U.S. patent application 20180080722 (applicant GE Oil & Gas UK Ltd., Newcastle-upon-Tyne, Great Britain):
"Apparatus and method for maintaining temperature in a sub-sea device, in particular in relation to fluid in the device. A material having a high latent heat on phase change  is used to release latent heat to the fluid when the fluid temperature decreases towards a threshold value. ... The use of a phase change material in accordance with the present invention is also advantageous with other types of sub-sea pipe (such as rigid pipe), apparatus and installations where a degree of temperature control is desirable."