I

__ General deterioration in splash zone

Mud line

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— Holes in flanges and web

j

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Figure 3-16. Corrosion of steel H-pile.

Figure 3-16. Corrosion of steel H-pile.

Figure 3-15. Corrosion of steel in various zones on waterfront structures.

electrical current to flow between the areas of high and low oxygen content with the result of accelerated corrosion. This effect is particularly severe on stainless steels and aluminum alloys. A difference in environment with resulting accelerated corrosion can also be created when a surface is partially covered by an electrically conductive material such as concrete.

Corrosion can also be accelerated by the action of bacteria. In the absence of oxygen, sulfate-reducing bacteria can grow, and the sulfides they produce can cause rapid attack on steel. Areas such as the inside of pipe piles that are not filled with concrete can become depleted in oxygen and are frequently the site of sulfate-reducing bacteria attack. Some bottom sediments are oxygen deficient and sulfate-reducing bacterial attack can occur on buried steel in such sediments. Sulfate-reducing bacteria attack is more likely to occur in polluted harbors but can occur whenever oxygen is depleted. The presence of sulfate-reducing bacteria can often be detected by the odor of rotten eggs produced by the bacteria. As sulfate-reducing bacteria attack usually occurs on the inside of pipe piles or below the mud-line, it is especially difficult to locate and repair open end pipe piles which are not concrete filled and typically used on offshore and waterfront structures, and experience indicates that the insides are generally not subject to corrosion.

Finally, corrosion can be initiated by the presence of stray currents, e.g., from nearby electrical power lines.

This area is usually more severe than below MLW.

Kick Out Failure Sheet Pile

General deterioration '' in splash zone

Wale

„Holes in steel

Figure 3-17. Corrosion of steel sheet piles.

General deterioration '' in splash zone

Wale

„Holes in steel

Figure 3-17. Corrosion of steel sheet piles.

3.4.2.2 Abrasion. Abrasion of steel structures can generally be recognized by a worn, smooth, polished appearance of the surface. Abrasion is caused by continual rubbing of adjacent moving steel surfaces, or by the exposure of structural components to wave action in areas of sandy bottom. Steel sheet piling and pile-supported structures are particularly susceptible to sand abrasion in exposed locations. Abrasion of steel structures is a problem because it removes both protective coatings and protective layers of corrosion products, thus accelerating corrosion.

3.4.2.3 Structural Connection Loosening. Structural connections joined together by rivets or bolts have a tendency to work loose over an extended period of time. Connection loosening can result from impact loadings of the type imparted by a vessel striking a pier or wharf fender system. Wave action and reciprocating machinery mounted on or below pier or wharf decks are other sources of possible connection loosening. Corrosion of bolts, rivets, nuts, washers, and holes can also contribute to connection loosening. Loosening of connections will tend to produce misalignment in mating surfaces, which, in turn, can result in distortion and stress concentrations in framing members.

3.4.2.4 Fatigue Failure. Fatigue failure results in the fracture of structural members as a consequence of repeated high loadings. Fatigue distress can be recognized by a series of small hairline fractures perpendicular to the line of stress in the member. Tubular connections of offshore platforms are particularly susceptible to fatigue failure. Fatigue cracks are difficult to locate. Since fatigue cracks represent an extremely dangerous condition in steel marine structures, extreme care must undertaken when inspecting structural members subjected to repetitive loadings, particularly high wave loading. Figure 3-18 illustrates the typical location of fatigue cracks.

Platform And Collision Offshore
Figure 3-18. Location of fatigue cracks in offshore platform.

3.4.2.5 Overloading. Steel structural elements are sensitive to impact damage from berthing vessels and other types of accidental overloading. Impact or collision damage can generally be recognized by the appearance of local distortion (deformation) of the damaged member. This damage is generally characterized by a sharp crimp or a warped surface as illustrated in Figure 319. Compression overloading damage of a steel pile is illustrated in Figure 3-20.

3.4.2.6 Foundation Deterioration. Loss of foundation material from around steel piles leads to accelerated corrosion and loss of column strength of the piles. The loss of foundation material is usually caused by the scouring of material from around the piles, as illustrated in Figure 3-21. Scouring can be caused by an increase in the velocity of the water passing by the piles or a change in the current's Jirection. If the piles thus exposed are not protected, eventual collapse of the structure is possible. A loss of foundation material in front of a sheet pile bulkhead may cause kick-out of the toe of the wall and result in total failure.

3.4.3 Typical Inspection Procedure

Inspection of a steel waterfront structure should proceed as outlined in Table 3-3.

forms, members are usually welded together. These welded connections must be carefully inspected as they are highly susceptible to electrochemical corrosion and fatigue cracking. Clean these welded connections of all marine growth and, depending on visibility, inspect visually or by touch. Again, record all information immediately in the inspection log.

3.4.4 Steel Structures Offshore

3.4.5 Equipment and Tools Required

When steel is used as a structural member in support towers and other ocean plat-

Concrete Cap

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