DNV-ST-0126: Offshore Blade NDT Overlay Requirements

What the standard covers

The overlay adds inspection after major maintenance, a tighter sensitivity for the leading edge, and equipment requirements suited to a marine site. It applies where offshore exposure drives the inspection plan, so a marine programme scans to it in addition to the base standard rather than instead of it.

What it demands

• NDT after every major maintenance intervention, not only on a calendar
• Higher leading-edge sensitivity, a 3 mm indication threshold against the base 6 mm
• Corrosion-resistant equipment certified for offshore use, IP67 and NEMA 4X
• A plan that holds up where site access is expensive and weather-limited

How it lands on a blade scan

The leading-edge threshold is the clause that changes the hardware. It tightens to 3 mm against the 6 mm base floor, so the scan has to resolve half the size at the same speed, and the method that returns a depth is the one that fits. A 5 MHz dual-element gauge maps remaining wall to plus or minus 0.1 mm on a crawler sealed to IP67 and NEMA 4X and rated to minus 20 °C, which is what an offshore site needs. The maintenance trigger also changes the rhythm, because a scan after every intervention is a campaign that follows the asset rather than the calendar.

How to apply it to a blade

Run these checks in order. By Step 5 the scan plan has the threshold, the geometry and the record it needs.

1. Fix the acceptance threshold. Start from the threshold the zone carries, not from the method you happen to own. IEC 61400-5 Section 11 asks for volumetric detection of indications 6 mm and larger in the laminate plane, and DNV-ST-0126 tightens the leading edge to 3 mm, so the threshold is set before the probe is chosen.
2. Read the geometry off the drawing. Measure the thickness and the curvature the probe has to travel. A spar cap up to 80 mm of unidirectional laminate and a root transition of changing radius are two different scan problems, and the radius decides how the array is coupled.
3. Choose the modality pair. Pick modules by defect geometry, not by habit. PAUT images planar separations that lie parallel to the plies, TOFD catches volumetric clusters whatever their orientation, and running both on a 256-channel system puts the two mechanisms in one pass.
4. Automate the scan path. Put the probe on an encoder-driven carriage and let the path, not the technician, set coverage. A 600 mm spar cap falls to one longitudinal pass at 500 mm/s on a linear array, where a single-element probe needs 20 parallel passes and runs about five times longer.
5. Lock the evidence chain. Write the coverage map, the daily calibration and the indication log to the blade serial while the scan is running. It is the difference between a 3-day type-certification audit and a 3-hour document review, and it is what ISO 9712 qualification is there to sign.

Nothing here is rounded up for effect. Each value is the one on the equipment sheet or in the field report behind it.