Ship piping and pipeline inspection: wall thickness, corrosion and erosion

Internal corrosion and erosion at bends, tees and downstream of valves

Wall loss inside a ship's pipe is rarely uniform; velocity, turbulence and where water sits decide where it concentrates. At a bend, the extrados, the outside of the curve, takes the impingement of the flow and wastes first, especially in sea-water and steam lines. At a tee, the branch connection and the run wall opposite the branch see turbulent flow. Downstream of a valve, orifice plate or reducer, the jet leaving the restriction strikes the wall a short distance along, so the worst reading is often past the flange, not at the valve itself.

In horizontal oil lines, the six o'clock position corrodes where water and sediment settle out; crude tanker cargo lines and fuel transfer lines are typical. Dead legs and low points in fire mains and bilge lines hold stagnant seawater and corrode from the inside. In sea-water cooling systems, erosion-corrosion combines with galvanic attack where copper-nickel, bronze valves and steel pipe meet. Steam lines lose wall downstream of control valves and traps, and condensate returns corrode at their low points.

These are where a sampling plan should start, and the pattern of findings is itself informative. A line thinned at its bends but not along its straight runs is behaving as expected. A line thinned uniformly suggests a change in the fluid or a failed internal lining, and the inspection should widen.

Minimum wall checks against class rules and owner's criteria

A thickness reading becomes a decision only when it is compared with a minimum. Classification society rules calculate a required wall from design pressure, add allowances for bending, manufacturing tolerance and a service-dependent corrosion allowance, and also set absolute minimum wall thicknesses for steel pipe by service and outside diameter. Most rules set heavier minima for pipes passing through tanks, such as bilge, air and sounding pipes through ballast or fuel tanks, ballast lines through fuel tanks and fuel lines through ballast tanks, because a leak there contaminates or floods a space nobody can see.

We do not decide which criterion applies; you or the class surveyor do. Tell us the minimum for each system, or send the relevant class rule table, and every reading will be reported against it, with values below the minimum and those approaching it flagged. Without a criterion we report measured wall against nominal and leave the assessment to your technical department. Pipe below its minimum is normally renewed; clamps, wrapping and doublers are temporary measures that class accepts, if at all, for a limited period.

Deck piping exposed to weather, pipe supports and clamps

Deck piping corrodes from the outside as well. Cargo piping, tank cleaning lines, fire mains, inert gas and vent lines take green water, salt spray and sun, and the damage concentrates where water is trapped: under supports and saddles, beneath U-bolts and clamps, at deck penetrations and sleeves, and at flanges where paint cannot reach. A pipe can look sound along its run and be wasted almost through on the underside of its support.

Supports cause their own failures. A missing or broken support lets vibration fatigue small-bore branches at their welds; a seized one stops thermal movement on heated lines and overloads the pipe. A clamp that has worn into the pipe leaves a local groove that gauging finds only if a reading is taken at the clamp. We inspect supports visually, release clamps where that can be done safely to gauge beneath them, and use magnetic particle testing on support welds and small-bore connections showing signs of cracking. Insulated lines are gauged where the insulation can be opened, because corrosion under insulation cannot be seen from outside.

Reporting with isometric sketches

Every line gauged is drawn as an isometric sketch, not to scale but faithful in routing, fittings, valves and flanges, with each measurement location numbered. Readings are tabulated against those numbers with nominal wall, the minimum applied and the measured minimum at each location, and low values are flagged. Lines are identified by system and by the ship's own line numbering where the piping diagrams provide it, so the report reads alongside the ship's drawings.

For renewal planning the report lists spools and fittings below their minimum, with locations, which is what a pipe shop needs to prefabricate. Photographs record external corrosion, support defects and anything found outside the scope. On repeat rounds, readings at each fixed location are compared with the previous round and the wall loss rate is shown, so the next inspection can be set before a line reaches its minimum rather than after it leaks.

Frequently asked questions