Does a dnv offshore case always need 1440 h of salt spray?

A: No. A shell under deck or in an internal walkway is served by 480 h; only continuous spray positions, such as the deck edge, justify 1440 h.

Can a C5-M case use carbon steel screws?

A: Not advisable. Carbon screws rust red around 720 h, and replacing the batch later costs more than starting with A4-70. Where carbon is structural, coat it and run a separate 720 h check.

Is a thicker dry film always safer?

A: Only to a point. Past 320 μm adhesion drops and impact lifts the coat in sheets. Build 240–320 μm in 3 coats, lowest of five readings per face at 80% of nominal.

What does 85% tensile retention measure?

A: The strength of the shell material, pulled after 1440 h at 35 ℃ in 5% sodium chloride and 24 h of recovery, against an unexposed reference. It says nothing about the coating.

Can the inspection interval stretch to 24 months?

A: For C3, 18 months is arguable. For C4 and C5-M it is not. Edge chalking turns to blistering inside 24 months.

DNV Offshore Case

Quick answer: A dnv offshore case takes its class from where it mounts: C3 under deck at 480 h salt spray, C4 on open deck at 720 h, C5-M in splash at 1440 h.

The Mounting Point Sets the Class
What 480, 720 and 1440 Hours Buy
Fasteners and the Dissimilar-Metal Trap
Deck Exposure Against the Splash Zone
Reading the 12-Month Inspection
FAQ

1. The Mounting Point Sets the Class

A 900 mm case bound for a 12 MW platform is judged at goods-in on three figures: where it hangs, how many hours of salt spray it survived, and how much dry film sits on it. Answer those and the class, the price and the inspection interval all follow.

Buyers invert the order: heavy wall first, coating second, class last. A 1440 h shell where 480 h suffices carries roughly 30% more cost and no extra life.

ISO 12944-2 names the bands C3, C4 and C5-M; C5-M is the 1998 label, mapped to C5 in the 2017 edition, which adds CX. Offshore projects tie each band to a spray duration and an inspection interval, so the band becomes a purchase specification.

Corrosivity class	Typical mounting point	Salt spray	Dry film (typical)	Inspection
C3	Under-deck shelter, internal walkway	480 h	160–200 μm	12 months
C4	Open deck centre, superstructure flank	720 h	200–240 μm	12 months
C5-M	Deck edge, within spray reach	1440 h	240–320 μm	12 months plus 6-month walk-by

2. What 480, 720 and 1440 Hours Buy

The three durations do not describe three paint formulations. They describe three allowances for damage: white rust over up to 5% of area at 480 h, scribe creep inside 2 mm either side at 720 h, no red rust at fasteners or corner fittings at 1440 h.

dnv offshore case rows on a white showroom plinth, black wheeled shells before one blue unit and orange cases behind

Film build follows the tier: 240–320 μm in 3 coats with a 4 h flash-off, the range workshops apply. Past 320 μm adhesion falls away and impact lifts the coat in sheets. Thickness is read at five points per face, lowest not below 80% of nominal.

Preparation sets the ceiling: Sa2.5 blasting with a 40–70 μm profile makes 1440 h repeatable, while residual salt above 20 mg/m² blisters a premium system at 600 h.

Tensile retention is measured separately, often confused with coating quality. A shell held at 35 ℃ in 5% sodium chloride for 1440 h, rested 24 h and pulled, must keep at least 85% of initial strength. A drop there means moisture has reached the filler. Cost does not scale with hours: 480 h to 720 h adds roughly 15%, 720 h to 1440 h adds roughly 35%.

3. Fasteners and the Dissimilar-Metal Trap

Corrosion rarely starts on a wall panel. It starts at a latch, hinge or screw thread. A4-70 stainless is the working default: molybdenum resists chloride, and the grade stays ductile at -25 ℃.

Stainless against aluminium is the trap. A screw threaded into an aluminium trim strip forms a galvanic cell the first time seawater lands, and the aluminium sacrifices itself. A 0.5 mm insulating washer under the head breaks the circuit, and it belongs on the case drawing.

Torque belongs on the drawing too. M6 screws at 8 N·m: lower and they work loose, higher and the thread galls, letting salt into the root. At least 10% are witnessed, and on a 1440 h order four screws come out at acceptance. One red thread rejects the batch.

4. Deck Exposure Against the Splash Zone

The gap between open deck and splash zone is not hours but wetting frequency. A splash-zone case takes over 20 wave hits a day, the salt film never rinses off, and ageing runs at roughly 1.5 times the open-deck rate.

dnv offshore case moulds on a workshop floor under a red gantry crane, blue racks between white columns

Lashing uses four points, with a 3 mm rubber pad so point load does not grind the corner through. Keep the case at or under 120 kg; above that the preload is recalculated and the pattern grows to six. In the splash zone it also stands 150 mm off the steel on 100 mm blocks, or water blisters the base in 48 h.

UV ageing runs 1000 h with colour shift held to ΔE ≤3, since the stencilled tag number is read by contrast. Low-temperature impact is a falling weight after 4 h at -25 ℃, with no corner crack permitted.

Environmental factor	Test condition	Threshold	Applies to
Neutral salt spray	35 ℃, 5% NaCl, continuous	480 / 720 / 1440 h	C3 / C4 / C5-M
UV ageing	1000 h cycle with wet period	ΔE ≤3	C4 and C5-M
Low-temperature impact	4 h at -25 ℃, falling weight	No corner crack	C5-M
Tensile after spray	1440 h mist, 24 h recovery	≥85% retention	All classes

5. Reading the 12-Month Inspection

Twelve months is where the measured loss sits: C3 and C4 shells lose under 10 μm of film a year, while C5-M shells chalk at the edges by month 12, hence the walk-by at month 6.

Six items are recorded: blistering, scribe creep, fastener rust, seal-face distortion, lashing wear and tag legibility. Anything past threshold enters repair within 30 days, before rust spreads and the repaired area quadruples.

Failure mode	How it is read	Threshold	Action
Blistering	Visual plus cross-cut	≤2 mm, ≤3 per m²	Abrade and recoat
Scribe creep	Width either side of scribe	≤2 mm per side	Local repair
Fastener rust	Threads after removal	Zero at 1440 h	Replace batch

Design life is 15 years; at year 10, below 120 μm of remaining film, replacement beats recoating. Material and coating records go to the class society with every case, so a replacement does not restart the paperwork.

Conclusion

Class selection for a dnv offshore case is a pricing of position: C3 at 480 h under deck, C4 at 720 h on open deck, C5-M at 1440 h in the splash zone. Get it wrong and film thickness, fastener grade and inspection interval inherit the error. The four lashing points, the 120 kg ceiling and the 12-month interval belong in the technical agreement.

This article covers case-side specifications only and does not address any handling, loading or use of the contents.