Mooring Layouts

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