What is submarine pipeline laying?
What is submarine pipeline laying?
Submarine pipeline is a kind of pipeline that continuously transports a large amount of oil (gas) on the seabed through closed pipelines. It is the main component of offshore oil (gas) field development and production system, and is also the fastest, safest, economic and reliable mode of offshore oil and gas transportation.
There are two common laying methods for subsea pipelines: one is pipeline dragging method, the other is pipe laying ship method. The towing method is generally suitable for the laying of submarine pipelines in landing or under sea section, beach and extremely shallow sea area or short distance submarine pipeline laying on the sea. The pipelaying method is to use a special pipelaying vessel, and a complete set of t-laying machines and vessels are required to cooperate with it. It is generally applicable to the offshore laying of submarine pipelines whose long-distance water depth can meet the draught requirements of the pipelaying vessel.
Offshore natural gas must be transported by offshore pipelines, and crude oil produced in shallow water can be directly loaded into oil tankers by production platforms. However, due to the crude oil produced in the deep sea, it will threaten the safety of the platform when large oil tankers dock on the production platform. Therefore, the single point mooring (SPM) specially used for berthing large oil tankers in the sea appears. In this way, there must be an oil pipeline connecting the production platforms and the SPM.
In the 1970s, after the development of large-scale oil and gas fields in the sea area, large-scale offshore oil and gas pipelines were built to directly transport the oil and gas produced to onshore oil and gas storage stations. As the global oil and gas industry is marching towards the ocean, the submarine oil and gas pipelines are increasingly becoming an important part of the global energy transportation artery.
Offshore pipelines include subsea oil and gas gathering and transportation pipelines, trunk pipelines and auxiliary pressurized platforms, as well as main pipes connecting pipelines and platforms. The transportation process is the same as the onshore pipeline, but because the offshore pipeline project is carried out in the sea area, the construction method is different from the onshore pipeline engineering.
A variety of methods have been developed for subsea pipeline construction, each of which has its ability to adapt to special conditions. The construction in shallow water area of landing section is different from that in deep water section. No matter what method is adopted, the basic tasks to be completed are: welding the connecting pipe section; inspecting the interface; coating the welding interface on site and wrapping the protective layer; laying the pipeline to the seabed without damaging the pipe.
The construction of submarine pipeline is greatly affected by the environmental conditions such as waves, rivers, wind and site conditions. Therefore, the construction should be carried out in the season with relatively calm wind and waves. In the process of submarine pipeline laying, the marine conditions may deteriorate and the operation has to be suspended. Therefore, the utilization rate of marine machinery and equipment in offshore construction is low, which is much higher than the cost of onshore pipeline construction.
With the increase of water depth, the laying of submarine pipeline has stepped into the open sea area. The influence of hydrology and sea conditions on the pipeline laying is also changing. The pipeline construction is becoming more and more difficult, and the influencing factors tend to be complex. It is necessary to comprehensively strengthen the ship resources, performance, design calculation and monitoring to deal with the construction influencing factors.
At present, due to the relatively backward technology and equipment, the laying depth of submarine pipeline in China is limited to the shallow water section. This has seriously affected the development of deep-water offshore engineering projects in China. However, with the completion and commissioning of China’s first deepwater pipelaying vessel, “offshore oil 201” will fill the gap in the field of deepwater pipe laying equipment in China, and will narrow the gap between China and developed countries in deepwater pipe laying technology. Its global leading overall technical level and comprehensive operation ability will also play an important role in the construction and installation of offshore engineering.
Types of submarine pipelines
The types of subsea pipelines are as follows:
- (1) Seamless steel pipe. It is generally used in small diameter subsea pipe tray with special requirements.
- (2) Longitudinal submerged arc welded steel pipe (SAW). The diameter is generally more than 16in and the maximum is 56in.
- (3) Straight seam high frequency resistance welded steel pipe (hferw). Due to the improvement of welding quality and low price, this kind of pipe has been more and more used in the submarine pipeline of more than 20 in.
