Maritime Inspection: Ship Thickness Measurement, Surveys and Marine NDT

Maritime inspection services: what we measure and inspect

Most maritime NDT comes back to one question: how much steel is left, and is it enough. Ultrasonic thickness measurement answers it for plating, stiffeners and piping, and it is the measurement that class rules, charterers and buyers all rely on to judge the hull structural condition of a ship. Around it sit the inspections that find what thickness readings cannot: cracks at fatigue-prone details, and local wall loss in systems such as scrubber piping that sit outside the hull survey.

Each service has its own page with the technical detail, grouped below into surveys and class work, compliance surveys, marine NDT and advanced NDT; this page shows how they fit together.

• Ultrasonic thickness measurement (UTM) of shell and deck plating, tank and hold structure, bulkheads, frames, longitudinals and brackets
• Gauging for special (renewal) surveys, intermediate surveys and the annual-survey follow-up of substantial corrosion, with close-up survey support
• Condition Assessment Scheme (CAS) readiness gauging, and gauging for the Condition Assessment Programme (CAP) charterers ask for on older ships
• Gauging on ESP ships: oil tankers, chemical tankers and bulk carriers, including double-hull and double-skin designs
• Pre-docking gauging and steel renewal estimates before the yard is contracted
• Condition gauging for pre-purchase inspections and for on-hire and off-hire surveys
• Ship piping and scrubber (EGCS) wash-water and overboard discharge piping inspection
• Ultrasonic tightness testing of hatch covers and other closing appliances
• Inventory of Hazardous Materials (IHM) surveys and maintenance, noise surveys and infrared thermography of switchboards and machinery
• Marine NDT: visual inspection, magnetic particle and dye penetrant crack detection, ultrasonic weld inspection, PAUT, TOFD, ACFM, eddy current and corrosion mapping

Compliance surveys

Surveys a ship needs for rules outside the hull survey: the hazardous materials inventory for EU port calls and recycling, noise levels for crew spaces, and thermal checks that catch electrical and insulation faults early.

Advanced NDT

Encoded and electromagnetic methods for what conventional NDT does poorly on a ship: stored weld images, flaw height in thick plate, cracks found through paint, and wall loss mapped over an area.

How thickness gauging fits renewal, intermediate and annual surveys

The class cycle sets the rhythm: annual surveys, an intermediate survey in place of the second or third annual, and a renewal (special) survey every five years, with two docking surveys in each five-year period. Gauging is light at the first renewal survey and grows with each one. For ESP ships the third renewal survey, the 15-year survey, brings heavy gauging of the cargo and ballast tanks, and the 20-year and 25-year surveys go further, with more transverse sections, more close-up survey and more readings.

Two consequences matter for planning. The rules allow a renewal survey to be commenced at the fourth annual survey and progressed through the following year, so gauging can be spread between port calls and the docking instead of all landing in the yard period. And areas found with substantial corrosion, where wastage has used up more than three-quarters of the allowable margin without exceeding it, are measured again at later annual and intermediate surveys, so one renewal report sets part of the scope for the next five years. Excessive corrosion, beyond the allowable margin, means renewal.

Typical findings are general wastage of plating and stiffeners, pitting on tank bottoms and horizontal surfaces, grooving corrosion alongside welds, and edge corrosion at cut-outs and free edges. The gauging plan targets the details each ship type is known for, and the previous report shows where to look harder.

Which gauging needs a firm approved by the ship's classification society (IACS UR Z17)

Thickness measurement credited to a class survey must be carried out by a firm approved by that ship's classification society under IACS UR Z17. Approval is held separately with each society, so the vessel's class decides which firms may do her survey gauging. On ESP ships the rules also require a survey planning meeting before gauging starts, attended by the surveyor, the master, the owner's representative and the thickness measurement firm, and the surveyor witnesses and controls the measurement.

A great deal of useful gauging is never credited to a survey and needs no class approval. The table sets out the usual position; where a charterer, financier or insurer sets its own requirement, that requirement governs.

One practical point: readings taken by a firm the ship's class has not approved cannot later be turned into survey readings. If one gauging is to serve as both the steel estimate and the survey record, the firm must hold that society's approval when the readings are taken.

Ship piping, scrubber piping and crack detection

Ship piping fails from the inside, so the outside view says little. Sea water cooling and ballast piping loses wall at bends, reducers and downstream of valves; fire mains corrode where water stands; cargo piping on tankers thins along the bottom of horizontal runs. Gauging a planned grid of points on each spool finds the thin sections before a leak does.

Scrubber wash-water and overboard discharge piping is the newer problem. The wash water is warm and acidic, the materials are mixed (coated or rubber-lined carbon steel, stainless and duplex grades, glass-reinforced plastic), and failures cluster at joints, at the distance piece to the shell and in the shell plating around the discharge. A leak there is a flooding risk, not a maintenance nuisance. Scrubber pipe inspection covers the metallic sections, and the dedicated page explains how.

Thickness readings do not find cracks. Magnetic particle crack detection does, at the details where fatigue starts: bracket toes, end connections of longitudinals at transverse webs and bulkheads, hatch corners, hopper knuckles and cut-outs. Penetrant testing covers non-magnetic materials, and ultrasonic testing of welds answers questions about defects below the surface.

Frequently asked questions