Technical Presentations at the July 2012 Meeting

1.1   'Risk Based Inspection Planning for Ageing Oil and Gas Assets', Steve Matthews, Plant Asset Management, Petrofac 

Risk Based Inspection (RBI) is a risk assessment and management process focused on loss of containment of pressurised equipment due to material degradation.  RBI plans use risk assessments to determine the interval, scope and method of inspections.  This should ensure that the same risk values produce the same ‘level’ of activity.  The long-term objective is to drive down risk through effective inspections and broader integrity management activities such as corrosion management. 

RBI applied to ageing facilities can provide several challenges; in particular, the study will generally coincide with a life extension scenario.  Furthermore, there will often be issues around document and data management, such as P&ID updates, inspection & corrosion management records and repairs & modifications traceability.  Finally, the duty holder may have to accept higher ’risk profiles’ and the need for increased inspection resources. 

A recent Case Study involved a client in mainland Europe.  The scope of work comprised onshore oil and gas production facilities, including buried pipelines and flowlines.  The RBI programme formed part of a wider initiative to move to international ‘good practice’ in Asset Integrity Management.  The work was a particular challenge due to the current status of facilities and records; pipelines had variable diameters, no external corrosion protection and were unpiggable.  There was also a general history of leakage but no records maintained. Surface facilities were subject to some inspections, however, there were general problems with fabric maintenance and piping integrity.  The RBI methodology was shaped during discussions with client and designed provide a robust, practical and sustainable approach.  It was based on a mixture of semi-quantitative analysis and qualitative assessment to reflect the general lack of data.  The methodology included a process for screening out low risk systems from further analysis. 

The Pilot study concluded that direct inspections of pipelines and flowlines should be carried out as part of a planned, risk-based excavation programme.  It was recommended that the inspections should initially focus on gathering data from High Risk pipelines and flowlines with low predicted remaining lives due to internal and external corrosion.  It was also recommended that relevant information on location and pipeline conditions should also be collected as part of pipeline interventions for investigation of leakages and repairs.  The introduction of additional inspection techniques (including Guided Wave Inspection) should technical evaluation and live demonstrations.  Finally, it was recommended that the client establish a piping inspection programme based on industry good-practice and enhance existing static equipment inspections by increasing use of ultrasonic wall thickness measurements. 

[A pdf version of this presentation has kindly been provided and is available to members from the Secretariat]. 

1.2   Engineering and Material Challenges for Dense Phase CO2 Pipeline Transport in High Pressure Dynamic Flow Mode’, Kumar Patchigolla, Cranfield University

In open literature, there is little information available with regards to the material corrosion, in relation to high pressure dense CO2 pipeline transport from single point sources, such as the power industry. A typical CO2 pipeline is designed to operate at high pressure in the dense phase. However, it is evident that although there is considerable experience of testing materials in lower pressure gaseous CO2 in the oil and gas industry, there is little understanding of the behaviour of pipeline materials when in contact with trace amounts of contaminants with CO2 captured either from power plants or the oil and gas industry.

In this particular project development, a dynamic dense phase CO2 corrosion rig has been built (conditions: ~90 bar, 40 oC and up to 5 l/min flow rate) in flow mode, to understand the effect of impurities (SO2, O2, H2, NO2 & CO) present in CO2 on the pipeline transport materials. This unique facility in the UK was developed via the MATTRANS project funded by the E.ON-EPSRC strategic partnership (EP/G061955/1). The test rig includes different metallic materials (X grade steel: X60, X70, X100 and duplex) to assess the corrosion of pipelines, and different geometry components (tubes, plates, charpy and tensile coupons), to assess ageing and decompression behaviour of polymeric seals (Neoprene, fluorocarbon, ethylene and Buna N) under water-saturated dense CO2.

[Kumar Patchigolla, Centre for Energy and Resource Technology, Cranfield University, Cranfield, MK43 0AL, UK, k.patchigolla@cranfield.ac.uk]

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3.1   Valiant Jetty Project: HMNB Clyde: Faslane: Cathodic Protection’, Brian Wyatt, Corrosion Control Affiliates Ltd, John Thirkettle, Thor Corrosion & Chris Lynch, Corrpro Companies Europe Ltd

Corrosion Control Affiliates Ltd (CCAL) were appointed by AMEC Group Ltd in September 2005 to undertake the Detailed Design of the Cathodic Protection systems for the steel piles and the reinforced concrete Jetty which form the Valiant Jetty Project for the berthing of the Royal Navy’s new Astute Class attack submarines. The key specialists responsible the design were John Thirkettle (Thor Corrosion), Chris Lynch (Corrpro Companies Europe Ltd) and Brian Wyatt (CCAL). At this stage the Concept Design phase of the cathodic design had been completed by the same team working under an earlier contract.

