Deepwater Pipeline

To ensure continuity of supply, E&P companies have to consider opportunities in ever increasing water depths. Assisting this are new technological advances, including pipeline manufacture and design that increase the technical feasibility of deepwater developments.

Some key Risks and Challenges of deepwater pipelines are:
  1. Wall thickness design
  2. Installation challenges
  3. Flow Assurance - Prevention of Hydrates
  4. System pressure test
  5. Geohazards
  6. Repair systems for ultra-deep waters
Deepwater challenge #1 – Pipeline collapse

Wall thickness design

Risks and Challenges:
  • High external pressure means pipeline collapse failure mode governs wall thickness design rather than pressure containment – buckling vs yielding
  • Manufacturing of thick wall pipes may limit project options and project feasibility. Can they be produced?
  • Can the pipeline be laid?
  • Project viability often depends on having an optimal wall thickness which provides adequate strength, can be manufactured and installed at an acceptable cost.
Pipeline collapse
  • Use of limit state design approach to optimize wall thickness design e.g. OS-F101 standards
  • Specify improved linepipe properties against collapse failure by improving compressive yield strength, balance between strength and toughness and improving linepipe roundness
  • Justify use of less conservative safety factors through technology qualification.
Collapse capacity of pipelines – design requirements
Deepwater challenge #2 – Installation


Risks and Challenges:
  • High strains coupled with high external water pressure with increased risks of local buckling and fracture
  • High pipelay vessel tensioner capacity required. More stringent requirements to tensioner holding
  • Ability to detect buckle and repair a wet buckle
  • Use of limit state design approach to check against local buckling, ovalisation and to optimize wall thickness/ pipe weight
  • Specify supplementary linepipe material properties for high strains and additional testing
  • Specify supplementary dimensional properties of the linepipes
  • Use of Engineering Criticality Assessments for girth weld assessments
  • Self propelled buckle detector crawlers with remote sensors (e.g. microwave buckle detector used on S7000)
Enhanced material and dimensional properties for fracture and buckling control

Challenge #3 – Hydrate plugs

Formation of Hydrate plugs

Risks and Challenges:

  • In the event of a small leakage, seawater ingress into a deepwater gas pipeline (external pressure > internal pressure) can lead to formation of hydrates.
  • Large Hydrate plugs may form – How large? How to intervene?

  • Passive prevention systems
– Insulation (coating, PiP, bundle)
– Trenching
– Parallel trenched pipelines

  • Active prevention systems
– Injection of methanol/glycol (MEG)
– Heating – electrical, circulation of hot water
– Dehydration of pipeline

  • Reactive - Hydrate remediation
– Depressurisation
– Injection of methanol/glycol (MEG)
– Heating

Deepwater Challenge #4 – Relevance of system pressure test

System Pressure Testing

Risks and Challenges:
  • Most pipeline codes requires field pressure testing after installation to 1.1 to 1.25 times design pressure. Where the external pressure is much larger than the internal pressure, is this test relevant and practical?
– The system test pressure may cause lateral buckling of the pipeline.
– Require huge compressors to empty the pipeline.

Pipeline lateral buckling
  • Adopt DNVGL-OS-F101 provisions for system pressure test waiver (Sec.5 B203)
Waiver for System Pressure Test (DNVGL-OS-F101)

Deepwater challenge #5 – Geohazards


Risks and Challenges:
  • Steep slopes
  •  Loose Sediments
  • Slope instabilities and mudslides
  • Turbidity flow
  •  Seismic activities
  • Earthquake causing seabed faults
  • Uneven seabed causing long freespans
  • Lack of suitable methods for seabed intervention

  • Adequate characterization of seabed and geomorphology
  • Optimise routing of pipeline considering potential hazards, safety, minimise pre and post lay intervention
  • Apply advanced analyses and design where hazards cannot be avoided. 
Freespan assessment based on DNVGL-RP-F105

Deepwater challenge #6 – Repairs

Damage and Repairs

Risks and Challenges:
  • Wet buckle during installation
  • Damage during operation
  • Ingress of seawater/ hydrate plugs
  • Unsuitable soil/ environmental conditions
  • Lack of diverless repair systems for ultra deepwater
  • Emergency response procedures must be established
  • Develop and qualify repair methods suitable for specific pipeline system and environmental conditions.

Deepwater challenge – Repairs


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