Oil and gas gathering and transportation is an oilfield production stage after geological exploration, oilfield development, drilling and oil production. The task of this stage is to concentrate, separate, measure, purify and process the oil well output from the wellhead of the oil well to produce oil, gas and by-products that meet the quality requirements, and then deliver them to users.

Offshore oil and gas gathering and transportation system includes offshore oil and gas production equipment system and engineering facilities that provide production sites and supporting structures for it. Offshore oil and gas gathering and transportation includes the production equipment and engineering facilities of the whole oilfield. These engineering facilities include wellhead platforms, production platforms, living platforms, oil storage platforms, oil storage tankers, oil storage tanks, single-point mooring and oil transportation terminals. According to the production capacity, oil field area, geographical location, engineering technology level and investment conditions of developed oil fields, different oil and gas gathering and transportation systems can be formed respectively.

With the development of offshore oilfield development projects from offshore to offshore, offshore oil and gas gathering and transportation has formed the following three types.

1. All-land gathering and transportation system

In the early stage of offshore oilfield development, artificial islands are built not far from the shore, wooden or concrete wellhead protection frames (platforms) are built, and drilling and oil production are carried out. The output of the oil well is collected, separated, measured, treated, stored and transported to the shore by the pressure of the oil well through the oil pipeline. This kind of production system with all gathering facilities on land is called all-land gathering and transportation system.

Generally, the offshore engineering facilities of the system are: (1) wellhead protection frame (platform) goes ashore through the subsea flowline; (2) The wellhead protection frame (platform) is connected with the land through the trestle; (3) The artificial island is connected with the land through the bank.

All-land production system only has wellhead protection frame (platform) and oil pipeline at sea, which greatly reduces the offshore engineering quantity and is convenient for production management. Land production and operation costs are relatively low and are less affected by climate. Compared with the offshore production system with the same production scale, it has good economic benefits. This system is usually suitable for offshore oil fields with shallow water and high reservoir pressure. This gathering and transportation method is often used in the development of beach oil fields in China.

2. Half sea and half land gathering and transportation system

With the increase of water depth and offshore distance, and the development and application of steel jacket platform in oilfield development site, onshore gathering and transportation system is no longer applicable. In order to solve the problems of low efficiency of long-distance oil-gas mixed transportation and insufficient reservoir pressure, oil-gas separation and some processing equipment are gradually put into the sea. Oil and gas produced by oil wells are separated at sea, and then transported ashore by crude oil booster pipe for treatment, storage and export. If the associated gas volume is small, except for the platform fuel, the rest will be vented and burned at sea. If there is a large amount of natural gas, oil and gas will be separated at sea and then transported ashore for further treatment. This kind of oil and gas gathering and transportation system, which only carries out the preliminary treatment of oil and gas at sea and puts the main oil and gas gathering and transportation equipment and storage and transportation work on land, is called semi-sea and semi-land gathering and transportation system. The system is suitable for oil fields with large oil field area and high output, suitable for laying pipelines on the seabed, and oil and gas production bases or oil terminals on land, especially for gas field gathering and transportation. Because it is difficult to solve the problem of offshore natural gas storage and treatment, general gas fields adopt semi-sea and semi-land gathering and transportation systems, such as Jinzhou 20-2 gas field in Bohai Bay, China.

3. All-sea gathering and transportation system

With the rapid development of world industry, the demand for oil is also increasing. In order to simplify the transportation of crude oil produced at sea after landing, oil storage tanks and oil terminals are built at sea with the help of modern marine engineering technology, so that oil and gas can be transported directly from the sea. In this way, the concentration, processing, storage and export of oil and gas are all put at sea, forming a whole ocean gathering and transportation system. This also makes the development of offshore oil fields develop to the far sea, deep sea and polar regions with harsh natural conditions. The whole sea gathering and transportation system can be fixed or floating; Wellhead production system can be on water or underwater. This gathering and transportation production system is suitable for small oil fields, marginal oil fields and large oil fields. It is not only suitable for the conventional development of oil fields, but also suitable for the early development of oil fields. This is the most adaptable and widely used gathering and transportation production system in the world today.

To sum up, offshore oil and gas gathering and transportation system has developed from all-land type to half-sea and half-land type, and from half-sea and half-land type to all-sea type. The fundamental difference between them lies in whether the production and processing facilities of gathering and transportation are placed at sea or on land. If all the production facilities of oil and gas gathering and transportation are put on land, it is called full prevention type. If all facilities are placed at sea, it is called all-sea type; If some facilities are placed on land and some facilities are placed on the sea, it is called semi-sea and semi-land type.

