The Facts & Myths About Shorted Casings

Transcription

1 The Facts & Myths About Shorted Casings Presented to TEG 292X [35,05] Direct Assessment Methodology Application 10 September 2006 by Earl L. Kirkpatrick, P.E. ELK Engineering Associates, Inc.

2 There Ain t No Such Thing As a Water Short!!! A direct metallic contact: Pipeline Steel to Casing Steel Or Test Lead Wire to Casing Steel IS A SHORT If there s water in a casing annulus, it may cause a potential shift on the casing, but IT IS NOT A SHORT

3 Office of Pipeline Safety Code of Federal Regulations External corrosion control: Electrical isolation. (c) Except for unprotected copper inserted in ferrous pipe, each pipeline must be electrically isolated from metallic casings that are a part of the underground system. However, if isolation is not achieved because it is impractical, other measures must be taken to minimize corrosion of the pipeline inside the casing.

4 ASSUMPTIONS Casing end seals keep rocks, dirt, and creepy-crawly critters out of casings, not water Moderately corrosive low resistivity soils Free Corrosion Potentials: -550 Bare Casing Pipe -450 Well Coated Pipeline* * Typically well coated pipelines exhibit lower P/S potentials than bare pipelines in the same soil conditions 2 ma/ft 2 required to polarize to -850 on (from -450) or 200 mv/ma polarization Linear polarization characteristics over a reasonable P/S potential range Ignore IR drops in water in the annular space

5 Therefore we may assume the following polarized P/S potentials, based upon assumed current densities per square foot of bare surface area. C.P. Current Density (ma/ft 2 ) Coated pipeline or casing (V) Base casing (V) Free Corrosion Potential

6 POST MOUNTED TEST STATION PIPELINE MARKER 4'-0" 3'-0" 2-#12 STR CU THW WHITE TEST LEADS CASING PIPE CASING INSULATOR (TYPICAL) CASING END SEAL (TYPICAL) CARRIER PIPE LOW END OF CASING NOTES : 1. TEST STATION REQUIRED AT ONE END OF CASING ONLY. INSTALL AT MOST CONVENIENT OR PROTECTED SITE. SKETCH b232901

7 Dry or water below the carrier pipe No pipeline CP current demand inside of the casing

8 Annulus partially full of water Holidays on bottom of pipe Minimal CP current demand

9 Annulus flooded All holidays in contact with the electrolyte Maximum CP current demand

10 SCENARIO I PIPE 2% Bare Bare Casing Pipe Condition Surface Area (ft 2 ) Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays 100 ft 12 Pipe ma ma Pipe Submerged ft 16 Casing Casing potential Casing Potential Pipe 418.9

11 SCENARIO I PIPE 2% Bare Bare Casing Pipe Condition Surface Area (ft 2 ) Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays 100 ft 12 Pipe ma ma Pipe Submerged ma ma 100 ft 16 Casing Casing potential Casing Potential Pipe

12 SCENARIO II PIPE 2% Bare Coated Casing 50% Bare Pipe Condition Surface Area (ft 2 ) Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays 100 ft 12 Pipe ma ma Pipe Submerged ft 16 Casing Casing potential Casing Potential Pipe

13 SCENARIO II PIPE 2% Bare Coated Casing 50% Bare Pipe Condition Surface Area (ft 2 ) Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays 100 ft 12 Pipe ma ma Pipe Submerged ma ma 100 ft 16 Casing Casing potential Casing Potential Pipe

14 SCENARIO SCENARIO III PIPE III 2% Bare Coated Casing 2% Bare Pipe Surface Area Condition (ft 2 ) 100 ft 12 Pipe Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays ma ma Pipe Submerged ft 16 Casing Casing potential Casing Potential Pipe

15 SCENARIO SCENARIO III PIPE III 2% Bare Coated Casing 2% Bare Pipe Surface Area Condition (ft 2 ) 100 ft 12 Pipe Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays ma ma Pipe Submerged ma ma 100 ft 16 Casing Casing potential Casing Potential Pipe

16 SCENARIO SCENARIO IV PIPE IV 2% Bare Coated Casing 1% Bare Pipe Surface Area Condition (ft 2 ) 100 ft 12 Pipe Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays ma ma Pipe Submerged ft 16 Casing Casing potential Casing Potential Pipe

17 SCENARIO SCENARIO IV PIPE IV 2% Bare Coated Casing 1% Bare Pipe Surface Area Condition (ft 2 ) 100 ft 12 Pipe Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays ma ma Pipe Submerged ma ma 100 ft 16 Casing Casing potential Casing Potential Pipe

18 SCENARIO SCENARIO V Coated Pipe V 5% Bare Coated Casing 2% Bare Pipe Surface Area Condition (ft 2 ) 100 ft 12 Pipe Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays 5% Holidays ma ma ma 5% Holidays Pipe Submerged ft 16 Casing Casing potential Casing Potential Pipe

19 SCENARIO SCENARIO V Coated Pipe V 5% Bare Coated Casing 2% Bare Pipe Surface Area Condition (ft 2 ) 100 ft 12 Pipe Resulting Polarized P/S Potentials 2 ma/ft 2 4 ma/ft 2 6 ma/ft 2 Holidays Holidays Holidays 5% Holidays ma ma ma 5% Holidays Pipe Submerged ma ma ma 100 ft 16 Casing Casing potential Casing Potential Pipe

20 If it ain t shorted don t call it a short