There are many areas of the world where
oil-producing countries do
not have nearby refineries. Crude oil must be
transported in
ocean-going
tankers. Offshore loading facilities load tankers
with crude oil from
coastal
tank farms where water depth will not permit
docking. Loading is by
pipeline
from land to an offshore mooring area. Oil is
pumped to a loading
station
with flexible hoses, which connect a sub-surface
pipeline, manifold
(PLEM)
to a buoy mooring facility. Tankers can be loaded
and sub sea valves
controlled
from this mooring facility.
Problems develop, however, in these systems
when
heated oil enters the
offshore pipeline, cools, and deposit paraffin
wax or asphaltine on
inside
pipe wall. Since this wax buildup hampers flow
and can plug the
pipeline,
frequent pigging was necessary. Wax removed by
pigging is deposited in
a receiver chamber at the PLEM.
A new monitoring system determines the extent
of
wax collected so that
it can be removed before flow reduction and
plugging occur. This system
can also determine status of subsurface valves
without requiring a
diver
or remotely operated vehicle.
Key
Components of the System
The magnetic sphere and valve position
monitoring
system (MSVP) is designed
to be compatible for integration into an export
terminal supervisory
control
and data acquisition (SCADA) scheme. It features
a control console
which
houses a remote terminal unit (RTU) similar to
those used in a main
telemetry
system. This arrangement allows the magnetic
sphere monitoring system
to
use an existing radio network for
communications. Data and other
information
concerning the MSVP is displayed on the control
consoles and also is
accessible
by the master station.
Onshore components consist of two non-intrusive
magnetic pig/sphere-signaling
units, which indicate arrival, departure, or
passage of a magnetic
detectable
pig or sphere.
One unit is a magnetic pipeline scraper passage
indicator designed for
above ground monitoring at launching/receiving
facilities, valve
locations,
or aboveground piping. Two of these units are
used in an operating
system.
The first is mounted directly on a
launch/receive
trap at the land section
of a pipeline. It announces arrival of a
magnetic sphere, which is
returning
from an offshore section. When activated, it
turns on a high-intensity
strobe light and an elapsed time counter. An
auto re-set with event
counting
capability also is featured on this unit.
A second unit is mounted directly after a block
valve and provides a
closure when activated by passage of a magnetic
sphere. Upon
activation,
data is sent to the SCADA system, which signals
an electronic console
on
a service vessel. This serves as a "wake-up"
call for the sub-surface
acoustic
and magnetic systems.
Control
Console
Systems
Service vessels have two identical control
console systems which can
be used during normal operations. Each console
contains:
(1) an RTU,
(2) a transceiver radio,
(3) an acoustic modem,
(4) a master central processing unit,
(5) valve, sphere, and sphere, static position,
status displays,
(6) remote/local valve control,
(7) four manual valve switches,
(8) eight timer relays for valve
open/close,
(9) power supplies.
Sub sea valve positions and the
sphere/scraper are monitored with
the console. Control consoles are operated from
pipeline control and
activated
by a radio link to the RTU. Subsurface valves
can be activated
manually,
if needed, with features on the console. All
other functions are
processed
through the master CPU and include sampling of
flow rate, mimically
displaying
progress of a sphere, early warning alerts,
receipt confirmation, and
static
sphere position. Included in the processing
stage is monitoring of
valve
positions which are displayed at a lower
position on the console front.
In the upper quadrant of the console front
panel, is a mimic display,
which
shows progression of a sphere. Below this are
manual actuators for
hydraulic
operation of valves if the radio link fails.
Communication with subsurface magnetic systems
is
by an acoustic link
from the PLEM. The PLEM has two 20in.valves, one
36-in. mainline valve,
and one 16-in. valve. Mounted directly to the
downstream side of the
mainline
valve is a scraper receiver trap for collecting
the sphere and wax or
foreign
materials.
