02. offshore anticorrosion design guideline _ 2012.07

MOU corrosion protection

A Design & Operation Planning Guideline

Jang, Hug Jin / Coating inspector &approval engineer

DNV Korea &Japan

© Det Norske Veritas AS. All rights reserved Slide 2 23 October 2014

Together we will cover

MOU corrosion protection

1. Concept

2. Basic aspects of FPSO corrosion control

3. Corrosion influences &rates

4. Design Corrosion Control System

Corrosion margins, Coating systems & Cathodic protection

5. Inspection, Maintenance & Repair Strategies

6. Recommendations


Corrosion Protection

CONCEPT

Fatigue load from 20 years world wide corresponds to 10years North Atlantic trade.

Hence it needs a higher standard of AC system also

However corrosion management for Offshore unit operating in certain peaceful

regions, such as West Africa, China & Brazil, can provide significantly increased

challenges compared to their North Sea area.

Because, the corrosion rate is affected by the environmental factors, such as

relatively high air temperatures, RH & cargo temperatures.

Then we have to consider Key corrosion protection design factor

And Impact on inspection, maintenance and repair coating during operation



CONCEPT

Even though there are over 200 FPSO/ Offshore unit operating worldwide,

Designing and implementing a effective Inspection, Maintenance & Repair (IMR)

system for a 20 year service still remains a major challenge.

Due to limited information for a 20 year continuous service.



CONCEPT

NB Offshore unit have traditionally been purpose built for

harsh environments, such as the North Sea, with

converted Offshore unit dominating the peaceful regions,

such as Asia, Australia, Brazil &West Africa.

As a result, NB have been designed with a major focus

on fatigue & ultimate strength.

However, it is forecast that 90% of Offshore unit will be

installed in mild climate area in the future

Offshore unit can face serious corrosion control

challenges relative to their North Sea counterparts

because of increased ambient temperatures & humidity.

Many of the current NB ‘mega’ Offshore unit will have

production capacities over 200,000 bopd (Barrels of Oil

Per Day).

Therefore, any lost production time can have serious

economic consequences.



CONCEPT

Hence, it is very important to design, implement &

manage a proper corrosion protection design.

This presentation provides some background & guidelines to facilitate the cost effective corrosion control design for NB FPSO hulls.

Specifically the focus is on:

Important factors related to corrosion control,

Combination of corrosion margins, coating systems

& cathodic protection,

NB inspection,

Operation inspection, maintenance & repair

strategies



BASIC ASPECTS OF FPSO CORROSION CONTROL

For carbon & low alloy steel FPSO hulls there are three principal types of corrosion

to be considered

General corrosion,

Pitting corrosion, including “in-line pitting attack” &“grooving corrosion”, and

Galvanic corrosion (e.g. at welds).

General corrosion = the ratio of corrosion attack depth to its width is less than 1

Localized corrosion = the depth of the corrosion exceed the width

Pitting corrosion = the ratio gets much greater than 1

Galvanic corrosion is generated through preferential corrosion of the weld deposit

due to galvanic action, e.g. between a weld & the base metal.




1. Corrosion consequences

There are a number of different consequences for each type of corrosion.

General corrosion can result in fatigue, buckling & leakage.

However, the frequency of such failures is low to medium and it is easy to

inspect. During operation, qualified surveyors & inspectors are able to manage

the general corrosion by maintenance coating

Localised corrosion (pitting & groove) can result in serious leakage of tank

contents (lead to pollution or explosion).

And it is more difficult to inspect compared to general corrosion.

2. Structural integrity &corrosion control

The structural integrity of Offshore unit can be achieved through the following:

Corrosion allowance (for given structural members)

Corrosion protective coatings

Cathodic protection (for seawater tanks & the external hull)




3. Relationship between corrosion & time

The deterioration of the FPSO hull will mainly be determined by corrosion due to

seawater & the conditions in the cargo spaces .

The corrosion rate will change with time. Typically three different corrosion Model :

Model A: Corrosion wastage rate decreases with time.

Model B: Corrosion wastage rate increases linearly with time.

Model C: Corrosion wastage rate increases non-linearly with time.



CORROSION INFLUENCES & RATES

The below table1 provides some basic corrosion ranges for cargo & ballast tanks.



