Assessing caisson inspection techniques

Battling corrosion and wear offshore is a challenge faced by many operators. Elia Barnett surveys the companies tasked with inspections to find out more about today’s solutions.

A Stork inspector. Photo from Stork.

Corrosion and material deterioration is an ongoing challenge for subsea offshore facilities. Caisson structures used for floatation and firewater capabilities require frequent inspections in order to maintain structural integrity.

If caissons are compromised it will cripple operations on a rig, says Levi Vaverka, senior project manager for Stork. One of the dangers of corrosion is that it will damage the structure to the point where the caisson could fall off and sink to the ocean floor. If the caisson comes off there is a huge risk of damage to the infrastructure around it, he says.

Another issue affecting caissons is holes formed due to corrosion. “If the caisson isn’t holding water because it’s got holes in it, then you’re going to lose a lot of pressure,” Vaverka says. “You’re not going to get the volume you should, the pressure you should, you won’t get what you should out of the fire suppression system.”

Challenges for internal inspection

Obtaining accurate data from the caisson inspection has proven to be the biggest challenge for operators. The methods with which the data was obtained have not always been accurate.

“Technology is not designed for this type of application,” Vaverka says. “The inspections that you get aren’t fantastic. Some engineers were having difficulty doing their varied discrete analysis of the structural integrity of that item. And some companies have said that there’s not much point doing these inspections unless we have valuable data.”

There are obvious inhibitors to obtaining an accurate assessment of the caisson’s condition. Access is always an issue. Many service companies report hearing from the operators that internal inspections are the most intrusive on the rig. In the case of a firewater pump system, the pump inside the caisson has to be removed in order to conduct the inspection, potentially disturbing operations. Marine gunk builds up both inside and outside the caisson. This makes the task of inspecting the caisson’s condition much more difficult and allows the potential of blockages building up inside the caisson. Equipment has a potential to get stuck down hole.

Challenges for external inspection

A Stork worker conducting an inspection.

External caisson inspection comes with its own set of challenges. While traditionally, it doesn’t shut down operations, obtaining accurate data from outside the caisson is difficult. External inspections are subject to weather conditions whereas internal inspection is not. Marine matter buildup on the outside of the caisson also hinders inspection. Inspecting from the outside doesn’t always catch internal degradation of the caisson, especially at the supports where there is overlapping material. Many service companies provide more than one avenue of inspection in order to obtain the most consistent data.

The methods traditionally used to inspection caissons involve simple visual inspection on both the outside and inside walls. While visual inspection is still a valid part of the process, service companies have developed tools to address specific challenges. New technologies and techniques have been developed in order to obtain the most accurate assessment on caisson degradation. Ultrasonic corrosion testing is becoming the standard tool for internal and external inspection for many service companies. Other technologies include visual inspection, 3D laser scanning, high-pressure cleaning, and ROVs for external deepwater inspection.

Stork 

Utrecht, Netherlands-headquartered Stork, an asset integrity management specialist, uses ultrasonic corrosion mapping and 3D laser scanning for nondestructive testing (NDT) internal inspection. They also have a visual camera with bright lighting to check for any corrosion. This method is conducted simultaneously in order to make sure the data is accurate.

“The laser is a beam of light on a rotating head,” Vaverka says. “As the unit goes down, the laser head is circulating. That pinpoint of laser light, when it reflects back from an object, the unit is able to see that. And it can tell based on the speed of light, how long that took, and how far something is. When you get no return, then there’s a hole. If you get a return, you can calculate how deep that return goes.”

Stork’s tool allows for both the laser scanning and ultrasonic corrosion testing. The method uses sound waves to travel through the steel walls. Based on the return, the data analyzed reveals the thickness of the steel walls.

Sonomatic

Both Stork and Warrington, England-based Sonomatic have a tool in their arsenal, which the companies say is able to squeeze into tight places.

Sonomatic, which specializes in advanced ultrasonic inspections and asset integrity solutions introduced its new internal caisson inspection tool at the 2015 SPE Offshore Europe conference in September. Scott Bulloch, topside project manager Sonomatic, says the tool was developed because operators were experiencing problems with data quality and reliability of inspection results with existing tools currently in the market place. Limited reliability of the inspection information from existing tools can lead to less than optimal integrity decisions, with costly outcomes for operators, he says.

Also tools becoming stuck within the structures were an issue, “This was having major impacts on operations offshore,” Bulloch says. “Our tool has several fail-safe mechanisms in it. With any loss of power, the whole tool collapses and we can retrieve it, we have other mechanisms that prevent it from getting jammed from unexpected obstructions, thereby lowering risk of downtime to the asset.”

Sonomatic’s tool can inspect both the dry section and submerged section with ultrasonic corrosion mapping to complete the internal inspection for remaining wall thickness, and can inspect welds from the corroded surface looking for fatigue cracking. The company also deploys subsea tools deployed by ROV for external inspections with various ultrasonic imaging techniques.

Innospection

Aberdeen, Scotland-based Innospection specializes in electromagnetic inspection technologies for its NDT solutions, specifically the Magnetic Eddy Current (MEC) technique.

Standard Eddy Current is only sensitive at the surface of the metallic material. “Modification to the standard Eddy current enables the deeper inspection of the metallic structures,” says Andreas Boenisch, managing director at Innospection. “Defect detection using the MEC technique is achieved by superimposing direct current magnetization with an Eddy current field i.e. an Eddy Current coil is used on the ferromagnetic material while the section of the ferritic steel component is magnetized at the same time.” With this technique, internal defects such as corrosion, pitting and cracks can be detected from the external surface.

The MEC technique can be used on caissons with an outer diameter of 10-42in, wall thickness up to 25.4mm and coating thickness up to 15mm, Boenisch says. This method requires only the removal of heavy marine growth prior to the inspection and can also be used in conjunction with other advanced inspection techniques such as ultrasonic testing.

AISUS Offshore

AISUS Offshore, also based in Aberdeen, specializes in remotely deployed visual and ultrasonic inspections. For the company, the high-pressure cleaning goes hand in hand with the ultrasonic inspection method because the data obtained from the ultrasonic is more accurate when the marine debris is removed.

“We use a zone 2 diesel driven high-pressured water jetting unit to remove all scale, marine growth and loose debris from the caisson interior,” says Stuart Lawson, managing director, AISUS Offshore. “The cleaning apparatus can operate at pressures between 6000-10,000 psi.”

The visual inspection method uses a remotely deployed camera that scans the internal walls. The camera is backed up with LED lights and lasers that looks for degradation, blockages, internal diameter changes, wall loss defects, corrosion, scale and any other anomalies identified during the inspection.

Conclusion

Caisson inspections are important for maintaining the integrity and safety of the rig. The inspection process has always posed a challenge because of hard to reach places that limit or hinder obtaining accurate data. The industry has developed many techniques to address these challenges both for internal and external topside and subsea inspection from visual inspection, 3D laser scanning, ultrasonic corrosion scanning, and electromagnetic currents. Often running one or more of these techniques increases the accuracy of the data obtained. As technology improves in these fields, so does the quality of data, and the ability to make decisions on how best to improve the life of offshore assets.