How are underwater pipelines laid? Subsea Pipelines: How It Works Offshore Subsea Pipelines Intersection Projects with Cables

PART 1. DESIGN STANDARDS

1. General Provisions

1.1. Offshore main gas pipelines must have increased reliability during construction and operation, taking into account special conditions (great sea depths, increased length without intermediate compressor stations, sea storms, undercurrents, seismicity and other factors).

Design decisions for laying offshore gas pipelines must be coordinated with the State Committee of the Russian Federation for Environmental Protection, Gosgortekhnadzor of Russia and local supervisory authorities.

1.2. Protective zones are established along the route of the offshore gas pipeline, which include sections of the main gas pipeline from compressor stations to the water's edge and further along the seabed within the continental shelf, at a distance of at least 500 m.

1.3. The diameter of the offshore gas pipeline and the value of the working pressure are determined from the conditions for the supply of natural gas to the Consumer on the basis of hydraulic analysis.

1.4. The service life of the offshore gas pipeline is set by the Project Owner. For the entire service life of the gas pipeline system, the reliability and safety of the structure and such influences as metal corrosion and fatigue of the materials used must be calculated.

1.5. The boundaries of the offshore section of the main gas pipeline are shut-off valves installed on opposite shores of the sea. Shut-off valves must be equipped with automatic emergency closing.

1.6. At the ends of each line of the offshore gas pipeline, units for launching and receiving cleaning devices and flaw detector projectiles should be provided. The location and design of these nodes are determined by the project.

1.7. The offshore gas pipeline must be free from obstructions to the flow of the transported product. In the case of using artificial bending curves or fittings, their radius must be sufficient for the passage of cleaning and control devices, but not less than 10 pipeline diameters.

1.8. The distance between parallel lines of offshore gas pipelines should be taken from the conditions of ensuring reliability during their operation, the safety of the existing line during the construction of a new line of the gas pipeline and safety during construction and installation work.

1.9. Offshore pipeline protection against corrosion is carried out in a complex way: by a protective outer and inner coating and cathodic protection means.

Anti-corrosion protection should contribute to the trouble-free operation of the offshore pipeline throughout the entire period of its operation.

1.10. The offshore pipeline must have an insulating connection (flange or sleeve) with a corrosion protection system for onshore sections of the main gas pipeline.

1.11. The choice of the offshore pipeline route should be made according to the criteria of optimality and be based on the following data:

· soil conditions of the seabed;

seabed bathymetry;

morphology of the seabed;

initial information about the environment;

· seismic activity;

Fishing areas

ship fairways and places of mooring of vessels;

areas of soil dumping;

water areas with increased environmental risk;

The nature and extent of tectonic faults. The technical and environmental safety of the structure should be taken as the main criteria for optimality.

1.12. The project must provide data on the physical and chemical composition of the transported product, its density, as well as indicate the calculated internal pressure and design temperature along the entire pipeline route. Information is also given on the temperature and pressure limits in the pipeline.

Permissible concentrations of corrosive components in the transported gas should be indicated: sulfur compounds, water, chlorides, oxygen, carbon dioxide and hydrogen sulfide.

1.13. The development of the project is based on the analysis of the following main factors:

direction and speed of the wind;

height, period and direction of sea waves;

speed and direction of sea currents;

level of astronomical high and low tide;

· storm surge of water;

The properties of sea water

temperature of air and water;

· growth of marine fouling on the pipeline;

seismic environment;

· Distribution of commercial and protected species of marine flora and fauna.

1.14. The project should include an analysis of the allowable spans and stability of the pipeline on the seabed, as well as the calculation of nozzles - limiters of the avalanche collapse of the pipeline during its laying at great depths of the sea.

1.15. The gas pipeline should be buried in the bottom in the areas of its landfall. The design elevation of the top of the pipeline buried in the ground (by weight coating) should be set below the predicted depth of erosion of the bottom of the water area or onshore section for the entire period of operation of the offshore pipeline.

1.16. In deep-water sections, the gas pipeline can be laid on the surface of the seabed, provided that its design position is ensured during the entire period of operation. At the same time, it is necessary to justify the exclusion of the rise or movement of the pipeline under the influence of external loads and its damage by fishing trawls or vessel anchors.

1.17. When designing an offshore pipeline system, all types of impacts on the pipeline that may require additional protection should be taken into account:

the occurrence and spread of cracking or collapse of pipes and welds during installation or operation;

· Loss of pipeline stability on the seabed;

· loss of mechanical and service properties of pipe steel during operation;

· unacceptably large spans of the pipeline at the bottom;

erosion of the seabed;

· strikes on the pipeline by anchors of vessels or fishing trawls;

earthquakes;

Violation of the technological regime of gas transportation. The choice of protection method is adopted in the project depending on local environmental conditions and the degree of potential threat to the offshore gas pipeline.

1.18. The following data should be reflected in the project documentation: pipe dimensions, type of transported product, service life of the pipeline system, water depth along the gas pipeline route, type and class of steel, the need for heat treatment after welding of girth field welded joints, anti-corrosion protection system, plans for the future development of regions along pipeline system routes, scope of work and construction schedules.

On the drawings, it is necessary to indicate the location of the pipeline system in relation to nearby settlements and harbors, the courses of ships, as well as other types of structures that can affect the reliability of the pipeline system.

The project takes into account all types of loads that occur during the manufacture, installation and operation of the pipeline system, which may affect the choice of design solution. All necessary calculations of the pipeline system for these loads are performed, including: analysis of the strength of the pipeline system during installation and operation, analysis of the stability of the position of the pipeline on the seabed, analysis of fatigue and brittle fracture of the pipeline, taking into account welded circumferential seams, analysis of the stability of the pipe wall against collapse and excessive deformations , vibration analysis, if necessary, analysis of the stability of the seabed base.

1.19. As part of the offshore gas pipeline project, it is necessary to develop the following documentation:

Specifications for pipe material;

Specifications for pipe welding and non-destructive testing, indicating the norms for permissible defects in welds;

· specifications for reinforced inserts to limit the avalanche collapse of the pipeline;

Specifications for external and internal anti-corrosion coating of pipes;

Specifications for the weight coating of pipes;

· specifications for the material for the manufacture of anodes;

· technical conditions for laying the offshore section of the pipeline;

· technical conditions for the construction of the pipeline when crossing the coastline and coastal protection measures;

· specifications for testing and commissioning of the offshore pipeline;

· technical conditions for the maintenance and repair of the offshore pipeline;

general specification of materials;

Description of construction boats and other equipment used.

When developing "Specifications" and "Specifications", the requirements of these standards and the recommendations of generally recognized international standards (1993), DNV (1996) and (1993), as well as the results of scientific research on this issue, should be used.

1.20. Design documentation, including test reports, survey materials and initial diagnostics, must be retained throughout the entire service life of the offshore pipeline system. It is also necessary to save reports on the operation of the pipeline system, on inspection control during its operation, as well as data on the maintenance of the offshore pipeline system.

1.21. Examination of design documentation should be carried out by independent organizations, to which the design organization provides all the necessary documentation.

2. Design criteria for pipelines.

2.1. The strength criteria in these codes are based on allowable stresses, taking into account residual welding stresses. Limit state design methods may also be used, provided these methods provide the reliability of the offshore pipeline system required by this code.

2.2. Calculations of the offshore gas pipeline must be made for static and dynamic loads and impacts, taking into account the work of welded circumferential seams in accordance with the requirements of structural mechanics, strength of materials and soil mechanics, as well as the requirements of these standards.

2.3. The accuracy of calculation methods should be justified by practical and economic feasibility. The results of analytical and numerical solutions, if necessary, must be confirmed by laboratory or field tests.

2.4. The calculation of the offshore gas pipeline is made for the most unfavorable combination of actually expected loads.

