Abstract: Structures such as free standing oil production platforms or subsea templates are secured to the sea bed by inserting a tubular pile in the seabed so as to form, with a part of the structure a pair of nested members. Fluid is then introduced into the innermost of the members to plastically deform the innermost member radially so that it engages with the outermost member and forms a mechanical connection between the members. A tool for effecting the plastic deformation comprises a mandrel for insertion into the innermost member and carrying a means to form a chamber within the innermost member and adjacent to its inner surface. The tool also includes a means of introducing fluid into the chamber to expand it radially and thereby deform the innermost member.