Amtec Guide to the Effect of Structural Behaviour on Coating Breakdown
Coating breakdown is common in high stress areas of the structure and in particular at structural details.
The increased manganese content of the high strength steel in ship’s structure (as compared with mild steel) results in ships with thinner plating and consequently, less stiffness. It therefore follows that the deflection of the ship constructed with high tensile steel whilst in high seas is increased. For example, the deflection on a high tensile inverted angle longitudinal on the hopper part of inner skin bulkhead between the transverse webs of a typical Aframax, under maximum loading maybe of the order of 15mm or 15,000 microns while the DFT of the coatings applied is perhaps of the order of 250 microns.
Customarily the ship structure is designed and assessed to post buckling behaviour, which means that some permanent deformation of the plate may occur. The difference in strain between high tensile (0.15%) and mild steel (0.10%) may be of the order of 50%. Coating breakdown is common at the weld toes of various details in the hull structure where yielding is exceeded (see below).
Photograph 1. Typical corrosion at the toe of the bracket in the end of a vertical girder at the tank top in the cargo tank of a double hulled tanker.
Photograph 2. Corrosion at the mid-span of the vertical girder at the centre line bulkhead in the cargo tank of a double hulled tanker.
Photograph 3. Corrosion in way of the buckled lower end of the web in a lower wing tank.
The stress concentration around specific designs of structural details may be more pronounced compared with other areas under the influence of buckling and shear stress. This may be noted around manholes fitted with ring stiffeners compared with those fitted with vertical stiffeners (see below). Similarly the phenomenon may also be exacerbated by the structural behaviour of the angle stiffener (torsionally unbalanced section) compared with a balanced section (i.e. T or slab section – see below). The stress behaviour at the free edge of the flange and twisting of the section overall when loaded, may affect the coating behaviour adversely at the mid span and at the end joint. This may occur on hard epoxy coatings which may have not been designed to sustain such stress and strain modes.
Photograph 4. Typical manholes with and without ring stiffeners.
Photograph 5. Inverted angle longitudinal through upper wing tank web cut out.
The structural failure mode for high tensile steel members located in the outside fibres of the hull girder, may be through buckling. This will also affect the hard epoxy coatings unless they have been designed for this type of steel failure mode.
The structure of a new ship will be stress relieved after launching and for approximately the initial 6 months in service. This means that the activity of stress redistribution around the hull will be more pronounced during this period and therefore the Operators should be very alert during this period to assure that the coatings are maintained intact and provide some necessary touch ups.
As coatings age they become harder and more brittle and as such can become more prone to cracking. It follows that if coatings have been designed with some fatigue behaviour in mind, under normal ship type cyclic stress in air, sea water and/or cargo environment, (oil or petroleum products) then the need for repair or even renewal of coatings in the tank maybe predicted.
Details of the connections of piping (ballast lines, cargo lines, heating coils and other hydraulic lines) to structural members require special attention at the design and fitting stage. The structural deformation of the hull structure and thus to attached members, is naturally different than for these piping systems. It follows that there will be some movement between the two surfaces which will result in crevice corrosion in way of their connections. The pictures below show initiation of the corrosion at stainless steel stanchions connected to stainless steel U-Bolts and corrosion of the stainless steel coils, which then propagated to inner bottom plating at the early stage of the vessel’s life.
Photograph 6. Corrosion initiation on stainless steel stanchion which is supporting stainless steel heating coils by stainless steel U bolts.
Photograph 7. Typical condition of the hydraulic pipes in cargo tanks.
Photograph8. Typical condition of stainless steel heating coils in way of stainless steel stanchions in cargo tanks. Note the red corrosion products at the crevice between the pipe and the stanchion.
Photograph 9. Ballast line attached to the flange web in an upper wing tank.