Ship & offshore inspection: seeing inside dark steel boxes without staging

A hull is a collection of huge boxes made of steel plates and stiffeners — longitudinals, transverses, frames and brackets — welded together. The main drivers of deterioration are corrosion from seawater and cargo, fatigue cracking from repeated wave loads, and deformation from cargo operations and berthing. Ballast tanks in particular are damp, dark spaces repeatedly filled with seawater; once the coating fails, corrosion accelerates.

Seagoing ships keep their class by passing surveys by a classification society (for example ClassNK, DNV, ABS or Lloyd’s Register). The basic cycle is annual surveys, an intermediate survey, and a renewal (special) survey every five years. In Japan, ships classed by a classification society registered with the government (e.g. ClassNK) are deemed to have passed the corresponding government inspections under the Ship Safety Act.

Especially demanding is the Enhanced Survey Programme (ESP) for bulk carriers and oil tankers. Under IMO’s 2011 ESP Code and IACS unified requirements, the extent of close-up survey and thickness measurement grows with ship age. A close-up survey means the surveyor examines structural details normally within reach of hand — including the upper parts of tall tanks.

Then there is the danger of enclosed spaces. Tanks can lack oxygen or hold residual cargo gases. IMO’s recommendations for entering enclosed spaces require ventilation, gas testing and attendants, yet accidents continue. Reducing the number of entries itself is the drone’s greatest value.

Inside ships, most drones are collision-tolerant caged aircraft. In narrow tanks crowded with stiffeners they keep flying after bumping into structure and capture images under their own lighting. Outside, standard multirotors cover the hull, hatch covers, and offshore flares and jackets.

Classification societies and remote inspection techniques (RIT)

IACS calls drones, ROVs and similar means of replacing close-up access remote inspection techniques (RIT) and publishes guidelines for their use (IACS Recommendation No. 42). The core idea: RIT does not replace the surveyor’s judgement — it is a means for the surveyor to see the structure. It counts as close-up survey only when the surveyor is satisfied the imagery shows condition to the same degree.

• Approved service suppliers: firms providing RIT are approved by the classification society as service suppliers. The procedural requirements for service suppliers are set out in IACS Unified Requirement UR Z17, which also covers thickness-measurement firms.
• Surveyor attendance: as a rule the surveyor is on site and examines the structure through real-time video.
• Image quality and location: adequate lighting, resolution and stable imagery are required, and it must be clear which member is being viewed.
• When in doubt, a person goes close: if the imagery reveals defects or is inconclusive, conventional close-up access follows.
• Each society’s guidance: DNV, ABS and other societies publish their own guidance and approval procedures for RIT and drones. The details differ, so confirm with the attending class society first.

A ship’s tank is among the most hostile places a drone can fly. No GNSS at all, a compass made useless by surrounding steel, total darkness, and narrow spaces full of stiffeners. Radio bounces off steel walls and may not reach beyond the next manhole.

• Confined spaces: choose caged aircraft that hold attitude after a collision. The manhole size decides which aircraft can enter. Confined spaces
• Darkness: onboard lighting is the only light. Rust dust stirred up by propellers can wash out images. Darkness
• GNSS-denied: use aircraft that localise visually or with LiDAR and can record which member in which compartment was imaged. GNSS-denied
• Magnetic disturbance: inside a steel box the compass cannot be trusted; fly in modes that do not depend on it. Magnetic disturbance
• Water, spray & humidity: freshly deballasted tanks are humid with water left at the bottom; outside there are waves and spray. Water, spray & humidity
• Wind & turbulence: shell plating at sea and flare stacks are windy, and air swirls above the deck. Wind & turbulence

Cargo tanks add one more barrier: flammable gas and oxygen deficiency. Typical inspection drones are not certified for explosive atmospheres, so tanks must be gas-freed and confirmed safe by measurement before flying. Even without a person entering, this step cannot be skipped. See also explosive atmospheres and oxygen deficiency & toxic gas.

Ship survey findings are recorded as coating condition, presence of substantial corrosion (wastage beyond 75% of the allowable margin), and any cracks or deformation; repairs are decided from these. Drone imagery helps in the steps before that: finding, locating and comparing with last time.

The framework comes from SOLAS and the ESP Code (IMO) and, under them, classification society rules and IACS unified requirements. Whether a drone can serve for close-up survey is governed by each society’s RIT guidance and its approved service supplier procedures, and ultimately the attending surveyor decides. For ESP ships, some societies also require the flag administration to accept RIT. Owners and managers agree the survey plan with class in advance, specifying where and how drones will be used.

For flying, the inside of a tank or hold is an indoor space and falls outside Japan’s aviation flight rules. But flights over the deck or around the hull are subject to aviation law, and in port you also need permission from the port authority, facility security officer and shipyard. Offshore, the operator’s safety rules and the coastal state’s regulations apply. Compare countries in world regulations.

• Skipping gas-freeing: even without people inside, a non-certified drone cannot fly in a tank with flammable gas. Ventilation, testing and permits remain.
• Under mud and rust scale: sediment on the bottom or thick scale hides the steel. Clean first.
• All thickness measurement: contact-sensing drones are improving, but taking every required reading by drone is not yet common practice.
• Confirming cracks: a suspicious line on video must be confirmed by close access or NDT to establish whether it is a crack and how far it extends.
• Rating and class: accepting the survey as close-up, requiring repairs and maintaining class are decisions for the surveyor and the society.

On the voyage before dry-docking, the crew opens a tank manhole and completes ventilation and gas testing. A drone carried on board flies the tank on its own. Its video and point cloud are overlaid on the 3D model from the last renewal survey, and only the changed areas reach the shore-based surveyor and the ship manager.

By the time the ship enters dock, it is already settled where staging goes and which plates will be renewed. Surveyors enter only where the drone has found something, and only when they must. Collapses in tanks become a thing of the past — that, at least, is how the editors see it.