Concrete mattresses offer more than just pipeline protection

Articulated concrete mat protecting subsea pipeline. Photo: Submar.There are many methods used to protect offshore pipelines: sand, grout or cement bags, burying, concrete or cathode coating, and trenching, to name a few. Most methods address ancillary issues like separation, support, erosion, expansion of infrastructure, pipeline corrosion, and ensuring that the pipeline infrastructure itself is not detrimental to the environment surrounding it. Only one form of pipeline protection technology however, found itself in the midst of a battle between the seafood industry of southeastern Louisiana, the oil and gas industry, and environmental agencies: concrete mattresses, also commonly referred to as concrete mats.

While the mats were effective at protecting and stabilizing pipelines in the navigable waters off southeastern Louisiana, some fishermen were reporting that the mats were ensnaring the shrimp trawlers’ nets, greatly reducing their harvest.

“It took a long time and a lot of resources to convince everyone that mats were a better solution than leaving pipelines exposed,” said Monique Roberts, offshore division manager of Submar, a company that has produced approximately 80,000 mats for the Gulf of Mexico over the last 20 years. “The situation was affecting two of Louisiana’s top industries [oil and gas; seafood].”

Following four years of intense battles, lobbying and at least two case studies, the Louisiana Department of Wildlife and Fisheries enacted General Permit 24 (GP24), in 2010, and concrete mattresses continued their industry reputation as one of the most reliable and cost-effective ways to protect existing or expanding pipeline infrastructure.

History

According to the ‘Concrete Mat Subsea Deployment Procedures’ published by the International Marine Contractors Association (IMCA) in September 2011, mattresses first emerged as a method of subsea pipeline support and stabilization during the 1970s. Rather than concrete, they were made of canvas bags filled with bituminous material and aggregates. This was not a formidable solution, as they would often split when submerged or would fail to take the pipeline shape due to their stiffness.

European companies were the first to widely develop the technology for concrete mats after environmental regulatory agencies began cracking down on the use of sand or cement bags that were then-prevalent in stabilization, capping, and abandonment.

Mats produced in the early 1980s marked the first usage of concrete to increase flexibility and versatility. Initially made from concrete cured in pre-made plastic pots that were interconnected with rope, this iteration was known as the link-lok mattress. Bituminous materials were prohibited by environmental regulatory agencies in the 1990s. Both the manufacturing technique and materials used needed further refinement to become the costeffective, environmentally friendly solution that the concrete mattress presents today.

Today, polypropylene rope is laid into a form, with concrete poured over it–no more plastic pots. The result? The wet-cast, 8-ft by 20-ft by 9-inthickarticulated concrete block mat that the pipeline industry knows today, with 4000psi compressive strength. Concrete density can range 1800-4800 kg/cu m, depending on the amount of applied weight needed. To protect the pipeline, the concrete is usually coated with a non-abrasive substance or pad that varies by manufacturer and product.

In the early 1990s, European companies began to license the technology behind their mats. Submar, headquartered in Houma, La., was one of the original companies to receive this licensing in 1990 and they were the first to bring the mats to the Gulf of Mexico.

The only major change made to today’s industry-standard size is that, at times, a 12-in. mat rather than the industry-standard 9-in. thickness can be approved for permit on a project-byproject basis by the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE). Water depth, install cost and diver efficiency are typically some of the major factors in determining if a 9-in. or 12-in. mat would be the most effective.

Concrete mats can also be tagged with information such as pipeline number, mat number, contact details, and location, so that in the event a mat is moved, the finder is alerted to return the mat to its proper owner or location as quickly as possible, so that the pipeline affected can be checked and tested if needed.

Premier Concrete regional engineer Patrick Bonorden explained that these concrete mats offer a low-cost, permanent solution to several of the issues greatest affecting the industry: permanent pipeline protection and environmental stability.

“The predominant technology is the mats; they’re generally used across the board,” he said.

Alternate protection

Deepwater mat with hole allowing for clamp. Image: Deepwater Corrosion Services.Depending on the project or pipeline, the protection these mats provide pipelines are among the industry’s best. The technology can be employed for a host of situations, including rig pads, capping, abandonment and separation.

Concrete or sand bags – also referred to as sling bags - are the closest offshore competitor to the concrete mat. Bonorden described the standard sling bag as a burlap bag that contains a 3:1 mixture of concrete and cement that hardens under water. Multiple bags are typically arranged in pallet-sized form for deployment.

While the usage of sand or concrete bags somewhat overlap with that of concrete mats – they, too, count abandonment, trenching, capping and spanning among their uses – some projects present obstacles that exclude sling bags as a viable option. David Flannery, in business development – Americas at Deepwater Corrosion Services, explains that while sand bags are very cost-effective, depending on the project or pipeline placement, it is the deployment that can sometimes present issues. There can be more guesswork involved in their placement over the pipeline than with concrete mats, leaving more room for error.