Second, offshore oil and gas gathering and transportation process

As the whole offshore oil and gas gathering and transportation system can realize all oil and gas gathering and transportation tasks, this section takes the whole offshore production platform as an example to introduce the main process flow and equipment of oil and gas gathering and transportation. Oil and gas gathering and transportation production includes oil and gas water separation, crude oil treatment, natural gas treatment, sewage treatment and other major production projects.

1. Oil and gas metering and oil and gas production process Oil is a mixture of hydrocarbons. In the stratum, oil, gas and water are generated by * * * *, and the composition of crude oil produced by each oilfield is different due to different conditions of oil and gas generation. In addition, oil also contains a small amount of impurities such as oxygen, phosphorus, sulfur and sand. The task of oil and gas production and treatment is to separate and purify oil well fluids and provide qualified commercial oil and gas for users. Because the composition and physical properties of oil and gas produced by each oilfield are different, the production and treatment processes are not exactly the same. For example, crude oil produced offshore in China generally does not contain sulfur and salt, so there is no desalination process. Some oil fields produce crude oil without water, so there is no dehydration link. Offshore crude oil processing includes oil and gas metering, oil and gas separation, crude oil dehydration and crude oil stabilization. Because water injection is widely used to supplement energy in offshore oil fields, crude oil dehydration is one of the main links in crude oil treatment.

2. Natural gas treatment

Natural gas separated from oil and gas still contains light oil, saturated water, carbon dioxide and dust at high temperature. If these substances are not treated, they will be wasted and the pipeline system will be blocked and corroded. Natural gas treatment mainly refers to dehydration, desulfurization and condensate recovery, and some natural gas also needs to remove carbon dioxide. The natural gas treatment of offshore platforms is generally to separate the gas separated from the high-pressure separator and the gas flashed at all levels into corresponding gas scrubbers, remove the liquid carried by the gas, and then enter the compressors with different pressure levels for staged pressurization to reach the design pressure. This is a typical four-stage separation gas compression and condensate recovery system. Condensate collected by gas scrubbers at all levels respectively enters the crude oil pipelines of flash tanks at all levels. In order to prevent natural gas hydrate from blocking the pipeline, ethylene glycol-gas contactor is used to absorb moisture in natural gas.

Due to the large investment, high quality and large space occupied by the natural gas treatment and compression system, some platforms often treat the natural gas separated from production, some of which are used as platform fuel and some of which are sent to the torch for venting and combustion. If the amount of gas is large, it can be transported ashore for further treatment. How to deal with natural gas should be selected after comprehensive evaluation. In general, the gas after gas compression and condensate recovery still needs further dehydration, desulfurization and condensate recovery. Dehydration mainly adopts natural cooling method, ethylene glycol chemical absorption method and compression cooling method, and light oil can be removed at the same time. Sulfur-containing natural gas also needs desulfurization and sulfur can be recovered at the same time. The offshore natural gas processing and production system is the same as that on land, so I won't go into details here.

3. Treatment of oily sewage

With the rapid development of world industry, the natural environment is polluted, which seriously affects the growth of living things and human health. At present, the world environmental protection agency stipulates that all oily sewage in the oilfield must be treated, and the oil content in the water is less than 15 ~ 50 mg/L before it can be discharged. Therefore, the sewage after dehydration of crude oil from offshore oil production platform and oily sewage produced in production must be treated by sewage treatment system.

4. Selection of production technology and equipment for offshore oil and gas gathering and transportation

Different from drilling technology and rig equipment, the design of oil and gas gathering and transportation production technology and the selection of main equipment are often designed and manufactured according to the composition, physical properties, output, oilfield development mode and the selection of oil and gas gathering and transportation system. For example, in an oilfield with high gas content and far offshore, it is technically difficult to choose a semi-sea and semi-land gathering and transportation system, and oil and gas are transported ashore for a long distance. Therefore, the oil and gas are transported ashore separately, that is, the oil and gas are separated and preliminarily treated on the offshore platform, and the oil and gas are comprehensively treated after landing; If the whole sea gathering and transportation system is adopted and all the oil and gas processing, storage and transportation equipment are put offshore, then the specific process flow and equipment model are obviously different from the former. Each oilfield is designed and installed according to the designed production technology, main equipment, engineering structure selection and scale, and generally designed according to the production content, which can be roughly divided into the following categories.