2.5. For an offshore gas pipeline, calculations should be performed separately for the loads and impacts arising during its construction, including hydrostatic tests, and for the loads and impacts arising during the operation of the offshore pipeline system.

2.6. When calculating strength and deformability, the main physical characteristics of steel should be taken according to the "Specifications for Pipe Material".

3. Loads and impacts.

3.1. In these standards, the following combinations of loads are accepted in the calculations of the offshore gas pipeline:

permanent loads;

· constantly operating loadings together with loadings of environment;

· permanent loads in combination with random loads.

3.2. The permanent loads on the offshore pipeline during its construction and subsequent operation include:

· the weight of the pipeline structure, including weight coating, marine fouling, etc.;

external hydrostatic pressure of sea water;

buoyancy force of the aquatic environment;

internal pressure of the transported product;

temperature influences;

backfill soil pressure.

3.3. Environmental impacts on an offshore pipeline include:

loads caused by underwater currents;

· Loads caused by sea waves.

When calculating the offshore pipeline for the period of construction, one should also take into account the loads from construction mechanisms and the loads arising in the process of hydrostatic tests.

3.4. Random loads include: seismic activity, seabed soil deformation and landslide processes.

3.5. When determining the loads and impacts on the offshore pipeline, it should be based on the data of engineering surveys carried out in the area of ​​the pipeline route, including engineering-geological, meteorological, seismic and other types of surveys.

Loads and impacts should be selected taking into account the predicted changes in environmental conditions and the technological regime of gas transportation.

4. Permissible design stresses and strains.

4.1. Permissible stresses in the calculations for the strength and stability of offshore pipelines are set depending on the yield strength of the metal of the pipes used using the design coefficient "K", the values ​​of which are given in

s additional £ K × s T ()

The values ​​of the design coefficients of reliability "K" for offshore gas pipelines.

Ring tensile stresses under permanent loads

Total stresses for constant loads in combination with environmental loads or random loads

Total stresses during construction or hydrostatic testing

Offshore gas pipeline

Onshore and coastal sections of the gas pipeline in the protected zone

Offshore gas pipeline, including onshore and coastal sections in the protected zone

0,72

0,60

0,80

0,96

4.2. The maximum total stresses caused by internal and external pressure, longitudinal forces, taking into account the ovality of the pipes, should not exceed the allowable values:

4.3. Pipelines should be checked for strength and local stability of the pipe section against external hydrostatic pressure. In this case, the internal pressure in the pipeline is assumed to be 0.1 MPa.

4.4. The value of the ovality of the pipes is set by the formula:

()

The permissible total ovality, including the initial ovality of the pipes (factory tolerances), should not exceed 1.0% (0.01).

4.5. The residual deformation in the offshore pipeline should not exceed 0.2% (0.002).

4.6. In areas of possible subsidence of the offshore pipeline, it is necessary to calculate the predicted curvature of the pipeline axis from its own weight, taking into account external loads.

4.7. The project should analyze all possible stress fluctuations in the pipeline in terms of intensity and frequency that can cause fatigue failure during the construction process or during the further operation of the offshore pipeline system (hydrodynamic effects on the pipeline, fluctuations in operating pressure and temperature, and others). Particular attention should be paid to sections of the pipeline system prone to stress concentration.

4.8. To calculate fatigue phenomena, it is possible to use techniques based on fracture mechanics when testing pipes for low-cycle fatigue.

5. Calculation of the pipeline wall thickness.

5.1. For an offshore gas pipeline, the pipe wall thickness should be calculated for two situations determined by the acting loads:

On the internal pressure in the pipeline for shallow, onshore and coastal sections of the gas pipeline located in the protected zone;

On the collapse of the gas pipeline under the influence of external pressure, stretching and bending for deep water sections along the pipeline route.

5.2. Calculation of the minimum wall thickness of the offshore gas pipeline under the influence of internal pressure should be made according to the formula:

()

5.7. When determining the wall thickness of pipes under the conditions of the combined effect of bending and compression, the value of the compressive yield strength equal to 0.9 of the yield strength of the pipe material should be taken into account in the calculations.

5.8. When using laying methods with full control of the bending deformation of the pipeline, the allowable bending deformation when laying the pipeline at sea depths of more than 1000 m should not exceed 0.15% (0.0015). In this case, the critical value of the bending deformation of the pipeline at such depths will be 0.4% (0.004).

6. Stability of the pipeline wall under the influence of external hydrostatic pressure and bending moment.

6.1. For ratio range 15D/t

()

()

In this case, the initial ovality of the pipe should not exceed 0.5% (0.005).

6.2. The external hydrostatic pressure on the pipe at the actual water depth is determined by the formula:

()

6.3. It should also be taken into account that at a pressure exceeding the critical value, local transverse collapse of the pipe can develop along the longitudinal axis of the pipeline.

The external hydrostatic pressure at which the propagation of the previously occurring collapse can occur is determined by the formula:

()

6.4. To prevent the development of collapse along the length of the pipeline, it is necessary to provide for the installation of collapse limiters in the form of stiffening rings or nozzles with an increased wall thickness on the pipeline.

The length of the limiters must be at least four pipe diameters.

7. Stability of the pipeline on the seabed under the influence of hydrodynamic loads.

7.1. Pipeline calculations should be carried out to check the stability of the pipeline position on the seabed during its construction and operation.

If the pipeline is buried in unstable soil and its density is less than the density of the surrounding soil, it should be determined that the resistance of the soil to shear forces is sufficient to prevent the pipeline from floating to the surface.

7.2. The relative density of the pipeline with a weight coating should be greater than the density of sea water, taking into account the presence of suspended soil particles and dissolved salts in it.

7.3. The value of the negative buoyancy of the pipeline from the condition of stability of its position on the seabed is determined by the formula:

7.4. When determining the stability of offshore pipelines on the seabed under the influence of hydrodynamic loads, the design characteristics of wind, water level and wave elements should be taken in accordance with the requirements
*.

It is allowed to assess the hydrodynamic stability of the pipeline using analysis methods that take into account the movement of the pipeline in the process of self-burrowing into the ground.

7.5. Maximum horizontal ( R x + R i) and the corresponding vertical Pz projection of the linear load from waves and sea currents acting on the pipeline, must be determined by the formulas *.

7.6. Calculations of the velocities of bottom currents and wave loads should be made for two cases:

· repeatability once in 100 years when calculating for the period of operation of the offshore pipeline system;

· repeatability once a year in calculations for the period of construction of the offshore pipeline system.

7.7. Friction coefficient values ​​must be taken from engineering survey data for the corresponding pounds along the offshore pipeline route.

8. Materials and products.

8.1. Materials and products used in the offshore pipeline system must meet the requirements of approved standards, specifications and other regulatory documents.

It is not allowed to use materials and products for which there are no certificates, technical certificates, passports and other documents confirming their quality.

8.2. The requirements for the material of pipes and fittings, as well as for shut-off and control valves must meet the requirements of the "Specifications" for these products, which include: product manufacturing technology, chemical composition, heat treatment, mechanical properties, quality control, accompanying documentation and marking .

If necessary, the "Technical Conditions" provides requirements for special tests of pipes and their welded joints, including in a hydrogen sulfide environment, in order to obtain their positive results before the start of production of the main batch of pipes intended for the construction of an offshore gas pipeline.

8.3. The "Specifications for pipe welding and non-destructive testing" should indicate the requirements for defects in welds, under which it is allowed to repair girth welded joints of the pipeline. It is also necessary to provide data on the heat treatment of welded joints or their concomitant heating after welding during pipeline installation.

8.4. For welding electrodes and other products, specifications for their manufacture must be submitted.

8.5. Tolerances for ovality of pipes during their manufacture (factory tolerance) in any section of the pipe should not exceed + 0.5%.

