Tension Buoyant Tower for Small Fields in Deepwaters
Authors: Perryman S.R. , Horton E.E. , Halkyard J.E. , Beynet P.A. 
Published: October 2008

The paper describes a concept for a deepwater platform capable of supporting surface wellheads and drilling or work over operations and/or production equipment for up to 18 wells. The fundamental principles are described along with the functions of the key components. The previous analytical studies and model tests which have contributed to the concept are shown to confirm and clarify the concept. The concept could be applicable to hydrocarbon reservoirs in water depths from 1500 ft to several thousand feet worldwide. The concept can be engineered, fabricated and installed using existing methods and infrastructure. Drilling operations are similar to jack-up drilling technology. The results show that by restraining the buoyant hull against heave, while permitting some amount of pitch, an economical system can be configured. A key factor is the arrangement of well and structural tubular axial stiffness's in relation to their distance from the center. The concept allows mean and axial tensions to be optimized to the extent that nonpremium threaded connectors may be utilized for the tendons and run by the rig.

INTRODUCTION

An offshore platform concept has been under development for several years which may help to permit production of hydrocarbons from small deepwater fields in an economical manner. The concept is commonly referred to as a TensionBuoyant Tower or TBT. The original goal was to provide a fixed platform type of drilling and production system for deep waters, i.e., greater than 1500 tl and capable of supporting up to 24 wells with associated equipment. Although it has been demonstrated through joint industry studies and model tests that it is technically possible to achieve this goal, there are not many reservoirs prolific enough to warrant such a large drilling and production system. Most reservoirs being discovered today in the deep waters of the Gulf of Mexico require only a handful of wells to produce them. Thus, there is a real need for an economical production system or these reservoirs will likely not be developed in the near term.

OBJECTIVES

The main objective of the TBT concept for small fields is to provide a low cost means of supporting the wellheads at the surface for direct access to the wellbore. This requires that the payload be minimized as much as possible. For example, the topsides should be limited to a small processing capability or a multi-phase pump flowing to a nearby processing system and a workover rig for tieback and maintenance, or if necessary a fixed platform rig for a surface drilling capability. The well strings should be restricted to having as few casing and tubing strings as necessary brought back to the surface to reduce their hanging weight, unlike the fixed platform type of well system in which all casing and tubing strings are brought back to the surface without regard to weight.

Another objective is to reduce installation cost by minimizing the marine construction equipment required to Mall the TBT. This is achieved by utilizing a floating rig to install the seafloor spacer template.