CORROSION INFLUENCES & RATES

There are some of the key operational factors that tend to significantly influence the

corrosion wastage for an FPSO.

These factors cover aspects such as temperature increases, oxygen in seawater,

chlorinity, humidity, temperature gradients, types of cargo oil, etc.

For example, a temperature increase of 10ºC can double the corrosion rate if all other

conditions are kept constant.

It means that we have to consider the corrosion rate of a ballast tank in a double-

bottom or side for an FPSO located in certain area

Where the viscous oil needs to be maintained at high temperatures (up to 80°C).

The high temperature of the oil tends to create a temperature increase in the ballast

tanks, particularly in the bulkheads to the oil storage tanks.

To further compound the matter, the increase in the temperature of the ballast tanks can

also lead to an increased bacterial corrosion.

In this context it can also be mentioned that bacterial corrosion can be exacerbated by

the organic material. the organic material could also be the protective coating.

Consider all these factor, it needs pre-qualified test for coating products


Corrosion Protection Temperature

A temperature increase of 10°C will double the corrosion rate if other conditions are constant (e.g. based on an ambient temperature of 20ºC)

Oxygen in the seawater 7.9 to 3.9 ml/l The corrosion rate will increase linearly with the oxygen content. However the oxygen solubility in seawater decreases with increasing temperature. Corrosion rate (μm/year) = 21.3 + 25.4 (O2 ,ml/l) + 0.356 (temp. °C)

Salinity of the seawater 32 to 36 ‰ Marginal or no effect on corrosion, because oxygen solubility in seawater is reduced slightly with increasing salinity. While increasing salinity will reduce seawater resistivity.

Chlorinity, 0/00 Chlorinity is related to the salinity by: Salinity = 0.03 + 1.805 * chlorinity

Flow effect Increasing flow rate will increase the corrosion rate. For a constant flow velocity V, m/s the corrosion rate will have a relationship: Corr. Rate (μm/year)= constant + constant * V 0.6

Tank sloshing or wave action Increased corrosion rate. The increase will depend on the tenacious/protective effect of the rust deposit on the steel surface.

Sulfide pollution in the in the seawater Sulfides in small amounts (ppm –level) can cause dramatic increase in the general corrosion effects. The presence of sulfides may also indicate microbiological corrosion (Sulphate Reducing Bacteria) NOTE : Sulfides may also initiate more insidious corrosion forms e.g. sulfide cracking corrosion mechanisms



Residual chlorine in ballast water

The presence of residual chlorine (0.5 to 1 ppm) in the ballast water will increase the general corrosion by about 100%.

Humidity 0 to 100%

In a tank with no free water the corrosion of steel will be negligible at humidity values below 60%. The amount of water in the atmosphere increases with increasing temperature. For high temperatures, e.g. 30°C and above, the humidity will lead to condensation in diurnal cycles in tropical and sub-tropical climates as the humidity will become higher than 100% at least during the night

Rust deposits

Rust can effect the progressive corrosion in basically the three ways: Models A, B and C (Figure 1). Coating damage

Coating will mitigate the corrosion in the areas were it acts as a barrier to water. The corrosion rate in an area with coating damage exposing bare steel will be as for the corrosion rate on any bare steel. Consequently exacerbated corrosion at coating damaged areas may occur, but is not the common effect.

Temperature gradients (e.g. due to hot bulkheads) A temperature increase of 10°C will double the corrosion rate, if other conditions are constant. This can result in corrosion rates of around 0.4 mm/year. Corrosion mitigation require adequate coating, which implies a coating system which is qualified for such high temperature service (cyclic temperatures up to about 60°C).



Properties of the oil in the storage tanks: water cut

The oil may contain a separate water phase. This water phase may be acidic. An acidity in the water phase (pH < 5) needs to be considered as this will cause serious corrosion of bare steel.

Oil: “Sour” “Sour oil” contains sulphides that may initiate localized corrosion forms, particularly in the tank bottom. Sulphides can also lead to the insidious corrosion forms which leads to cracking (e.g. hydrogen induced cracking)

Oil: “Sweet” “Sweet” oil contains CO2 which results to increased acidity in the water phase.

Cargo washing The practice and procedures for cargo washing should be considered also in relation to the possible effect on the corrosion and corrosion protection measures status of the tank.

