A NEW APPROACH TO BOLT PROTECTION

Pic: rusting flange bolts on platform pre-commissioning

Corrosion-inhibiting sprayable thermoplastics (CIST) provides a radically new approach to corrosion control in bolted systems. A range of solutions are offered to protect such structures but, especially offshore, corrosion often starts even before a project is complete. And, once corrosion takes hold, it is very hard to stop without constant and expensive maintenance.

There are, of course, more reasons than just corrosion for failure in flange joints and other bolted assemblies.

  • Fatigue Cracking
  • Incorrect preloading
  • Poor design
  • Poor materials
  • Poor assembly
  • Corrosion fatigue
  • Hydrogen embrittlement
  • Stress corrosion cracking
  • Galvanic corrosion
  • Pitting/Crevice corrosion
  • Atmospheric corrosion

The most common cause for failure in bolts is fatigue cracking – and this can be as a result of a number of factors, wrong preloading, poor connection design or improper assembly. Corrosion fatigue, hydrogen embrittlement, stress corrosion cracking – it’s long list but, no matter how much we would like to have a single remedy for every possible situation, it is certainly beyond the scope of a coating, applied after manufacture and assembly, to be able to address all these problems!

Yet, if we examine a number of the factors involved, we can see that the effects of corrosion have to be taken into consideration at all levels. An assembly of separate components,almost certainlymade from a range ofdifferent materials,would be prone to galvanic corrosion.Thus the bolt material would need to have a free corrosion potential that is more positive than the base material of the flange, otherwise the corrosion effect on the much smaller surface area of the bolt would be severe.

Pic: corrosion on flange bolts

In fact, in the offshore industry,there has been increased use of corrosion resistant alloys in piping systemsbut, because compatible bolt materials are expensive and often in short supply, low alloy steel [ASTM A193 B7] bolts are in common use. Severe, rapid corrosion of a B7 bolt will result unless isolating sleeves can be installed and constant vigilance is required in a system where such incompatibilities exist.This table shows a typical group of candidate bolting materials and the likely corrosion effect that would result from their use.

Pic: bolt material diagram

If galvanic corrosion is the most likely mechanism for the acceleration of damaging corrosion in bolted systems, other factors also exist. Pitting, crevice corrosion and stress corrosion cracking all need to be taken into account, even for corrosion resistant alloys, and, where coating systems are used to protect low alloy materials, damage to the coating from stress in use or during assembly is common.

Corrosion affects the system in two ways: Primary safety risk, by reducing the load-bearing ability of the bolt and hence the integrity of the system - with high potential for human, environmental and economic damage, and secondary economic risk from the economic and production consequences of seized bolts and the need to cut them out.

Thus, eliminating corrosion as a factor in the consideration for bolted system survival would be a significant breakthrough in both safety and economics and, if such a breakthrough was in parallel with a reduction in environmental impact, it would seem an extremely worthwhile objective. Yet the introduction of sprayable thermoplastics appears to offer just such a combination and it is quantifying the value of this approach that is of such importance for its general acceptance.

How CIST works

The key difference between the solution offered by corrosion-inhibiting sprayable thermoplastics and other solutions for problems in bolted systems is its multi-functional, whole system approach. The material, a solid at normal ambient temperatures, is melted in the specially-designed system and sprayed on to the substrate providing a perfectly-fitting barrier coating, preventing ingress of moisture and contaminants. At the same time, a built-in inhibiting oil is deposited on all contact surfaces during application and continues to be released throughout the lifetime of the system, working its way into every pit and cavity within the encapsulation, preventing corrosion and stopping any that may already be present.

The material is non-hazardous, easy to remove and is both re-usable and recyclable. The system provides:

  • Protection for whole system
  • Passive barrier protection
  • Active corrosion inhibitor
  • Easy access to substrate
  • Re-usable non-toxic material

This gear shaft, left outside for more than three yearsin a hot, salty environment, shows in practical terms how the system works.

