acomAVESTA CORROSION MANAGMENT

Corrosion Problemsin the Oil Industry

AbstractThis paper describes some examples of corrosion fail-ures in the oil industry from the Norwegian sector of theNorth Sea. The reported problems are mainly con-centrated on seawater applications, however, someexamples from water injection systems and productionsystems are included.

byRoy Johnsen

R&D Manager, Materials Technology, Statoil, Postuttak, N-7004 TRONDHEIM, Norway

All rights reserved. Comments and correspondence can bedirected to Sten von Matérn, Technical Editor, Avesta AB,S-77480 Avesta, Sweden. Tel. +46(0)226-818 00.Telex 40976 AVESTA S, telefax +46(0)226-545 07. No 3-1989

2

acom No. 3-89

IntroductionFailures of structures and equipment on an offshoreinstallation can be very costly due to loss of production.More important failures may present a hazard to plat-form personnel, as was the case with the AlexanderKielland accident in the Norwegian sector of the NorthSea in 1980. If a failure leads to ignition of oil or gas onthe installation, the consequences may be catastrophicwith a total loss of the installation and loss of humanlives, as in the Piper Alpha oil platform disaster last yearin the British sector.By investigating the causes of failure, further hazardsand lost production may be avoided. In the Norwegiansector there has not been reported any systematicevaluation of damages on offshore installations causedby metallurgical failures. On the British sector, however,Britoil has published a paper describing their experi-ence over the last ten years from five offshore installa-tions and one onshore production terminal linkeddirectly by a pipeline to an offshore unit.

According to the results of the investigation which ispresented in Table 1 and 2, the most frequently re-corded failure mechanism is corrosion. Table 3 detailsthe various corrosion mechanisms experienced (1).

Table 1: Types of failure on offshore-/onshore installa-tions from a review performed by Britoil (1).

Table 2: Main items involved in the failuresinvestigated by Britoil (1).

Table 3: Types of corrosion related failures in thereview performed by Britoil (1).

Type of corrosion % of totalfailures

CO2 related 9H2S related 6Preferential weld corrosion 6Pitting (unclassified) 4Erosion 3Galvanic 2Crevice 1Impingement 1Stress corrosion 1

Seawater systemsThe seawater systems which include the water injectionsystem, the ballast water system, the firewater system,parts of the fresh water making system and the coolingwater system are among the most important auxiliarysystems on an offshore installation. However, experi-ence has shown that seawater saturated with oxygenis one of the most corrosive environments to whichmetallic materials can be exposed. As a result of thisexperience from offshore installations under operation,the material philosophy for seawater systems has beenchanged during the last ten to fifteen years (2).

Cement lined pipeIn the project phase for the Statfjord A platform whichstarted in 1975, cement-lined carbon steel was se-lected for general seawater service. This choice wasmainly based on experience from onshore installationsin the USA. The Statfjord B and C platforms were alsodesigned with high percentage of seawater piping sys-tems made from cement-lined carbon steel.Soon after system start-up leaks in the piping systemswere experienced. The leaks were frequently repairedwith patches or large pipe sections were replaced. Afterfailure in a 3" nipple in the ballast water system onStatfjord A in 1982, a review of inspection routines andmethods was performed. In addition several studieswere started to review materials and ways of improvingreliability of the ballast water system. A material reviewon the use of cement-lined piping, performed by Mobil(the operator of the field until 31.12.86), showed thatcement lined pipes corroded most frequently in fieldjoints, in sections with turbulence or high velocity, insections with cracks or defects, in the cement lining andin areas where metallic couplings to more noble alloyscould initiate galvanic corrosion.

Based on material and piping design reviews it wasdecided to replace parts of the ballast water system onStatfjord A with titanium grade 2 using 6 Mo type valves(UNS S31254). The titanium parts should be internallycoated with polyurethane to reduce the cathode/anodesurface area and by that avoid galvanic corrosion inunreplaced cement lined carbon steel piping material.The titanium installation was performed in 1986.

Although inspections have shown large areas withcement lined pipes in good condition after more thannine years in service, Statoil will not recommend thismaterial for new offshore projects.

