An assessment of three offshore platforms in the Valhall field of the North Sea is described in this paper. Ultimate strength and reliability analyses were performed for the jacket-type platforms for four levels of sea floor subsidence. The nonlinear structural models of the platform elements, joints, and associated soil-structure interaction are summarized in a companion paper by Gebara et al., 2000, “Assessment of Offshore Platforms Under Subsidence—Part I: Approach,” ASME J. Offshore Mech. Arct. Eng., 122, pp. 260–266. Similarly, environmental loads and reliability acceptance criteria are presented by Pawsey et al., 1998, “Characterization of Environmental Loads on Subsiding Offshore Platforms,” OMAE98, Lisbon, Portugal, and Stahl et al., 1998, “Acceptance Criteria for Offshore Platforms,” OMAE98, Lisbon, Portugal, respectively. This paper describes the procedures employed and results obtained for ultimate strength and reliability analysis. Ultimate platform strength was calculated using a pushover analysis in which environmental loads for increasingly rare events were applied incrementally until the platform failed. Failure was defined as formation of a limiting mechanism in the platform structure or foundation. Structural reliability analysis was performed to estimate the annual probability of platform failure as a function of subsidence level. The platform structural system was modeled in terms of two major subsystems that were assessed as likely to fail: (i) the platform deck, and (ii) the platform jacket and piles. Uncertainties were included in the loads, structural components, and soil strengths based on industry experience and data. First and second-order reliability methods (FORM/SORM) were used to estimate the annual failure probabilities. Consideration was given to the correlated nature of the failure modes due to the common environmental loads. The reliability results were compared with predefined acceptance criteria to judge the suitability of continued operations. [S0892-7219(00)00304-6]

Issue Section:
Technical Papers
Keywords:
structural engineering, failure analysis, oceanic crust, soil, mechanical engineering
Topics:
Event history analysis, Failure, Failure mechanisms, Probability, Stress, Tensile strength, Waves, Offshore platforms, Soil, Uncertainty, Shear (Mechanics), Reliability
1.
Krieger, W. F., Banon, H., Lloyd, J. R., De, R. S., Digre, K. A., Nair, D., Irick, J. T., and Guynes, S. J., 1994, “Process for Assessment of Existing Platforms To Determine Their Fitness for Purpose,” OTC Paper No. 7482, Proc., Offshore Technology Conference, pp. 131–140.
2.
Bea, R. G., Ramos, R., Valle, O., and Valdes, V., “Risk-Based Hurricane and Earthquake Criteria for Design and Requalification of Platforms in the Bay of Campeche, Mexico,” Proc. 17th International Conference on Offshore Mechanics and Arctic Engineering, Lisbon, Portugal.
3.
Stahl, B., Aune, S., Gebara, J., and Cornell, C. A., 1998, “Acceptance Criteria for Offshore Platforms,” OMAE98, Lisbon, Portugal.
4.
Gebara
,
J.
,
Dolan
,
D.
,
Pawsey
,
S.
,
Jeanjean
,
P.
, and
Dahl-Stamnes
,
K.
,
2000
, “
Assessment of Offshore Platforms Under Subsidence—Part I: Approach
,”
ASME J. Offshore Mech. Arct. Eng.
,
122
, pp.
260
–
266
.
5.
Pawsey, S., Driver, D., Gebara, J., Bole, J., and Westlake H, 1998, “Characterization of Environmental Loads on Subsiding Offshore Platforms,” OMAE 98, Lisbon, Portugal.
6.
Aggarwal, R. K., Dolan, D. K., and Cornell, C. A., 1996, “Development of Bias in Analytical Predictions Based on Behavior of Platforms During Hurricanes,” OTC8077, Proc., Offshore Technology Conference, Vol. 2.
7.
PMB Engineering, Inc., 1996, “Hurricane Andrew Effects on Offshore Platforms,” Phase II Joint Industry Project, Final Report, San Francisco, CA.
8.
Madsen, H. O., Krenk, S., and Lind, N. C., 1986, Methods of Structural Safety, Prentice-Hall, Englewood Cliffs, NJ.
9.
Risk Engineering, Inc., 1994, “RELACS-Reliability Analysis of Components and Systems,” Ver.2.0, Boulder, CO.
10.
Jardine, R. J., and Chow, F. C., 1996, “New Design Method for Offshore Piles,” Marine Technology Directorate 96/103, U.K.
11.
American Petroleum Institute, 1996, “Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms,” API RP2A-WSD, 20th Ed. Suppl. 1.
12.
PMB Engineering, Inc., 1995, “CAP—Capacity Analysis Program,” San Francisco, CA.
Copyright © 2000
 by ASME
You do not currently have access to this content.