After over 50 years of deepwater development, the platform concept has settled into four functional categories: semisubmersibles, TLPs, spars, and ship-shaped FPSOs. Each of these notions offers specific functionality with varying cost/benefit tradeoffs.
This article will describe the tension leg platform (TLP) development, one of four major platform types.
A tension-leg platform (TLP) or an extended tension leg platform is a vertically anchored floating structure that is often utilized for offshore oil or gas production in water depths greater than 300 meters (about 1000 feet) but less than 1500 meters (about 4900 ft). Therefore, offshore wind turbines could potentially benefit from tension-leg platforms.
Historical Development
For almost 40 years, the Tension Leg Platform (or TLP) has been utilized for deepwater oil and gas field development. Conoco installed the first TLP in 1984 at Hutton Field in the United Kingdom (UK) Sector in the Central North Sea. This TLP was implanted at a depth of 486 feet.
Since 1984, an additional 26 TLPs have been erected around the world, including 18 in the Gulf of Mexico (GOM) of the United States, two in the North Sea (in the Norwegian Sector), four in West Africa (two each in Angola and Equatorial Guinea), and one each in Indonesia and Brazil.
Critical Components of Tension Leg Platform
Hull
TLP hulls are typically square, with four vertical columns joined by a horizontal ring pontoon. Mini-TLPs, on the other hand, have a smaller water plane area with a single central column or four closely spaced tiny columns and an extended submerged pontoon structure with three or four radiating pontoons to give multiple baselines for tendon attachment. In all circumstances, the hull’s job is to provide buoyancy and stability.
Topsides
Topsides contain all the production, drilling, and utility systems accommodations for oil and gas drilling and production. Topsides are multi-level decks in offshore oil and gas facilities and come in modular and integrated designs. Once integrated, the deck and hull are structurally connected, forming a wholly integrated continuous floating structure.
Production Risers
The ability to give enough controlled motions that rigid top-tensioned production risers that support relatively ordinary dry surface production trees may be used. These risers are supported by the topsides (or hull) structure via a tensioning system (usually designed as several hydraulic or pneumatic tensioners) that supports the comparatively minimal vertical motion between the production lines.
Export Risers
The flow of processed oil and gas from the TLP is routed through export risers to a pipeline system beneath the sea. Exports risers are top-tensioned stiff risers, like production risers, flexible risers (using flexible pipe), or Steel Catenary Risers (SCRs). SCRs are steel pipes hung from the TLP to the seafloor in a catenary design, allowing the TLP and riser to move freely without requiring a top tensioning system.
Tendons
Tendons are used to permanently direct the TLP to the seafloor and control the TLP’s horizontal excursions (or offset), heave, roll, and pitch motions. To maintain the TLP’s stability and position, the tendons must always be in a specified range of tension. Typically, the four-column hull configuration will have eight to twelve tendons (two or three tendons per column). In comparison, the mini-TLP hull variants will have six or eight tendons (with two tendons for each of the three or four horizontal legs).
Foundations
Early TLPs, including Jolliet in the GOM, relied on subsea Template(s) piled on the seafloor to secure the tendons’ lower ends. Since Mars, all GOM TLPs have used a driven vertical pile as the base for each tendon. Different foundations have been employed in other regions, particularly where the soils deviate from the typical GOM sediments. Large gravity-based caissons and suction pile foundations are examples of this.
Wellhead
The wellhead for each well is placed on the seafloor directly beneath the TLP. It links the riser to the sound casing system for TLPs that support top-tensioned risers and surface trees.
Tendon Design Philosophy
Conceptual Design
During the early design calculations, conceptual design transfers the functional requirements into the floating structures of architectural and engineering characteristics. It incorporates technical feasibility studies to identify critical aspects such as length, width, depth, draft, hull form, anchoring system, and well and riser systems to satisfy environmental criteria, functional needs, and installation feasibility of a tension leg platform. In addition, initial lightship weight estimates and anchoring pretension are included in the conceptual design.
Materials
The specified platform materials’ strength, toughness, and fatigue resistance must be compatible with predicted fabrication procedures and each key point’s inspectability throughout service. For example, the steel used for the tendons may be of greater strength than structural steel, which will affect the tendon manufacture and inspection, as well as the type and service of the tendons.
In a marine environment, the tendons are subjected to significant cycle fatigue stresses that are superimposed on the mean stress tensile load. Therefore, the material’s final qualities must be acceptable to meet the requirements for strength, toughness, corrosion resistance, and corrosion fatigue.
Wind Forces
A tension leg platform has extended natural periods in the surge, sway, and yaw, which can be accelerated by wind energy. Therefore, the impacts of the entire wind spectrum, including steady and variable winds, should be considered in evaluating wind-induced platform loads and reactions.
Ice Loads
Because of the increased frontal area, superstructure ice can influence tendon tension and increase local wind loads. In addition, wave-induced motions of floating ice can impose local impact forces, which should be factored into building design.
Wave Impact Forces
Engineers should assess wave slap and wave slamming forces for their local influence on structural or flotation parts and, if necessary, include them in the overall solution of the equation of motion. They should be considered while designing column structure. For example, wave slap stresses on the columns might cause tendon “ringing” responses.
Earthquakes
Operators should establish ground acceleration time records for TLP sites where earthquakes are a threat. Vertical ground motion is far more critical than a horizontal ground motion for TLP tendon tension responses.
Fire & Blast Loading
Unexpected fires or explosions can be disastrous for offshore structures that deal with hydrocarbons. Explosions can create an overpressure that destroys offshore infrastructure. Fires also produce heat loads on nearby structural elements, resulting in deformations and unexpected strains. As a result, the design of an offshore structure should thoroughly consider possible detrimental loadings.
