Subsea pipelines play a crucial role in the offshore oil and gas industry, providing an efficient means of transporting hydrocarbons from offshore production facilities to onshore processing plants. This blog post delves into the technical aspects of various subsea pipeline installation techniques, construction methods, protection requirements, health, safety and environmental implications, and legal, quality, and commercial considerations for offshore construction.
The S-lay method is an established technique for subsea pipeline installation, particularly in shallow to moderately deep waters. The process involves the following steps:
- Pipe preparation: Before installation, the pipes undergo preparation, which includes pre-welding, bevelling, and applying anti-corrosion coatings.
- Lay vessel setup: The lay vessel is equipped with a stinger, a structure that extends from the back of the vessel and supports the pipeline as it is lowered to the seabed. The stinger's angle can be adjusted to control the bending radius of the pipeline.
- Pipe assembly: Pipe sections are placed on the lay vessel's firing line, where they are aligned and welded together. High-quality welding is crucial to ensure the structural integrity of the pipeline. Both manual and automatic welding techniques can be employed, depending on project requirements and pipe dimensions.
- Non-destructive testing (NDT): Following welding, the welds undergo NDT to detect any flaws or defects. Common NDT methods include radiographic testing (RT), ultrasonic testing (UT), and magnetic particle inspection (MPI).
- Field joint coating: After NDT, the weld area is cleaned, and a protective coating is applied to the field joint, providing corrosion resistance and mechanical protection.
- Pipeline installation: As the vessel moves forward, the welded pipeline is lowered to the seabed in an S-shaped curve, supported by the stinger. The pipeline's tension is carefully controlled to prevent overbending or excessive stress on the pipe.
- High productivity: The S-lay method enables simultaneous welding, NDT, and field joint coating, which contributes to increased productivity and shorter installation times.
- Cost-effective: This method is particularly cost-effective for large diameter pipelines and long-distance installations, as it requires fewer specialised equipment and vessels compared to alternative techniques.
- Versatility: The S-lay method is suitable for a wide range of pipe sizes, materials, and water depths, making it a popular choice for diverse offshore projects.
- Deck space: The S-lay method requires substantial deck space on the lay vessel for pipe storage, handling, and assembly. This constraint can limit the pipe-laying capacity and necessitate additional pipe transportation logistics.
- Water depth limitations: In deeper waters, maintaining the S-curve shape and controlling the tension on the pipeline can become challenging. For deep and ultra-deep water installations, alternative methods such as J-lay or reel-lay may be more suitable.
Overall, the S-lay method offers a proven and reliable solution for subsea pipeline installation in a wide range of scenarios. Engineers and project managers must carefully consider the project-specific requirements and constraints to determine the most appropriate installation technique.
The J-lay method is a specialised technique for subsea pipeline installation that is well-suited for deep and ultra-deep water environments. The process involves the following steps:
- Pipe preparation: Similar to the S-lay method, pipes undergo pre-welding, bevelling, and anti-corrosion coating application before installation.
- Lay vessel setup: The lay vessel is equipped with a J-lay tower, a vertical structure that supports the pipeline as it is assembled and lowered to the seabed. The tower often features adjustable tensioners to maintain precise control of pipe tension during installation.
- Pipe assembly: Pipe sections are lifted vertically and positioned within the J-lay tower, where they are aligned and welded together. Due to the vertical orientation, the welding environment is typically more stable, allowing for high-quality welds.
- NDT: Welds are inspected using NDT methods such as radiographic testing (RT), ultrasonic testing (UT), or magnetic particle inspection (MPI) to detect flaws or defects.
- Field joint coating: After successful NDT, the weld area is cleaned, and a protective coating is applied to the field joint.
- Pipeline installation: The pipeline is lowered to the seabed in a J-shaped curve, with the J-lay tower supporting the weight of the suspended pipe. This reduces tension on the pipeline, allowing for installation in deeper waters.
- Precise control: The J-lay method allows for precise control of pipe tension and curvature, making it suitable for deep and ultra-deep water installations.
- High-quality welding: The vertical positioning within the J-lay tower provides a stable welding environment, which can result in higher quality welds compared to other methods.
- Handling capacity: The J-lay method can accommodate larger pipe diameters, heavier wall thicknesses, and more complex pipe designs, such as pipe-in-pipe or pipe bundles.
