MOORING DESIGN & MOORING ANALYSIS IN INDIA
Our naval architects and consultants utilize best industry practices to provide solutions for mooring analysis such as spread mooring and single point mooring systems, which includes turrets, CALMs, SALMs, towers, submerged buoys, and more. We have vast experience in mooring consultation of harbor and jetty moorings, ships, Barges,accommodation vessels small ships, and for various other offshore structure
MOORING DESIGN CAPABILITY
We are able to analyze frequency and fully coupled, time domain mooring analysis for floating vessels, FPSOs and FSOs
We are able to analyze frequency and fully coupled, time domain mooring analysis for floating vessels, FPSOs and FSOs
- Spread mooring
- Single point mooring
- Semi-Submersible Platforms
- Tendon Leg Platforms
- Drill ships
- Accommodation vessels
- Mooring during offshore operation
- SPARs
MOORING SERVICES
We can provide expertise in the following areas:
- Wire-Chain Catenary Mooring analysis
- Taut Polymer Mooring Analysis
- Pile/Anchor Specification
- Air Gap Calculation
- Coupled Mooring-Riser-Vessel Analysis
- Mooring Fatigue Assessment
- Hydrodynamic Vessel Motion and mooring assessment
MOORING ANALYSIS PROCEDURE FOR OFFSHORE BARGE
Barge Loading Condition
Barge loading condition in operating condition shall be defined approximately and any variation in consumables, deck load etc shall be considered for worst load case scenario.
Inertia
Moments of inertia are assessed using the estimated gyration radii of the vessel,
Roll inertia radius,
Pitch and Yaw inertia radii,
weight, m = displacement
Inertia is then considered as follow:
Roll inertia radius,
Pitch and Yaw inertia radii,
weight, m = displacement
Inertia is then considered as follow:
Vessel Roll Damping
The calculation of roll motion is a key point of any ship motion calculations. It is governed by the roll damping, which is caused by two main contributions:
The wave radiation (linear) and the viscous effects on the hull (quadratic) for sea-keeping analysis
Add an additional quadratic viscous damping (BQ)
With:
Cd is quadratic damping coefficient (equal to 0.05 for this barge without a bilge keel),
ρ is sea water density (equal to 1025 kg/m3),
B is the breadth of the vessel,
L is the length of the vessel.
The wave radiation (linear) and the viscous effects on the hull (quadratic) for sea-keeping analysis
Add an additional quadratic viscous damping (BQ)
With:
Cd is quadratic damping coefficient (equal to 0.05 for this barge without a bilge keel),
ρ is sea water density (equal to 1025 kg/m3),
B is the breadth of the vessel,
L is the length of the vessel.
Additional Linear Damping for Low Frequency Motions
Additional linear damping can be calculated by BUREAU VERITAS formula for spread moored vessel:
It should be noted that Maxx, Mayy, Maψψ are the diagonal terms of the asymptotic added mass matrix of the vessel. And Koxx, Koyy, Koψψ are the diagonal terms of the mooring stiffness matrix [Ko] evaluated at the average position of the vessel.
It should be noted that Maxx, Mayy, Maψψ are the diagonal terms of the asymptotic added mass matrix of the vessel. And Koxx, Koyy, Koψψ are the diagonal terms of the mooring stiffness matrix [Ko] evaluated at the average position of the vessel.
Wind and Current Loads
Wind loads can be calculated with wind tunnel test, API RP 2SK guidelines etc.Projected area and wind force application point are required for wind force determination.
The steady state force (for bow or beam environment) due to wind acting on a moored floating unit can be determined using following Equation,
Fw = Cw × Σ (Cs×Ch×A) ×Vw²
Where:
Fw = wind force, lbs (N),
Cw = 0.0034 lb/(ft² • kt²) (0.615 N• sec²/m4)
Cs = shape coefficient,
Ch = height coefficient,
A = vertical projected area of each surface exposed to the wind, ft² (m²),
Vw = design and speed, knots (m/sec).
