What Is a Tension-Leg Platform?

Tension leg platform is a kind of offshore oil and gas development engineering facilities and equipment. Its principle is to use a semi-compliant and semi-rigid platform to generate buoyancy that is much larger than the structural self-weight, so as to balance with the pre-tension. Environment, and its own upright buoy structure can also make it have good sports performance. There are 25 tension leg platforms under construction and active in the world, which can be distinguished and classified according to the overall structure, the location of the tree, the function and the application method.

Tension leg platform

The first generation of tension leg platform, that is, the traditional type of tension leg platform, has a long application time, a wide distribution range, a large number of platforms, and a mature design theory. It has played an important role in the development of tension leg platforms.

Since 1984, a total of 11 traditional tension leg platforms have been built and put into production in the world. There have been no major accidents such as overturning, sinking, etc., and they have an excellent work record. This has strengthened the industry's confidence. In the 20 years of its development, researchers and engineering technicians from various countries around the world have accumulated rich design and application experience and technical data, laying a solid foundation for the development of the tension leg platform in the future.

Among the 11 traditional types of tension leg platforms that have been built, the five tension leg platforms that Shell Oil Company successively built between 1994 and 2001 have a certain representative, namely Auger, Mars, Ram, Ursa, and Brutus.

Through the production practice of the first-generation tension leg platform, it has further proved the excellent motion performance and economy of the tension leg platform in semi-rigid and semi-flexible areas in the deep sea. However, it is also found that there are still some shortcomings in the traditional tension leg platform structure.

In extremely deep waters with a water depth of more than 1200m, as the length of the tension tendon increases, the problem of the tension leg's own weight becomes too large, and the tension of the tension tendon in deep water changes, which affects the positioning of the platform performance.

In terms of reducing the cost, improving the stress situation and the sports performance, the body structure of the traditional type tension leg platform still needs to be further improved.

The differential frequency load is a slowly changing force, and it will resonate with the motion in the plane of the tension leg platform, which also changes slowly. Also, windy

The main idea of the tension leg platform design is to make the platform semi-compliant

At present, there are 21 tension leg platforms under construction and in service in the world. The basic working principles of these tension leg platforms are the same, but the structural forms and application methods are different. In order to clearly distinguish them, here are three ways to distinguish them. Twenty-one tension leg platforms are classified, and several platforms will be selected from each category for a detailed introduction in future articles.

1.Classify by overall structure

It can be divided into two categories, namely the first-generation tension leg platform and the second-generation tension leg platform;

2.Different classification according to the location of the tree

Can be divided into two categories of wet tree platform and dry tree platform.

3.Classify by function and application

It can be divided into three categories: high-tension tension leg platform, mini-tension tension leg platform, and wellhead tension leg platform;

Tension leg platform according to the overall structure

In the 20 years since 1984, the optimization of the tension leg platform structure has been a hot issue. In order to further reduce the cost of the tension leg platform and improve its adaptability, stability and safety, research institutions and oil companies around the world have continuously proposed new forms of tension leg platforms and put them into the actual production field for inspection, thus forming A diverse family of tension leg platforms. According to the stage of the evolution of the tension leg platform structure, they can be roughly divided into two categories, namely the first generation tension leg platform and the second generation tension leg platform.

The first generation tension leg platform is the earliest tension leg platform, and it is also the largest number of tension leg platforms in the world today. There are currently 12 active and under construction platforms, accounting for more than half of the world's total number of tension leg platforms. Keep growing. The overall structure of the first-generation tension leg platform has been described earlier, and is not repeated here. In order to distinguish it from the tension leg platform of other structural forms developed later, it is also called a traditional type of tension leg platform.

