Petcoke hold cleaning: why it's so hard to remove and how to get holds back to grain clean

Why petcoke is so difficult to clean

Petroleum coke is a by-product of oil refining. It comes in two broad forms that clean very differently:

• Green (raw) petcoke still contains a share of volatile hydrocarbons. It's oily and sticky, and the fine dust bonds to steel and coating, especially if the hold is warm. This is the one that causes the trouble.
• Calcined petcoke has been heated to drive off most of the volatiles. It's drier and dustier, abrasive, and mostly comes off with good sweeping and washing, though it still stains.

Green petcoke leaves a thin hydrocarbon film over everything it touches. Water alone can't break that film, because oil and water don't mix. Hit it with 200 bar of plain seawater and you'll remove the loose coke above the film and polish the film itself. That's why holds look "clean but grey" after a heavy wash, and why surveyors rub a white cloth on the frames and hold it up.

The geometry makes it worse. Petcoke dust settles on every horizontal surface: hopper tops, the upper side of frame brackets, stiffener flanges and the hatch cover girders. Most of those are high up, and a jet from the tank top has lost much of its force by the time it reaches them.

What chemical is used for petcoke cleaning?

For green petcoke you need an alkaline degreasing cleaner formulated for hydrocarbon residues. Several marine chemical suppliers sell products marketed specifically for petcoke and coal. The chemistry varies, but the principle is the same: the cleaner emulsifies the oily film so that the following high-pressure wash can lift it off the steel.

How to choose and use one properly:

• Match it to the coating. Strong alkalis can soften some paint systems. Check the product data sheet against your hold coating, and test on a small area if you're unsure.
• Check MARPOL status. Wash water containing a cleaning agent can only be discharged at sea if the agent isn't harmful to the marine environment. Get the supplier's declaration and keep it on board.
• Follow the dilution and dwell time exactly. Too weak and it won't cut the film. Too strong and you're paying for chemical you're about to wash off, and risking the coating. Dwell time is usually measured in minutes, not hours.
• Apply it to a surface it can work on. Many petcoke products are designed to go onto a dry or nearly dry surface. Spraying them onto a freshly soaked hold dilutes them before they start.

What about acid cleaners? They're for mineral scale and cement, not oily residue. Using an acid on petcoke film achieves little and adds risk.

Petcoke to grain: a hold cleaning procedure

This is the sequence that works for us on a typical green petcoke to grain change. Adjust it to the chemical manufacturer's instructions.

1. Maximise removal during discharge. Get the stevedores to sweep down hopper tops and ledges while their equipment is still in the hold. Petcoke left on board is petcoke you have to bag by hand.
2. Clean hatch covers and coamings. Before anything else, so nothing falls back in.
3. Dry sweep thoroughly. Tank top, hoppers, ladders and platforms. Remove sweepings and record quantities for the Garbage Record Book.
4. Clear the bilge wells. Remove solids now so the wash doesn't choke them.
5. Seawater wash, top down. Remove the loose coke and dust. This is a rough wash. You're not expecting the hold to look clean yet.
6. Let surfaces drain and dry off as far as practical. Check the chemical's instructions. Most work best on a surface that isn't streaming wet.
7. Apply the petcoke chemical. Use the application method and order the manufacturer recommends, often bottom-up with an air-driven sprayer or a combi-gun with chemical injection. Cover every surface, including the upper frames and the underside of brackets.
8. Respect the dwell time, and don't let it dry. If the chemical starts to dry on the steel, it can leave its own residue. Work in sections sized to what you can wash off in time.
9. High-pressure wash. Wash the emulsified film off, top down. This is where pressure matters. A unit around 200–250 bar will do most of it; stubborn areas on older coatings may need more.
10. Inspect, then repeat locally. Rub a white cloth on frames and brackets at several heights. Treat and wash again wherever it comes away black.
11. Fresh water rinse. Remove chemical residue and chlorides, top to bottom.
12. Dry, ventilate and pre-inspect to grain standard.

How do you clean petcoke from frames and brackets?

This is where most of the time goes. The tank top and lower hoppers are easy to reach. The upper side frames, the brackets at the top of each frame and the under-deck structure are 10 to 15 metres up on a Supramax and higher on bigger ships.

Options, from cheapest to most effective:

• Combi-gun or cannon from the tank top: fine for the first wash and chemical application. Poor for the final HP wash on high surfaces.
• HP lance on extension poles: helps up to a few metres above the hopper.
• Cherry picker: good access if one can be landed in the hold and the port allows it.
• Staging: slow to rig and strike, and often impractical in the time available.
• Rope access: technicians descend from the coaming with an HP lance and work each frame at arm's length. It's usually the fastest way to get close-range cleaning on the upper structure without heavy equipment.

How long does petcoke hold cleaning take?

Longer than most other cargoes, and the upper structure drives the number. As a rough guide, budget at least two to three times the effort of a normal post-coal clean if you're going to grain. A ship's crew that also has to keep watches will struggle to take five holds from green petcoke to grain clean on a short ballast leg. That's a job for a riding crew or a shore gang with proper HP equipment.

Petcoke to alumina, and other next cargoes

• Petcoke to petcoke or coal: normal clean is usually enough. Sweep, wash, dry.
• Petcoke to alumina: alumina is usually shipped to a grain clean standard or better, and receivers are very sensitive to carbon contamination, since even small black specks show up in a white powder. Treat it at least as seriously as grain, and pay particular attention to the hatch cover undersides, where black dust falls straight into the cargo.
• Petcoke to cement or fertiliser: grain clean standard is normally expected.

A few things that save time next time

• Barrier coatings. Britannia's guidance notes that a temporary barrier coat applied before loading dirty cargoes such as green petcoke protects the paint and is easier to remove than the residue itself. If you trade petcoke regularly, it's worth the conversation.
• Wash as soon as possible. Petcoke film gets harder to shift as it ages and warms.
• Keep the coating healthy. Residue bonds far more stubbornly to rough, broken-down coating than to sound paint.

If you've got a petcoke-to-grain change coming up and the timeline looks tight, send us the details — previous cargo, next cargo, ship type and where she'll be — and we'll tell you what's realistic.