The ultimate value of a properly run hydrostatic test, and particularly of installed pipe at pressures reasonably above normal operating (I believe for many reasons, some as discussed by cvg on this thread), is undeniable. When hydro-testing equipment (e.g. huge presses with hydraulic cylinders and conveyors etc.) that utilize hydraulic end pressure to make and maintain a water seal is used to test at least full length pipes or pipes without complicated appendages etc., a Shop hydrostatic test is not necessarily cumbersome to large manufacturers at even very high pressures. Domestically (in the USA) however, I understand the available equipment for such routine testing is however limited to 120” diameter. AWWA C200 contains the clause, “Special sections that cannot be tested in a hydrostatic testing machine may be hydrostatically tested by welding on heads, or by use of blind flanges, or as specified by the purchaser.” When alternate Shop hydro-testing methods are considered for pipes or specials, e.g. in effect building a pressure vessel at a factory every time an individual pipe or special is tested, I believe the logistics, schedule, costs, and occupation of fabrication facilities associated with such testing are affected quite significantly. I believe in such cases it would be more common (as suggested by AWWA Manual M11etc.) to utilize other forms of non-destructive testing such as magnetic particle (MT) or Ultrasonic (UT) testing for such special or unusually large Shop work, and perhaps subsequently also special individual, e.g. double-seal testing port arrangements or special joint testers to check the leak-tightness e.g. of field welds etc. As far as supporting pipes full of water in the field, AWWA Manual M11 of course provides guidance on stresses that are developed at various locations for various spans and configurations of saddle or ring girder supports etc. As far as disposing of hydrostatic test water, the issues here are understandable as there are nearly 40,000 gallons (nearly 150m3) per stick of pipe involved; however, one system I believe some Contractors have used to address such issues over the years is as previously mentioned testing long pipelines in sections, that involve in essence moving test bulkheads (either on the ends of shorter test sections, or periodically fastened up inside the pipe), and then use smaller by-pass piping/pumping arrangements that can in essence be used to move (repeatedly re-use a smaller given quantity of test water), instead of testing thousands of feet of pipe that would obviously require flushing a huge quantity of water at some point down a sewer etc.
Regarding the double vs. single welding, and assuming the reference is to lap welds, there are various criteria that are considered in the decision that include: ability to air test the annular space of completed joint, longitudinal stresses, installation constraints and access to the interior or the exterior. More information would likely be needed to offer any more specific response, and perhaps the pipe manufacturer and/or Contractor would likely be helpful in that regard.