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7/24/2019 OTC 6602 - Often Overlooked Data From Offshore Subsea Survey
1/6
OTC 66
Often Overlooked Data Available From a Typical
Offshore Subsea Survey
M.W. Mateer, ARCO Oil Gas Co.
This paperweePresentedtthe?srdAnnualo~ i nHouston,ex=,May6-9,191
Thi s paper waa sel ect ed f or preaent eti on by t he OTC Program Committee f ol lowing r eview of informat ion cont ained i n an abstr act submit ted by t he aut ho s). Content s of t he psper ,
aa p resented, hava no t been rev iewed by the Of fsho re Technology Conference and a re sub jact to co rrac tlon by the au thor a ). The mater ial , aa p resan ted , does not naceeaar il y ref lect
any pos it ion o f the Offshore Technology Conference or i ts o ff ic ers. Psrmlas lon to copy 1s res tr ic ted to an aba trac t o f not mo re than 300 words . I ll us trat ions may no t be cop ied. The abs trac t
should contain conapicuoua acknow ledgment of where and by whom the papar ia p resented.
ABSTRACT
Regul ar subsea i nspect i on of off shore
structures to assess the conditi on of cathodi c
protect ion systems is a way of 1i fe in the Gul f
of Mex ico.
In an effort to col lect more useful
data, new i nspect i on procedures were
i mpl emented i nto the ARCO Oi 1 and Gas Sout hem
O stri c t plat form inspect ions to see i f i t was
possi bl e to routi nel y cal cul ate the current
output of anodes using the anode resistance
method i n NACE RP-01-76.
By t ri al and err or , a
potent i al sampl i ng techni que was di scovered
which al l ows the col l ection of the kind of
potent i al data needed to cal cul ate anode
current wth an accuracy simlar to that
possibl e w th di rect measurement w th a gauss
meter. The speed at which this data can be
taken makes thi s survey techni que no more
costl y than a normal potenti al survey whi l e
provi di ng much more useful data.
Keywords:
cathodi c protect i on, anode,
potent i al survey.
I NTRODUCTI ON
A typi cal underwater potenti al survey
consi sts of measuring the potential of the
structure agai nst a standard reference
electrode ( usual l y Ag/AgCl ) in order to
i nsure the vi abi 1ity of the cathodic protection
system
Although a great deal of data can be
obtained, i t i s usual l y necessary to 1imt the
scope of the potential survey to keep the time
and cost reasonabl e.
When pl anni ng for the
ARCO Southern O strict 1989 pl atform surveys, a
11l ustrat i ons at end of paper.
compl ete rethi nki ng of our pl atform survey
requi rements was done to try and take the best
advantage of the changes caused by the new MMS
pl atf orm survey requi rements.
As par t of thi s rethi nki ng, the i dea of
using potent i al measurements to survey anode
current output was di scussed w th the survey
contractor.
I t was fel t that a usabl e
corre 1at ion coul d be found between anode
potent i al and anode current output usi ng the
cal cul ati on method recommended i n NACE RP-O -
76. This was cons idered to be an easy
addit i on to our survey that could be
accompl i shed w thout an i ncrease i n effort.
The mai n unknown factor was the proper l ocati on
of the potential survey poi nts needed to gi ve
the proper anode and cathode potent iaIs for the
cal cul ati on.
Ideas on how to obtai n the needed
data were devel oped and added to the survey
procedures pr i or to the star t of work. I n
order to have a standard against whi ch to
compare the resul ts, an underwater gauss meter
was obtai ned for taki ng anode current
measurements di rect l y from the anode supports.
Ouri ng the surveys, di rect current measurements
and anode potent i al s were taken on 57 anodes on
13 di ff erent pl atf orms.
ANOOE CURRENT CALCULATIONS
In theory, the output of an anode can
be found by using the equati on:
I = E/ R
(1)
where I i s the cur rent , E i s the potent i al
di ff erence between the anode and cathode and R
is the ci rcui t resi stance. Since the current
density i s much hi gher at the anode than the
383
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z
OFTEN OVERLOOKED OATA AVAI LABLE FROM A
cathode and the resistance in the stee1 is
negl igi bl e, the ci rcuit resistance can be
cons i dered to be the anode to el ectrol yte
resi stance.
This can be found by using a
modi fi ed form of Ow ghts equati on: (for deep
water)
R = 0.0627(f /L)[ ln(2L/r ) - 1]
(2)
Where R is the anode res istance in ohms, L is
t he l engt h of t he anode i n i nches, r i s t he
radius of the anode in inches and ~ i s the
r esi st i vi t y of t he wat er i n ohmcm To
cal cul ate the output of an anode then, one must
know the potent i a1 di ff erence between the anode
and the structure, the resist ivi ty, and the
length and radius of the anodes. To simpl i fy
and i mprove data gatheri ng, the fo1l ow ng
assumpti ons were made.
