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October 11, 2011 Page 21 <br />4.8 Structures on Metal Footings /Skids <br />4.8.1 Subsidence Control Plan for Structures on Metal Footings /Skids <br />Some wellhead equipment such as compressors, pumps, soundproof housings for <br />compressors /pumps /separators /transformers, produced water tanks, and methanol tank have <br />metal footings or skids. Such footings are typically placed directly on a gravel pad to absorb <br />machine vibrations. No specific subsidence damage criterion was found for such structures <br />during AAI's literature review on the subject. Considering the range of surface deformations <br />predicted by the subsidence analysis, the above - mentioned installations are unlikely to be <br />affected by undermining. Hence, such structures may be fully undermined by retreat mining <br />methods with appropriate monitoring, as outlined below. <br />4.8.2 Monitoring Plan for Structures on Metal Footings /Skids <br />plan: <br />To ensure that the control plan is effective, AAI recommends the following monitoring <br />The structure owners should be informed as mining approaches, and data generated <br />during monitoring should be shared with them. <br />• Visual monitoring should be performed weekly as long as the retreat line is within a 10° <br />vertical offset angle of the structures. Any appearance of tilt or change in elevation in <br />these structures and any large cracks that may destabilize the structures should be noted. <br />• Structural anomalies reported by the surface owner(s) will be investigated. <br />4.8.3 Worst Possible Consequences of Damage to Structures on Metal Footings /Skids <br />Horizontal strains in excess of the allowable strain levels have the potential of damaging <br />gas or water pipes that are connected to compressors and pumps, potentially disrupting <br />production operations and leaking gas or water. Damage to the soundproof housings for <br />compressors /pumps /separators could twist or bend the metal panels. High strains may not cause <br />any damage to the produced water tank, but may damage the water pipes connected to it, <br />potentially allowing leakage of produced water. Large subsidence - induced strains could tip <br />methanol storage tanks, rupture the connected pipes, and possibly lead to methanol spills. <br />4.8.4 Structures on Metal Footings /Skids Damage Mitigation Plan <br />In case monitoring indicates onset of damage or potential for damage to the structures on <br />metal footings /skids, the following mitigation plan should be adopted. <br />Appropriate personnel at mine and regulatory authorities, as well as the owners of the <br />structure, should be informed in case anomalies are observed that may indicate an <br />affected structure. <br />• The retreat plan should be re- evaluated to ensure that operations will not further damage <br />the structure in question. <br />Agapito Associates, Inc. <br />