Getting to the bottom of subsea repairs
Older pipelines need more attention, and operators need a repair strategy.
Depressed oil and gas prices and the resulting cuts in capital spending for exploration and production have made headlines in the petroleum industry. Shrinking cash flows have had similar impacts on other on aging offshore infrastructure. Many pipelines designed to be replaced 10 to 15 years after the first flow continue to operate after 20 or even 30 years of service.
Corrosion can cause pinhole leaks and small cracks that can be detected through regular inspection by divers or remotely operated vehicles (ROVs), and repaired during planned maintenance cycles. Pipelines also can sustain serious damage from dragged anchors, subsea mud slides, platform legs being lowered at the wrong coordinates, or by structures toppled and swept away by hurricanes. Severely damaged pipelines must be shut down and major repairs must be made as quickly as possible to minimize impact on the environment and to restore product flow.
Most subsea pipelines are located in shallow water. Skilled divers using specialized tooling, clamps, and connectors usually perform repairs in depths less than 1,000 feet. In depths greater than 1,000 feet, the same tasks must be accomplished using specially modified tools and equipment operated by ROVs, because this depth is out of dive range.
Minor pipeline repairs
Regular pipeline inspection may detect bubbles, liquid, or gas flowing from a pinhole leak, or inspectors might observe a dent or serious external corrosion that might result in pipeline failure. Before repairs can be made, a service vessel with divers-or ROVs if in deep water-is deployed to find the location and extent of the damage before deciding on a repair approach. A subsea inspection tool may be required to evaluate the damage, and a subsea dredger may be needed to expose the pipe for visual and instrumented inspection.
Minor repairs are performed using clamps to seal off the localized damaged area of the pipeline. Structural smart clamps come with elastomeric seals and metal grips to stop the pipeline from leaking while providing structural attachment to the parent pipeline. Such structural clamps are considered permanent repairs. If a temporary repair is needed-in cases where the pipeline will be replaced or shut down in the near term-split-sleeve seal clamps without metal grips can be used, also known as nonstructural clamps. Standard diver-actuated clamps for pipelines from four to 24 inches in diameter and with a range of pressure ratings are available from supplier inventories. Many of these clamps can be modified for ROV installation quickly.
The first step in performing a minor repair is to dredge out enough material around the pipeline to provide space to place the clamp around the damaged section of pipe. Next, any protective concrete and fusion-bonded epoxy (FBE) must be removed by divers or ROV-operated tools. If the pipe has a welded seam, it must be milled off to create a smooth and round surface for the clamp. The clamp is then lowered by crane from the service vessel and placed around the prepared pipeline. The clamp is configured with ROV-compatible bolts. In deep water, this operation is completed using a work-class ROV. Whereas, in shallow water, the divers can turn the bolts by hand as long as the seals are properly in place.
In some cases, when large clamps are needed, or space is tight around the pipeline, a hydraulic module is mounted on the clamp to facilitate activation. The clamp is then tested to verify seal integrity and the dredging tool is used to backfill material beneath the pipeline. Sand bags or grout bags also may be placed under the clamp to compensate for its weight and to prevent sagging of the pipeline.
Major pipeline repairs
Major repairs require the damaged section of the pipeline to be removed so a replacement "spool section" can be installed using connectors that match the diameter of the pipeline. This more complicated operation requires the pipeline to be shut down, stopping its flow and the revenue it generates. Such severe damage often occurs suddenly due to an accident or extreme weather conditions. Pipeline failures also can pose a threat to the environment, so operators typically respond to them quickly.
As with minor repairs, visual and instrument inspection by divers or ROV is required for major repairs to fully assess the damage. Dredging also is usually required to expose the pipeline over the section to be removed and replaced. Spool sections can range in length from six feet to more than three miles.
For shallow water repairs carried out by divers, the operation usually proceeds with the pipeline resting on the sea floor. During deep-water repairs, subsea pipeline lifting frames are placed on either side of the section to be removed and provide support to the pipeline throughout the repair operation. For sections longer than 100 feet, four lifting frame skids are recommended, two per side—but a minimum of two are required. When the lifting frames elevate the pipe, the service vessel crane is used to support the center of the damaged section during the cutting operation. Concrete and FBE must be removed from the areas to be cut, including sufficient length to hold the pipe end flanged connectors.
