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Offshore Production Technology Mooring Design (part b) Marine & Offshore Technology
15

Mooring Layouts

Jan 23, 2018

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Engineering

David Seow
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  • 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