Patent ID: 7003838

Claim:
A hydraulic pig advance system comprising: a control volume chamber having a first region and a second region, said chamber comprising hydraulic fluid; a first force transmitting member moveably mounted in said chamber; a first fluid line having a first end in fluid communication with the first region of said chamber and a second end; a second fluid line having a first end in fluid communication with the second region of said chamber and a second end; a fluid control loop having a first loop section in fluid communication with the second end of said first fluid line, said first loop section being distinct from said first fluid line, and a second loop section in fluid communication with the second end of said second fluid line said second loop section being distinct from said second fluid line; a first directional flow control device positioned in said fluid control loop intermediate said first loop section and said second loop section so as to be capable of permitting fluid flow through said first directional flow control device to said first loop section and said first fluid line or to said second loop section and said second fluid line, depending upon the selective positioning of said first directional flow control device; a second directional flow control device positioned in a fluid discharge section, said fluid discharge section being separate from but in fluid communication with said fluid control loop, so as to be capable of permitting fluid flow through said second directional flow control device from said first loop section and said first fluid line or from said second loop section and said second fluid line, depending upon the selective positioning of said second directional flow control device; and a pig housing having a first end in fluid communication with said second directional control valve such that fluid can flow from said control volume, through at least one of said first loop section or said section loop section and said second directional control valve into said housing.