Transcript
Offshore Production Technology
Mooring Design (part b)
Marine & Offshore Technology
Mooring Design
• To distribute the loads in the individual lines as equally as possible
• To give sufficient redundancy to the overall system.
Mooring Pattern
Mooring Pattern
Mooring Layout
The important factors are:
• Strength of each mooring line– Breakage tension of 3,130 to 5,160 kN (320 to 525
tonnes)
• Seabed topography and soil friction
• Prevailing directions of wind, waves and current
• Proximity of other fixed structures on the seabed such as templates and pipelines
Mooring Layout
The important factors are (continued):
• Proximity of other fixed structures in the water column, such as risers and riser mid-water arches, etc.
• Other storage or drilling vessels moored in the vicinity
• Future operational activities in the field (e.g. well workover).
Offshore Mooring Patterns 1
Offshore Mooring Patterns 2
Mooring Line Configurations
Mooring Analysis
Design criteria
• Normally designed for the 100-year storm conditions, i.e. for the combination(s) of wave height, wind and current velocities (1/100 storm conditions – statistics).
• Conditions are established by extreme value analysis and extrapolation based on environmental data measured over a sufficient length of time.
Mooring Analysis
Typical values of waves in 100-year storms are:
100-Year Significant
Wave Height (m)
Associated Wave Period
(seconds)
West of Shetland 18 20
Northern North Sea 16 17
Gulf of Mexico 13 16
Philippines 11 15
Brazil 7 14
Mooring Analysis
Tensions and excursions
• Design of the mooring system should allow floater remains within acceptable limits of horizontal distance (or excursion) whilst subject to the worst environmental loads.
Regions of Vessel Offsets vs Line Tension
Mooring Analysis
Traditional mooring and riser systems design
• Traditionally, uncoupled behaviour of the two systems, and each system is analysed independently.
• The motion of FPSO is calculated taking into account the mooring system only; the motion obtained is then imposed as an input to design of riser system.
Mooring Analysis
Integrated mooring and riser systems design
• System stiffness increases with:– Increase in riser number– Size of the risers– Water depth
• Nowadays, stiffness and damping contribution from the risers are included in fully integrated analysis of the mooring and riser systems.
Mooring Analysis
Broadly what does mooring analysis involve?
• design criteria (e.g. 100-year storm)
• tensions in lines, excursions anticipated/allowed
• integrated mooring and riser design
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