An arrangement for fixing two concentric tubes (1, 2) together by means of one or more pressure inducing members, preferably each formed as a pad (6); the member acts to press the tubes together; a support (3), preferably formed as a collar fixed inside the outer tube (1), is provided opposite the or each location of the or each member. The support is designed to maintain the circularity of the inner tube so that it can be readily withdrawn from the outer tube without jamming. This is useful for off-shore installations.