Offshore Wind Turbine Transition Piece Mounting

Case Study 2: Offshore Wind Turbine Transition Piece Mounting

Scenario

A monopile foundation for the Vineyard Wind II project (Massachusetts, USA) required welding a 120-mm-thick transition piece to the pile flange. Space was constrained inside the submerged splash zone, limiting weld accessibility to single-sided fillet welds only. Certification required compliance with DNV-OS-C102 and ISO 15614-1, with fatigue life validation under 10⁸ cycles. Dynamic wave and turbine thrust loads induced high bending-shear coupling, and underwater wet welding was prohibited — all welds were performed dry in a cofferdam.

Given Data

• Shear Force: 412,000 N
• Bending Moment: 125,600,000 N·mm
• Effective Length of the Weld: 320 mm
• Allowable Stress in the Weld: 110 MPa (reduced per DNV-RP-C203 for high-cycle fatigue in seawater; base AWS value derated by 27%)

Calculation

Using the same combined-stress formula:

(s_{\text{min}} = \frac{\sqrt{V^2 + 9M^2 / L^2}}{f_w \cdot L \cdot \sqrt{2}})

• Numerator: (\sqrt{(412{,}000)^2 + 9 \cdot (125{,}600{,}000)^2 / (320)^2})
  • (V^2 = 169{,}744{,}000{,}000)
  • (M^2 = 1.5775 \times 10^{16}); (9M^2/L^2 = 9 \cdot 1.5775 \times 10^{16} / 102{,}400 \approx 1.387 \times 10^{12})
  • Sum ≈ (1.5567 \times 10^{12}); √ ≈ 1,247,700
• Denominator: (110 \cdot 320 \cdot \sqrt{2} \approx 110 \cdot 320 \cdot 1.414 \approx 49{,}773)
• (s_{\text{min}} = 1{,}247{,}700 / 49{,}773 \approx 25.07, \text{mm})

Result and Decision

Calculated minimum fillet weld size = 25.07 mm → nearest standard size is 25 mm, but AWS D1.1 §3.8.2 limits single-sided fillets on ≥12 mm base metal to ≤10 mm unless qualified procedure exists. To meet both structural and procedural compliance, engineers split the joint: a 10 mm fillet plus a 16 mm partial-penetration groove weld (qualified per ISO 15614-1), achieving equivalent throat area and satisfying DNV fatigue assessment. Final weld detail passed ultrasonic testing (UT) and strain-gauge validation under simulated 100-year storm loading.

Lesson

Weld size calculators provide theoretical minima — real-world fabrication constraints (accessibility, qualification limits, joint geometry) often override pure strength-based results; always validate against applicable procedure qualifications and constructability before finalizing the weld detail.