Cupro-nickel pipesPiping systems made from 90/10 CuNi or 70/30 CuNialloys have been used on several installations in theNorth Sea. According to information from the suppliersthese alloys have good properties in chlorinated sea-water. However, although practical experience showsthat CuNi-alloys do not suffer from corrosion undercertain service conditions, a lot of failures have beenreported. These failures have often been caused byerosion, polluted seawater, low iron content (< 1.3 wt%)or residues of carbon inside the pipe.

Parts of the seawater system on Statfjord B and C wereoriginally designed with 90/10 CuNi. After six to twelvemonths of service a number of leaks were discovered inthis system on Statfjord B. The leaks were mainlylocated close to elbows and tees or near reducers.

Type of failure %

Corrosion (all types) 33Fatigue 18

Mechanical damage/overload 14

Brittle fracture 9

Fabrication defects (excluding welding) 9

Welding defects 7

No defect (ie analysis and quality testing) 10

Item %

Production pipework and vessels 18Pumps and compressors 14

Downhole production equipment 12

Drilling and wireline equipment 12

Conductors and casing 10

Cranes and lifting equipment 9

Water injection, equipment and pipework 6

3

acom No. 3-89

A laboratory investigation showed presence of an ironrich second phase in grain boundaries with an increasein magnetic permeability. To overcome this problem,90/10 CuNi should be purchased in solution annealedcondition. If welding is needed it should be performedas rapidly as possible to reduce exposure time in thetemperature range 500-550°C.Another example is taken from a production platform atthe Ekofisk field with parts of the seawater systemmade from 90/10 CuNi. These include cooling system,oil coolers for the diesel engines and oil water treatmentsystem. According to the design specifications, thewater velocity should be lower than 3 m/s and the tem-perature below 11°C. After eight months in service,leakage in a pipewall with thickness 2 mm was dis-covered. Later on several other leakages were dis-covered. The reason for these failures were depositcorrosion caused by low water velocity (3).

Another failure was reported from the flotel "Kosmos"owned by Jahre Offshore A/S. After less than one yearin operation corrosion damages were discovered on aseawater system made from 90/10 CuNi, operating witha velocity of 1.8 m/s and max. temperature 30-40°C.The wall thickness was 3.5 mm.The reason for this failure was galvanic corrosioncaused by galvanic coupling between valves made fromstainless steel and the pipings made from 90/10 CuNi. Inthis couple, the piping material will act as an anode com-pared to the passive stainless steel. In earlier literature,CuNi-alloys and stainless steels have been presentedwith approximately equally free potentials in seawater.As can be seen from Table 4, this is not the case.Depending on the relative area difference between thetwo metals such couplings can cause severe corrosiondamages on the CuNi-alloy. At the Kårstø terminal,which is an onshore plant operated by Statoil, the cool-ing water is chlorinated seawater with max. tempera-ture 20°C. The piping system and the tubings in the heatexchangers are made from 90/10 CuNi and 70/30 CuNi.After less than one year in operation corrosion failuresoccured in both the tubings and the piping system. Thisfailure was caused by a combined effect between cor-rosion and erosion.

On the basis of the reported failures and on the fact thatcost and weight of these alloys exceed that of otherrelevant alloys (stainless steel with 6%Mo and glassfibre reinforced plastic (GRP)), Statoil does not recom-mend CuNi alloys for seawater piping systems.

Table 4: Potential development (mV SCE) of differentalloys in natural seawater flowing with velocity1 m/s and temperature 10°C.

Stainless steel pipesStainless steel pipes have been used for seawater sys-tems even before the offshore activity started in theNorth Sea. However, the experience from the use of thesocalled "stainless steel" which in most cases was AISI316L, varied. The alloy frequently suffered from local-ized corrosion. The reason for the positive result insome cases, was often caused by the not intendedcathodic protection form less noble alloys coupled toAISI 316L.

The selection of stainless steel Avesta 254 SMO (UNSS31254) for the whole seawater piping system on theGullfaks A platform, was a step into a new world for theoil industry. The order included approximately 40000 mwith pipes from 1/2-36" in diameter, 5000 flanges and23000 fittings. The total weight of the order wasapproximately 700000 kilos. In addition, several of thevalves for the seawater system are made from 254SMO. Later, 254 SMO has been selected for the sea-water system on the Gullfaks B and C, Oseberg andSnorre platforms.After four to five years in service at the Gullfaks field theexperience from the use of 254 SMO in seawater sys-tem, is very good. Some failures have been reported.Examinations have shown that exchange of alloys veryoften has been the reason for the failures. On severaloccasions AISI 316L has been used instead of 254 SMO.In these cases the quality control system has notworked properly. Corrosion failures in welds have beencaused by the use of not recommended filler metal. Thisexchange of alloys has been the most serious problemwhen using 254 SMO.

