Inspect riser splash zone corrosion with machine vision

The fastest corrosion on a fixed platform sits on the risers in the splash zone: tide and waves wet and dry the steel thousands of times a year, and every coating defect becomes a corrosion site. On a riser that is internally cladded with Inconel 825 and coated outside with fusion-bonded epoxy under a polypropylene jacket, you cannot open the pipe for internal NDT, and stripping coating offshore is slow and costly. This guide sets out what an external camera system can find on such a riser: which imaging bands reveal each corrosion signature, the pixel and cooling-time numbers that size the rig, the strip pattern that covers the zone, and the point where NDT has to take over.

Can a camera see corrosion through an intact coating?

No — imaging sees the coating surface and nothing behind it, so you detect corrosion where it announces itself: blisters, cracks, disbondment edges, chalking and rust staining — and you leave wall-thickness numbers to ultrasonics or another volumetric method.

Polypropylene and FBE are opaque at visible, SWIR and thermal wavelengths at the thickness a riser jacket uses, so no camera reads steel condition under sound coating. What published work on riser splash-zone inspection consistently shows is that the coating itself carries the signal: the daily wet-dry cycling finds every holiday, crack and disbonded pocket, and those defects host the corrosion. The same literature reports that large areas of splash zone can be inspected optically without stripping coating or clearing marine growth. That is the job here: image coating condition patch by patch, log it against position, and dispatch NDT to what the cameras flag.

Which imaging band finds each corrosion signature?

Use three bands: visible light for blistering, cracking, chalking and rust staining; long-wave infrared near 8-14 µm for disbondment revealed by cool-down contrast after a heat pulse; and SWIR for retained moisture that visible light reads as ordinary staining.

Pick the LED band per defect with machine vision light wavelengths as the reference. UV adds a fourth look on ageing coatings, where chalking and some degradation products scatter or fluoresce — see why UV imaging reveals surface defects.

The visible band in this guide is carried by the LS-CMR-0166, a 12 MP, 1/1.7-inch rolling-shutter camera delivering 4000 × 3000 pixels at 30 fps over USB3.

The thermal band is carried by the LS-IRC-0044, a 640 × 512, 8-14 µm camera with an 88.5° × 73.2° field of view and 50 Hz output over GigE.

The SWIR band is carried by the MV-CI013-GS-NN, a fanless 1.3 MP camera on the InGaAs IMX990 sensor with a GigE interface.

Both families sit in the area scan cameras and infrared cameras categories, and the camera finder filters them by interface, resolution and shutter.

How much detail do you resolve at the working distance?

Plan on 0.5 mm per pixel for the visible camera and 7.2 mm per pixel for the thermal camera at a 3.0 m standoff, which puts a 2 mm coating crack across 4 pixels and a 25 mm disbondment across 3.5 pixels.

Worked example: the visible strip

Detection needs about 3 pixels across the smallest feature, so a 2 mm crack needs a pixel pitch of 2 mm / 3 = 0.67 mm or finer. Design to 0.5 mm per pixel for margin:

1. Strip width: 4000 px × 0.5 mm/px = 2000 mm — a 2.0 m wide strip.
2. Strip height: 3000 px × 0.5 mm/px = 1500 mm, so each frame covers 2.0 × 1.5 m.
3. Crack sampling: 2 mm ÷ 0.5 mm/px = 4 px across, above the 3 px threshold.
4. Focal length: a 1/1.7-inch-class sensor has an image width of roughly 6.7 mm, and f ≈ WD × s / FOV = 3.0 m × 6.7 mm / 2000 mm ≈ 10 mm. That is wide-angle territory; run your sensor choice through the calculator below and buy the lens the output names.

Worked example: the thermal patch

IFOV = 88.5° / 640 px = 0.138° per pixel ≈ 2.4 mrad. At 3.0 m one pixel covers 2.4 × 10⁻³ rad × 3000 mm = 7.2 mm, so a 25 mm disbondment spans 25 / 7.2 = 3.5 px — detectable, not measurable. Halve the standoff to 1.5 m and the pitch drops to 3.6 mm: the patch then spans 6.9 px and a 10 mm crack reaches 2.8 px. The same IFOV arithmetic drives infrared camera selection by IFOV for electrical targets, and it transfers unchanged.

How does thermography flag disbondment under the coating?

Heat the surface 5-10 K above ambient, then film the cool-down: a disbonded patch under about 3 mm of polypropylene separates from sound coating after roughly 90 s, because heat needs t ≈ d²/α to cross depth d.

The physics: sound coating dumps its heat into the steel behind it, so it cools fast. A disbondment is a thin air gap that blocks that flow, so the patch above it stays brighter during cool-down and heats faster during the pulse. Polypropylene diffuses heat at α ≈ 1 × 10⁻⁷ m²/s, so:

• Disbondment at the steel, under d = 3 mm of jacket: t = (3 × 10⁻³)² / 1 × 10⁻⁷ = 90 s. Heat for 30-60 s, then film a 2-3 minute cool-down.
• Defects near the FBE line at d = 0.3 mm: t = (0.3 × 10⁻³)² / 1 × 10⁻⁷ = 0.9 s. A millisecond-class flash shows these almost immediately.
• The LS-IRC-0044's 50 Hz rate is far more than the physics needs. Log at 5 Hz and a 2-minute watch still yields 600 frames.

