In the fall of 1991, Texaco successfully tied back three subsea template wells to a deepwater conventional platform atGarden Banks Block 189. Since these were Texaco's first subsea wellhead tiebacks in the Gulf of Mexico, Texaco solicited recommendations from various contractors (as well as from other operators) in regard to design and operational considerations which would increase the chances of a successful tieback project. The considerations presented in this paper involve drilling operations, template design, platform design and actual tie back operations.
Since the early 1970' s, operators have employed subsea wellhead tiebacks as a means of retaining costly exploration wells and later utilizing them as an early production source for a new platform installation. One desirable aspect of this approach is that it allows development drilling and platform construction to occur concurrently. In the span of the last few decades as worldwide exploration drilling has extended into deeper waters, this concept has become increasingly more common among an ever-increasing number of operators. Just as the drilling technology has changed to meet the new challenges, considerations have also changed for template design, jacket design and ultimately tieback operations to be used on these deepwater projects. The Gulf of Mexico now has two conventional platforms located in excess of 700 feet of water which have successfully incorporated a multi-wellhead template/subsea tieback system. The design philosophy used at both of these installations varies greatly from those earlier projects completed in shallower water depths. One of the most important contrasts is that the planning must focus on using a diverless system when performing the wellhead tiebacks. This is significant in that the average water depth of the eleven tentative tieback operations presently being planned worldwide is approximately 550 ft.
This paper updates earlier presentations by incorporating new information gained from recent experience in designing tiebacks for these deeper water depths. One of the most important aspects in planning deepwater tiebacks is to put equal emphasis on all phases of the operation. Proper design considerations should begin prior to the drilling of thefirst discovery well and continue throughout the planning of the template and platform jacket structures. If the first three aspects of the project arecarefully approached and given equal importance the final phase involving the actual subsea wellhead tieback will be performed more efficiently with a greater probability of success.
Every subsea well which has the remote possibility of being incorporated into a deepwater platform tieback project must be properly designed at the very initial stages. This begins with the choice of the subsea wellhead system to be used for drilling the well. All conventional subsea wellhead systems offer the option to tieback to platforms. In determining which system is better suited for the project, more consideration for the decision needs to be given to the tieback features of the system as compared to the drilling features.