8.6. Connectors intended for offshore pipelines shall be factory tested with a hydraulic pressure of 1.5 times the working pressure.

8.7. The following welding consumables can be used for automatic welding of pipe joints:

ceramic or fused fluxes of special compositions;

· Welding wires of special chemical composition for submerged arc welding or shielding gases;

gaseous argon;

special mixtures of argon with carbon dioxide;

self-shielded flux-cored wire.

Combinations of specific grades of fluxes and wires, grades of self-shielded flux-cored wires and wires for shielded welding, must be selected taking into account their resistance in a hydrogen sulfide environment and be certified in accordance with the requirements of the "Specifications for Pipe Welding and Non-Destructive Testing".

8.8. For manual arc welding and offshore pipeline repair, basic or cellulosic electrodes should be used. Specific brands of welding electrodes must be selected taking into account their resistance in a hydrogen sulfide environment and be certified in accordance with the requirements of the "Specifications for Pipe Welding and Non-Destructive Testing".

8.9. The pipe weight coating shall be steel mesh reinforced concrete applied to individual insulated pipes at the factory in accordance with the requirements of the Pipe Weight Coating Specification.

The class and brand of concrete, its density, the thickness of the concrete coating, the mass of the concreted pipe are determined by the project.

Steel reinforcement should not form electrical contact with the pipe or anodes, and should not extend to the outer surface of the coating.

Sufficient adhesion must be provided between the weight coating and the pipe to prevent slippage under the forces that arise during the laying and operation of the pipeline.

8.10. Reinforced concrete coating on pipes must have chemical and mechanical resistance to environmental influences. The type of fittings is selected depending on the loads on the pipeline and operating conditions. Concrete for weight coating must have sufficient strength and durability.

Each concrete pipe entering the construction site must have a special marking.

PART 2. PRODUCTION AND ACCEPTANCE OF WORKS

1. General Provisions

During the construction of offshore gas pipelines, experience-tested technological processes, equipment and construction equipment should be used.

2. Welding of pipes and methods of control of welded joints.

2.1. Pipe connections during construction can be performed using two organizational schemes:

· with preliminary welding of pipes into two- or four-pipe sections, which are then welded into a continuous thread;

welding of individual pipes into a continuous thread.

2.2. The welding process is carried out in accordance with the "Specifications for pipe welding and non-destructive testing" in one of the following ways:

· automatic or semi-automatic welding in shielding gas with a consumable or non-consumable electrode;

· automatic or semi-automatic welding with self-shielded wire with forced or free formation of the weld metal;

· manual welding by electrodes with a covering of the basic type or with a cellulose covering;

· electrocontact welding by continuous fusion with post-weld heat treatment and radiographic quality control of welded joints.

When welding two or four pipe sections on the auxiliary line, automatic submerged arc welding can also be used.

"Specifications" are developed as part of the project by the Contractor and approved by the Customer on the basis of conducting studies on the weldability of a pilot batch of pipes and obtaining the necessary properties of welded ring joints, including their reliability and performance in a hydrogen sulfide environment, and conducting the appropriate certification of welding technology.

2.3. Before starting construction work, welding methods, welding equipment and materials accepted for use must be certified at the welding base or on the pipe-laying vessel in conditions close to construction conditions, in the presence of the Customer's representatives and accepted by the Customer.

2.4. All operators of automatic and semi-automatic welding, as well as hand-held welders, must be certified in accordance with the requirements of DNV (1996) or taking into account additional requirements for the resistance of welded joints when working in a hydrogen sulfide environment.

Certification must be carried out in the presence of representatives of the Customer.

2.5. Welders who must perform underwater welding must additionally undergo appropriate training, and then special certification in a pressure chamber with simulated natural working conditions on the seabed.

2.6. Welded ring joints of pipes must comply with the requirements of the "Specifications for Pipe Welding and Non-Destructive Testing".

2.7. Circumferential welded joints are subjected to 100% radiographic control with duplication of 20% of the joints by automated ultrasonic control with recording the test results on tape.

Upon agreement with the Customer, it is allowed to use 100% automated ultrasonic testing with a tape recording of 25% of duplicate radiographic testing.

Acceptance of welded joints is carried out in accordance with the requirements of the "Specifications for Pipe Welding and Non-Destructive Testing", which should include the norms for permissible defects in welds.

2.8. Circumferential welds are considered accepted only after they have been approved by the Employer's representative based on review of radiographic images and records of ultrasonic testing results. Documentation with records of the results of the welding process and the control of welded pipe joints is kept by the operating organization of the pipeline throughout the entire service life of the offshore pipeline.

2.9. With appropriate justification, it is allowed to connect the pipeline strings or repair work on the seabed, using docking devices and hyperbaric welding. The underwater welding process shall be classified by appropriate tests.

3. Corrosion protection

3.1. The offshore gas pipeline must be insulated over the entire outer and inner surface with an anti-corrosion coating. Pipe insulation must be carried out in factory or basic conditions.

3.2. The insulating coating must comply with the requirements of the "Specifications for External and Internal Anti-Corrosion Coating of Pipes" for the entire service life of the pipeline in terms of the following indicators: tensile strength, relative elongation at operating temperature, impact strength, adhesion to steel, maximum peel area in sea water, fungus resistance, resistance to indentation.

3.3. The insulation must withstand breakdown tests at a voltage of at least
5 kV per millimeter of thickness.

3.4. Insulation of welded joints, valve assemblies and shaped fittings must, in terms of its characteristics, comply with the requirements for pipe insulation.

Insulation of the connection points of electrochemical protection devices and instrumentation, as well as restored insulation in damaged areas, must ensure reliable adhesion and corrosion protection of pipe metal.

3.5. When performing insulation work, the following must be carried out:

quality control of the materials used;

· step-by-step quality control of stages of insulation works.

3.6. During the period of transportation, handling and storage of pipes, special measures must be taken to prevent mechanical damage to the insulating coating.

3.7. The insulating coating on the pipeline sections completed by construction is subject to control by the cathodic polarization method.

3.8. Electrochemical protection of the offshore pipeline system is carried out using protectors. All electrochemical protection equipment must be designed for the full life of the offshore gas pipeline system.

3.9. The protectors must be made of materials (alloys based on aluminum or zinc) that have passed full-scale tests and meet the requirements of the "Specifications for the material for the manufacture of anodes" developed as part of the project.

3.10. Protectors need to have two connecting cables with a pipe. Bracelet-type protectors are installed on the pipeline in such a way as to avoid mechanical damage during transportation and laying of the pipeline.

The drain cables of protective devices should be connected to the pipeline using manual argon-arc or capacitor welding.

Upon agreement with the Customer, manual arc welding with electrodes can be used.

3.11. On the offshore pipeline, potentials must be provided continuously over its entire surface during the entire period of operation. For sea water, the minimum and maximum values ​​​​of protective potentials are given in. These potentials are calculated for sea water with a salinity of 32 to 28%o at a temperature of 5 to 25°C.

Minimum and maximum protective potentials

3.12. Electrochemical protection must be put into effect no later than 10 days from the date of completion of the pipeline laying.

4. Landfalls of the pipeline

4.1. The following construction methods may be used for pipeline landfall:

· open excavation works with the installation of sheet piling on the foreshore;

· directional drilling, in which the pipeline is pulled through a pre-drilled well in an offshore area;

tunnel method.

4.2. When choosing a pipeline construction method at the landfall sections, the relief of the coastal sections and other local conditions in the construction area, as well as the equipment of the construction organization with the technical means used to carry out the work, should be taken into account.

4.3. Pipeline landfalls using directional drilling or a tunnel must be substantiated in the project by the economic and environmental feasibility of their use.

