Heave-Restrained Platform Reduces Costs and Eases Operations
Authors: White Charles N. , Goldsmith Riley G. , Triantafyliou Michael S. 
Published: December 2007

The "well-systems-centered" design approach described in this paper integrates existing offshore platform and drilling technology. A simplified deepwater well tieback system has been developed to provide drilling, completion, workover, and production capabilities similar to those of conventional fixed platforms. It combines the roles of well risers and platform tethers and uses mudline suspension subsea connections to create a well-tethered platform. This latest evolution of the tension-leg platform (TLP) concept eliminates the need for many costly components [riser tensioners, subsea blowout preventers (BOP's), and the separate tension-leg mooring system with foundation templates, piles, mooring tendons, and connectors]. The riser/tethers of this well system form the core of the heave-restrained platform (HRP).

This paper presents a general description of the HRP design and recent developments of the well-system components and running procedures. Emphasis is on the integrated design process that accounts for the critical aspects and costs of safe deepwater drilling operations while ensuring an efficient production facility. We present results of design studies that provide guidance for configuring efficient platforms.

Introduction

Well-Systems-Centered Design. Past TLP design has emphasized optimized vessel and mooring system performance. However, the primary function of an offshore production facility is to allow commercial development of hydrocarbon reservoirs through efficient drilling and production. Deepwater drilling and well-system operations can dominate field development costs. Thus, new concepts must take a well-systems-centered design approach to permit the most effective drilling and production operations.1,2

The 1970's concept of the vertically moored platform3 used well conductors as tension-leg mooring elements. However, the Hutton TLP design separated mooring and well functions.4 Following the Hutton lead, the major divisions for TLP design customarily have been vessel design (hull or hull and deck structure with marine systems), mooring system (foundations and tethers), and facilities (quarters and processing). Well-system design and implementation responsibilities form a small separate division or are included in facilities design. Selection of these divisions and definition of their interfaces inherently fixes certain features and thus limits design flexibility.

Production-system designs, such as the original Hutton TLP system, are uneconomical for developing many deepwater fields. The HRP concept required a departure from traditional division of design responsibilities. It was necessary to re-evaluate traditional TLP systems in light of the deepwater challenges and then to redefine the major design subsystems.

Deepwater Well System Challenges.

In the late 1970's and early 1980's, operating solutions were developed for the Hutton TLP and similar conceptual TLP designs. As offshore frontiers moved to deeper waters, operators realized that TLP designs face several problems that were not critical with the shallow-water Hutton TLP.

The TLP's fundamental operational premise - the wells extend to the platform - provides many production operation advantages over subsea completions. Conventional platform drilling operations with surface BOP's also provide improved operating efficiencies. However, the Hutton well systems that provided these features5 are inappropriate for deepwater drilling operations in abnormally pressured reservoirs because only a single, independent well-control barrier is provided. Deepwater TLP's must use subsea BOP's or have multiple casing strings tied back to the surface to achieve the conventional two-barrier system for safely developing abnormally pressured reservoirs. Appendix I of Ref. 1 explains this problem.

Double-wall riser systems, such as the Auger TLP well system,6 provide safety and flexibility to work over and sidetrack with surface BOP's. As water depth increases, the double-walled riser system has progressively greater impact on overall TLP system cost. Higher riser loads, and the larger deck structure needed to support these loads, require a bigger hull for greater buoyancy. Expensive foundations and tether systems are required to moor the larger TLP's.

Deepwater Well System Challenges.

In the late 1970's and early 1980's, operating solutions were developed for the Hutton TLP and similar conceptual TLP designs. As offshore frontiers moved to deeper waters, operators realized that TLP designs face several problems that were not critical with the shallow-water Hutton TLP.

The TLP's fundamental operational premise - the wells extend to the platform - provides many production operation advantages over subsea completions. Conventional platform drilling operations with surface BOP's also provide improved operating efficiencies. However, the Hutton well systems that provided these features5 are inappropriate for deepwater drilling operations in abnormally pressured reservoirs because only a single, independent well-control barrier is provided. Deepwater TLP's must use subsea BOP's or have multiple casing strings tied back to the surface to achieve the conventional two-barrier system for safely developing abnormally pressured reservoirs. Appendix I of Ref. 1 explains this problem.

Double-wall riser systems, such as the Auger TLP well system,6 provide safety and flexibility to work over and sidetrack with surface BOP's. As water depth increases, the double-walled riser system has progressively greater impact on overall TLP system cost. Higher riser loads, and the larger deck structure needed to support these loads, require a bigger hull for greater buoyancy. Expensive foundations and tether systems are required to moor the larger TLP's.