Determining thermal conditions in a pipeline

A system, comprising:

• a pipeline that carries a first fluid at a first temperature, the pipeline comprising a tubular conduit that comprises a bore that carries the first fluid, and a layer of insulation installed over an exterior surface of the tubular conduit;
• a bypass conduit that is external to the layer of insulation and is fluidly coupled to a source of a second fluid and to the tubular conduit, through the layer of insulation, and into the bore, the bypass conduit configured to circulate the second fluid at a second temperature different than the first temperature into the bore to mix with the first fluid, the second fluid separate from the first fluid and comprising a composition substantially similar to the first fluid in the bore;
• and
• a thermal image detector configured to detect a thermal gradient, due to mixing of the first and second fluids, between the first fluid carried in the bore and at least one of the tubular conduit or the layer of insulation at a particular location of the pipeline to indicate a presence of at least one of water or water vapor between the exterior surface of the tubular conduit and the layer of insulation at the particular location of the pipeline;
• at least one pump in fluid communication with the bypass conduit and configured to forcibly circulate the second fluid through the bypass conduit and into the bore;
• and
• a control system configured to perform operations comprising:
• in response to the second fluid forcibly circulated through the bypass conduit and into the bore by the pump, determining that the detected thermal gradient exceeds a threshold value.