Method of inserting a device in a pipe to reduce the internal cross-section of the pipe

A method of reducing the opening of a first flow modifying device (3) set in a pipe (1) involves introducing a second device (45) into the pipe (1) ahead of a foam pig (4). The fluid flow in the pipe (1) moves the foam pig (4), which inturn moves the second device (45) along the pipe (1) until it engages with the first flow modifying device (3), reducing it's internal cross-section. The foam pig (4) then passes through the opening in the device (34) by the continuing action of fluid pressure.