Wind shielding in accordance with acceptable methods shall be considered as per API RP 2SK guidelines.
For API method,The equations presented are convenient for calculating wind and current forces for bow and beam environments. For environments approaching from an oblique direction, Equation is used to evaluate wind forces.
Where,
= force due to oblique environment, lbs (N),
= force on the bow due to a bow environment, lbs (N),
= force on the beam due to a beam environment, lbs (N),
= direction of approaching environment (degrees off bow).
The steady state force (for bow or beam environment) due to wind acting on a moored floating unit can be determined using following Equation,
Fw = Cw × Σ (Cs×Ch×A) ×Vw²
Where:
Fw = wind force, lbs (N),
Cw = 0.0034 lb/(ft² • kt²) (0.615 N• sec²/m4)
Cs = shape coefficient,
Ch = height coefficient,
A = vertical projected area of each surface exposed to the wind, ft² (m²),
Vw = design and speed, knots (m/sec).
Wind shielding in accordance with acceptable methods shall be considered as per API RP 2SK guidelines.
For API method,The equations presented are convenient for calculating wind and current forces for bow and beam environments. For environments approaching from an oblique direction, Equation is used to evaluate wind forces.
Where,
= force due to oblique environment, lbs (N),
= force on the bow due to a bow environment, lbs (N),
= force on the beam due to a beam environment, lbs (N),
= direction of approaching environment (degrees off bow).
Current Load
Fcx = Ccx×Σ S×Vc²
Fcy = Ccy×Σ S×Vc²
Where:
Fcx = current force on the bow (N)
Fcy = current force on the beam (N)
Ccx = ½ * ρWater * Shape Coefficient (considered 1) = 502.7 Nsec²/m4
Ccy = ½ * ρWater * Shape Coefficient (considered 1) = 502.7 Nsec²/m4
S = wetted surface area of the hull (m²)
Vc = design current speed (m/sec)
The graph below shows the value of coefficient multiplied with surface area at different angle of incidences.
Fcy = Ccy×Σ S×Vc²
Where:
Fcx = current force on the bow (N)
Fcy = current force on the beam (N)
Ccx = ½ * ρWater * Shape Coefficient (considered 1) = 502.7 Nsec²/m4
Ccy = ½ * ρWater * Shape Coefficient (considered 1) = 502.7 Nsec²/m4
S = wetted surface area of the hull (m²)
Vc = design current speed (m/sec)
The graph below shows the value of coefficient multiplied with surface area at different angle of incidences.
Mooring Lines Properties
The main characteristics of the mooring wire ropes like Number of Lines, Length on Each Drum,Diameter,Weight in Air, Weight Submerged, Axial Stiffness and MBL etc shall be defined.
Anchor Data
Anchor arrangement and holding capacity in soft or hard clay shall be defined.
Mooring Winches
Mooring winch location, pull capacity on different layers, MBL etc shall be defined.
Wave Model
Wave is modeled with JONSWAP spectrum using significant wave height Hs, the peak period Tp, and the shape parameter Gamma.
Operating waves
For the operating position significant wave height range of that particular month or period shall be used. Each Hs shall be allocated three Tp based on the Noble Denton guidelines. Gamma can be used as 3.3.
Extreme waves
Based on the operational periods, the monthly extreme value at field location shall be used which can be extracted from metocean report. The Tp used for standoff position shall be taken with margin i.e as +/- 30% of the associated Tp of monthly extreme value.
Analysis Methodology
Capabilities of the mooring system shall be computed using quasi-dynamic simulations of the vessel and the mooring system under combination of wind, wave and current environmental loads. Simulations shall consider combining wave frequency and low frequency responses of the system to get the maximum vessel motions and mooring line tensions.
Simulations shall performed for 3 hours duration environments. “Quasi-dynamic analysis” is performed, meaning that full dynamic motion of the vessel is considered but line tensions are estimated from vessel dynamic motions with static restoring curves.
