Design and Modelling of Riser Turret Mooring Systems - AOG · 6 - – AOG2015 Perth 11-13 March Mooring design analysis of an FPSO moored through a disconnectable, bow mounted, Riser

Design and Modelling of Riser Turret

Mooring Systems

Dr.Fabrizio Pistani, Floating Systems Group, Perth

AOG2015 Perth WA March 11-13

2 - www.woodgroup.com – AOG2015 Perth 11-13 March

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

Presentation outline


RTM concept

Buoyant column about 100m

Universal Joint connection

Chain table for mooring

Risers connected at mid

length


De-coupling of motions

Reduction of loads on the FPSO arm

RTM concept


Evasion from tropical cyclones

RTM sinking under water surface

Will re-connect when cyclone passes

All RTM columns are in Australian waters

RTM concept


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

Project Scope


Traditionally => Extensive model test campaigns

Presently => Numerical tools

Design


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


Screening Analysis

Ariane: Fast Limited

Detailed Analysis

Orcaflex: Accurate Time consuming

Software Packages


Calibration

Model test results used to

calibrate the numerical

models

Statically

Pull models for mooring

stiffness

Dynamically

surge extinction and

model test runs


Calibration – Orcaflex model

Mooring stiffness curve

and characterisation


Calibration

Dynamic calibration:

surge extinction

Adjustment of the

damping values

Discrepancies due to non

linearities


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


Calibration RTM Column Modelling in Orcaflex

DNV-RP-C205 recommended coefficients for

drag with reduction factors for finite length

cylinders

Disconnected model: Calibration of hydrodynamic parameters


Modelling the link in Ariane

Ariane

Orcaflex

Overcoming limitations


Calibration – Ariane model

Ariane Calibration

Static : pull models for

mooring stiffness

Dynamic: surge extinction


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


Results The 100 year return period

steady current dominated

non-cyclonic case is governing

Environments:

1yr and 100yrs non-cyclonic

2yrs cyclonic conditions

some environments with sea and swell


Tandem offloading


Tandem offloading: results Maximum loads in the offloading hawser for different environments and tankers


Disconnection analysis


Disconnection analysis Selection of the right moment and tug assistance force for disconnection


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


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


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?


Thanks to:

The MCS office in Perth

(now WoodGroup Kenny)

Azad Hessamodini

Johann Melillo

Design and Modelling of Riser Turret Mooring Systems - AOG · 6 - – AOG2015 Perth 11-13 March Mooring design analysis of an FPSO moored through a disconnectable, bow mounted, Riser

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