Inspecting In-Service Tanks

As appeared in the July issue of Tank Storage Magazine: Diakont’s inspection crawler technology provides reliable data and reduces tank downtime to ensure tank integrity                                                                                                                                       


     01 Summary scan map of a complete tank 

     02 One of Diakont's robotic tank inspection tools 

     03 Robot operator screen 

     04 Interior view of a filled gasoline tank from a Diakont Robot 



A KEY fact of life for operators of petroleum and petrochemical facilities and terminals with tank storage is performing recurring inspections that assess the integrity of those tanks,determine what maintenance or repair is needed, and confirm that the tanks are suitable to stay in service. These inspections are conducted as a matter of professional prudence, but also to meet underwriting requirements, and to comply with regulations; which often follow an industry code such as API 65 or EEMUA 159. Many of these inspections are quite routine, conducted from outside the tank – such as inspections of the roof structure and any seals if applicable, external appurtenances and structures, and the shell.

However, the internal structures, particularly the tank floor plates, are inaccessible for inspection from outside. So as a result, the common practice for inspecting these internal elements is to conduct an out-of-service tank ‘turnaround’, wherein the tank is drained, cleaned of sludge, and degassed of vapour – to allow for personnel to conduct the inspection manually via confined space entry. The timing of these out-of-service turnaround inspections may be determined based upon a certain calendar-driven duration of time, or alternatively by using risk-based inspection (RBI) methodology that applies various available conditional information to risk-rank the tanks within the operator’s fleet, such that these out-of-service turnarounds can be better prioritised.


The Challenge of Reliable Data

The challenge arises in the fact that, historically, reliable data about the condition of tank floor plates has been challenging to obtain while a tank remains filled and in operation. As a result, it is common for tanks to be taken out of service, only to be found to be in good condition, while other tanks may develop unanticipated problems that cause unplanned downtime. It is typical that >75% of tanks taken out of service for a turnaround are found to be in sufficiently good condition that they could have continued safe operation for three, five, or even 10+ years. This absence of actionable information about the condition of the tank floors for tanks while in-service results in a misalignment between tank farm operator maintenance budget expenditures, versus actual value outputted from the maintenance activities (this assumes, of course, that the desired value is optimally contributing toward a safe, reliable, economical, and regulatory-compliant tank farm).

Many attempts have been made over the last decades to develop and implement technology to fill this information void. Qualitative assessments using methods such as acoustic emission have been implemented in some cases, and onstream inspection robots have been used to take point-ultrasonic readings. However, none of the available legacy inspection tooling and service implementation really delivered nondestructive evaluation (NDE) data with the quality and coverage area provided by out-of-service inspections.


Rising to the Challenge

The US-based, globally operating company Diakont took on this challenge, upon request from a global integrated petroleum company in 2014. Diakont started from a ‘clean sheet’ approach in the development of the onstream inspection technology and associated service offering, establishing the following four-part criteria:

  • Ensure minimal impact on facility operations, with the tank remaining in active operation with valves open throughout the inspection (excluding briefly during tooling deployment and retrieval).
  • Ensure personnel and process safety, with tooling that is certified-safe at the most stringent level for the application, considering the typically highly flammable environment (without relying upon non-failsafe protection such as a nitrogen blanket, or depth sensors).
  • Employ provable, high-coverage, multiple-method, position-encoded NDE, that is reliable in real-world tank conditions, including in the ‘critical zone’ area adjacent to the floor-to-shell weld (unlike other ultrasonic-only or phased-array technologies which experience signal loss in the typical deposits that are present on the tank bottom), and also complete other internal assessments such as roof underside condition, coating condition, and measuring out-of-plane settlement.
  • Operate safely and reliably in real-world tank conditions, with semi-autonomy, considering zero-visibility and floor sludge, with a robust ‘simultaneous localisation and mapping’ (SLAM) navigation system that builds a ‘digital twin’ map of the tank and floor geometry.

Diakont was somewhat uniquely suited to addressing this challenge, as a vertically integrated technical energy services company, that also designs and manufactures its own inspection tooling. The development program ran from 2014–2019, including incremental testing throughout on non-flammable tanks.

The Diakont service offering uses a rugged crawler-type tool, lowered to the floor of the tank via a roof manway hatch. Both MFL and an array of UT sensors serve to interrogate the tank floor plates, identifying and then sizing any indications. A brush and jetting system displaces sludge, and 3D imaging sonars on the front and rear are used for obstacle avoidance. The entire system uses multiple methods of protection to achieve the hazardous flammable-environment certification, including pressurisation with nitrogen, and intrinsic safety. This system is used in tanks with various designs and roof structures, and with contents from crude oil to gasoline, as well as alcohols and various petrochemicals.

The service is now mature, with multiple global crews conducting inspections for a variety of forward-thinking petroleum companies, and a healthy backlog that continues to drive scale-up. Diakont was presented with an award by Tank Storage Magazine in 2020 of top ‘Emerging Technology’ for this new, modern solution for ensuring tank integrity whilst the tanks remain filled and in-service.

Could this be the magical solution that means that out-of-service tank turnarounds no longer need to occur,  with their associated high cost, vapour emissions, and confined space entry? Unfortunately, no – it does not. Having the new capability to conduct reliable, effective inspections of the complete tank does not mean that occasional maintenance during a tank’s operating life does not still need to occur. Coatings degrade, internal appurtenances fail, and the unstoppable forces of chemistry convert tanks’ strong structural steel into oxides through the process of corrosion. But now having the ability to conduct internal inspections whilst a tank remains filled adds a very powerful new tool to tank integrity managers’ toolboxes.


Part of an Integrity Programme

So how does this fit into a typical tank farm operator’s integrity program? One must first consider their tank riskranking. Tanks that are known to be in poor condition should still be taken out of service for necessary repairs. However, tanks that are in good condition, with expected good internals and floor, can complete their required inspections while the tank remains filled and inoperation, validating their condition, and in most cases saving substantial direct cost and downtime loss. And then for all the tanks in unknown condition or with typical ageing-based degradation, onstream internal inspection provides the information that the facility tank integrity manager needs in order to risk-rank tanks appropriately, and schedule turnarounds such that only tanks that need repairs are taken out of service.

Implementing onstream robotic internal inspection into a facility’s tank integrity management program has the potential to save substantial operational expense, while also increasing personnel safety, environmental stewardship, and also increasing uptime and predictability. Many facilities can reduce the number of out-of-service turnarounds conducted each year, extending operational cycles when possible. For a facility with tens of tanks, this has been shown that it can result in a direct savings of at least millions of dollars per year.

Now that the technology for onstream robotic petroleum and petrochemical tank internal inspection has been evolved to the point that it is a reliable, safe solution that produces high-coverage, provable NDE data, the industry has reached an inflection point with regard to how these tank assets can be managed. With this service now integrated into tank operator integrity managers’ portfolio of tank management capabilities – management expectations for tank farm uptime, leak avoidance, and economy are increasing. Storage tanks are critical elements of the industrial infrastructure, but they are only serving their purpose when in operation. Diakont’s technology and service offering keeps tanks in service for a greater percentage of time, while ensuring that they operate in a more-predictable manner, and ultimately contributes to safety while also providing a means of real cost-savings to operators.


Learn more about Diakont's Storage Tank Floor Inspection Service