The Thames Barrier has just undergone a comprehensive
maintenance overhaul. Chris Leede,project manager of
Monitran, explains how part of that overhaul included the
fitting of sensors to monitor bearing wear and other
parameters

Operated by the Environment Agency,
the Thames Barrier is London’s main
defence against flooding. It became
operational in 1982 and protects parts of the
city against abnormally high tides and
tidal/storm surges.

The barrier divides the river into four nonnavigable
spans (closest to the river’s banks)
and six navigable spans; of which four are
61m wide and two are 30m wide. The nonnavigable
spans have falling radial gates, in
that they lower into the water to form a
barrier, whereas the navigable spans have
rising sector gates which, in their open
positions, lie flat with the river bed.

Each of the four main gates weighs more
than 3300t, presents a 20m high barrier when
in its defence position and can hold back
loads of up to 9000t.

During its almost 30-year history the
barrier has been closed more than 400 times.

Of these closures about 80 were against tidal
surges and about 40 were to prevent
rainfall/fluvial flooding.Most though have
been for maintenance and test purposes.

To adopt their defence positions the main
gates rotate through about 90°; the
maintenance position requires a further 90° of
rotation. The bearings of the main gates have
only ever experienced quarter and half turns
since the early 1980s, and any wear will reflect
this life of partial rotations. Under a contract
awarded in early 2010 by the company
responsible for the barrier’s maintenance and
upgrade projects,Monitran was tasked to
design, install and commission a system to
measure bearing movement and gate position.

Also, the system had to be a permanent
fixture and would require some equipment to
be placed on the outer faces of the bearings;
and therefore be subjected to the full force of
a tidal river.

Four eddy current probes are used to make
distance measurements as the gate/bearing
rotates through 180°. Positioned at 90°
intervals around the end of the bearing (see
Figure 1), the probes were optimised for
maximum sensitivity over a 10mm
displacement range and positioned a nominal
5mm away from the chamfered face of the
end of the inner bearing.

The bracket seen at 6 o’clock in Figure 1
also holds a pressure sensor – used to derive
water depth (tide height). The cables for the
pressure sensor and eddy current probes
terminate in the junction box seen at 10
o’clock. This box (stainless steel and sealed to
IP68) also contains an inclinometer for
recording gate position/angle. In some
instances, most notably for the larger gates
and where space permitted,Monitran
adopted a Russian Doll approach with the
junction boxes.

A single cable feeds through the centre of
the bearing and into the inner trunnion
support structure; and then on into a GRP
junction box enclosure that contains the eddy
current probe drivers. In addition, the inner
face of each bearing is fitted with an
accelerometer in order to measure vibration
levels; and those of the larger gates also have
water leak detectors.

All signals and power lines from/to the
sensors converge at a single cabinet (on one of
the barrier’s ‘decks’). The cabinet has six
displays which, between them, show the
displacements being recorded by the 48 eddy
current probes, the outputs of the 12 pressure
sensors, the gate positions (as either 0, 90 or
180°) and the status of each of the eight water
leak detectors. BNC connectors make
available the accelerometer’s outputs for
vibration monitoring or analysis.

The overall project was something of a
diversion from Monitran’s core business, in
that the company is recognised as an OEM of
vibration, displacement and proximity
sensors. The Thames Barrier project
warranted far more than just the provision of
standard or custom products, and Monitran
had to assemble a team of skilled engineers
and put them through training in order to
work in confined spaces and on floating
platforms. Accordingly, there was a relatively
steep learning curve – but this was offset by
many things the company did to reduce risks
during the installations. For example, as much
circuitry as possible was wired, enclosed and
pressure-tested at Monitran’s HQ in High
Wycombe before going out to the barrier.

In addition, a complete installation and
commissioning was done on a single bearing
before the others were tackled. Doing so led to
the early detection of a problem with a watertight
bulkhead, and gave its manufacturer
time to devise an alternative method of
construction.

Monitran has undertaken a variety of
smaller projects during recent years – all the
while honing its project management skills
(including the use of specialist subcontractors),
but the Thames Barrier project
is considered to be its most prestigious, and at
times challenging, to date.