The Project is nearing its completion in 2012. Following a sale of the civil engineering construction business from AMEC to Morgan Sindall our Client is now the Morgan Sindall / AMEC Valiant Jetty Joint Venture. The cathodic protection design is fully complete and documented; it has been subject to Independent Peer Review (by another MCF Member) .

The Valiant Jetty project incorporates the following elements and cathodic protection provisions:

·        Jetty External Walls, Cantilevers and Base Slab (reinforced concrete)              Impressed Current

·        Jetty Restraint Piles (Steel)               Galvanic Anodes

·        Linkspan Dolphin Support Piles (Steel)              Impressed Current

·        Viaduct Support Piles (Steel)               Impressed Current

·        Shore Abutment ‘H’ Piles (Steel)               Impressed Current

·        Jetty Support Building Podium Piles (Steel)               Impressed Current

·        Subsea Anchor Piles (Steel)               Galvanic Anodes

·        Sea Water Intake Systems (Steel)               Galvanic Anodes

·        Fender Outstands (Steel)               Galvanic Anodes

The key issues of the CP design can be summarised as follows:

1.      All of the CP systems were designed and installed as “defence in depth” provisions to supplement a robust design that did not rely on CP to deliver the full life of the project components; a combination of corrosion durability (for the reinforced concrete jetty) and rigorous inspection regimes (for both concrete and steel) were put in place to deliver the full design life (50 years) for the structures.

2.      Thus the time to initiation of corrosion for the low water cement ratio and gbbs and silica fume cement replacement concrete was determined in laboratory testing to be in excess of 50 years; the CP provisions were to supplement this as “defence in depth”.

3.      Steel piles in sea water were provided with “sacrificial steel” allowances in accordance with the corrosion rates in BS 6349-1 and in all except the JSB Podium pile were coated with glass flake epoxy coatings and subjected to planned routine inspections in particular to detect “Accelerated Low Water Corrosion” (ALWC) and other forms of Microbially Influenced Corrosion (MIC).

4.      The impressed current system for the reinforced concrete jetty is understood to be the largest CP of new construction steel in concrete project in the UK. It provides CP (or “cathodic prevention”) to the external envelope of the Reinforced Concrete (RC) floating pontoon Jetty exposed to sea water and marine spray; thus all the immersed surfaces, the external walls and the cantilevered section of the deck have received CP. The protected surface area (of concrete) is some 13000 m2. The corrosion threat in a hollow RC structure is to the external (extrados) steel, this steel will be the first exposed to chlorides that will, eventually, diffuse through sound concrete and which theoretically could diffuse more quickly than determined in the laboratory testing by virtue of unavoidable construction defects in such a large and complex structure. Whilst the extrados steel may be in oxygen depleted, water saturated and chloride rich concrete, the intrados steel is most likely to be in relatively dry, oxygen rich and chloride free concrete. This is the “hollow column” scenario first proposed by Nigel Wilkinson in the Concrete in the Oceans programme in the 1970s and could lead to corrosion of the anodic extrados steel whilst the intrados steel is cathodic.

5.      The RC Jetty CP system has been designed to comply with BS EN 12696 (recently revised in 2012 to include immersed and buried RC elements); the design is compliant with the 2012 Revision of the Standard.

6.      The RC Jetty CP system utilised, for the atmospherically exposed above water elements, MMO/Ti anode expanded ribbons cast into the extrados cover; the anodes were pre-assembled onto plastic spacers to form frames which ensured no short circuits between anode and steel reinforcement (cathode). The jetty is constructed as a number of discrete cells and each cell was treated as a discrete anode zone above the water level, with individual anode arrays, multiple anode/cable connections, multiple cathode connections and multiple monitoring provisions in each cell wall. The deck is cantilevered and it was determined to include the cantilever section of the deck into the CP system due to its enhanced exposure to wind borne chlorides. The cantilever deck anode frames were separate from those for the walls, allowing variation in current density applied, and each cell cantilever was again treated as a discrete anode zone, with individual anode arrays, multiple anode/cable connections, multiple cathode connections and multiple monitoring provisions in each cell. This entire system was based on well established reinforced concrete cathodic protection or “cathodic prevention” principles and practice for new reinforced concrete structures in harsh environments.

