Dr Sarath Obeysekera

Case study for Cololmbo dockyard/  Mazagon combination 

For a product/chemical tanker in the ~30,000 DWT range:

• Fillet welds: typically 60–75% of total weld length
• Butt welds (groove welds): about 25–40%

Why fillet welds are so high:

• Stiffeners to plating (longitudinals, frames, girders)
• Brackets, supports, outfitting attachments
• Internal structures in cargo tanks and ballast spaces

Even though butt welds are fewer, they consume a large share of time and QA effort (full penetration, radiography, UT, class approval).

Where fillet welds are concentrated

During block construction between keel laying and launching, fillet welding is heavily used in:

• Panel lines (stiffener-to-plate welding)
• Sub-assemblies and block fabrication
• Double hull structures (critical in tankers)
• Engine room foundations and supports

Shipyards like Colombo Dockyard PLC and Mazagon Dock Shipbuilders Limited already rely heavily on semi-automatic fillet welding (FCAW/MAG) in these stages.

Should you mechanize or go robotic?

We should mechanize aggressively, but robotics must be selective.

1. Where mechanization makes immediate sense

High return, low complexity:

• Long straight fillet welds on panel lines
• Flat/horizontal stiffener welding
• Repetitive sub-assemblies

Use:

• Gantry welding systems
• Carriage-mounted fillet welders
• Multi-head panel line welders

 This alone can boost productivity 2–3×.

2. Where robotics becomes viable

Only where geometry is consistent and repeatable:

• Panel line stiffener welding (robotic gantry systems)
• Standardized blocks (series production)
• Double-bottom structures with repetitive layouts

Robotics works best in flow-line production, not job-shop chaos.

3. Where robotics struggles (don’t force it)

• Inside tanks (confined, variable geometry)
• Outfitting stage
• Erection stage after block assembly
• Repair and rework areas

Here, skilled welders remain irreplaceable.

Strategic implication (important for your earlier policy thinking)

Given:

• Labour shortages
• Possible migration to the Persian Gulf
• Rising wages

The smart approach is:

• Mechanize all repetitive fillet welding
• Retain skilled welders for:
  • Butt welds
  • Complex joints
  • QA-critical work

This reduces dependence on large volumes of labour while protecting quality.

Practical benchmark for planning

For a 30,000 DWT tanker:

• Total weld length: ~1,500–2,500 km (rough order)
• Fillet portion: ~1,000–1,800 km

If you mechanize even 40–50% of that fillet welding, you:

• Cut labour demand significantly
• Improve consistency
• Reduce rework

Bottom line

• Expect ~70% fillet welding by length in such a vessel
• Full robotics is unrealistic
• Hybrid model (mechanization + skilled labour) is the winning formula

Regards

Dr Sarath Obeysekera