Sludge tanks The accumulation of sludge/deposits in tanks can be expected to exacerbate bacterial corrosion in the tank bottom



FPSO hull arrangement &‘corrosion zones’

Offshore unit consists of a number of tanks & void spaces. The tanks may contain

ballast water, stabilised oil, unstabilised oil, slop tanks, methanol tanks &/ or

combination tanks.

As a result, Different AC system shall be considered for each space.

In case of cargo tank, it is divided into three corrosion zones (upper, middle & lower)

Hence Corrosion margins, coating systems & cathodic protection systems shall be

recommended in consideration of each corrosive environment



Each of these zones is further sub-categorised into horizontal and vertical surfaces and

plating and stiffening members.

Corrosion rates for different zones, orientations & structural types are provided in the

below table.

These rates can be multiplied by environment condition & operational condition



An overview of the commercial & operational impact, from different corrosion protection

measures, is provided in Table 3.

If owner spend

300,000 dollar

during construction,

they may save

millions dollar

during operation



DESIGN CORROSION CONTROL SYSTEM

Design corrosion margins

Generally, corrosion model B (linear increase) will be used

in corrosion wastage over time.

However, there is a need to evaluate the total impact of the

environmental conditions & the operational factors when

establishing the corrosion margin.

Corrosion allowances for traditional ship designs are based

on 10-year service, and are provided in various

Classification Society standards, e.g. DNV Rules

Generally, the ballast tank corrosion allowance will be in the

range from 1mm to 3.5mm for one sided corrosion

exposures. And For members with two sided corrosion

exposure the corrosion allowance will be in the range of

2mm to 7mm.




Design corrosion margins

To apply these corrosion rates to account for corrosion over

a 20 year service would result in twice the values for the 10-

year service, which is typically an unrealistic approach.

Therefore we have to consider :

Corrosion allowance as a safety factor for critical item

Plan a corrosion protection system to avoid corrosion

wastage, and

Develop an appropriate IMR plan to ensure correct &

consistent coating maintenance and anode renewal.




Coating systems

Quality of the coating applied at the NB stage will be the deciding factor for the

subsequent maintenance during operation.

A high performance coating a long service life & result in the minimum maintenance.

The high performance coating will imply high initial costs.

However, the benefit will be the optimal life cycle cost for the operation of the FPSO.

=> Also low coating maintenance requires

The result for wrong coating maintenance work can be an exceedingly short service

life of the AC performance and rapidly increasing maintenance/repair costs.




Coating systems

For example,

a high performance corrosion protection system costs can be around US$ 4 to 8 million for a

VLCC sized NB FPSO compared to US$ 1~2 million for a low quality coating system (based on

surface area of 200,000㎡).

The costs for re-coating only one tank (2000 ㎡ at 3 times the cost of work at a dockyard) during

operation can be around US$100,000 to US$200,000.

If you also consider the impact from lost production, then could easily imply $5,000,000 delayed

revenue (10% of 200,000 Barrels of Oil Per Day at $25/barrel for 10 days).




Some typical coating systems used by the marine industry are listed in Table 5.

It is important to be aware that coating of an FPSO is a very complex process which

represents a considerable workload & logistics task in the yard.

A successful coating application work will require extensive planning before

contracting.

Normally this is not part of the Classification Society’s scope of work, unless

specifically requested as a part of the contract or additional class notation,

Quality control of the surface preparation & application is essential to ensure that the

applied coating system will provide the expected performance.

It is impossible to achieve a high quality coating during maintenance in service.




Cathodic protection

Sacrificial anodes provide cathodic protection (CP) in the part of the ballast tanks

which are submerged .The design of the CP system is done based on providing

protection for a given service life. Sacrificial can here be retrofitted during the

service life.

For the external hull, the protection will be a combination of coating & sacrificial

anodes, which is necessary to achieve a 20 year service life.

For the external hull there are two alternatives:

Sacrificial anodes, or Impressed current protection.

The critical issue is to plan for a renewal interval of the anode system during service

life.

The cathodic protection current demand will be highest at the end of the service life.

It is important to ensure that anodes are retrofitted in the tank bottoms to prevent

fitting corrosion on bottom area.



INSPECTION, MAINTENANCE & REPAIR STRATEGIES

A corrosion protection design can only be cost effective with proper IMR program..

