Internal corrosion allowance
A corrosion allowance of 3 mm is generally recommended for carbon steel piping, unless higher corrosion allowances are required. However, each system should be evaluated and the selected corrosion allowance be supported by corrosion evaluations. All piping classes in carbon steel grades in NORSOK L-001 have a corrosion allowance of 3 mm for standardization reasons.
For submarine pipeline systems a maximum corrosion allowance of 10 mm is recommended as a general upper limit for use of carbon steel. Carbon steel can be used in pipelines where calculated inhibited annual corrosion rate is less than 10 mm divided by design life. Otherwise corrosion resistant alloys, solid or clad or alternatively flexible pipe, should be used. For pipelines with dry gas or non-corrosive fluids, no corrosion allowance is required. Corrosion during installation and testing prior to start-up shall be considered.
Corrosivity evaluations in hydrocarbon systems
Evaluation of corrosivity shall as a minimum include
- CO2-content,
- H2S-content,
- oxygen content and content of other oxidising agents,
- operating temperature and pressure,
- organic acids, pH,
- halide, metal ion and metal concentration,
- velocity, flow regime and sand production,
- biological activity,
- condensing conditions.
A gas is considered dry when the water dew point at the actual pressure is at least 10 °C lower than the actual operation temperature for the system. Materials for stagnant gas containment needs partikular attention.
NORSOK M-506 is a recommended practice for the evaluation of CO2 corrosion.
A corrosion evaluation with inhibition should be based on the inhibitor availability, considered as the time the inhibitor is present in the system at a concentration at or above the minimum dosage.
The percentage availability (A %) is defined as:
- A % = 100 x (inhibitor available time)/(lifetime)
- Corrosion allowance (CA) = (the inhibited corrosion allowance) + (the uninhibited corrosion allowance)
- CA = (CRinhib x A %/100 x lifetime) + (CRuninhib x {1 – A %/100} x lifetime
where
CRinhib = inhibited corrosion rate.
CRuninhib = uninhibited corrosion rate (from NORSOK M-506 or other model).
At the design stage an assumption may be made that inhibition can decrease the corrosion rate to 0,1 mm/year. The inhibited corrosion rate shall, however, be documented by corrosion tests at the actual conditions or by relevant field or other test data. It should be noted that to achieve the target residual corrosion rate, high dosages of inhibitor may be required.
The inhibitor availablity to be used in a design calculation depends on the planned corrosion Management programme, including corrosion monitoring and corrosion inhibition. Unless defined otherwise, an inhibitor availability of 90 % shall be used. Maximum inhibitor availability shall not exceed 95 %. A 95 % inhibitor availability requires that a qualified inhibitor is injected from day one and that a corrosion Management system is in place to actively monitor corrosion and inhibitor injection.
The inhibited corrosion rate includes the effect of glycol and/or methanol injection. Lower inhibited corrosion rates with glycol and/or methanol can be used when documented by tests or other relevant documentation. The effect of any inhibitor depends on reservoir conditions which may change during production time.pH stabilisation can be used in condensed water systems to reduce the corrosion rate. pH stabilisation is only applicable in combination with glycol in sweet systems. NORSOK M-506 does not apply for this case, and a corrosion rate of 0,1 mm/year shall be used for design purposes, unless field or test data are available.
Corrosion inhibitors may have low efficiency and are not recommended to reduce corrosion of carbon or low alloy steels in production wells, subsea trees and subsea piping systems.
Use of corrosion inhibitors in process systems is not recommended, but can be used provided the inhibitor in each process stream satisfies the inhibitor supplier’s minimum recommended concentration for each stream and flow rate. Due to complex geometries and normally high flow rates, there is an increased risk for high
inhibited corrosion rates locally in process systems compared to pipelines, which will influence the need for inspection and maintenance.
In pipeline systems carrying hydrocarbons with condensed water, the corrosivity may be reduced by application of inhibitors in combination with pH adjustment as an alternative to inhibitors alone. The combined effect of inhibitors and pH adjustment shall be qualified and documented by corrosion tests unless relevant documentation exists.
Vessel materials for oil separation and gas treating systems shall be selected based on the same corrosivity criteria as for hydrocarbon piping systems. Vessels manufactured in solid CRAs, internally CRA clad or weld overlayed, will not need additional internal corrosion protection systems.
Galvanic corrosion between CRA equipment and the vessel wall in internally paint coated (lined) carbon steel vessels shall be addressed in case of coating damages. As a minimum CRA support brackets shall be painted. Other protection methods like cathodic protection may be considered.
Possibility for “sour” service conditions during the lifetime shall be evaluated. Sour service definition, Metallica materials’ requirements and qualification shall be according to ISO 15156 (all parts).
Drying or use of corrosion inhibitors shall not relax the requirement to use “sour” service resistant materials if the conditions otherwise are categorised as “sour” by the above documents.
If sand production and/or particles from well cleaning and squeeze operations are expected, an erosion evaluation shall be carried out. The evaluation should be based on DNV RP O501.
External corrosion protection
- Splash zone protection
- Use of coating
- Cathodic protection
- Corrosion protection of closed compartments
- Insulation, atmospheric exposure
- Galvanic corrosion prevention
Internal Corrosion Protection
- Carbon steel welds Mixture
sumber:
NORSOK STANDARD
Material Selection, 2004
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