4.4. During the construction of the pipeline on the coastal section with the use of underwater earthworks, the following technological schemes can be applied:

· a pipeline string of the required length is made on a pipe-laying vessel and pulled to the shore along the bottom of a previously prepared underwater trench using a traction winch installed on the shore;

· The pipeline string is manufactured onshore, hydrostatically tested and then pulled into the sea along the bottom of an underwater trench using a traction winch installed on a pipe-laying vessel.

4.5. The construction of the offshore pipeline in the coastal areas is carried out in accordance with the requirements of the "Technical Specifications for the Construction of a Pipeline at the Crossing of the Shoreline", developed as part of the project.

5. Underwater excavation

5.1. The technological processes of developing a trench, laying a pipeline in a trench and filling it with soil should be maximally combined in time, taking into account the drift of the trench and the reshaping of its transverse profile. When backfilling underwater trenches, technological measures should be developed to minimize the loss of soil outside the trench.

The technology for the development of underwater trenches must be agreed with environmental authorities.

5.2. The parameters of the underwater trench should be as minimal as possible, for which increased accuracy in their development should be ensured. The requirements of increased accuracy also apply to backfilling of the pipeline.

In the zone of transformation of sea waves, more gentle slopes should be assigned, taking into account the reformation of the cross section of the trench.

5.3. Parameters of an underwater trench in areas whose depths, taking into account
surge and tidal fluctuations in the water level, less than the draft of earth-moving equipment, should be taken in accordance with the standards for the operation of sea vessels and ensuring safe depths within the boundaries of the working movements of earth-moving equipment and ships serving it.

5.4. Temporary stockpiles should be kept to a minimum. The location of the storage of the developed soil should be chosen taking into account the minimum environmental pollution and agreed with the organizations that control the ecological state of the construction area.

5.5. If the project allows the use of local soil for backfilling the trench, then during the construction of a multi-line pipeline system, it is allowed to fill the trench with the laid pipeline with soil torn from the trench of the parallel line.

6. Laying from a pipe-laying vessel

6.1. The choice of offshore pipeline laying method is based on its technological feasibility, economic efficiency and environmental safety. For deep seas, S-curve and J-curve laying methods using a pipe-lay vessel are recommended.

6.2. The laying of the offshore pipeline is carried out in accordance with the requirements of the "Specifications for the Construction of the Offshore Section of the Pipeline" developed as part of the project.

6.3. The pipe-laying vessel, prior to the start of construction work, must undergo tests, including tests of welding equipment and non-destructive testing methods, equipment for insulating and repairing welded pipe joints, tensioning devices, winches, control devices and control systems that ensure the movement of the vessel along the route and the laying of the pipeline to the design marks.

6.4. In shallow water sections of the route, the pipe-laying vessel must ensure that the pipeline is laid in an underwater trench within the tolerances determined by the project. To control the position of the vessel relative to the trench, scanning echo sounders and all-round scanning sonars should be used.

6.5. Before starting the laying of the pipeline in the trench, the underwater trench should be cleaned and control measurements should be made with the construction of the longitudinal profile of the trench. When pulling the pipeline along the seabed, it is necessary to perform calculations of traction forces and the stress state of the pipeline.

6.6. Traction means are selected according to the maximum design traction force, which in turn depends on the length of the pipeline being dragged, the coefficient of friction and the weight of the pipeline in water (negative buoyancy).

The values ​​of the coefficients of sliding friction should be assigned according to engineering surveys, taking into account the possibility of immersion of the pipeline into the ground, the bearing capacity of the soil and the negative buoyancy of the pipeline.

6.7. To reduce traction during laying, pontoons can be installed on the pipeline, which reduce its negative buoyancy. Pontoons must be tested for strength against hydrostatic pressure and have devices for mechanical slinging.

6.8. Before laying the pipeline in the deep water section, it is necessary to perform calculations of the stress-strain state of the pipeline for the main technological processes:

start of laying

· continuous laying of the pipeline with a bend on S-shaped or J-shaped curve;

laying of the pipeline on the bottom during a storm and its rise;

Completion of installation work.

6.9. The laying of the pipeline should be carried out strictly in accordance with the construction organization project and the work execution project.

6.10. During the laying of the pipeline, the curvature of the pipeline and the stresses arising in the pipeline must be continuously monitored. The values ​​of these parameters should be determined on the basis of load and deformation calculations before the pipeline is laid.

7. Coastal protection measures

7.1. The fastening of the coastal slopes after laying the pipeline is carried out above the maximum design water level and must ensure the protection of the coastal slope from destruction under the influence of wave loads, rain and melt water.

7.2. In the course of coastal protection works, experience-tested environmentally friendly designs should be used, technological processes and work should be carried out in accordance with the requirements of the "Technical Specifications for the Construction of a Pipeline at the Crossing of the Coastline and Coastal Protection Measures".

8. Construction quality control

8.1. Construction quality control should be carried out by independent technical departments.

8.2. To achieve the required quality of construction work, it is necessary to ensure quality control of all technological operations for the manufacture and installation of the pipeline:

· the process of delivery of pipes from the manufacturer to the installation site must guarantee the absence of mechanical damage on the pipes;

· quality control of concreted pipes should be carried out in accordance with the technical requirements for the supply of concreted pipes;

· incoming pipes, welding materials (electrodes, flux, wire) must have Certificates that meet the requirements of the technical conditions for their supply;

· when welding pipes, it is necessary to carry out systematic step-by-step control over the welding process, visual inspection and measurement of welded joints and check all circumferential welds by non-destructive control methods;

· insulating materials intended for assembly joints of pipes should not have mechanical damage. Quality control of insulating coatings should include checking the continuity of the coating using flaw detectors.

8.3. Marine earth-moving equipment, pipe-laying barges and their service vessels must be equipped with an automatic orientation system designed to constantly monitor the planned position of these technical equipment during their operation.

8.4. The control of the depth of the pipeline in the ground should be carried out using telemetry methods, ultrasonic profilers or diving surveys after the pipeline is laid in the trench.

If the depth of the pipeline in the ground is insufficient, corrective measures are taken.

8.5. During the laying of the pipeline, it is necessary to control the main technological parameters (the position of the stinger, the tension of the pipeline, the speed of the pipe-laying vessel, etc.) for their compliance with the design data.

8.6. To control the state of the bottom and the position of the pipeline, it is necessary to periodically conduct a survey with the help of divers or underwater vehicles, which will reveal the actual location of the pipeline (erosion, sagging), as well as possible deformations of the bottom along the pipeline caused by waves or underwater currents in this area.

9. Cavity cleaning and testing

9.1. Offshore pipelines are subjected to hydrostatic testing after being laid on the seabed in accordance with the requirements of the "Specifications for Testing and Commissioning of the Offshore Gas Pipeline", developed as part of the project.

9.2. Preliminary testing of the pipeline strings on shore is carried out only if the project provides for the production of pipeline strings on shore and their laying in the sea by dragging methods towards the pipe-laying vessel.

9.3. Prior to hydrostatic testing, it is necessary to clean and control the internal cavity of the pipeline using pigs equipped with control devices.

9.4. The minimum pressure during hydrostatic strength tests is assumed to be 1.25 times higher than the design pressure. In this case, the hoop stresses in the pipe during the strength test should not exceed 0.96 of the yield strength of the pipe metal.

The holding time of the pipeline under the pressure of the hydrostatic test must be at least 8 hours.

The pipeline is considered to have passed the pressure test if no pressure drops were recorded during the last four hours of testing.

9.5. The tightness test of the offshore gas pipeline is carried out after a strength test and a decrease in the test pressure to the calculated value during the time necessary to inspect the pipeline.

9.6. Removal of water from the pipeline must be carried out with the passage of at least two (main and control) piston-separators under the pressure of compressed air or gas.

