Background
The bedplate of a marine slow-speed two-stroke engine is one of the largest single structural components in any commercial application. For a typical 8-cylinder engine with 950 mm bore, the bedplate is approximately 17 m long, 4 m wide, and 1.5 m tall. It weighs 150 to 200 tonnes when fitted with main bearings. The bedplate is shipped as a single piece (typically) from the engine manufacturer to the shipyard, transported in special carriers, and lifted into the engine room by gantry crane.
The bedplate’s structural function is multiple:
- Main bearing support: transfers radial loads from the crankshaft to the hull
- Tie rod anchorage: provides the lower attachment for tie rods that hold down the cylinder cover and absorb gas pressure forces
- A-frame foundation: supports the A-frame and column structure above
- Oil sump: contains the engine’s main lubricating oil supply
- Hull interface: chocked or bolted to the ship’s double-bottom structure
- Reaction support: absorbs engine reaction torques and propeller thrust through the connection chain
The structural design must accommodate gas-pressure forces (during combustion), inertial forces (from reciprocating piston masses), torsional loads (from crankshaft torque variation), and thermal loads (from engine warm-up and shut-down cycles). All of these load streams must be sustained for the engine’s design life of 100,000+ hours, the equivalent of 25 years of continuous service.
This article describes bedplate construction, materials, and design considerations.
Construction methods
Welded fabrication
Modern marine bedplates are welded fabrications of structural steel plate. Construction starts with:
- Side girders: longitudinal plates running the bedplate’s full length, forming the outer walls
- Transverse partitions: vertical plates at each main bearing location, dividing the bedplate into bays
- Top plate: the flat upper surface where the A-frame sits, with cutouts for main bearing pockets
- Bottom plate: the lower closure, bolted or welded to the hull double-bottom
- Internal stiffeners: webs and gussets reinforcing the main load paths
Plates are typically 30 to 80 mm thick depending on local stress. Welded joints are full-penetration welds inspected by ultrasonic and radiographic methods.
Cast iron bedplates
Older or smaller engines may use cast iron bedplates: a single large casting weighing similar to a fabricated bedplate but with simpler geometry. Cast iron’s good vibration damping is an advantage; the difficulty of casting very large pieces and the weight per unit strength are disadvantages. Cast iron bedplates are not used for the largest modern engines.
Composite construction
Some specialty engines use composite construction: fabricated steel main structure with cast iron sumpwell or end-pieces for damping, oil-tightness, and complex geometry. Composite construction adds cost but can resolve specific design constraints.
Main bearing pockets
Each main bearing site on the bedplate has a precisely machined pocket:
Pocket geometry
The pocket is a semi-cylindrical housing into which the lower bearing shell sits. Pocket diameter is typically 1.0 to 1.4 m for large engines. The pocket has machined faces top and bottom for bearing cap interface, and side faces for retaining bolts.
Bearing cap
A separate bearing cap is bolted on top of each pocket. The cap completes the cylindrical bearing housing and is removable to allow main bearing inspection and replacement. Cap bolts are heavy stud bolts, typically 60 to 100 mm diameter, pre-tensioned by hydraulic tensioning equipment.
Bearing shell
The bearing shells (white-metal lined steel back) sit between the bedplate pocket and the cap. Shells are split into upper and lower halves and are designed for replacement during overhauls.
Pocket alignment
The series of bearing pockets along the bedplate must be precisely aligned with each other to support the crankshaft without inducing bending stress. Alignment is achieved during fabrication by:
- Final machining of all pockets in a single setup on a large boring mill
- Checking pocket centreline straightness with optical or laser systems
- Verifying with a mandrel passed through all pockets
Pocket alignment tolerance is typically 0.05 to 0.15 mm across the bedplate’s full length.
Tie rod anchorage
Tie rods running from the cylinder cover down through the engine structure transfer cylinder gas-pressure forces directly to the bedplate. Each cylinder typically has 4 to 8 tie rods.
Anchorage geometry
The bedplate has heavy anchorage pads where tie rods attach. The pads are typically reinforced with additional plate stiffeners that distribute the tie rod loads into the surrounding structure.
Tie rod tension
Tie rods are pre-tensioned to a value greater than the maximum cylinder gas force. This pre-tensioning ensures that the tie rod path remains in compression at all times during operation, even during peak combustion. Pre-tension is typically 1.5 to 2× the maximum gas force.
Stress concentration
The tie rod attachment points are stress concentration sites on the bedplate. Fabrication detail (weld profile, gusset placement, plate transition) is critical. Fatigue cracks initiating at tie rod attachments are a recognized failure mode.
Oil sump integration
The bedplate doubles as the engine’s main lubricating oil sump:
Sump volume
A typical large slow-speed engine has 30 to 80 cubic metres of oil capacity in the sump, providing sufficient reserve for circulation through bearings, piston cooling, and other lubrication points.
