Flooded Member Detector

Cygnus Instruments manufactures ultrasonic Flooded Member Detection (FMD) equipment also referred to as Flooded Member Inspection (FMI) designed to identify water ingress in subsea structural members. This category includes the Cygnus FMD System for ROV, the Cygnus FMD System for Diver, and a range of supporting FMD accessories, including the FMD umbilical cable, system test piece, and probe holder handles.

Flooded member detection identifies a through-wall defect in a subsea member by determining whether water has entered the structure often the result of a weld failure or crack, particularly following extreme weather events or impact. Left undetected, this kind of water ingress can compromise structural integrity. Historically, gamma radiation FMI has been used to inspect entire subsea platforms; the Cygnus ultrasonic FMD system offers operators and inspection contractors a simpler, more affordable alternative particularly suited to interim and one-off inspections.

How the Cygnus FMD System Works

Each Cygnus FMD system consists of an ultrasonic transducer and transducer handler, a subsea electronics unit, and reporting software used at the surface. Once the transducer’s contact sensor confirms correct alignment with the member, the system runs a test and returns a clear DRY (green) or FLOODED (red) result, along with the accompanying A-scan waveform for further verification. Results are logged and can be exported as PDF or Excel reports for analysis and client reporting.

The system uses a purpose-designed algorithm combining an advanced peak-detection algorithm, low-distortion/low-noise data acquisition, and advanced signal processing engineered to achieve a high Probability of Detection (PoD) and reduce false readings. A measurement can only be taken once the contact sensor confirms transducer contact, and the reporting software alerts the surface team if contact is lost mid-inspection. Every system includes a System Test Function to confirm reliability before deployment.

Diver-Deployed and ROV-Deployed FMD Systems

Cygnus offers the same core FMD technology in two deployment configurations, so operators can choose the option that fits their inspection team.

Cygnus FMD System for Diver

Purpose-built for use by dive teams, this system (Cygnus FMD System for Diver) features a self-aligning transducer handler that maintains correct contact with the target member regardless of the diver’s angle of approach a benefit when a diver has only one hand free. Because the system is straightforward to learn, owners and operators with varying levels of ultrasonic testing experience can use it without relying on specialist external contractors.

Cygnus FMD System for ROV

Designed for integration with remotely operated vehicles, this configuration (Cygnus FMD System for ROV) uses the same self-aligning transducer handling solution, ensuring correct member contact even when the ROV approaches at an angle. Like the diver system, it is owner-operable and does not require specialist third-party contractors to run.

Notably, the same FMD unit can be used interchangeably by both divers and ROV teams a design choice that lets operators deploy one system across different inspection methods rather than investing in separate equipment for each.

Built for Subsea Reliability

FMD equipment from Cygnus is designed to the highest standards and supported by a three-year warranty. The transducer solution is compact enough to reach difficult inspection points such as K-nodes, where other inspection methods cannot operate, and a range of options is available to suit different working depths and requirements. Cygnus Instruments was incorporated in 1983 and has since built four decades of experience in ultrasonic testing, with all equipment manufactured in the UK to the company’s own quality standards.

FMD Accessories

The Flooded Member Detector range is supported by a set of accessories available within this category:

FMD Umbilical Cable — available in a 120m length, connecting the subsea unit to the surface electronics.
System Test Piece — used to run a full system test of the Cygnus FMD system before deployment.
Probe Holder Handles — a selection of handle options for the Cygnus FMD probe holder, including cable protectors and self-aligning transducer handler components.

Full accessory listings are available in the FMD Accessories category.

Why Choose Cygnus for Flooded Member Detection

Cygnus Instruments applies its ultrasonic testing expertise spanning ultrasonic thickness gauging and ultrasonic leak detection to flooded member detection, giving customers a single, experienced supplier for subsea inspection, repair and maintenance (IRM) and asset integrity management (AIM). For a deeper look at how and why flooded member detection is used across subsea operations, see our Flooded Member Detection industry page. For questions on any product in this range, contact our team or explore our technical support resources.

Frequently Asked Questions

What is a Flooded Member Detector (FMD)?

A Flooded Member Detector is an ultrasonic testing instrument used to check whether a subsea structural member — such as a leg, brace, or K-node on an offshore platform has water inside it. Water ingress usually signals a through-wall defect like a weld failure or crack, often caused by extreme weather or impact damage, and can compromise structural integrity if left undetected.

What is the difference between Flooded Member Detection (FMD) and Flooded Member Inspection (FMI)?

There is no technical difference FMD and FMI are two names for the same inspection method. Cygnus uses both terms interchangeably to describe ultrasonic testing that identifies water ingress in subsea structural members.

How does the Cygnus FMD system work?

The system uses an ultrasonic transducer and handler connected to a subsea electronics unit and surface reporting software. Once a contact sensor confirms correct alignment with the member, it runs a test and returns a simple DRY (green) or FLOODED (red) result with a supporting A-scan waveform, which can be logged and exported as a PDF or Excel report.

Is ultrasonic FMD as reliable as gamma radiation testing?

Gamma radiation FMI has traditionally been used for full-platform inspections, but ultrasonic FMD offers a simpler, more affordable alternative — particularly well suited to interim or one-off inspections rather than whole-platform surveys. Cygnus’s system uses advanced peak-detection algorithms and low-noise data acquisition specifically engineered for a high Probability of Detection (PoD) and to minimize false readings.

What's the difference between the Cygnus FMD System for Diver and the Cygnus FMD System for ROV?

Both use the same core ultrasonic FMD technology and self-aligning transducer handler, which maintains contact with the member regardless of approach angle. The Diver version is built for hand-held use by dive teams; the ROV version is configured for integration with remotely operated vehicles. Functionally, they deliver the same DRY/FLOODED result.

Can the same FMD unit be used by both divers and ROVs?

Yes. Cygnus designs the same FMD unit to work interchangeably across both deployment methods, so operators can use one system for multiple inspection approaches rather than buying separate equipment for divers and ROVs.

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