TABLE 1.
Recommended extent and Location of Survey
Structural Element | Extent of Inspection | Scan Orientation | Notes |
Main Legs |
100% from (-) 10'0" to mud line. |
Outboard side. |
Node crotch readings may be taken if required, not required for this procedure. Scan all leg anodes if fitted. |
Risers |
100% from (-)10'0" to mud line. |
Orientation Unimportant. |
Ensure ground connection on correct side of insulating device if fitted. |
Conductors (If Installed) |
One vertical scan in conductor area over entire platform depth. |
Orientation Unimportant |
Try to select a shielded conductor on the inside of the bundle. |
Vertical, Horizontal and Vertical Diagonal Framing |
25% of anode carrying members at each platform level. |
90° from anode orientation if fitted, if no anodes orientation unimportant. |
If anodes installed at 6 o'clock, scan at 3 or 9 o'clock. |
Skirt Piles and Guides (If Installed) |
25% of guides over entire length to mud-line. |
Outboard side. |
Use tip contact probe to stab exposed pile above guide if specified. |
Anode (Scans) |
At least 25% of anodes at each platform level. |
Scan face furthest from supporting member. |
If scan is difficult point read at either end and in center of selected anodes. |
Anode (Cleaning) |
At least 2 anodes per platform level. Increase to 4 on 8 pile structures on >200 feet of sea water. |
Clean (Water Blast) one end 6" and center 12" band full circumference. |
Select anodes for cleaning based on potential surveys, pick one high and one low at each level. |
Recording Potentials - For convenience a spreadsheet is used (Lotus 123). The survey software includes a module to allow potentials to be directly dumped into the spreadsheet at a user selectable rate. A single keystroke toggles data entry on or off. Macros in the spread sheet error trap bad readings. The readings will automatically scroll down columns until interrupted. manual input into the spread sheet is possible during data transfer for notations, etc.
TABLE 2.
Spreadsheet Input Template (with actual data)
POLASCAN INPUT TEMPLATE |
Ctrl+Shift |
= Auto Log Readings |
CTRL+PgUp for EXAMPLE |
Alt+Shift |
= Stop Auto Log |
|
Shift+Shift |
=Manual Entry |
PLATFORM: ABC123-B | PLAN ELEVATIONS: (ENTER WATER DEPTHS) |
DATE: 05-31-96 | 1 | 2 | 3 | 4 | 5 |
TECH: AJL |
-- |
-156 |
-218 |
-253 |
-288 |
SCAN 1 | SCAN 2 | SCAN 3 | SCAN 4 | SCAN 5 | SCAN 6 | SCAN 7 | SCAN 8 |
STARTWD |
-218 |
-218 |
-156 |
-218 |
-218 |
-184 |
-184 |
-184 |
ENDWD |
-156 |
-156 |
-288 |
-156 |
-156 |
-156 |
-156 |
-184 |
MEMBER TYPE |
VDB |
ANODE |
LEG |
VDB |
ANODE |
VDB |
ANODE |
HB |
MEMBER ID |
B1-A1 |
B1-A1 |
A1 |
B3-A3 |
B3-A3 |
A3-B3 |
A3-B3 |
A3-A4 |
CALIBRATE |
0.001 |
|
0.002 |
0.002 |
|
0.003 |
|
0.003 |
|
-0.922 |
|
-0.927 |
-0.943 |
|
-0.993 |
|
-0.961 |
|
-0.922 |
|
-0.927 |
-0.944 |
|
-0.990 |
|
-0.960 |
|
-0.922 |
|
-0.927 |
-0.944 |
|
-0.991 |
|
-0.960 |
|
-0.922 |
|
-0.927 |
-0.946 |
|
-0.990 |
|
-0.960 |
|
-0.922 |
|
-0.927 |
-0.947 |
|
-0.992 |
|
-0.960 |
|
-0.923 |
|
-0.927 |
-0.948 |
|
-0.987 |
|
-0.959 |
|
-0.923 |
|
-0.927 |
-0.948 |
|
-0.981 |
|
-0.957 |
|
-0.923 |
|
-0.927 |
-0.948 |
|
-0.979 |
|
-0.958 |
|
-0.923 |
|
-0.927 |
-0.949 |
|
-0.978 |
|
-0.957 |
|
-0.923 |
|
-0.927 |
-0.950 |
|
-0.977 |
|
-0.957 |
|
-0.924 |
|
-0.927 |
-0.952 |
|
-0.977 |
|
-0.956 |
|
-0.925 |
|
-0.927 |
-0.952 |
|
-0.979 |
|
-0.955 |
|
-0.925 |
|
-0.927 |
-0.953 |
|
-0.982 |
|
-0.954 |
|
-0.924 |
|
-0.927 |
-0.954 |
|
-0.982 |
|
-0.954 |
|
-0.924 |
|
-0.927 |
-0.956 |
|
|
-0.984 |
-0.954 |
|
-0.925 |
|
-0.928 |
-0.958 |
|
|
-0.985 |
-0.954 |
|
-0.926 |
|
-0.926 |
-0.960 |
|
|
-0.984 |
-0.953 |
|
-0.926 |
|
-0.927 |
-0.959 |
|
|
-0.985 |
-0.954 |
|
-0.925 |
|
-0.927 |
-0.961 |
|
|
-0.987 |
-0.954 |
|
-0.925 |
|
-0.927 |
-0.963 |
|
|
-0.988 |
-0.955 |
|
-0.926 |
|
-0.928 |
-0.966 |
|
|
-0.985 |
-0.954 |
Data Analysis
The data analysis phase consists of the following steps:
• Spreadsheet Organization
• External Data Input
• Anode Current and Current Density Computation
• Remaining life Computation
• Final Reporting
Spreadsheet Organization - The raw field data sheets are imported into an organization template which performs the following functions and calculations. This is a macro driven process and takes only a few minutes for each field template.