- Lower installation rates: Due to the vertical assembly process, the J-lay method typically has lower installation rates compared to the S-lay method.
- Specialised vessels: J-lay installations require specialised vessels with J-lay towers, which can limit availability and increase project costs.
The J-lay method offers a robust solution for deep and ultra-deep water subsea pipeline installations. It provides precise control over pipe tension and curvature, enabling successful installations in challenging environments. However, the method's limitations, such as lower installation rates and the need for specialised vessels, must be considered when selecting the most appropriate installation technique for a specific project.
The reel-lay method is an efficient and cost-effective technique for subsea pipeline installation, particularly suited for small to medium-diameter pipelines and flexible products like umbilicals and flowlines. In this method, pre-welded pipe joints are spooled onto a large reel on the installation vessel. The pipeline is then unspooled and straightened as it is deployed onto the seabed.
- Reel-lay process: The reel-lay process begins with welding and non-destructive testing of pipe joints onshore or on the installation vessel. The pipe joints are then spooled onto a large reel on the vessel. During installation, the pipeline is unspooled from the reel, passes through a straightening system to remove any residual curvature, and is guided by a stinger or ramp onto the seabed. The installation vessel moves forward, maintaining tension and controlling the pipeline's position and orientation.
- Reel-lay market and vessels: The reel-lay market caters to the installation of small to medium-diameter pipelines, as well as flexible products such as umbilicals, flowlines, and risers. Reel-lay vessels are specifically designed for this method, featuring large reels, straightening systems, and handling equipment. Key players in the reel-lay market include TechnipFMC, Saipem, and Subsea 7, among others.
- Special considerations: The reel-lay method has certain unique considerations that must be addressed during pipeline installation. These include:
a. Reel capacity: The reel's capacity dictates the maximum length of the pipeline that can be installed in a single deployment. This factor depends on the pipeline's diameter, wall thickness, and material properties.
b. Residual stress and fatigue: The pipe bending and straightening process can introduce residual stresses and fatigue. Engineers must ensure that the pipeline's material properties and design factors are compatible with the reel-lay process to avoid compromising the pipeline's integrity.c. Onboard welding and inspection: In cases where pipe joints are welded and inspected onboard the vessel, adequate space and facilities must be provided to accommodate these operations and ensure the pipeline's quality.
The reel-lay method offers an efficient and cost-effective solution for subsea pipeline installation, particularly for small to medium-diameter pipelines and flexible products. With careful consideration of reel capacity, residual stresses, fatigue, and onboard welding and inspection requirements, the reel-lay technique can successfully support a wide range of subsea pipeline projects.
Bundles and Towed Installation
Bundles and towed installation is an integrated approach for installing subsea pipelines, umbilicals, and control systems within a single structure. The process typically involves the following steps:
- Bundle design: Bundles are custom-designed for each project, taking into account pipeline diameter, wall thickness, insulation requirements, and the inclusion of additional components such as control lines or umbilicals.
- Bundle fabrication: Bundle components are fabricated onshore in a controlled environment. The pipes and umbilicals are assembled into a single structure, supported by a carrier pipe or beam structure. The bundle assembly is typically insulated and may also include heating systems for flow assurance.
- Towhead structures: At each end of the bundle, towhead structures are attached to provide connection points for the subsea pipelines and umbilicals. Towheads also contain valves, manifolds, and other necessary subsea equipment.
- Towing methods: The completed bundle is launched into the water and towed to the installation site using surface or submerged towing methods. Surface towing involves floating the bundle on the water surface, while submerged towing uses buoyancy elements to control the bundle's depth.
- Bundle installation: Once the bundle reaches the installation site, it is carefully lowered onto the seabed using anchor handling vessels or installation barges. The towheads are then connected to the subsea infrastructure.
- Efficient installation: Bundles and towed installation enable the simultaneous installation of multiple components, reducing offshore installation time and costs.
- Onshore fabrication: Fabricating bundles onshore allows for greater quality control and reduced weather-related risks during assembly.
- Flow assurance: Bundles can incorporate insulation and heating systems to maintain optimal flow conditions for hydrocarbon transportation.
- Limited to relatively shallow water depths due to towing constraints.
- Requires specialized equipment and vessels for bundle fabrication, towing, and installation.