Specific inspection plan of corrosion margins, coating protection & cathodic

protection shall be prepared.




Corrosion margins

With a good corrosion protection design the FPSO global

strength may be based on a gross thickness approach.

A net thickness (gross minus corrosion margin) approach

may be considered for local strength calculations

It was recommended to use corrosion margins as an

additional level of safety factor whenthere be a breakdown

in either the coating or cathodic protection systems.

Generally there is a poor link between what corrosion

margins were used during design & what thickness loss

may be accepted during operation.

If pitting intensity is less than 20% then a localized 40%

loss in thickness may be accepted.

If the pitting intensity is greater than 20% then only

localized 20% loss in thickness is acceptable. For main

deck & bottom plating with 0.4L a thickness of 20% is

acceptable provided adequate buckling strength remains.




Corrosion margins

The current large NB FPSO designs have extremely large stillwater bending moments

that dominate the global strength design, So the buckling criteria can easily govern.

Hence, it is important to specify a essential corrosion margins used for IMR program.




Coating protection

MOU Maintenance coating requires close attention to

planning procedure to ensure adequate coating

quality.

The result of inadequate coating maintenance work

can be an exceedingly short service life AC coating

The maintenance strategies focus on the coating

maintenance is that corrosion wastage (general

corrosion) is not considered a problem when the

coating breakdown is within the range 3%




However, to ensure that corrosion wastage does not become serious,

it is then necessary to plan that the coating maintenance be carried out prior to a 3%

coating breakdown situation.

Since there are many variables that can impact the performance of the coating system

it is difficult to predict the progress of coating breakdown..

However, a coating breakdown of less than 3% within the first 2 to 3 years of operation

typically indicates that the coating will function adequately for an extended service life.




As a part of the cost optimization of the corrosion protection system, it is also

important to understand the relationship between coating condition & Class survey

As shown in Table 6. If the general corrosion breakdown exceeds 20% then annual

inspection is required,

which requires that tank to be taken off line, cleaned, gas freed &made available for

inspection.

This can be an expensive process, considering the impact from lost production.

The selected maintenance coating product needs to be qualified or to have

documented performance. In this context the service life of the maintenance system

needs to be considered.




Alternative coating system for maintenance




For maintenance coating the following characteristics

should be documented:

1) Adequate corrosion protection

2) Need for qualification of paint system

3) Compatibility with existing coating

4) Requirement for surface preparation

5) Application conditions (temperature & humidity).

6) Curing period &curing period between coats

7) Number of coats to provide full coating thickness.

DNV has established a qualification testing for ballast

coatings

The requirement for surface preparation (e.g. use of

water jetting or alternatives to dry blast cleaning) can

be considered.




Cathodic protection maintenance

Replacement of anodes needs to be considered and this can be determined by

measuring the potential as described in Table 9.

At a point where the anode mass is reduced by about 80% the anode needs to be

replaced.

For the external hull the need for maintenance of the anodes/impressed current

system requires planning at the design stage. This is necessary to ensure that

anode retrofitting can be achieved in a cost effective manner as needed during the

service life of the system

The cathodic protection current demand will be highest at the end of the service life



In summary

Corrosion design, implementation & management during operation for a NB FPSO

intended for a 20 year service life remains a major challenge.

The key points are as follows:

There are many different environmental factors that influence corrosion and

these need to be understood in order to select an appropriate corrosion control

system.

A combination of coating, cathodic protection &corrosion allowance is

recommended for NB Offshore unit.

High performance coating systems (COAT III) are recommended as a cost

effective solution due to their life cycle cost advantages.

Quality control of the coating application during fabrication &maintenance is

critical to ensuring that it functions as planned.

A coating breakdown of less than 3% within the first 5 years generally indicates

that the coating system will function adequately for an extended service life.

Corrosion allowances are primarily required as an additional safety factor if a

good quality painting system is applied & maintained. In this case a corrosion

allowance based on a 10-year service is considered sufficient.

Systematic maintenance shall be developed that facilitate the development of a

cost optimal coating protection system


Hug Jin JANG / Coating Engineer, Production Support

E-mail: [email protected]

http://www.dnv.com/

02. offshore anticorrosion design guideline _ 2012.07

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