managing rt scope on a process plant construction project

8/3/2019 Managing RT Scope on a Process Plant Construction Project

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Managing RT scope on a Process Plant Construction Project

Process Industry Projects, particularly Petrochemical ones, invariably face one bottleneck: Completion of

NDT scope – Rather Radiography (RT) scope if one has to be very specific. Overruns in BOQs,

manpower, equipment and machinery are always there and always get covered but the resultant increase

in Radiography scope is the thing to look at – Within no time, we might be having a mountain to climb

and because they are the last men standing, the Quality Control/Assurance guys face all the heat.

Here we are presenting some guidelines which may be adopted before radiography itself a critical path:

1. It is foremost important to have complete and authentic radiography scope in your hand. Total number

of weld joints along with their details like size, thickness, service location must be made available. In

ideal circumstances, an accurate welding BOQ would form the basis of this scope but always expect a

few updates and revision to upset the applecart. Hence, constant follow-up will have to be ensured.

2. Every process plant has systems and sub-systems and all these have different priorities. Get the systems

handover priority from the relevant team, usually the pre-commissioning or operations teams. This would

enable you to prioritize radiography work.

3. Put in some effort to develop a system to track Welding and Radiography progress. Details may vary

from project to project but once you have done that, dedicate one guy for to keep this system updated. A

more thoughtful approach would involve work in close liaison with the execution/construction team so

that all Welding progress translates into effective and efficient Radiography Progress.

4. Repair rate is critical as repairs invariably lead to time losses, repeat work, penalty shots and ultimately

increased Radiography scope. Be proactive and keep welding quality in check. Consider this – If you

have an original RT scope of 10,000 joints, there will be a difference of 1800 joints between a 2% repair

rate and a 20% repair rate. Like Welding/RT progress, keep checks on welding quality as well – Firstly

because that is your job as a QA/QC guy and secondly because it will save you from embarrassment.

Calculate Repair Rate on a weekly basis and keep updating your remaining scope.

5. Developing the infrastructure for radiography is critical. A significant portion of weld joints will be

shop welds and separate bunkers can easily ensure seamless, interruption-free RT of these joints. Better

planning could increase the number of field welds and your life would be easier. Field welding is always

more difficult and consequently has greater repair rate – Radiographing these joints would come at the

expense of interrupting construction activities and would not make life easy for anyone.

6. Barricades and site evacuation are always a source of contention with the construction guys.

Radioactive sources of lesser strength can reduce this considerably but will involve a trade-off between

efficiency and time. Spending time on optimizing this area can save precious time.

7. RT time is priceless – Never should it go wasted. Radiography bottleneck concerns will grow as the

project nears completion. By that time, other teams (Pre-commissioning, Operations, Construction) will

have greater stakes and will pounce on any time window. In most of the cases, their parallel operation

would always be possible and RT would automatically become a low-priority issue.

Operations and Pre-Commissioning teams loathe RT and Construction teams merely tolerate it because

they have no choice. Make sure that you plan for those days when no help exists to avoid any last moment

embarrassment.

Should things do not work out that way, be prepared to spend cash. Get the management to approve a few

million dollars for more advanced NDE techniques but remember that in principle, all these techniquesare being chosen merely because they do not interfere with the activities of other teams. Otherwise they

carry scores of challenges of their own.

Making the right choice is important. However, it has a simple answer – Ultrasonic Testing (UT). Of

course these are of various types but the biggest advantage they offer is exactly what we mentioned



earlier: These techniques do not require evacuation of the area. At the end it boils down to the right type

of Ultrasonic Testing Technique and in case you do not have unlimited reservoirs of cash (which is

usually the case), the timing of mobilizing the Ultrasonic Testing teams at your site.

Time of Flight Diffraction (TOFD) TOFD saves time. Full stop. This is its greatest attribute. It has its limitations though and wouldn’t work

for pipes of diameters less than 6inches and thickness less than 12mm. However, if cleaned meticulously,

joints of lesser thicknesses and sizes too can be scanned using this technique. However, it has major

limitations, some of which are listed below:

• The technique does not work on pipe fittings.

• The minimum diameter of pipe-to-bend welds that may be scanned using this technique is 8inches.

• The technique requires at least 300mm of straight, clean surface on either side of the weld joint. Hence,

any welds adjacent to supports or having less than 300mm of straight clean surface around the weld may

not be tested.

Fig.1 TOFD scanning in progress

Whenever you begin liaison with any NDE Services Provider, ensure mentioning the acceptable code

against which TOFD pass/fail criteria has to be performed.

Fig.2 Principle of TOFD technique



Fig.4 ASME Piping Calibration Blocks

Like TOFD, Pulse-Echo also requires clean, free space on both sides of the joint. Unlike Pulse-Echo, this

is not about having 300mm on both sides but having 3.5 times the thickness of the weld joint on either

side. The centre line of the weld joint has to be marked for reference using a centre punch. Capping has tobe chipped off if enough of space is not available for scanning in order to cover the whole weld.

Data interpretation though takes a bit longer than TOFD. If following American codes, the usual

acceptance criteria are the ones mentioned in ASME B31.3. In certain cases, Phased Array Ultrasonic

Testing may be used as an alternative to Pulse-Echo.

Close Proximity Radiography Close Proximity Radiography should not have been here in the first place. It is after all radiography and

does require barricades. However, the combined use of specially designed shielding material with low

strength sources, usually Selenium-75, ensures that this cordoning off is as low as 2m in most cases.

Like UT, it has its limitations with the low strength source ensuring that thicknesses higher than 35mmare beyond its range. However, some firms have developed shielding material for Iridium-192 as well

which lets them scan joints of thickness up to 65mm. Unlike UT though, Close Proximity RT has no

issues with shapes and locations – it just requires cooled, completed joints and can be a valuable resource

when time is of paramount importance. Most critically, it is much cheaper than Ultrasonic Testing

techniques.

A careful analysis shows that in crunch situations, it offers a valuable middle ground between

radiography and UT and if managed properly, a combination of these techniques can easily ensure that

most joints can be scanned without interfering with construction progress.

Summarily, it can be said that the key to managing your NDT scope is having complete command of

the scope and progress based on which plans should be prioritized. There will always be bottlenecks butan intelligent selection of various techniques can change a grave situation to a manageable one. Such a

selection would see engaging the right technique being employed at the right time.

managing rt scope on a process plant construction project

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