This document specifies requirements, design and test methods for straight lengths of factory made thermally insulated pipe-in-pipe assemblies for directly buried district cooling distribution systems, comprising a service pipe from DN 15 to DN 1200, rigid polyurethane foam insulation and a casing of polyethylene. The pipe assembly can also include the following additional elements: measuring wires, spacers and diffusion barriers.This document applies only to insulated pipe assemblies, for continuous operation with water at various temperatures (1 to 30) °C and a maximum operation pressure of 25 bar.The design is based on an expected service life with continuous operation of a minimum 50 years.