Pic: peeled back gear shaft

Peeling the encapsulating the thermoplastic away from the complex shape of the shaftreveals a surface gleaming with inhibitors – and no sign of corrosion anywhere. The diagram shows the different ways in which the system works to prevent corrosion.

Pic: diagram showing protective mechanisms

A New Standard of Protection?

It is the whole system approach, together with the dual, active/passive performance, that sets a new standard for bolted system protection. The fact that it is a worthwhile new approach was recognised by DNV in 2001 when they contacted Enviropeel about becoming part of their long-term bolting materials testing programme. This programme, testing for which started in 2003, is to be over 10 years and is with the participation of DNV, ConocoPhillips, BP, Enviropeel and PetroBras.

Surprisingly enough, the prime motivation for the programme was that the performance of bolted systems has been subject to very little systematic research, although some work has been done on bolting materials and coatings. In many cases, design engineers have had to rely on manufacturers’ claims for bolt performance statistics without a full understanding of how a system using such fastening materials would react in all environments. For example, an oil company with excellent results using PTFE on a phosphate primer in the Middle East, found that this coating system was ineffective in the North Sea.

It was felt by DNV that, although the main focus of the programme was to test bolting materials for optimum performance in marine conditions, so many failures were occurring that it was also important to seek remedies in situations where corrosion had already taken place, and be able to provide on-site protection for existing systems where problems were likely to occur; hence the inclusion of the Enviropeel CIST in the programme.

Pic: CIST test-piece rack Splash zone exposure site

Special test pieces were designed and two exposure sites were designated, one splash zone site in North Carolina and the other at Bergen in Norway.

Inspection is programmed at regular intervals, with removal and testing of any failing samples. After 18 months, samples in the North Carolina splash zone were performing so badly that all but two were removed.

Pic: Bolt and nut failure after only 18 months

The fact that one of the two surviving systems was our thermoplastic coating was very satisfying for us but it may be of more overall significance that, despite a two year search and input from manufacturers and professional engineers, nearly all the systems failed after less than two years in splash zone conditions.

Anti-Corrosion Performance

Apart from the ongoing tests with DNV, a number of tests on the material’s resistance to UV, exposure to cryogenic temperatures and longevity in the field have been successfully undertaken. Above all, however, it is the anti-corrosion performance of the system that is most relevant in determining the validity of the CIST approach.

Pic: Test pieces from 3000 hour ASTM B117 showing protected and unprotected areas

These test pieces, showing results of 3000 hour hot salt fog testing, show how effective CIST is in preventing corrosion. Outside the CIST protection the standard carbon-steel of the trial joints can be seen to be heavily corroded, with exfoliating rust. Stripping off the protective coating reveals a corrosion-free substrate, with bolts and nuts in their original condition.

The developmentof CIST has been measured in a number of ways:

  • ASTM and other testing of specific qualities and functions
  • Long-term testing with world leading authorities, such as DNV
  • Long-term application programmes
  • Programme development for recognised standard for sprayable thermoplastics
  • Application development with major companies and organisations

ASTM and other testing of specific qualities and functions

A wide variety of testing has been undertaken to establish the suitability of materials and equipment. Testing continues as specific requirements come to light and, as the Company adopts a policy of continuous development, new developments are being tested all the time.

Long-term testing with world leading authorities, such as DNV

As noted, we are currently in the 6th year of the ten-year testing process with very satisfactory results so far. As the programme develops, bolted test pieces that have failed with their original coating are to be coated in Enviropeel and returned to the test zones to test the system’s ability to arrest corrosion once it has occurred – a key element in providing a solution for world-wide problems with flanges, valves and other bolted systems as well as providing insight into its performance in other areas, such as for stored, stand-by and transit protection.

Long-term application programmes

The Company has adopted a strategy of seeking long-term application programmes in specific target areas in order to audit progress and provide background data on all aspects of CIST applications. A key motivator in the early stages of product development was to provide a long-term ability to preserve steel structures of all kinds with a system that could be applied as a short-term reactive remedy but which would also form part of a long-term pro-active asset maintenance programme.

