Pipelines
From Subseapedia
Contents
Design Codes and Regulatory Requirements
- Design shall be governed by the most stringent requirements defined by:
- Local regulations (e.g. MMS – US, HSE – UK)
- Project requirements
- Company standards and specifications
- Local codes and standards
Route Selection
- Route influenced by
- Obstructions such as rocks, mud slides, pipelines, platforms, etc.
- Other users of the seabed
- Limitations of construction equipment
- Need detailed survey data to better understand surrounding environment
- Two main types of route survey:
- Geophysical - shape of seabed and soil types
- Geotechnical - strength of seabed soil
- Survey data are inputs for detailed design and installation
- Two main types of route survey:
Mechanical Design
Wall Thichness
- Pressure containment
- Static and dynamic loading
- Local buckling (external pressure design)
- Design codes (U.S. applications)
- ASME B31.4 (liquid pipeline systems application)
- ASME B31.8 (gas transportation application)
- API RP 1111 (hydrocarbons transportation application)
- Others - Flavor of the region
- Collapse – Bending moment and external pressure
Span Assessment
- Maximum allowable span length
- Static stress Analysis
- Static stress criteria
- Installation, hydrotest and operation conditions
- Vortex induced vibration (VIV)
- Fatigue criteria
- Operation conditions
- Static stress Analysis
Span Mitigation
- Mitigation strategies:
- Sandbags
- Concrete mattresses
- Structure supports
- Trenching
- Rock dumping
- VIV Suppression - if the maximum span length is governed by VIV and the static stress criteria is satisfied
Thermal Expansion and Buckling
- Middle-line expansion
- Upheaval buckling
- Buried pipelines
- Lateral buckling
- Pipeline laid on seabed
- End expansion - fixed end vs. free end
- Pipe burial and rock dumping
- Thermal expansion loops
- Lateral buckling control - buckle initiator
- Pipeline installed over a sleeper
- Pipeline installed with buoyancy (add to previous)
On-Bottom Stability
- Pipeline stability - Analysis to ensure pipeline
- Movement is within allowable tolerances for all foreseeable environmental/operational conditions
- Metocean Data - Design currents and waves
- Hydrodynamic Forces
- Drag, Inertial and Lift
- Soil Lateral Resistance
- Soil friction
- Resistance due to pipe embedment
- Analysis tool (e.g. American Gas Association on-bottom stability program)
- Mitigation
- Concrete weight coating or just increase wall thickness
- Trenching
- Weight clamps
- Anchoring
Material Selection
- Basic Considerations:
- Strength
- Pressure
- Tension
- Environment
- Material compatibility with Internal Content
- Availability of Material
- Stell, CRA, Flexibles
- Pipeline Life or Afterlife
- Strength
Corrosion
- Types of Corrosion
- Internal
- External
- Mitigation of Corrosion by Coatings
- Cathodic Protection
- Internal Corrosion Control
Internal Corrosion Protection
- Material selection
- Use corrosion inhibitor chemicals
- Consider chemical effects on all parts throughout the system
- Internally coat the line with corrosion resistant material
- Can be costly
- May have a limited life
- Monitor corrosion rate
- Online corrosion monitoring (corrosion spools, corrosion coupons, iron counts)
- Intelligent pigging
External Corrosion Protection
- External corrosion coating
- Fusion Bonded Epoxy (FBE)
- Three layer Polyethylene (3LPE)
- Elastomeric Coatings: Polychloroprene, ethylene propylene diamine (EPDM)
- Cathodic Protection
- Must be in a continuous conductive medium
- Sized to accommodate up to 25% coating breakdown or less over field life
- May be impressed current or sacrificial anodes
Installation
- Installation Methods
- S-Lay Vessel
- J-Lay Vessel
- Reel-Lay Vessel
S-Lay Method
- Suspended pipe shaped like S-Curve
- Pipeline must be tensioned to hold its shape & avoid buckling
- Pipe laying up to 60"
- 40' or 80' pipe sections welded together horizontally onboard vessel
J-Lay Method
- Suspended pipe shaped like letter "J"
- Almost vertical tower supports pipe onboard vessel
- Pipe enters water at high entry angle - no overbend
- Deep water applications
Reel-Lay Method
- Pipe is welded together onshore and spooled onto a large diameter reel - plastically yielded
- Pipe must be straightened offshore during un-spooling
Pipeline Testing and Commissioning
- Hydrotesting
- Defined by code, regulation and project requirements
- Serves as final pipeline integrity check
- Usually tied to installer's credibility
- Commissioning is a planned process to prepare the pipeline for operations phrase.