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AUGUST/SEPTEMBER 2015
OFFSHORE DESIGN &
ENGINEERING EQUIPMENT
SUPPLEMENT
Platforms
Better level
measurement
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Supplement 
Platforms
Better level
measurement
In today’s economic climate of low oil prices and reduced demand, producers are under pressure to
maximise the efficiency of their existing offshore assets by minimising maintenance, reducing costs,
and maximising process availability. ODEE reports.
U
nplanned shutdowns caused by
the failure of equipment or
underperforming processes are a
significant cause of lost production and
inefficiency. Modern industrial automation
plays a critical role in understanding the
health of equipment and increasing
visibility into production process. This
helps to identify potential problems at an
early stage so that timely action can be
taken before production is interrupted.
To support efficiency improvement
programmes a greater amount of reliable
and accurate information is needed by
operators to better control and optimise
offshore processes. The latest
measurement devices can support this
requirement, especially those
incorporating diagnostics for monitoring
device health, and wireless capability that
enables quick and easy installation and
removes the need for costly wiring that
adds weight and causes disruption
during installation.
Many critical processes on offshore
platforms and FPSOs rely on accurate
level measurement to optimise processes
and maintain plant availability. These
include separators, scrubbers and
storage tanks, where the traditional
approach is to use mechanical float
switches or displacers to monitor liquid
levels. But these mechanical devices are
affected by changes in media density
and have moving parts that wear or get
stuck and require scheduled
maintenance. Regular recalibration is
required to maintain accuracy. However,
the need for more reliable continuous
measurements is migrating users to
modern, low maintenance technologies
such as guided wave radar (GWR), noncontacting radar and vibrating forks.
For many offshore applications, GWR
is the technology of choice for level
measurement. GWR provides an
accurate, reliable and maintenance free
way to measure level in a wide range of
applications. GWR level transmitters are
not density dependent and measurement
accuracy is unaffected by high
turbulence or vibrations. Since there are
no moving parts to stick or wear,
maintenance costs are reduced and the
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Supplement 
Platforms
problems of false readings, which can
result in hazardous situations, are
avoided.
In a GWR installation, the device is
mounted on top of the tank or chamber
with a probe extending to the full depth
of the vessel. A low energy pulse of
microwaves is sent down the probe and
when the pulse reaches the media
surface, a reflection is sent back to the
transmitter. The transmitter measures the
time taken for the pulse to reach the
media surface and be reflected back and
an on-board microprocessor accurately
calculates the distance to the media
surface using ‘time-of-flight’ principles.
Diagnostic values such as Signal
Quality can be remotely monitored while
the device remains in service. By tracking
Signal Quality Metrics, plant personnel
can automatically monitor for process
build up on the probe and for unwanted
surface conditions. This awareness
lowers maintenance costs by enabling a
preventative maintenance programme
that reduces or eliminates manual
checks.
For installations where there are
obstructions in the tank, non- contacting
radar is often considered as an
alternative. Non-contacting radar doesn’t
have a probe and works by sending a
signal through the vapour space that
bounces off the surface and returns to
the gauge. Like GWR, changes in
pressure, temperature, and vapour space
conditions have no impact on the
accuracy. In addition, radar devices have
no moving parts, therefore maintenance
is minimal.
Vibrating fork level switches are ideal
for low level and high level alarm
applications supporting applications
requiring overfill prevention. The switches
operate on the principle of a tuning fork,
whose frequency varies depending on
whether the fork is immersed or dry. The
latest generation vibrating fork level
switches feature built in diagnostics and
fast update rates that make them
suitable for a wide range of critical
applications. In addition, the design of
the fork is such that they require very
minimal maintenance, ideally suited to
offshore installations.
The continuous data provided by these
intelligent measurement sensors can be
integrated into a platform’s existing
automation system to increase visibility
into the process. Status information and
device diagnostics are easily accessible
from the control room, reducing
maintenance requirements.. With the
growing number of unmanned platforms,
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a fully automated system also provides
the opportunity to manage and operate
offshore assets remotely from an onshore
control room using satellite
communications. Onshore operations are
able to monitor critical levels and predict,
detect and correct conditions that could
potentially lead to unplanned downtime.
Wireless makes it easy to install
level instrumentation
In many offshore applications, the lack of
an existing cabling infrastructure has held
back the installation of additional level
instrumentation. The cost of installing
new wiring, which can include ducting
and cable trays, can be significant and
involve third party contractors. This all
adds up to increased risk, and weight on
the platform. There is also the
inconvenience and issues of performing
the work whilst the site is operating.