Working in the North Sea on the joint Chevron/ConocoPhillips platform, the Britannia since 2003 has allowed an unparalleled continuity of development.

On Britannia, manufacturing faults on bolts supplied for the construction of the pipeline infrastructure had caused them to fail in as little as six months. With a particular problem on the smaller flanges and valves because of the disproportional effect of corrosion on low-diameter connections, a rolling programme of applications was started on all bolted connectors on 4 inch nominal pipes and below – although over the years, the programme had spread to include even the largest joints.

More recently, in 2008, a bridge linked platform was added to the Britannia field. With corrosion on bolted systems before the structure was commissioned, CIST was planned as part of the construction programme with more than 8000 joints coated and an ongoing programme of applications on the main platform and vendor packs planned until 2012.

Video: flange applications on North Sea platform

At first the Britannia operators kept a watching brief to see if the Enviropeel CIST lived up to its claims. After the first year, everybody was very happy with what had been accomplished and when, during the second year of the project, some applications from the previous year were stripped off and the substrates examined, it was clear that bolt protection was excellent.

Pic: stripped application

Because of this experience, when a new bridge-linked platform was added to the Britannia field and commissioned in 2008, CIST was chosen to encapsulate an initial 8000 flanges with further work on vendor packs and pipework supports planned until 2012. Not only was the Enviropeel protection found to be effective but its cost-effectiveness compared with bolt replacement was also demonstrated by their maintenance engineers:

Pic: cost comparison between CIST and bolt replacement

Not only did the use of CIST reduce costs by up to 72% in the first placebut, without it, all the costs would have to be repeated again and again as the bolts corroded without adequate protection.

Programme development for recognised standard for sprayable thermoplastics

A joint NACE/Enviropeel programme is currently being developed in the US to provide performance benchmarks for consultants and engineers based on standardised tests with bolted systems.

Application development with major companies and organisations

Much of the focus of this presentation has been on bolted system protection because of the particular problems that such systems present. But a philosophy of providing flexible ‘whole system protection’, whether it be for flanges offshore, stored engineering products or standby equipment, has lead to unexpected developments.

Pic: stored components

Australia

Working with BHP Billiton and Dampier Salt in Western Australia, an engineering company, who pioneered the use of Enviropeel in Australia, has been able to establish the remarkable ability of CIST to not only prevent corrosion but also the ingress of contaminants and abrasives into rotating systems such as bearings. Following a three-year testing programme a CIST system has been specified for the protection of conveyor bearings at BHP Billiton and Dampier, who are part of the Rio Tinto Group. Data collected by BHP Billiton showed stored equipment failures were reduced from 40+ % to zero and conveyor bearing lifetimes were increased by more than 500%.

Diagram: BHP Billiton performance data

Pic: Bearing housings

On the ocean

Development of special units with stainless steel components and twin gas detectors was required for use on the decks of LPG Carriers that had previously required constant maintenance of the deck pipework to prevent bolt and flange corrosion.

In the USA

Pic: wellhead ASTM B117 test results

Test results witha large international oilfield services company on the protection of wellheads have shown the potential for CIST to protect these most complex structures.

Mobile units designed to service land-based wellheads are being pioneered in Libya for a local oilfield services company.

Equipment

CIST uses specially designed units to heat and pump the material, which is delivered to the substrate via spraygun and hose. Units vary in size from hand-portable units to high volume zone-2 units, equipped to operate in hazardous areas.

Pic: zone 2 unit and operator

Conclusion

Experience with corrosion-inhibiting sprayable thermoplastic system proves that working to protect a system as a whole rather than a sum of its parts is extremely worthwhile. The adoption of a more integrated, holistic approach offers significant advantages, where one solution can be shown to be effective for a variety of problems.The CIST material is re-usable, recyclable and non-toxic, reducing waste and helping reduce our impact on fragile ecosystems. Prolonging system life cycles already provides sound economic justificationfor the use of systems like CIST but significant reductions in hazard to personnel,as well asincreased environmental safety should make it a priority.