- Not suitable for all pipeline designs and configurations.
Installation of Flexibles
Flexible pipelines, also known as unbonded flexible pipes, are used in subsea applications where conventional rigid pipelines are not suitable due to dynamic loads, complex seabed terrain, or thermal expansion requirements. The installation process typically involves the following steps:
- Flexible pipe manufacturing: Flexible pipes are composed of multiple layers, including an inner carcass, pressure sheath, tensile armour layers, and an outer protective layer. These layers provide flexibility, strength, and protection against external damage.
- Reel-lay or S-lay installation: Flexible pipelines can be installed using reel-lay or S-lay methods, depending on the project requirements and available installation vessels.
- Umbilical cables and bonded hoses: In addition to flexible pipelines, other flexible components such as umbilical cables (for power, communication, and control) and bonded hoses (for fluid transfer) can also be installed.
- Adaptability: Flexible pipelines can accommodate complex seabed terrain, thermal expansion, and dynamic loads, making them suitable for challenging subsea environments.
- Reduced installation time: The flexibility of the pipes allows for faster installation compared to rigid pipelines.
- Lower maintenance: Flexible pipelines can better withstand fatigue and stress, reducing the need for maintenance and repairs.
- Higher material and manufacturing costs compared to rigid pipelines.
- Susceptible to damage from external threats, such as dropped objects, fishing gear, or anchor drags.
- Limited service life compared to rigid pipelines, depending on operating conditions and material selection.
Landfalls mark the critical transition point between subsea pipelines and onshore facilities. Various techniques are used to achieve a secure connection while minimising environmental impact and ensuring pipeline integrity.
- Pull ashore into cofferdam: A cofferdam, a temporary watertight structure, is constructed around the landfall area. The pipeline is pulled ashore through a trench and into the cofferdam, where it is connected to the onshore facilities. Once the connection is complete, the cofferdam is removed, and the trench is backfilled.
- Pull ashore from onshore construction site: In this method, the pipeline is assembled onshore and pulled into the water using winches or other pulling equipment. This technique is typically used for smaller pipelines or when environmental constraints limit access to the landfall area.
- Directionally-drilled landfalls: Directional drilling technology allows for a more precise and controlled installation of pipelines under sensitive coastal areas, minimising environmental impact. The pipeline is pulled through a pre-drilled hole, connecting the subsea and onshore sections.
Tie-ins are essential to connect separate pipeline sections or to join new pipelines to existing infrastructure. Several methods can be employed depending on project requirements and water depth:
- Flanged connection by diver: Divers connect the pipeline sections using bolted flanges. This method is generally used in shallow water installations and requires skilled divers to perform the task efficiently.
- Hyperbaric welding: In this technique, divers perform welding operations within a hyperbaric chamber, which is pressurised to match the surrounding water pressure. This method is suitable for deeper water installations and provides a high-quality welded connection.
- Diverless tie-ins: Remotely operated vehicles (ROVs) or other remote-controlled equipment are used to connect pipeline sections without the need for human intervention. This method is preferred for deep and ultra-deep water installations, where deploying divers is impractical or unsafe.
Pre-commissioning activities ensure the pipeline's operational readiness and verify its integrity before hydrocarbon transportation commences. Key pre-commissioning steps include:
- Gauging and flooding: The pipeline is cleaned and flooded with water to remove debris and test its water-tightness.
- Hydrotesting: The pipeline is pressurised with water to confirm its structural integrity and verify that it can withstand the required operating pressure.
- Dewatering, air, and vacuum drying: Following hydrotesting, the pipeline is dewatered using air or vacuum drying techniques to remove residual water and prepare it for hydrocarbon transportation.
- Testing of valves and controls: Valves, control systems, and other associated equipment are tested to ensure proper operation and functionality.
By understanding and implementing appropriate landfall techniques, tie-in methods, and pre-commissioning procedures, engineers can ensure a safe and efficient transition from subsea to onshore facilities while maintaining the integrity of the pipeline system.
Quality, Safety, Health, and Environment (QSHE)
In subsea pipeline construction, managing Quality, Safety, Health, and Environment (QSHE) is of utmost importance to ensure the overall success of the project, minimise risks, and comply with legal and regulatory requirements. QSHE management is a comprehensive approach that covers multiple aspects of the construction process.
