Background
The continuous survey regime was introduced because the alternative, the special survey, requires the ship to be out of service for a substantial period (often several weeks for the hull special survey, including dry-docking) and to undergo extensive opening up of machinery items in a short window. The disruption to commercial trading and the cost of crew overtime and contractors during a single survey window were significant. By spreading the work, the continuous regime allows much of it to be done while the ship is on hire, fitted into idle periods such as ballast voyages, port stays, or scheduled lay-up.
This article describes the structure of the CSH and CMS regimes, the role of the Chief Engineer’s CMS as a practical extension that further reduces surveyor attendance for routine machinery items, alternative survey arrangements that integrate condition-based monitoring and predictive maintenance with the class regime, and the underlying IACS Unified Requirement Z framework that harmonises the practice across class societies.
Continuous Survey vs Special Survey
A special survey is a thorough, intensive survey carried out every five years which renews the certificate of class. The special survey examines the hull structure, machinery systems, and equipment to a far greater depth than the annual or intermediate surveys. Tank internals are opened up for visual examination of structural members, pipe systems are pressure tested, machinery is opened up for examination of running surfaces and bearings, and electrical systems are inspected including insulation resistance testing.
In the absence of an alternative arrangement, all of these items would fall due simultaneously at the special survey. The continuous survey regime breaks the work into smaller, manageable packages, each individually credited as the surveyor attends and verifies the item. At the end of the five-year cycle, all items are confirmed and the special survey is, in effect, completed by aggregation.
The special survey itself remains a regulatory milestone; the class certificate is endorsed at its successful completion regardless of which survey methodology is used. The continuous regime is a method of fulfilling the special survey, not an exemption from it.
CSH: Continuous Survey of Hull
The CSH covers the hull structural items that would otherwise be examined at the hull special survey. The list typically includes:
Internal structural examination of cargo holds, ballast tanks, fuel and lubricating oil tanks, void spaces, cofferdams, chain lockers, and forepeak and afterpeak structures. The examination addresses corrosion of plating and stiffening members, condition of coatings, integrity of welded connections, condition of access arrangements, and freedom from cracks at structural discontinuities.
Hatch covers and coamings, including ultrasonic tightness testing per the IACS Recommendation No. 14, weathertightness gaskets, cleating and securing arrangements, and chain or wire operating gear.
External hull plating, particularly in way of the boot-topping and the underwater hull, requiring drydocking attendance unless an in-water survey is granted.
Sea chests, sea valves, overboard discharges, and side shell penetrations.
Cargo system items on tankers and bulkers including tank vapour locking, structural members in way of cargo loading and discharge, and the condition of cargo line piping where structural support is involved.
Steering gear compartment including the rudder horn, rudder bearings, and steering gear seatings, with the rudder pintles and bearings opened up for measurement.
The CSH is programmed by the operator with the attending classification society surveyor, taking into account the trading pattern of the vessel. Tank surveys, requiring tank cleaning and gas-freeing, are clustered to minimise duplication of preparation work. Drydock items are reserved for the next scheduled drydocking under the intermediate survey cycle.
CMS: Continuous Survey of Machinery
The CMS covers the machinery items that would otherwise be examined at the machinery special survey. The list is extensive and varies by ship type, but typical items include:
The marine diesel engine main engine, with each cylinder unit opened for examination of piston, cylinder liner, piston rings, connecting rod, crosshead and bottom-end bearings, and main bearings on a rotational basis. The crankshaft deflection measurement and bearing clearance verification.
The auxiliary engines and generators with similar opening-up of cylinders and bearings.
The reduction gears, propulsion shafting, stern tube bearings, propeller, and tail shaft.
Steering gear hydraulic system, including pumps, rams, control system, and emergency steering arrangements.
Anchor handling and mooring equipment including windlasses, mooring winches, and warping ends.
Boilers and pressure vessels, with internal examination of waterside and fireside surfaces. Hydrostatic testing if required.
