Use Of 316 in Offshore Water Systems

Use of 316L in Water systems

Offshore.

Possibilities & Limitations

By

Torfinn Havn, Ztrong AS, Norway

Use of 316L in water systems offshore.

Possibilities and limitations.

Agenda

1 What is 316

2 Properties

3 Limitations

4 Extended possibilities

5 Examples

Use of 316 in water systems offshore.

Possibilities and limitations.

What is 316

The 316 is an alloy belonging to the AISI 300 series, which have minimum of 16% Cr and 6% Ni. The alloying elements are balanced

to give an austenitic structure.

The 316 consists of 16-18 % Cr, 10-14% Ni and 2-3% Mo

The 300 series are available as sheet, plate, all wrought forms and castings. The most widely used group of stainless steels (e.g.304).

The brittle and detrimental sigma phase is not a problem for 316. But any additional Cr or Ni increases rapidly the risk for sigma

formation.

Protecting films formed by Cr, Mo and O atoms cause the high corrosion resistance


Properties

Corrosion of stainless steels might be due to

General corrosion

Pitting corrosion

Intergranular corrosion

Stress corrosion cracking

The problems experienced offshore with 316 are pitting and stress

corrosion cracking.

Both mechanisms are due to local breakdown of the protective films.

On the borderline corrosion.

PRE = %Cr + 3.3(%Mo + 0.5%W) + 16 %N

Use of 316L in water systems offshore. Stress Corrosion Properties.

Use of 316L in water systems offshore

Pitting corrosion is due to local breakdown of the protective films.

Chloride ions are main responsible for the breakdown

Increased temperature contributes

Presence of oxygen increases the electrochemical potential and leads to increased pitting risk

2H+2e- = H2Oxide layer Cr m On + Mo,W

thickness 20-40

Steel + Cr + Mo + N + W

Oxide layer Cr m On + Mo,W

thickness 20-40


Typical overvoltage curve.

log current

crit

repass


log current

crit

repass

Increased

Chloride

content &

tempe-

rature

Increased

oxygen

content &

chlorite

(NaOCl)


log current

crit

repass

Increased

Chloride

content &

tempe-

rature

Increased

oxygen

content &

chlorite

(NaOCl)

By cathodic protection or impressed current protection


log current

crit

repass

By cathodic protection or impressed current protection

F++

e

e

OH-

OH-

Use of 316L in water systems offshore. Limitations.

Can be used for fresh water only. Not for sea water.

Can not be used for produced water if there is oxygen present (O2 > 20-40 ppb).

Can not be used at temperatures > 60 C (140 F) due to risk of chloride induced external stress corrosion

cracking.

Use of 316L in fresh water systems offshore

A survey was undertaken to assess the limitations for 316L piping for fresh water in living quarters.

Both cold and hot water systems were needed in the living quarter

Some small amounts of chloride must be anticipated due to two reasons; either the fresh water is produced

offshore by evaporation technique, reversed

osmosis technique, or the water is transported by

ships over the sea.

Example 1

Existing material was CuNi pipe

Example 1

CuNi pipe for fresh water suffered from external

corrosion due to thermal insulation, 10 year in service

Existing material was CuNi pipe

Example 1

Wall thickness 0.9 mm External corrosion of

CuNi pipe 1 OD and original wall

thickness of 2.5mm.

Externally insulated.

Service 10 years.

On a North Sea

platform.

Some locations had

leakages through the

wall thickness.

Experience with CuNi pipe

Example 1

Strong demand: All bathrooms in offshore living quarters

shall have fresh water of minimum 55 C within 5 seconds.

This means that hot water is circulating 24 hours a day

in the looped pipe system.

The circulation of the water is damaging to CuNi piping

which at some locations suffer from internal erosion

corrosion.

Externally the thermal insulated CuNi suffers from general

corrosion.

Lesson learned; CuNi piping beneath thermal unsulation must be painted.