“With sand bags, it’s kind of an art form; with mats, it’s more obvious how they’re applied,” he continued. “They’re very stable, both dimensionally and against the environment.”

Then, should the need for removal arise, as in the case of a pipeline inspection, sling bags must be removed – and replaced – one by one. The bags are not always permanent solutions, either, as they are not as stable as mats and run the risk of not maintaining their shape.

In direct contrast, concrete mats are recognized throughout the industry as stable, versatile, and easy to apply. After risk assessments have been performed and any necessary type of permits obtained, in most cases, the mats are transported to the pipeline, where teams of divers and/or remotely operated vehicles (ROVs) are ready to assist. The crane operator negotiates the mats into final position with guidance from the diver or ROV supervisor. Once settled, quick-release frames are commonly implemented to allow the divers to detach the load from the crane.

“You use the mat because it’s one piece, and it’s articulated to do what it needs to do,” Premier’s Bonorden said.

Perhaps the most famous example of how critical concrete mats can be to the industry was immediately following the Deepwater Horizon oil spill in 2010. BP used ROVs to build a subsea staging area with Submar’s mats. Roberts explained that BP needed to be able to immediately deploy equipment to assess and address the situation. This concrete mat platform was a holding ground for the underwater tools and equipment necessary for quick access and dispatch to different areas. More than 4,800 sq ft of concrete mats were deployed to the area, creating a large, stable platform. This allowed the ROVs to survey the area, access tools and equipment, and respond– without being affected by the dips, dives, and obstructions of the uneven ocean floor. It was a watershed moment in demonstrating the usefulness of concrete mats. Sling bags would have been incapable of smoothing the bottom of the ocean floor.

“We’re directly affected and have a vested interest: It’s a way we could help,” Roberts was quoted as saying to Houmatoday.com following this project with BP. “Our projects are meant to be more of a long-term solution. We don’t want to have to deal with this again.”

This use of concrete mats is perhaps the biggest example as to how this technology can be creatively utilized to serve a particular need, but many companies are working to innovate the concrete mat even further.

Flannery explained how, in order to provide cathodic protection (CP) to pipelines in unstable conditions, Deepwater Corrosion’s offshore concrete mat design includes casting small, interconnected aluminum anodes directly into the concrete.

A recently completed project off the coast of California required a tailormade solution for an older pipeline needing protection for another 20-30 years. In California, a permanent CP monitoring system replaced the usual anodes in the concrete to protect and extend the life of the pipeline. The CP monitoring system was connected to the pipeline by a clamp, and in order to protect the clamp from trawling, a four-six block hole was created in the mat. While this particular project was completed in more shallow water – around 300 ft deep – Flannery contends that because concrete mats can be used in nearly any water depth, this technology could be employed much further offshore.

There are a few instances in which products are still in danger of moving under concrete mats, primarily in water less than 300 ft deep or where pipelines are located at the mouth of a river. Of course, without any protection at all, the unthinkable could happen: pipelines could jump or break. Furthermore, the environment around the pipeline could be harmed indefinitely from exposure. Clearly, pipeline protection is critical to a pipeline’s longterm integrity and success.

Deep water

Industry leaders are finding that water depth does not negatively impact the concrete mats, so they regularly stretch the depths into which the mats are installed.

More concrete mats have been used “around 5,000 ft,” Bonorden said “That’s now starting to get to 7,000 and even down to 10,000 ft. They can also be used all the way up to the shoreline.”

Both Premier and Submar have current mat installation projects at 7,000 ft in the Gulf of Mexico. Submar will be shipping mats this month to the Keathley Canyon, about 250nm southwest of New Orleans.

Environmental impact

Twenty-ft single-release, ROV-friendly frame with concrete mat load. Photo: Submar.In addition to being used for pipeline separation and pipeline crossing, another notable advantage to using concrete mats – and one that extends beyond the industry—is the product’s inherent environmental friendliness. Concrete was cited as environmentally sound by the IMCA, and the mats are used to combat erosion beneath and surrounding pipelines. In cases where the soil under the pipeline is eroding, the mat can be used to span that area, thereby halting erosion underneath. It is generally regarded as the technology that is the least obtrusive to operations and, equally importantly, the environment and vegetation below and around the mat.

In New Orleans, the banks of the Mississippi River have been lined with a form of concrete mats since at least the 1930s for flood control. Premier’s Bonorden pointed to last year’s report that the New Orleans branch of the U.S. Corp of Engineers were once again lining the river with concrete mats to maintain stabilization and to keep the river from diverting, estimating that the group had been doing so since around the 1970s.

“Although there are no pipelines involved, it is still erosion control,” Bonorden said. “Once they realized they could do it in inter coastal waterways, it was used up and down the coast for the same purpose.” “In all reality, you’re not just protecting the pipeline,” Submar’s Roberts said. “You are also protecting nature from the pipeline.” OE

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