Production from fields developed in deeper and colder waters requires that the hydrocarbons brought to the surface be at the highest possible temperature to eliminate asphaltenes, paraffins and hydrates from blocking a production line. Good insulation allows a less viscous fluid to be produced at higher rates, extend shut-in period and reduce stress during expansion and contraction of pipes. Ideally, the annular space between the riser and production line should be isolated with a special, low thermal conductivity fluid. Further, the insulation fluid must be environmentally acceptable to mitigate the effects of any spills or other unexpected events. The recent development of a lightweight hybrid riser for a deepwater completion in North Sea required a thermal insulation fluid that was both cost effective and environmentally acceptable. This paper describes a unique joint project to develop a lightweight riser and insulating fluid. The development included the building of a special test facility, which featured a 20-m long (60 ft) full size aluminum riser with electrically heated production pipe, which was filled with insulation fluid. The test piece was immersed into the sea for several months exposure. The authors will detail several formulations, both mineral oil and water-base fluids including simple techniques to measure thermal conductivity, diffusivity and heat capacity. These data are compared to a specially developed computer model and field data from long-term exposure test. Other applications of the thermal fluid is drilling through permafrost and insulating transportation lines. Introduction With the development of fields in Alaska, it was necessary to prevent the melting of permafrost. In the early 1980's thermal insulating fluids were developed1 with diesel oil as the base fluid. This fluid was used also in double-wall pipelines to transport oil at the highest possible temperature. With the development of fields in deeper and colder waters there is need for good insulation to produce hydrocarbons and prevent paraffines, asphaltenes deposits, as well as hydrates blockage, specially in shut-in situation. Insulation also reduces thermal expansion and contraction caused by temperature fluctuations. Sub-sea construction of drilling riser, top tension risers and hybrid riser towers in deep and ultra-deep waters required modern lightweight materials and better, more environmentally acceptable, thermal insulators. In deepwater applications, risers have to be supported by buoyancy in the form of distributed buoyant material, submerged tanks, or a floating structure with some form of pontoons or a hull. Lightweight materials have the advantage of minimizing the amount of buoyancy required. Steel and low-density titanium, composite and aluminum are the primary materials used for risers. Among the lighter materials, aluminum is the most cost-effective and has sufficient mechanical properties for use in riser construction.2,3 The thermal insulation fluid has to be non -corrosive for these materials. Manufacturing of an aluminum riser tube Although aluminum is the most cost-competitive material for a hybrid riser, precipitation hardenable alloys offer the highest strength. Specifically, they obtain the strength through precipitation of hardening particles formed from the alloying elements. Welding of those alloys necessitates their exposure to high temperatures where the strength is lost. Softening of the heat affected zone, change in microstructure, loss of alloying elements in the welding zone and excessive deformation during welding are major drawbacks for using aluminum alloys. However, it is possible to overcome these problems by using a solid state welding method, called Friction Stir Welding (FSW).