7.      The Jetty CP system, for the immersed surfaces, utilised 10 No. MMO/Ti tubular rod anodes projecting vertically below the outer longitudinal walls of the jetty, distributed based upon the calculated current demand along the jetty length. The anode spacing from the concrete based slab, by way of a GRP clad steel mounting tube, was optimised using the well established Beasy modelling program. The anode (and cathode for the steel mounting tube) cables were ducted within the walls to above water level and then into the electrical/mechanical spaces within the Jetty via cofferdams. Each Cell lower wall and (Port and Starboard half) base slabs are provided with multiple negative connections and multiple monitoring provisions. The entire system was based on well established combinations of reinforced concrete cathodic prevention principles and offshore cathodic protection practice; it was based in part on the proven Murchison platform CP system of some 25 years ago.  

8.      The Jetty CP system was powered by 2 No. Power Supply and Monitoring System Enclosures (PSMEs) which provide each immersed anode with individual 15A 12V smoothed dc supplies and provide 4No. identical supplies for Port/Starboard and Fore/Aft subdivisions of the atmospheric zones for the upper walls and cantilevers. In the junction boxes within each (port and Starboard side of each) cell there are simple resistive controls to modify current delivery cell to cell and upper wall to cantilever. All the monitoring provisions were cabled back to the relevant PSME and are data logged therein. The system is controlled to meet the CP criteria in BS EN 12696 which for the immersed parts of the Jetty the relevant criterion is for steel/concrete potentials to be in the range -720 to -1100mV Ag/AgCl/0.5M KCl Instant OFF. However, for reasons related to compatibility with the CP systems of berthed vessels the optimum range of protection is -720 to -800mV generally over the immersed structure but with more negative values permitted near to the impressed current anodes.  

9.      The steel pile CP system utilised MMO/Ti tubular rod anodes in two configurations. For the offshore Viaduct bents and the Dolphin for the Linkspan bridge to the Jetty, the sea bed is covered in deep soft silt and the anodes are constructed into “rings” with multiple anode tubes on a ring main cable. These “mud anode rings” were laid with diver assistance into the silt and both ends of the anode ring main are run, in steel conduit, up one of the piles being protected to a PMSE on the structure above. The Dolphin has 4No.such mud anode rings and the Viaduct bents have 2 No. All have fixed dual Ag/AgCl/0.5M KCl and Zinc monitoring electrodes.  

10. For the inshore Viaduct piles and the bare piles of the Podium the sea bed is load bearing hard shingle over rock. For this application “sea anode sleds” were placed, with multiple MMO/Ti tubular anodes with individual cable connections being run back to the PSME above; the foundation for the sleds were fibre reinforced concrete. These sleds were placed by boat.  

11. Both “mud anode rings” and “sea anode sleds” are rated at 30A 18V and are individually controlled by their respective PSME; there are in total 24 individual channels, 10 No. for the bare podium piles and 14 No. for the coated Viaduct and Dolphin piles. All piles are inter-connected with negative returns between all elements.  

12. The steel pile impressed current CP system was designed to have capacity to provide current to the existing base buried and immersed structures which are in electrical continuity with the steel piles due to their common electrical earthing provisions.  

13. The overall steel pile CP system was designed to established cathodic protection principles and practice within the requirements of both BS 7361 and BS EN 13174. Due to the known likelihood of ALWC, the protection criterion adopted for the steel piles in loch water was -950 to -1100mV with respect to Ag/AgCl/0.5M KCl.  

14. Certain other steel in water elements were protected by galvanic anodes using Al-Zn-In alloy anodes. These include the restraint piles for the Jetty which are protected with bracelet anodes similar to those used extensively in offshore retrofits and cable anchors, fender outstands and sea water intake frames for which conventional trapezoidal anodes had their steel cores welded directly to the structures concerned.  

15. The difference in optimum protection levels between the reinforced concrete Jetty and the steel piles has lead to a need to electrically isolate the steel in loch water structures from the Jetty and the electrical earthing systems of the Jetty from those of the Viaduct and the general base facilities. This has been achieved by the use of Polarisation Cell Replacement (PCR) devices, passive electronic devices widely used for the same purpose in isolating cathodically protected pipelines from the copper earthing systems of pump stations and terminals, which allow typically +/-2V dc potential difference to be maintained across them with no current flow but have a large current conductance capacity above this low dc value and for all ac voltages. PCRs have been installed to isolate the Jetty earthing system from that of the viaduct and the base facilities. Services (fluids, gasses and communications) and the Linkspan bridge have all been provided with electrical isolation in order to ensure isolation between the reinforced concrete Jetty and the steel in loch water piles etc.  