Use of 316L in fresh water systems offshore

The chloride content of various types of fresh potable water

which must be anticipated:

ppm chloride

Water from Norwegian lakes 0 10

Water produced by evaporation 5 15

Water produced by reversed osmosis 10 75

Water transported by ship from land 50 - 300

20

40

60

80

1 10 100 1000 10000 100k

ESCC

Pitting

Safe region for

316 SS

ppm Cl-

C

Safe use of 316L in fresh water with chloride as function of temperature

104 F

140 F

176 F

68 F

Safe use of alloys in fresh water with chloride as function of temperature

104 F

140 F

176 F

68 F 20

40

60

80

1 10 100 1000 10000 100k

ppm Cl-

Ti gr 2

Superduplex 25Cr/ 6Mo

Vanlig duplex 22Cr

316L

C

Ti gr2

25Cr/6Mo

22Cr

316L

The new material selection concluded with:

Use 316 SS in cold fresh water piping up to 40 C (104 F)

Use Ti Gr 2 in hot fresh water piping 40 C (104 F) - 60 C (140 F)

Never use CuNi piping in the fresh water system (due to external corrosion under insulation and constant high velocities in the hot

part of the piping system)

In general when 316L is not as corrosion resistant as service requires,

then use:

A higher alloyed material as 6Mo, 25Cr or Inconell 625

Titanium

A solution based on 316L in a combination with a sacrificial material or cathodic protection.

Example of a technical solution based on 316L with a sacrficial material

A produced water tank was to be designed in the top of the drilling shaft

on Sleipner A platform in the North Sea

5m

1.62m

5m

Extended possibilities

Example 2

Example of a solution based on 316L with a sacrficial carbon steel.

T operation ( 40 F min , 50 F normal, 185 F max)

Medium: Produced Water with oil

5m

1.62m

5m

Candidate materials:

1 Carbon steel + lining + paint + sacrificial anodes

2 6Mo / 25Cr

3 316L + anodes

4 Ti Gr 2

5 GRP

Example of a solution based on 316L with a sacrficial material

Materials Initial

cost k$

Comments

1 CS + lining

+ paint +

anodes60

High maintenance

cost, anodes do not

work > 60 C

2 6Mo / 25 Cr 230 Pitting corrosion must

be assumed

3 316L + CS

anodes

130 Limited experience

4 Ti Gr 2 400 Technical superior

5 GRP Many inlets & outlets,

falling objects,

stiffness

The design of the carbon steel anodes for 316L tank (half the tank)

Anode current output =135mA/m2 x 11m2 + 0.12 A (self corrosion = 8.3% x 135 x 11)

= 1.62 A

Protection potential = - 300mV (SCE)

Anode potential = -620mV +(log 1.62A log 0.12) x 60 mV/decade

= -553 mV (SCE)

Driving potential = -300 mV - - 553 mV = 253 mV

Anode size = 350mm x 350 mm x 35 mm

Anode resistance R = 30 ohmcm/ 2 x35cm = 0.43 ohm

Current output I = U/R = 0.253 / .43 A = 0.59 A

N1 = 3 anodes are needed

Anode mass for 20 years = (1.62x56x103x3600x8700x20)/(2x96500)= 294 kg

N2 = 294 kg/ (32.8 kg x 0.9) = 10 anodes are needed

The design of the carbon steel anodes for 316L tank (half the tank)

Seen from above.

After filling the tank with sea water, the anode potentials were measured:

Measured anode potentials

-560

-550

-540

-530

-520

-510

-500

1 2 3 4 5 6 7 8 9 10

SC

E (

mV

)

Average

Design

After filling the tank with sea water, the potentials were measured:

Measured potentials on Stainless Steel at mid distance between anodes

-530

-520

-510

-500

-490

-480

-470

-460

-450

-440

-430

-420

1 2 3 4 5 6 7 8 9 10

SC

E (m

V)

The design potential = -300 mV SCE

After filling the tank with sea water, the potentials were measured:

Bottom of tank stainless steel

-388

-386

-384

-382

-380

-378

-376

-374

-372

1 2 3 4 5

The design potential = -300 mV SCE

After filling the tank with sea water, the current output was measured:

Values spanned from 250 mA to 327 mA for the anodes.

Average value was 282 mA.

The design was based on 150 mA from each anode.

The readings just a few hours after water filling were judged not

representative. Most probably deposits on the surface will build-up and the

protecting current will decrease.

After 15 years in service the carbon steel anodes are proposed replaced.

This means that the protecting current is higher than 135 mA/m2 most

probably 20/15 x 135 = 180 mA/m2.

