Research on the seismic fortification level of offshore platform in Bohai Sea and adjacent areas.

API code is introduced at first, and then a comparison of seismic character and seismic hazard between Bohai and Southern California is carried out. The analysis indicates both the seismic frequency and intensity and the seismic hazard of Bohai are much weaker than that of Southern California. API code states the strength level and deformation level of permanent structures in Southern California takes 200 a and several hundred to a few thousand years respectively. But in the reference codes in China, the seismic levels take 500 a and 10 000 a for strength design and deformation design, and it seems too conservative. In China, the deformation level of class A structure takes 2%–3% probabilities of exceedance in reference period 100 a, and that of class B and C often takes 2%–3% in 50 a. Now that offshore platforms may cause server subsequent risk, it is safe to take 1% in 30 a as its deformation design level. On the basis of the above analyses and social economic level and the consistency with present codes, the strength design level and deformation design level of Chinese offshore platforms is suggested to take 200 a and 3 000 a respectively. [ABSTRACT FROM AUTHOR]