At offshore oil and gas fields around the world, there are thousands of kilometers of subsea pipelines connecting drilling rigs and production platforms to wellheads and onshore facilities. These represent billions of dollars of investment by companies over many years. Owners of these high-value assets must understand the cost implications of ignoring the effects of corrosion. There are many advantages of planning for corrosion control and mitigation, two of which are that the life of an asset can be extended and that maintenance time and costs are reduced.
Marine Pipelines and Offshore Infrastructure
The marine environment is a harsh one, and pipelines are exposed to a range of external physical, climatic and chemical effects that can cause corrosion and degradation to the outside of the pipes. Not to mention, the fluids flowing through a pipeline are themselves corrosive to the inside surfaces. (To learn more about seawater corrosion, read An Intro to Pipeline Corrosion in Seawater.)
Monitoring the impact of corrosion on subsea pipelines and offshore structures is a critical aspect of ensuring pipeline integrity. A key way to minimize corrosion is to employ appropriate protection technologies. Companies such as Deepwater Australasia (DWA), Carboline and Independent Maintenance Services Pty Ltd (IMS) supply products and services that meet the varied challenges of offshore and deep-water oil and gas operations around the world.
To enhance the effectiveness of the work of companies like DWA, IMS and Carboline, the Australasian Corrosion Association (ACA) works with industry and academia to research all aspects of corrosion in order to provide an extensive knowledge base that supports best practices in corrosion management, thereby ensuring all impacts of corrosion are responsibly managed, the environment is protected, public safety is enhanced and economies are improved. (You may be interested in reading: Corrosion Knowledge Management versus Corrosion Management: An Essential Tool for Assets Integrity Management.)
I recently spoke with representatives from these companies about the offshore and subsea corrosion management challenges and current protection strategies.
The Oil and Gas Industry Goes Deeper
Most of the world's shallow water oil and gas deposits have been found. As the demand for oil has increased, exploration companies have been looking at reservoirs in deeper and deeper waters. The cost of floating facilities and platforms compared to deep-water reservoirs is extremely high, so projects with equipment located on the sea floor are becoming more common.
Figure 1. Offshore technician and support ship.
According to David Flanery, Business Development Manager at DWA, the method of corrosion protection selected depends on the materials used to construct offshore infrastructures. Pipelines are often epoxy or concrete encased, whereas a platform usually has large amounts of exposed steel. Subsea assets often require protective systems that include special coatings with a long-duration operational life, sacrificial cathodic protection systems, or combinations of these.
Ricky Collins, Sales Manager Australasia at Altex Coatings, a regional Carboline supplier, stated that manufacturers have developed insulating products that have been designed to withstand the rigors of deep-water operations. These materials have been specifically engineered for use with subsea pipelines.
Surface coatings and other corrosion prevention methods are usually maintained by companies such as IMS. The scope of the work these companies carry out on offshore structures ranges from general maintenance work, surface preparation and maintenance coatings, to spot blasting and painting.
According to Jan Sikora, Operations Manager at IMS, all of the work his company does to keep offshore structures in optimal condition is planned proactively by the asset owners. “Regular inspections are carried out to determine the condition of an offshore installation and then the asset owner plans the schedule and scope of works to be carried out by us,” he added.
There is a variety of methods for securing a pipeline on the seabed. The depth of the water above the pipe determines whether it must be buried or weighted to keep it in place. In general, if the water depth is less than 50 meters, most countries require that pipelines be laid in a trench.
Extreme Subsea Challenges
The working and operating environment for equipment and pipelines in the deep ocean are vastly different from those of coastal activities. The temperature of seawater at depths of thousands of meters drops to around 2°C (35.6°F). Oil from deep wells can be as hot as 176°C (349°F). As the hot oil comes up from the well it travels through the much colder pipeline and the fluid in the pipe can quickly cool.
At approximately 21°C (70°F), the water and gas mixtures in the pipe can form gas hydrates or paraffins. If the buildup of paraffins is too great, it can ultimately block the pipeline. Such blockages can be extremely costly to clear and, if a pipeline ruptures, can cause catastrophic damage to equipment and the environment.
Subsea flow assurance is a term used in the offshore oil and gas industry to describe processes that ensure that subsea pipelines and equipment maintain oil flow. It is therefore essential that appropriate insulating materials are applied to infrastructures in order to maintain or at least slow down the heat loss from the fluids being transported. Manufacturers of surface coatings have worked to develop suitable materials to handle the extreme conditions of deep water activities.
“An offshore production field is a very complex system,” Flanery said. “Ideally, all the different components and their separate corrosion protection needs should be carefully planned at the design stage.”
For example, oil and gas flows from the reservoir through the subsea tree and typically to a manifold or pipeline end termination (PLET) via a jumper pipe. (A jumper is a short flexible or rigid length of pipe used to connect a flowline to other components.) Fluids pass along the pipeline to a production platform for processing before being sent to a tanker or onshore facility for further processing.
At times there can be a design gap between the corrosion protection systems of two adjacent assets, such as a flowline and a manifold. This can occur because each specialist company manufactures its specific component and different contractors lay them on the seabed. The corrosion protection system for each asset may not be properly communicated between companies and the operator may not take a holistic oversight of the field.
