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CVI and GVI for Hull Maintenance Explained | Deep Trekker

Rachel Doornekamp   |   July 21, 2021

Consistent inspection is crucial for the overall safety, efficiency and structural integrity of ship hulls and offshore structures. Ships can be inspected regularly underwater - without being put into dry docks - with visual inspections. Visual inspections consist of visually documenting various waypoints on the hull to check for any structural defects, biofouling, marine growth or other issues. Visual inspection is widely considered to be the most economical and fastest way to obtain an early assessment of the condition of a vessel and its components.

This consistent underwater visual inspection is an important task to confirm and improve upon the operations safety and efficiency of ships and other floating vessels. With consistent visual inspection operators can determine maintenance schedules and workloads best suited to each vessel while minimizing cost and downtime.

While occasional dry-docking will always be necessary, underwater inspections can help reduce the number of dry-dockings required. Minimizing dry-docking time is important in order to reduce downtime and save money over time.


Close Visual Inspections (CVI) for Hulls

CVI or close visual inspections are crucial for hull maintenance. A detailed and close up visual examination, a CVI is used to identify areas requiring more in depth inspections. More specifically a CVI is an extensive examination of a specific area, section or component to detect and potentially identify damage, failure or irregularity.

CVI typically makes use of additional equipment to enhance the quality of the inspection. Normally available lighting is often supplemented with a direct source of bright light such as auxiliary lights or flashlight, or perhaps even sonar tools in particularly murky and turbid waters.

Furthermore, CVI often entails surface cleaning to gain clear access to all components of the area being inspected. Specific tools such as probes, gauges and measuring tools are also often used to acquire further insight into the hull or structure. Finally, more extreme access procedures may be required to ensure close inspection to all aspects of the component being inspected.

The qualifications applicable to CVI can be both PCN or CSWIP. A CVI is suitable for a number of structures and components including:

1. Hull

CVI allows users to thoroughly examine the ship hull to ensure safe and efficient operation, while also meeting any applicable shipping standards. Inspectors are looking at the entirety of the hull, searching for any defects, damage or other imperfections that could pose an issue for the ship. Regular hull inspections not only ensure safe operation but allow for shipping teams to make informed decisions regarding cleaning, painting and maintenance schedules.

2. Anchors

Anchors are crucial for the safe operation of shipping vessels. Anytime the ship needs to stay afloat in a stable, stationary position a well-functioning anchor is required. Operators inspect the anchor and accompanying submerged chain look over all aspects from the head to shank to the fluke.

3. Propellers

Without properly functioning propellers ships cannot make their way across bodies of water with any sort of control or direction. The structural integrity and correct operation of propellers is therefore imperative for safe and effective operation. Consistent inspection of the entire propellers including blades and hubs is essential.

4. Sea Chests

Sea chests provide an intake reservoir from which piping systems can draw raw water. Typically containing grating and baffle plates, sea chests require inspection to prevent damaging clogs. Blockages that decrease the availability of water for onboard operations can cause damage in components that require water for cooling.

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General Visual Inspections (GVI) for Hulls

General Visual Inspection (GVI) is quite simply a visual inspection of overall condition with a report following the inspection to report on the status of the structure.

GVI is conducted further back than a CVI and looks at more of an overview of the entire area. Typically completed at approximately an arm’s length away from the component, operators are searching broadly for any defects or issues. While a mirror may be required to enhance visual access to exposed areas, a GVI is conducted under normally available lighting conditions. Daylight, flashlight or ROV lights are usually the only light used for GVI and sometimes nearby structures or objects may have to be moved out of the way to gain access.

GVI looks to detect a number of things including:

1. Defects

Defects on a ship can occur for a variety of reasons. From accidental impacts to normal wear and tear, ships and related structures can take a beating on the job. Inspection confirms the safety of the vessel while also providing operators with the information needed to make accurate decisions regarding shipping, maintenance and painting schedules.

2. Biofouling

Biofouling refers to the accumulation of plants, algae, animals and microorganisms on a ship. In addition to the potential transfer of invasive species, biofouling can negatively impact the hydrodynamics, speed and fuel efficiency of a vessel.

3. Cracks

Cracks anywhere on a ship pose a serious risk to watertight structures. Catching signs of a crack as early as possible is imperative to safe operation.

