A dual walled pipe string structure defines an annular cavity between the pipe walls into which hard granules may be packed to oppose wall collapse when inserted to high pressure well depths. Segments of pipe thus constructed will reduce heat transfer across the pipe string allowing for the conveyance of drilling mud from a surface cooler to a downhole tool with diminished elevation in temperature. The rate of cooled drilling mud transfer to the tool may be controlled to match the heat transfer rates and the heat generated at the tool. To further counteract downhole wall pressures, inert gas at a corresponding pressure may be introduced into the interstices between the granules.
