From the initial stage of submarine pipeline engineering, China began to study and introduce pipeline transportation technology of submarine high pour point and high viscosity crude oil. From Chengbei and Bozhong 28- 1 in Bohai Sea to Bozhong 34-2/4 Oilfield and Wei 10-3 Oilfield in the early 1980s, submarine oil pipeline projects all involved how to solve the technical problems of crude oil transportation. According to the characteristics of oilfield crude oil, in cooperation with Japanese and French petroleum engineering circles, safe and reliable engineering countermeasures have been studied and adopted, and relevant design, construction and operation management technologies have been learned and introduced. Subsequently, in several oilfield development projects developed in Bohai Bay and Beibu Gulf, several submarine oil pipelines were designed and laid, forming a complete set of submarine pipeline transportation technology for high pour point and high viscosity crude oil in China. Through a large number of engineering practice applications and tests, it is proved that this technology is practical and reliable.
First, the transportation process
Aiming at the pipeline transportation of high pour point and high viscosity crude oil, many drag reduction and viscosity reduction methods have been adopted in oil fields and export pipeline projects at home and abroad, such as dosing, emulsification and viscosity reduction, water suspension transportation, viscoelastic liquid film and so on. , and conducted a lot of research and experiments, but due to various technical and economic reasons, it has not been widely used. At present, the most practical and reliable method is heating the transportation process to reduce viscosity and prevent solidification.
For high pour point crude oil, in order to prevent crude oil from freezing during pipeline transportation, the temperature of crude oil in the pipeline is always kept above freezing point by heating.
For high viscosity crude oil, heating is used to reduce viscosity, meet the pressure drop demand of pipeline and save pumping energy consumption. Of course, when the hot oil transportation process is adopted, the thermal insulation pipeline structure is generally adopted accordingly.
(A) Process simulation analysis
Generally, the simulation calculation of the transportation process of submarine oil pipeline involved in offshore oil field development projects should calculate the pressure drop, temperature drop, liquid retention and some necessary process parameters under different conditions (pipe diameter, transportation capacity, inlet temperature, etc.) according to the annual output forecast provided by oil field geological development (and considering certain design factors). ). According to this, the best pipe diameter is selected, and the process parameters (conveying pressure, temperature, etc.) are determined. In different production years).
In recent years, the simulation calculation and analysis of crude oil pipeline transportation process are generally carried out by computer simulation program. China Offshore Oil introduced PIPEFLOW software from Canadian NEOTEC Company, which is similar to popular commercial software such as PIPESIM and PIPEPHASE, and compiled various calculation methods, partial correction coefficients and reference databases for designers and analysts to choose from.
(2) Selection of thermal insulation materials and determination of thickness
Thermal calculation is a very important link in the submarine pipeline for transporting hot oil, and the heat transfer coefficient k of the pipeline is a comprehensive expression of the thermal condition of the pipeline. In addition to the influence of pipeline structure, buried temperature conditions, thermal conductivity of insulation materials and thickness of insulation materials are three major factors.
From the calculation and analysis results, because the change of ground temperature is not obvious, it is necessary to pay attention to the influence on K value only when the throughput is low.
The performance of insulation materials and the thickness of insulation layer are the most critical factors affecting the K value and the final temperature of the pipeline. At present, the insulation materials selected in China are the same as those most commonly used abroad, all of which are polyurethane foam. This is an organic polymer foam that can form an open-cell or closed-cell honeycomb structure. Its advantages are low thermal conductivity (≤ 0.03 W/m2 h℃), low density (40 ~ 100 kg/m3), low water absorption (≤3%), good chemical stability, mature industrial production and relatively cheap price. Considering the thermal insulation effect, of course, the greater the thickness of thermal insulation layer, the better. However, when the thickness of insulation layer reaches a certain value, the increase of insulation effect and thickness is no longer linear, but increases very gently. Especially for submarine pipelines, the increase of insulation layer thickness means the increase of outer diameter. For long-distance pipelines, if the outer diameter is increased by one grade, the increased steel pipe consumption and construction cost are very considerable. Therefore, it is reasonable to choose the insulation layer thickness of 50 mm according to the calculation analysis and optimization design.
(3) Calculation and analysis of shutdown and restart
The calculation and analysis of shutdown and restart is an important content in the process design of submarine pipeline for high pour point and high viscosity crude oil, which will directly affect the safety and reliability of pipeline transportation.