The low frequency motion shall be computed in time domain while the wave frequency motion shall be calculated in frequency domain. The global dynamic motion is the superposition of the low frequency motion and the wave frequency motion, the stiffness shall be computed for the sum of the WF and LF motion.
Simulations shall performed for 3 hours duration environments. “Quasi-dynamic analysis” is performed, meaning that full dynamic motion of the vessel is considered but line tensions are estimated from vessel dynamic motions with static restoring curves.
The low frequency motion shall be computed in time domain while the wave frequency motion shall be calculated in frequency domain. The global dynamic motion is the superposition of the low frequency motion and the wave frequency motion, the stiffness shall be computed for the sum of the WF and LF motion.
Hydrodynamic Analysis
Diffraction radiation method evaluates hydrodynamic loads on a structure, submitted to regular waves and enables to get accurate RAOs operation of the vessel. Newman Approximation shall be considered for consideration of shallow water effects on drift loads.
Environmental Load Cases
The maximum allowable operational environmental conditions shall be estimated for each environmental direction assuming Wind and current are applied collinear to the wave direction and Directions are considered as coming from 0 to 337.5 deg and every equal intervals are screened.
Design Criteria
All mooring system criteria are majorly checked against the requirements of API RP 2SK, Design and Analysis of Station Keeping Systems for Floating Structures .
Mooring Line Design Strength
The tension in the mooring lines shall be checked against API RP 2SK requirements. The tension must therefore remain lower than:
50% of the MBL under intact condition
70% of the MBL under damage condition
50% of the MBL under intact condition
70% of the MBL under damage condition
Winch Brake Capacity
The maximum tensions in the mooring lines shall be lower than the winch brake capacity of mooring winches at first layer .
Anchor Holding Capacity
The maximum tension in the mooring line shall be lower than ultimate holding capacity of that particular type of anchor in specified ground conditions.
Anchor Uplift
No uplift at the anchor shall occur under intact condition. This criterion shall not be considered for damaged conditions (one line failure).Anchor uplift is checked against class requirements.Vertical load at the anchor should remain less than 20% of anchor’s wet weight.
Barge Clearance with Existing Facilities
The minimum horizontal clearance between barge and existing offshore structures shall shall be specified for intact and damage conditions.
Barge Excursion for mooring line clearance
For mooring line, a minimum horizontal clearance of 10m shall remain under intact condition and a minimum clearance of 3m with mooring line will be considered under damaged conditions.
Mooring line vertical Clearance with Subsea Facilities
Minimum vertical clearance of 15m shall be considered between mooring lines and existing pipelines for all the range of tension from allowable environments.
If midline buoys are installed, the vertical force should not exceed 70% of the buoyancy capacity to ensure visibility.
If midline buoys are installed, the vertical force should not exceed 70% of the buoyancy capacity to ensure visibility.
FPSO HOOK UP PROCEDURE
FPSO General Configuration
Normally FPSO are hooked-up at ballast draft, with level trim. Based on ballast draft loading condition, the final pretension of the mooring lines shall be set to calibrated 10 to 15% of mooring chain MBL. With such pretension at ballast draft, lower pretension will be found for the “100% loaded” loading condition.
Tugs General Requirements
A minimum three tugs are recommended for the hook-up operations. An additional Installation vessel is used to install the mooring lines.
A bare minimum of two tugs are required to hold FPSO position on site, typically the two of the ocean tugs used for towing. However, a safe minimum of three tugs shall be used for hook-up. An additional Installation vessel is required to support during the hook-up operations.
The positioning tugs and assisting installation vessel are arranged in following way
Lead tugs, tied to the bow of the FPSO Stern tugs, tied to the stern of the FPSO Support installation vessel, located on site to support the hook-up operation.
A bare minimum of two tugs are required to hold FPSO position on site, typically the two of the ocean tugs used for towing. However, a safe minimum of three tugs shall be used for hook-up. An additional Installation vessel is required to support during the hook-up operations.
The positioning tugs and assisting installation vessel are arranged in following way
Lead tugs, tied to the bow of the FPSO Stern tugs, tied to the stern of the FPSO Support installation vessel, located on site to support the hook-up operation.