- (4) Threaded welded steel pipe. This kind of pipe has not been generally recognized by the offshore pipeline industry because of the welding quality problems.
Design requirements of submarine pipeline
The design standard of submarine pipeline system is the code for submarine pipeline system (SY / T 10037-2010). In order to design a safe and reliable submarine pipeline, it is necessary to obtain the following information and data:
- (1) Transportation fluid: including oil and gas type, flow rate, composition, density, viscosity, dew point, temperature, pressure, specific heat capacity and thermal conductivity.
- (2) Engineering geology: types and mechanical properties of seabed soil, vertical and horizontal friction characteristics of seabed, composition of surface soil structure of seabed, seabed landform, profile map of submarine pipeline route and seismic intensity.
- (3) Oceanographic Data: Bathymetric Chart, current, wave, tide, ice, water temperature, salinity of sea water, distribution of animals and plants in sea area and visibility of sea water.
- (4) Meteorological data: wind direction and wind speed, temperature, air pressure and visibility.
- (5) Other information: charts of the sea area, submarine cables or pipelines to be crossed, underwater obstacles, navigation channels, fishing areas and relevant laws and regulations of the government.
Characteristics of offshore pipeline engineering
The main characteristics of offshore pipeline engineering construction are as follows:
- ① Construction investment is large. To lay a medium diameter pipeline in the general sea area requires a large professional fleet composed of pipe laying vessels, ditching vessels and more than 10 tugs for auxiliary operation. In addition, there is a need for ships to supply materials, equipment and fuel. The cost of renting a professional fleet is the main cost in the construction of offshore pipelines. Due to the high cost, the construction cost of offshore pipelines is 1-2 times higher than that of similar pipelines on land.
- ② High construction quality is required. Whether in the construction period or after the commissioning, the maintenance of the offshore pipeline is much more difficult than that of the onshore pipeline, so the quality of the offshore pipeline construction should be ensured.
- ③ The construction environment is changeable. The sea state changes rapidly and violently. If the wind and waves are too large, it is difficult for the construction fleet to maintain stability. In this case, it is often necessary to lower the construction pipeline to the seabed and resume the construction after the storm.
- ④ The construction organization is complex. In the construction of offshore pipelines, the prefabrication of pipelines, the supply of spare parts, fuel and fresh water of the fleet all depend on the shore base; the determination of the position and moving direction of the fleet also relies on the close cooperation of the radio stations on the shore base. Therefore, the offshore pipeline construction has the characteristics of joint construction by land and sea.
Submarine pipeline laying process
The project of laying pipelines for transporting oil and natural gas on the sea floor, which also includes offshore positioning, pipeline laying and trenching.
Guide the pipelaying vessel to move along the route direction and determine the position of the construction fleet in the sea area. The method of positioning on the sea is to set up more than two directional radio stations with known longitude and latitude on shore, and the directional radio stations transmit microwave directional signals. The working ship is equipped with a radio direction finder, which can accurately measure the angle between the ship and the radio stations on the shore, so as to accurately measure the position of the ship. Microwave can be used to transmit signal in offshore operation, and 200m radio long wave is generally used for offshore operation. These two methods can achieve the accuracy required by the positioning of pipe laying operation.
Offshore pipeline laying is developed from the construction method of onshore pipeline crossing rivers and lakes. There are three main methods of pipe laying: laying by pipe laying boat, laying by traction method and laying by drum boat. The specific operation method should be determined according to the pipe diameter, seawater depth, sea state and distance from the shore.