In some cases, however, 254 SMO has suffered fromcorrosion. Plate type seawater evaporators for freshwater production made with titanium plates and 254SMO type shell are installed on one of the Statfjord plat-forms. After one year in service some corrosion prob-lems were identified in this unit. Crevice corrosion wasobserved in the fresh water maker vessel door flangeseal face where it comes into contact with a nitriterubber door gasket. Corrosion was also observed on254 SMO surface of a back plate in line with the titaniumplates in the evaporator section. The reason for the cor-rosion attack on 254 SMO was that the critical crevicecorrosion temperature had been exceeded since theservice temperature was 48°C. Another corrosion fail-ure at the same temperature level has been reported byNorsk Hydro (4).

During the last ten years several research projects havebeen performed to examine the behaviour of stainlesssteel alloys in seawater with and without chlorination.Critical crevice corrosion temperature of UNS S31254depends on a lot of parameters like chlorination level,crevice geometry, alloy composition, welding condition,etc.

Exposure period (days)Alloy 1 7 13 32 50 80 90

90/10 CuNi -215 -215 -210 -210 -210 -210 -210254 SMO -100 -180 220 280 300 310 312AISI 316L -100 70 100* 120* 140* 140* 140*Titanium - 50 30 180 250 290 305 310Hastelloy C -150 200 275 300 310 310 313Inconel 625 -150 210 270 300 305 310 312

* localized corrosion

4

acom No. 3-89

Table 5 shows results from tests in chlorinated seawaterat 8°C and 30°C with flanged connections made from254 SMO and duplex stainless steel SAF 2205 (4).

According to own experience Statoil does not recom-mend the use of 254 SMO in chlorinated seawater attemperatures exceeding 30-35°C.

Table 5: Crevice corrosion of flanged connection ofSAF 2205 and 254 SMO in chlorinated sea-water (4).

Test Chlorine Seawater Number ofNo. level, ppm temp., °C attacks

1 0.2 8 SAF 2205: 4254 SMO: 0

2 0.5 8 SAF 2205: 4

254 SMO: 0

3 0.2 30 SAF 2205: 5

254 SMO: 0

4 0.5 30 SAF 2205: 6

254 SMO: 1

5 1.0 30 SAF 2205: 6

254 SMO: 4

Pump materialsThe most commonly used pump materials for seawaterhandling on offshore installations are NiAl-bronze andduplex stainless steel (SAF 2205). So far only one proto-type pump made from 254 SMO has been corrosiontested in seawater. This test has been performed byFrank Mohn A/S during the last three years. The testresults so far are acceptable with only minor corrosionattacks reported.The experience from the use of pumps made from NiAl-bronze which can be used with velocities up to 20 m/swithout erosion problems, is in most cases acceptable.The reason for the high resistance against erosion is thedevelopment of an aluminium rich oxide film on themetal surface. If the pH in the seawater is reduced, thestability of the oxide film will be reduced. This can be theresult if the seawater contains traces of sulphide pollu-tions. In such cases selective corrosion can occur. Thismeans that iron and aluminium are corroded, while cop-per rich phases are left in the surface. On the Statfjord Aplatform the impeller on one of the seawater pumps inthe ballast water system corroded very fast due to sul-phide pollutin in the water.

Duplex stainless steel suffers from localized corrosionin seawater under certain conditions even at tempera-tures in the range of 10-20°C. To prevent corrosionattacks a duplex stainless steel pump should be equip-ped with sacrificial anodes both for the external andinternal parts of the pump in contact with seawater.Frank Mohn A/S has during the last five to ten yearsdelivered several pumps to the offshore industry madefrom duplex stainless steel and equipped with sacrificialanodes. According to their experience, the pumpsbehave well with intact anodes, while corrosion attackshave been reported on parts without cathodic protec-tion (5).

