FLEXIBLE VS RIGID PIPELINES– Oil & Gas Trainings

by Dr. Al Taver

www.globalenergypanel.com

Pipeline Material options

• Rigid Pipelines (Carbon Steel)

• Flexible Pipes – Composite Materials

Material Type

• Carbon Steel Pipes- Seamless- Longitudinal Submerged arc welded (LSAW)- ERW (Electrical Resistance Welded)- HSAW (Helical or Spiral Welded)

• Flexible pipe- Non-Bonded- Bonded

Spec - Codes

• Rigid Pipelines

- As per API 5L

• Flexible Pipelines

- As per API RP 17B

Non-bonded Flexible pipe

1.Interlocked carcass

2.Thermoplastic Inner Tube

3.Zeta pressure Carcass

4.Double cross wound tensile Armors

5.Thermoplastic outer jacket

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Bonded Type

Non-bonded Type

CriteriaLine pipe selected should fulfill the following technical requirements of project:

• Suitability for service conditions- strength - Corrosion Resistance- Design Condition- Design Life

• Suitability for Installation- Routing- Fabrication and installation

• Environmental Criteria

• Proven track record

• Availability of vendors

General Advantages

• Flexible Pipe- Easy to store, Handle, Transport, Install and retrieve.- Adapts well to the seabed topography

• Rigid pipes- Characterized by its OD, wall thickness and material grade for a given dia, thickness and grade the mechanical behavior is fully defined.

Parameters for Technical comparison

• Pipeline Design and engineering- Length/Routing- Throughput- Design Life- CP system- Free span correction and crossings

• Installation• Operation, repair and maintenance• Reusability• Vendor availability• Safety

Pipeline Routing and Length

• Major factors affecting pipeline routing is permissible bend radius for direction changes

• Permissible bend radius depends on

- Allowable bending stress

- Installation requirement

• Normal Bend radius

- For Rigid pipes 700-1000m

- For Flexible Pipelines 10D

Throughput/Size

• Friction Loss

* Rigid pipe

- Surface roughness Rs=0.038mm

- Parameters considered are velocity, Reynold’s number

* Flexible Pipe

- Vendors recommendation for smooth bore Rs=0.005mm

10/15/2013

Design Life

• Flow MaterialTreated de-oxygenated sea water with oxygen scavenger

* Rigid Pipe- Corrosion for rigid option 2.0 mil/year average- Total corrsion/25 years = 2.0X25=50mil=1.3mm

- Provide Corrosion allowance* Flexible Pipe

- Expected design life of non bonded pipe covering all aspects 20-25 years.

Corrosion Protection-Flexi pipe

• Inner structural layers protected from environment by thermoplastic sheathing and no external corrosion coating required

• Steel layers are protected by sacrificial anodes at ends.

• Any damage to external sheathing then requirement of anodes increase

10/15/2013

Corrosion Protection-Rigid Pipe

• External surface provided with corrosion coating along with sacrificial anode type CP for corrosion control

Pipeline Free Spans -Flexible pipe

• Conforms generally to gradual variations in sea bed profile and thus minimise free span corrections

• For steep seabed variations, free span correction will be required such cases allowable free span when compared with rigid pipe is very small

Pipeline Free Spans-Rigid pipe

• In order to control stress level, free span lengths must be controlled with in design values

• Correction techniques involving installation of grout bags.

• Allowable span is more than Flexible pipes.

Crossings

• For crossings, both flexible and rigid are comparable and require similar correction technique.

Installation

• Flexible Pipe- Smaller Installation Spread required- Less Installation time

• Rigid Pipe- Installation spread requirements higher- More installation time

Operation, Repair and Maintenance- Flexible pipes

• Once damaged, flexible pipes are rather expensive to repair

- Any damage to external sheathing, water ingress into structure of flexible pipe will cause damage to inner layers.

- No known procedure for repairing flexible pipe in sub sea condition.

- The damaged sections to be cut and ends of pipe prepared to install new end fittings.

- Additional pipe lengths to be added so as to joint it on board and lower it to sea bed.

- Repair jobs depends solely on the Manufacturer

Operation, Repair and Maintenance- Rigid pipes

• Rigid pipes are less prone to mechanical damage. Higher mechanical strength and concrete coating provides mechanical protection.

• Repair jobs are simple, well proven, permanent in nature and economical.

Reuse of pipes

• An advantage normally associated with flexible pipe is retrieval and reuse at new location.

• Rigid pipes it is not possible

Summary

• Rigid Pipes

- Established and proven system.

Superior in:

- Availability

- Mechanical Design

- Repair and Maintenance

- Safety and Resilience

Summary

• Flexible pipes

- Retrieval and reusability

- Less lay spread requirements

- Less installation time

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Disclaimer

The information contained in these course/notes has been compiled from various sources and isbelieved to be reliable and to represent the best current knowledge and opinion relative to thesubject.

Global Energy Panel offers no warranty, guarantee or representation as to its absolutecorrectness or sufficiency.

Global Energy Panel has no responsibility in connection therewith; nor should it be assumedthat all acceptable safety and regulatory measures are contained herein, or that other oradditional information may be required under particular or exceptional circumstances.

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