NDT of welds: how to win quality of results with cost of inspection

Daniel CHAUVEAU - Alexis BLETTNER - Thierry RAGUENET
Institut de Soudure - ZI les jonquières - 57365 Ennery - France
Contact

1 - ABSTRACT

Replacement of g-rays of hyperbaric welds on sea lines by X-rays radioscopy. The system was designed to be remote operated up to 300 m under the sea level and the time of inspection was reduced by a factor 4.
Replacement of X-rays testing pipe welds by automated multichannels ultrasonic testing. An inspection rate of 100 meters of welds per day has been reached. Handling of pipes out the shop has been avoided as night working leading to economise 30 % on the cost of inspection.
Replacement of ultrasonic testing by eddy current on grids welded in austenitic steel. The time of inspection was reduced by a factor 6.

2 - INTRODUCTION

Non destructive testing, as applied during both the construction and operation stage, has always been an integral part of any quality assurance or plant reliability program. However now, due to the need to increase more and more the competitiveness but keeping safe products, NDT shall ensure quality and repeatability at the lowest cost.

3 - TESTING TIE IN WELDS ON PIPELINES IN HYPERBARIC CONDITIONS - SUBSTITUTION OF X-RAYS BY RADIOSCOPIC TESTING

3.1 - SCOPE
Manual welding in dry hyperbaric environments is now well established for underwater pipeline connections and repairs. Although hundreds of welds have been produced successfully by skilled welder divers, there is considerable incentive to develop automatic hyperbaric welding to eliminate variability in quality and properties and reduce the cost of qualifying personnel under simulated hyperbaric conditions, while improving working environments.
STOLT COMEX SEAWAY (SCS) now STOLT OFFSHORE had designed, build and tested THOR 2 (TIG Hyperbaric Orbital Robot), representing a second generation automatic hyperbaric welding system with associated integrated pipe preparation module. In order to reduce safety hazards and the risk of damage, and therefore minimise the high cost that could be involved in the repair of underwater pipelines, welds must be inspected by a suitable underwater non-destructive testing technique (NDT), the most common of these being radiography. The most current method, using radioactive isotopes, involves taking radiographs once the weld is completed. This method is slow due to the time needed to expose and process films could be dangerous because of the radioactivity level involved and could lead to administrative problems (exportation of radioactive isotopes and storage on the barge).
The use of radioscopy can facilitate this task and reduce inspection time.

3.2 - DESCRIPTION OF THE TESTING SYSTEM

Fig 1a: Schematic view of the hyperbaric system and a photo of the radioscopic system

Fig 1b: Example of a radioscopic image obtained on a weld
The system developed in partnership with SCS allows the inspection of pipeline welds in hyperbaric habitat and can be operated from the surface up to 350 m water depth and can be derived up to 600 m.
It is designed to operate with the X-RAY source and the X-RAY detector positioned outside the pipe : inspection technique known as Double Wall Single Image technique.
The system consists of a Control Station designed to work on a boat or a barge. The following modules (X-RAY Source and Imager, scanner, high voltage generator, power and control unit) have been designed to work subsea under 30 Bars external pressure (figure 1):

The X-RAY source is a metal-ceramic tube rated at 225 kV and 10mA with a dual spot of 0.6 and 2 mm.
The X-RAY imager includes a X-RAY image intensifier and a camera head. The X-RAY image intensifier is a 6" (150 mm) single field image intensifier tube. The camera head consists of a Vidicon tube and a video amplifier.
The scanner consists of 2 support carriages travelling around the pipe on a light flexible tracks as used with SCS automatic welding system. One carriage supporting the X-RAY imager is driven by stepper motors. The second carriage supporting the X-RAY Source is mounted opposite to the first carriage.
The Control Station, architectured around a PC computer, drives the General Control Unit and the Image processing system featuring frame grabber and frame processor boards.

3.2.1 - BENEFITS
The main advantages of radioscopy inspection are :

weld quality assessment in real time and better image quality than gamma rays,
quick data access and speeding-up of the inspection,
possibility to transmit X-RAY images at distance (cable, optic fibre, modem...),
low consumable cost (no film),
radio-protection problems easier to solve leading to improve the safety of personals,
possibility to work in hostile environment in remote mode,
possibility to work in full automation (diverless) for deep site (600 m),
large quantity of data stored in a small volume without risk of deterioration.

