Evolution of the Tension Leg Platform

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30752, “The Tension Leg Platform: From Hutton to Big Foot,” by Steven J. Leverette, Leverette Offshore, and Stephen B. Hodges, Shell (Retired), prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4-7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The complete paper is a comprehensive discussion of the development and deployment of the tension leg platform (TLP), one of the four major platform types that also include floating production, storage, and offloading (FPSO) vessels; semisubmersible floating production systems; and spar platforms. The authors summarize the evolution of the TLP during a nearly 4-decade span and provide a retrospective of the progression of TLP technology, including hull shapes, tendon connectors, flex elements, and riser systems. A Design Driven by Function Although the technology involved may be impressive, the authors remind that essentially it is merely a means of supporting a payload economically in deep water within required motion limits. The ultimate objective is to provide the most cost-effective, safe, reliable platform to meet functional requirements. After almost 50 years of deep water development, platform concepts have broadly stabilized into four categories of functionality: semisubmersibles, TLPs, spars, and ship-shaped FPSOs. Each of these concepts brings unique functionality with different cost/benefit tradeoffs. The driver behind the TLP concept was straightforward from an operator’s perspective: Provide a platform that behaves like a fixed platform with regard to the wells (i.e., dry trees; direct vertical access to wells; minimal tensioner stroke, allowing an array of wells in close proximity without giant tensioner systems such as those found on drilling semisubmersibles; and enabling export risers) in water depths much deeper than any fixed platform. This functionality, however, comes at a cost. While a semisubmersible may look similar in terms of hull and topsides, a TLP designed for the same payload requires 10 to 30% more displacement to provide the pretension in the tendons that keeps them in tension even in the most severe environmental conditions. The TLP also features 10 to 30% more freeboard to account for set-down at large offsets. While a semisubmersible, spar, or FPSO rides the tide and waves, a TLP acts like a fixed structure and, furthermore, is pulled down in the water geometrically at offset positions. The cost of this phenomenon must be offset by improved functionality. Thus, a large TLP with a full drilling rig and 24 wells can be an excellent choice for a large, centralized reservoir that can be drilled from a single location, but it makes little sense in developing a group of small reservoirs spread out over a region.