Technical Presentations at the April 2012 Meeting
Copper alloy mesh for aquaculture cages has been
commercially used in Japan, Chile and Australia. Demonstration sites exist
in Turkey, Korea, Hawaii and China. This
presentation gave an overview covering both the history of its development
and also described the diverse test programmes which are on-going to
quantify its benefits. The
latter range from laboratory and sea water immersion corrosion studies,
Dome trials for copper leaching, new designs in submerged cages, and
evaluations in food safety, growth performance and disease.
The original trials were carried out in Scotland in 1976
using copper-nickel and, since that time, other alloys including
proprietary brass alloys have been successfully developed. Several
different designs of cage are undergoing development trials. The
copper cages are designed as an alternative to nylon nets, which
are coated with anti-fouling copper compounds. As 50% of world fish
consumption is now farmed, the potential market is high.
Subsea umbilicals are used for control and
operation of subsea oil and gas production facilities. Typically an
umbilical consists of steel tubes, electrical cables, fiber optic cables,
weight or strength elements and fillers. Super duplex stainless steel tube
material has been used for almost 20 years within the umbilical industry.
However, a recent failure on two orbital welds of 25Cr super duplex
of an umbilical installed in the South China Sea was reported.
From these results some umbilical end users recommended the use of
thermoplastic coating to avoid crevice corrosion on 25Cr super duplex
stainless steel above 20°C. However,
this limit is based on results obtained for super duplex stainless steel
in aerated natural seawater and very little is known on the
micro-environment formed by the confined seawater between metallic tubes
and polymer matrix of an umbilical.
This work reports corrosion potential and oxygen
content measurements in the confined zone between the metallic tubes and
the polymer matrix of an umbilical. These
measurements were performed using micro-electrodes on a 2 meter long real
umbilical at 30°C in heated natural seawater.
From the measurements, it is shown that the oxygen content in the
confined zone is rapidly consumed probably due to the passive current on
the stainless steel tubes and then remains below 2 ppm over one year
exposure period. From the
open-circuit potential measurements performed in the confined zone, it is
clear that the open circuit potential remained below -150 mV/AgCl.
This translates an absence of electrochemical effect of the biofilm
in the confined zone. Visual
and metallographic examinations of the tubes after exposure confirmed the
results obtained by the microelectrodes and clearly indicate that no
corrosion initiation occurred on superduplex stainless steel (base metal
and welds) under these experimental conditions.
Weld Corrosion in a Gas Compression System’, Dave Moore, Lloyd’s
Register EMEA, Aberdeen
A case study was considered where a
pinhole leak had occurred in a weld in a 14-inch diameter process gas
system line. This was in an
offshore oil and gas production facility on the UK continental shelf. It resulted in the immediate shutdown of the gas system and
subsequent shutdown of all production.
The facility shutdown lasted for more than three months while
repairs were carried out. Causes
of failure were discussed, and the reasons why inspection had failed to
detect the corrosion.
Lessons learned: Weld
corrosion should be considered during design.
This case study highlights the challenge of predicting where weld
corrosion will occur, and the challenge of detecting weld corrosion.
Hence: the integrity management industry should accelerate
cascading the most recent research findings on inspecting preferential
weld corrosion to the practitioners "in the field".
Integrity providers, NDT companies and clients need to work closely
together and accept that on critical systems where serious defects are
located, that it should result in the deployment of advanced techniques
that are capable of higher resolution with recordable results to allow for
more definitive analysis of data. Integrity
engineers and inspection technicians should appreciate the practical
limitations of manual UT scanning. This requires a paradigm
shift in the minds of many technicians and inspection engineers who have
long experience of "accurate" results from the manual UT shear
wave tool, which makes them inherently resistant to appreciating its
limitations. Pressure system inspection should
utilise more quantitative methods for assessing any weld defects
identified by routine inspection. Essentially, this means greater use of
TOFD and radiographic inspection.
Marine structures are frequently monitored or surveyed as
part of an ongoing integrity management program to obtain data on
corrosion potentials, anode consumption and in some cases field gradients.
One objective is to determine how effective the CP system is at providing
protection to the structure and another is to verify that sacrificial
anodes are being consumed at a rate consistent with design assumptions.
However it is not economic, or in many cases even feasible, to measure
potentials on a complete structure or to collect data from all anodes, so
the CP engineer is required to infer the condition of the overall
structure from the available data.
For structures with complex geometry, the accuracy of data
collected may not be reliable (eg field gradient measurements being
affected by other nearby metal surfaces), and problems with access can
make reliable orientation and placement of the probe on the
structure/anode difficult to achieve. These issues require the CP engineer
to use his judgment to determine which data to use and which to discard.
Computer modelling has been used in the design of CP systems
firstly to optimize the design, and secondly to provide assurance to the
operator that requirements for anode life are met, and that the system
will sustain the potentials required to protect the structure (ie it
determines the distribution of potentials across all surfaces).
Tools to Ensure the Quality of Duplex Stainless Steel Components’,
Roger Francis, Rolled Alloys
It has long been understood that the heat treatment of duplex
and superduplex stainless steels is critical to obtain the optimum
structure and the desired properties. Over the last twenty years
there have been a number of cases where inadequately heat treated
components have been delivered by the manufacturer and then subsequently
identified as defective further down the supply chain. In some cases the
problem was identified and resolved prior to fabrication and installation,
while in others fittings have leaked in service due to poor microstructure
from incorrect heat treatment.
Common to all these cases is that the cast and batch
production test certificate indicated that the goods met specification
requirements in all respects. Hence the similitude between cast and batch
specific test pieces and the production parts has been called into
question. The use of additional testing when specifying these alloys
is common but there is no agreement on what these tests should be.
There has been extensive discussion on how best to test
individual components non-destructively to detect unsatisfactory material.
Some have suggested that magnetic measurement of the ferrite content is
adequate, whilst others believe the test to be insufficiently discerning,
resulting in too many good parts falsely being identified as “suspect”
and causing unnecessary remedial action.
The present paper describes the procurement specifications
used by the authors’ company to ensure adequate properties in service.
The paper addresses the strengths and limitations of magnetic ferrite
measurements and shows how the readings are affected by manufacturing
route, product form, surface roughness and radius of curvature. The
paper goes on to show how the test can be used to identify material that
may contain sigma phase and that in-situ metallography is then required on
these suspect areas to either release the part or condemn the part to
remedial heat treatment. The results of five years successful
experience with this combination of tests are discussed.