The Bullwinkle platform is not only the world's tallest offshore platform but also the second tallest manned structure ever built (fig. 1). Installed during 1988 in the Gulf of Mexico in 1353 feet of water and weighing 75,000 tons, its design evolved over a five year period including the fabrication and installation phases. This paper documents the design process from conceptual work through detailed installation engineering.
In October 1983 Shell Offshore Inc. (SOI), a subsidiary of Shell Oil Co. (SOC), drilled the discovery well on a prospect code named "Bullwinkle" in Green Canyon Block 65. Work began immediately in SOC on structural concepts for platforms in 1,100 to 1,500 feet of water as discussed by G. H. Sterling et al in Reference 1, a companion paper to this one. Tonnage, installation and cost estimates were prepared for all options. A project schedule (fig. 2) was developed with a target date o May 1, 1988 for the jacket load out and early June, 1988 for the start of installation.
Economic analysis based on exploration data projected development of a 60 well slot, fixed, self-contained drilling and production platform in 1350 feet of water, Table 1 lists the history of the Bullwinkle platform design from the start of conceptual engineering to final installation checking. About 37 man-years of engineering and 5 man-years of' computerated drafting were expended by SOC in the design of the platform. This is exclusive of significant engineering efforts on the part of the fabrication and installation contractors plus does not include any of SOI's project engineering.
Structural concepts for Bullwinkle originated from previous work on deep water platforms, Cognac (1,025' of water), the originally planned 900' Boxer platform and Eureka (700'), plus a review of available literature from in house and joint industry studies (ref. 2, 3,4, 5, 6 and 7). Framing considerations weighed such factors as:
existing and planned fabrication yard capabilities;
using only existing launch barges, Le. two piece jacket, or a new launch barge...how big?;
installation plans;
inplace structural dynamics for both storm and fatigue loadings;
deck size and loading requirements; etc..
A simple computer program was written (calibrated to existing structures) to estimate first natural period of trial structures based on: water depth, surface dimensions, leg batter, deck mass, lateral load at the surface, estimates of leg area and mass distributions. The concentrated lateral load set at the surface of the water represented an estimate of the dynamic wind, wave and current force for the concept under consideration. The previously noted studies were a valuable source of data. Limiting the first natural period to less than six seconds and extrapolating weights from the platform dimensions yielded an efficient means of comparing alternative configurations. Over fourteen different concepts and or configurations were screened in this manner.
