Minimal Tripod Structure Installed in Deeper Waters: Ingredients for Successful Project Execution
Authors: Kallaby Joseph , Billings C.M. , Doing Mark H. 
Published: October 2008

Minimal structures have traditionally been successfully used in the Gulf of Mexico in shallow waters, well under 400 ft. The tight economics of marginal fields in deeper waters, of 400 ft. to 600 ft., have always posed a challenge to operators. Designing a minimal platform in these water depths, to be structurally robust yet efficient, providing for the owner's operational needs, yet economical, has been a challenge to design engineers for some time.

The successful design of a minimal structure in 428 ft. of water depth, fabricated and installed within the target budget and schedule, is the subject of this paper. The development of rational design criteria, in cooperation with the Minerals Management Servcie, which accounts for wave dynamics and fatigue, is presented. The use of recently developed innovative installation aids is discussed. The economics of the tripod design concept is compared to other concepts.

Introduction

Minimal structures abound in the Gulf of Mexico in waters under 300 ft., yet, less than a handful are in over 400 A. of water depth. The oil and gas industry has sought, and continues to seek, economical alternatives to developing the many marginal fields in water depths of 400 ft. to 600 ft., and beyond. Platforms in such water depths must be structurally robust to provide for the additional demands of dynamic amplification and fatigue needs. Their design must balance strength against costs for profitable production.

The successful design fabrication, and installation in 1995 of the Blanco tripod platform in 428 ft. of water depth in Vermilion Block 395, is an example of one such platform. It is an essential step to confii the economic viability of the minimal platform concept in deeper waters and to encourage future development of marginal fields in these waters.

Considerations for Marginal Fields.

Low gas prices! Marginal fields! Minimal reserves! Refurbished equipment! The Project Engineer working development projects in the Gulf of Mexico is faced with a formidable task. More and more often, company's management is asking for economical development of "marginal" fields in the face of low product prices and flat price forecasts. In shallow water fields (less than 200'), the Project Engineer has been able to choose from several economical alternatives such as caissons, licensed-name minimal structures, as well as refurbished salvaged jackets and decks. However, as development activity moved into deeper water, operators are finding prospects that fall into the "marginal" category due to smaller reserves, development costs associated with medium water depths of 300' to 600', and continued depressed price forecasts.

The Vermilion Block 395 development provided a challenge to find a structural concept that would support multiple wells, processing and compression equipment, as well as provide for personnel needs and for well workover capabilities, in water depths that precluded the use of jack-up drilling rigs. Several design concepts were solicited from engineering f irms, and preliminary bids were received for fabrication and installation.