General Arrangement of the Hook-Up
Lead tugs are used to tow the FPSO to site, while the stern tugs are used to stabilize the FPSO fishtailing during sailing and installation.
Installation Vessels used on site to recover the chains and ensure the connection of the chains with the FPSO.
Installation Vessels used on site to recover the chains and ensure the connection of the chains with the FPSO.
General Hook-Up Sequence
The general hook-up procedure is as follow:
On-site, the positioning tugs are used to guide the FPSO to final location in order to start connection of the mooring lines. Assisting installation vessel is used to pick up the lines and help transfer the chains to the FPSO. First inner line of spread mooring system is recovered and the line hook-up procedure is followed. This mooring line is hooked up with 50% of final pretension. Same procedure would be repeated for other three lines at the four corners and on inner most locations. At the end of this stage, these four lines are connected and their tension shall be adjusted to get the final tension.
To compensate new mooring line tension,At each new line connected, Lead Tug and Stern Tug shall work to keep the FPSO around +/-15m offset from its final location .
The line hook-up procedure would be repeated for next set of middle lines .These Mooring lines also shall be re-adjusted to get the final pretensions. In this way, The line hook-up procedure would be repeated for next set of lines. So basic is that starting from inner most lines tugs shall work on next set of lines. At the end of this stage, all mooring lines are connected. Once all the mooring lines are connected and tensioned, adjustment of the mooring line tension shall be performed to get the correct positioning of the FPSO (midship location, riser porch location and FPSO heading). The following sequence is to be used to hook-up each mooring lines to the FPSO.
Retrieve mooring line end using the subsea or pennant buoy (at the line end location). Chain end is recovered and locked onto the installation vessel. Buoys and pennant wires are stored on AHT deck Moves towards FPSO stern corner, relocating the line up to fairlead of mooring line. Installation vessel shall (with the end of the chain) move as close as possible towards FPSO, so that it will have minimum distance from the appropriate fairlead. Meanwhile on the FPSO, mooring wire and winch sockets are prepared and connected to fairlead. The mooring wire shall be placed through the chain stopper (open) and stored into the chain chute. At the end of the mooring wire, A messenger line is attached to the socket. Once the installation vessel close to the FPSO, the synthetic rope (messenger line) free end is sent to the installation vessel (while the other end remains attached to the socket). Once the messenger line is connected to a winch on the installation vessel, the mooring wire is paid-out from the mooring winch through the chain stopper and fairleads; and the synthetic rope in paid-in from the AHT to guide the open spelter socket from the FPSO to the installation vessel. Once the socket at end of the mooring wire is on-board the AHT deck, the open spelter socket is connected to the last chain link. Once the chain has passed 15 links inside the chain stopper, chain stopper shall be closed and messenger line shall be retrieved which is ready to be used onto the next line. Then the chain stopper shall be opened and the mooring winch is used to set the chain at 50% of final pretension.
On-site, the positioning tugs are used to guide the FPSO to final location in order to start connection of the mooring lines. Assisting installation vessel is used to pick up the lines and help transfer the chains to the FPSO. First inner line of spread mooring system is recovered and the line hook-up procedure is followed. This mooring line is hooked up with 50% of final pretension. Same procedure would be repeated for other three lines at the four corners and on inner most locations. At the end of this stage, these four lines are connected and their tension shall be adjusted to get the final tension.
To compensate new mooring line tension,At each new line connected, Lead Tug and Stern Tug shall work to keep the FPSO around +/-15m offset from its final location .
The line hook-up procedure would be repeated for next set of middle lines .These Mooring lines also shall be re-adjusted to get the final pretensions. In this way, The line hook-up procedure would be repeated for next set of lines. So basic is that starting from inner most lines tugs shall work on next set of lines. At the end of this stage, all mooring lines are connected. Once all the mooring lines are connected and tensioned, adjustment of the mooring line tension shall be performed to get the correct positioning of the FPSO (midship location, riser porch location and FPSO heading). The following sequence is to be used to hook-up each mooring lines to the FPSO.
