Tragically, lives were lost, wildlife remains threatened, and lingering environmental damage is imminent! The catastrophic, and devastating Horizon Oil Rig disaster clearly demonstrates the importance of stringent quality control, and inspection measures that are necessary throughout the oil, and gas construction, and production industry. Oil and gas drilling rigs are composed of a multitude of complex high pressure piping systems, that consist of hundreds of welded joints. Although a cause has not yet been determined, one cannot rule out the possibility of a ruptured line due to a failed weld, as a plausible cause of the initial Horizon rig explosion.
In the early 70s I personally experienced a near disaster at an Imperial Oil refinery in southern Ontario Canada. The weld securing a 36 inch hydraulic slide valve to the bottom of a reactor on a crude unit had cracked. The reactor contained 600 tons of catalyst, and was operating at a temperature of 1200 degrees fahrenheit. If the slide valve separated from the vessel, there was no way of stopping the hot catalyst escaping from the reactor, which could potentially melt the structure supporting the reactor, leading to catastrophic damage. Fortunately the reactor was safely shut down avoiding a possible disaster. Later an inspection of the failed weld, reviled that the valve had been welded (9 years earlier during the original construction) using the incorrect welding rod. It is a miracle the weld held as long as it did, considering the harsh conditions in which the valve operated on a 24/7 schedule. The following paragraphs will provide an overview of quality control inspection.
The quality control inspector monitors qualified welders, and fitters to assure that welding procedures are properly applied, from initial joint preparation to the completed weld. Quality control inspection may be as simple as a visual (VT) observation, or a complex combination of nondestructive inspection procedures that include leak testing, ultrasonic (UT), liquid penetrant (PT), and x-rays (RT).
Visual Inspection (VT) - The importance of visual inspection is often over looked. A visual test (VT) can provide a lot of information about a weld. Weld defects visible to the naked eye such as cracks, porosity, incomplete fusion, inclusions, undercut, overlap, and incomplete penetration can be observed with a simple visual exam. A weld passing a visual exam will have a much higher probability of passing further Non-Destructive Evaluation (NDE) methods.
Leak Testing - Leak testing welds is done on vessels, and piping systems built to hold a pressurized liquid, or gas. The vessel, or piping system is usually pressurized above its design operating pressure and held at that pressure for a specific amount of time. The common test mediums are air, gas(usually nitrogen), or water. These tests are performed mostly on new construction and are part of the ASME code.
Ultrasonic (UT) - Ultrasonic weld inspection utilizes high frequency sound waves out of the range of human hearing to penetrate different materials. Sound waves are generated by piezoelectric transducers which transform electrical vibrations into mechanical vibrations and vice-versa. Transducers are selected to match the thickness, type, temperature, and configuration of the material to be tested. Reflected sound waves are processed on a computer, and referenced to standard parameters (normally flat bottom holes carefully machined on specimens of the same material) of given shape and size. A qualified inspector can determine if a weld is free of defects or inconsistencies. Additional techniques may be required to determine acceptance or rejection. Ultrasonic testing is capable of detecting thin interfaces (that X-Rays cannot detect) so that both testing methods complement each other. Ultrasonic testing is one of the most widely used methods of nondestructive testing. Its primary purpose is to detect and characterize internal discontinuities. UT can also measure thickness, detect surface discontinuities, and bond characteristics.
Liquid Penetrant (PT) - Liquid Penetrant weld inspection is a method of detecting and locating defects that are present on the surface of a weld. A penetrating liquid dye is applied to the cleaned surface, and will seep into surface discontinuities. After a predetermined amount of time, the excess dye is removed. A developer is then applied that acts like a blotter drawing remaining penetrant out of the any discontinuities. Liquid Penetrant inspection is used on both magnetic and non magnetic materials like aluminum, stainless steel, magnesium, titanium, chromium , etc., and will detect extremely small cracks. There are three different types of penetrant used with both visible and fluorescent methods. These are classified by how they are removed from the test surface: solvent removable, water washable, and post-emulsifiable. Solvent removable types are most common, and ease of use makes them ideal for "on site" inspections.
X-Rays (RT) - Radiographic weld inspection is performed by directing a radiographic source (an x-ray tube or a radioactive isotope) to all, or part of the weld to be inspected, and exposing a radiographic film to the opposite side of the source for a predetermined time. The resulting film provides information on the internal features of the weld. Any defects that are larger than the acceptable parameters is cause for rejection. All the relevant parameters including accept or reject are then recorded on an X-Ray inspection report sheet. The report, and the processed film are usually turned over to the customer at the completion of the job.
“Remember” a welders consistent professional performance, and dedicated attitude, is the first major step in the prevention of defective welds.
Written by Brian Chalmers