Design of Offshore Pipelines_Materials

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Presentation Reference Number Here

DESIGN OF OFFSHORE PIPELINESMaterials


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LINEPIPE MATERIALS


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Steel

Carbon Steel

Cost - A$ 1000 - 1500 per tonne Standard length is 12.2m joints (40 ft)

Main types of line pipe

Electrical Resistance Welded (ERW)

Submerged Arc Welded (SAW)

Seamless

Spiral welded


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Linepipe Manufacturing ERW


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Linepipe Manufacturing UOE SAW


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Carbon Steel Linepipe

Design conditions

Maximum / minimum pressure

Maximum / minimumtemperature

Fabrication requirements

Corrosivity of produced fluids

Design Life

Required Properties

Line pipe size

Strength Toughness

Weldability

Corrosion Resistance


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Line Pipe Properties


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Line Pipe Material Properties

Toughness

Ductile fracture resistance (mainly

gas lines) Brittle fracture resistance (low

temperature lines)

Test methods

Charpy tests (small scale)

CTOD (small scale)

Drop Weight Tear Tests (largescale)


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Fittings

Pipeline Fittings Includes; valves, flanges, bends, tees, etc

Similar materials selection issues apply

Should have compatible strength to line pipe Non-flanged components should be weldable

Valve materials should be selected to ensure integrity ofseals and compatibility


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Low Temperature Services

Toughness extremely important (eg Artic service) orclose to wells with significant Joule Thompson cooling.

Brittle fracture is potentially catastrophic

Transition temperature for material should be low

For


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Corrosion Resistant Line Pipe Material

Corrosion resistant in certain environments

Solid or clad / lined corrosion resistant line pipe

13% Cr (weldable grades)

Duplex stainless steel

Super austenitic stainless steel

Others (titanium, etc)

(flexibles)


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Flexible Pipe1. Interlocked steel carcass resists to

hydrostatic pressure, to radialcompression during installation andsupports the inner thermoplastic sheath. Itis generally manufactured with stainlesssteel AISI 304 or 316.

2. Inner thermoplastic sheath promotes

sealing, preventing internal fluids (oil, gasor water) from permeating to the externallayers. It is manufactured with nylon or asimilar material.

3. Interlocked steel pressure layer: resiststo internal and to hydrostatic pressure

and to radial compression. It is usuallymanufactured with carbon steel.

4. Double crosswound tensile armours:resist to axial forces, to internal pressureand to torsion.

5. External thermoplastic sheath: protectsthe internal layers against externalagents, like corrosion and abrasion, tomaintain the double crosswound tensilearmours tied and assure the sealing. It isusually manufactured with a polymer, likenylon.

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5

Good for short lengths / small to medium diameters.

Avoid metrology

Corrosion resistant

Significantly more expensive than steel for longlengths

Static or dynamic


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WELDING


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Pipeline Welding

Common Types

Submerged arc welding, SAW (double jointing); Shielded metal arc welding, SMAW (using cellulosic or low hydrogen

basic electrodes); Gas metal arc welding, GMAW.

Varying bevel preparations.

Productivity generally very important.

Quality also very important.


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Pipeline Welding

Welding may be manual, mechanised or semi-automatic. Weld inspection by AUT or X-ray Acceptance criteria based on workmanship or ECA.


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PIPELINE CORROSION


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Corrosion Risk

North Sea experience

22% of incidents are caused by corrosion

40% of these resulted in LOC (loss of containment)

80% of LOC incidents caused by internal corrosion

Australian experience

External corrosion in riser splash zones poor coatingperformance and inspection practices

SRB related internal corrosion - untreated hydrotestwater and/or infected well fluids

Varanus Island explosion believed to be a result ofpipeline corrosion.


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Topics

Internal and External Corrosion Types of Corrosion

Sweet

Sour Microbial Induced Corrosion

Corrosion Prevention

Inhibition

Coatings

Cathodic Protection

Corrosion Monitoring


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The Corrosion Process

All corrosion processes are electrochemical in nature, and in general require a metallicsurface in contact with an electrolyte (water). Involve anodic and cathodic reactions.

Anodic reaction involves the dissolution of metal into the electrolyte as positively chargedions.

Electrons remaining in the metal lattice migrate to the cathode and are discharged,

reacting with oxygen and/or hydrogen ions.

