Offshore Wind Tower Flange Connection Reinforcement

Case Study 1: Offshore Wind Tower Flange Connection Reinforcement

Scenario A major European offshore wind developer was retrofitting a 120-m-tall monopile transition piece in the North Sea. The existing flange-to-shell connection exhibited fatigue cracking under cyclic wave and turbine loads. Structural analysis indicated that increasing the fillet weld throat thickness from 5 mm to 6 mm would improve fatigue life by 42% per IIW recommendations. Constraints included limited access (only 3-hour weather windows), strict ISO 5817 Class B tolerances, and mandatory preheat control due to high-strength S355NL steel (t = 42 mm) to avoid HAZ embrittlement.

Given Data

• Required throat thickness: 6.0 mm
• Root opening: 0.5 mm (measured via UT prior to welding)
• Weld angle: 45° (standard for symmetric double-bevel preparation on flange interface)

Calculation The Fillet Weld Size Calculator uses the geometric relationship:

Leg length = √2 × (throat thickness + root opening × cos(weld_angle/2))

Substituting values:

• cos(45°/2) = cos(22.5°) ≈ 0.9239
• Root opening contribution = 0.5 mm × 0.9239 ≈ 0.462 mm
• Effective throat = 6.0 mm + 0.462 mm = 6.462 mm
• Leg length = √2 × 6.462 mm ≈ 1.4142 × 6.462 ≈ 9.14 mm

Result and Decision The calculated weld leg length of 9.14 mm was rounded to 9.0 mm (per AWS D1.1 §3.5.2, leg lengths are specified in 0.5-mm increments and must not be less than required). Welders used controlled-short-arc GMAW with pulsed current, 1.2-mm ER100S-G wire, and interpass temperature ≤ 150°C. Post-weld NDT (PAUT + MPI) confirmed full fusion and throat thickness ≥ 6.0 mm across 100% of the 3.2-m circumference weld.

Lesson Root opening — often overlooked in field measurements — directly increases effective throat; neglecting it risks undersizing the leg length and compromising fatigue resistance in cyclic-loaded connections.