Retrieve mooring line end using the subsea or pennant buoy (at the line end location). Chain end is recovered and locked onto the installation vessel. Buoys and pennant wires are stored on AHT deck Moves towards FPSO stern corner, relocating the line up to fairlead of mooring line. Installation vessel shall (with the end of the chain) move as close as possible towards FPSO, so that it will have minimum distance from the appropriate fairlead. Meanwhile on the FPSO, mooring wire and winch sockets are prepared and connected to fairlead. The mooring wire shall be placed through the chain stopper (open) and stored into the chain chute. At the end of the mooring wire, A messenger line is attached to the socket. Once the installation vessel close to the FPSO, the synthetic rope (messenger line) free end is sent to the installation vessel (while the other end remains attached to the socket). Once the messenger line is connected to a winch on the installation vessel, the mooring wire is paid-out from the mooring winch through the chain stopper and fairleads; and the synthetic rope in paid-in from the AHT to guide the open spelter socket from the FPSO to the installation vessel. Once the socket at end of the mooring wire is on-board the AHT deck, the open spelter socket is connected to the last chain link. Once the chain has passed 15 links inside the chain stopper, chain stopper shall be closed and messenger line shall be retrieved which is ready to be used onto the next line. Then the chain stopper shall be opened and the mooring winch is used to set the chain at 50% of final pretension.
Tugs Requirements
Lead and Stern Tugs
The tugs used for the hook-up operations are usually the same the one used for the towing operation. Towing is usually designing but in any case, following requirements are needed for the Lead and Stern Tugs.Bollard pulls of tugs are calculated based on the force required to keep the FPSO in position with only two lines connected in specified wave heights, wind and current load in beam and quartering directions.
The tugs used for the hook-up operations are usually the same the one used for the towing operation. Towing is usually designing but in any case, following requirements are needed for the Lead and Stern Tugs.Bollard pulls of tugs are calculated based on the force required to keep the FPSO in position with only two lines connected in specified wave heights, wind and current load in beam and quartering directions.
Installation Vessel
An Installation vessel(INSTALLATION VESSEL) is required to support the hook-up operation.
The installation vessel shall be fitted with Anchor Handling systems. The installation vessel shall be able to fit a studless link hydraulic stopper. Installation vessel shall be fitted with auxiliary winch to heave the messenger line with drum capacity of about 50m.
The installation vessel shall be fitted with Anchor Handling systems. The installation vessel shall be able to fit a studless link hydraulic stopper. Installation vessel shall be fitted with auxiliary winch to heave the messenger line with drum capacity of about 50m.
MEASURES AND TOLERANCES
Positioning System
Recommended systems for the operation are:
DGPS1 with an accuracy of this system lower than +/- 1m. Gyrocompass with an accuracy lower than +/- 0.2 degree. A DGPS1 surface positioning system shall be used on the FPSO to position the vessel in the target box. A survey Gyrocompass shall be sued to install the FPSO with the proper heading. An additional DGPS1/2 system shall be placed at the riser porch tie-in location to monitor the good location of the riser connection. Positioning systems shall be set with Projection UTM Zone 48, local datum.
Recommended systems for the operation are:
DGPS1 with an accuracy of this system lower than +/- 1m. Gyrocompass with an accuracy lower than +/- 0.2 degree. A DGPS1 surface positioning system shall be used on the FPSO to position the vessel in the target box. A survey Gyrocompass shall be sued to install the FPSO with the proper heading. An additional DGPS1/2 system shall be placed at the riser porch tie-in location to monitor the good location of the riser connection. Positioning systems shall be set with Projection UTM Zone 48, local datum.