Since 1984, traditional types of tension leg platforms have been continuously developed in production practice, and their theoretical research and engineering applications have matured. In the 1980s, the production and application of Hutton and Jolliet platforms provided rich data accumulation and excellent work records for traditional tension leg platforms. Since the 1990s, traditional tension leg platforms have continued to develop rapidly. SnorreTLP and HeidrunTLP were completed in 1992 and 1995, respectively, bringing the number of tension leg platforms in the North Sea to three. From 1994 to 2001, Shell Oil Company In the Gulf of Mexico, five traditional tension leg platforms were successively manufactured, namely AugerTLP, MarsTLP, Ram Pan LP, UrsaTLP and BrutusTLP; in 1999, BP also built the company's first tension Leg platform MalinTLP; In 2003, Unocal built WestSenoTLP in the waters east of Kalimantan Island, Indonesia, and introduced the tension leg platform to Asian waters for the first time. These tension leg platforms maintain a number of world records for the performance of the tension leg platform. Among them, HeidrunTLP has a displacement of 290310t, which is the largest tonnage in the world's active tension leg platform. SnorreTLP has a daily output of 190,000 barrels of oil (1 barrel = 158.9873dm3) And natural gas 3.2 × 10 ^ 6 m3, maintaining the world record for the production capacity of the tension leg platform; UrsaTLP's working water depth exceeded the kilometer mark, and by 2004 it still maintained the world record for the working depth of the tension leg platform. Among the first generation tension leg platforms are Hutton, Jolliet, Snorre A, Auger, Heidrun, Mars, Ram, Powell, Ursa, Marlin, Brutus, WestSeno A, and WestSeno B.

The second generation tension leg platform appeared in the early 1990s, and it was developed on the basis of the first generation tension leg platform. The second generation tension leg platform inherits the excellent motion performance and good economic benefits of the traditional type tension leg platform, while optimizing and improving the structural form, making the tension leg platform more suitable for deep sea environments, and reducing the construction cost. The world's marine engineering community is very enthusiastic about developing the second-generation tension leg platform, and major companies have proposed a wide variety of platform design solutions. In general, the second-generation tension leg platform currently in production is divided into three series: the SeaStar series tension leg platform designed by Atlantia, the MOSES series tension leg platform designed by MODEC, and the ABB company. Designed extended tension leg platform (ETLP for short). The structural forms and characteristics of these second-generation tension leg platforms will be described in detail in later chapters. The second-generation tension leg platforms are: SeaStar, TLP, MOSES TLP, ETLP, Morpeth, Allegheny,

Typhoon, Matterhorn, Prince, MarcoPolo, KizombaA, KizombaB, and Magnolia.

In addition, in addition to the above-mentioned tension leg platforms that have been put into practical production, in the past 20 years, researchers and engineering technicians around the world have also proposed many valuable design schemes, and carried out around these schemes. Extensive and in-depth research and experiments. Although due to various reasons, these platform design schemes have not yet entered the production field, but understanding them will be of great benefit to open up people's ideas and better conduct the next research. Therefore, two new types of tension leg platforms are selectively introduced in the following articles, in order to inspire readers.

Tension leg platform Xmas tree position

According to the installation location of the tree, the tension leg platform in the world today can be divided into two categories: wet tree platform and dry tree platform.

The wet tree platform (wettreesplatform) has oil trees located on the sea floor. An independent full set of production and processing facilities are installed on the platform to support a certain number of subsea oil wells. Subsea oil wells are connected to production facilities on the platform through flexible oil pipelines and steel catenary risers (SCR), and all production activities on the platform are carried out through these pipelines. The advantage is that the tree is located on the sea floor, which reduces the load on the platform and does not need to build a bulky platform body, thereby reducing the overall cost of the platform. Because vertical tensioned risers are not installed, there is no need to consider the platform draft. The effect of the changes on the production riser, which simplifies the design of the platform. The wet tree platform is very suitable for oil fields with wide distribution areas and scattered oil points. It consists of a flexible submarine pipeline system consisting of flexible oil pipelines and SCRs, and a wet tree platform as the manifold center, which can control a wider area. In addition, the wet tree platform's production reserve capacity has great flexibility. New equipment and subsea oil wells can be easily added to the existing production system, which is convenient for the long-term development of oil fields. Hutton, Jolliet, Snorre A, Auger, Heidrun, Mars, Ram / Powell, Ursa, Marlin, Brutus, Prince, Kizomba A, Matterhorn, WestSeno A, MarcoPolo, Magnolia, Kizomba B and WestSeno B have been completed.