The temperature drop analysis after shutdown is the final determination of pipeline safety time. After the pipeline with hot oil transportation technology is stopped, with the loss of oil storage heat, crude oil will solidify from the pipe wall to the center of the pipe, and the thickening of condensate oil layer and the latent heat released during condensation will delay the whole solidification process. The solidification time of the stored oil depends on the heat preservation conditions of the pipeline, the heat capacity of the oil, the temperature when the transportation is stopped and the cross-sectional diameter. Generally speaking, the larger these values, the longer the full-face solidification time. Generally speaking, the thickness of condensate layer is a variable value in the axial direction of the pipeline, and the thickness of condensate in the last section of the pipeline is usually taken as the control value of safe shutdown time.
For the heated and transported crude oil pipeline with high pour point and high viscosity, it is expected that the pipeline cannot resume oil transportation within the safe shutdown time. The most effective measure to ensure the safety of the pipeline is to replace the oil in the pipeline with water or low pour point oil when it begins to solidify.
Restart analysis after shutdown is to consider the most unfavorable working conditions and environmental conditions that may occur after pipeline shutdown. At this time, in order to restore the oil flow, it is necessary to calculate the required restart pressure, put forward the measures to realize the restart, and add the necessary equipment and facilities.
In general, the restart pressure (p) is calculated by the following formula:
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Where: p is restart pressure (PA); P is the pipeline outlet pressure (pa); Di is the inner diameter of the pipeline (m); τ is the yield stress (Pa) of crude oil at shutdown ambient temperature; L is the possible solidification length of the pipeline (m).
(4) hydrate and erosion prevention measures
The oil pipeline involved in offshore oil field development project is a kind of pipeline different from land long-distance crude oil pipeline and offshore crude oil pipeline. It is the internal gathering pipeline of the oil field, where the crude oil produced from the wellhead platform is mixed with gas and water and transported to the central processing platform or floating production storage device. This submarine pipeline is accompanied by water and gas extracted from the wellhead and belongs to mixed transportation pipeline. For this kind of oil pipeline, heating and conveying technology and thermal insulation pipeline structure are also adopted.
In the process design of this mixed oil pipeline, in addition to the simulation calculation and analysis usually needed to purify crude oil pipeline, slug flow analysis and hydrate and corrosion prevention analysis should also be added.
Slug flow is an important problem in the process of oil-gas mixed transportation. At present, there is a general analysis and calculation method to judge whether there is serious slug flow and how to determine the length of slug flow in normal transportation period. In the process of pigging, because there is a certain amount of stagnant liquid in the pipeline, liquid slug flow will be formed before pigging. Slug flow caused by pigging must be considered in the design of downstream separation equipment. Generally, a certain buffer capacity is designed to keep the container running between the normal liquid level and the high liquid level alarm line to ensure normal production.
Hydrate is a hidden danger that affects the operation of submarine mixed transportation pipeline, especially in the following three working conditions. Therefore, the measures to prevent hydrate formation are put forward: ① Low throughput, in order to prevent hydrate formation, it is required that the oil and gas temperature in the pipeline should always be above the hydrate formation temperature during transportation. However, under the condition of low throughput, the temperature drops rapidly, and according to the hydrate formation curve, hydrate may be formed. At this time, antifreeze (hydrate inhibitor) such as methanol should be injected in time to prevent hydrate formation; (2) During the shutdown process, in the long-term shutdown state, because the oil and gas temperature in the pipeline has dropped to the ambient temperature, and the pressure in the pipeline remains at a high pressure, hydrate may be generated. At this time, the measures should be taken, one is to release the pipeline pressure, the other is to inject hydrate inhibitor into the pipeline; (3) Restart, generally after shutdown, requires that the start-up pressure is higher than the normal operating pressure, and at this time, the temperature is often very low, and hydrate is easily generated. At this time, hydrate inhibitor should be continuously injected until the temperature in the pipeline reaches the normal operating temperature.
Corrosion prevention is a problem that can not be ignored in the process design of oil-gas mixed pipeline. For multiphase pipelines, if the flow velocity exceeds a certain value, the solid particles contained in the liquid will strongly scour and corrode the inner wall of the pipeline, especially at the sharp bends such as risers and expansion bends of submarine pipelines. Therefore, the maximum velocity to avoid scouring should be calculated in the design, and the formula is:
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Where: Ve is the erosion speed (ftlft=0.3048m). /s)；
Pm is the density of multiphase mixture in the conveying state (lb 65438+lb 0 = =0.453592kg).