• Error trapping of all numbers.
• Organize data columns to always be in shallow to deep order.
• Generate single number anode and effective cathode potential list (the most negative anode potential is returned and paired with the average cathode value at the ends of the member to which the anode is attached.
• Prepares graphic output of all scans, sets limits to scale all scans on platform the same.
• Prepares a tabular listing of scans.
External Data Input - Table 3. shows additional information which is reqUired in order to perform a full data analysis along with the source and the criticality of the information
TABLE 3.
INFORMATION | SOURCE | NOTES | CRITICALITY |
DATE INSTALLED |
COMPANY |
FOR PLATFORMS OVER 10 YEARS OLD ONLY THE YEAR IS REQUIRED, FOR NEWER PLATFORMS MONTH REQUIRED. |
HIGH |
FRAMING DRAWINGS |
COMPANY |
STICK DRAWINGS OK IF SURFACE AREA IS KNOWN, OTHERWISE DIMENSIONED DRAWINGS WILL BE REQUIRED. |
HIGH |
ORIGINAL CP DESIGN CRITERIA |
COMPANY |
SURFACE AREA, CURRENT DENSITY AND DESIGN LIFE, THESE CAN BE BACK CALCULATED BUT ADDS TIME TO REPORTING PHASE. |
MODERATE |
ORIGINAL ANODE SIZE |
DRAWINGS |
LENGTH, WIDTH, HEIGHT, WEIGHT, CORE SIZE. |
HIGH |
ORIGINAL ANODE NO. |
DRAWINGS |
CAN BE PHYSICALLY COUNTED. |
HIGH |
ORIGINAL ALLOY |
COMPANY |
CAN BE DEDUCED FROM SURVEY |
LOW |
RETROFIT |
COMPANY |
DETAILS OF RETROFIT: WHEN? NUMBER / TYPE OF ANODES? |
HIGH |
WATER RESISTIVITY |
DATABASE |
DEEPWATER HAS IN-HOUSE DATABASE WITH THIS INFORMATION. |
HIGH |
SCAN DISTANCE |
OBSERVED |
DISTANCE OF PROBE FROM MEMBER (ROV SURVEYS), OR PROBE SENSING CORRECTION DISTANCE. |
HIGH |
WATER DEPTH |
MEASURED |
GENERAL DEPTH AT WHICH ANODE POTENTIALS ARE RECORDED. |
MODERATE |
ANODE CONSUMPTION |
MEASURED |
CIRCUMFERENCE, PITTING FACTOR RECORDED ON CLEANED ANODES. |
MODERATE |
ANODE LENGTH |
MEASURED |
-- |
HIGH |
Anode Current & Current Density Computation - Each surveyed anode has current output calculated in the spreadsheet using the same calculations as would be used to estimate the output of an anode in a new design. The only change is that the "2L rather than the 4l=L" version of Dwight's Equation is used. This is based on field matched data described by Mateer1. Thus:
Anode Resistance R = (r 12pl) (In(2ll r)-1) Ohms ............... (1)
where:
R = water resistivity (W-cm),
L = anode length (cm)
r = effective anode radius (cm)
Resistance is applied to Ohm's law to estimate current output:
Anode Current I =V IR ............................................................ (2)
where:
V = Effective driving voltage (Anode potential minus cathode potential)
R =Anode resistance from equation (1)
Current Density is simply computed by dividing the total current output for each level of the structure by the surface area of exposed steel in that same area: this will give an average current density.
Finite accuracy of these derivations is estimated to be +1- 15%, but the relative accuracy is much better than this and can give excellent comparisons between different platforms and different areas on the same platform. Current losses to steel below the mud line are ignored when computing current density on the lower elevations of a structure. This results in higher than actual numbers in this region but we have no good basis for how to correct this at this time.
Remaining Life Computation - Using the information on how long the anodes have been in service combined with the calculated current output above, with a correction made for increased output dUring the polarization period, the total number of ampere-hours delivered to date is estimated. An efficiency of 1100 ampere-hours / lb. is used to estimate the weight which should have been consumed. This number is compared to the weight loss based on physical measurement. If these values agree within +/- 20%, the remaining life to 90% consumed is calculated by straight weight loss / year extrapolation. If numbers do not agree within +/- 20%, no remaining life estimate is made for that anode.
Passive or low potential anodes are not considered (operating potential below (-) 0.930 V vs Ag/AgCI would be considered abnormally low). Any remaining life computation giving a result in excess of 20 years is reported as > 20 years, rather than the actual value.
Final Reporting - Final reporting will provide the following:
• Anode current / potential.
• Graphs of all anomalies or areas of interest.
• Disk containing all graphs for viewing.
• Potential summaries by level.
• Current density by level.
• Estimated remaining life by level.
Summary
The survey method described can be achieved offshore with little or no cost increase over a regular potential survey. The additional information can be used to plan retrofits more accurately4. The surveys are also showing CP designers how conservative early Gulf Of Mexico design criteria are (also how unpredictable early anode quality was). As we gather more information and begin to use new CP design methods5, the importance of these types of survey will emerge and will encourage further improvements in the technology.
References
1. American Petroleum Institute API- RP2A "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms" Section 14.0
2. M. W. Mateer "Often Overlooked Data Available from a Typical Offshore Subsea Survey," CORROSION/91 Paper No. 233
3. M. W. Mateer, K. J. Kennelley. "Designing Anode Retrofits for Offshore Platforms," Materials Performance Volume 33 NO.1 (January 1994)
4. W. H. Hart, S.Chen, D. Townley. "Sacrificial Anode Cathodic Protection of Steel in Sea Water," CORROSION/97 Paper No. 474