Anodic react ion:

MMn+ + ne-

M stands for a metal and n stands for the number of electrons

that an atom of the metal will easily release, i.e. for iron andsteel: FeFe2+ + 2e-

Cathodic reactions:

O2 + 4 H+ + 4e-2H2O (oxygen reduction in acidic solution)

1/2 O2 + H2O + 2e-2 OH- (oxygen reduction in neutral or

basic solution)2 H+ + 2e-H2 (hydrogen evolution from acidic solution)2 H2O + 2e

-H2 + 2 OH

- (hydrogen evolution from neutralwater)


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Sweet Corrosion

Carbon dioxide (CO2) corrosion results when CO2 dissolves in water toform carbonic acid (H2CO3). The acid lowers the pH and sufficient quantitieswill promote general corrosion and/or pitting corrosion of carbon steel.

Corrosion rates depend on:

Partial pressure of CO2 (Increasing pressure increases CR)

pH (lower pH increases CR)

Temperature (CR increases with temperature up to the point where stableprotective films are formed)

Saturation of fluid with iron ions Fe+ (Saturation decreases corrosion rate)

Flow regime

Hydrocarbon wetting

Inhibitors (including glycol)

Corrosion rates can be predicted using variations of De Waard-Milliamsmodel with correction factors.


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Sour Corrosion Process

H2S produced from some reservoirs.

Some designs consider possibility of future H2Sproduction even if not predicted based on reservoir tests.

Dissolution of hydrogen sulphide into the water phase

Metal attack to form iron sulphide and hydrogen gas


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Sour Corrosion Process

Hydrogen in the atomic stage is produced as part of the corrosion process.

In the presence of H2S it exists for a sufficiently long time at the steelsurface to become absorbed into the steel. (The presence of sulfidepoisons the metal surface reducing the ability for absorbed atomic

hydrogen at the metal surface to form H2, thereby increasing the rate atwhich atomic hydrogen diffuses into the metal lattice.)

Once inside the steel the hydrogen atom is free, unless trapped, to diffuse.

Diffusion rate dependant on lattice dilation (i.e. at highly stressed/strained

zones), solubility (i.e. differences in microstructure), concentration (i.e.towards the outer surface) and temperature.

If too much hydrogen is present at too high stresses in a susceptiblemicrostructure the result will be hydrogen embrittlement cracking and lossof internal integrity.

The trapped hydrogen atom will recombine to molecular gas, and becapable of exerting very high internal pressures.


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Sour Corrosion

Sulphide stress cracking, SSCThis is a form of hydrogen stress cracking that involves embrittlement of the metal byatomic hydrogen. High strength steel and hard weld zones are particularly prone to SSC.

Hydrogen induced cracking, HIC

This consists of planar cracking resulting from pressurisation of trap sites by hydrogen.This is typically seen in steels with high impurity levels. Note that HIC may occur without

externally applied stresses. When it occurs close to the surface it may result in blistering.

Stress oriented hydrogen induced cracking, SOHIC

This consists of staggered small cracks formed perpendicular to the principal stress(residual and applied) resulting in a ladder-like crack array linking small pre-existingfeatures akin to HIC. SOHIC is facilitated by high hydrogen concentration and local

stresses at and above yield strength.

Stepwise cracking, SWC

This is cracking that connects hydrogen-induced cracks on adjacent planes in the steelwall. SWC is dependent upon local straining between the HIC, and embrittlement of thesurrounding steels by dissolved hydrogen.

Sour Service Resistance is obtained by keeping the hardness of basemetal, heat-affected zones and weld metal at sufficiently low levels,and by improving steel cleanliness.


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Microbial Corrosion

Microbiological induced corrosion (MIC) is caused bythe presence of sulphate-reducing bacteria (SRB). TheSRBs feed on fatty acids (present in formation water)

and a range of hydrocarbons to produce sulphideswhich are corrosive.

SRBs can be introduced through formation water or

untreated seawater. Corrosion local to SRBs causes pitting.

The presence of SRBs can lead to rapid pipe walldamage. SRBs present in anaerobic conditions cause

the majority of MIC failures.


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Corrosion Control

Internal

Processing of pipeline fluid - ie de-water

Material selection - CS or CRA?

