Offshore Structures and Materials
What you must understand
Offshore structures fall into fixed (jacket, gravity-based, monopile), floating (semi-submersible, FPSO, spar), and subsea categories, each with distinct corrosion exposure zones: atmospheric, splash, tidal, submerged, and mudline/buried. Candidates should know the structural role of members (legs, braces, conductors, risers, mooring chains) and how design and fabrication details drive corrosion: weld metallurgy and heat-affected zones, hydrogen-induced cracking in high-strength steels, fatigue at welded joints, and sour service limits per NACE/ISO material requirements.
Material selection spans carbon steel with coatings and CP, duplex and super-duplex stainless steels, corrosion-resistant alloys for risers and tubing, and non-metallics. Understand why duplex is favored where chloride SCC resistance and strength are needed, and why carbon steel remains the default for large structures when protected. Fabrication quality - welding, post-weld treatment, coating application - is often the true determinant of service life.
Traps in questions
Questions may imply higher strength steel is always better; in fact higher strength increases SCC and hydrogen cracking susceptibility. Another trap: assuming stainless steel needs no CP or coating - it can suffer crevice corrosion and pitting in stagnant chlorinated seawater.
Worked example / rule of thumb
Splash zone members: specify a high-build coating system or corrosion-resistant sheathing, because CP is ineffective there. Rule of thumb: protect the submerged zone with CP plus coating, the splash zone with coating/barrier, and the atmospheric zone with coating and inspection.