DNV-RU-SHIP Pt.2 Ch.2 Machinery Systems: Propulsion, Boilers, Hydraulics, and Piping Rules

DNV-RU-SHIP Part 2 Chapter 2 defines the classification requirements for machinery systems on DNV-classed vessels — propulsion plants, auxiliary machinery, boilers and pressure vessels, hydraulic and pneumatic systems, and all associated piping. For ship designers, machinery engineers, and class surveyors, this chapter is the primary reference that determines what must be designed, tested, and documented for class approval. This article covers the major technical areas, type approval requirements, and the non-conformances most commonly raised at new-building surveys.

1. Scope and Structure of Pt.2 Ch.2

Part 2 of DNV Rules for Ships covers machinery and systems across four chapters:

Chapter 2 is the broadest of the machinery chapters and the starting point for any vessel machinery classification scope. It establishes the system-level requirements — what redundancy is needed, what performance criteria must be met, and what documentation must be submitted to DNV for approval.

2. Propulsion Systems

2.1 Main Engine Requirements

DNV-RU-SHIP Pt.2 Ch.2 §2 covers propulsion plant requirements. The main engine (diesel, gas turbine, or dual-fuel) must meet:

• Type approval: all main engines shall hold DNV type approval (TA) prior to installation; TA certificate scope must match the engine configuration on board (fuel type, cylinder count, rating)
• Power rating: the maximum continuous rating (MCR) must be verified by factory acceptance test (FAT) and confirmed at sea trial
• Starting capability: the engine must be capable of starting from cold within 30 minutes for vessels with DYNPOS or similar notations
• Reversing: for direct-drive propulsion, the engine must reverse within 15 s from full ahead to full astern under DNV-RU-SHIP Pt.2 Ch.2 §2.4

2.2 Propulsion Redundancy — Vessels with DYNPOS Notation

Vessels with DYNPOS-AUTRO or equivalent DP2/DP3 notation require propulsion redundancy beyond basic machinery class. The requirements cascade from DNV-OS-D101 and Pt.2 Ch.2 together:

• DP2: loss of any single component shall not cause loss of position-keeping capability — thruster redundancy at equipment level
• DP3: loss of any single fire or flood zone shall not cause loss of DP capability — full system redundancy at zone level

2.3 Shafting and Propeller

Propeller shaft design is governed by DNV-RU-SHIP Pt.2 Ch.2 §3. Key requirements:

Tail shafts (water-lubricated or oil-lubricated stern tubes) require periodic withdrawal and inspection. The inspection interval depends on the shaft seal type and classification notation — typically 5 years for oil-lubricated, 2.5 years for water-lubricated without enhanced survey credit.

3. Auxiliary Machinery

3.1 Auxiliary Engines and Generators

Electrical power generation requirements under Pt.2 Ch.2 §4 are closely linked to Pt.2 Ch.2 electrical sections and DNV-RU-SHIP Pt.4 Ch.7:

• Capacity: the generating plant must supply the essential services with the largest single generator out of service
• Emergency generator: must start automatically within 45 s of main power loss; must be independent of the main machinery space (separate room, separate fuel tank)
• Load shedding: the power management system (PMS) must shed non-essential loads before generators trip on overload
• Black-start sequence: the starting sequence from dead ship must bring essential services online within 30 minutes

3.2 Steering Gear

Steering gear requirements are among the most safety-critical in Pt.2 Ch.2 §5. SOLAS and DNV classification requirements together mandate:

• Two independent power units, each capable of moving the rudder from 35° one side to 30° the other side in ≤28 s
• Manual takeover from the bridge within 2 min of hydraulic power failure
• Emergency steering from local control in the steering gear room within 2 min
• Oil level alarms and pump failure alarms alarmed and indicated on the navigation bridge

4. Boilers and Pressure Vessels

DNV-RU-SHIP Pt.2 Ch.2 §6 covers fired and unfired pressure equipment. This section has significant interaction with EN 13445 (pressure vessels) and PED (Pressure Equipment Directive) for EU-flagged vessels.