The drytreesplatform's Christmas tree is located on the platform, and the vertical production riser is directly connected to the tree located on the platform's wellhead deck. The excellent motion performance of the tension leg platform makes it a great advantage in installing a dry tree system. Because the relative movement between the platform and the production riser is small, a simple structure and low cost riser tensioning device can be used. The production activities of the dry tree platform are mainly carried out through top tensioning risers. The advantage is that the subsea oil well and the surface dry tree are directly connected through the production riser directly. A rig can be installed on the platform to make the tension leg platform perform the drilling and completion functions by itself, avoiding the need to call other drilling facilities in the long-term oilfield development. The problem of disrupting platform production. In addition, because the tree is located on the platform, it is convenient to maintain and easy to manage, and it also saves the hardware cost of connecting the subsea tree to the upper body of the platform. Built are Morpeth, Allegheny, and Typhoon.

It should be noted that most of the existing tension leg platforms in the world are the central platforms in the sea area where they are located. In addition to some tension leg platforms, a dry tree system is installed on the platform, which can carry out exploration and well control work. At the same time, it is also connected to the subsea production system or other satellite platforms of nearby oil fields through flexible oil pipelines and SCRs as the center of its oil processing and output. In this case, these tension leg platforms combine both dry and wet tree systems. Therefore, the criterion for classifying each tension leg platform is to see if the platform has the ability to support a dry tree system.

Tension leg platform functions and applications

At present, the functions and application methods of tension leg platforms are very flexible. If this is used as a standard, the 21 tension leg platforms in the world can be divided into three categories: high-tension tension leg platforms, mini-tension leg platforms, and wellhead tension leg platforms. .

Largedeck load leg platform (largedeckloadTLP) is the oldest type of these three types of tension leg platform. It is a large and expensive form of tension leg platform that can support a set of high-capacity crude oil processing facilities. . There are currently 9 high-load tension leg platforms worldwide, of which 3 are located in the North Sea oil field and 6 are located in the Gulf of Mexico. Because the pretension of the tension leg well limits the sloshing movement of the platform, the well control facility can be installed on the upper body of this platform to facilitate equipment maintenance and repair work. Historically, the main reason why this production system has been favored by the industry is that it can install a dry tree oil recovery system. However, due to its high cost and incompatibility with extremely deep water environments, people have gradually lost interest in building high-load tension leg platforms. When the working water depth exceeds 1200m, the excessive weight of the tension tendon is the most important problem for the high-load tension leg platform. Those belonging to the high-load tension leg platform are: Hutton, Snorre A, Auger, Heidrun, Mars, Ram / Powell, Ursa, Marlin, and Brutus.

Mini-TLP is not a simple and traditional type of tension leg platform. It has been through structural improvements to the upper body, columns and tension leg system of the platform to achieve optimization of various parameters. The goal of smaller tonnage for larger payloads. Compared with the traditional tension leg platform of the same size, the mini tension leg platform has the advantages of small size, low cost, good flexibility, and low environmental load. It is very suitable for developing small and medium oil fields. And unlike the large-load tension leg platform, the mini-type tension leg platform can work stably in extremely deep water environments, which is why it can gradually replace the high-load tension leg platform and occupy the most important reason for the construction of the current tension leg platform. . Among the mini-tension leg platforms are: Morpeth, Allegheny, Typhoon, Prince, Matterhorn, Marco Polo, and Magnolia

Tension Leg Wellhead Platform (TLWP) is an economical tension leg platform. Unlike the first two tension leg platforms, the wellhead tension leg cannot be used for independent production work. On its platform, only the well control facilities are installed, while other oil production and processing facilities are installed in an auxiliary production near the platform. Facilities, such as FPSO (floating production oil storage unit). TLP and FPSO are connected through pipelines to form a complete offshore oilfield development system. This combination makes full use of the advantages of small relative movement between the tension leg platform body and the production riser system and excellent sports performance. In addition, FPSO has the advantages of flexible movement, large loading capacity, and relatively low cost. Therefore, the tension leg platform undertakes drilling. And wellhead operations, while crude oil processing, storage, and transportation are performed by FPSO. This system has been tested in practice and has proven to be an effective and economical method of offshore oil and gas development. It is very suitable for oilfields that do not have or lack submarine pipeline systems and permanent bases, and that require drilling, completion, and well maintenance work. Zone use. The wellhead tension leg platforms are: Jolliet, Kizomba A, West Seno A, Kizomba B, and West Seno B.