Pumps including the cargo pumps, bilge pumps, ballast pumps, fire pumps, and general service pumps.
Electrical machinery including main alternators, electric propulsion motors, and emergency switchboard.
The CMS programme is again drawn up with the attending surveyor and integrated with the operator’s planned maintenance schedule.
Chief Engineer’s CMS
The Chief Engineer’s CMS, often abbreviated CE-CMS, is a recognised arrangement under which routine machinery items can be inspected and signed off by the Chief Engineer rather than requiring a class surveyor attendance. The Chief Engineer must hold an appropriate certificate of competency (typically Class I or equivalent), be employed in that capacity by an operator approved for the arrangement, and complete the survey in accordance with the approved survey procedure.
The arrangement covers items that are routine in nature, well within the competence of an experienced Chief Engineer, and do not present novel inspection challenges. Typical CE-CMS items include auxiliary pumps, heat exchangers, smaller compressed air receivers, hydraulic system components, and basic electrical equipment.
The Chief Engineer’s report, photographs, and supporting records are submitted to the class society for review. The class surveyor retains audit rights and may attend any item even when nominally allocated to the Chief Engineer. Items found defective during CE-CMS must be referred to the class surveyor for further action.
The arrangement reduces surveyor attendance time, lowers cost, and incentivises quality maintenance practice on board. It is contingent on the operator’s overall classification record being satisfactory; class societies may withdraw the privilege if the operator’s port state control detentions or major non-conformities accumulate.
Survey Item Planning
Survey item planning is the operator’s responsibility under the continuous regime. The operator must produce, at the start of each five-year cycle, a survey programme showing each CSH and CMS item, the date of its previous attendance, the latest date by which the next attendance must occur, and the planned date in the current cycle. The programme is filed with the class society and updated as items are attended.
In practice, the survey planning is integrated with the planned maintenance system running on board. Maintenance and survey events are coordinated so that machinery is opened only once for both maintenance and class examination. Tank surveys are coordinated with cargo schedules so that gas-freeing time is not wasted.
The operator’s superintendent and the chief engineer maintain the master programme. The Marine Survey Coordinator role, common in larger operators, exists specifically to manage this work across a fleet.
Alternative Survey Arrangements
Alternative survey arrangements (ASA) are bespoke programmes approved by the class society that depart from the standard CSH/CMS framework to take advantage of specific operator capabilities. Common alternatives include:
Planned Maintenance System (PMS) approval, under which routine machinery items are examined according to the operator’s planned maintenance schedule, with surveyor verification at audit intervals rather than at every event. The PMS itself is approved by class.
Risk-Based Inspection (RBI) programmes, particularly for piping and pressure systems, where the inspection frequency and method is determined by a risk analysis combining likelihood of failure (based on corrosion data, operating history, and material) with consequence (based on system criticality and potential safety or environmental impact).
Condition Monitoring System (CMS-CM) arrangements, distinct from the survey CMS, under which selected machinery is monitored continuously by sensors (vibration, temperature, lubricating oil analysis, performance) and survey credits are granted based on monitoring data trends rather than on physical opening-up.
These arrangements are specific to the operator and the ship, are documented in approved procedures, and are subject to surveyor audit at defined intervals.
Condition-Based Monitoring
Condition-based monitoring (CBM) refers to the use of sensors and analytics to track the actual condition of machinery rather than assuming a fixed deterioration rate from the time-based maintenance interval. The principal CBM techniques in marine engineering are:
Vibration analysis on rotating machinery, using accelerometers mounted on bearing housings and casings. The vibration spectrum reveals bearing defects, misalignment, imbalance, and looseness. The trend over time indicates deterioration.
Lubricating oil analysis following ASTM and ISO standards, sampling oil from the main engine sump and other oil systems and analysing for wear metals, additive depletion, contamination, and viscosity. Wear metal trends correlate with bearing and ring wear.
Performance monitoring comparing measured engine output, fuel consumption, exhaust temperatures, and combustion pressures against baseline values. See marine engine performance monitoring and combustion analysis for the specific techniques.
