Development of Environmental Combination Design Criteria for Fixed Platforms in the Gulf of Mexico
Authors: Wen Y.K. , Banon H. 
Published: October 2008

The objective of this ongoing API sponsored research study is to develop design criteria for selection of winds, waves, and currents in the Gulf of Mexico to be used in design of offshore structures. A probabilistic methodology is developed that leads to the joint probability distribution of hurricane induced winds, waves and currents for a generic site. Both analytical and statistical (simulation) procedures are used to generate the joint probability information, where the statistical input is based on the historical hurricane information in the Gulf. Three different methods are compared for selection of the design combination criteria:ignoring the platform response and simply focusing on the joint probability of the environmental events,aiming for a target return period for the platform response, andaiming for a target reliability for the platform.

For fixed platforms, the results are shown to be similar based on the three selection methods; this conclusion may not be valid for other climate areas. The project Technical Advisory Committee's recommendation for fixed platforms is a combination of the 100-year maximum wave height, the 100-year wind, and 54% of the 100-year current or roughly a 2.0 knot current. This, on the average, leads to an annual safety index of 3.0 for the platform.

Background and Introduction

Selection of the storm (or hurricane) environmental conditions for design of a platform is an important part of the design procedure. This selection process involves two distinct steps:selection of an appropriate return period, andadopting a criteria for selection of waves, winds, and currents.

Both the return period and the design combination criteria affect the strength of a platform and thus directly impact the reliability of the design.

The generally accepted design practice for fixed platforms in the Gulf of Mexico (GOM) is to use a wave height with a 1% annual probability of exceedance, i.e., the l00-year return period. The design combination criteria, however, are not as well established. The present API recommended practice (RP2A, 18th Ed.) suggests using the specified recurrence interval (e.g., 100-year) wave height, and the currents and winds likely to coexist with the design wave.

The objective of this ongoing study, which is funded by the API, is to quantify the environmental combination criterion for design of fixed platforms in the Gulf of Mexico. The criterion should account for the joint probability of occurrence of winds, waves, and currents. This requires a storm hindcasting model that can predict the magnitudes and directions of winds, waves, and currents as a storm passes by the site.

Early in the project, it was decided that such a combination criterion should be platform dependent, i.e., the design criterion for fixed platforms should not be the same as that for compliant platforms such as TLPs or FPSs. The reason is that fixed platforms are inherently more sensitive to waves than winds and currents. Compliant platforms, are less susceptible to waves because of their dynamic motion.