The results of removing water from the gas pipeline should be considered satisfactory if there is no water ahead of the control piston-separator and it left the gas pipeline intact. Otherwise, the passage of the control piston-separator through the pipeline must be repeated.

9.7. If the pipeline breaks or leaks during testing, the defect must be eliminated and the offshore pipeline retested.

9.8. The offshore pipeline is put into operation after final cleaning and calibration of the internal cavity of the pipeline, initial diagnostics and filling of the pipeline with the transported product.

9.9. The results of the cavity cleaning and pipeline testing, as well as the removal of water from the pipeline, must be documented in acts in the approved form.

10. Environmental protection

10.1. In marine conditions, all types of work require a careful selection of technological processes, technical means and equipment that ensure the preservation of the ecological environment of the region. It is allowed to use only those technological processes that will ensure the minimum negative impact on the environment and its rapid recovery after the completion of the construction of the offshore gas pipeline system.

10.2. When designing an offshore gas pipeline system, all environmental protection measures must be included in a properly approved environmental impact assessment (EIA) plan.

10.3. When constructing a system of offshore gas pipelines, it is necessary to strictly comply with the environmental requirements of Russian standards. In water areas of commercial fishery importance, it is necessary to provide for measures for the conservation and restoration of biological and fish resources.

The dates for the start and end of underwater earthworks using hydromechanization or blasting are established taking into account the recommendations of the fisheries protection authorities, based on the timing of spawning, feeding, fish migration, as well as the development cycles of plankton and benthos in the coastal zone.

10.4. The EIA plan should include a set of design, construction and technological measures to ensure environmental protection during the construction and operation of the offshore gas pipeline system.

In the process of developing an EIA, the following factors are taken into account:

· initial data on natural conditions, background ecological state, biological resources of the water area, characterizing the natural state of the region;

· technological and design features of the offshore gas pipeline system;

· terms, technical solutions and technology for performing underwater technical works, a list of technical means used for construction;

· Assessment of the current and predicted state of the environment and ecological risk, indicating the sources of risk (technogenic impacts) and probable damages;

· basic environmental requirements, technical and technological solutions for environmental protection during the construction and operation of the offshore gas pipeline and measures for their implementation at the facility;

· measures to ensure control over the technical condition of the offshore gas pipeline system and prompt elimination of emergencies;

monitoring of the state of the environment in the region;

· the size of capital investments in environmental, social and compensation measures;

· Evaluation of the effectiveness of the envisaged environmental and socio-economic measures and compensations.

10.5. During the operation of the offshore gas pipeline system, it is necessary to predict the possibility of a pipeline rupture and product release with an assessment of the expected damage to the sea biota, taking into account the possible accumulation of fish (spawning, migration, feeding period) near the pipeline system site and to implement protective measures for the pipeline and the environment provided for such cases by the project.

10.6. To protect and preserve the natural environment in the sea area and in the coastal zone, it is necessary to organize constant supervision over compliance with environmental measures during the entire period of anthropogenic impact caused by the construction and operation of the offshore gas pipeline system.

Attachment 1 . Mandatory.

Designations and units of measurement

D - nominal diameter of the pipeline, mm;

t - nominal thickness of the pipeline wall, mm;

s x - total longitudinal stresses, N / mm 2;

s y - total hoop stresses, N/mm 2 ;

t xy - tangential shear stresses, N/mm 2 ;

K - design coefficient of reliability, taken according to;

s t - the minimum value of the yield strength of pipe metal, adopted according to state standards and specifications for steel pipes, N / mm 2;

P - design internal pressure in the pipeline, N/mm 2 ;

Ro - external hydrostatic pressure, N / mm 2;

Px - drag force, N/m;

Рz - lifting force, N/m;

Ri - inertial force, N/m;

G - pipeline weight in water (negative buoyancy), N/m;

m - reliability factor, taken equal to 1.1;

f is the coefficient of friction;

Рс - calculated external hydrostatic pressure on the pipeline, taking into account the ovality of the pipe, N / mm 2;

Рсг - critical external pressure for a round pipe, N / mm 2;

Ru - external pressure on the pipeline, causing the fluidity of the material

pipes, N / mm 2;

PP - external hydrostatic pressure at which the pipe collapse that occurred earlier will spread, N / mm 2;

e o - allowable bending deformation for the pipeline;

e c - critical deformation of the bend, causing collapse as a result of pure bending of the pipe;

u- Poisson's ratio;

E - Young's modulus for pipe material, N / mm 2;

H - critical water depth, m;

g - acceleration of gravity, m / s 2;

r- density of sea water, kg/m 3 ;

U - ovality of the pipeline;

R - permissible radius of curvature of the pipeline when laying at great depths of the sea, m.

Annex 2.
Recommended.

Technical terms and definitions

Offshore gas pipeline - a horizontal part of the pipeline system located below the water level, including the pipeline itself, electrochemical protection devices on it and other devices that ensure the transportation of gaseous hydrocarbons under a given technological regime.

Protected zone of coastal sections of the gas pipeline - sections of the main gas pipeline from coastal compressor stations to the water's edge and further along the seabed, at a distance of at least 500 m.

Pipe elements - details in the construction of the pipeline, such as flanges, tees, elbows, adapters and valves.

Weight Coating - a coating applied to a pipeline to provide it with negative buoyancy and protection against mechanical damage.

Negative pipeline buoyancy - downward force equal to the weight of the pipeline structure in air minus the weight of the water displaced in the volume of the pipeline immersed in it.

Minimum yield strength - the minimum yield strength specified in the certificate or standard to which the pipes are supplied.

In calculations, it is assumed that at the minimum yield strength, the total elongation does not exceed 0.2%.

Design pressure - pressure, taken as a permanent maximum pressure exerted by the transported medium on the pipeline during its operation and for which the pipeline system is designed.

pressure surge - accidental pressure caused by failure of the steady state flow in the piping system shall not exceed the design pressure by more than 10%.

Overpressure - the difference between two absolute pressures, external hydrostatic and internal.

Test pressure - normalized pressure at which the pipeline is tested before putting it into operation.

Leak test - hydraulic pressure test, which establishes the absence of leakage of the transported product.

Test of endurance - hydraulic pressure test, which establishes the structural strength of the pipeline.

Nominal pipe diameter - the outside diameter of the pipe specified in the standard to which the pipes are supplied.

Nominal wall thickness - pipe wall thickness specified in the standard to which pipes are supplied.

Offshore pipeline reliability - the ability of the pipeline to continuously transport the product in accordance with the parameters established by the project (pressure, flow, and others) for a specified period of operation under the established control and maintenance regime.

Permissible stresses - maximum total stresses in the pipeline (longitudinal, ring and tangential), allowed by the standards.

Burying the pipeline - the position of the pipeline below the natural level of the seabed.

Depth value - the difference between the levels of the upper generatrix of the pipeline and the natural level of the seabed.

The length of the sagging section of the pipeline - the length of the pipeline that is not in contact with the seabed or with support devices.

Offshore pipeline laying - a complex of technological processes for the manufacture, laying and deepening of the offshore pipeline.

Annex 3.
Recommended.

Regulatory documents used in
development of these rules and regulations:

1. SNiP 10-01-94. "The system of normative documents in construction. Basic provisions" / Ministry of Construction of Russia. Moscow: GP TsPP , 1994

2. SNiP 2.05.06-85 *. "Main pipelines" / Gosstroy. M.: CITP Gosstroy, 1997

3. *. "Rules for the production and acceptance of work. Main pipelines" / Gosstroy. Moscow: Stroyizdat, 1997

4. SNiP 2.06.04-82 *. "Loads and impacts on hydraulic structures (wave, ice and ships)" / Gosstroy. M.: CITP Gosstroy, 1995

5. "Safety Rules for the Exploration and Development of Oil and Gas Fields on the Continental Shelf of the USSR", M.: "Nedra", 1990;

6. "Safety regulations for the construction of main pipelines". M.: "Nedra", 1982;

7. "Rules for the technical operation of main gas pipelines", M.: "Nedra", 1989;

8. US Standard "Design, construction, operation and repair of offshore pipelines for hydrocarbons", AR I - 1111. Practical recommendations. 1993.