Positioning Tolerances
The positioning of the FPSO using above equipment shall be as follow:
FPSO horizontal position tolerance is a box with +/-5m sides from reference point FPSO heading tolerance is +/-2 deg from reference heading, with a maximum accuracy of +/- 0.5 deg
The positioning of the FPSO using above equipment shall be as follow:
FPSO horizontal position tolerance is a box with +/-5m sides from reference point FPSO heading tolerance is +/-2 deg from reference heading, with a maximum accuracy of +/- 0.5 deg
Mooring Lines Pretensions
The chain tension will be confirmed by a chain angle measurement gauge that will be fitted to each mooring chain outboard of the chain chutes.ROV survey between the distance between the fairleads and the mooring lines touch-down point on the seabed can be used as confirmation results of the proper tensioning.
The chain tension will be confirmed by a chain angle measurement gauge that will be fitted to each mooring chain outboard of the chain chutes.ROV survey between the distance between the fairleads and the mooring lines touch-down point on the seabed can be used as confirmation results of the proper tensioning.
Tensions Tolerance
The chain angle accuracy shall be measure with accuracy of 0.5 deg. If ROV assistance is used, the distance from fairleads to TDP shall be measured with an accuracy of 5m.Tension calculation using above measurements shall be +/- 5MT from recommended pretension.
The chain angle accuracy shall be measure with accuracy of 0.5 deg. If ROV assistance is used, the distance from fairleads to TDP shall be measured with an accuracy of 5m.Tension calculation using above measurements shall be +/- 5MT from recommended pretension.
Mooring Chain Twist
The installation shall minimize the twist of the mooring lines. The twist tolerances shall be as follow:
There shall not be more than 1 twist every 100m. Not more than 3 twists all along the mooring lines shall be occurring along the whole line. A ROV survey of the mooring lines shall be conducted to ensure that allowable twist and no kinked chain link occurs along the mooring lines. Marking of the chain can be used to help control of the twist during ROV survey.
The installation shall minimize the twist of the mooring lines. The twist tolerances shall be as follow:
There shall not be more than 1 twist every 100m. Not more than 3 twists all along the mooring lines shall be occurring along the whole line. A ROV survey of the mooring lines shall be conducted to ensure that allowable twist and no kinked chain link occurs along the mooring lines. Marking of the chain can be used to help control of the twist during ROV survey.
Emergency Response Plan
The FPSO approach will be timed for the slack water prior to the start of the weaker ebbing tidal stream. As an overall statement, weather condition shall not exceed for mooring lines hook-up operation:
Significant Wave Height less than specified Wind speed less than specified Current velocity less than specified Otherwise, Mooring Line Hook-up is aborted and the line is retrieved and laid on seabed by the Support Tug. Mooring lines hooked-up remains attached to the FPSO. The Lead and Stern Tug sets back into a towing configuration in order to face the up-coming weather.
The FPSO approach will be timed for the slack water prior to the start of the weaker ebbing tidal stream. As an overall statement, weather condition shall not exceed for mooring lines hook-up operation:
Significant Wave Height less than specified Wind speed less than specified Current velocity less than specified Otherwise, Mooring Line Hook-up is aborted and the line is retrieved and laid on seabed by the Support Tug. Mooring lines hooked-up remains attached to the FPSO. The Lead and Stern Tug sets back into a towing configuration in order to face the up-coming weather.
FPSO Preparation
Prior to FPSO entering the field, a dedicated rigging team will board the vessel to make final preparations for the hook-up works.
The Tow Master will be in overall charge in Positioning of the FPSO, he will liaise closely with the Clients Offshore Installation Manager to instruct the different tugboats for the approach and to hold the FPSO in position during the hookup.
Mooring winches shall be used for the tensioning of the mooring chain legs. Chain angle measurement gauge shall be prepared.For the hookup operation, synthetic ropes and appropriate equipment will be prepared on deck at the fore and aft bundles of the FPSO. All safety equipment shall be as per SOLAS specification, National Regulation and International applicable standards mooring hookup.
Prior to FPSO entering the field, a dedicated rigging team will board the vessel to make final preparations for the hook-up works.
