Preparing the published article.
Underwater Inspection: What a Survey Can and Cannot Establish
How to scope a hull, intake, or aquaculture inspection so the report states coverage, evidence type, and what remains unknown.
Write the Inspection Scope and the Water Baseline First
An underwater inspection is commissioned to answer a question, so the scope has to name the question before anyone gets in the water. Start by agreeing exactly what is examined: a hull, a sea chest or intake, a mooring chain, a net cage, or a pontoon. Then record what the inspection must support, because a routine condition check, a pre-purchase assessment, a damage investigation, and a documented record for a later decision need different levels of evidence.
- Asset identity and the exact areas in scope, with a plan or drawing reference for every named location.
- The question the survey must answer, written as one sentence, plus the asset owner's decision that depends on it.
- Known history: previous repairs, last survey date, known damage, and any operational restriction already in force.
- Site conditions at the time of work: turbidity, water temperature, depth, current, and the reference used to judge visibility.
- Acceptance criteria: which defects are reportable, which are not, and who decides on follow-up work.
Record the environmental baseline honestly, because it bounds what the survey can establish. Poor visibility is an operating condition to write down at the pre-job meeting, not an explanation offered afterwards. Keep two distinctions in the scope as well: a dock or shore-based inspection differs from an in-water inspection in access quality, and a survey that documents condition is not the same as one that certifies fitness for continued service.
What Hull and Intake Inspection Can and Cannot Establish
For a hull survey, record how each finding is positioned on the asset. State whether the work was done by a diver or a remotely operated vehicle, and list the sensors carried: a camera, a thickness gauge, a cathodic protection probe, or a cleaning tool. Tie every reported location to a named structural feature and a frame or station reference, not to somewhere along the port side.
- Coverage achieved as the proportion of the planned area actually examined, with unexamined portions listed.
- Positioning method: whether the position came from a measurement, a reference mark, or an estimate.
- Items checked: coating breakdown, pitting, deformation, impact marks, weld condition, fouling coverage, anode condition, and the area inside sea chests and gratings.
- Media: still images, video, and any measurement record, delivered with the naming convention agreed before the dive.
The limit matters as much as the result: video shows that something is present, but does not by itself establish remaining thickness, structural reserve, or an interval to the next inspection. Ask which findings were measured and which were observed. A thickness reading is a point measurement, so its value depends on where and how many points were taken.
Intakes, Nets and Moorings Need Their Own Tolerances
For sea chests and cooling intakes, the question is usually whether flow can be restored and whether the structure is sound, so the plan must state how much of the screen is examined and how a partial view is reported. Ask for the tooling needed for cleaning, because a survey before cleaning and one after cleaning answer different questions. Record whether entry was into a confined space and what the permit required.
| Report item | Evidence to request |
|---|---|
| Defect list | Location reference, description, images, and whether measured or observed |
| Coverage map | Areas examined, areas partially examined, and areas not examined |
| Measurements | Instrument type, calibration record, number of readings, and where each was taken |
| Media set | Original files with timestamps, not only edited highlights |
| Limitations | Conditions that restricted the survey and what they affected |
For nets, cages, and moorings, the tolerances differ: mesh condition, collar and sinker-line attachments, chain wear, and biological growth each need their own method, and the target may be a crevice in a confined opening or a large structure in open water. Ask the provider to state the assessment standard applied; if there is none, treat the description as condition reporting rather than a pass or fail judgement. Set criteria against the structure's function: whether openings still meet a stated dimension, whether a load-bearing attachment shows deformation, or whether cleaning can restore service.
Close the report with the same discipline it started with. The final paragraph should answer the original question in one sentence, list what was verified, list what remains unknown and why, and name the areas that were not inspected. A survey that states its unknown regions is more useful than one that fills them with an estimate.
Photo credits: the images in this article are licensed reference photographs of the relevant robot class, credited with author and licence in each caption; they are not photographs of the specific equipment discussed. Licence and author details are recorded with each image.