9. Norwegian Standard "Det Norske Veritas" (DNV) "Regulations for Subsea Pipeline Systems", 1996

10. British standard S8010. "A practical guide for the design, construction and laying of pipelines. Submarine pipelines". Parts 1, 2 and 3, 1993

11. API 5 L . "US specification for steel pipes". 1995

12. API 6 D . "US Specification for Pipe Fittings (Valves, Plugs and Check Valves)". 1995

13. US Standard AS ME B 31.8. "Regulations for Gas Transportation and Distribution Pipeline Systems", 1996

14. US standard SS-SP-44. "Steel flanges for pipelines", 1990

15. International standard ISO 9000"Quality Management and Quality Assurance", 1996

First offshore oil pipelineappeared in the early 50s of the last century in connection with the beginning and development of oil production in the Caspian Sea. At the same time, the firstoffshore gas pipelines. All of them served to transport oil and gas from production sites to onshore areas.

Today, these tasks performed by underwater pipelines have been supplemented by the functions of efficient offshore main pipelines designed to ensure the transportation of gas and oil over long distances. Their construction, despite the technological difficulties and high cost, is fully justified in today's turbulent political environment. The principle is simple. Some countries want to have a stable income from the sale of oil and gas, while others are guaranteed to receive products without interruption along the hydrocarbon transit route. Offshore pipelines completely eliminate all geopolitical risks associated with transit through other countries.

Construction of offshore pipelines begins at the pipe manufacturing plant, where a three-layer coating consisting of epoxy, adhesive and polyethylene is applied to the outer surface of the pipes. In the same place, to increase the throughput of the pipe and additional insulation, a special red-brown epoxy paint is applied to the inner surface. The next steps are the installation of cathodic protection against corrosion and concreting by covering the pipe with a layer of concrete applied to a reinforced frame or wire mesh, sometimes filled with iron ore. At the same time, the mass of one 12-meter pipe can reach 24 tons.

Concrete additionally protects the pipe from mechanical damage, and iron ore as a filler makes the structure heavier and allows it to lie stably on the seabed. Only the ends of the pipes remain unprotected for subsequent welding.

Welding of pipes to the main line of the gas pipeline and subsequent insulation of the joints is carried out on a special pipe-laying vessel, which is a large non-self-propelled barge moving with the help of a tugboat and a special anchor winch.

Undoubtedly, the most vulnerable point of offshore main pipelines is the underwater joint. That is why its isolation is given maximum attention.The underwater joint isolation technology includes the following steps:

Shot blast cleaning of steel pipe joints by supplying cast iron shot, which is thrown with force onto the surfaces to be cleaned by a special shot blasting unit. This is the most effective way to remove scale, rust and other contaminants from pipe joints.

induction heating joints of pipes before insulation, providing higher productivity, faster and more uniform heating - in comparison with the use for these purposes.

Installation of a thermoshrinkable cuff TIAL-MGP- today one of the most reliable solutions that provides reliable long-term insulation of joints of underwater pipelines. It is produced according to the classical TIAL cuff shrinkage technology.

Primer is applied first:



The cuff is shrinking with propane burners:


Quality control using a spark flaw detector and an adhesive meter.


Installation of the lining casing, inside which the PPU components are poured.



The technology used to isolate the underwater joint using the TIAL-MGP collar is in demand and is widely used in the construction of modern offshore underwater pipelines.

TIAL materials included inRussian Maritime Register of Shipping , chapter - objects of observation,Offshore subsea pipelines:

Sleeve for anti-corrosion protection

weld

At present, the issue of laying the second line of the Nord Stream (Nord Stream) has become topical. The laying of the pipeline on the seabed provides for the work of pipe-laying vessels.

Pipe-laying vessels use a variety of pipeline-laying methods. These main methods include the methods of laying pipelines using the S-Lay, J-Lay and Reel-Lay methods. Each of these methods has its own characteristics. Fig. 1-6 shows the layout of pipelines by each of the methods, with its own advantages and disadvantages.

Tensioners - a device for creating pipeline tension forces; S-lay barge - a pipe-laying barge operating according to the S-lay method; Stinger - stinger (lowering boom): Sagbend region - pipeline bend area; Seabed - seabed; Touchdown point - the point where the pipeline touches the bottom; Unsupported span - unsupported span; Waterline - water level; Overbend region - a section dangerous from the point of view of a possible fracture of the pipeline.

S-lay pipe laying is mainly practiced in shallow water, and the laying speed of this method is approximately 6.5 km/day. Bending moments in this laying method become a major factor. Therefore, a long and large tensioner is required.

The method is unacceptable for laying pipelines at great depths. A tensioner and stinger are needed to reduce bending moments.

Before laying the pipeline on the seabed, each segment of the pipeline is welded, inspected and covered with a protective layer, passing through the welding, inspection, coating stations on board the vessel.

The assembled pipeline is lowered from the stern of the vessel, the tension force is provided by the tensioner, and the pipeline itself is supported by the launching boom, and the curvature of the pipeline descent is strictly controlled. The pipeline is then bent under its own weight and laid on the bottom.


Fig.3. J-Lay Pipeline Laying Vessel


Fig.3. J-Lay pipeline laying vessel.

J-Lay Tower - a tower for laying pipelines using the J-Lay method; J-Lay DP Vessel - a vessel equipped with a dynamic positioning system, from which the pipeline is laid using the J-Lay method; Thrusters - propeller-steering columns; Unsupported span - unsupported span; Sagbemd region - pipeline bend section; Seabed - seabed; Touchdown point - the point where the pipeline touches the bottom; Waterline - water level.

While the S-lay method is only suitable for shallow water, the J-lay method can be used in deeper water. This is possible due to the relatively short section of sagging pipeline and the lower required tensile forces during installation.

Installation and installation is carried out in an almost vertical manner, with the pipeline laid on the seabed with a single bend radius. The paving speed is 3.2 km/day. When laying, each pipe segment is first raised to a vertical position and then they are welded together.

Inspection and coating is also carried out on board. When the vessel moves along the route, the pipeline slowly sinks to the bottom. Since the pipeline, unlike the S-lay method, has only one bend, the risk of structural damage due to a bend in the pipeline is minimal.



Water - water level; Touchdown point - the point where the pipeline touches the bottom; Tensioner - a device for creating tension forces; Stinger - stinger; Reel - drum; Reel-Lay Barge - a barge for laying pipelines using the Reel-Lay method; Pipeline - pipeline.

The Reel-lay method of laying the pipeline is considered the most effective. The paving speed is 3.5 km/h. It is suitable for laying pipelines with a pipe diameter of less than 18 inches and a pipe diameter to wall thickness ratio (D/t) between 20 and 24.

The main advantage of this method over the previous ones is that the entire production process, including welding, inspection and coating, is carried out on shore and not on board, which significantly reduces production time and costs.

Before laying the pipeline, it is wound on a large-diameter drum mounted on board the vessel. From this drum, the pipeline is laid to the bottom.

From time to time, innovative designs of pipe-laying vessels appear, such as the Lewek Constellation.

Undersea pipeline contractors are increasingly opting to use a variety of pipe-laying methods on board ship, as oil and gas field infrastructure for the most part consists of different pipelines that require different pipe-laying methods. This imposes specific requirements on pipe-laying vessel projects: more flexibility in the use of various technologies, more cost-effectiveness for operations at any depth, equipment suitable for the installation of various pipelines.