- Law: Compliance with local, national, and international laws and regulations is critical for any subsea pipeline project. These legal requirements cover a wide range of topics, including environmental protection, worker safety, quality assurance, and operational standards. Ensuring compliance helps avoid fines, penalties, or potential project delays.
- Quality Assurance (QA): QA is a systematic approach to ensure the pipeline's design, materials, fabrication, and installation meet predefined quality standards. QA practices include inspections, audits, and performance monitoring. It is essential to maintain a high level of quality throughout the project to prevent issues such as pipeline leaks, equipment failures, or operational disruptions.
- Health, Safety, and Environment (HSE) management: HSE management is a holistic approach to minimise risks associated with health, safety, and environmental factors. Companies typically establish HSE management systems, which include policies, procedures, and guidelines for risk mitigation, incident response, and continuous improvement.
- Risk assessment: Identifying, assessing, and managing risks is an integral part of QSHE management. Risk assessments involve evaluating potential hazards, their likelihood, and potential consequences. Appropriate mitigation measures are then implemented to minimise risks to an acceptable level. Regular risk assessment reviews help identify new risks and ensure the effectiveness of existing mitigation strategies.
- Health: Protecting the health of workers involved in subsea pipeline construction is a top priority. This includes implementing measures to prevent occupational illnesses, providing adequate training and personal protective equipment, and ensuring access to medical services. Regular health monitoring and prompt response to health concerns help maintain a safe and healthy workforce.
- Safety: Ensuring the safety of workers and equipment is critical during subsea pipeline construction. Safety measures include developing and implementing safety management systems, providing safety training, conducting regular safety audits, and promoting a safety-conscious culture within the organisation.
- Environmental: Subsea pipeline construction can have significant environmental impacts, particularly in sensitive marine ecosystems. Environmental management includes assessing potential impacts, implementing mitigation measures, and monitoring environmental performance. Companies must also adhere to environmental regulations and obtain necessary permits and approvals.
Effective QSHE management in subsea pipeline construction is crucial to ensure compliance with legal and regulatory requirements, maintain high-quality standards, protect worker health and safety, and minimise environmental impacts. By addressing these aspects systematically and proactively, companies can reduce risks, optimise performance, and ensure the long-term sustainability of their projects.
Accurate surveying is crucial in subsea pipeline construction to ensure proper route selection, pipeline positioning, and safe installation. Survey data is vital for engineering design, construction planning, and environmental impact assessments. The survey process encompasses a range of methods and operations, which are outlined below.
- Bathymetric surveys: Bathymetric surveys map the seabed topography, providing essential information for route selection, pipeline design, and installation planning. Multibeam echo sounders and side-scan sonars are commonly used for these surveys, offering high-resolution data on water depth and seabed features.
- Geophysical surveys: Geophysical surveys detect subsurface features and potential hazards, such as buried objects, rock outcrops, or shallow gas pockets. Methods used in geophysical surveys include sub-bottom profiling, magnetometer surveys, and seismic reflection surveys. These surveys help identify areas requiring further investigation, such as geotechnical sampling or additional geophysical data acquisition.
- Geotechnical surveys: Geotechnical surveys provide data on the seabed's soil properties, such as strength, density, and stratigraphy. Cone penetration tests (CPTs), in-situ vane shear tests, and sampling with gravity or piston corers are common techniques used in geotechnical surveys. This information is vital for pipeline design, anchor selection, and assessing the need for seabed intervention, such as trenching or rock removal.
- Environmental surveys: Environmental surveys identify sensitive habitats, species, and potential impacts of pipeline construction on the marine environment. These surveys often involve underwater video or still photography, sediment sampling, and water quality measurements. Environmental surveys are crucial for obtaining permits and ensuring compliance with environmental regulations.
- Pre-installation surveys: Prior to pipeline installation, pre-installation surveys are conducted to verify the selected route and identify any potential hazards or obstructions. These surveys typically include a combination of bathymetric, geophysical, geotechnical, and environmental survey methods.
- As-laid surveys: As-laid surveys are performed during pipeline installation to ensure that the pipeline is correctly positioned and laid according to design specifications. These surveys often involve real-time monitoring using underwater positioning systems, such as acoustic positioning or remotely operated vehicles (ROVs) with high-definition cameras.
