OTC 6602 - Often Overlooked Data From Offshore Subsea Survey

7/24/2019 OTC 6602 - Often Overlooked Data From Offshore Subsea Survey

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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

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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.

OTC 6602 - Often Overlooked Data From Offshore Subsea Survey

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Transcript of OTC 6602 - Often Overlooked Data From Offshore Subsea Survey