Global Design And Analysis Of The Auger Tension Leg Platform

ABSTRACT The global design and analysis of tension leg platforms encompasses an essential, if not transparent, role in the TLP design process. The essential aspect is the sizing and proportioning of the TLP principal dimensions such that the platform meets the functional requirements of the project and responds to environmental forces in a cost effective manner. The global design effort is transparent in the sense that there is not a single piece of hardware which is under the direct purview of the global designer, yet, the designers of all of the TLP systems and subsystems are directly affected by and rely upon the results of the global design and analysis function. This paper focuses on the analytical methods used in the global design of the Auger TLP and the interaction and impact which these results have upon the detailed design of the various TLP systems (e.g. hull, deck, tendons, foundation, production risers, steel catenary risers, well bay gantry, tank and ballast system, etc...) SYSTEM OVERVIEW The Auger TLP is a self-contained facility with complete production and drilling capability as well as personnel accommodations. The unique aspect of the Auger TLP centers around the drilling system which is operated in an analogous manner to a deepwater semi-submersible with a guidelineless exploration drilling rig. The Auger TLP has a fixed drilling rig, free standing individual seafloor wellheads and a lateral mooring system (LMS) to position the TLP drilling rig over any individual well. The TLP has two primary modes of operation, the drilling mode and the hurricane ready (storm) mode. In the drilling mode, the TLP is positioned over a well and is fully operational. In the hurricane ready mode, the TLP is centered in the well pattern and personnel are evacuated. In addition to the standard TLP global design and sizing issues pertaining to platform displacement, column spacing, draft, deck elevation, etc..., the complex interaction of the tendons, risers and lateral mooring system in the drilling and hurricane ready modes of operation requires special consideration. ENVIRONMENT AND RESPONSE-BASED CRITERIA In addition to conventional Gulf of Mexico design environmental conditions (see Table 1), response-based hurricane design criteria were established for the Auger TLP using the approach described in reference 2. By accounting for the joint probability of occurrence of the wind, wave, and current components of the seastate as well as the responses of the platform, long term probability distributions were derived for maximum and minimum tendon tension and for platform offset. The 100-year return period responses were estimated from these distributions and the seastates most likely to generate these responses were identified. The criteria were based on a hindcast database of hurricane seastates which includes the 35 worst hurricanes between the years 1900 to 1984 in the deep water Gulf of Mexico. The independent variables input to the global response model were significant wave height, spectral peak wave period, peakedness of the wave spectrum, mean wave direction, mean wind speed, mean wind direction, current profile, and mean current direction.