Subsea structural members are designed to stay sealed and dry. When extreme weather or an impact breaches a member, water can enter it, and because this happens inside the structure, the flooding often goes unnoticed unless it is checked for. Flooded member detection is how inspection teams find it.
Drawing on its long experience in ultrasonic testing equipment, Cygnus Instruments developed the Ultrasonic Flooded Member Detector (FMD) system as an alternative to the gamma radiation FMI technique. The system can be used by a diver or mounted onto a remotely operated vehicle (ROV). It is deployed quickly by an on-platform dive team or ROV pilot and requires little specialist expertise.
This guide explains how flooded member detection works, where it is used, and how the Cygnus FMD systems for divers and ROVs compare. To view the full product range, visit the Flooded Member Detector category page. For technical advice on choosing the right configuration for your inspection programme, contact our team or call +44 (0) 1305 265 533.
What Is Flooded Member Detection?
Flooded member detection, also referred to as flooded member inspection (FMI), establishes whether the inside of a subsea structural member is dry or has been breached by water ingress. Water can enter a sealed member as a result of extreme weather conditions or impact. Because the flooding occurs inside the structure, it would frequently go unnoticed without a dedicated detection method.
Cygnus uses ultrasound to make this determination. The Ultrasonic FMD system returns a clear DRY or FLOODED result, supported by A-scans for verification, giving inspection teams a definitive outcome from each test. Cygnus offers this ultrasonic approach as an alternative to the gamma radiation FMI technique.
How Flooded Member Detection Systems Are Used
The Cygnus FMD system is deployed either by a diver or by mounting it onto a remotely operated underwater vehicle. This lets inspection teams choose the method that suits the structure and the dive plan.
After deployment, the system runs a test through the FMD reporting software at the surface. The result is displayed simply as DRY (Green) or FLOODED (Red), accompanied by A-scans for further verification. The results are then logged for analysis and can be exported as a PDF or as an MS Excel-compatible CSV report.
A key part of the process is the probe handling solution, which self-aligns with the member to ensure correct contact has been made regardless of the angle or direction of approach. The system only takes a measurement when the contact sensor detects correct probe contact and alignment. Any loss of contact during a test is indicated on the reporting software at the surface. A built-in System Test function lets operators verify the reliability of the system before it is deployed.
Applications of Flooded Member Detection
Flooded member detection is used to identify water ingress in subsea structural members caused by extreme weather conditions or impact. It is well suited to offshore structural inspections, as the system can be deployed by on-platform dive teams or ROV pilots. The compact probe design gives access to difficult locations such as K nodes, where other inspection methods may be unable to perform, and the system inspects members with diameters from 200 mm to 2000 mm, making it versatile across a broad range of subsea structures.
Industries Using Flooded Member Detection
Cygnus FMD systems are applied to offshore and subsea structures, where structural members are exposed to extreme weather and impact and must be checked for water ingress. Deployment is carried out by on-platform dive teams or ROV pilots, which makes the systems suited to offshore and subsea inspection programmes.
Flooded Member Detection is one of the three ultrasonic testing disciplines Cygnus provides, alongside Ultrasonic Thickness Testing and ultrasonic leak testing.
Cygnus Flooded Member Detection Systems
Cygnus offers two FMD systems, one configured for divers and one for ROVs, supported by a range of FMD accessories. Both systems share the same core ultrasonic technology and reporting workflow.
Cygnus FMD System for ROV
Overview: The Cygnus FMD System for ROV is designed for mounting onto remotely operated vehicles that inspect subsea members for water ingress. It combines intelligent, advanced software with an electronics interface that is truly simple to use. Its innovative, fail-safe design means it can be operated by operators of varying experience.
Purpose: To carry out flooded member detection from an ROV, returning a simple DRY (Green) or FLOODED (Red) result with A-scans for verification. The system comprises a probe with probe handler, a subsea electronics unit and reporting software.
Key Features: The ROV system is easy to use, with a self-aligning probe handling solution that ensures correct contact with the member even when approached from an angle, and results are logged at the surface for easy review. It is cost effective because it is simple to learn and can be operated by a range of operators, which removes the need for external contractors. It is also rugged, available in high-grade acetal for the harshest environments and supplied with a 3-year warranty. Its reliability comes from a purpose-designed algorithm that combines advanced signal processing, an advanced peak detection algorithm, and low-distortion, low-noise data acquisition to deliver a high Probability of Detection, while a System Test function supports pre-deployment checks. Finally, the compact probe makes the system versatile, reaching hard-to-reach areas such as K nodes, with options available to suit different working depths.