A 6-in. tube containing five flux-gate
magnetometers and one inductive
receipt sensor are mounted directly to the
outside of a receiver. An
early
warning sensor and inductive device is located
ahead of the offshore
main
line. All magnetic sensing units are coupled to
a sub sea processing
unit,
which is contained in a 6-in. by 180-in.
silicone oil filled tube.
Communication to the surface is by a link from
an
underwater acoustic
modem mounted on the PLEM and powered by two
lithium battery packs
wired
in parallel. A transceiver transducer sends and
receives data to and
from
a surface acoustic transducer, which is lowered
or raised by a winch
during
operations. An umbilical cable connects the
transducer to a control
console.
Valve positions are monitored with twin
magnetic
sensors. One is at
a fully open position and the other is at a
fully closed position.
Sensors
are code so that they are identifiable by the
CPU. Valve status
indicators
are located in the lower quadrant of the control
console's front panel.
A subsurface electronic package consists of
magnetic sensing modules,
valve position modules, magnetic flux gate
detection unit,
microprocessor
unit, acoustic data system, connecting cables,
and a lithium battery
pack.
Inflatable spheres have a spring-loaded
magnetic
constellation that
provides a strong magnetic signal to the outer
wall of the pipeline.
System
Overview
An operating sequence begins when the control
room
turns on an output
in the RTU. This shows that the confirm launch
magnetic sensor is
activated.
Two status output signals are shown by magnetic
sensors and hardwired
onshore
into the PLC. This gives control room
accessibility. An indication that
a sphere is in the trap is given with the first
status signal. Start up
or launch confirm is shown by the second signal.
A control console RTU communicates with the
master
station and connects
a master CPU via analog and digital I/O.
Information sent to and coming
from the master station is transmitted through
these connections.
Existing
radio frequencies and protocol are used so the
master station has
access
to data within the system.
Valves are controlled with eight interposing
relay
outputs on the RTU.
Control is directed by the PLC via RTU when in
remote mode operation.
Communication between master CPU and underwater
sub sea processor uses
an acoustic data line. A StarTrak master CPU
manages display control,
interface
to an RTU, and interface to the acoustic link.
The sub sea CPU controls
underwater power management, interface to valve
position sensors,
interface
to magnetic sensing modules, interface to
magnetic flux-gate detection
unit, and interface to the acoustic data
link.
Immediate sphere location is determined by flow
rate in the pipeline.
A PLC will send the flow rate in bbl/hr to an
analog output in the RTU.
Sphere position during operation is shown on the
control console's
display
panel. A master CPU controls the display and
data is sent to RTU via
I/O
connections. Approximate sphere position in the
pipe is shown from the
time of launching until an early warning sensor
detects it. Upon
detection,
the underwater unit is powered up by acoustic
data link and the exact
position
is determined and displayed.
Analog input to the RTU shows location of a
sphere
after its detection
and a numeric display on the control console
shows location as measured
from a block valve, its final destination. Each
valve position has two
sensors, one for open and the other for closed.
Four valves are
monitored
by serial data link. A sub sea processor
activates the serial data link
and reports all four valve positions when
queried.
Sequence of initiating a query begins when PLC
communicates to the RTU
and turns on an output. Output is detected by a
master CPU which, in
turn,
powers up an acoustic link and requests valve
status, Then a sub sea
processor
acknowledges the valve status request and powers
a serial data link to
acquire status of each valve. Serial data link
for the valves is
powered
down and data transmitted to the surface by
acoustic link.
When a master CPU receives this
data,
eight outputs connected
to the RTU are activated. At this time, valve
status can be read by the
PLC. Valve status is activated only when
requested by the PLC or a
"Request
Valve Status" push button on the control console
is activated. Early
warning
and receive confirmations from magnetic sensors
are shown on the mimic
display. The master CPU activates outputs
(inputs to an RTU) to show
passage
of a sphere and underwater battery status is
monitored.
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