/cubic feet l cubic feet = 2083 1685× 10-2m3.
); C is the empirical coefficient, which is 100 for continuous operation and 125 for intermittent operation.
Erosion rate is a function of mixture density. The greater the density of the mixture, the smaller the erosion rate, and the smaller the density of the mixture, the greater the erosion rate. In order to ensure that the pipeline is not eroded, the fluid velocity in the pipeline should be controlled below the calculated minimum erosion velocity.
(5) Operation management
For submarine crude oil pipelines with high pour point and viscosity, special attention should be paid to the following operational management issues.
1. Initial startup
The first operation generally adopts the following steps: ① preheat the pipeline with hot water or hot diesel oil to establish a temperature field suitable for operation; (2) After the measured outlet temperature meets the design requirements, the well shall be started and put into production as required.
2. Stop transmission and restart.
Parking is generally divided into two categories: emergency parking and planned parking. Different shutdown conditions will lead to different restart methods. In order to ensure the restart of the pipeline after shutdown, a high-pressure restart pump is generally installed on the wellhead platform.
A. Short-term shutdown means that the lowest temperature of the fluid in the pipeline is above a certain design value (such as the freezing point of crude oil), so that wellhead oil and gas can directly enter the pipeline or be started by the high-pressure pump.
B. For long-term parking, the high-pressure pump should be started before parking to complete the fluid replacement operation in the pipeline. If you don't prepare in advance, it will be an unexpected and sudden shutdown. Once the transportation is stopped for a long time, the inside of the pipeline drops to the ambient temperature, and the crude oil precipitates wax and solidifies. At this point, start the high-pressure pump, replace the crude oil with diesel oil, and then follow the initial start-up steps.
3. Pig iron
In the normal production process, pigging operation should be carried out frequently according to the production situation to remove wax deposition and stagnant liquid in the pipeline, so as to improve transportation efficiency and reduce corrosion.
4. Pressure injection of chemical solution
During normal transportation, the following chemicals should be injected:
Anti-scaling agent-prevents scaling in the pipeline due to the water content in crude oil, which will reduce the transportation capacity;
Paraffin inhibitor-prevents wax in crude oil from condensing and depositing in the pipeline;
Preservative-can form a protective film on the inner wall of the pipeline, isolate corrosive liquid from the inner wall of the pipeline and play a protective role;
Antifreeze-methanol, etc. To prevent hydrate formation.
Second, the thermal insulation submarine pipeline structure
For the submarine pipeline with high pour point and high viscosity using hot oil transportation technology, it is the most common and practical to make the oil transportation steel pipe into thermal insulation structure in order to slow down the temperature drop along the pipeline. We have widely applied the submarine thermal insulation pipeline structure and formed a complete design and construction technology.
(a) Types and characteristics of structures that have been applied
Submarine steel pipe insulation pipeline structure (not involving flexible hose submarine pipeline here) can be summarized into two types: one is double-layer steel pipe insulation structure; The second is the single-layer steel pipe thermal insulation structure.
1. Double-layer steel pipe insulation structure.
Or multi-wall pipe structure, and the cross section of the pipe body is shown in Figure 15-3. In this type, there are three forms.
Figure 15-3 Double Steel Pipe Insulation Structure
Figure 15-4 Double-layer steel pipe insulation structure with packing flange
The first form: the pipeline structure is shown in figure 15-4. A single pipe joint (usually12m or 40ft in length) is equipped with a strong sealing flange at each end. In the annular space between the inner and outer pipes, foaming materials are injected to form a closed water stop and heat preservation unit. The inner and outer pipes of the unit are connected into a whole by sealing flanges at both ends, and the expansion and contraction of the inner pipe are constrained by the sealing flanges, so that the inner and outer pipes do not move relative to each other. When laying pipes at sea, the outer pipes of two adjacent pipe joints are connected by two half-tile puppets. The advantage of this form is that if the outer pipe or joint of the pipeline is damaged, the insulation failure will be limited to a minimum. The disadvantage is that the workload of interface welding is large, and the pipe laying method can not improve the speed, resulting in high engineering cost.
Figure 15-5 Double-layer steel pipe insulation structure with special joints
Figure 15-6 Double steel pipe thermal insulation structure with relatively movable inner and outer pipes.
