Monitoring sand erosion in offshore pipework using permanently installed ultrasonic sensors
Dr. Robert Phillips, Applications Engineer (email@example.com)
Sand erosion in pipes on upstream assets can happen rapidly causing critical safety issues and incurring significant financial costs. Manual ultrasonic inspection is typically used alongside other techniques to determine the extent of internal erosion. In this blog we discuss the use of permanently installed ultrasonic sensors to complement manual inspection campaigns.
The challenges with inspecting for erosion in offshore pipework
Currently, acoustic sand detectors are used to detect the levels of sand production and these detectors can be coupled with manual ultrasonic thickness (UT) measurements. When sand levels are suspected to be high, either spot UT readings or gridded UT scans are used to provide information about the wall thickness in areas suspected of damage. The locations for inspection are guided by computational fluid dynamics (CFD) modelling and Risk Based Inspection (RBI) programmes. This allows inspection to be targeted at high-risk areas, such as elbows and T-junctions. Due to the manual nature of UT readings, the data quality can lead to inaccurate determination of the internal pipe erosion rate.
With high erosion rates, or when structures are close to the end of their design life, there is a requirement for frequent inspections. This requires skilled inspectors to travel out to the asset and utilise bed space. It can also require insulation or coatings to be removed from the pipework or vessels, or scaffolding to be erected. All of this comes at a significant cost to the operator.
Permanently installed ultrasonic thickness monitoring sensors have recently been adopted to overcome some of the issues associated with manual ultrasonic inspection. The sensors provide highly accurate, trendable data from the same location each time, which can be used for predictive maintenance. In addition they do not have the costs associated with facilitating inspections.
The Inductosense WAND solution is a new approach to permanent monitoring
The Inductosense WAND sensors are permanently installed, ultrasonic sensors. They offer accurate repeatable, ultrasonic thickness measurements from the same location each time. They differ to other solutions on the market as they do not have wires or batteries and are less than 1mm thick. They are activated wirelessly by a handheld data collector or by a remote data collector and a measurement can be acquired in a few seconds. Each sensor has an RFID associated with it and measurements can be tracked over time to provide accurate rates of internal wall loss. Personnel on the asset are able to collect the data and the sensors can be installed underneath coatings or insulation and also activated in hard-to-reach areas.
A key advantage of the WAND system is the simplicity of installation and data collection. The sensors are low cost and require no maintenance. WAND is more than an order of magnitude more cost effective per condition monitoring location (CML) than alternative permanently installed sensors on the market.
Permanent monitoring to complement an inspection campaign
The Inductosense WAND technology has been utilised offshore to complement existing inspection campaigns. In one such case, large amounts of sand erosion was noted periodically and flow restrictions had been implemented to limit the risk of pipe damage based on the acoustic sand monitor information. An NDT service provider was undertaking manual ultrasonic thickness measurements to keep on top of the progression of the internal wall loss. Acoustic sand detectors were used to estimate the quantity of sand in the production. Although the acoustic sand detectors measure the quantity of sand in the produced content, it does not provide details on the size and hardness of the sand particles. This makes it difficult to infer the rate of erosion that this will cause. The manual ultrasonic measurements gave an accuracy of around +-0.5mm, and accurate erosion rates could not be extracted from these measurements.
CFD models were used to identify where the erosion is most likely to occur on the elbows of flow lines. Matching this information with manual ultrasonic thickness data, locations were identified for the WAND sensors to be installed. The sensors are single-point ultrasonic thickness sensors, and must be placed in areas where the erosion is expected. The localised nature of sand erosion elevates the possibility of the sensor missing the erosion pathway. In this case, the use of multiple sensors were used and in some cases manual UT scans were performed around the area at a less frequent interval.
Both scanning and single point thickness readings are used in industry to monitor wall loss. In the case of sand erosion, the choice of technique is particularly important due to the localised nature of some erosion pits. By combining area coverage with gridded scanning and high quality data provided by the monitoring solution, this provides great confidence to the operator on the condition of their asset.
The frequency of the data collection was adjusted to fit with the information from the acoustic sand monitors. Readings were taken from over 100 installed sensors using the WAND handheld data collector. Technicians on the asset were able to quickly collect the data which was then remotely analysed by the NDT service provider. This solution allowed the linking of the acoustic sand monitor data with the accurate wall loss information from the ultrasonic sensors, thus informing the true effect in terms of wall loss that came from the presence of sand in the production.
By using technicians on the asset to quickly collect data from locations that require frequent measurements this reduces the number of trips to the asset required by specialist NDT personnel. This was particularly important during the Covid-19 pandemic where it became more difficult to get personnel out. Customers can typically see a return on investment of a WAND-100 sensor system within a period of one year.
Wider scale deployment to support optimisation of inspection teams
The unit price for the WAND sensors in larger volumes is <£100 and the sensors can be installed in a few minutes. This offers the potential for wider deployment at CMLs across an asset. With the use of the ECHO45 extension accessory they can be applied underneath jacketed insulation and with the use of the REACH400 they can be accessed at height.
The WAND data collector can be used by anybody to collect the data, however the WAND-RDC Remote Data Collector can be retrofitted to automatically collect the data from the already installed sensors at locations where either a high frequency of measurement is required, or the access is difficult.
Unlike more expensive permanent thickness monitoring sensors on the market, wide scale, accurate thickness monitoring is now a possibility with the WAND solution. When compared to manual UT, the cost savings depend on the required frequency of inspection of each location, but offshore typically come at the point when the 2nd or 3rd measurement would have been taken. In addition there is the added benefit of having more accurate, digitised corrosion/erosion data that can be used to undertake predictive maintenance and reduce the costs associated around unscheduled downtime.