Design Approaches To The Main Parts Of Tension Leg Platforms
Tendon Systems
The tendon system includes the tendons and any ancillary components required for functioning, such as load measurement devices and inspection or monitoring equipment. The tendon system restricts the platform’s mobility in response to wind, waves, current, and tide within defined limitations.
Tendons connect platform points to corresponding seafloor foundation points. The tendons are ideally under a continuous tensile load that provides a horizontal restoring force when the platform is displaced laterally from its still water position by constraining the platform at a draft more profound than necessary for expelling its weight.
Foundations
The TLP foundation system refers to the foundations that connect the tendon legs to the seafloor. The foundation structure’s design should ensure that allowable stress, displacement, and fatigue limits are not exceeded during and after installation. Leg templates and well templates on a single foundation piece are attached using piles, suction anchors, gravity, mud mats, or a combination of these methods.
Fabrication And Installation
The following are the four basic ways of platform fabrication:
Deck Float Over
The deck is built independently from the hull in one piece, floated over it, lowered, and joined to it using controlled ballast and jacking methods. Deck outfitting is typically accomplished before deck mating.
Modules
The deck facilities are stacked modules on top of the hull using this technology. Modules can be configured to “float” on sliding supports or carry global loads between columns. In the latter instance, the global loading between columns should be taken by a structural frame connecting the columns. This is usually done before the final tow to the installation site at the last outfitting facility.
Integral Deck and Hull
The deck and hull are built together in this way. The builder may undertake deck outfitting concurrently with the building of deck subassemblies or after deck and hull construction. This construction requires a sufficiently deep dry dock or a sound, sheltered Deepwater site.
Deck Lifting
It occurs when a single deck piece is raised and integrated offshore.
Transportation
To avoid structural damage, vessels should take precautions during sea shipment. Towing or carrying on a mobile heavy-lift vessel are two options for transportation. Escort tugboats should be considered to prevent damage. Transportation stability requirements should be chosen based on the time, duration, and location of the route, as well as the degree of damage protection and control provided. The capacity to outrun or seek safe harbor during a storm will substantially impact the transportation’s motion requirements.
Corrosion Protection
Steel should be safeguarded from the effects that change a refined metal to a more chemically stable form, such as oxide, hydroxide, or sulfide. Overprotection may induce hydrogen embrittlement and is required to protect the coating layers. Coatings, cathodic protection, corrosion allowance, and corrosion monitoring are corrosion protection systems.
People Also Ask?
What Is The Main Idea Behind The Design Of The TLP?
The basic principle underlying the TLP’s design is to ensure that the vertical forces acting on the platform are balanced, that is, that fixed and variable platform loads plus tendon tension are equal to the platform’s displacement.
The TLP concept was straightforward: provide a platform that behaved like a fixed platform in terms of wells (i.e., dry trees; direct vertical access to wells; allowing an array of wells nearby without giant tensioner systems like those found on drilling semisubmersibles, enabling export risers in water depths as much more profound than any fixed vessels.
What Is The Difference Between TLP and Spar Platforms?
Compared to the next-closest design under decreasing vertical motions, a riser tensioner on a TLP will have a maximum stroke of 1 to 2 m. In contrast, a spar riser in identical conditions may have a stroke of 10 m or more relative vertical motion. Furthermore, the TLP riser stroke is predominantly geometric (rather than wave-induced, as is the case with a spar), which is related to the difference in the effective length of risers vs. tendons.
What Is Tension Leg Mooring System?
As the name implies, the ‘tension leg’ mooring system comprises tubular steel legs. The legs are made up of several tubular steel elements known as tendons. The steel legs are tensioned by buoyancy in the floating offshore unit. Because of the high tension in the tension legs, horizontal offsets are limited to a small proportion of the water depth. In addition, heave, roll, and pitch motions are negligible due to the tendons’ high axial stiffness.
Furthermore, the platform deck is built on top of the TLP’s hull—the topside of a TLP is comparable to that of a typical production platform, with a deck that houses the drilling and production equipment, besides that is the power module and living quarters.
How Does Buoyancy Affect The Mooring System?
A TLP’s fundamental architecture consists of four air-filled columns forming a square supported and connected by pontoons, comparable to a semisubmersible production platform.
The buoyant hull supports the platform’s topside while an intricate mooring system maintains it. The buoyancy of the platform’s hull counterbalances its weight, necessitating clusters of tight tendons or tension legs to anchor the structure to the foundation on the seabed. Piles are driven into the seafloor and then hold the foundation in place.
What Are Some Of The Innovations Of the TLP Floating Production Facility?
More recent designs include the E-TLP, which has a ring pontoon connecting the four air-filled columns; the Moses TLP, which centralizes the four-column hull; and the SeaStar TLP, which has only one central column for a hull—the SeaStar TLP is a widely used floating production facility because TLPs are ideal for a wide range of water depths.
Conclusion
In this scenario, we consider the semisubmersible floating production systems. The goal is to create the structure, a practical concept in which the integrity of the platform design, construction, and maintenance on the water are considered. Specifically, the tension leg platform is a notion made possible by technological improvements.
Reliable platform concepts such as the Tension leg platform ring unique functionality and addresses design modifications needed to extend service life with some emphasis on environmental constraints such as sea growth on structural materials.
We hope that by delving into various concepts, ideas, and possibilities in this article, we have demonstrated an efficient production system that incorporates cutting-edge technology that will be able to improve the costs associated with deep ocean exploration and drilling, thereby bettering project development time and financial risk.