C7a Mooring Layouts_2020

1. Offshore Production Technology
Mooring Layout
Marine & Offshore Technology

2. Learning Outcomes
1. Explain the factors affecting mooring layout
2. Describe the different mooring patterns and mooring
line configurations
3. Explain the purpose and method for conducting
mooring analysis

3. 1. Explain the factors affecting mooring
layout

4. Mooring Layout
• To distribute the loads in the individual lines as equally as
possible
• To give sufficient redundancy to the overall system.
Mooring Pattern

5. 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

6. 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).

7. 2. Describe the different mooring
patterns and mooring line configurations

8. Mooring Patterns

9. Offshore Mooring Patterns 1

10. Offshore Mooring Patterns 2

11. Mooring Line Configurations

12. 3. Explain the purpose and method for
conducting mooring analysis

13. 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.

14. 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

15. 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.

16. Regions of Vessel Offsets vs Line Tension

17. 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.

18. 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.

19. 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

20. Key Questions
1. Explain the factors affecting mooring layout
2. Describe the different mooring patterns and mooring line configurations
3. Explain the purpose and method for conducting mooring analysis