| Abstract Scope |
Due to the world's insatiable energy appetite, the amount of infrastructure deployed by the oil and gas (O&G) industry over the last 40 years is truly remarkable. In the upstream O&G business (finding and extracting), the easy developments disappeared in the 1970s. Subsequently, large finds became increasingly offshore or far from the developed world. This necessitated placing structures in the oceans to produce the oil and gas, and/or using long-distance transportation of the extracted products via pipelines or ships. Water depths for offshore developments have increased from hundreds of meters in the 1980s to about 3000 meters today. Pipelines can exceed 1500 km. Oil tankers and LNG ships can be about 400 meters long.
Over the decades, the O&G infrastructure has become increasingly complicated, and the welding engineering community has been constantly challenged to find reliable solutions. This talk will highlight some of the welding and metallurgical developments that have enabled the O&G industry to deliver the world's energy needs. A few examples are as follows.
During the 1980s, offshore structures consisted of welded space frame designs called jackets. With steel thicknesses up to about 75mm and total weights ranging from about 10,000-20,000 tons, thousands of jackets have been built. Submerged arc welding became widely used and heat inputs were often between 3-5 kJ/mm. To ensure adequate weld toughness, fracture mechanics technology emerged during this era. In 1983, during the construction of Exxon’s Odin oil platform, a small but persistent number of low heat affected zone (HAZ) toughness measurements occurred during weld procedure qualification. This was named the local brittle zone (LBZ) problem. The incident initiated 10-15 years of study on subjects such as HAZ metallurgy, weld toughness, and structural integrity. LBZs motivated the use of advanced electron microscopy for steel HAZs which led to an unprecedented gain in the understanding of HAZ metallurgy. LBZs motivated structural integrity studies (including large scale tests) that demonstrated small regions of low toughness microstructure can compromise integrity. Furthermore, the LBZ issue is credited with two decades of improvements in thick-section steel manufacturing and with maturing products generally known as microalloyed steels and high strength low alloy steels. Due to these engineering developments, no LBZ-related failures of offshore structures have been experienced.
A second example of enabling welding technology in the O&G industry involves mechanized pipeline welding. Pipeline construction by manual or semi-automatic processes is laborious. Beginning around 1980 and evolving over the subsequent two decades, a variety of mechanized welding technologies emerged that were exclusively aimed at girth welding of long-distance pipelines. Key enabling factors included increasingly sophisticated welding power supplies and the use of pulsed gas metal arc welding (PGMAW). Despite the challenge of girth welding in the 5G (fixed) position, the new systems evolved into outstanding performers. This mechanized technology accomplishes excellent throughput by using narrow bevel preparations and high travel speeds. The pulsed arcs were critical to puddle control in all circumferential positions, limiting heat input to produce the desired weld metallurgy, and still creating enough penetration to keep lack-of-fusion defects to a minimum. Excellent strength and toughness properties are produced, and thousands of miles of pipelines have been built using this technology.
A third example of unique technology involves a metallurgical conundrum. In 2013, two relatively new 28” x 95 km carbon steel pipelines in Kazakhstan experienced failure by sulfide stress cracking. The cost was estimated at $3.6B. The pipe steel was sour-grade-qualified X60, and the plates were produced by thermomechanical control processing (TMCP) which is widely accepted as a high-quality material choice. The through-wall cracks were traced to very thin (200 – 500 microns) local regions of high hardness microstructure (~300 HV 100g) on the inside surface of the pipe. These areas were named local hard zones (LHZs). The failure investigation determined that the LHZs comprised a narrow stripe-like geometry of several meters long, 20-50 mm wide, and a few hundred microns thick. It was determined that LHZs are created when spray water cooling during TMCP plate processing interacts with surface oxides that, unintentionally, remain on the plate after high pressure deoxidation cleaning. To mitigate future occurrences of LHZs, a qualification protocol was developed and has been successfully implemented. |