- Post-lay surveys: After pipeline installation, post-lay surveys are conducted to verify the final pipeline position, assess the quality of the installation, and identify any areas requiring remedial action, such as additional trenching or rock dumping for stabilisation. Post-lay surveys can also provide valuable data for future pipeline integrity assessments.
Survey methods and operations play a crucial role in subsea pipeline construction, providing essential data for route selection, engineering design, and installation planning. By employing a combination of bathymetric, geophysical, geotechnical, and environmental survey methods, engineers can optimise pipeline routes, ensure safe installation, and minimise environmental impacts.
Seabed modification is a critical aspect of subsea pipeline construction, ensuring the safe and efficient installation of pipelines and protecting them from potential damage caused by uneven or unstable seabed conditions. Various seabed modification techniques are employed to create a stable foundation for the pipeline, mitigate risks, and minimise environmental impacts.
- Sweeping: Sweeping is a technique used to clear the pipeline route of debris, loose sediment, or small rocks. Towable or remotely operated vehicles (ROVs) equipped with brushes or water jets are commonly used to sweep the pipeline route, creating a smooth and level surface for installation.
- Rock removal: In areas with rock outcrops or large boulders, rock removal may be necessary to create a suitable pipeline route. Techniques for rock removal include mechanical excavation, hydraulic cutting, or drilling and blasting, depending on the size and nature of the rocks.
- Protection: Protecting the pipeline from external threats, such as dropped objects, fishing gear, or anchor drags, is crucial for maintaining its integrity. Protection methods include burying the pipeline beneath the seabed, installing protective coverings, or using cable protection systems.
- Rock dump: Rock dumping involves placing layers of graded rock over the pipeline to provide additional support, stability, and protection. This technique is particularly useful in areas with soft seabed conditions, where trenching may not be feasible or in areas where the pipeline needs to be raised above the seabed for thermal insulation or corrosion protection.
- Concrete mattresses: Concrete mattresses are precast concrete slabs, often reinforced with steel or synthetic fibres, that are laid over the pipeline to provide protection and support. These mattresses are typically used in areas with uneven seabed conditions, high current velocities, or where additional protection is required, such as crossings or near subsea structures.
- Protective structures: Various types of protective structures can be installed over or around the pipeline to provide additional support and protection. Examples include articulated concrete blocks, frond mattresses, and grout bags. Protective structures can be customized to suit specific project requirements and seabed condition's.
- Crossings: In cases where the pipeline needs to cross existing infrastructure, such as other pipelines or cables, special crossing structures may be required. These structures can be pre-fabricated or constructed in situ and typically include supports, spacers, or protection elements to prevent damage to both the pipeline and the crossed infrastructure.
Seabed modification techniques play a crucial role in subsea pipeline construction, ensuring the safe and efficient installation of pipelines and protecting them from potential damage. By employing a combination of sweeping, rock removal, protection, rock dumping, concrete mattresses, protective structures, and crossing solutions, engineers can create a stable foundation for the pipeline, mitigate risks, and minimise environmental impacts.
Post-lay trenching and burial
Post-lay trenching and burial are critical steps in subsea pipeline construction to provide stability, protect the pipeline from external threats, and comply with regulatory requirements. These processes involve creating a trench along the pipeline route, lowering the pipeline into the trench, and backfilling to cover the pipeline. Various trenching and burial techniques are employed based on the seabed conditions, pipeline characteristics, and project requirements.
- Ploughing: Ploughing is a common trenching technique that involves using a towed or tracked plough to cut a trench in the seabed. The pipeline is guided into the trench, and the displaced soil is typically used to backfill the trench. Ploughing is suitable for a wide range of soil conditions and can be adapted for various pipeline sizes and configurations.
- Jetting: Jetting is a trenching technique that uses high-pressure water jets to fluidise the seabed soil, creating a trench for the pipeline. The pipeline is then lowered into the trench, and the fluidised soil settles back, providing support and cover. Jetting is particularly effective in soft to medium soils and can be performed using towed or ROVs.
- Cutting: Cutting is a trenching technique that involves mechanical or hydraulic cutting tools to excavate the seabed material. This method is used in areas where ploughing or jetting is not feasible, such as in hard or rocky soils. Cutting can be performed using specialised trenching machines, excavators, or ROVs, depending on the project requirements and site conditions.