The Tow Master will be in overall charge in Positioning of the FPSO, he will liaise closely with the Clients Offshore Installation Manager to instruct the different tugboats for the approach and to hold the FPSO in position during the hookup.
Mooring winches shall be used for the tensioning of the mooring chain legs. Chain angle measurement gauge shall be prepared.For the hookup operation, synthetic ropes and appropriate equipment will be prepared on deck at the fore and aft bundles of the FPSO. All safety equipment shall be as per SOLAS specification, National Regulation and International applicable standards mooring hookup.
FPSO SPREAD MOORING METHODLOGY
Following steps are considered for complete mooring analysis
- FPSO hydrodynamic analyses,
- Mooring quasi-dynamic analyses,
- Mooring lines dynamic analyses,
FPSO first order motion
3D panel method is best suited to determine the added mass and radiation damping of structures, as well as waves loads in finite depth : Stokes 1st order loads (Froude-Krylov terms), diffracted wave loads.
Wave periods between 3 to 50 seconds shall be studied from 0° to 360° with a 10° heading step.The first order motions shall be computed in the frequency domain.
Roll motion is governed by the roll damping, which mainly depends on wave radiation and viscous effects on the hull. For sea-keeping analyses, the standard procedure is to model these two components of roll damping as the sum of linear term accounting for the wave radiation damping and additional viscous damping is added using the Ikeda-Tanaka-Himeno formulas, including eddy making, friction, lift and the contribution of bilge keels.
Stochastic linearization is considered one of the most accurate methods of linearization of damping quadratic term because it iterates on roll velocity standard deviation to adapt linearized damping to environmental conditions.
Wave periods between 3 to 50 seconds shall be studied from 0° to 360° with a 10° heading step.The first order motions shall be computed in the frequency domain.
Roll motion is governed by the roll damping, which mainly depends on wave radiation and viscous effects on the hull. For sea-keeping analyses, the standard procedure is to model these two components of roll damping as the sum of linear term accounting for the wave radiation damping and additional viscous damping is added using the Ikeda-Tanaka-Himeno formulas, including eddy making, friction, lift and the contribution of bilge keels.
Stochastic linearization is considered one of the most accurate methods of linearization of damping quadratic term because it iterates on roll velocity standard deviation to adapt linearized damping to environmental conditions.
Extreme quasi-dynamic analyses
Methodology
A full time domain analysis is used. “Quasi-dynamic analysis” means that lines tensions are estimated from FPSO dynamic motions but with quasi-static restoring load curve.The low frequency motion is computed from a time domain simulation of 3 hours duration, the wave frequency motions are then added from the linear RAO (computed from a frequency domain simulation). The stiffness is computed for the sum of the wave frequency and low frequency motion.
Second order wave loads
The Newman approximation can be considered to calculate non-linear second order wave loads. This approximation is considered valid for shallow water depth and maximum draft. However a sensitivity analysis on one case shall be performed using Full QTF matrix.
Low frequency damping
- Viscous / drag loads on the hull provides major contribution to the low frequency damping for all the motions. Estimation is derived from a standard linear formulation (ITTC).
- Damping from mooring lines dynamic: shallow water combined with semi-taut mooring and limited FPSO motions leads to small contributions to the total damping
- Wave drift damping is also rather small because wave height and low frequency motions are reduced. Then influence of current on wave drift damping is also neglected.
- Seabed effects : motions on the mooring lines close to the seabed are too limited to induce friction effects leading to low frequency damping
The associated levels of damping correspond to 3% of the critical damping in surge and sway, and 5% in yaw.
Intact load cases matrix
Extreme weather data shall be provided for every regular azimuth angle. Wind, wave and current are considered collinear and environments are then applied at every step.
Extreme weather data shall be provided for every regular azimuth angle. Wind, wave and current are considered collinear and environments are then applied at every step.
Damaged conditions
In order to predict the maximum offset, cases having the most loaded line for each bundle of mooring lines shall be considered broken. In order to predict the maximum tension, another case where the second most loaded line considered broken is added.