1.
Anodes were assumed to have the
ori gi nal manufactured di mensions unl ess survey
measurements showed a change had occurred.
2.
Structure potent i al s near the anodes i n
the survey were cons i dered to be adequate for
the cal cul ati on.
3.
Anodes potenti al s were taken i n several
di fferent pl aces on the anode to eval uate the
effects of geometry on potenti al .
These assumpti ons simpl i fi ed the data gatheri ng
and al l owed us to survey more anodes than woul d
have been possi bl e otherwse. For our
cal cul at ions the resist iv ity of the sea water
was assumed to be 20 ohmcm
The radi us was
assumed to be:
r = C12~
(3)
Where C is the circumerence of the anode cross
sect i on.
To i mprove the qual i ty of our
cal cul ati ons, a correcti on was made to the
measured anode potenti al to compensate for the
IR
drop caused by the mnimum two i nch (5.08
cm separati on between the anode surface and
the Ag/ AgCl reference.
Thi s di st ance i s set by
the const ruct ion of the reference cel l . To
compensate, two di ff erent anode resistance
cal cul ati ons were made, one usi ng the radi us
val ue determned from equati on 3 (R) and one
for a radius 2 inches (5.08 cm greater . (R+2)
The di ff erence between the two was considered
to be the resi stance of the el ectrol yte between
the anode and the reference cel l . To correct
the anode potenti al , f i rst the anode output was
cal cul ated using the measured anode potenti al ,
Then the calculated current is used to f i nd the
IR
drop between the anode and the reference by
the equati on;
IR
drop = Current x [(R+2)-R] (4)
Thi s
IR
drop i s then added back to the
measured anode potenti al and the current
cal cul ati on repeated w th the corrected
potent i al .
TYPICAL OFFSHORE SUBSEA SURVEY
OTC 6602
EXPERIMENTAL
Thi rteen pl atforms were surveyed for
anode performance as one part of our regul ar
pl atf orm surveys.
Al l pl atforms were
convent i onal 4, 6 and 8 pi l e designs set i n 30
to 90 feet of water i n OCS water s in the Gul f
of Mexico.
Platform ages var ied from 5 to 12
years.
Al l anodes were trapezoi ds i n cross
sect ion measur ing 8x 8 x 8. A few typical
anodes on each pl atform were measured for
actual di mensi ons. Our procedure was to
measure the potenti al of each anode three
t imes, once in the mddle on the side facing
away from the pl atform and once at each end.
Structure potenti al s were taken on the l egs as
near as possi bl e to the surveyed anodes. Many
of the surveyed anodes were al so cl eaned and
photographed so that we coul d correl ate
appearance to anode potenti al and current
output .
Each surveyed anode was measured for
current output w th the gauss meter four times.
Each anode standoff was measured tw ce, once
w th the meter fl i pped each way, to compensate
for any effect of the earths magnet ic f i el d.
The average of the two readi ngs for each
standoff was taken as the correct value. The
current val ues for each standoff were then
added to get the total current for the anode.
Someti mes the marine l i fe on the stand off was
too thi ck to al l ow the sensing cl i p to f i t
over the standoff .
In
these cases the di ver
removed the growth w th a chi ppi ng hamner. In
cases where fl uctuati ng or unstabl e
measurements were seen, repeated readi ngs were
taken unti l a stabl e number was achi eved.
RESULTS AND DI SCUSSI ON
Anode Potenti al Survey
A total of 57 anodes on 13 di f ferent
pl atforms were measured for current output and
potent i al .
Typical l y three or four anodes on
each structure were sel ected for test ing. The
potenti al s and vi sual examnati ons showed a
w de var iat ion in anode performance. Figure 1
shows the di stributi on of anodes by thei r
measured potenti al .
Over 65% of the anodes
surveyed had potenti al s more posi ti ve than -
1.00 volts vs an Ag/AgCl reference. The l owest
potenti al anode had a measured potenti al of -
0.951 volts vs Ag/AgCl. The mean potent ial of
the anodes was -0.991 volts . This was
surprising si nce the expected potential of the
anodes was -1.05 to - 1. 13 vol t s. Thi s i s ver y
di scouragi ng and l eads us to bel i eve that the
number of poorl y performng anodes i n servi ce
is greater than we had previously bel ieved. In
spi te of this , structure potent ial s were
uni formy good.
Evi dentl y the hi gh potenti al
anodes were produci ng more than thei r share of
the CP current.
As seen below thi s did turn
out to be the case.
