Potential sources of uncertainty in the thermal conductivity needle probe laboratory test

Thermal resistivity of soil is a key parameter for the design of power transmission cables for offshore wind farms and requires a reliable determination. It can be determined by a number of different tests, including the thermal needle probe. This test is commonly performed to ASTM D5334 but a number of variables described and considered in this paper are either permitted or not described by the standard in how the test is executed. Consequently, there is potential for a number of sources of uncertainty in the thermal conductivity measurement. A study was performed across three laboratories to investigate the repeatability of the needle probe test, on a North Sea clay and two types of quartz sand with differing fines content. Verification testing of the test equipment was performed on reference materials and classification testing was performed to review the potential for any inherent variability of the soils. Uncertainties associated with the test equipment, test method and interpretation of the raw temperature versus time data are considered. This paper identifies where potential sources of uncertainty were observed and makes suggestions for future work to deepen and broaden the understanding of uncertainty of thermal conductivity measurements from the thermal needle probe test.