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Oilfield Technology
January
2016
micro-organisms can result in a mixture that, over a long
period of time, can cause significant corrosion.
Finding new, affordable materials with the right levels
of resistance is crucial in reducing the cost of harnessing the
most chemically impure reserves.
Having an in-house team of materials science
specialists who stay up to date with cutting-edge advances
in metallurgy is absolutely key in order to offer the latest in
high-performance yet cost-effective alloys.
Of course, this emphasis on innovation needs to be backed
up by extremely robust quality controls, and new products and
materials pass through exhaustive testing, simulating in-situ
conditions and ensuring beyond doubt that performance will
be at the required level. The cost of a sub-sea valve failing, both
financially as well as in terms of environmental impact, means
this demand for reliability is non-negotiable.
Approval testing
Before any valve can enter service, prototypes must be
rigorously tested in conditions that simulate and go beyond
the pressures, temperatures and dynamic movements that a
valve will face once in situ. The mandatory tests are specified
in ISO 10423 and ISO 13628-4, however most operators’
policy is to exceed the level outlined in the standard to
provide an additional safety margin.
Testing for the new valves involves high and low
temperature cycling – that is opening and closing – the valve
through multiple operations at the pressure and temperature
limits far more than is required by the standards. In reality,
many of these valves will be used on applications where
they will only be cycled a limited number of times in their
operational lifespan.
Through significant investment in software and
hardware much of the testing has been automated. The
pressure sources to the test valves are controlled through
the use of automated valves and pressure sensors which are
also digitally linked to the temperature cabinets. Manual
operation of valves uses digital stepper motors and torque
transducers to replicate valves being operated while in
service by remote operating vehicles (ROV’s).
In addition to product validation testing, strain
gauging is used to perform design validation on certain
key components. For example, the theoretical loading
on valve spindles can be compared to the actual loading
experienced during operation.
Cost considerations
The decision over whether or not to extract oil and gas from
any given well boils down to a relatively simple question –
will the cost of extracting a market-ready product from the
well leave enough of a profit margin to make the exercise
worthwhile? Equipment costs make up a significant share of
the overall cost of extraction.
For this reason, the affordability of any solution will always
be a big consideration. While it may be theoretically possible
to extract from even more challenging reserves if cost was
removed from the equation, there are economic limitations.
Currently, these limitations mean that the effective
maximum depth for subsea projects is around 3000 m
below sea level. However, this has increased rapidly in
recent years, and it is widely predicted that 3600 -3700 m
will soon be commonplace. Valves that can stand up to
the high pressures and temperatures will be essential in
allowing access to these reserves.
Evolvingdesignmethods
Over the past few years the industry has been assessing
the design standards needed to allow equipment to be
fabricated that can safely handle higher pressures and
temperatures than those at the current limits.
Why? Because traditional equations used to calculate the
sizes of valve casings have been based on linear elastic stress
analysis methods – an approach which assumes no yielding
occurs within the casing. These are perfectly acceptable for
some heavier wall sections and simple internal geometry
but where weight and space are at a premium alternative
methods are needed. It is also problematic to address
structural discontinuities sections of the valve casing where
sharp corners lead to highly stresses area – using elastic
methods.
The industry is encouraging the use of elasto-plastic
methods – an analysis with a defined limit to the elastic
stress and fracture mechanics based fatigue – an analysis
which examines how a crack might grow during the
life of the valve. This presents challenges for smaller
manufacturers as it requires availability of detailed material
data such as elevated temperature tensile and fatigue
crack growth rate mechanical properties. Of course, a
further challenge is developing the technical capabilities to
perform these evaluations, and both of these will require
investment by valve manufacturers in the years ahead.
Ultimately, oil and gas is set to continue to play a
critical role in meeting the world’s energy and materials
needs for many years to come, and there will be a
continuation of the drive to extract from increasingly
challenging reserves in terms of temperature, pressure and
chemical impurities.
Valve technology plays a crucial role in allowing these
energy sources to be reached, both by ensuring that they
are not economically prohibitive, but also – and most
importantly of all – that the process poses minimal threat to
safety or the environment.