The second form: the inner and outer pipes at both ends of the insulation pipe joint are connected by special joints, as shown in figure 15-5. It was first put forward by Shell Oil Company and later developed into a patented product of Italian Snamprogetti Company, which has been put into use in some submarine pipeline projects. Obviously, this form retains the advantages of the first form and overcomes its disadvantages. On the pipe-laying ship, like laying a single-layer steel pipe, multiple welding stations can carry out assembly line operation, which greatly improves the speed of pipe laying at sea. The problem with this form is that the joint is a patented product with high cost. The patented product has been applied to the subsea oil pipeline of Huizhou 26- 1 oil field in the eastern South China Sea.
The third form is shown in figure 156. In this form, the inner tube and the outer tube can move relatively. When connecting at sea, after the inner pipe interface is welded, add the interface insulation material, and then pull the outer pipe to butt joint, so it is not necessary to use the half-tile pipe. Relatively speaking, it can reduce the workload of offshore welding and improve the speed of pipe laying. Through the cooperation with Japanese companies, CNOOC China introduced this technology of thermal insulation submarine pipeline design and offshore installation, which has been adopted in many submarine oil pipelines that have been laid.
2. Single-layer steel pipe insulation structure.
The difference between this structure and the double-layer steel pipe insulation structure is that the outer sheath pipe does not use steel pipe. According to the different coat materials, it can be divided into the following five types.
First, the high-density polyethylene sheath. High-density polyethylene (HDPE) is an ultra-high molecular weight polymer and an excellent material to prevent water vapor from passing through. This ultra-high molecular weight improves the mechanical properties of steel pipes, such as wear resistance, impact resistance, tear resistance and overall physical strength. Compared with steel pipe jacket, this prefabricated jacket system has the characteristics of light weight and no need for anti-corrosion protection. Thermal insulation foam exposed at both ends of the pipe joint is protected by heat-shrinkable polymer end caps, and heat-shrinkable sleeves are also used for water sealing and corrosion protection of field joints. The jacket system has been applied by European and American companies to submarine pipeline projects in the Arabian Gulf and offshore Gabon. In recent years, the applied water depth has reached 43m.
Second, lock the spiral steel sheath. The characteristic of this jacket is that the steel consumption is much lower than that of the conventional steel pipe jacket. There is no need to butt weld the field interface, and the foam insulation material exposed at the end of the pipe joint is still protected by the heat-shrinkable end cover. This jacket system has been widely used abroad, and the maximum applied water depth has reached 55 m.
Thirdly, molding polyurethane sheath. This sheath combines anti-corrosion materials and PVC foam insulation materials (Figure 15-7). Its advantages are: ① The pipeline can maintain good flexibility and can be laid by winding boats. ② If the jacket is damaged on the seabed, almost no thermal insulation material is exposed to water, unlike other systems, which will soak the whole pipe joint. ③ Ensure high reliability of foam drying.
Figure 15-7 Molded Polyurethane Sheath Insulation Structure
Figure 15-8 Rubber Sheath Insulation Structure
Fourth, rubber jacket. Similar to molded polyurethane sheath (figure 15-8). Except that the jacket consists of PVC foam and rubber layer. Each layer of PVC is about 5 ~ 8 mm thick, and the rubber layer is 1mm thick. The number of layers depends on the thermal insulation requirements, but the outermost layer of PVC foam should be covered and protected with a thick rubber layer.
Fifth, cancel the sheath system. The thermal insulation material applied outside the steel pipe of oil pipeline is waterproof and has good thermal insulation performance, and can resist high hydrostatic pressure and strong mechanical damage. This structure should be said to be a real single-layer steel pipe insulation structure.
(2) Key technologies of design and construction
Most of the submarine steel pipe insulation pipelines built in China are double steel pipe insulation structures. The design and construction technology of thermal insulation structure was introduced from Japan by CNOOC.
Key technologies in the design of 1.
The key technology of submarine pipeline design with double steel pipe insulation structure is the structural analysis of flat pipe and the overall analysis of riser telescopic bending system.
For the structural analysis of flat tubes, the computer analysis program "DPIPE" developed by Nippon Steel Company of Japan is applied. The structural model of the analysis program is shown in figure 15-9.
Figure 15-9 Flat Pipe Structure Analysis Model
A, a, a'-the fixed point of the outer tube; Anchoring points (partitions) between the inner tube and the outer tube of B, B', E and E'; D-the fixed point of the inner tube; KB，KB？ -spring constant; Wf-friction load with soil; A-A-A'—— fixed part (outer tube); Li+lm, Li '+lm '- movable part (outer tube)
In the figure, the spring stiffness KB, KB? It is obtained by the overall analysis model of riser expansion bending and flat pipe connection explained later.