4. Discolouration

Discoloration - particularly on the hull - can be a sign of a potential defect. Even if the spot is harmless, discoloration can indicate a new paint job is required. A smooth coat of paint on the hull helps keep fuel costs as low as possible by maximizing the hydrodynamics of the vessel.

It is important to note that GVI does not fall under any zonal inspection.

Considerations When Selecting a GVI or CVI for Hull Maintenance

Both CVI and GVI are valuable processes to ensure safety, efficiency and structural integrity. As suggested by the names, GVI are more broad, while CVI are more in depth and specific. For instance, when there is a notable finding on a GVI, a further CVI can be carried out after to gain additional insight into the defect or irregularity.

Inherently, visual testing is part of all test methods. CVI however relies more on additional tools such as probes, lighting and caliper tools to add insight to the visual findings. Regardless of type of inspection, visual testing methods are one of the most important tools an inspector can use.

Overall Goal To gain a general view of the component or hull and make note of any defects or irregularities. To conduct a detailed inspection of the component or hull and identify defects or irregularities.
Lighting Basic lighting is often used - may use some additional lighting. Typically uses additional lighting to create optimal viewing conditions.
Tools Basic access tools are utilized to gain general insight into the hull. Advanced tools such as calipers, probes and gauges are used to get a thorough view of the hull.
Distance Inspections are conducted within an arm’s length. Inspections are conducted in a very close and detailed view.
Access Basic access to the hull or component is required. Mirror may be used to gain a visual of hidden sections. Complete access is required to conduct a CVI.

Tools for Conducting Hull Maintenance Inspections

There are several useful tools that operators can use to conduct hull maintenance inspections. While underwater inspections can certainly be completed by human divers, submersible Submersible ROVS or remotely operated vehicles provide a safe and convenient alternative.

ROVs can safely enter a variety of hazardous and turbid environments without risk to human life. Furthermore, a submersible vehicle can be deployed at a moment’s notice with one to two employees - allowing teams to save time and money during operations.

Additional tools can be integrated into ROVs to enhance both GVI and CVI procedures for optimal inspections.

1. Sonar

Deep Trekker vehicles can also be outfitted with specialized tools to further enhance underwater inspection. Sonar, for instance, has proven to be incredibly useful in turbid conditions. With sonar, operators can still have a complete view of submersed structures and hulls, regardless of underwater conditions. ROV pilots with sonar have a strong advantage in that they can complete accurate and thorough inspection no matter the surrounding underwater environment.

2. Caliper Attachments

Measuring tools also allow inspectors to gather important information about their structure or hull. The Caliper Attachment adds value to inspections by enabling operators to measure surfaces, objects, defects or marks within 2.5mm of accuracy by comparing the spot to the caliper measurement tool.

3. Thickness Gauges

The Cygnus Thickness Guage can be integrated into the ROV to evaluate the thickness of steel on hulls or structures. The ultrasonic thickness probe presses on the surface, taking an accurate measurement even through marine growth or coatings. Evaluating the thickness of steel structures can help to ensure structural integrity.

4. Additional lights

Users can add auxiliary lights to their vehicles to augment their underwater inspections in a wide variety of underwater environments. Ship inspections can often take place in dark or deep water making supplementary lighting helpful.

5. Cameras

Auxiliary cameras can provide ROV operators with additional views and angles with minimal maneuvering. Additional cameras and even zoom cameras enhance visual inspections greatly while underwater.

Deep Trekker ROVs

Deep Trekker offers three different ROVs, the DTG3, REVOLUTION and PIVOT. The battery operated submersible vehicles are camera equipped to provide users with reliable eyes underwater. Deep Trekker also offers the DT640 MAG Utility Crawler. The magnetic wheeled robot is perfect for ferrous metal surfaces, driving directly on the ship hull or underwater infrastructure to conduct convenient inspections. Deep Trekker provides a wide range of accessories and toolsto maximize operations.


Aiming to provide solutions for every mission, Deep Trekker makes tough tasks easy. Constantly seeking to develop the most durable, innovative, portable and affordable underwater ROVs and submersible robots to solve a suite of underwater problems, Deep Trekker is on the cutting edge of submersible technology. When you buy an ROV from Deep Trekker, you buy the team, the ideas, the vision and the support.

As always, our team of industry professionals is here to answer any questions you may have about how submersible robots can aid in your next hull maintenance inspection. When you’re ready to get a vehicle of your own, reach out to get your own customized quote .

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