Key specifications: 0.5 MHz high-power piezo composite single-element probe; measuring range 200–2000 mm member diameter; resolution 1 mm; depth rating 300 msw; operating temperature 0°C to +75°C.
Applications: Subsea structural member inspection from an ROV; offshore inspection programmes; inspection of hard-to-reach joints such as K nodes.
Industries: Offshore and subsea structural inspection.
Cygnus FMD System for Diver
Overview: The Cygnus FMD System for Diver is used by divers to detect water ingress in subsea structural members using ultrasound. Its easy-to-use interface provides a clear DRY or FLOODED result.
Purpose: To enable a diver to perform flooded member detection underwater. After deployment, the system runs a test via the FMD reporting software at the surface and shows a simple DRY (Green) or FLOODED (Red) result with A-scans. The results are logged for analysis and can be exported as a PDF or as an MS Excel-compatible CSV report.
Key Features: The diver system is easy to use, with a responsive, self-aligning probe handling solution that confirms correct contact with the member regardless of the angle or direction of approach. Its clever, fail-safe design is easy to learn and can be used by owners and operators with varying levels of experience, which makes it cost effective by removing the need for expensive external contractors. It is rugged, designed to the highest standards and supplied with a 3-year warranty. The same purpose-designed algorithm, combining advanced signal processing, advanced peak detection, and low-distortion, low-noise data acquisition, delivers a high Probability of Detection. Measurements are only taken when correct probe contact is confirmed, and a System Test function supports pre-deployment checks. The compact probe design also allows testing in hard-to-reach areas, including K nodes.
Key specifications: 0.5 MHz high-power piezo composite single-element probe; measuring range 200–2000 mm member diameter; resolution 1 mm; depth rating 300 msw; operating temperature 0°C to +75°C.
Applications: Diver-led inspection of subsea structural members for water ingress; offshore structural inspection.
Industries: Offshore and subsea structural inspection.
FMD Accessories
The FMD systems are supported by a range of FMD accessories. The FMD Umbilical Cable is available in 120 m length, and a 50 m Deck Cable (13 kg, 290 x 380 x 490 mm) is used alongside it. A System Test Piece is used to run a full system test of the Cygnus FMD system, and a selection of Probe Holder Handles is available for the Cygnus FMD probe holder.
Diver Flooded Member Detection
For diver-deployed inspection, the Cygnus FMD System for Diver detects water ingress in subsea structural members using ultrasound and presents a clear DRY or FLOODED result. The diver configuration includes a Communications PSU (Power Supply Unit) that provides both power and communications to the diver, a stainless-steel D-handle, and a cable protector that gives extra cable protection and an attachment point for divers. It also uses the 120 m umbilical cable, the self-aligning, lightweight acetal probe handler, and the System Test Piece, which is used to run a full system test.
ROV Flooded Member Detection
For ROV-deployed inspection, the Cygnus FMD System for ROV mounts onto remotely operated vehicles inspecting subsea members and determines whether water ingress is present. ROV-specific options include a choice of T-Bar and Fishtail handles and a cable protector, alongside the self-aligning, lightweight acetal probe handler. The system connects through the 120 m umbilical cable and the 50 m Deck Cable (13 kg, 290 x 380 x 490 mm), and the System Test Piece is used to run a full system test before deployment.
The system’s data link uses RS232 full-duplex, which is suitable for fibre multiplexers.
Key Benefits of Cygnus FMD Systems
Every Cygnus FMD test returns a clear, definitive DRY or FLOODED outcome, backed by A-scans for verification. A purpose-designed algorithm combines advanced signal processing, advanced peak detection, and low-distortion, low-noise data acquisition to deliver a high Probability of Detection and eliminate false readings. Measurements are only taken when the contact sensor confirms correct probe contact; any loss of contact is flagged on the surface reporting software, and a System Test function lets teams verify the system before it goes in the water.
The fail-safe design is simple to learn and can be used by operators with varying levels of experience, without relying on external contractors. The compact probe reaches difficult geometry, including K nodes. Results are logged at the surface and can be exported as a PDF or as an MS Excel-compatible CSV report, and the rugged construction is backed by a 3-year warranty.
Related Inspection Technologies
Cygnus’ expertise extends across the wider ultrasonic testing range. For corrosion and wall-thickness measurement, see the Ultrasonic Thickness Testers range, including the Underwater UT Testers used by divers and on ROVs. For application context, explore the Offshore and Subsea inspection pages, or browse the full Flooded Member Detector range. To discuss an FMD system for your inspection programme, contact Cygnus Instruments.
