THE INDUSTRY IS WELL VERSED in corrosion mapping of vessels and pipework, but storage tanks are also susceptible to internal and external corrosion. So, inspection plays a major role in their integrity management.

Our robotic system has a range of methods for inspecting storage tanks while in service. In-service inspection is a cost-effective alternative to out of service inspection for situations where the likelihood of degradation requiring repair has been assessed as low. It can provide significant benefits to tank operators: removing the need for shutdowns; maintaining critical storage capacity; reducing outage personnel requirements, eliminating hazards associated with personnel entry to tank internals, and improving knowledge of tank floor conditions.

In service storage tank floor inspection

Internal or external corrosion may lead to failure of the floor of a storage tank, with severe consequences. Historically the floors have been inspected using tools and techniques that rely on emptying, cleaning, and then internal entry, with the tanks taken out of service.

The robotic tank floor inspection capability is part of a comprehensive inspection service. It can be completed while the tank remains in operation and is aligned to the requirements of API 653 (Tank Inspection, Repair, Alteration, and Reconstruction, a standard developed and published by the American Petroleum Institute (API) covering the inspection, repair, alteration, and reconstruction of steel above-ground storage tanks used in the petroleum and chemical industries) and EEMUA 159 (Above ground flat bottomed storage tanks, a guide to inspection, maintenance and repair).

The key features of this robotic system are:

  • Advanced ultrasonic immersion transducers system;
  • Integration with proprietary inspection project management and analysis software;
  • Suction and discharge pumps (optional);
  • Umbilical cord and carrier with over 100m cable length;
  • Advanced navigation system;
  • Purge system for products below 37.5°C flashpoints;
  • Camera and light system (optional); and
  • Temperature range –20 to 50°C.

Figure 1 shows the steps taken to inspect and evaluate the integrity of the tank floor.

Assessment and planning

Working with the client to plan the inspection of the tank floor, operational and integrity requirements should both be considered.

Successful in-service inspection of tanks relies on comprehensive planning of all operational aspects. This relies on working closely with the tank operator to establish critical design, operational and safety data to develop a detailed project plan. This covers logistics, equipment and manpower as well as site requirements to facilitate the inspection work.

Planning and assessment is similar to the approach used for non-intrusive inspection of pressure vessels, which in turn is based on DNV-RPG103 (Recommended Practice for Non-Intrusive Inspections). This entails developing a detailed understanding of the degradation threats and associated risks to define the most appropriate tank inspection strategy.

There are two different approaches. Type A inspection applies in situations where there is a low probability of degradation based on previous inspection history. Type B inspection applies when there is some degradation expected but it is not expected to be such as to threaten integrity in the medium term.

In both cases, detailed inspection requirements (i.e. probability of detection, accuracy and coverage) are defined for each zone of the tank. Finally, inspection plans, defining the inspection technique(s), coverage and locations for inspection are developed.

Screening

Short range ultrasonic testing is used to screen the annular ring for any degradation in advance of the robotic inspection. Acoustic emission is also used to find high activity regions and help to evaluate whether to apply a Type A or Type B inspection.

The acoustic emission technology is still under evaluation. The idea is to use it to confirm that corrosion is not active, in which case a Type A inspection will be applied. For tanks where a Type B strategy applies, the acoustic emission is used to identify areas of corrosion activity for prioritisation of coverage for the ultrasonic inspection. This ensures that the sample inspection includes representative areas of corrosion.

Quantitative robotic inspection

With the inspection strategy and plan in place, the robotic inspection is deployed.

An acoustic navigation system is used to track and locate the robot inside the tank. The robot is also equipped with an array of ultrasonic transducers, to capture ultrasonic data from the tank floor.

In a typical tank floor inspection, millions of ultrasonic signals are captured. A bespoke module has been developed as part of the proprietary integrity management software to extract wall thickness values from the large number of ultrasonic datasets, using advanced signal processing techniques to manage quick analysis of the dataset. Once the wall thickness values have been extracted, they are used for statistical evaluation of the tank floor.

Evaluation

Corrosion has been found to show statistically regular behaviour in a wide range of situations including on tank floors. This allows for a sampling approach, in which the results from a limited inspection can be used to estimate the condition in other areas. Sonomatic has considerable experience in developing and applying statistical methods for analysis of inspection data, and was primary author of the Recommended Practice for Statistical Analysis of Inspection Data developed by the HOIS Joint Industry Project to develop good practice for NDT in the oil and gas industry. The methods in this document are used as a basis for statistical analysis of tank floor inspection data.

This process covers:

  • Automated analysis of the data using advanced signal processing algorithms to obtain wall thickness values;
  • Derivation of wall thickness distributions;
  • Identification of applicable statistical fit type and parameters; and
  • Estimation of minimum wall thickness and probabilities for limiting conditions.

The use of sample inspections supported by statistical analysis conforms to the requirements of API 653 for internal inspection of tank floors.

Actual evaluation of the wall thickness values depends on the type of inspection applied.

For a Type A inspection, the idea is to confirm the absence of degradation and this is typically done by examining the wall thickness values extracted from the ultrasonic signals. A Type B inspection uses the wall thickness values to identify the underlying statistical distribution and then estimates minimum remaining wall thickness for the un-inspected area. It is important to note that depending on the inspection performance, extreme value analysis can be used for evaluation. Using the estimates of the minimum wall thickness, the remaining life assessment is derived.

Key aims of the novel inspection service are to provide information which allows effective integrity management decisions to be made, and to support clients’ integrity requirements by providing fitness for service and remaining life assessments based on inspection data.

In the case of tanks this covers complete assessments in accordance with API 653 or EEMUA 159, and all levels of assessment from simple hand calculation checks on remaining wall thickness to advanced nonlinear finite element analysis in accordance with the Level 3 requirements of API 579.

A key part of the approach to fitness for service assessment is application of statistical methods where the inspection has been performed on a sampling basis, as detailed in the previous section.

The fitness for service and remaining life assessments are used to make recommendations on any repair and maintenance requirements as well as defining future inspection intervals.

Author information: Chris Gregory, Business development manager at Sonomatic; Ian Daniel, Business development manager at Sonomatic; Mark Stone, Integrity services manager at Sonomatic