Abstract: For heat insulation of pipeline bundles under water the invention proposes to fill the space between one or more gas or fluid conducting lines and a surrounding carrier pipe with a crude oil fraction, preferably a kerosene fraction, with gelling chemicals, so that the oil is viscosified in the bundle on or near the destined location. The dynamic viscosity after gellation is preferably between 10 and 1.000 Pa.S. The oil is preferably a kerosene fraction. The invention also relates to the way of filling the said space and to means for keeping pressure on the gel by a duct in the carrier pipe, filled with gas and having a pig movable therein and in contact with the gel so that the gas keeps the pig pressed onto the gel.