Laying by pipelaying vessel
This is the most commonly used method. The operation process is to ship the pipes to the pipelaying vessel after adding cement weighting layer through the onshore prefabrication plant, and then assemble and weld the pipes section by section. When the pipe laying vessel moves forward, the welded pipe section slides into the sea from the stinger at the stern of the ship. In the whole process of pipe laying, the length of pipe sliding must be synchronized with the displacement of the ship, at the same time, the pipe laying ship must be in a relatively stable state. For this reason, 4-6 anchors are arranged around the front and rear of the pipe laying vessel. The ship can be stabilized by adjusting the tightness of the anchor cable, and the ship position can be moved by adjusting the length of the anchor cable. When the pipe section slides from the tail of the stinger to the sea bottom, it is suspended in the sea water to form an S-shaped bending from the upper arch to the lower bending, which makes the pipe section subject to complex bending stress. Therefore, there are enough tensioners on the ship to clamp the pipe section so that it can not slide freely, and make the pipe slide down with the displacement distance of the ship.
This method first appeared in the 1950s. When developing oil and gas fields in shallow water area, a river channel that can pass through shallow water vessels is usually opened manually, and the pipes are assembled on a pipe laying barge assembled by pontoon. When the barge moves backward, the welded pipe section slides into the water. This kind of pipe laying barge has gradually developed into a large pipe laying vessel. In 1956, the first large pipelaying vessel was put into operation. In 1979, the semi submersible “Castro” pipelaying vessel successfully laid a 500mm diameter pipeline in 608m deep sea area when building a gas pipeline from Algeria in Africa to Europe and Italy through Tunis Strait through Tunisia.
In recent years, traction method has been widely used in laying pipelines in shallow water. This method does not require the use of pipelaying vessels and ditching vessels, and can reduce many auxiliary vessels with less cost. The operation method is generally as follows: firstly, the pipes are assembled into 1-2 km long pipe sections on the coast, and then the pipe sections are towed into the sea by tugboats, and then the pipelines are towed to the predetermined places section by section, and then connected in the sea to form a whole pipeline.
In the calm sea area, the pipe section can be floated on or under the water surface; in the sea area with strong wind and waves, it can be towed on the sea floor. However, attention must be paid to ensure the construction quality. In case of failure during construction, large vessels are still needed to remove them.
Reel ship laying
The pipe is wound on a large diameter drum on the ship. When laying, the pipe is unwound from the drum and sunk into the sea bottom. This method is usually used to lay pipes with small diameter, and its advantages are fast progress and good continuity. This method is now being studied to lay pipes with a diameter of 400 mm.
Used in World War II reel laying. This method was used in 1944 when the wartime oil pipeline across the English Channel was rapidly laid. At that time, the armored flexible pipes made of copper tin lead alloy were wound on the reel of the ship, just like laying submarine cables. The pipelines were laid on the sea floor, and 12 pipelines with a diameter of 77 mm were laid.
In order to accurately open the pipe trench on the position of the pipe and reduce the excavation earthwork as much as possible, the method of laying the pipe first and then trenching is generally adopted. It is an important safety measure to open a trench on the sea floor and bury the pipeline in the trench.
This method was used in the construction of the gas pipeline from Ekofisk oil field in the North Sea to Emden in the Federal Republic of Germany in the mid-1970s. The pipeline is 442 km long and 920 mm in diameter. About 150 km of the pipeline is buried in a trench. After the pipeline is laid on the seabed, a skid type ditcher is “mounted” on the pipeline, which is towed and moved by the ditcher, and the high-pressure water and compressed air are supplied to the skid type ditcher through the hose from the ditcher. According to the required groove shape and depth, several groups of nozzles are arranged on the ditching machine. The high-speed water from the nozzle impacts the soil under and on both sides of the pipe, making it into mud. At the same time, air is injected to flush the mud out of the ditch. After the trench is opened, the pipeline will sink into the bottom of the trench by its own weight. The trenching vessel can excavate hard seabed with a depth of 2.1-4.6 meters, and can operate under water at a depth of 100 meters.
Laying construction method of submarine pipeline
Pipelaying ship method is the most commonly used method for laying submarine pipelines, which can provide a complete offshore prefabrication plant. This plant enables the continuous assembly of pipes into pipes along the selected route. The pipelaying vessel can be a standard barge or a semi submersible pipelaying vessel. These vessels can only operate on the sea without freezing for a long time. If they work together with auxiliary tugs and supply vessels, they can be self-sufficient and work continuously for several months at a time.