16. Interaction testing has been completed to confirm that there are no excessive potential changes due these new CP systems on buried and immersed structures on the base, some of which have their own CP systems and some which do not. It has not been necessary to make any revisions to the new CP systems in order to meet the established interaction potential shifts in BS 7261. Late in the project, an outfall close to the Podium that had been intended to be abandoned at an earlier stage of the project was determined to have continued life; this was incorporated into the Podium CP system.  

17. The impressed current CP systems have been designed specifically to be safe for full current output operations with divers in close proximity with the CP system and in particular, the anodes. Many impressed current CP systems suffer inadequate performance due to them being switched OFF during diving operations for safety reasons, with diving operations being for extended period or operators simply omitting to switch the CP system ON after completion of diving works. Diving operations are anticipated to be frequent around the Jetty and the limiting of individual anode current outputs, limiting output voltage, limiting ac ripple have all been used to meet the full requirements of the latest International Marine Contractors Association (IMCA) Code of Practice for the Safe Use of Electricity Under Water. Field measures have proven that the field gradients around the anodes of the CP systems do not present threat to divers.  

Thanks are due to the Morgan Sindall / AMEC Valiant Jetty Joint Venture and their Client, the Defence Infrastructure Organisation of the MOD, for their cooperation and support throughout the Project and their permission to present this information to the Marine Corrosion Forum.

3.2     Aluminium Alloys for the Marine Environment, Clive Tuck, Lloyd’s Register EMEA

Aluminium alloys are normally selected for engineering applications for their low-density, high strength-to-weight ratio, high-thermal conductivity, acceptable weldability and good corrosion behaviour in seawater.  The different types of alloys are grouped into non-heat-treatable and heat-treatable alloys, depending on the microstructures which can be developed to give enhanced strength. For marine applications, the main alloys of choice are those of the 5xxx-series (Al-Mg) because of their superior corrosion resistance in seawater.   Some of the 6xxx-series (Al-Mg-Si) alloys can also be used.   Aluminium alloys of other types, particularly those containing copper, show inferior corrosion performance in seawater. 

As with other alloys which readily form passive oxide films in air, corrosion of aluminium alloys is largely by a localised mechanism. General corrosion is rarely seen, but it can occur when the environment pH is lower than 4 or greater than 8.5.  Localised corrosion occurs in seawater due to breakdown of the protective oxide film in isolated areas, and this results in the development of pits on the surface or extended areas of corrosion in crevices. The initiation of such corrosion is caused by chemical attack by chloride ions at defective points in the passive oxide film.  Intergranular corrosion in Al-Mg can be severe and this can cause general break-up of the surface in layers, which is called exfoliation corrosion.  Controlled manufacturing processes and specific alloy tempers have been developed which are known to give some Al-Mg alloys resistance to exfoliation. 

Another type of corrosion which is commonly encountered is galvanic corrosion, particularly when aluminium components are connected directly to brass, copper or graphite. Normal methods of prevention can be used, such as painting the surface of the more noble material, electrically isolating the two metals, or applying cathodic protection.  However, the latter must be carefully in its design, as local high pHs can be generated on the aluminium at cathodic potentials, which can produce a general breakdown of the passive film.  

Other corrosion types encountered in aluminium alloys are stress corrosion, fatigue corrosion, sustained load failure and microbiologically-influenced corrosion. 

Although bare aluminium alloy surfaces can develop a good protective patina after exposure to seawater, organic coatings can be applied to produce a physical barrier between the aluminium and its environment. The effectiveness of a coating depends as much on surface preparation as on coating selection. There are a number of pre-treatments that can be applied, the choice of which will depend upon the coating system used. 

In general, aluminium alloys can be welded by the fusion-welding process although newer techniques such as friction-stir welding (FSW) are also used.  However, the strength properties of the heat affected zone (HAZ)  produced during these joining processes are normally lower than that of the parent material, although the effect is less significant when FSW is used.  This means that the design of welded structures cannot make full use of the as-manufactured mechanical properties.  Also, poor quality welding can result in the presence of crevices which can become corrosion initiation sites. Therefore, continuous welding should be used as much as possible, the welds being able to act as physical barriers to water ingress.  

Prevention of corrosion relies on the correct choice of alloy and good design.  Coatings can also be used to provide an effective barrier, although this relies on the correct application of a conversion coating which is able to modify the passive film in order to ensure good adhesion.

 

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