Fig.7. Innovative pipe-laying vessel "Lewek Constellation" laying the pipeline using the Multy-Lay method.

Aligner Wheel - leveling drum; 3000 mT Main Crane - main crane with a lifting capacity of 3000 t; 4x1200 mT Storage reels - four drums for storing pipelines weighing 1200 tons each; 2x1250 mT Carousels - two underdeck swivel drums for pipelines weighing 1250 tons each; 60 mT PLET (pipeline end termination) handling system and work station Moon pool 19 m L x 8 m W - a mine with a free water surface, dimensions: length 19 m, width 8 m; 2x600 mt Winches - two winches with a pulling force of 600 t; 2x20mT Storage Reel - two drums for storing pipelines weighing 20 tons each; 125 mT Secondary Winch - auxiliary winch with a force of 125 t; 2 WROVs TMS (Thether Management System) - two underwater remote-controlled vehicles (UA) with a UA cable control device; Helipad Sikorsky 61N&S92 - helipad for Sikorsky 61N and S92 helicopters; Optional J-Lay Module - an optional module for laying pipes using the J-Lay method; 2x400mT Tensioners - two devices for creating tension forces of 400 t each; Rigidpipe Straightening - straightening device; 80mT Crane - crane with a lifting capacity of 80 t.

The presence on board the vessel of the PA provides the possibility of inspection and, if necessary, carrying out underwater work. PA is a necessary component of the equipment of the pipe-laying vessel. A free-water mine with hoisting equipment placed is also a complex engineering structure.



Fig.8. Mine with free water surface and lifting equipment of the vessel "Lewek Constellation". The equipment of the mine must ensure the operation of the ROV at a depth of 4000 m in severe weather conditions.

Cursor Winch - winch; Latch Beam and Subsea Snubber - retractable beam and shock absorber for underwater work; Cursor Frame - frame; HPU (Hydraulic Power Unit) for Hatches and Skidding Pallet - hydraulic drive for hatch covers and pallets moving along guides; Active Heave Compensation ROV Winches - tripping winch for ROVs with active roll compensation; Umbilical Sheave - umbilical sheave; Cursor Sheave - pulley; Cursor Rails and Parking Pads - guides and trays for injection molding machines; Latch Beam Umbilical Winch - umbilical winch; Fall Safe Foldable Top Moon Pool Hatch - a safe folding top hatch of a mine with an open water surface; Skidding Pallet - a pallet moving along guides; ROV Moon Pool is a shaft with an open water surface for lowering and raising ROVs.

Text: Oleg Gubarev

The development of oil and gas fields located on the shelf is impossible without the construction of pipelines. In modern offshore oil fields, some subsea pipelines connect individual offshore platforms with a central reservoir and a floating berth, which is equipped for tanker mooring, others connect the reservoirs directly with the onshore oil storage.

The offshore pipeline construction technology provides for the following stages: excavation, preparation of the pipeline for laying, its laying, backfilling and protection from damage.

The need to bury offshore pipelines is due to the fact that otherwise they can be damaged when moving coastal ice, trawls, ship anchors, etc. During earthworks, devices are used that develop a trench, both from the surface of the water and in a submerged position. The former include floating dredgers, jet installations, clamshell dredgers, pneumatic and hydraulic soil pumps. To the second - various kinds of autonomous devices operating under water.

So, in Italy, the S-23 dredger was created, which can develop trenches at a depth of up to 60 m. Digging of a trench is carried out by a milling ripper at a speed of up to 130 m/h in soils of medium density. The parameters of the trench to be torn off are as follows: depth - up to 2.5 m, width along the bottom - from 1.8 to 4.5 m.

In Japan, a bulldozer and an excavator have been developed for working under water at a depth of up to 70 m. The bulldozer weighing 34 tons has a powerful engine and moves on tracks. Unlike dredgers, it can develop dense soils.

The underwater excavator is designed for excavation of trenches during the construction of offshore pipelines, foundation pits for the foundations of various offshore structures and dredging. The speed of its movement along the bottom is 3 km / h. The excavator is operated by two operators from a surface vessel.

Before laying, a protective coating is applied to the pipeline and it is surcharged against ascent. World experience in the construction of offshore pipelines has shown that the best protective coating for them and at the same time a weight is a concrete coating.

Laying of offshore pipelines is carried out by pulling, or from the sea surface by gradual build-up.

The pulling scheme is shown in fig. 4. The pipeline 1 moves along the roller descent path 5. The traction force along the cable 2 is transmitted from the winch installed on the vessel 3. The vessel is held by anchors 4. The pulling method is simple, ensures the laying of the pipeline exactly along the route. However, it is applicable when laying pipelines up to 15 km long.

The scheme of laying from the sea surface with a gradual build-up (Fig. 5) is the most widely used. The pipe-laying vessel 4 is fixed on anchors 6, each of which can withstand a force of up to 10 tons. A stock of concreted pipes is created on the vessel, sections of which are 36 m long and delivered by special transport vessels. The length of the pipe-laying vessel makes it possible to connect sections in a 180 m long string.

Pipeline 1 is laid as follows. On the ship 4, the next lash is welded, the joints are insulated, concreted and equipped with floats 2. The lash is joined to the end of the pipeline laid earlier and held by the tensioner and a special rigid attachment 3. The angle of inclination of this attachment is chosen so as to minimize the stress in the lowered pipeline. The joint is isolated and concreted, after which the whips are lowered into the water on pontoons. Unslinging of pontoons is carried out automatically at a predetermined depth.

The ship "Suleiman Vezirov" with a displacement of 8900 tons per day can lay 1.2 km of welded pipes with a diameter of 200 ... 800 mm under water. The pipe-laying vessel of the Vartsila company with a displacement of 41,000 tons allows laying up to 2.5 km of a pipeline with a diameter of 530 mm per day at a depth of up to 300 m. The stock of pipes is enough for them to work for 5 ... 10 days.

Laying offshore pipelines with a preliminary excavation of the trench is associated with significant costs. Trenching at sea costs a hundred times more than on land. In addition, it is quite difficult to accurately lay the pipe in a trench from the side of a ship rocking on the waves.

It is cheaper and easier to bury a steel pipeline already laid on the bottom into the ground. For this, special underwater pipe deepening units have been designed. Their main element is a trolley that rolls along the pipe.

Fig 4 - Scheme of pulling through the pipeline: 1 - pipeline; 2 - cable; 3 - the vessel on which the winch is installed; 4 - anchors.

Figure 5 - Scheme of laying the pipeline by a pipe-laying vessel: 1 - pipeline; 2 - floats; 3 - rigid prefix on which the end of the pipeline lies; 4 - pipe-laying vessel; 5 - crane; 6 anchors.

Various deepening devices are fixed on the trolley: jet nozzles, plows, cutters or rotary wheels. The energy for their drive is supplied from the ship via a cable line, which reaches a length of 1 km or more. Recently, pipe deepeners are equipped with underwater cameras, which makes it possible to control their operation from the surface.

Rock placement is most commonly used to protect offshore pipelines from damage in the coastal zone. The stone is dumped from the side of barges with inclined bunkers and vibrators. Vessels with a smooth deck are often used, overboard of which stones are dumped by a bulldozer. The accuracy of such filling is low. Therefore, at present, the role of a bulldozer is performed by special shields controlled by hydraulic cylinders connected to a computer. Such devices allow high-quality backfilling of the pipeline with waves as high as a two-story house and wind speeds up to 15 m / s.