One way of overcoming these
constraints is to use wireless enabled
level measurement devices. Wireless
vibrating fork and radar transmitters have
all the functionality and features of their
wired equivalents but with the additional
benefits and convenience provided by
wireless technology. On existing
installations, replacing older technology
with the latest level measurement
devices reduces maintenance and
enables accurate measurement. Where
no cabling exists, wireless presents an
unparalleled way to add visibility in a
wide range of offshore applications.
Wireless level transmitters are quick and
easy to install. They can be accessed
from the gateway – configured and
reliably transmitting data to a control
system or data historian in just a few
hours.
Separator applications
In separator applications, real time
performance data enables process
control to optimise product quality. For
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Supplement 
Platforms
condensate oil and water production
tank applications, reliable real time data
enables monitoring between times when
the tank requires emptying, minimising
the risk of spills.
For example, Rosemount 5300 GWR
from Emerson provided a solution for
level measurements on a Floating,
Production, Storage and Off-loading
(FPSO) vessel, where there were a
number of challenging applications.
These included medium pressure, low
pressure and high pressure gas
compression scrubbers, glycol contactor
suction scrubber and glycol contactor.
The changing process conditions and the
presence of foam and vapour as well as
dirty sticky fluids made this a very difficult
level application.
The sensitivity of the Rosemount 5300
allowed it to distinguish between a
hydrocarbon layer and water. As the
hydrocarbon layer builds up, the GWR
can track it and report its presence as
well as the water level. This increases the
accuracy of the level reading and allows
the scrubber to operate more efficiently.
The ease of obtaining echo curves from
the Rosemount unit was a key to
understanding the extent of the
hydrocarbon layer build-up.
The GWR was proven to be a highly
reliable device well suited for the widely
varying process conditions of the FPSO.
The more accurate and reliable level
readings increased the safety of the
platform operations. Shutdowns due to
level measurements were eliminated,
which in turn led to increased production.
Reduced shutdowns and increased
production have resulted in significant
cost savings.
Flare knock-out tanks
Flare knock-out tanks serve as recovery
units and allow the offshore platform to
flare (burn) vapours. The tanks usually
contain water with oily contaminants and
there may be rapid changes to the
contents. They are usually part of
emergency management system and
operators face environmental fines for
overspill so accurate level measurement
is critical.
The traditional mechanical systems
used to measure level in flare knock-out
tanks can be inaccurate since they are
sensitive to changes in density and are
costly to maintain. Capacitive systems,
which can also be used for these
applications, are expensive and time
consuming to install as they require
calibrating. Their need for frequent
maintenance and inherent potential for
inaccuracy makes capacitive systems a
less attractive option for these
applications.
Both non-contacting and guided wave
radar measurement devices can be used
for continuous level measurement in flare
knock-out tanks. They can be installed
using either a stilling well or direct
connection and commissioning is usually
completed with just a few clicks. Radar
based level measurement devices are
immune to vapour space changes,
independent of density changes and
require minimal maintenance. and are
therefore ideally suited to this application
The effective and accurate measurement
they provide enhances safety and
reduces the risk of spillage and
environmental damage.
For example, an offshore oil platform
operator in the South China Sea was
struggling to meet pollution requirements
and had to re-evaluate their control
scheme. Natural gas and water was
separated from the crude oil and the gas
was vented and combusted in a flare
stack. However, not all the gas was
combusted and its condensate
accumulated on top of the seawater in a
vessel below the stack.
The operator was using a mechanical
float-based system to measure the total
overall level in the vessel, with the aim of
removing the condensate for treatment
rather than dumping it in the sea.
Measurement was hampered by the
surrounding wave action, readings were
unreliable and frequent maintenance was
needed.
Installation of a Rosemount 3302
guided wave radar allowed the operator
to accurately measure both the overall
level and condensate interface
irrespective of local conditions or sea
state. The operator was able to identify
the quantity of condensate, and pump it
out and process as fuel. The seawater
could be discharged to the ocean
without contaminants. These types of
measurements could easily be done now
with an integral wireless guided wave
radar.
Summary
Improved level measurement can support
the need to maximise the efficiency of
existing offshore assets, whilst reducing
operation costs. Replacing traditional
level measurement with modern guided
wave or non-contacting radar devices
improves accuracy and reliability. Being
able to continuously monitor a process
helps to optimise processes and the
increased visibility enables operators to
identify problems much earlier that could
interrupt production. With the availability
of wireless enabled level devices
achieving these improvements has
become much easier, with many of the
cost and installation obstacles
removed. 
For further information please visit:
www2.emersonprocess.com
38 | AUGUST/SEPTEMBER 2015 | OFFSHORE DESIGN & ENGINEERING EQUIPMENT
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