The Impact of Dynamics on Template Design in Offshore Wind Projects 

In a commercial-scale offshore wind farm, numerous jacket foundations must be installed. With each jacket requiring multiple pin piles, this leads to a large number of piles that need to be installed in series. Since this cannot be done in a one-off operation and taking into account the high costs of expensive assets, one cannot wait for good weather. Therefore, having good workability is crucial for both offshore wind contractors and developers. 

Defining workability

Workability is defined as the percentage of time that the operation can take place. By determining the duration for an operational cycle and the acceptable sea states, one can assess the workability at a given location and for a specific period. Simply stating that we achieve a workability of, for example, two meters of significant wave height and wave periods of up to seven seconds does not provide a full picture. These limiting environmental conditions must be overlaid on the metocean data of the location of interest and based on the duration required to complete our operation. Then, the workable weather windows are defined, showing a workable percentage of time per day, month, or year when compared with the metocean data of the location for a given period. The illustration below, shows the process of defining workability for a project. Firstly, the locations of interest are defined. Following, the historical weather data are extracted and finally, an acceptable weather limit is selected to be the design condition that gives sufficient workability for the operation. 

Importance of high workability

 High workability is essential for a successful and cost-efficient pin pile installation campaign. Some factors that can limit the installation campaign include the installation assets used to install the piles safely and accurately. One such asset is the pre-piling template, which can ensure the pile’s verticality when driving and the center-to-center distance of the piles.  

Pre-Piling Template Design 

It is common practice to have mission equipment that are not limiting factors of the operation but in case these assets are limiting the operation, they should have acceptably high workability. During the pre-piling template design, several aspects are considered, spanning from loads on the piles and template, template stability, overturning moments, method of installation, etc. 

The template design is highly influenced by the design loads acting from the pile motions. In an ideal world where resonance is never an issue, the template size would proportionally increase with the increase of the piles and environmental conditions considered. However, these loads and moments can be drastically increased when the system enters resonance. 

Resonance in Pre-Piling Templates 

Resonance occurs when the system’s natural frequency is excited. The factors that contribute to resonance in the pre-piling template example are: 

• Pin pile installation sequence, thus system’s stiffness 

• Pile stick-out length above the template 

• Mass of pile and hammer 

• Soil conditions and pile penetration 

• Water depth 

• Wave periods 

All but the last are parameters of the system. However, the wave periods are the external mean that acts on the system. Having bigger pile lengths and weights, difficult soil conditions (no self-penetration, thus longer free pile length), and water depths affect the natural period of the system, bringing it closer to the typical wave periods expected in a pin pile installation campaign. An example is illustrated below, where the effect of relative pile length change on the natural frequencies and loads on the pile support are shown. 

Options to address resonance 

Thus, one of the following three options has to be chosen: 

1. Adjust the piling template design (for example, adjust template height to reduce the free pile length) 

2. Adjust the external periods that should be considered (reduce workability) 

3. Accept the system’s response and external periods and design a template that accounts for resonance (CAPEX increase) 

Overall, pin pile is a complicated topic where the template design decisions are based on a multi-criteria consideration. Higher CAPEX can lead to lower OPEX and cycle times but there is always an optimum direction based on project’s specifications, timeline, risks and many other factors.

Download TWD’s Jacket Track Record 

By collaborating closely with our clients and fabricators, TWD delivers robust foundation installation tools and smart methods tailored to the specific needs of each project. Our comprehensive Jacket Foundation Track Record download highlights TWD’s contributions to the successful installation of jacket foundations for offshore wind campaigns worldwide. Are you curious to learn more about tailored equipment design and innovative installation methods for jacket foundations? Download TWD’s Jacket Foundation Installation Track Record.