The Problem It Solves
An LNG tank is a structure within a structure. The annular space refers to the roughly 1-meter gap between the outer concrete wall and the inner steel wall, filled with a resilient blanket and perlite insulation which is permanently sealed before the tank ever enters service. Once sealed, it cannot be accessed again for the 20–40 year life of the tank. Yet this is precisely where the most critical structural movements occur. Without monitoring equipment installed during construction, operators have no visibility into one of the most mechanically significant regions of the entire asset.
What It Monitors
As the inner steel wall cycles between cryogenic and ambient temperatures and between full and empty states, three distinct types of movement occur relative to the stable outer wall:
- Vertical (settlement): the inner wall sinks under the weight of the stored LNG
- Horizontal (radial): varying fill levels create pressure changes that expand or contract the inner wall, shortening the annular space
- Rotational: uneven thermal shrinkage causes the inner wall to twist relative to the outer wall — unpredictable and undetectable by any other means
Each axis represents a distinct failure mode. None can be inferred from the others. The TMMS measures all three simultaneously.
Why Fiber Optics
Conventional electrical sensors are not viable in the annular space for four reasons:
Cryogenic survival. The system must withstand -169°C permanently. Conventional electronics degrade, crack, or drift at these temperatures. Fabry-Perot fiber optic sensors have minimal moving parts and are rated for the full life of the structure.
Intrinsic safety. No electrical current is permitted near LNG. Fiber optic sensors transmit via light, eliminating ignition risk entirely — a regulatory requirement, not just a design preference.
EMI immunity. The pumps, compressors, and switchgear of an LNG terminal generate electromagnetic fields that corrupt conventional sensor readings. Fiber optic signals are unaffected.
No-maintenance design. A sealed, inaccessible space makes conventional sensor maintenance impossible. The TMMS is designed to operate without any intervention for the full 20–40 year tank life.
System Architecture: From Sensor to Control Room
Depending on the tank design, sensors are installed either at the bottom of the annular space, below the corner protection, or just above it. Sensors are placed around the tank, along its perimeter (for instance, every 60°, 90°, 120°) during construction, before the annular space is sealed. All fiber optic cables exit through a single counter-flange and junction box at the tank roof, connecting to a monitoring cabinet. Data feeds into existing terminal SCADA systems via standard protocols including Modbus.
Critically, the TMMS isolates differential movement between the inner and outer walls from overall tank movement. Without this isolation, readings would blend true inner wall behavior with whole-tank settlement — making it impossible to distinguish a structural problem from normal operation.
Regulatory Context
Governments worldwide are increasingly mandating Tank Movement Indicators for LNG facilities. The TMMS is the only technically credible way to demonstrate compliance with movement limits in a space that cannot be physically inspected. For operators, it functions simultaneously as a safety system, a structural health record, and an audit-ready compliance instrument.
Summary
The TMMS solves a monitoring problem unique in structural engineering: how to continuously instrument a permanently sealed, explosive, cryogenic space for 40 years without maintenance or recalibration. Fabry-Perot fiber optic sensors, permanently embedded during construction, reporting all three axes of inner wall movement in real time and is the only solution capable of delivering this data. For LNG terminal operators, that makes it irreplaceable.
