Monopiles with transition pieces

How does Monopiles with transition pieces work, step by step?

1. 1

   Step 1: Fix the ground model and size the pile

   Everything starts with the site investigation. Boreholes and cone testing at or close to every foundation position build a ground model across the array, because seabed conditions vary from one position to the next and a single design applied everywhere is either wasteful or unsafe. The designer combines that model with the turbine loads, the water depth, the wave and current climate and the required fatigue life, then sets the diameter, the wall thickness distribution and the embedment for each position. Positions are often grouped into a handful of design families so that fabrication stays repeatable. The output is a pile schedule that tells the yard what to build and the installation contractor what to expect at each location.

2. 2

   Step 2: Fabricate, coat and load out

   Plate is rolled into cans, the cans are welded into sections and the sections are welded into a complete pile, with the circumferential welds inspected throughout because they carry the fatigue duty. External coatings and anodes go on for corrosion protection, internal secondary steelwork is fitted, and the pile is marked up so the crew offshore can read penetration from the deck. Transition pieces are outfitted separately and arrive far more complete than the piles, with platforms, ladders, boat landing and internal fit-out already installed. Load-out from the fabrication yard onto the transport barge or the installation vessel is itself an engineered operation, and quay strength, crane reach and sea fastening all govern how many units go out per trip.

3. 3

   Step 3: Prepare the position and set the spread

   Before the vessel arrives the position is surveyed and cleared. Unexploded ordnance survey and clearance, boulder clearance and a check for wrecks and archaeology all happen ahead of the installation campaign, and any obstruction found is dealt with rather than driven through. The installation vessel arrives on position, jacks up or holds station on dynamic positioning depending on its type, and the crew set out the gripper frame that will hold the pile vertical. Scour protection, usually a graded rock layer, is placed either before or after the pile depending on the design, and that choice belongs to the designer.

4. 4

   Step 4: Upend, stab and grip the pile

   The pile is lifted from its cradle horizontally, upended to vertical using the main crane and an upending arrangement, and lowered through the gripper frame until its toe reaches the seabed. In most ground the pile self-penetrates several metres under its own very considerable weight before any driving starts. Verticality is monitored continuously while the gripper holds the pile, because a lean corrected early is trivial and a lean discovered late is not. Heavy offshore lifts of this scale are planned by the installation contractor and independently reviewed before the campaign starts.

5. 5

   Step 5: Drive to the design penetration with noise mitigation running

   The hammer is placed on the pile head and driving begins with a soft-start sequence at reduced energy, so that any marine mammals in the area move off before full energy is applied. Noise abatement is deployed and confirmed working before that point. Driving then proceeds in a controlled sequence while blow counts and penetration are logged continuously, giving a driving record that is compared against what the ground model predicted. Where the record departs from expectation the contractor stops and the designer assesses it. Refusal on an unexpected hard layer, or unexpectedly easy driving, are both engineering events rather than production problems.

6. 6

   Step 6: Install the transition piece and make the connection

   With the pile driven, the transition piece is lifted on and set over or into the pile head. Any lean left in the driven pile is corrected here, and the transition piece is held and adjusted until it is vertical within the tolerance the specification sets. The connection between the two is then made permanent. Grouted connections, bolted flange connections and combinations of the two are all used, and the choice is the designer's and is made long before the vessel sails. Whatever the arrangement, the connection is a controlled operation with its own procedure, its own hold points and its own records.

7. 7

   Step 7: Complete the outfitting and hand over the interface

   The secondary steelwork is checked and completed, corrosion protection is confirmed, the internal platforms and access arrangements are made safe, and the J-tube and cable entry are made ready for the array cable that will follow in a separate campaign. Navigation aids and lighting are commissioned. The foundation is then surveyed as-built, and the position, orientation, level and flange condition are recorded because the turbine that lands on it later has to arrive at a known interface. That handover pack is what the turbine installation contractor works to.

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   Step 8: Survey, monitor and close out

   After installation the seabed around the foundation is surveyed to confirm the scour protection is where it should be, and repeat surveys through the life of the project track whether it is moving. Structural monitoring instruments fitted on a sample of foundations give the designer real behaviour to compare against the assumptions. The driving records, the as-built survey, the connection records and the inspection reports are assembled into the foundation file for that position. On a large array this happens dozens or hundreds of times, and the discipline of the paperwork is what keeps the campaign repeatable rather than heroic.