Offshore Structural
Monitoring
Fixed Structures
Mecon
has developed a novel method for monitoring the structural
health of jackets (cross-braced tubular steel structures that
support oil platforms in the North Sea and elsewhere). Based
on structural acoustics, the method uses a sparse array of
fixed transducers to monitor an entire structure. With
funding from the UK Health & Safety Executive (HSE),
the system has been proved on a laboratory scale model. The
next step will be to demonstrate it offshore.
View
jacket monitoring presentation.
Floating Structures
Mecon
has just completed an intensive study of possible acoustic
methods of monitoring the structural health of Floating
Production, Storage & Offloading Facilities (FPSOs). The
report is soon to be published on the UK HSE's website at
http://www.hse.gov.uk/research/rrpdf/rr328.pdf .
The methods would also be suitable for monitoring tankers
and other vessels.
Civil Structures
Railway Bridge structural inspection
In the UK, Network Rail has responsibility for some 28,000
railway bridges. The normal process of visual inspection can
fail to pick up structural degradation, particularly in
steel bridges with brick or concrete facings. Close
inspection may be needed, and in case of doubt it may be
necessary to monitor a bridge's dynamic response. In either
case the carriageway under the bridge has to be
closed, and if monitoring devices are fitted this may take
days. This is too slow and expensive an approach to apply
wholesale.
Network
Rail needs a non-contact method to measure a bridge's
behaviour quickly and without closing the carriageway. Mecon
has been pioneering the use of lasers for rapid diagnosis of
a bridge's structural condition.
Web-based Geotechnical Monitoring
Data from large networks of geotechnical sensors are
automatically gathered, processed and made available to
clients via a secure website. The BT telephone exchange and
Central Post Office in Cambridge, UK are currently being monitored for
subsidence, shock and vibration during
large-scale building works close to their premises. The
system will be in place for several years.
Applications of the system in London and elsewhere are planned for
2005.
Production Testing
A Company manufacturing wind turbine blades from a mixture
of plastic and wood composites was having difficulties in
production testing these large structures for delaminations
and voids. Mecon carried out a series of investigations on
tap testing techniques, resulting in a design for a small
hand-held transducer system. Mecon manufactured and supplied
the units to the production testing team, allowing several
blades to be tested simultaneously, rapidly and accurately .
Pipeline Integrity
Gas Distribution Pipeline
Advanced Pressure Testing
Pressure testing new gas mains has been made vastly
quicker and more accurate by the incorporation of an
acoustic temperature measurement system researched and
designed by Mecon. Gas mains can now be tested before they
are buried, and the effects of temperature variation can be
removed from the results (with an accuracy of 1 milliKelvin).
Built by Advantica, the system is in regular service with
Transco. For more details, view a recent
presentation on the technique.
Pipe Bedding Condition
Assessment
Buried pipelines can be damaged if the supporting material
is washed from under them, or if they are laid across a sill
of hard material and the surrounding soft material
subsequently subsides. Mecon carried out a feasibility study
which showed that measurements of the cut-on frequencies,
damping, and mode shapes of pipe vibration modes can be used
to characterise both the condition of the pipe wall and the
surrounding bedding material.
Ultrasonic Water Mains
Condition Measurement
The
UK has thousands of miles of water mains, made of materials
ranging from cement, through cast iron, to modern plastics.
The water companies need to be able to assess the state of
these pipes in situ, rapidly and economically. Mecon has
developed a range of ultrasonic transducer arrays for
measuring water pipe size and wall thickness, and detecting
corrosion, cracks, leaks, joints, valves, linings and other
features. The arrays are battery-powered and miniaturised,
designed to be carried inside instrument packages that can
pass through kilometres of water pipe while information is
continuously recorded. They are currently on trial
with a major water company.
The picture shows a 16-channel array control board with
one of its ultrasonic transducers.
Dynamical
Modelling & Analysis
Undersea Pipeline Dynamics
(1): Seabed Slamming
Very strong cross-currents can cause a pipeline laid on the
seabed to go into self-excited oscillation, slamming
repeatedly against the seabed and eventually damaging the
pipe. This little-known phenomenon is not normally a problem
for pipeline engineers, since very strong cross-currents are
needed for it to be strong enough to lift a pipeline.
However in some areas of the world, cyclonic storms are
strong enough and frequent enough for the problem to be
taken seriously. Mecon carried out a thorough analysis of the
phenomenon to predict the risk of damage to a specific
pipeline offshore Australia.
A numerical analysis of the
dynamics was combined with a Monte Carlo analysis based on
sea-state statistics to estimate both the fatigue life of
the pipeline and
the uncertainty in it. On the basis of Mecon's
recommendations the anchoring arrangements for the 40km line were changed, saving the client several million dollars.
Scientific
Software
Undersea Pipeline Dynamics
(2): Piling Shock
Pipelines are normally assembled onshore, together with
switchgear which allows pipeline flows to be controlled
remotely. The manifolds carrying the switchgear are piled
into the seabed. The contractor must decide whether to
attach the delicate switchgear before the assembly is towed
out to sea and sunk (and risk shock damage during piling) or
after piling (incurring the added costs of using divers to
install the switchgear on the submerged manifold)
Mecon produced a rapid dynamical modelling program to
allow the contractor to predict peak shock levels at the
switchgear during piling, so that this decision can be made
more sensibly.
Undersea Pipeline Dynamics
(3): Modelling Deepwater Pipe Laying
Vessels like Stena Offshore's Apache unreel
continuous steel pipes of diameters of 16in from
a gigantic reel carried amidships. The pipe passes over a
J-shaped "stinger" at the stern before falling vertically to
the seabed. The pipe undergoes considerable deformation, and
variations in pipe laying rates, sea state and other factors
can cause appreciable changes in stress. Mecon produced and
supplied software to calculate the stresses analytically
(i.e. exactly). This is quicker and more accurate than
modelling with finite elements, and does not require a
separate model to be built for every case.
Noise &
Vibration
Wind Turbine Noise
Abatement
Complaints about noise
from onshore wind farms are common in the UK, and Mecon has
been involved in noise reduction work for many years. Reducing the
noise radiation from a large built-up structure is not
straightforward, and requires a proper understanding of the
acoustics of the structure. Mecon limited has used two
different analytic tools for doing this: Noise Transmission
Path Audit (TPA), and Statistical Energy Analysis (SEA).
Following a detailed analysis, clients have been able to
apply noise reduction methods cost-effectively. For
further details, view a
presentation on noise transmission path audit .
Seismic Source Noise
Reduction
The
search for oil and gas offshore usually involves letting off
explosions of compressed air a metre or two below the water
surface. These bangs have to be loud enough so that the
echoes returning from oil and gas reservoirs (up to several
kilometres below the surface) can be picked up by underwater
microphones. When this is carried out close to shore, the
result can be disturbance both to wildlife and to residents.
The photo shows a "gun barge" operating in the Dutch
fenlands.
Mecon carried out an investigation into whether the noise
disturbance to the environment can be reduced without
hindering the search for oil & gas.
Read more.
