2. Steel Line Pipe

With the term line pipe we refer to the pipe pieces that are welded together onboard a pipelaying vessel to form the pipeline. Line pipe is categorised be the production method as follows

2.1. Seamless (SMLS) Pipes

  • Mandrel Mill: Maximum size approx. 7 “ OD, max WT ~25 mm
  • Plug Mill: Medium sizes (max. 16”), max WT ~45 mm
  • Pilger Mill: Heavy wall, max 40” OD, max WT 100 mm

Advandages of seamless pipes for use in subsea pipelines

  • No weld hence no additional longitudinal weld hardness and toughness issue
  • High flexibility in grades and heat treatment conditions e.g Normalized (N) Quenged and tempered (QT)
  • With proper grade suitable for low temperatures

Disadvantages of seamless pipes for use in subsea pipelines

  • Rolling for mandrel and plug mills is OD controlled
  • Wall thickness tolerances up to +/-15 %
  • Eccentricity (at pipe ends) 
  • ID tolerances (at pipe ends)
  • Lengths (for heavy wall) i.e. weight restriction.
  • Surface condition (slivers, scabs etc)

2.2. High Frequency Welded (HFW) Pipes

HFW pipe is produced with OD from 2'' to 24'' and with WT up to 22mm. High frequency welding is a type of pressure welding done either condictive or inductive (without contact)


  • Accurate dimensions
  • Accurate pipe length
  • Accurate wall thickness


  • Weld hardness
  • Weld area thickening (at ID)
  • Toughness of weld
  • Not suitable for low design temperatures ( Td < -10/-20 C)
  • Risk on loss of contact (ERW)

2.3. Submerged Arc Welded (SAW) Pipes

SAW weld pipe can be produce with dimeters ranging from 16” - 64” OD. SAW weld pipes are made from flat plates that are bend and form to a circular section and the welded using Submerge Arc Welding


• Grades & materials

– Corrosion

• Technical issues

• Line pipe coating systems