New structural members that are light and durable are anticipated to reduce the running and maintenance costs of structures exposed to harsh marine environments, such as offshore oil production facilities, thereby reducing their lifecycle cost. This study aims to investigate the applicability of new materials to marine structures, focusing on their durability. To this end, a 5-year exposure test has been conducted beginning in 1999 on 3 types of specimens (for corrosion observation, tension testing, and joint strength testing) made of 21 selected materials (6 nonferrous metals, 8 steels, 4 composite materials, and 3 rope materials). The specimens have been exposed at 3 sites: Okinotori-shima and Miyako-jima, corrosive environments with high temperature and humidity, and a thermo-hygrostatic room in a laboratory. Having completed the natural exposure tests in 2004, the authors conducted strength tests and observation thereafter toward 2005. This paper summarizes the results of such tests and observation conducted so far, while making a final assessment of each material, and refers to the tasks ahead based on these results. No marked differences were observed between the results of exposure at Okinotori-shima and Miyako-jima. Both islands were therefore found to be similar environments in terms of corrosion. The corrosion states of specimens were classified into five Categories: I (no corrosion), II (slight corrosion), III (light corrosion), IV (corrosion with strength loss), and V (corrosion with significant strength loss). Only reference specimens of ordinary steel were designated as Category V. Those designated as Category IV included textile ropes and Ni steel and coated steel panels with damaged coating film. Most other new materials were designated as Categories III or less with no strength losses. Materials designated as Categories I to III were subjected to surface observation using optical and electron microscopes and element distribution analysis over a cross section using an electron beam probe microanalyzer. The progress of corrosion in each material over the five years has thus been elucidated. Within the range of the 5-year exposure test, most of the selected new materials pose no problems in regard to durability. However, marine structures are more vulnerable to alternate stresses than general structural members on land because of constant waves and pulsating gales on the sea. The authors intend to investigate the durability of structural members under continued or cyclic stress as a subject for the future.

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