Combined Oceanographic Criteria for Deepwater Structures in the Gulf of Mexico
Authors: Forristall G.Z. , Larrabee R.D. , Mercier R.S. 
Published: October 2008

The strongest winds, waves, and currents do not necessarily occur at the same time in a storm, and it is unnecessarily conservative to set design criteria as if they did. We hindcast the most important hurricanes to affect the northern Gulf of Mexico in the twentieth century using the ODGP-2 model for winds and waves and the Mellor-Durbin turbulence closure model for currents. To account for the effect of the joint occurrence of wind, wave and current, we used generic structure models of a rigid platform, a compliant tower, and a tension leg platform to integrate forces. Response functions for the structures were developed through numerical simulations in random waves. Multivariate regression analysis then produced simple polynomial equations for the responses which could be applied to the entire database of oceanographic hindcasts. For the fixed platform, the 100 year base shear found from using this method was approximately 15% less than that which resulted from applying the 100 year oceanographic conditions for winds, waves and currents simultaneously. For a given force level, the hindcasts indicate that a larger proportion of the total force is due to the current than has been thought previously. For the TLP, combined oceanographic criteria which include directional effects were developed. The most likely sea state to cause the 100 year minimum tendon tension was found to occur near the eye of a hurricane.

1 PURPOSE AND SCOPE

The environmental forces which a structure must resist cannot be predicted deterministically, so the design for them is necessarily probabilistic. In the face of this uncertainty, a rational determination of the design force level can be made by balancing the probability of failure during a project's planned life against the cost of strengthening the structure. The environmental parameters which would cause the design force then become the oceanographic criteria for new designs.

Forces on fixed structures in moderate water depths are largely due to waves, and reliable statistics for waves in the Gulf of Mexico have been available at least since the work of Ward et al (1978). The novel concepts which are being considered for production structures in deep water attract substantial forces from winds and currents in addition to waves. Recent measurements (Price et al, 1991) have shown that hurricane generated currents in deep water can be as large as 7 ft/sec. Deep water oceanographic criteria should thus explicitly include wind and current as well as waves.

The design force for many platform types is dominated by waves and a 50 or 100 year return interval wave height is a common design level condition. By extension, one could combine the 100 year wave with the 100 year wind and current. This approach is, however, unduly conservative since the largest wind, wave and current values in 100 years are not expected to occur at precisely the same time.

Joint statistics of wind, wave and currents can be developed through hindcasting, but the statistics cannot be used to develop criteria without knowing the relative importance of the various parameters.