a method to set a hollow device into an undersea flow pipe uses a flexible pig driven by a flow flowing into said flow pipe to displace a hollow device to a point where a stop means halts a displacement of said hollow device , thereby setting said hollow device into position . a method to retrieve a hollow device into an undersea flow pipe uses a flexible pig driven by a flow flowing into said flow pipe to retrieve said hollow device from a location where said hollow device operates halted by a stopping means .