Corrosion Mapping of Coated and Clad Offshore Assets: Advancements in Electromagnetic Technology

Introduction

Offshore asset owners have had to contend with corrosion issues in the splash zone (the section of a marine structure that is periodically in and out of water due to the action of waves or tides) for decades, but a new generation of electromagnetic technology is addressing this challenge by delivering precise, high-resolution data that provides critical insights into asset integrity.

Taking on the Inspection Challenge

Pipes, such as risers, caissons, and conductors, that run through the splash zone region of an offshore asset can be subject to process-induced internal surface corrosion threats – often small, deep, and isolated in nature and notoriously difficult to locate and identify quickly. The external surface of these pipes and other structures are also alternately wetted by corrosive seawater, then exposed to air, and under continuous assault from the elements. Therefore, and because of the limited effect of cathodic protection within the splash zone, the pipes are at high risk of localized and general external corrosion.

Riser operators recognize that a leak could be catastrophic in terms of environmental impact as well as the economic cost of fines, cleanup and remediation, and the intangible cost of a compromised reputation. They want to prevent containment issues while maximizing uptime and output and extending the service life of their assets. Whereas structural asset owners demand detailed condition information to understand the structural overall condition of the asset to guarantee the asset integrity.

Some owners have chosen to absorb the cost of “over-engineering” splash zone assets—an approach meant to ensure the intended service life can be met but often results in components that are heavier and more expensive than they need to be for the environment in which they are used. The industry has also employed coatings or clad material to protect the surface of components in the splash zone. While coatings or claddings are successful in slowing the pace of external corrosion, they make it more difficult to inspect components for material weaknesses.

Nondestructive testing (NDT) methods, including visual inspection, ultrasonic testing, pulsed eddy current, magnetic flux leakage, and radiography, have their own individual limitations when used to establish the integrity of the metal that lies beneath protective coatings.

While these NDT methods are useful to some degree, they generally fall short in providing the precise information owners need for superior asset integrity management. The splash zone area – with its typical external marine growth, surface protection condition, and changing sea level position – cause limitations for sound-based techniques to achieve required sound coupling. The typical heavy wall thickness and required standoff inspection is above the flux leakage inspection capabilities. The required corrosion defect size detection is not possible by the physics of pulsed eddy current technique. Visual inspection is only for external condition potential; however, it is limited there because of the marine growth and constant changing sea level condition. Radiography inspection has its practical limitations for safe overboard operations and changing condition within the splash zone.

The Value of a New Approach

The need for a better option led to the recent introduction of a new generation partial saturation, high-frequency eddy current-based technology for splash zone inspection. The integration of the high-resolution electromagnetic technology implemented into a splash zone suitable solution is the result of a collaborative effort between two industry companies: one providing the advanced inspection technology and the other facilitating the inspections in complex and demanding environments.

The scanner used in the inspection process is a nonintrusive inspection (NII) tool that incorporates multiple unique high-resolution sensor arrays and operates using high-frequency sensors to detect and size internal and external defects. This system improves on the previous electromagnetic technology, delivering greater magnetic field strength, multiple frequencies, multiple modes, twice the resolution, and increased signal-to-noise ratio due to its onboard data storage capability.

Using this NII approach, it is possible to capture precise data on pipe with a wall thickness of 26 mm (1 in) through 4 mm (about 0.16 in) of Monel cladding and up to 15 mm (about 0.59 in) of external coating.

The improved technology enhances sensitivity, enabling a greater than 90% probability of detection related to flaw type. Unlike manual ultrasonic inspections, which sometimes have difficulty recognizing small pits, this method can detect down to 5 mm (about 0.2 in) diameter at a depth threshold of 20% wall thickness.

The electromagnetic technology is carried in a splash zone operational system, which can also integrate ultrasonic sensors that provide added data where ultrasound can be applied; the multiple technique concept is built to integrate other suitable NDT methods (e.g., radiography) to enhance the process and provide even greater level of integrity data for the splash zone assets.

Deploying the Tool

Using rope access support, a crew of four can rapidly deploy the tool with a typical umbilical length of 70 m. However, the tool has a depth rating of 300 m. The all-in-one solution with onboard electronics means the topside-attached umbilical is significantly lighter, reducing cable handling complications during deployment.

Using an advanced, integrated cleaning system that removes marine growth across a 200 mm (about 7.87 in)-wide area prior to inspection, the tool is lowered by gravity along the surface of the pipe. When it reaches the bottom of the area to be scanned, it is retrieved upward along the clean path, rapidly scanning for external and internal corrosion, recording encoded data, and therefore identifying the precise location of flaws. Although cleaning the surface streamlines the inspection process, there is no need for the surface to be completely free of debris. The electromagnetic technology enables the tool to capture accurate data even when light marine growth or other surface matter is present.

An on top of the pipe set up indexing guidance ring supports the tool, which is moved by the crew in 200 mm (about 7.87 in.) overlapping swaths around the riser until the scan is completed. The width of the swath and the speed of the process allows rapid testing. For example, a 30-in riser can be inspected with approximately 12 scans.

In addition, to support a single crew and single deployment operations, the inspection system is integrated with a front cleaning system, either high pressure water jetting or mechanical rotating ‘flaps.’

Fully Integrated Inspection

The inspection data gathered using the scan system provides a complex electromagnetic signal data on resolution down to 2 mm output, which is used for analysis of defect position (external or internal), defect dimension, and respective wall loss. The data is transferred into C-Scan condition maps of the pipe in general, but also showing the external and internal data separated. Individual indications are analyzed for sizing. For reliable defect sizing analysis of the electromagnetic method, the sensor resolution was substantially increased, which supports the defect dimensional analysis and, therefore, the wall loss analysis accuracy. Despite coatings or clad material in place, the electromagnetic technology is adaptable in applied magnetic field strength and induced alternate current frequency to allow inspection and defect sizing equal to non-coated/non-clad surfaces.

Using the high-resolution sizing data gathered, an engineering team can make recommendations on the asset's integrity and damage mitigation, including through pressure wraps. It can also produce data to enable a fitness-for-service evaluation to predict the remaining safe service life and develop an appropriate inspection timeline that ensures flaws and weaknesses are closely monitored. With this critical performance data in hand, owners can make informed decisions, scheduling maintenance and repair activities within a safe service window to maximize productivity and avoid unplanned downtime.

Resolving Limitations

The partial saturation high frequency eddy current technique is very sensitive for detecting localized flaws, such as isolated pits with diameters from ¼ T or smaller and wall loss from 10% general corrosion flaws inside or outside the pipe wall. However, the technique is ‘relative’ in that it does not measure absolute thickness. It works by comparing the received signal data against reference sample defects calibrations. As such, the tolerances for defect sizing depend on the comparison of dimension of the detected defects versus the calibration defects and therefore the wall loss. The typical accuracy of the comparison related technique is +/- 10%, however, with the doubled resolution new sensor technology and signal data algorithm, the accuracy reaches +/- 5%.

Conclusion

The advanced partial saturation high frequency eddy current technology demonstrates the mapping and sizing capabilities in harsh offshore environments and additionally copes with inspection through coatings and clad material. It provides the possibility to gather high resolution and reliable data, which makes integrity assessments meaningful.