The main operation sequence of pipelaying vessel is as follows: weld the single pipe (12m long) or “double pipe” (2x12m, 3x12m) one by one, check each weld joint, process the interface with anti-corrosion insulation layer, thermal insulation layer and concrete protective layer; slowly put it into the seabed through the launching chute (stinger), and move the anchor cable of the pipelaying vessel once every laying a certain distance. Most pipelaying vessels are towed by high-powered tugs.
Floating pipe laying method
First of all, the pipe is welded into many long sections on the shore, and a number of pontoons are tied to the pipe section, and then the pipe section is dragged to the laying point. Attention should be paid to the influence of wind and waves and the bending deformation of pipelines during the towing process after launching, so as to avoid damage in the towing process. Hold one end of the laying section with a barge until it is aligned with the lower end and welded. The buoy is removed according to the sequence specified in the process, and the pipe is lowered to the seabed. Repeat these steps until the pipe is laid.
Bottom drag method
The bottom dragging method needs to be prefabricated into a certain length of pipe section on the shore and stacked on the pipe rack. Then the pipe section is hoisted to the track for launching the pipeline, and welded with the previous section. After welding, drag down a pipe section. When a certain towing section is reached (depending on the capacity of the tugboat), it is towed to the predetermined laying position. After all pipe sections are in place on the seabed, the butt ends of pipe sections shall be lifted out of the water surface for welding, and then laid down to the seabed after connection; or underwater connectors shall be used for connection. In this way, the towing pipe is completed by a tugboat equipped with a winch. The length of the pipe to be towed depends on the drag force of the winch and the friction force between the pipe section and the seabed. The bottom dragging method is suitable for short pipelines, ready-made assembly sites on shore and calm wind and waves.
Coiled pipe laying method
The reel laying method is to wind the pipe onto a large diameter reel, transport it to the laying site by barge, connect it with the laid pipe section in front, release it from the reel, straighten it, and then lower it to the seabed. This method can not use concrete cover. The diameter of the pipe is not easy to be too large. This method is very fast and is most suitable for laying submarine hose.
Vertical or large angle catenary pipe laying method
When the vertical or large angle catenary pipe laying method is used, the shape of the pipe is similar to that of the letter J. therefore, it is usually called the J-type pipe laying method. This method is suitable for the following two situations:
- (1) When it is necessary to connect the subsea wellhead with the processing facilities by using a short oil pipeline, the traditional pipe laying method is not economical and suitable.
- (2) In order to limit the excessive bending, a long stinger is required for deep-water pipe laying, which cannot be met by general pipelaying vessels. Generally, the derrick of semi submersible drilling platform is used to lift the connected pipe section into vertical state. After the pipeline reaches the seabed, the drilling platform is towed away from the drilling position, and the pipeline is in catenary shape. As the tug moves, the segments are constantly joined together and lowered to the seafloor.
Trenching and pipe laying
The purpose of trenching and burying submarine pipelines is to increase the stability of in-situ bottom setting and provide protection under fishing trawls and other activities. The influence of fishery activities on the submarine pipelines without trenching and burying is quite significant.
The influence of fishing activities (trawling load) on submarine pipelines has not been stipulated in compulsory regulations at home and abroad. Traditionally, people define whether the pipeline can bear the trawl load according to the pipe diameter, that is, when the pipe diameter is not less than 16cin, the submarine pipeline can bear the trawl load, so there is no need for trenching protection.
In general, in the areas where the high-speed current will cause displacement of the pipeline, where the pipeline is gradually damaged due to scouring, and in the area where anchoring and fishing operations may damage the pipeline, trenching and pipe laying should be adopted. Trenching can be done by suction dredger or ditching plough. The plough is usually towed by tugboat, which is more suitable for hard soil than dredger. When ditching with these two methods, pipe laying can be carried out at the same time. Some of the trench backfill is completed by the natural movement of seabed mud and sand.
Source: China Submarine Pipeline Manufacturerufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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