A method of sealing an annulus between inner and outer pipe sections of a pipe-in-pipe system includes positioning a sealing mass in the annulus in contact with the inner and outer pipe sections. Deforming the sealing mass occurs, for example by shearing and compression, by effecting relative longitudinal movement between the inner and outer pipe sections. Fixing the inner and outer pipe sections against reverse relative longitudinal movement to maintain deformation of the sealing mass is then performed. The inner pipe section and a displaced outer pipe section may be fixed by welding them to respective pipes of an adjoining pipe-in-pipe structure. Opposed ramp surfaces, each being similarly inclined relative to the longitudinal direction, extend into the annulus from respective ones of the pipe sections such that the sealing mass may be compressed between the ramp surfaces.