Subsea productionsystemsDuring the last five to ten years the use of subsea pro-duction systems have increased around the world. Suchsystems can produce oil and gas from one well or from acluster of wells, and are connected to a platform or toshore by a flowline. According to results from an investi-gation published in 1988, more than 250 productionwells in the North Sea will be subsea completed withinthe next ten years.

In the Norwegian sector of the North Sea, at the NorthEast Frigg field which is a gas field operated by ElfAquitaine Norge A/S, the first field development wasbased on a subsea production concept. The field cameon stream in 1983 (6). Later, Statoil has started produc-tion from subsea completed wells at the Gullfaks field(satellite wells) and at the Tommeliten field (templatesturcture).

Localized corrosion on stainless steelOn subsea completed wells the main equipment likeXMAS-tree, pipings, hydraulic cables, etc., are sub-merged in seawater. The corrosivity of the seawater hasnot always been taken into consideration. In somecases this has caused corrosion attacks. Localizedcorrosion on equipment made from stainless steelhas been the most frequently occuring corrosionmechanism (7).

At the North East Frigg field the hydraulic lines, used foroperating valves on the XMAS-trees, consisted of flex-ible hoses and hard piping. By quick couplings the flex-ible hoses were connected to the manifold distributionvalve and to the hard piping. The hard piping was madefrom AISI 316L stainless steel and was clamped to thetemplate structure by means of rubber clamps. This iso-lated the stainless steel pipes and its couplings from thecathodic protection system.The first evidence of problems with the hydraulic lineswere discovered in 1985 when blisters were found onseveral of the nitrile rubber hoses during inspection.All the hoses and its couplings were subsequently re-placed by a synflex type the same year. Some of thenew couplings were made from AISI 304 stainless steelinstead of AISI 316L. During 1985 and 1986 several cor-roded connections were discovered. Later in 1986 allthe original hydraulic piping were bypassed usingflexible synflex hoses connected at each end to thestructure which was cathodically protected.

The main reason for the corrosion damages was local-ized corrosion on not seawater resistant stainless steeland the fact that the hydraulic piping system wasisolated from the cathodic protection system of thestructure. On some occasions the corrosion rate wasincreased by galvanic contact between AISI 304 andAISI 316L.The same type of corrosion has been reported fromother subsea installations. To prevent corrosion goodelectrical contact must be provided between the differ-ent alloys in order to protect all parts by means of thecathodic protection system.

5

acom No. 3-89

Hydrogen embrittlement of highly alloyedmaterialsFor applications in seawater which require high strengthand corrosion resistance, highly alloyed materials havebeen used extensively during the last years. The mostcommon application is bolting materials. In additionalloys are also used for valve stems, bodies, bonnets,etc. submerged in seawater. Some highly alloyed ma-terials are succeptible to hydrogen embrittlementunder certain conditions. The most important par-ameters are the stress level of the component, the pro-tection level from the cathodic protection system andthe microstructure of the component/alloy. Table 6shows the relative resistance to hydrogen embrittle-ment of some highly alloyed materials tested in hydro-gen and helium (8).

Table 6: Relative resistance to hydrogen embrittle-ment (9).

Table 7: Results of slow strain rate tests of high alloyedmaterials (9).

Strain Rate = 5 x 106 sec-1

Alloy H2 Pressure(MPa)

StrengthRatio*

INCOLOY 903 35 1.00AISI 316 69 1.00OFHC COPPER 69 1.006061-T6 69 1.00INCOLOY 802 48 0.99

7075-T73 69 0.98A-286 69 0.97RA330 48 0.95Be-Cu (ALLOY 25) 69 0.93

AISI 310 69 0.93AISI 347 35 0.91ASTROLOY 35 0.90HASTELLOY X 35 0.87FM INCONEL 718 48 0.86AISI 1020 69 0.79

Ti-6Al-4V (ann.) 69 0.79INCONEL 625 35 0.76AISI 1042 (norm.) 69 0.75HY 100 69 0.73MONEL 400 48 0.65

MP35N 69 0.50INCONEL 718 69 0.46AISI 4140 69 0.40RENE 41 69 0.27

INCONEL X-750 48 0.2617-7 PH (H 1050) 69 0.23AISI 410 69 0.22250 MARAGING 69 0.12

* Notched strength ratio (H2/He) at room temperature

Bolts made from Monel K-500-a precipitation harden-ing copper-nickel alloy-have been widely used for sub-sea application in the North Sea. However, during thelast years some failures caused by hydrogen embrittle-ment have been reported. Failures of Inconel 625 boltshave been reported during the last year. The mainreason for this failure was a not acceptable micro-structure caused by the heat treatment, which was notperformed according to specifications combined with acertain stress level and cathodic protection.