A variety of methods for cutting the pipeline are available, including diver-operated "broco"-type cutters; mechanical cutters, which are best for small-diameter lines; diamond wire cutters, which are used on larger pipelines; and chop saws, which are more reliable than wire cutters and are designed for ROV use.
After the damaged pipe section is cut and removed, the exposed pipe ends must be cleaned to bare metal. Any weld seams must be milled down, and burrs must be removed from the pipe ends—both internally and externally—to ensure that connectors can be installed properly. In addition, a pipe end check tool is used to confirm the pipe’s roundness. Smart flange connectors are then attached to the exposed pipe ends.
Smart flanges are mechanical connectors that provide metal grips and elastomeric seals, and are available in classes 600 through 2,500, and could be rated up to 10,000 psi. After the repair is complete, smart flanges can be tested before product flows into the pipeline and without over-pressuring the pipeline, avoiding possible damage to other sections of the aged pipeline.
The spool piece is lowered by the service vessel crane, aligned with the pipe, and the flanges are connected with bolts tightened with the right amount of torque to ensure that the ring-type joint (RTJ) gasket achieves a positive seal. In shallow water, divers carry out this operation. In deep-water repairs, the spool piece can be configured with double grip and seal connectors with hydraulic actuators that do not require extensive bolt tightening by the ROV. These double grip and seal connectors provide structural support and high-pressure seals. When activated with hydraulics, they are locked in place by mechanical slips. The spool piece with connectors is placed in an installation frame, which mates with the pipeline lift frames already in place. A laser alignment system is used to line up the spool section with the main pipe, and then the repair connectors are stroked by activating the hydraulic cylinders with ROVs. After installation is complete, the connectors are tested without over-pressuring the pipeline.
The installation frame is then removed, and the dredger is used to level the seabed beneath the repaired section. Concrete or steel mud mats are placed on the seabed to compensate for the additional weight of the connectors. The pipeline is lowered to the seabed, and the lifting frames are retrieved to the surface. The pipeline can then be put into service.
Prepare for pipeline failures
Given the consequences of subsea pipeline failure, it is important for operators to establish regular pipeline inspection, maintenance, and repair programs to maintain asset integrity. Even after taking these precautions, pipeline failures can occur. To be prepared to respond to these incidents, pipeline repair experts recommend that operators maintain emergency pipeline repair inventories including one clamp and two smart connectors for every size of pipeline in operation. For deep-water pipelines and remote locations, it is good practice to have pipeline lift tools as well as ROV tools for pipeline repair kept on hand and in good working order.
In addition, it is important for operators to understand that pipeline repair is a complex operation that requires engineering, logistics, and deployment of a range of equipment besides clamps and connectors, including a vessel to support lifting, diving, and ROV operations, as well as dredgers, ROVs, and associated tooling. Qualified divers with proper training in pipeline repair, and ROV operators experienced in similar operations, must be assembled and managed, including support staff. Additionally, specialized equipment may need to be modified or built from scratch and deployed on short notice to meet the project’s requirements.
Because of these factors, project management is an important aspect of pipeline repair operations. Pipeline owners can benefit from having a single project manager who can effectively coordinate the activities of numerous service disciplines. Suppliers that can provide a range of pipeline repair services and equipment also can improve efficiency through better communication, a common safety program, and shared accountability for results. It also is important for pipeline repair contractors to have facilities close to each area of operation to enable quick response and ongoing support for the repair effort.
Meeting pipeline repair challenges
As pipeline systems age and capital budgets for replacement lines are reduced, operators face the challenges of maintaining product flow and protecting the environment. Fortunately, technology and services are available for regular pipeline inspection and maintenance as well as unplanned repairs, in shallow and deep-water conditions. To prevent lengthy downtime, operators should have plans in place to repair their pipeline, and should build relationships with pipeline repair experts who can respond quickly when a leak is detected or a pipeline fails.
John Charalambides is director of business development at Oceaneering Connection Systems.
Original content can be found at Oil and Gas Engineering.