Thermography using infrared cameras to detect hot spots in electrical equipment and bearings.
Acoustic emission and ultrasonic monitoring for steam trap performance, valve internal leakage, and tightness of pressure boundaries.
CBM data is used both for maintenance decision-making and, where the alternative survey arrangement permits, as evidence supporting class survey credits.
Predictive Maintenance Integration with Class
Predictive maintenance, the next step beyond CBM, applies analytics (regression, machine learning) to the monitoring data to forecast future condition and the optimal intervention point. The classification societies have, over the past decade, developed approval frameworks that allow predictive maintenance outputs to inform survey planning.
The DNV Smart Ship and Asset Lifecycle approach, the Lloyd’s Register Cyber-Enabled Ships notation, and the ABS Smart Function programme are examples. Each provides notations to vessels using qualifying data analytics and condition monitoring frameworks, and each in turn permits adjustment of class survey scope based on the data.
The benefits to the operator are reduced unplanned downtime, optimised maintenance spend, and extended intervals between physical opening-up of machinery. The challenges are data quality, sensor reliability, and the cybersecurity exposure of the connected systems. The IMO has issued guidance on cyber risk management which class societies incorporate into the related notations.
IACS UR Z Surveys
The International Association of Classification Societies (IACS) maintains the Unified Requirement Z series, which harmonises the survey requirements across the recognised class societies. The principal documents are:
UR Z3 governs periodical survey of bulk carriers, incorporating the Enhanced Survey Programme (ESP) which mandates close-up survey of structural members, thickness measurements, and ballast tank examination at defined intervals based on age.
UR Z10 series governs the survey of tankers, again under the ESP framework with thickness measurement and tank examination provisions.
UR Z7 covers the hull surveys for general cargo ships and other vessel types not covered by ESP.
UR Z17 governs the procedural requirements for class surveyors including their qualifications, the manner of survey, and the documentation produced.
UR Z20 covers the rules for periodical surveys of equipment for the carriage of liquefied gases in bulk, applied to LNG and LPG carriers.
The UR Z framework ensures that a vessel surveyed by one IACS class society at a port of call is recognised as having met equivalent standards under any other IACS class. The IMO recognises the IACS unified approach in its various circulars on the recognition of class societies under the SOLAS, MARPOL, and Load Line conventions.
Related Wiki Articles
- Classification Society
- Marine Spare Parts and Maintenance Management
- Marine Engine Performance Monitoring
- Marine Pressure Vessel Inspection
- Marine Engine Crankshaft and Main Bearings
- Marine Diesel Engine
- Port State Control
- Marine Cathodic Protection and Hull Coatings
- Marine Engine Cylinder Liners and Pistons
- Marine Reduction Gears
References
- IACS Unified Requirement Z3, Periodical Hull Surveys of Bulk Carriers
- IACS Unified Requirement Z7, Hull Surveys of General Dry Cargo Ships
- IACS Unified Requirement Z10.1, Hull Surveys of Oil Tankers
- IACS Unified Requirement Z10.2, Hull Surveys of Chemical Tankers
- IACS Unified Requirement Z10.5, Hull Surveys of Double Hull Oil Tankers
- IACS Unified Requirement Z17, Procedural Requirements for Service Suppliers
- IACS Unified Requirement Z20, Periodical Surveys of Liquefied Gas Carriers
- IACS Recommendation No. 14, Hatch Cover Securing and Tightness
- IMO Resolution A.1104(29), Survey Guidelines under the Harmonized System of Survey and Certification (HSSC)
- IMO Resolution A.1120(30), Survey Guidelines under the HSSC
- IMO Resolution A.744(18), Enhanced Survey Programme for Bulk Carriers and Oil Tankers
- ISO 13374, Condition Monitoring and Diagnostics of Machines
- ISO 17359, Condition Monitoring and Diagnostics of Machines - General Guidelines
- ASTM D6224, Standard Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment