Corrosion Barriers &Assurance

Agenda

• Summary of Corrosion Threat

• Corrosion Barriers/Prevention

• Assurance

• Seawater Water Injection System

Summary of Corrosion Threat• CO2/Sweet Corrosion• H2S/Sour Corrosion• O2 Corrosion• Microbial Influence Corrosion (MIC)• Galvanic Corrosion• Flange Face Corrosion• Internal Chloride Stress Corrosion Cracking• Weld Corrosion• Crevice Corrosion• Atmospheric Corrosion• Corrosion Under Insulation• External Stress Corrosion Cracking• Hydrogen Induced Stress Corrosion Cracking

Corrosion Barriers/PreventionsCorrosion barriers aims at reducing the effect of conditions that can lead to corrosion. This can be done by putting measures in place such as;• Material selection• Application of chemicals such as oxygen scavengers and

corrosion inhibitors• Pipeline design• Avoiding the use of de-similar metals.• Application of coating.

Material SelectionThis is a step in the process of FEED design. The main goal is to minimize cost while meeting product performance.Material option• Carbon Steel and low alloy steel• Austenitic SS• High-alloy austenitic SS• Duplex SS• Nickle base alloys

Application of ChemicalsCorrosion Inhibitors A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. • At the design phase it is assumed that the inhibitor efficiency is sufficient to reduce the

residual corrosion rate to 0.1 mm/yr. • In the case of high corrosivity fluid (i.e. CR > 5mm/yr) the company standard considers

that the inhibitor efficiency is not able to reduce the corrosion rate to 0.1 mm/yr and a maximum inhibitor efficiency (E) 95% should be imposed for design purpose and the residual corrosion rate will be calculated as CRinh = (1-E)*CR.

CA= (CRinh*DL*CIA) +CR*DL*(1-CIA)where:

CRinh is the residual corrosion rate equal to 0.1mm/yr or (1-E)*CR in case of high corrosivity fluid. DL is the design life. CIA is the corrosion inhibitor availability. In the design phase this value is assumed to be equal or greater than 90%.

Other Chemical Injected Includes;Oxygen Scavengers. Added to enclosed packaging to help remove or decrease the level of oxygen in the package. They are used to help maintain product safety and extend shelf life.Biocide. Used to control the level of microbiological organism within the system, which can lead to Microbial Induced Corrosion.

Oxygen Barriers• Deaerator. This is a mechanical device that is used for the removal of

oxygen. It’s intended use is for the reduction of oxygen concentration down to less than 100ppb

• Chemical Scavenges. These are chemicals, usually with ammonium bisulphite (ABS) used to reduce the oxygen concentration to less than 20ppb.

Microbial Control Systems• Chlorination. As a minimum the seawater inlet is chlorinated usually

after the coarse filters• Biocide. Similar to chlorination, biocide are applied to control microbial

activities. It is recommended that biocide are injected down stream of the deaerator.

• Pigging. Regular pigging where possible, can reduce the effect of microbial activity.

Erosion/Solid ControlTo control solids and flowrate that can lead to erosion corrosion within the pipeline system, several controls are put in place;

• Flowrate. This involves setting a minimum and maximum flowrates to limit solids settling or eroding the pipe walls

• Coarse filter. This filters are installed in the inlet off the seawater injection to prevent suspended solid and organic particles large than 2 mm from entering the injection system

• Fine filter. Similar to coarse filter, the fine filter is used to prevent residual solid particles size below 50 microns.

Galvanic Corrosion Control

This type of corrosion can be controlled or made harmless by;• By avoiding the combination of larger area of old material

which have become more noble over long period of time, with a small area of new material which are more active

• Use of more noble weld consumer than the base material• Addition of corrosion inhibitors• Insulating coupling points• Application of metallic coating on the one of the materials.

Crevice Corrosion ControlThis type of corrosion can be reduced by;• Avoiding crevices when possible.• Design of pipeline to avoid deposition.• Application cathodic protection

Atmospheric Corrosion ControlMainly caused by the present of oxygen and water on the base metal surface. This type of corrosion is controlled by;• Coating of the base material• Cathodic protection.

ASSURANCEThis is a process or procedure performed to ensure that the barriers put in place are effective against corrosion threat. As a minimum the monitoring should include;

• Corrosion monitoring

• Oxygen monitoring

• Bisulphite residual measurement

• Chloride monitoring

• Total suspended solid monitoring

• Flowrate and fluid velocity

CORROSION MONITORINGCorrosion Coupons. Corrosion coupons are a minimum for corrosion monitoring. Corrosion coupons should be remove not less than 90 days and not more than 180 days after insertion, and all coupons should be exposed for the same time.

Types• Strip Coupon • Flush Disc Coupon

Deterioration mechanism

Corrosion probes. Secondary options include electrical resistance (ER) and linear polarisation resistance (LPR) probes.

Main function is to measure metal loss due to corrosion/erosionTypes1. Electric Resistance(ER) Probe (Flush and Tubular ER Probe )

2. Linear Polarized Resistance (LPR) Probe

Electrical Resistance (ER) ProbeER probe is a device that provides a basic measurement of metal loss, while the probe is

in-situ and permanently exposed to the process.

It measures the change in ohmic resistance of a corroding metal element expose to the

process stream. Wire Loop ER Probe

Flush ER Probe

ER probes have all the advantages of coupons, plus:• They are applicable to all working environments gases, liquids, solids,

particulate flows.• Direct corrosion rates can be obtained.• Probe remains installed in-line until operational life has been exhausted.• They respond quickly to corrosion upsets and can be used to trigger an alarm.

ER probes are available in a variety of element geometries, metallurgies and sensitivities and can be configured for flush mounting such that pigging operations can take place without the necessity to remove probes. The range of sensitivities allows the operator to select the most dynamic response consistent with process requirements. 

Corrosion rate calculation based on data retrieved from a probe

A T40 probe, which has a life span of 10mils, is used in a process application. October, 29th 2013, the probe had a reading of 72 divisions and on November 23rd 2013, the probe had a reading of 95 divisions. The corrosion rate calculation for this period will be;

Corrosion rate (mpy) = 0.365 x (Reading final – Reading initial) x Probe span

Time in days

0.356 x (95 – 72) x 10 mils

25 days

= 3.36 mpy

1mpy = 0.0254 mm/yr

3.36mpy = 0.0853 mm/yr

Mechanism Corrosion Rate (mm/year)  Low Moderate High Severe

General Below 0.025 0.025 to 0.126 0.127 to 0.254 Above 0.254

Linear Polarized Probes

Linear polarization resistance monitoring is an electrochemical method of measuring corrosion.It measures the relationship between the electrochemical potential and current generated between the electrodes in the process fluid which allows the calculation of corrosion rate.LPR TheoryR= V/IResistance measure if inversely proportional to corrosion rate

LPR probe with 20mV applied between the electrode

Electrode

Process fluid

• The advantage of the LPR technique is that the measurement of corrosion rate is made instantaneously.

• This is a more powerful tool than either coupons or ER where the fundamental measurement is metal loss and where some period of exposure is required to determine corrosion rate.

• The disadvantage to the LPR technique is that it can only be successfully performed in relatively clean aqueous electrolytic environments. LPR will not work in gases or water/oil emulsions where fouling of the electrodes will prevent measurements being made. 

Oxygen MonitoringThis should be carried out by a manual colorimetric method no less than once per shift, complemented by online monitoring by, e.g. an Orbisphere electrode. The latter should be calibrated at intervals of not less than 30 days.

Orbisphere

Bacterial Monitoring• Monitoring should be carried out for both planktonic and sessile

bacteria, for both sulphate reducing bacteria (SRB) and general aerobic bacteria (GAB) also known as general heterotrophic bacteria (GHB).

• Monitoring should take place at a consistent time ideally before biocide treatment but certainly not during.

• Biocide residuals should be measured once per month where a suitable analytical method is readily available.

Bisulphite residuals• These should be measured at the same

time as the manual oxygen residuals.• A suitable method is the Hach drop

count SU-5 kit or other equivalent.

Chlorine monitoringThis should be carried out by a manual colorimetric method based on diphenylene diamine (DPD) at the inlet and outlet of the deaerator tower. Chlorine residual should be measured at the same time as oxygen measurements.Total Suspended Solids MonitoringTSS monitoring should typically be carried out weekly, but at not less than monthly frequency.

Seawater Injection System

Sea Water Injection system corrosion threats, barriers and assurance