Example of a solution based on 316L with a sacrficial material

Experience of the produced water tank after 15 years in service

1 No pitting corrosion in the tank

2 Anodes are to be replaced in 2009 or 2010

3 The tank concept has worked as planned

Stainless steel 316 piping protected by carbon steel in sea water

The current demand for 316 SS surfaces:

Biological surface layer disappears if:

The temperature of sea water is above 30-40 C

There is hypochlorite injection

0.5 5 mA/m2

Without a biological surface layer:

150 mA/m2

With a biological surface layer:

Example 3 Results from testing

Experiment for testing of protection length with sea water.

Pump

Access for internal potential measurements (SCE)Carbon Steel

R

Isolation

Water Reservoir

Resistance

Stainless Steel AISI 316L

2 ND Pipe

Isolation

Sea water at 2 m/s velocity

Stainless steel 316 piping protected by carbon steel in sea water

Development of potential

as function of distance from carbon steelUnchlorinated water, resistance from 0 ohm - 30 ohm

-0,6000

-0,5000

-0,4000

-0,3000

-0,2000

-0,1000

0 100 200 300 400 500 600 700 800 900 1000

Distance from c-steel [cm]

Ele

ctr

oc

he

mic

al p

ote

nti

al v

.s. S

CE

[V

]

SS and CuNi Piping protected by carbon steel in sea water

Galvanic current in connection vs. resistance

0

100

200

0 10 20 30 40 50

Resistance [ohm]

[ m

A/m

2 ]

SS316 Chlorinated seawater

SS316 Seawater

CuNi Chlorinated seawater

CuNi Seawater

Calculated protection length based on

results from 2" pipe experiments

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14 16 18 20 22

Pipe Diameter [inches]

Pro

tec

tio

n le

ng

th [

m]

Tafel curve for Stainless steel 316 piping in sea water

Tafel curve for Stainless steel 316 piping in sea water

The secret

for the SS / CS

combination

Tafel curve for carbon steel in sea water

Use of 316 SS protected by carbon steel in chlorinated sea water,

current demand = 10 mA/m2

Lined deoxygenation tank

Carbon steel pipe 200mm long, 24 mm wall thickness, current

output = 200 mA/m2, surface = 0.2 m2 . Design life time = 20 years

Electrical resistor of 20 ohm

Static mixer 2000 mm long, 4m2 internal surface

25Cr piping

Cost difference Static mixer in Inc 625 (UNS

N10625) compared to 316 SS (UNS S31603)

= 74 000 USD

Sea water for water injection

Example 4

Use of 316 SS protected by carbon steel in chlorinated sea water,

current demand = 10 mA/m2

Cost difference:

Static mixer in Inc 625 (UNS N00625)

compared to 316 SS (UNS S31600)

= 74 000 USD

Example 4

Carbon steel corrosion spool piece design

To avoid galvanic corrosion between different materials, it is common to install isolation spool pieces between the metals.

The spool pieces are typical of length 5xND 10xND and internally lined

The spool pieces are expensive and the lining may flake off

Example 5

TiCarbon steel Isolation piece

U = R I (V)

R = x L / A () = resistivityL = length of spool piece

A = cross section

R = resistance

= paint film

How the Isolation Spool piece works

Carbon steel corrosion spool piece design

Example of corrosion spool piece:

Piping upstream fire hose reel stations are typical made of 6Mo, 25Cr or Titanium, while the hose reel stations include CuNi piping and valves in Cu

alloys

A carbon steel corrosion spool piece can be designed instead of an isolation spool piece.

A typical length of a corrosion spool piece is 0.4 m with a wall thickness of 15 25 mm depending of diameter and life time.

A 6 ND spool piece of 25 mm wall thickness can protect a CuNi pipe for 30 years and a stainless pipe for much longer time.

Conclusions

1 Stainless steel of 316 quality can not stand chlorides and high

temperature at the same time

2 With cathodic protection, the limits for use of stainless steel 316 can be

considerably extended

3 The cathodic protection can be by use of sacrificial carbon steel pipe or

by sacrificial anodes.

4 Based on available data, a safe and reliable design can be made

5 A 2 ND 316 stainless steel pipe can be protected up to 14m length

6 The carbon steel will work as sacrificial anode up to high temperatures

7 The carbon steel potential is low enough to protect 316 SS, but not so low

that hydrogen evolution take place and limit the potential length due to high

ohmic potential drop

8 Stainless steel 316 with carbon steel protection save significant costs

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