Oil flow paths
Oil drains from each main bearing, crosshead, piston rod cooling, and other points into the sump. Each return path is through machined drains or open returns within the bedplate cavities.
Oil suction
Oil is drawn from the sump by the engine’s main lubricating oil pump, typically through a strainer at the sump bottom. Suction pipes are routed along the bedplate to the pump location.
Sump baffles
Internal baffles within the bedplate prevent oil sloshing during ship motion. Baffles also create separate drain compartments for different parts of the engine, useful for oil sample diagnostics.
Oil cooling
Some bedplate designs include integrated oil cooling passages. Cooling water flows through tubes welded into the bedplate cavities, transferring heat from the oil to the cooling water. More commonly, oil is cooled in a separate heat exchanger external to the bedplate.
Hull interface
The bedplate is mounted to the ship’s double-bottom structure through chocking and holding-down bolts:
Chocking
Chocks fill the gap between bedplate bottom and hull tank top. Modern practice uses epoxy chocking, with a few cast iron supports for critical points. The chocking distributes engine weight evenly into the hull and accommodates differential thermal expansion.
Holding-down bolts
Holding-down bolts (or studs) pass through the bedplate’s holding-down flanges and into the ship’s foundation girders. Each bolt is typically 60 to 100 mm diameter, pre-tensioned to clamp the bedplate to the hull. Total holding-down bolt count is typically 30 to 60 for a large engine.
Holding-down flange
The bedplate’s outer edges include heavy flanges for holding-down bolt attachment. The flanges spread the bolt loads into the bedplate structure and provide convenient surfaces for chocking.
End mounts
The forward and aft ends of the bedplate may have additional support arrangements: thrust block flanges, A-frame extensions, or auxiliary mounts. These end attachments transfer specific loads (propeller thrust, generator weight) into the hull.
Structural analysis
Modern bedplate design uses finite element analysis (FEA) to verify:
Static stress
Stress under steady-state engine operation, including:
- Crankshaft weight and bearing loads
- Tie rod pre-tension forces
- A-frame and column dead weight
- Self-weight of the bedplate
Maximum static stress is typically below 80 MPa, well within the steel’s yield strength.
Cyclic stress
Stress amplitude under cyclic engine operation, with cylinder gas pressure varying from scavenging (low) to peak combustion (high) once per revolution. Cyclic stress at the highest-loaded points is the principal fatigue driver.
Allowable cyclic stress for the engine’s design life of 10^9 cycles is typically 30 to 50 MPa.
Hot spots
FEA identifies stress concentration regions: tie rod attachments, bearing cap stud locations, the transition between holding-down flange and main bedplate body, weld terminations. Local detail design (gussets, smooth transitions) reduces hot-spot stress to acceptable levels.
Modal analysis
The bedplate has its own structural natural frequencies (separate from the crankshaft torsional modes). Modal analysis ensures that bedplate natural frequencies do not coincide with cylinder firing frequency or its low-order harmonics, avoiding bedplate resonance.
Service inspection
Visual inspection
At each major overhaul, the bedplate is inspected for:
- Visible cracks, particularly at known stress concentration points
- Holding-down bolt looseness or shifting
- Chock condition: cracks, oil leaks, or shifting
- Oil sump condition: deposits, sludge, baffles
- Internal weld condition where accessible
- Surface corrosion on inside surfaces
Magnetic particle inspection
Magnetic particle inspection (MPI) is applied to:
- Tie rod anchorage areas
- Main bearing cap stud locations
- Holding-down flange areas
- Suspect areas based on sister-ship experience
Repair options
Cracks detected during inspection may be repairable by:
- Crack-stopper holes drilled at crack tips to prevent propagation
- Welded repair with ground crack and re-weld (requires careful technique)
- Doubler plates added over the cracked area (last-resort approach)
Severe damage may require bedplate replacement, a major slipway task that effectively rebuilds the engine.
Related Calculators
- Bedplate Stress Calculator
- Main Bearing Load Distribution Calculator
- Tie Rod Pre-tension Calculator
- Bedplate Weight Calculator
- Holding-Down Bolt Sizing Calculator
- Oil Sump Volume Calculator
See also
- Crosshead Diesel Engine Architecture Overview
- Engine Alignment and Bedplate Flexure
- Engine Torsional Vibration Analysis
- Two-Stroke Marine Diesel Engine Fundamentals
References
- MAN Energy Solutions. (2023). Bedplate Construction and Service Manual. MAN Energy Solutions.
- WinGD. (2023). X-Series Bedplate Engineering Specifications. Winterthur Gas & Diesel.
- DNV. (2023). Rules for Classification of Ships, Pt.4 Ch.4: Rotating Machinery — Bedplate Design.
- Hughes, O. F. (2010). Ship Structural Analysis and Design. SNAME.
- Lloyd’s Register. (2022). Guidance Notes for Marine Engine Bedplate Inspection.