For buried pipelines, the friction load Wf between the pipeline and soil is calculated by the following formula:
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Where: w = r' hdoμ is the friction coefficient; Do is the outside diameter of the pipeline; Ws is the underwater unit weight of the pipeline; r？ Is the underwater bulk density of soil; H is buried depth.
For the overall analysis of riser expansion and bending system, the large-scale three-dimensional pipeline structure analysis program "PIDES" software developed by Nippon Steel Company of Japan is adopted.
Figure 15- 10 is an engineering example of establishing a three-dimensional structural analysis model based on this software.
Figure 15- 10 Schematic diagram of structural analysis model of riser telescopic bending system
Figure 15- 1 1 Case diagram of combined analysis of working conditions
For the established system structure analysis model, it is necessary to make a full and necessary combination analysis of various load cases according to the requirements of the code and the actual situation of the project. Generally, the loads to be considered include functional loads (pressure, temperature, mass, etc. ), environmental load (wind, waves, water, ice, etc. ), special load (such as earthquake) and platform displacement are added to the riser, and the load imposed by riser expansion and contraction.
Figure 15- 1 1 gives an example of combined analysis of riser expansion system, and the direction of load action is an important factor to be considered.
2. Key construction technologies
The onshore prefabrication and offshore installation technology of submarine pipeline with double steel pipe insulation structure introduced from Japan is mainly characterized in that a single pipe joint (12m long) is fixed on the inner pipe during prefabrication, and there is a certain air layer between the insulation material and the inner wall of the outer pipe, so that the inner and outer steel pipes can move each other. Only when the length is 2km or 1km, a rigid anchoring flange is set to form a watertight compartment in the annular space. In this way, when the pipe laying method is installed offshore, the pipe joint connection will be as shown in the previous figure 15-6. After the welding of the inner pipe is qualified, it will obviously reduce the welding workload of the outer pipe interface and improve the speed of pipe laying at sea by adding the anti-corrosion coating and corresponding insulation materials and adopting butt welding of the outer pipe.
(3) Application in the first-phase submarine pipeline project of Penglai 19-3 oilfield in Bohai Sea.
Submarine pipeline with double steel pipe insulation structure has been proved to be safe and reliable by many domestic engineering practices, but there are also some shortcomings such as large amount of steel used at sea and slow installation speed, which leads to high project cost. The research and application of single-pipe thermal insulation structure is the development direction of thermal insulation submarine pipeline technology.
Among them, the single-pipe thermal insulation structure with locking spiral steel plate (thickness 1mm) as jacket was successfully applied in the first-phase submarine pipeline project of Penglai 19-3 Oilfield operated by Phillips Company in 2002. Figure 15- 12 shows the cross-sectional structure of the thermal insulation pipeline.
China Offshore Oil Corporation is studying the trial-manufacture of single-pipe thermal insulation pipeline with high-density polyethylene (pe) as sheath. This technology has been applied abroad for a long time. Combined with the specific situation of our country, especially when the water depth of Bohai Sea is less than 30m, or even the water depth of many beach oil fields is less than 5m, this thermal insulation structure is economical and reliable, and the materials and technologies used can be localized and localized, which has a good application prospect.
Figure 15- 13 shows the cross-sectional structure of PE jacketed thermal insulation pipeline under development.
Figure15-12 PL19-3 Cross-sectional Structure of Submarine Pipeline
Figure 15- 13 Cross-sectional Structure of Polyethylene Sheathed Insulation Pipe
Table 15-3 gives the technical parameters of the developed thermal insulation pipeline.
Table 15-3 Technical Parameter Table of Thermal Insulation Pipeline
Of course, in the true sense, the outer sheath system should be eliminated for the single-pipe thermal insulation structure pipeline, and the thermal insulation material with good waterproof, thermal insulation performance, hydrostatic pressure resistance and mechanical damage resistance should be applied outside the oil transmission steel pipe. There is no doubt that this is the ultimate direction of this technology development. At present, a submarine thermal insulation oil pipeline with a diameter of 254 mm and a length of about 8.7 km in Huizhou 26- 1 North Oilfield (water depth is about 120 m) in the east of the South China Sea, after in-depth investigation and bidding, has already possessed the practical engineering foundation, and its technical feasibility and price acceptability have reached a good conclusion.