Technical Presentations at the April 2002 Meeting
2.1 'Tungum Alloy (Tubing) – `An Old-fashioned Material in a Modern World`’, Bill Inglis (Tungum Hydraulics Ltd.)
The Writer has chosen this title because, in many ways, copper-alloys are perceived to be “old-fashioned” in the face of advanced stainless steels and nickel alloys. But the fact remains that, properly chosen, a copper-based alloy can outperform, and thus be cost-effective over such “exotica”. However this requires that engineers banish the mindset of “the newer the material the better”. The evolving history of the use of TUNGUM Alloy amply illustrates this point
TUNGUM, is a trade-name for aluminium-nickel-silicon-brass, to BS.EN 12449 ref. CW700R, symbol Cu.Zn13Al1Ni1Si1. It was originally conceived in 1918 as a gold-coloured alloy to be used for the manufacture of cosmetic jewellery, cigarette cases and occasionally more substantial items such as balustrades. However, in the early 30’s, investigative work proved that the material possessed attractive physical properties to justify its adoption as an engineering material and the rights to it were transferred to the current owners who started the present company in 1933.
TUNGUM Alloy’s first major application was in tube form for the various hydraulic and breathing air services in the Wellington Bomber. Thereafter it was extensively used throughout the R.A.F.’s “fleet” until “TSR2” was abandoned in 1965. By then the Army and Navy had adopted our tubing being widely use by these services up to the present day. Commercially, it has seen, and is seeing successful application in marine, offshore and industrial applications for hydraulic and pneumatic control and instrumentation circuits. Its high level of resistance to marine corrosion is particularly notable, often outlasting the equipment or structure to which it is attached !
This Lloyds and DNV approved material has, during its history, been cast, forged, stamped, rolled into plate and sections and drawn into wire and tubing. However, the material has found its niche principally in tube form, although it is regularly forged to make tube connectors and valve bodies. Its high strength-to-weight ration (for a copper alloy), excellent ductility, high resistance to fatigue and shock, and low magnetic signature are all attractive features to an engineer. Being spark-proof against itself and other metals and possessing excellent cryogenic features ensure the materials adoption in the safety-conscious industrial gas industry.
TUNGUM Alloy’s special features demonstrate that an “old-fashioned copper alloy” can still hold its own in a modern engineering World.
2.2 ‘Recent developments in the use of high strength cupro-nickel alloys in off-shore engineering’, Clive Tuck (Langley Alloys, Meighs Ltd.).
High Strength Cupronickels have been developed over a period of sixty years since it was discovered that the addition of aluminium to Cu-Ni produced a strengthening effect. They have properties with particular benefit for marine engineering and the latest alloy of their type to be developed, MARINEL, has strength equivalent to B7 carbon steel and a proven record of hydrogen embrittlement resistance.
Recent applications in offshore oil and gas projects are presented to show the particular advantageous aspects of high strength cupronickel materials. Resistance to hydrogen embrittlement, anti-galling characteristics, corrosion resistance, high strength and biofouling resistance have made them very suitable for subsea connectors. MARINEL is widely used in this application, replacing K-500 and 17-4 PH which suffer from hydrogen embrittlement and crevice corrosion susceptibility. The same set of properties is required for fasteners and the ease of machining MARINEL, together with its ability to sustain imposed loading, make it more suitable than other high strength materials as a marine fastener.
The presence of aluminium in high strength cupronickels imparts a resistance to hydrogen sulphide corrosion and MARINEL has been successfully used to bolt equipment for down-hole deployment.
The more recent developments in deep water have produced some surprises, which have resulted in the realisation that design criteria are needed, which are different from those previously applicable for offshore engineering. The main problem with deep water relates to the higher hydrostatic pressures, which cause a greater solubility of calcareous deposit. This restricts the cathodic protection’s effectiveness in terms of efficiency and throwing power. Attempts to increase the protective range of anodes results in greater release of hydrogen (causing higher risk of hydrogen embrittlement) and higher degrees of biofouling. Thus, for ‘fit and forget’ solutions, MARINEL offers distinct advantages in its unique hydrogen embrittlement immunity and biofouling resistance.
4.1 ‘Erosion-corrosion of materials for drill bits offshore’, Myrna Reyes & Anne Neville (Corrosion and Surface Engineering Research Group, Department of Mechanical and Chemical Engineering, Heriot Watt University)
Drilling tools in the oil industry are subjected to severe environments, where highly abrasive rock as well as high rotary speeds and weights can accelerate degradation. It is often assumed that the principal degradation mechanism in offshore drilling is that of the cutting action of the diamond inserts or abrasion caused by the cuttings. However, the problem encountered is not simply a case of high stress abrasion but also involves erosion-corrosion issues. The presence of corrosive elements in the drilling mud used as lubricants and coolants injected at high velocities cause severe wear and erosion in service. The different nozzles direct the mud jets down-hole and sideways onto the bit, cooling it and allowing cuttings to be moved upwards. The high velocity of the jet impinging on the side blades can degrade the material supporting the diamond inserts.
In this talk, an analysis of the main degradation mechanisms occurring in drill bits is presented. An experimental study has been conducted in which the performance of current materials and candidate materials has been examined with a view to improving the life of drills for subsea applications.
4.2 ‘Corrosion and cracking effects on the structural safety of ships’, Iain Kennedy (Marine Structures, QinetiQ, Rosyth)
The QinetiQ Marine Structures Group based in Rosyth, is a Structural Engineering consultancy serving the Marine Defence, Marine Transport and Offshore Oil and Gas Industries. Amongst its many interests it has carried out work on the structural integrity of naval ships for the Ministry of Defence.
It is accepted that corrosion and cracking occurs in marine platforms and that emphasis is placed on the design and survey of a vessel. However, in managing both the risk of cracking and the frequency of repair, an understanding of the design, loading and environment is required in order to predict the vessel’s structural integrity and minimise the extent of unnecessary repair. The presentation examines these aspects using a case study to illustrate the benefits of applying a risk-based approach to managing structural integrity.
The case study examines a particularly virulent form of corrosion pitting in ships associated with sulphate reducing bacteria (SRB). SRB pitting corrosion rates can exceed 1 mm / year and can rapidly reduce the section thickness of ship plate in between scheduled refits. In addition, there have been concerns that SRB corrosion may cause local embrittlement and increased crack propagation rates due to the production of hydrogen during the corrosion process. Work carried out at QinetiQ has shown that the SRB environment does not cause embrittlement and has no observed effect on crack propagation rate. However, the reduction in plate section thickness due to SRB pitting, can have a significant effect on the ultimate strength of a ship.
Ship Ultimate Strength Analyses, carried out by QinetiQ, are able to show the effect of plate thinning on ultimate strength at positions along the ship in both hogging and sagging conditions. These results are compared with the design bending moments and factors of safety are generated. Using these validated models, it is possible to make quantitative decisions on which areas of the ship are sensitive to reductions in section thickness. Conversely, there may be areas of the ship that are less sensitive to plate thinning and therefore require less repair attention.
Ultimately, understanding the effect of corrosion on ship ultimate strength may give rise to improved repair regimes and lead to potential cost savings for platform owners/operators.