Conoco Inc. has performed a research project to look atthe succeptibility to hydrogen embrittlement of cathod-ically protected bolt materials. As can be seen from theresults in Table 7, Ferralium 255, Inconel X-750, Inconel718 and Monel K-500 are embrittled, while copper-beryllium and A-286 do not suffer from hydrogenembrittlement (9).

Alloy Yield Tensile Elonga Reduc- Time to(MPa) (MPa) tion tion failure

(% in of area (hours)2 in.) (%)

Monel K-500Air 579 965 25 36 36.3Seawater 600 993 27 42 39.8CP* 586 662 4.2 8 6.1CP 8 days** 565 634 3.5 7 4.1

Ferralium 255Air 759 896 26 76 32.8Seawater 793 903 24 65 28.0CP* 745 862 10 21 11.2CP 8 days** 793 876 7 22 19.1

Inconel X-750Air 634 1034 - 45 33.4Seawater 710 1103 22 50 32.2CP* 655 1062 - 14 16.3CP 8 days** 648 1034 8 12 14.4

Inconel 718Air 655 1041 27 43 32.9Seawater 648 1048 27 46 32.3CP* 627 1027 27 36 33.9CP 8 days** 655 972 15 13 19.0

BerylliumAir 931 1027 10 32 15.7***Seawater 979 1076 11 45 22.7CP* 917 1034 10 47 16.4CP 8 days** 889 1041 9 47 22.0

A-286Air 903 1158 17 44 20.6Seawater 910 1165 15 50 19.3CP* 924 1158 16 44 21.3***CP 8 days** 889 1158 17 37 21.4* Cathodically protected with aluminium anodes.** Coupled to aluminium anode for 8 days prior to testing

coupled to aluminium.*** One specimen.

Corrosion failures inother systemsCO2-corrosionCorrosion problems related to carbon dioxide in theproduced fluids have been a source of failure in produc-tion equipment. Damages have been observed in down-hole equipment, tubings, flowlines between the well-head and the process unit and in the process unit itself.The problems are caused by the corrosive effect of car-bonic acid produced by the dissolution of carbondioxide gas in water phases. The corrosion rate isaffected by temperature, pressure, pH, water composi-tion and the velocity of the fluid. Even if the iron car-bonate corrosion product formed may give some pro-tection to the steel, this is often removed at regions withhigh turbulence or velocity, forming the "mesa-type"corrosion. This type of corrosion has often been ob-served on the sides of production tubings or down-stream of obstacles, bends etc. causing turbulent flow.

Up to now the DeWard & Milliams nomogram, shown infigure 1 (page 6), has been the most widely used to pre-dict the corrosion rate of carbon steel in CO2-environ-ment.

6

acom No. 3-89

Since this nomogram does not take into considerationthe development of a protective layer under certainconditions, it gives a conservative estimate of the cor-rosion rate. During the last ten years several million dol-lars have been spent trying to develop a more accurateprediction tool. However, the DeWard & Milliams nomo-gram is still in use.

Conclusions

Figure 1DeWard & Milliams nomogram for calculations of corrosionrates (mm/year) as a function of partial CO2 pressure (bar) andtemperature (°C).Example: Corrosion rate 0.1 mm/year with 0.1 bar CO2 at 7°C.

Water injection systemsAnother source of failure has been associated with pref-erential corrosion of carbon manganese weld metal inseawater injection systems when used to weld typicalcarbon steel pipework.In pipework made from ASTM A106 Grade B material(0.25% C, 0.29-1.06% Mn) and ASTM A105 fittings(0.25% C, 0.6-1.06% Mn), corrosion attack has beenconcentrated in the weld metal with the wastage follow-ing the fusion boundaries. Corrosion rates up to 4 mm/year have been reported. No obvious damage to theadjacent parent metal has been observed.