What lighting works on a curved, glossy coated riser?

Light the strip at a grazing angle with paired bar lights so blisters and cracks throw shadows along the curve, and cross polarize light and lens to kill the specular sheen that glossy polypropylene returns off a cylindrical surface.

Work the bar pairs 15-30° off the surface tangent and keep them outside the camera's specular reflection cone. Bright field fills the frame evenly and reads staining well; dark field catches every raised edge and crack. The difference is the one drawn below.

Strobing does three jobs at once: it freezes the strip while the camera shuttle moves, it swamps ambient sun, and a 30-60 s high-duty-cycle run doubles as the thermography heat pulse. A structured projector earns its slot where coating has already fallen off, building a depth map of the pitting underneath. The special lights category holds all three types, the light finder filters by size and voltage, and food washdown lighting selection lists the ingress questions to settle before mounting anything in a spray zone.

The LS-SPC-0129 strobe runs from 48 V. The LS-SPC-0040 bar light spans 200-400 mm at 24 V for the grazing pair. The LS-SPC-0103 structured projector covers a 200 × 150 mm patch at 24 V.

How do you cover the splash zone in overlapping strips?

Two camera stations set 180° apart with nine imaging rows at 25% overlap cover a 10 m zone on a 508 mm outside diameter riser: 18 visible frames per pass, which is 0.6 s of raw capture at 30 fps.

The example takes a 20-inch riser (OD 508 mm) and a 10 m band between your lowest waterline and your highest wetting line; substitute your own figures.

1. Circumference: π × 0.508 m = 1.60 m. One 2.0 m wide frame at 3.0 m wraps the whole visible half with margin, so two stations 180° apart close the ring.
2. Vertical advance: a 1.5 m strip at 25% overlap steps 1.5 × 0.75 = 1.125 m per row.
3. Rows: 10 m / 1.125 m = 8.9, so 9 rows.
4. Frames: 9 rows × 2 stations = 18 per visible pass. At 30 fps that is 18 / 30 = 0.6 s of capture — positioning and tide windows set the pace, not data.

The thermal camera works at 1.5 m: the 3.6 mm pitch gives a 640 × 512 px = 2.3 × 1.85 m patch, stepping 1.85 × 0.75 = 1.39 m per row, so 10 / 1.39 = 8 rows from one side. Schedule it around low water: a surface wet with spray cools by evaporation and the disbondment contrast drowns. Tag every frame with elevation and clock angle so consecutive surveys align pixel to pixel — corrosion coordinate mapping for internal vessel inspection shows the same registration idea.

What must the rig survive in the splash zone?

The zone cycles between salt spray, immersion and full sun every day, so the cameras live in housings rated for that exposure, the window is the wearing part, and cleaning goes into the schedule rather than the hopes.

Rate the housing as an assembly — window, glands and mounts — not by the camera label behind it. A salt film on the window costs more contrast than any lighting change buys back, so budget a rinse or wipe per pass. Condensation after a cold night fogs windows from inside; vent or purge the housing and check the camera's operating range against the temperature swing it will see — camera operating temperature range explains what happens outside it. Sunlit coating beside deep shadow is a high-dynamic-range scene; if the visible frames clip, the capture and fusion techniques in HDR imaging for surface inspection recover both ends.

When does machine vision replace or back up NDT?

Vision screens the whole zone on every pass but cannot size metal loss under sound coating, so it replaces the visual survey round and feeds NDT: every flagged patch gets confirmed and measured with ultrasonic thickness or a volumetric method.

• Routine external monitoring of coating condition — vision, repeated on the survey interval.
• A flagged blister or disbondment — NDT at that location, then vision tracks the repair between passes.
• Wall thickness under intact coating — outside optics contribute nothing; this stays NDT.
• The Inconel 825 bore — no external camera sees it, so internal clad integrity stays with internal methods; how to inspect internal features with machine vision covers the cases where optics do reach an interior.

The framing is the same one argued for steel billets in vision or NDT for surface inspection: optical screening first, volumetric confirmation second. For the choice of companion NDT method on risers specifically, the riser splash-zone NDT method selection guide compares the volumetric techniques.

Which parts make up a riser splash zone inspection system?

Start from three cameras — visible, LWIR thermal, SWIR — a strobe or bar-light pair behind a sequencing controller, housings matched to the exposure and a coordinate log per frame; the parts below fill the capture and lighting roles for a first rig.

Capture

• Visible: the LS-CMR-0166, 4000 × 3000 px over USB3, with a wide lens sized from the strip width.
• Thermal: the LS-IRC-0044, 640 × 512 px at 8-14 µm, its IFOV checked against your smallest target patch.
• Moisture: the MV-CI013-GS-NN as the second look on blisters and staining.

Lighting

• Paired bar lights at grazing angle, crossed polarizers on light and lens.
• A strobe rated for 30-60 s runs as the heat pulse.
• A light controller to sequence pulse, capture and ambient rejection.

Environment and records

• Housings, glands and windows rated for the splash-zone exposure, cleaning in the plan.
• An elevation × clock-angle log per frame so surveys align.

If this is your first vision build rather than a retrofit, selecting a machine vision system for parts inspection walks the sizing decisions step by step. Size the camera side in the camera finder, start the build at the configurator, and send your riser dimensions and zone height through the contact page if you want the layout checked before you order.