“You cannot just look at a pipeline in isolation,” Flanery said. “It is always part of a much larger system.”
Typical offshore pipelines are composed of 12-meter lengths of pipe welded end-to-end on a pipe lay vessel. Each joint is covered with a factory applied anti-corrosion coating, except for approximately 60 centimeters at each end. These areas are left bare to prevent the heat from welding operations from damaging the coating. Once the girth weld is completed between the two joints an uncoated area of approximately 1.2 meters remains. (The causes and cures of corrosion on welded pipes is examined in Causes and Prevention of Corrosion on Welded Joints.) Most pipelines are designed to use a field-applied joint coating, typically in the form of a heat shrinkable sleeve.
Managing Cathodic Protection on the Ocean Floor
Cathodic protection (CP) is a technique used to control the corrosion of a metallic surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. (Learn about The Basics of Cathodic Protection.)
Figure 2. Clamping an anode to an underwater pipeline.
The most common CP system for pipelines use bracelet anodes that are clamped onto the pipeline approximately every 10 joints, or 120 meters. The anode is bonded to the pipeline via small wires, or bonding straps, fastened to studs welded directly to the pipeline.
Regular inspections are a requirement of any company operating an offshore field, and they must be able to certify that there is no danger of a pipeline rupturing. For compliance, usually the entire length of the pipeline needs to be surveyed every five years.
One method of monitoring a pipeline's CP system is called electrode field gradient (EFG) measurement, where a remotely operated vehicle (ROV) or diver swims along the entire length of a pipeline to record the field gradient of the pipeline’s CP system. The field gradient is used as an indication of cathodic protection activity. The field gradient strength is a function of the distance between the reference electrode array and the pipeline. However, all pipeline surveys must include periodic 'stabs' along its length to recalibrate the EFG readings.
“While towed or autonomous underwater vehicles can be used, you cannot really tell how good a pipeline is without contacting it,” Flanery said.
One of the latest methods for surveying pipelines is to install CP test stations at a regular, calculated interval, similar to those for onshore buried pipelines. This enables a more rapid and accurate pipeline survey using minimal survey equipment aboard a survey vessel. An ROV or diver is required to make contact readings at these test stations using a special probe.
This method allows the survey vessel to plan stops along the pipeline corridor and drop a diver or ROV into the water only at those locations. The diver or ROV 'stabs' the test station and this is correlated with the readings from an EFG probe to determine the integrity of the CP system at that point. Next, a nearby anode can be located and stabbed. During both contact measurements the voltage gradient is recorded.
From these readings, the survey crew can use onboard pipeline CP attenuation modelling to determine the next appropriate survey site and report on what actions may need to be taken immediately or planned to maintain optimal operations.
DWA has a range of corrosion control and monitoring equipment that can be quickly deployed on-site and easily added to a pipeline to enhance the effectiveness of the monitoring program.
Specialized Coatings for Deep Sea
Several deep-water pipeline coatings are premium-grade, tough, resilient glass syntactic polyurethane elastomers that provide the required thermal insulation properties and are 100 percent solids 'cast in place' materials. (For a background see Coatings for Marine Applications & Offshore Platforms.)
“The term '100 percent solids' implies a coating in solid form, but this is misleading,” said Collins. The term actually means that the coating contains no solvents or volatile organic compounds (VOCs). Zero VOC coatings pose no fire hazard and a low health risk while the coating is being applied. They are also environment friendly as hazardous organic solvent vapor is not generated or released into the air.
Figure 3. Preparing the steel of an offshore structure for priming and spray coating.
‘Syntactic foams’ contain the right combination of resins, pigments and glass spheres to provide the necessary properties to handle the environment and application parameters. Standard lightweight insulation is not suitable for the deep-water environment. They cannot endure long-term water exposure and low temperature, and more importantly due to extreme pressures (3000-6000 psi) at these water depths, most insulation materials will simply collapse and not survive the 25-year life expectancy of the equipment.
Collins added, “A product like Carboline's Carbotherm® 735 will handle all these conditions and yet is flexible enough to tolerate movement, bending and vibration during shipment, installation and operation.”
Essential Rope Climbing Skills for Offshore Platforms
Working on the structural cross members of an offshore platform requires a unique combination of skills, but also additional safety precautions. IMS staff need to be good corrosion prevention technicians as well as proficient abseilers (rope climbers).
“Both of these skills are very important in our job and we emphasize that safety is observed in all aspects of our work,” Sikora said. “When we find the right person with the appropriate corrosion qualifications, we train them in rope access. To ensure the safety of our workers is not compromised, we also hire experienced Level 3 abseilers and then train them in corrosion prevention techniques.”
Comprehensive planning is the priority when dealing with the constraints and challenges of offshore corrosion control. “Once we get to an offshore site, if we forget something, it is hard to arrange delivery of more materials or tools,” Sikora added. “You can't just jump in your van and drive to the local hardware store.”
The weather and access can also impact work at an offshore site. There is often limited space for the workers and all their equipment on an offshore platform and sometimes the workers must travel on and off the platform every day, which restricts the actual working hours available.
Fortunately, polyurea and polyurethane coatings and primers have been developed to have rapid cure times so that structures can be covered quickly. “With an effective protection system and regular maintenance, an offshore field should have an operational service life of up to 40 years,” Flanery added.