A TLP Reduces Costs and Eases Operations by "Well Systems-Centered" Design
Authors: White C.N. , Goldsmith R.G. , Triantafyllou M.S. 
Published: October 2008

Another stage in the evolution of the Tension-Leg Platform concept for deep waters has been achieved through a "weIl systems-centered" design approach that integrates existing offshore platform and drilling technology. The simplified well system described in this paper provides drilling, completion, workover and production capabilities similar to those of conventional fixed platforms. It combines the roles of weIl risers and platform tethers and utilizes mudline suspension type subsea connections to create a "weIl-tethered platform". This latest evolution of the Tension-Leg Platform (TLP) concept eliminates the need for many costly TLP components such as riser tensioners, subsea BOP's. and the separate tension-leg mooring system with its foundation templates and piles, mooring tendons, and connectors.

The "riser/tethers" of this well system form the core of the Heave Restrained Platform (HRP). Feasibility and component designs have been established for a configuration that holds a circular array of riser/tethers locked to the periphery of a moonpool within a central flotation column of the vessel. This TLP configuration restrains the platform from heaving but allows compliant pitch and roll responses. Stresses in the riser/tether strings (that both hold the platform in position and protect wellstream tubulars) are minimized at the vessel and near the seafloor by the unique configuration and motions characteristics of the HRP. An auxiliary spread mooring plays a key role in the concept and in stress management for the tie-back components by tightly restricting offsets. This significantly diminishes the requirements for subsea connectors and tapered transition joints or flexjoints.

Drilling operations are safely conducted from the HRP vessel, using a surface BOP, like operations from a conventional bottom-founded platform. 11l1s versatile well system also permits surface completion type tie-backs from wells that have been predrilled from a conventional floating drilling unit. Once wells are completed, their risers are locked to the periphery of the moonpool such that the surface weIlheads and trees do not move relative to the platform, thus, easing production operations.

The paper presents a general description of the HRP design and recent developments of the well systems components and running procedures. However, emphasis is on the integrated design process which accounts for the critical aspects and costs of safe deep water drilling operations, while ensuring the generation of an efficient production facility. Results of design studies that provide guidance for configuring efficient platforms are presented to ease the engineer's steps toward application.

CONCLUSIONS & RECOMMENDATIONS

The Heave-Restrained Platform incorporates a weIl-tethered platform weIl system to simplify deepwater drilling, completion and workover operations. The combined mooring/weIl system design eliminates the need for much of the expensive, specialized TLP well system and mooring system equipment; thus, reducing overall cost. Further cost savings are also seen in platform size reductions that result from combining the roles of risers and tethers. This drilling and tie-back system provides a low risk of blowouts because it provides at least two containment barriers for safe well control during all operations where significant abnormal pressures are encountered.