The offshore fatigue-corrosion challenge

Over a service life of around 25 years, an offshore wind turbine accumulates hundreds of millions of load cycles from wind, waves, rotor rotation, start-ups and shutdowns. Seawater adds to these loads, accelerating crack initiation and growth as soon as corrosion protection deteriorates. On welded structures, fatigue cracks almost always start at the weld toe, where geometric stress concentration, welding micro-defects and tensile residual stresses combine.

Critical areas on fixed-bottom wind

• Transition piece / monopile connection: highly loaded circumferential welds and flanges
• Tower circumferential and longitudinal welds: structural steels such as S355 to S460
• Jacket nodes: K, T and X tubular joints, where stress concentration peaks
• Flanges and transition connections: areas with high mean stress

Floating wind: new critical areas

Floating wind changes the nature of the problem. The floater is a hull-type structure under constant wave-induced motion, rotor loads transmitted through the tower and restoring forces from the mooring lines. Each of the three main floater families has its own weak points:

• Semi-submersibles: welded joints connecting the central column to the outer columns, braces and pontoons, heave plates; recent research on the fatigue reliability of semi-submersible floaters singles out these tubular joints as a major concern
• Barges: hull-to-tower-base connections, stiffeners and local reinforcements beneath mooring attachment points
• Tension leg platforms (TLP): tendon porches and connectors and tendon girth welds, which DNV-RP-C203 addresses explicitly because of their high mean stress

On the mooring side, chains and ropes are governed by their own standards; HFMI concerns the welded structural connections: fairlead foundations, chain stopper supports, padeyes and tendon fittings. These areas carry significant cyclic loads and are often submerged, so they are hard to inspect and repair without towing the unit back to port. Building a fatigue reserve at the fabrication stage therefore has direct economic value.

What IIW 2016 and DNV-RP-C203 say

The IIW 2016 recommendations (Marquis and Barsoum) govern HFMI treatment of weld toes. The benefit is expressed as additional FAT classes and increases with the steel's yield strength: about four classes for steels from 235 to 355 MPa, more for higher-strength grades. The claimable benefit is reduced as the stress ratio R rises and the maximum stress is capped; the S-N curve of the treated detail uses a shallower slope (m = 5).

DNV-RP-C203 (fatigue design of offshore steel structures, April 2016 edition) covers hammer peening, of which HFMI is the mechanised high-frequency version. Its Table 7-1 gives a factor on fatigue life of 0.011·fy for steels with a yield strength below 350 MPa and 4.0 above; the maximum S-N class that can be claimed is C1 or C depending on non-destructive testing. The RP also sets clear limits:

• avoid any compressive overload, which would wipe out the induced residual stresses;
• use the method only on members whose failure would have no substantial consequences;
• carry out fatigue testing if the fabricator lacks experience with the process, as the effect depends on the tool and workmanship;
• the improved S-N curves apply in air and in seawater with cathodic protection, not under free corrosion;
• given workmanship uncertainties, the RP advises against relying on this gain in general design: it often serves as a reserve, a reinforcement or a repair method.

In practice, the credit given to HFMI is agreed with the certification body on the basis of IIW 2016, a qualified procedure and full traceability. Always check the current edition of the documents applicable to your project.

The French floating wind context

Within a few years, France has moved from demonstrators to commercial projects. In the Mediterranean, three pilot farms of three turbines each are testing different concepts: Provence Grand Large off Port-Saint-Louis-du-Rhône (tension leg floaters), commissioned in 2025; EFGL off Leucate and Le Barcarès (semi-submersibles, 3 × 10 MW), which delivered first power in May 2026; and EolMed off Gruissan (30 MW, barge-type floaters), commissioned in spring 2026. On the tender side, the first commercial floating farm in South Brittany (AO5, around 250 MW) was awarded in 2024, and two 250 MW floating farms in the Mediterranean (AO6, Gulf of Fos and Narbonnaise) at the end of 2024. The AO10 tender, published in June 2026 and incorporating the projects originally planned under AO9, covers around 10 GW across eleven projects, roughly half of it floating, with winners due to be named in February 2027.

This change of scale means floaters built in series at port facilities, at rates that require weld finishing to be integrated into the production flow rather than handled case by case.

Operational constraints

• At the yard: treatment of critical joints before coating, integration into the assembly schedule, 100% visual inspection of the treated area
• Offshore: access by crew transfer vessel, limited weather windows, safety training required by operators
• Equipment: portable, rugged tooling suited to hard-to-reach areas
• Documentation: weld / operator / parameter traceability, consistent with certification body requirements

Our approach

Kronos Engineering, based in Bordeaux, masters HFMI and all the weld finishing processes presented on this site. Our engineers have been practising special processes since 2009 and work in more than 50 countries. We help you identify eligible details, write an IIW 2016-compliant procedure, prepare test specimens and document every treated weld, at the fabrication yard or on site.

To find out more, visit our HFMI process page. Preparing a floater, a repair or a life extension? Contact us: response within 12 business hours.

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