Another way to protect offshore pipelines from damage is to lay asphalt over the trench. Asphalting of the seabed is carried out using a floating asphalt plant. From its deck, the finished mixture is fed to the bottom through a vertical pipe, in the center of which a heater pipe passes so that the asphalt does not have time to cool due to contact with relatively cold water. At the bottom, the asphalt is leveled and compacted by an automatic device similar to those used for asphalting squares and streets. In one pass of the stacker, an asphalted area with a width of 5 m and a thickness of 85 mm appears on the bottom.

When designing and constructing pipelines in the Arctic, specialists need to solve a number of unique challenges that the oil and gas industry has not yet encountered when implementing projects in other regions of the world. These include ice gouging, ice erosion of the bottom, ice flow to the shore, stability of coastal soil, and ice melting. Often there is a need to develop special methods and equipment designed to work in remote regions (in the absence of any kind of infrastructure), with a limited duration of the construction season, in severe weather conditions and difficult ice conditions.

Specificity of the Arctic

All of the above factors must be taken into account when designing pipelines in addition to the volumes of oil or gas pumped, the strength of the soil and the stability of the seabed. Other factors include environmental conditions such as sea depth, temperature, marine life, type of work performed (for example, marine transportation of hydrocarbons or industrial exploitation of a field).

Plowing of the seabed occurs when ice hummocks move under the influence of wind or a neighboring ice field, while the keel of the hummock is in contact with the bottom. Ice erosion of the bottom is formed during the spring thaw, when water from overflowing rivers enters the surface of the sea ice and seeps into the sea through polynyas and cracks. Seeping water creates whirlpools that affect the seabed and the underlying pipelines.

The coastline and barrier islands are exposed to mobile ice during its freezing or opening. As a result, surges are formed along the coastline, the maximum height of which can be at the level of the waterline or the level of the coast, which leads to the emergence of ice blocks on the coast.

In the section of the offshore pipeline, when connecting to the onshore pipeline, some distance must be provided in its design to protect the pipeline from damage when ice comes ashore. Gravelling, loading, revegetation are necessary to prevent accelerated erosion of the site at the site of the pipe landfall.

When calculating the pipeline landfall distance, the retreat of the shoreline must also be taken into account. In shallow water, the bottom soil freezes in winter. Under the layer of moving ice is permafrost. The thermal effect of the pipeline on frozen ground must also be taken into account in the design so that thawing of the soil does not affect the integrity of the pipeline.

Installation of structures

Despite extensive experience in building pipelines in various regions of the world, experience in building pipeline systems in the Arctic is limited to three projects: Northstar, Oooguruk and Nikaitchuq. All three pipelines were laid off the ice during the winter construction season. The pipelines were buried to avoid damage from ice gouging.

In Arctic conditions, equipment placed on ice during the winter construction season was used to lay pipelines in shallow water. Although no deep-water Arctic pipelines have been built so far, pipe-laying barges have been used at great depths in subarctic regions (where there was no ice).

In non-Arctic regions, which are nevertheless subject to ice gouging, industry experience in pipeline construction has been accumulated on the Russian shelf (on Sakhalin Island), where laying was carried out from ships. As part of the Sakhalin-2 project, platforms were installed at the Piltun-Astokhskoye and Lunskoye fields, connected to the shore by a pipeline system with a total length of 262 km. In addition to being designed to withstand earthquakes, the pipelines were buried 35 m to avoid damage from ice gouging.

When determining the depth of pipelines, it is necessary to take into account a number of factors, such as coastal erosion, the movement of dunes, as well as the plowing of the seabed by ice hummock keels. To clarify the magnitude and frequency of ice gouging and erosion in the design of pipeline systems, it is necessary to use special programs designed to study the seabed. Usually, vessels equipped with multibeam side and bottom profiling sonars are used for their implementation. In case of ice erosion of the bottom, helicopters are used before the open water season.

After collecting the data, it is necessary to process and analyze them to develop appropriate design criteria. In the past, data processing was a laborious and lengthy process. Currently, special computer programs are used for this purpose. Detailed databases are created that contain information on each object, indicating its location, depth, width, length, etc. Each such data set contains the most important parameters used in the design and covers a wide range of depths with information on the frequency and magnitude of ice gouging.

Depth prediction

The following factors influence the parameters of pipeline systems deepening: ice gouging depth, trench geometry, deformations under gouging furrows, soil type and its shear strength. The main task is to eliminate and study the uncertainties associated with depth calculations. To do this, it is necessary to determine the design plowing depth based on field data and physical constraints such as soil and ice strength, and then determine the effect of ice on the soil and the load on the pipeline using coupled (refined soil model) and uncoupled (simplified soil model) analyzes. Typically, the pipeline is designed so that it does not come into contact with the keel of the ice hummock. Also considered are the loads on the trench and soil during pipeline laying and pipeline design criteria in terms of deformations and loads that affect the structural integrity of the pipeline.

The associated model is a 3D model in which the soil is modeled as a continuum and the plowing process is explicitly modeled in the soil environment. Unbound models are primarily 2D cantilever models in which the ground is modeled by springs. The transient displacement (deformations under the plunging furrows) superimposed on the base of the springs models the effect of the plowing process on the pipeline in uncoupled models; spring characteristics are a simplified representation of the behavior of the ground environment in terms of load/displacement curves.

Collaborative industry projects provide a better understanding of ice plowing processes and the required depth of pipelines. A recently completed study, Assessing and Addressing Risks in Pipeline Construction, which is one of the joint industry projects, aimed at creating engineering models, developing design procedures and summarizing best practices in the field of protection of pipelines from keel loads. Under the leadership of the Canadian Center for Hydraulic Research, a collaborative industry project was recently completed to model the interaction of ice hummock keels and the seabed. The purpose of this study was to study the process of ice gouging and its parameters - strength, depth and their relationships in sandy soil conditions.

Mechanical integrity of pipelines and its monitoring

Pipeline leak detection systems are divided into software and hardware systems. As part of software systems, data is collected from sensors that are commonly used in the operation of pipelines (pressure, temperature, flow sensors) to detect and localize potential leaks based on software algorithms. Leak monitoring hardware systems use sensors that are not associated with the normal operation of pipelines. Evaporative monitoring and fiber optic technologies are being introduced to improve currently available monitoring software systems.

Innovation Frontiers

It is possible to lay pipelines over short distances through wells drilled using trenchless laying methods. Such methods can be divided into two main categories: directional drilling methods and microtunneling.

Directional drilling is used in the construction of river crossings and the laying of short sections of pipelines through inaccessible terrain. With this method, the directional drilling rig is located on one side of the river. It drills the same well as when drilling for oil. The well is usually drilled to a depth of the order of several meters below the ground surface with access to the other bank. The pipeline or bundle of pipelines is then pulled through the well. This method minimizes surface damage: the volume of overburden per linear meter of pipeline allows the pipeline to be buried to a depth of several meters.

It should be noted that when laying the pipeline by this method, drilling mud (bentonite) is used. The exit of the drilling fluid in unpredictable places and the contamination of the environment with the fluid are the main disadvantages of this method. In addition, the use of directional drilling may be problematic due to the characteristics of the soils in the Arctic.

The use of this method is limited by the stability of the borehole walls and the force required to push the drill string into the borehole during drilling, and after it is completed, to push the pipeline through the borehole. Greater lengths can be achieved by building caissons in shallow water every 2 km of the route. This technology has been successfully applied to connect the Mittelplate platform in the German sector of the North Sea to onshore facilities via an 11 km pipeline.

Push-pipe microtunneling technology has been used at landfall sites (eg the Europipe section). However, with this method, the length is still limited to a few kilometers, mainly due to the need to push a support pipe from one end of the pipeline. Conventional tunneling technology using tunneling machines allows the creation of support structures directly behind the face, as a result of which it is possible to increase the length of the tunnel itself. To do this, its diameter should be several meters (necessary for installing equipment). However, the use of this method for the construction of pipelines, according to experts, can hardly be called practical.

 

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