4.1 Design Pressure and Wall Thickness

Shell thickness for cylindrical pressure vessels is calculated using the basic membrane formula:

4.2 Safety Valves

All pressure vessels and boilers must have safety valves sized to relieve the full rated input without the working pressure exceeding 10% above the design pressure (or 0.1 MPa above, whichever is greater). DNV-RU-SHIP Pt.2 Ch.2 §6.4 requires:

• Minimum two safety valves on steam boilers rated >50 kW
• Safety valves to be direct-spring loaded (no weight-loaded or pilot-operated valves unless specifically approved)
• Safety valve lift test at each annual survey (periodic survey interval)

4.3 Boiler Surveys

Fired boilers require a complete internal inspection at class renewal (5 years). DNV survey requirements include:

• Internal examination of water-side surfaces for scale, pitting, and corrosion
• UT thickness measurement of drum shells and headers where corrosion is observed
• Hydraulic pressure test to 1.25× design pressure at each class renewal if the boiler has not been opened and inspected since the last test
• Burner combustion analysis and safety shutdown system test (low water level, high steam pressure, flame failure)

5. Hydraulic and Pneumatic Systems

Hydraulic systems on ships power steering gear, stabilisers, hatch covers, cranes, and deck machinery. DNV-RU-SHIP Pt.2 Ch.2 §7 defines design and testing requirements.

5.1 Design Requirements

5.2 Pneumatic Systems

Compressed air systems (starting air, control air, service air) under Pt.2 Ch.2 §7.5:

• Starting air receivers: design pressure ≥30 bar; annual safety valve test; hydraulic test at 1.5× DP at 5-year survey
• Air compressors: oil-free for control air systems (instrument air quality per ISO 8573-1 Class 2.4.1 or better)
• Moisture separators and dryers mandatory on control air circuits to prevent water accumulation in pneumatic valves

6. Piping Systems — Overview

Piping detailed design falls primarily under DNV-RU-SHIP Pt.2 Ch.4, but Pt.2 Ch.2 §8 defines system-level requirements that govern pipe class selection and routing:

6.1 Pipe Classes

Class I pipes require DNV-approved materials (traceable to EN 10204 3.1 or 3.2 material certificates), pressure testing to 1.5× design pressure, and radiographic or ultrasonic testing of butt welds in steam lines >100 mm OD.

6.2 Bilge and Ballast Systems

Pt.2 Ch.2 §9 defines the bilge pumping capacity requirements. The minimum bilge pump capacity must be capable of pumping through the bilge main:

7. Type Approval and Plan Approval

DNV-RU-SHIP Pt.2 Ch.2 §1.4 defines what must be submitted to DNV for approval before fabrication commences:

8. Cross-Reference Map

9. Common Non-Conformances at New-Building Survey

• Type approval certificate scope mismatch — engine TA certificate covers one fuel type (HFO) but vessel is built for LNG/dual-fuel; the TA must specifically cover the installed configuration
• Safety valves not sized for full input capacity — designers size relief valves for normal operating pressure without verifying they can relieve the maximum boiler heat input rate; results in overpressure during survey lift test
• Starting air receiver capacity insufficient — the receiver capacity must sustain 12 consecutive start attempts for reversing engines; undersizing is common when main engine MCR is increased during FEED without recalculating starting air volume
• Bilge wells not accessible for pump suction — non-return valves or blanked connections in bilge mains discovered at inclining experiment or bilge pump test; often installed during construction to protect from flooding and never reconnected
• Hydraulic hose assemblies without hydrostatic test records — Class I hydraulic hoses must be tested at 1.5× working pressure with test report; hose assemblies procured without TA and test records are rejected at survey
• Oil-fired boiler combustion analysis not performed — annual surveys require combustion efficiency check (flue gas O₂/CO analysis) to confirm correct air/fuel ratio; ships often renew class without this measurement
• Emergency steering time not measured — sea trial must include emergency steering transfer (bridge to local control) and timed from loss of steering to resumption of control at rudder; if not measured at sea trial, the vessel is non-class until a witnessed test is completed
• Fuel oil double-block-and-bleed valves missing at machinery space bulkhead penetrations — quick-closing remote shut-off valves are required by SOLAS and DNV at the tank top/bulkhead crossing for all fuel lines to machinery space; these are frequently omitted on the shop-fabricated piping arrangement