The appearance of the anodes varied i n
l ine w th the measured potent ial . Those w th
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OTC 6602
MATEER 3
the most negati ve potent i al s rough and pi tted
whi l e the more posit i ve anodes have a much
smoother appearance.
Figures 2 & 3 show this
di ff erence i n appearance. None of the anodes
surveyed had so much meta 1 l oss as to change
thei r overal 1 shape.
The presence of mari ne
growth on the anodes did not appear to effect
thei r potenti al .
A potential di f ference of 10
to 15 m 11ivol ts was consistentl y observed
between the center and end potent i a1 readi ngs.
Thi s i s to be expected due to the higher
current dens i ty at the ends compared to the
center.
Anode Current Measurements
Measured current outputs for the anodes
ranged f ran 0.2 amps to 2.75 amps.
Surprisingl y, the current for each standoff was
never equa1 and somet i rm?swas very di f ferent.
In some cases 75
of the current was found to
travel through one standoff. A visual
i nspecti on of these anodes found some
separation of the alumnum from the core at the
l ow current end. Perhaps some extra resi stance
from a poor steel / al umnum bond i s responsibl e
for reduci ng the current. The most probabl e
cause for the core separation is poor casting
qual i ty.
For a given structure, a strong
rel ati on exi sted between the potent i al ,
appearance and measured output of the anodes.
The anodes w th the most negati ve potent i al s
i nvari abl y had the hi ghest current output and
the most corroded appearance. The expected
1i fet i mes of the anodes al so varied
accordi ngl y.
Measured Versus Predi cted Current
After the survey was compl eted and the
data reported, the predi cted currents were
cal cul ated and compared to the measured val ues.
The overal 1 correlati on between the measured
and predicted currents was not as good as
hoped. Table 1 contains the data from al l
anodes surveyed.
The compari son between
measured and predi cted anode output i s shown i n
Fi gure 4.
The 1i ne represent i ng the l east
squares f i t of the measured current data is
very close to the prediction but the data
scatter is large.
The probable cause of thi s
scatter is the lack of cathode potential
readi ngs close enough to the surveyed anodes.
Most of the structure potenti al s were taken on
the l egs, whi l e the anodes were l ocated
primari l y on the cross members. Our
expect.ati on that l eg potenti al s wouI d be
adequate for cal cul ati ng anode output was not
supported for most of the anodes.
anode currents for thi s pl atform i s much better
than for al 1 the anodes surveyed. The slope of
the least squares f i t 1ine for the measured
currents i s 28. 25 amps/vol t (0. 0354 ohms)
compared to the 27. 85 amps/vol t (0.0359 ohms)
derived frcm Dw ghts equati on. The curve f i t
of the measured data to the prediction is also
very good for thi s plat form The accuracy of
the calculation appears to be as good as the
expected accuracy obtai ned from using the gauss
meter.
These resul ts confi rmed our beli ef that
the poor correl ati on for the other pl atforms
surveyed was due to i naccurate cathode
potent ial data. Future surveys w 11 include
the potent i al of the members supporti ng the
anodes so that accurate cathode potenti al s can
be used i n the cal culati on.
ONCLUSI ON
Dw ghts equati ons can be used to
cal cul ate accurate anode current output from
structure potent i al survey data usi ng the
method gi ven i n NACE RP-01-76, section 5,
appendix 4. The qual i ty of data required for
accurate calculations is wel 1 w thin the reach
of a typfcal structure potenti al survey,
provided the fol l ow ng steps are taken:
*
Anode potent i als shoul d be taken in the
mddle of the anode rather than the ends.
*
The measured anode potenti al shoul d be
corrected for IR drop before the current is
calcul ated.
*
Cathode potent ial s shoul d be taken on
the member support i ng the anode.
*
The geometry of at least some of the
anodes shoul d be measured. Ori gi nal di mensi ons
may be used i f the anode shape is found not to
have changed substanti al l y since i nstal l ati on.
A visua1 i nspection of the anodes was
suff Ici ent i n most cases.
2. The measured potent i al s of t he anodes
i n thi s survey var i ed wdel y. Less than hal f
of the anodes surveyed met or exceeded the
potent i al expected for the a110YS used. Ei ther
the qual ity control for anode manufacture is
not as good as expected or the long term
performance of these anodes i s not wel 1 known.
3. Al 1 anodes sur veyed had unequa1
currents i n the two standoffs w th some anodes
havi ng 75% of thei r current i n one support.
