1. Introduction to offshore pipelines

1.1. Use of subsea pipelines

Pipelines are a huge scope for any offshore oil and gas project. Subsea pipelines transport oil and gas from subsea wells to the platform and subsequetly gas or oil from the platform to the coast for futher process and distribution. There are also large pipeline for the transportation of gas or oil from one country to another.

Pipelines are classified in three categories:

  • Infield Pipelines transport fluids within the field. They are often called flowlines or feeder lines. Infield Pipelines carry a mixture of oil, gas and water from the subsea wells to a manifold or directly from the well to the process platform. A smaller number of infield lines carry processed water from the platform to injection wells for disposal.
  • Export Pipelines transport processed oil or gas from the platform to the coast. If the pipeline carries a mixture of oil and gas  then it is a multi-phase pipeline. If the pipeline carries only oil or only gas, then it is an sigle-phase pipeline.
  • Transmission pipelines carry oil or gas from one coast to another mainly, the same way as a tanker transfers oil for trading purposes.

1.2. Typical pipeline diameters

It is always useful to have a rough idea of typical pipeline diameters for each type of pipeline

 Diameter [Inches]Length [km]
Gas lift flowlines: 2″ 
Water injection:6″sss
Oil production flowlines  
Export Pipelines14″ 
Oil production from manifold12″ 
Water injection to Manifold10″ 
Test flowline8″ 

3.1. Pipeline types

Steel pipelines

Grade from X52 to X70. Diameters from 2” to 48”. Wall thickness up to 42 mm. Stell pipeline transport products that are aggressive to steel

Flexible pipelines

Composite pipe structure. Diameters from 2” to 20”. Flexible are used as infield pipelines for aggressive products. Low cost installation in reel-lay. High pipeline manufacturing cost and few suppliers.

Duplex pipelines

Special pipeline which is resistant to aggressive products. Expensive to fabricate and to weld

Cladded or lined with stainless steel

Steel pipeline with liner. Less expensive then Duplex. Fabrication and welding remains expensive

Steel chromium pipelines

Steel with 13%Cr. Less expensive to fabricate. Can withstand aggressive fluids. Expensive welding

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)

Advantages

  • Accurate dimensions
  • Accurate pipe length
  • Accurate wall thickness

Disadvantages

  • 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

5. Linepipe Coating Systems

5.1. Asphalt Enamel

Consist of a multi layer hot applied (250 C) asphalt enamel

2 fibre glass reinforcement layers

Asphalt impregnated outer layer

Thickness is 4-6 mm

Cheap

Excellent bonding with CWC

Fusion Bonded Epoxy Coating (FBE)

Single layer epoxy powder coating

Thickness ~120 microns

Good cathodic dis-bondment

Also used as anti-corrosion layer in the multi-layer coating systems

No bonding with concrete coatings ( Additional rough coat required )

3 Layer Coatings (3LPE & 3LPP)

Fusion Bonded Epoxy

Adhesive (Co-polymer)

Mechanical protection layer (PE or PP)

Good cathodic dis-bondment characteristics

High mechanical protection

“Rough coat” on PE/PP bonding with CWC + tension

Multi-Layer Coatings (4LPE or PP)

Insulation purposes

Fusion Bonded Epoxy 120 Microns

Adhesive (Co-polymer) 150 micron

PU / PP Foam layer < 50 mm

PE or PP outer layer < 5 mm

Multi-Layer Coatings (SPU & GSPU)

Insulation purposes

Fusion Bonded Epoxy 120 Microns

Multi layer SPU/GSPU “unlimited”

Expensive

Concrete Coating (CWC)

Buoyancy and mechanical protection purposes..

Application methods:

1 Impingement: Spraying the concrete onto the anticorrosion coating

2 Compression :A continues slab of concrete is wrapped around the pipe

Pipe in Pipe ( PIP)

Insulation purposes

PU Foam inside pipe insulation Unlimited thickness

3LPP / 3LPE / FBE outer coating

Expensive

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