stress concentrations compared to a welded version (see figure on the right) by means of larger rounding radii.  

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.    

OC4 Jacket
Jacket design for a wind energy plant
Modalanalyse des OC4 Jackets

Our services

On the part of SKI Ingenieurgesellschaft the following services can be provided:

  • Natural frequency analysis
  • Load simulations in the ultimate limit state
  • Calculation of fatigue loads in time and frequency domain
  • Buckling analyzes für einzelne Rohrsektionen
  • Calculations for welding and bolted connections
  • Calculation of Ring flange connections
  • Berücksichtigung von Wellenlasten
  • Design of Grouted Joints and pile-sleeve connections
  • Local stress and fatigue calculations for pipe nodes
  • Stress and stability checks for the pipe profiles

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