(1) wellhead module. Equipped with wellhead Christmas tree, test separator, manifold and heat exchanger, etc.

(2) Oil and gas processing module. Generally, it includes production separator, electric dehydrator, crude oil stabilizer and its supporting pipelines, instruments, storage tanks, heat exchangers, etc. It's all set

(3) Natural gas processing module. Generally, separators, scrubbers, compressors and light oil recovery devices are provided.

(4) Sewage treatment module. There are oil separation flotation, precipitation separation, filter and its pressurized water pump and its auxiliary equipment.

In addition, there are power generation and distribution modules, living modules, water injection modules, compression modules and so on. The design of these modules needs a self-contained system, while considering the connection with other systems. Some production module equipment can be put into trial operation after being installed on land. After hoisting in place on the platform and connecting the water, electricity and pipeline systems, it can be put into trial operation in an all-round way, reducing the offshore engineering quantity and facilitating production management. When designing the module scale, the platform area, construction hoisting capacity and production safety requirements should also be considered.

Three. Offshore gathering and transportation platform facilities

When people sail in the vast sea, sometimes they suddenly find some buildings looming in the distance. You will be overjoyed and think you have seen a mirage. Only when the ship approached did it become clear that these behemoths made of steel stood high on the sea. Whether it is a typhoon attack or a wave beating, it is like a loyal sentry guarding the vast sea. These steel structures are offshore oil production platforms. The main difference between offshore oil and onshore oil is to build a platform first, and then drill wells to produce oil. Generally speaking, a platform is a fixed place where people live and produce at sea.

At first, people can only explore and develop oil in the ocean near the sea, and use wood to build an operating platform for drilling and oil production. With the progress of science and technology, people hope that the platform will be safer, more durable and more suitable for the harsh deep-sea conditions, and gradually use concrete or steel to build the working platform. Later, jack-up drilling platforms and drilling ships were invented. These two kinds of equipment are actually ships. The former has no self-propelled ability and needs to be towed by other ships, while the latter has self-propelled ability. After drilling and completion, the drilling platform or drilling ship sails for Araiba. At present, most offshore platforms are oil and gas production platforms. The connotation of facilities on these platforms is no different from that of onshore oil fields, but they are more complex, safer and more reliable. Figure 37- 1 shows that all facilities are located offshore, in which the central processing platform centrally measures oil and gas from surrounding wells through submarine pipelines and is equipped with safety devices, and then separates and purifies oil, gas and water, and the qualified crude oil is transported to the oil storage platform, and the treated water is reinjected or discharged through the wellhead platform, and natural gas is generally vented and burned; The main function of oil storage platform is to store crude oil and transport it to users regularly through shuttle tankers; The power platform is mainly powered by diesel generator set, natural gas turbine generator set, heating boiler and other equipment; Life platform provides rest and life for employees; There are trestles for workers to walk between platforms, and pipelines and cables such as fresh water, steam and fuel are also attached to them. Of course, according to the geographical location of the oil field in the ocean, all kinds of facilities should not be built at sea. If it is close to land, the oil and gas water treatment platform and oil storage platform will be built on land. Even if they are all built at sea, some facilities can be properly combined on a platform according to the situation. Wellhead platform is actually equivalent to onshore oilfield metering station, which is responsible for single well oil gathering, daily oil and gas production metering and water injection. Floating production and storage tanker is equivalent to the joint station of onshore oil field, which is responsible for oil-gas-water separation, purification and oil storage. Its power and life system are also on board. This greatly reduces the number of offshore fixed platforms and investment. If the oil field rapidly reduces production or loses production value, the floating production storage tanker can be transferred to other oil fields for further use.

Figure 37- 1 Early offshore oil production system

Fourth, submarine oil pipelines.

The competition of offshore oil is the competition of capital and technology. Oil and gas transportation develops with exploration and development. According to the characteristics of oil and gas storage and transportation, the modes of oil and gas field development engineering facilities in Shanghai are basically divided into two types, namely, all-sea type and half-sea and half-land type. The so-called all-sea type means that oil and gas production, processing, product storage and export of oil and gas fields are all carried out at sea; The so-called semi-sea and semi-land type means that oil and gas production is carried out at sea, oil and gas treatment is carried out at sea or on land, and product storage and export are carried out after landing.