Chemical inhibition

Use of corrosion allowance (with chemical inhibition)

Internal coatings (not generally effective risks withcoating girth welds)

External

External coatings

Cathodic protection


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External Corrosion Control

External Coatings

Coatings isolate metal from contact with the surrounding environment

First defense against corrosion

A properly selected, applied, and installed coating should provide99%+ of the protection required, supplemented with cathodicprotection

Required Properties of Coatings

Effective Electrical Insulator

Effective Moisture Barrier

Ease of Application to Pipe

Ability to Resist Development ofHolidays with Time

Ability to Withstand Handling, Storage,

and Installation

Resistance to Disbonding when under

Cathodic Protection

Ease of Repair / Field Joint

Compatibility


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Cathodic Protection There are two types of systems

galvanic where the anode is made from a more reactive metal than thesteel, ie anode is sacrificed to protect the steel.

impressed current where the driving voltage is supplied by an external

power source, ie transformer rectifier (TR) unit.


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How Does CP Work

How Does Cathodic Protection Stop Corrosion?Cathodic protection prevents corrosion by converting all of the anodic (active) sites on themetal surface to cathodic (passive) sites by supplying electrical current (or free electrons)from an alternate source.

For pipelines usually this takes the form of galvanic anodes which are more active thansteel. This practice is also referred to as a sacrificial system, since the galvanic anodessacrifice themselves to protect the structural steel or pipeline from corrosion.

In the case of aluminum anodes, the reaction at the aluminum surface is:

4Al => 4AL+++ + 12 e- (4 Aluminum ions plus 12 free electrons)

and at the steel surface,

3O2 + 12e- + 6H20 => 12OH-

(Oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions)

As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen isarriving, no corrosion occurs.


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Sacrificial Anodes Design

Subsea usually Al/Zn/In alloy or pure zinc

Bracelet type, stand-off

Sled mounted

Sizing Considers:

Weight of alloy (alloy capacity, mean currentrequirements)

Anode end of life resistance - based onsurface area

Anode spacing

Issues to Consider : Current drainage - local structures

Interface with onshore pipeline

Hydrogen damage

Cathodic Protection


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Cathodic Protection


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Typical External Coatings

Surface preparation is very important in the applicationof all coatings

Coating Offshore Onshore

Asphalt Enamel With Concrete Coating Not suitable

FBE Good, but needs anti-slip & extra thickness

Good, but prone todamage

3LPE/PP Good Good

C C i


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Concrete Coating (Not Corrosion Related)

For negative buoyancy

For mechanical protection

Various densities used (2,240 3,400kgs per cubic meter)

Reinforcement types

welded wire mesh welded steel cages

Application Methods

impingement(high velocity spray)

wrap (Compression coat)

C t C ti


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Concrete Coating

Concrete Curing

Cage Reinforcement Installation and Concrete Mixing

Cement, Iron Ore, Sand and Water

Anti-Corrosion Coated Pipe

Sand Berm

Weighing Machine

Fog Cure

OD measurement

Fi ld J i t C ti


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Field Joint Coating

Cold tape wrap, heat shrink sleeve or FBE

Generally field joint infilled with PU foam (used touse mastic) to provide continuous outsidediameter

Fi ld J i t C ti T


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Field Joint Coating - Tape

Surface preparation limited to cleaning and wire brushing to surface qualitySA 3.

Installation time 3-4 minutes.

Wrapping tape fabricated from PVCor polyethylene with a self-adhesivelayer. Total thickness 1.52.0 mm.

The tape is cut to size, and appliedto the field joint as a cigarette wrap,with the overlap at the top of thepipe.

Customary to use three wraps, oneto cover the exposed steel and two(narrow) wraps to bridge the

gaps to the adjacent factory-appliedcoatings, overlapping byapproximately 50 mm.

Field Joint Coating HSS


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Field Joint Coating - HSS

Surface preparation limited to cleaning and wire brushing to surface qualitySA 3.

Installation time 3-4 minutes.

Heat shrink sleeves are manufacturedfrom radiation-crosslinked polyethylenetape, provided with a self-priming sealant.The total thickness is 1.52.5 mm.

Application similar to tape, except thatthere is only one wrap.

Shrinking on to the joint is carried outusing the yellow flame of a gas torch,applying the heat from the centre of the

joint area and outwards. A typical shrinking ratio is 20 25% and,

after shrinking, the overlap to the factory-applied coating should be at least 50 mm.

Summary


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Summary

Experience shows that corrosion can be controlled, butthere will always be some residual risk.

Coherent corrosion control & monitoring programs arepart and parcel of the corrosion management.

The monitoring methods used must always be designed

to be fit-for-purpose.

Design of Offshore Pipelines_Materials

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