Offshore wind design

A LICenergy UK concept design study for a UK Round 3 offshore wind farm operator produced two innovations for the cost-effective connection of a transition piece (TP) to a monopile (MP). The first was a lean-steel ‘mini-skirt’ seal alternative to traditional grout connections. A secondary steel ‘cage’ encircling the monopile was also developed to support boat landing stubs and anti-corrosion anodes without the need for large welds on primary steel.

( Text content written by Twenty6 for Luke Fussell, LIC Energy )

Client challenge

Long, steel-heavy grouted connections between TPs and MPs were once the industry standard. However, in recent years, many have shown slippage problems caused by a design code error. Remedies have created extra problems.

Standards now require the addition of shear keys covering at least the central half of a grouted connection (seen vertically), plus axial welds to resist torsional loading. The result is hundreds of expensive metres of additional welding. This can introduce primary steel defects.

To resolve these problems, a UK Round 3 wind farm operator asked LICenergy UK for a cost-efficient alternative using minimum secondary steel as part of a far-reaching concept design study.

Our response

‘Mini-skirt’ seal

The LICenergy UK team has extensive design experience with bolted flange connections. We used them in Rampion Offshore Wind Farm designs where the flange was combined with a traditional grout skirt to share loads and support secondary steel attachments. On a Round 3 development, however, this approach was simply not lean enough for the client’s tight budgetary requirements. We then developed a dramatically scaled-down lean-steel version of the traditional long grouted connection which was completely non-structural. The resulting 0.5m ‘mini-skirt’ provides as a cost-efficient seal, using proven technology and installation methodology. In this way, the bolted connection is reliably sealed away from the corrosive marine environment for the duration of its life.

Secondary steel support cage

We also developed a secondary steel support ‘cage’. This encircles a monopile and carries boat landing stubs and an anode cage, with significant weight savings compared to a grout skirt.

Traditionally, stubs and corrosion protection systems are welded for convenience to the steel skirt of grouted connections. With the skirt removed this is no longer possible. Welding large attachments directly to the MP is not recommended, as they are vulnerable to damage from large forces involved in pile hammering.

With boat landing and CP systems welded to the ‘cage’ instead, only a few small attachments to MP primary steel are needed. These low-profile brackets are ground-flush to remove fatigue sensitivities.

Benefits

The ‘mini-skirt’ and ‘cage’ combination was designed to: –

• Provide an effective seal
• Reduce secondary steel use dramatically
• Reduce installation time
• Minimise the potential for pile hammering damage to welded attachments
• Minimise grouting operations in difficult offshore conditions
• Minimise the presence of people in a demanding H and S environment

Note on concept design study methodology

Our review covering a wide variety of options used across the industry and considered: –

• Manufacturing cost estimates for primary steel and secondary steel attachments
• Installation costs: – Installation vessel size; Number and size (weight) of offshore lifts; Installation time (grout curing)
• Weather window needs – down time
• Corrosion protection
• Cable routing – external J-tubes v internal routing
• Sealing requirements
• O and M
• Health and Safety – from transport and installation to eventual decommissioning