Of the many deepwater platform concepts that were developed as deepwater platform technology matured, the TLP has been the most frequently selected for deepwater construction projects. Over the past two decades, TLP technology has proven to be a safe and reliable means for establishing deepwater real estate for supporting drilling, production, and transportation operations. As the deepwater exploration province evolves from a frontier area toward maturity, the commercial and technological environment of field development concept selection has changed and has opened opportunity for secondgeneration deepwater platform concepts, such as the SeaStar® Mono-column TLP. This concept has been used to successfully exploit deepwater opportunities in the Gulf of Mexico including AGIP's Morpeth Field, AGIP's Allegheny Field, Chevron's Typhoon Field, and now TotalFinaElf's Matterhorn Field. This paper briefly describes these four projects and then discusses the continuing evolution of this technology. Introduction In parallel with the beginning of exploration for oil and gas reserves in deep water, major oil companies began developing platform concepts to exploit deepwater discoveries. Tension-Leg Platform (TLP) technology emerged as a cost-effective means for providing stable deepwater real estate for drilling and production operations. Over three decades, the Offshore Industry invested enormous amounts of money and manpower to develop the design and analysis methods, software, specialized hardware, fabrication techniques, and installation equipment necessary to safely construct and operate TLP's. Early TLP's resembled semisubmersible Mobile Offshore Drilling Units (MODU's), in which the catenary mooring system had been replaced by a tension-leg mooring that eliminated vertical motions. Figure 1 presents a summary of TLP's installed to date. The structural reliability of a tendon, like the structural reliability of an airplane wing, is an essential safety issue, that receives focused attention throughout the project. Note that there has never been an operational tendon failure on any of these fifteen TLP's, three of which have been operating for a decade or more. The number of service years that tendon system components have delivered, with zero failures, is very impressive, measuring in the tens of thousands of service years. As exploration drilling proceeded in deepwater and a mix of large and small fields were found, it became apparent that there was a need for a new generation of less expensive TLP's, which would permit economic development of heretofore sub-marginal fields. In response to this need, Atlantia developed the SeaStar mono-column TLP, thanks to the help of funding from the U.S. Department of Energy. The primary geometric difference between a SeaStar TLP and a conventional TLP is the former's single column. The single column allows deck and hull to be separately optimized, resulting in a most versatile and cost-effective solution. The tension leg mooring was the key to allowing the platform size to be reduced while maintaining the favorable motion characteristics needed for the safe operation of facilities and risers. A freely floating platform, such as a spar, experiences relatively large angular motions that must be accommodated with respect to the earth-bound risers.