Laboratory investigations with welded samples haveshown the occurrence of galvanic cells with the parentplate acting as a cathode and the HAZ and weld metalacting as an anode. This explains the selective attack ofthe weld zone. A series of corrosion trials have beencarried out in an attempt to optimise welding par-ameters using a number of alternative consumableswith various percentages of the elements Mn, Ni, Mo, Crand Cu. The trials indicate that the consumable contain-ing 0.5 %Ni and 0.4%Cu (ANS E 8018 G) appears to givethe most satisfactory performance from the weldingand corrosion resistance point of view. Further work hasbeen performed by the Welding Institute/CAPSIS in anindustry sponsored research project.

The amount of dissolved oxygen is another source forcorrosion attack in seawater injection systems. Accord-ing to the specification the oxygen content should notexceed 10-20 ppb in the injection water. However, dueto failure in the deaeration equipment and/or increasein the injection rate during certain periods, this criticalvalue can be exceeded. Combined with a water velocityin the range 3-5 m/s this can cause severe corrosionproblems in injection tubings and downhole equipmentmade from carbon steel.

¢ Corrosion is the most common cause of failure ofmaterials on offshore structures.

¢ Severe corrosion failures have been reported onboth cement-lined carbon steel piping and CuNi-piping for seawater systems.

¢ No severe corrosion problems have been reportedfrom the use of austenitic stainless steel Avesta 254SMO in seawater systems after four to five years inservice at the Gullfaks A platform.

¢ The main problem with the use of 254 SMO in sea-water systems is the mix-up of alloys and consum-ables for the welds.

¢ Submerged parts of seawater pumps made fromduplex stainless steel have to be protected by sacri-ficial anodes to prevent localized corrosion.

¢ Pumps made from NiAl-bronze suffer from corrosionin polluted seawater.

¢ All parts of a subsea production system (includingthe hydraulic lines) have to be cathodically pro-tected to prevent corrosion.

¢ High strength, high alloyed materials suffer fromhydrogen embrittlement under certain conditionsincluding cathodic protection in seawater.

¢ Severe corrosion attack on carbon steel tubings/equipment in seawater injection system caused bygalvanic corrosion in welds or too high level of dis-solved oxygen, has been reported.

References1. P. Nelson, J. R. Still: Metallurgical failures on offshore

oil production installations. Metals and Materials.September 1988.

2. Ø. Strandmyr: Operational experience from the Stat-fjord platforms. Presented at Holmenkollen ParkHotel, Oslo. 26-27.10.1988.

3. L. Lunde, R. Johnsen: Seawater resistant alloys (inNorwegian). Ing. Nytt nr 18-20, 1986.

4. R.E. Lye, R.S.Hansen: Seawater corrosion views andexperience. Presented at Holmenkollen Park Hotel,Oslo, 26-27.10.1988.

5. N. Nilsen, V. Dagestad: High alloyed materials in sea-water pumps (in Norwegian).

6. P. Tobiassen, N. Gil: Practical experience from NorthEast Frigg after four years in production. Presentedat the subsea seminar at Royal Garden Hotel, Trond-heim, 26-27.01.88.

7. E. A. Molinari: Subsea production systems-corrosionprevention. As above.

8. M. Jooston et al.: Material considerations for criticalservice subsea XMAS-trees and tubing hangers. Asabove.

9. LH. Wolfe, M. Jooston: Failures of Nickel/Copperbolts in subsea application. SPE Production Engin-eering. August 1988.

This paper was published at the 11th ScandinavianCorrosion Congress, Stavanger 1989. It is reprinted herewith the kind permission of the Author.

7

acom No. 3-89

acom is distributed free of charge to personsactively involved in the development of the processingindustry and other areas where stainless steels are im-portant.

to Sten von Matérn, Avesta AB, S-774 01 Avesta, Sweden

acom appears four times a year, and we welcomeyour application, as well as additional applicationsfrom your friends and colleagues.

Name: ___________________________________________________________ Position: __________________________________

Company: ________________________________________________________Companyactivity: ___________________________________

Mailing address: ______________________________________________________________________________________________

Postcode/City: _______________________________________________________________________________________________

Country: ____________________________________________________________________________________________________

£ Please, add my name to your address list acom No. 3-89

Nermans 9111046 12.89