Evolution of the Tension Leg Platform
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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 deepwater 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.
In comparison with a spar, which is the next-closest design in reducing vertical motions, a riser tensioner on a TLP will exhibit a maximum stroke of 1 to 2 m, whereas a spar riser in similar conditions may stroke 10 m or more of relative vertical motion. Additionally, the TLP riser stroke is primarily geometric (rather than wave-induced, as for a spar), associated with the difference in effective length of risers vs. tendons. In most cases, the riser hangoff is at the deck, as opposed to the tendon hangoff near the keel. The riser pendulum is longer than the tendon or hull pendulum, so the top has to move upward relative to the TLP when the TLP offsets laterally.
In the 1970s, numerous patents were filed for TLP technology, including such ideas as drilling wells through the tendons and self-installing TLPs.
Although TLPs have been used to develop large reservoirs with up to 24 dry-tree wells and a large drilling rig, as originally envisioned, they also have been deployed successfully for smaller fields with tender-assisted drilling or only workover capability, as well as pure subsea developments. Designs also have evolved to adapt and exploit various field-development criteria. Roughly half of the 30 existing TLPs are of a conventional four-column design with a ring pontoon, including those deployed during most of the first decade of TLP development. However, as is described in the complete paper’s chronology of the 30 existing TLPs at the time of writing, other design concepts were developed to optimize TLP cost and effectiveness.
Table 1 of the complete paper lists all existing TLPs, identifying their functional type, design type, and other basic characteristics. Fig. 1a of the complete paper shows them on a timeline, color-coded by design type (conventional or other proprietary design concept). Fig. 1b of the complete paper shows them on a timeline sorted by functional type (TLP, wellhead TLP, tender-assisted-drilling wellhead TLP, and subsea TLP) and color-coded by region. These figures provide a thorough overview of the evolution and diversification of the TLP concept as a viable floating production system for a range of field-development drivers and can be referenced while reading the chronology offered in the complete paper. Fig. 1 of this synopsis illustrates the displacement and water depth of the 30 TLPs on a timeline showing the range of size and the progression of water depth.
Characteristics of TLP Systems: Overview
The TLP’s ability to limit vertical motion greatly is what makes it unique. The TLP is held in place by vertical tendons with a high axial stiffness. A TLP does have a vertical response coupled with large lateral offsets, which is termed “setdown.” The system resembles an upside-down pendulum; as it moves horizontally, it is constrained to follow an arc. There are vertical motions caused by stretching of the steel tendons as the result of wave loads, but, because of the high stiffness of the tendons, these motions are small. This stiffness, coupled with the large mass of the system, does result in resonant vertical motions at frequencies typically higher than wave frequencies (typically 2- to 4-second resonances), but these are not excited by much energy and the amplitudes are too small to be of much consequence to functionality. However, such vibrations are important for tendon fatigue and do have some contribution to extreme loads in the tendons.
In the horizontal direction, the TLP is a compliant system. Because of the pretension in the tendons (typically 20 to 30% of displacement), the TLP is lighter than its displaced volume of water. Its horizontal response to wave forces is, therefore, somewhat livelier in surge and sway than an equivalent free-floating body. This is mediated in design by using a deeper draft than is typical for ships and drilling semisubmersibles.
The small vertical motions enable top-tensioned risers and short-stroke tensioners, but the TLP concept also enables much smaller vessels than would be feasible in extreme sea states and provides a good support platform for steel catenary risers used for connecting to subsea wells and export pipelines, therefore enabling other uses for the TLP.
The evolution of the TLP has involved many talented people and organizations over the course of the past half-century. It has created a close-knit community where competition has existed side-by-side with collaboration and support. No one individual or company can claim exclusive ownership; progress has involved a broad spectrum of contributors. The authors cover many of the main issues experienced in this period in the complete paper, but readers should be aware of many complementary efforts during the development of the TLP concept, a list that includes the following:
- Improved understanding of the ocean environment
- Better model-test facilities to characterize real ocean waves
- Numerical-simulation technology to simulate global performance and structural responses
- Instrumentation technologies to facilitate measuring of tendon tension
- Development of lifting and jacking technologies to integrate decks and hulls
- Technologies to transport ever-larger facilities
The authors salute those who have contributed to the development of the TLP concept and thank those who have helped gather the information for the complete paper.
Evolution of the Tension Leg Platform
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