Design and Modelling of Riser Turret
Mooring Systems
Dr.Fabrizio Pistani,
Floating Systems Group, Perth
AOG2015 Perth WA March 11-13
Presentation outline
The concept
Design philosophy and motivations
The software tools
Calibration of the numerical models
RTM column modelling
Some results
Tandem offloading
Disconnection
Conclusions
Further Developments
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RTM concept
Buoyant column about 100m
Universal Joint connection
Chain table for mooring
Risers connected at mid
length
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RTM concept
De-coupling of motions
Reduction of loads on the FPSO arm
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RTM concept
Evasion from tropical cyclones
RTM sinking under water surface
Will re-connect when cyclone passes
All RTM columns are in Australian waters
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Project Scope
Mooring design analysis of an FPSO moored through a
disconnectable, bow mounted, Riser Turret Mooring system RTM
Analysis scope aimed to verify that the mooring system meets all
operational, survival, and installation requirements
Tasks:
Connected strength assessment (including tandem offloading)
Disconnected strength assessment
Service life (fatigue) assessment
Disconnection / reconnection analysis
Mooring / risers interference assessment
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Design
Traditionally => Extensive model test campaigns
Presently => Numerical tools
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Scope
Design the mooring system for a vessel
that is already existing…
…but will be refurbished and installed:
• With a different mooring layout
• On a different site
• With a similar RTM column
Using:
model tests carried out for the original
configuration for calibration of the new
system layout;
in order to avoid repeating model tests
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Software Packages
Screening Analysis
Ariane: Fast  Limited
Detailed Analysis
Orcaflex: Accurate  Time consuming
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Calibration
Model test results used to
calibrate the numerical
models
Statically
Pull models for mooring
stiffness
Dynamically
surge extinction and
model test runs
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Calibration – Orcaflex model
Mooring stiffness curve
and characterisation
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Calibration
Dynamic calibration:
surge extinction
Adjustment of the
damping values
Discrepancies due to non
linearities
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Calibration
Dynamic calibration of Orcaflex model with connected tests
• Low frequency motion of the vessel (standard deviation of X and Y offset) showed
good correspondence between the tests and the Orcaflex numerical model
• Mean offsets were also generally close, but tended to be underestimated for
collinear or near collinear cases
• Mean yaw heading and yaw oscillation generally matched well. In fatigue
environments, the yaw oscillation was somewhat higher in the Orcaflex runs
• Good correspondence was found between the UJ angles and loads and maximum
line tensions
• Reasonable match when no additional damping was considered
• Actual measured wave spectrum had to be used
• Transverse motions not replicated
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Calibration
RTM Column Modelling in Orcaflex
Disconnected model: Calibration of hydrodynamic parameters
DNV-RP-C205 recommended coefficients for
drag with reduction factors for finite length
cylinders
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Modelling the link in Ariane
Overcoming limitations
Orcaflex
Ariane
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Calibration – Ariane model
Ariane Calibration
Static : pull models for
mooring stiffness
Dynamic: surge extinction
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Calibration
Ariane Dynamic calibration with connected results
• Ariane used to model low frequency motions for screening purposes: only vessel
offsets were compared
• The low frequency motion of the vessel showed reasonable correspondence
• Mean offsets for collinear cases tended to be underestimated
• The mean yaw heading and yaw oscillation found in Ariane matched reasonably well
• The Ariane results were comparable to the Orcaflex simulation results
• Separate calibration of low frequency damping levels for Ariane is required; best
match obtained by considering % of values suggested in hydrodynamic study
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Results
Environments:
The 100 year return period
1yr and 100yrs non-cyclonic
2yrs cyclonic conditions
some environments with sea and swell
steady current dominated
non-cyclonic case is governing
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Tandem offloading
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Tandem offloading: results
Maximum loads in the offloading hawser for different environments and tankers
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Disconnection analysis
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Disconnection analysis
Selection of the right moment and tug assistance force for disconnection
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Conclusions
• The verification of the mooring design has been successfully carried out without the
need for an additional campaign of model tests
• The use of two different software packages provided means for screening a large
number of cases and then refine the investigation with additional details
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Conclusions
• The availability of the refined model allowed to investigate details of the procedures
for disconnection and reconnection providing important insight
• Overall WoodGroup Kenny Perth has delivered a mooring design of a complex system
supporting the client also in the installation and procurement phases
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Further Development
• Coupled analysis
• Usage of load RAOs for the vessel
• Account for different periods and dynamics of the floating boadies
• VIV/VIM in disconnected conditions
• Still not well established
• Time consuming
• Lack of in field measurements
• CFD as alternative?
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Thanks to:
The MCS office in Perth
(now WoodGroup Kenny)
Azad Hessamodini
Johann Melillo
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