It is an inevitable trend for mankind to develop deep-sea oil and gas for a long time to come. In this environment, with the emergence and development of offshore storage and transportation technology, the general trend of offshore oil development in the world is to develop in depth. The submarine pipelines in the world have developed from short-distance submarine pipelines in oil and gas fields to various long-distance submarine pipelines from platforms to land. Great progress has been made in the design and construction technology of submarine pipelines. At present, there are more than 50 large deepwater oil and gas fields, including more than 200 deepwater platform facilities and 1 000 underwater devices. The development of deep-water technology constantly refreshes the latest level of deep-sea operations in the world: drilling depth reaches 3050m, oilfield production depth is 2192m, and submarine pipeline depth is 2150m.

Six, floating production storage and unloading device

Floating production and storage devices can be transported independently or temporarily, and then transported to land by tanker. 1976, Shell Oil Company introduced the concept of FPSO for the first time, which was modified from an oil tanker in the waters near Castren. FPSO is a British floating production storage &; The abbreviation of unloading means "floating production storage unloading device". It integrates production, processing, storage and transportation, living and power supply, and has the characteristics of high investment, high risk and high return (Figure 37-2). In the following 26 years, the first 15 was its conceptual formation stage, and it entered a rapid development stage after 9O. Most of the original FPSO were modified ships. In this regard, Singapore's shipyards have been relatively successful and won most of the modified ship projects. At present, the construction market of FPSO is mainly dominated by Japanese and Korean shipyards and Singapore shipyards. As FPSO is a typical high value-added ship, its cost is as high as hundreds of millions of dollars. In recent years, China shipyard has also begun to actively participate in this market.

Figure 37-2 Schematic Diagram of FPSO Work

Statically, as of February 2008, the number of FPSO in service was 139, of which 54 were newly built, accounting for 38.85%, and 85 were renovated, accounting for 6 1. 15%. 32 ships were ordered, of which 1 1 was newly built and 2 1 was rebuilt, accounting for 34.38% and 65.63% respectively. Both new construction and renovation have experienced two peaks: 1997- 1999, and since 2003. FPSO in service were basically built after 2000, about 80% of the ships were within 10 years, and most of them can be used for at least 10 years, so the demand for renewal is relatively small. Among FPSO in service, Brazil, China, Britain, Australia, Nigeria and Angola are the most widely distributed countries, with the numbers of 22, 15, 13, 12, 16 respectively. Among FPSO orders, Brazil still has the largest number of 9 ships, followed by Britain, India and Nigeria, with 5 ships, 4 ships and 3 ships respectively.

Seven. development trends

Mr Einar Holmefjord, a Norwegian expert, pointed out in his speech entitled "The Present Situation of Norwegian Marginal Oilfield Development and Research Activities -DEMO 2000" that "yesterday, we used a platform based on gravity for drilling and production, today, we use a floating production system and underwater facilities, and tomorrow, we will directly transport the well fluid from the seabed to the shore for treatment without any offshore facilities". Mr Einar Holmefjord's words concisely summarized the present situation and development trend of offshore oil abroad. In order to develop marginal oil fields, floating production facilities and underwater tieback technology are increasingly adopted abroad, and a series of supporting technologies have been developed, such as underwater mixed transportation technology, deep-water large-displacement mixed transportation pump, underwater power supply and distribution system, underwater working robot, underwater horizontal Christmas tree, underwater manifold and underwater multiphase metering technology. New technologies and equipment of superstructure, including oil and gas gathering and transportation facilities and water treatment facilities, are constantly emerging, such as multiphase turbine technology and seawater deoxidation technology. These technologies have been applied, and some technologies have matured. The development of deepwater and ultra-deepwater oilfields is the biggest challenge for offshore oilfield development abroad. Some areas such as Ormen Lange, Voring plateau and At 1antic Margin have water depths of 600 ~ 1400 meters, while Angola, Gom, New Foundland and Brazil have water depths of 1500 ~ 3000 meters. Deep water has the characteristics of low temperature, ultra-high static pressure and complicated physical properties of the medium in the riser caused by temperature and pressure changes, which is easy to cause problems such as plug flow, wax deposition and hydrate in the riser section, and once problems occur, it will cause great losses and harm. In order to solve the flow safety problem of deepwater media in pipelines, a new discipline-flow safety has been formed in recent years. At present, the research on deepwater technology carried out by foreign companies includes multiphase flow in riser, SPAR model platform, deepwater mooring system, light combined riser, electric heating tube technology, hydrate inhibition technology (development of kinetic inhibitors) and so on. Solving the technical problems of deepwater oilfield development is the development trend of offshore oil technology abroad.