Depending on which maritime body you ask, there are somewhere between 50,000 and 100,000 large ships sailing the world’s oceans, with cargo vessels, passenger ships, and warships making up the bulk of this number. Commercial vessels are required to have their hulls inspected on an annual basis, and military vessels generally follow a similar inspection regimen. There are various global and national bodies which provide certification standards, such as ABS (American Bureau of Shipping) and Lloyds of London. The goal of inspecting large vessels is to protect the owners of the ships, the crews, the passengers, the companies using the ships, and the insurance industry.
There are a variety of different certification standards, but the standard ABS inspection regimen is fairly typical for the industry. The ABS regimen is known as HIMP, which stands for Hull Inspection and Maintenance Program. HIMP has a three-tiered system of inspections, with annual inspections, three-year inspections, and five-year inspections. The three- and five-year inspection tiers encompass all the inspection areas of the lower tiers, while adding further areas of the ship to the inspection list. HIMP inspections cover the entire vessel, but the underwater portion of the survey is the trickiest for ship owners. Underwater surveys can be conducted by human divers, but increasingly this job is being handled by remote-operated vehicles (ROVs).
On a commercial vessel, the diver or ROV must provide visual data on the stern and rudder bearings, the sea suctions and sea valves, the propellers, and – most time-consuming - the hull plating. The inspection must look at any markings on the hull, all inlets and discharges, the rudder, the propeller, and all other objects that protrude from the hull. Corroded or damaged areas must be examined closely, and although thickness testing is not automatically part of the HIMP, any area found to be damaged or corroded is likely to be examined internally and have the thickness tested at the affected spot(s).
Military vessels follow a similar inspection regimen, and many military vessels will also have stencil-marked areas of the hull which need to be looked at. Inspections will check the hull and other secondary areas of the vessel for biofouling, to assess the need for having the ship’s bottom scraped. Inspections generally involve several waterline-to-waterline underwater traversals of the vessel, following the seams of welded sections. Propellers need to be inspected on both their front and rear facings.
In the early years of ROVs, their use in underwater hull inspections was relatively rare because the low video quality of early vehicles made their results of marginal utility to inspectors. Today however, with true HD displays and cameras on even consumer-level ROVs, these useful vehicles are an integral tool for large hull inspections. Since inspection divers need to be ABS-certified, many surveying companies are finding that it is highly useful to conduct a pre-inspection using an ROV only. This pre-inspection can find any problems that already exist so that they can be mitigated before the actual inspection, preventing a time-consuming, and expensive, temporary suspension of the vehicle’s certification when the inspector finds significant problems.