- Cable trenching: In cases where the pipeline needs to be installed alongside or near subsea cables, specialised cable trenching techniques may be required. These techniques can include ploughing, jetting, or cutting, with additional considerations for cable protection and maintaining safe separation distances.
- Trench transitions: Trench transitions are areas where the pipeline changes its position within the trench, such as at crossings, turns, or changes in seabed elevation. These areas require careful planning and execution to ensure the pipeline remains supported and protected throughout the transition.
- Backfilling: After the pipeline is placed in the trench, backfilling is performed to cover and stabilise the pipeline. Backfilling can be achieved using the displaced soil from trenching or by importing additional material, such as rock or sandbags. Proper backfilling provides support, protection, and thermal insulation for the pipeline, ensuring its long-term integrity and performance.
Post-lay trenching and burial techniques play a crucial role in subsea pipeline construction, providing stability, protection, and regulatory compliance. By employing a combination of ploughing, jetting, cutting, cable trenching, trench transitions, and backfilling, engineers can ensure the safe and efficient installation of pipelines in various seabed conditions and project requirements.
Diving and ROV operations
Diving and ROV operations are essential aspects of subsea pipeline construction, providing critical support for installation, inspection, maintenance, and repair tasks. These techniques offer unique capabilities for working in the challenging underwater environment, enabling safe and efficient execution of various pipeline-related activities.
Diving and Equipment:
- Surface-supplied diving: Surface-supplied diving is a common method used in subsea pipeline construction, where divers are supplied with breathing gas from the surface through an umbilical hose. This method enables divers to work at depths up to 50 meters (165 feet) and is suitable for a range of tasks, such as inspections, tie-ins, and repair works.
- Saturation diving: For deeper operations, saturation diving is employed, allowing divers to work at depths of up to 300 meters (984 feet) or more. In this method, divers live in a pressurised habitat on the surface or on a diving support vessel and are transported to the work site in a pressurized diving bell. Saturation diving enables longer working periods at depth and is used for complex tasks that require extended time on the seabed.
- Diving equipment: Divers working on subsea pipeline projects require specialized equipment to ensure safety and efficiency. This includes diving suits, helmets, communication systems, underwater tools, and safety gear. Additionally, diving support vessels are equipped with decompression chambers, gas supply systems, and handling equipment for deploying and recovering divers.
- Inspection-class ROVs: Inspection-class ROVs are small, lightweight vehicles used for visual inspection and basic intervention tasks. These ROVs are typically equipped with high-definition cameras, lights, and basic manipulators, enabling them to perform tasks such as pipeline route surveys, as-laid and post-lay inspections, and simple maintenance works.
- Work-class ROVs: Work-class ROVs are larger, more capable vehicles designed for heavy intervention tasks, such as pipeline installation support, tie-ins, repair works, and asset recovery. These ROVs are equipped with powerful manipulators, advanced tooling, and high-capacity lifting systems, allowing them to perform complex tasks in challenging environments.
- ROV support systems: ROVs require dedicated support systems, including launch and recovery systems (LARS), tether management systems (TMS), and control cabins. These systems ensure the safe and efficient deployment, operation, and recovery of ROVs during subsea pipeline construction projects.
In conclusion, diving and ROV operations play a crucial role in subsea pipeline construction, offering unique capabilities for working in the challenging underwater environment. By employing a combination of surface-supplied and saturation diving techniques, as well as inspection and work-class ROVs, engineers can safely and efficiently execute various pipeline-related tasks, ensuring the successful completion of subsea pipeline projects.
In summary, subsea pipeline construction is a complex and multifaceted process that involves various installation techniques, such as S-lay, J-lay, bundle and towed installations, and flexible pipeline installations. These methods are tailored to specific project requirements and environmental conditions. Landfalls, tie-ins, and pre-commissioning tasks ensure seamless integration with onshore facilities, while quality, safety, health, and environmental considerations are paramount to project success. Seabed modifications, post-lay trenching and burial, and diving and ROV operations support pipeline installation and protection, while survey methods ensure accurate positioning and monitoring. By employing a combination of these techniques and technologies, engineers can successfully design, install, and maintain subsea pipelines in a wide range of scenarios, ensuring the safe and efficient transportation of oil, gas, and other resources.