Test pieces have been inspected with this system and the conventional hyperbaric method using g-RAY source and films. The Real-time Radiography system provides significantly better results up to 20 mm pipe wall thickness.
For a 20" weld, the inspection time (excluding scanner installation) is expected to be less than 4 mn instead of several hours for the gammagraphic system. Due to the very expensive cost of the hour of the lay barge and despite the high cost of the investment of the equipment, this new process can be competitive since the 30^th welds tested

4 - TESTING WELDS IN WATER PIPELINES - SUBSTITUTION OF X-RAYS BY ULTRASONIC TESTING

4.1 - SCOPE
A boilermaker (small size company) has been charged to manufacture the first four kilometres of tubes required for a water pipeline.
This order has been placed meanwhile a pipe mill was erecting a new plant with the required capacity : the large diameter (> 2 m) and thickness (16 to 18 mm) exceed the capacity of the existing plant. The engineering supervising the project had firstly requests x-ray testing. Taking into account the production rate requirements, the type of non-destructive testing method to be selected was of great importance. As a matter of fact, it was impossible to transport the tube to a single testing unit. X-rays testing led to handling difficulties and the radio safety requirements would have led to a slackening in production rate.
That is why a mobile computerised ultrasonic testing system ensuring the data record, delivering real time results and mobile in the shop, has been proposed.
This proposal has been accepted by the engineering supervising the all project. The latter has accepted to depart from the radiographic testing required by the API standard. These techniques allowed a good evenness of the welding beads and satisfactory execution repetitiveness, thus entailing the limitation of welding defects.

4.2 - DESCRIPTION OF THE TESTING SYSTEM (FIGURE 2)
The system is equipped with a DC motor allowing moving the holding system up to 150 mm/s. The position of the system is known through an optical encoder. The ultrasonic equipment is organised around a microcomputer, an ultrasonic board and a multiplexer developed especially for that purpose.
The operator can observe the A-scan during testing on one channel chosen between the eight channels. The system is equipped with an adjustable threshold. Any echo exceeding this threshold and within the testing gate releases a visual and sound alarm.
A part of the screen is reserved for typing the data concerning the file and the information required for tracing the testing.


Fig 2: General view and details of the ultrasonic system

For testing longitudinal welds, the head motorization is used, meanwhile concerning girth welds, the tube is rotated by means of a turning gear.
The welds have been tested in conformity to the standard API 5 L and the testing reports have been edited downstream.
The manufacturing made in 3 shifts is controlled in one 8-hour shift. Calibration is performed as a minimum, at the beginning and at the end of a shift.

4.3 - BENEFITS
The system used doesn't present main technical innovative items compared to existing equipments on the market. The innovations are on the design of the system leading to a low cost, the reporting software giving a real time sentencing according API 5L and last but not least the use of such a system on the shop of a small size company.
The direct cost for one meter of weld inspected is equivalent to the cost of an X-ray inspection. Handling of pipes out the shop has been avoided as night working leading for the manufacturer to economise at least 30 % on the indirect cost of the inspection. Due to the quick access to the results, the manufacturer has got the advantage to correct quickly his welding process (leading to avoid a certain amount of repairs).

5 - REPLACEMENT OF ULTRASONIC TESTING BY EDDY CURRENT ON DEBRIS'S FILTERS GRIDS WELDED IN AUSTENITIC STEEL.

5.1 - SCOPE
The entrance of the water pumping of the nuclear power plant have grids acting as debris's filters in order to avoid that undesirable objects flow through the water circuit.
These grids are manufactured by welding up together strips of austenitic steel with a defined step. The grids have a final diameter of 2 m and are manufactured on a specific machine with a cylinder shape by crossing wires and rods and then cut and flatten.

After a period of work in a Japanese plant, several strips have been lost in the water circuit leading to an ultrasonic test of the whole grids and the other ones supplied in the same series.
The manufacturer has offered to supply new grids but before sending he had to discover the origin of the non-quality and to set a reliable NDT method to provide perfect new grids.

5.2 - BENEFITS
The grids have more than 2000 welds. The examination of such welds by ultrasonic testing is difficult and requests very small ultrasonic probes. A team of 5 people during a week has been necessary in Japan to test the whole grids.
Eddy current have been assessed and found very efficient. The principle of testing is illustrated in figure 3 and the typical signals observed are shown in figure 4. A whole grids is tested by an operator within 1 or 2 days depending of size and number of defective areas detected.

Fig 3: Principle of eddy current testing

Fig 4: Typical signal obtained during grid testing