scope:

SUMMARY

1. An LRFD RP 2A platform design document including commentary has been assembled during the project. It should be available for release during 1986 to appropriate API and industry committees for review. Much of the data base for this document has been analyzed and summarized in the report.

2. To date, the PRAC 22 study has reviewed six platforms for calibration. This includes Platforms A through C which emphasized wave loading, Platform D which studied deep water dynamic response, and Platforms E and F (reviewed this past year) primarily for seismic calibration. The platform calibration studies confirm the applicability of LRFD methodology and the impact on member interaction ratios expected from component reliability studies.

3. During the 85-22 Project a review was made in cooperation with the LRFD Tubular Strength Task Committee chaired by John Cox of the statistical data base on tubular strength especially tests performed since the original 1980 LRFD calibration. This review provided the data base in the document which will be referenced in the LRFD RP 2A for any future review. The study this year confirmed the component reliabilities found in earlier work but refined several of the checking equations to provide more consistent reliabilities over the full range of practice. Some changes affected the nominal column stability equation, local buckling (strength) and bending strength for members with larger D/t ratios. The hydrostatic capacity was also reviewed to reflect recent API and industry tests.

4. The 1.35 load factor recommended for environmental load effects was reviewed to check the sensitivity to platform life, geographical location and wave force exponent. Guidelines were provided to allow more consistent values where platform characteristics fall outside the range encompassed in the 1.35 factor.

5. Considerable effort in the 85-22 Project covered the LRFD provisions for seismic design. The brute-force calibration of a 0.9 seismic load factor was confirmed in the Brown & Root (85-55) study. This compared members interaction ratios, connections and piles for two different platforms. On the average, LRFD is similar to present WSD provisions. The proposed 0.7 pile resistance factor will give results which fall midway between the 1.25 and 1.50 range of pile safety factors found in practice.

6. The PMB study done for this project produced estimates of seismic uncertainties. The safety indices calculated from these results by the author along with a corresponding pilot study showed safety indices consistent with other parts of the LRFD formulation, However, some of the variables in the PMB study lacked adequate statistical data and the modeling of inelastic structure response did not adequately reflect the resistance bias expected from this important component of the platform seismic capacity. The PMB study is available as a separate document from the API.