The expected cause i s poor adhesion between the
core and the al umnum
Fortunatel y, one pl atform i n the survey
was found to have very consistent cathode
potenti als over the entire structure, i ncludi ng
l egs, nodes and cross members. Because of
thi s, we deci ded to survey every anode on this
st ructure as a test of the cal cul at i on. Data
f or t hi s pl at form i s gi ven i n Fi gure 5. The
correl ati on between cal cul ated and measured
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TABLE 1
SURVEY POTENTIAL ANO CURRENT OATA
ANOOE
ANOOE
CORRECTEO CATHODE MEASUREO CALCULATE
POTENTIAL
POTENTI AL POTENTI AL CURRENT CURRENT
IN VOLTS
IN VOLTS
IN VOLTS IN AMPS
IN AMPS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
-0.951
-0.952
-0.958
-0.963
-0.963
-0.964
-0.966
-0.968
-0.971
-0.972
-0.972
-0.972
-0.973
-0.974
-0.975
-0,975
-0.976
-0.979
-0.979
-0.979
-0.980
-0.981
-0.981
-0.982
-0.983
-0.987
-0.990
-0.990
-0.993
-0.995
-0.995
-0.996
-0.997
-0.998
-0.998
-0.999
-1.000
-1.000
-1.001
-1.002
-1 003
-1.003
-1.004
-1.004
-1.004
-1.004
-1.004
-1.007
-1.007
-1.009
-1.009
-1.010
-1.016
-1.033
-1.034
-1.041
-1.043
-0.953
-0.953
-0.965
-0.970
-0.965
-0.965
-0.969
-0.970
-0.974
-0.977
-0.976
-0.974
-0.978
-0.978
-0.987
-0.977
-0.982
-0.985
-0.982
-0.982
-0.983
-0.987
-0.985
-0.989
-0.988
-0.996
-0.996
-0.994
-0.998
-1.003
-1.003
-1.004
-1.004
-1.004
-1.006
-1.007
-1.008
-1.009
-1.009
-1.010
-1.012
-1.012
-1.014
-1.010
-1.010
-1.008
-1.009
-1.009
-1.013
-1.017
-1.017
-1.015
-1.019
-1.046
-1.047
-1.052
-1.053
-0.945
-0.950
-0.921
-0.935
-0.945
-0.957
-0.945
-0.945
-0.947
-0.935
-0.945
-0.947
-0.945
-0.945
-0.934
-0.961
-0.945
-0.934
-0.960
-0.953
-0.950
-0.945
-0.948
-0.945
-0.945
-0,935
-0.945
-0.959
-0.948
-0.945
-0.945
-0.945
-0.957
-0.935
-0.945
-0.945
-0.947
-0.934
-0.950
-0.956
-0.945
-0.953
-0.945
-0.960
-0.959
-0.961
-0.958
-0.985
-0.958
-0.957
-0.945
-0.964
-0.985
-0.970
-0,955
-0.980
-0.945
0,525
0.25
1.4
1.55
0.47
0.20
0.54
0.52
0.70
1.05
0.80
0.37
1.05
0.77
2.60
0.50
1.18
1.30
0.53
0.70
0.62
1.25
0.80
1.40
1.13
1.90
1.20
0.95
1.10
1.66
1.62
1.60
1.40
1.35
1.75
1.75
1.75
2,00
1.65
1.72
1.80
1.85
2.20
1.37
1.20
0.90
1.00
0.43
1.25
1.70
1.77
1.05
0.55
2.75
2.75
2.25
2.20
0.24
0.09
1.21
0.98
0.56
0.22
0.66
0.71
0.76
1.17
0.86
0.74
0.92
0.91
1.48
0.46
1.02
1.42
0.60
0.82
0.92
1.17
1.02
1.21
1.21
1.70
1 41
0.99
1.40
1.61
1.61
1.63
1.30
1.93
1.71
1.73
1.71
2.10
1.64
1.51
1.85
1.64
1.93
1.41
1.41
1.32
1.41
0.67
1.53
1.67
2.01
1.42
0.94
2.12
2.56
1.99
3.02
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NODE POTENTI L DISTRIBUTION
%OF ANODES PER GROUP
951-975 976- 1000
1001- 1025 1026- 1050
ANODE POTENTIAL -MILLIVOLTS )
Fig. node potential distribution.
Fig. 2GOOd anode.
F ig . 3Bad anode .
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388
ANODE OUTPUT COMPARISON
C LCUL TED VS ME SURED
AMPS OF CURRENT
3. 5
I
MESURED CURRENT
+CALCULATED CURREN -
DATA FROM ALL PLATFORMS SURVEYED
Fi g. 4Anode ou tp ut compar ison- dat a fr om al l platf orms sur ve yed .
ANODE OUTPUT COMPARISON
C LCUL TED VS ME SURED
AMPS OF CURRENT
3. 5
- .. - - . .. . _. _r_.
MEASURED CURRENT
+ CALCULATf=D CURRENT
DATA ON SINGLE PLATFORM
Fig . 5Anode out pu t comper feo n-d eta on ei ngl e plat form.