In order to predict the maximum offset, cases having the most loaded line for each bundle of mooring lines shall be considered broken. In order to predict the maximum tension, another case where the second most loaded line considered broken is added.
Transient conditions
For the intact cases where the maximum tensions occur (one case per FPSO loading), transient analyses are performed in order to quantify the transient effect of one-line failure.
The simulation to be selected should satisfy the following criteria:
Using the same sets of Airy waves and wind simulations can be repeated. During these simulations, the line should be broken at different times equally distributed between the two instants identified.
For the intact cases where the maximum tensions occur (one case per FPSO loading), transient analyses are performed in order to quantify the transient effect of one-line failure.
The simulation to be selected should satisfy the following criteria:
- The maximum tension is the closest to its design tension in intact conditions, and
- The maximum occurs while the low frequency component of the tension is increasing.
Using the same sets of Airy waves and wind simulations can be repeated. During these simulations, the line should be broken at different times equally distributed between the two instants identified.
Extreme Lines Dynamic Analyses
The time series of six D.O.F FPSO motions will be imposed concurrently with the wave and current effect onto the mooring line. The analysis will be carried out in the range of the times that cover the maximum line tension and FPSO motions.
The two FPSO loading conditions in the intact and damage cases will be considered.
The time series of six D.O.F FPSO motions will be imposed concurrently with the wave and current effect onto the mooring line. The analysis will be carried out in the range of the times that cover the maximum line tension and FPSO motions.
The two FPSO loading conditions in the intact and damage cases will be considered.
Sensitivity Analyses
Additional cases will be performed to check the sensitivity of the mooring system in response to the slight variation conditions as shown below.
Anchor installation tolerances Sensitivity
Sensitivity analyses are carried out in intact condition only. The following anchor installation tolerance is carried out:
Full QTF Sensitivity
Sensitivity analysis on the two worst cases for each of the 100% and 10% Loaded Conditions will be performed using Full QTF matrix. This is to compare against the same cases where Newman approximation is considered.
Hs / Tp Sensitivity
+/- 1s of Tp will be the sensitivity case to investigate the effects of the mooring system due to changes in Tp and Hs. It will be tested only for the cases that lead to maximum line tension for both 100% and 10% Loaded Conditions.
Non-Collinear Sensitivity
For the cases that lead to maximum line tension for both 100% and 10% Loaded Conditions, variation of the wind and current direction from the wave direction shall be considered.To investigate the effect of the mooring system due to non collinear environment. These conditions shall be considered
+/- 1s of Tp will be the sensitivity case to investigate the effect to the mooring system due to changes in Tp. It will be tested only for the cases that lead to maximum offset for both 100% and 10% Loaded Conditions.
Wind Spectrum Sensitivity
Sensitivity analysis on the worst case for each of the 100% and 10% Loaded Conditions will be performed using different wind spectrum. This is to compare against the same cases where another wind spectrum is considered.
Additional cases will be performed to check the sensitivity of the mooring system in response to the slight variation conditions as shown below.
Anchor installation tolerances Sensitivity
Sensitivity analyses are carried out in intact condition only. The following anchor installation tolerance is carried out:
- Pretension in mooring lines (±2°)
Full QTF Sensitivity
Sensitivity analysis on the two worst cases for each of the 100% and 10% Loaded Conditions will be performed using Full QTF matrix. This is to compare against the same cases where Newman approximation is considered.
Hs / Tp Sensitivity
+/- 1s of Tp will be the sensitivity case to investigate the effects of the mooring system due to changes in Tp and Hs. It will be tested only for the cases that lead to maximum line tension for both 100% and 10% Loaded Conditions.
Non-Collinear Sensitivity
For the cases that lead to maximum line tension for both 100% and 10% Loaded Conditions, variation of the wind and current direction from the wave direction shall be considered.To investigate the effect of the mooring system due to non collinear environment. These conditions shall be considered
- With wind @ -30° from waves and current @ +45° from waves.
- With wind @ +30° from waves and current @ -45° from waves.
- With wind @ -45° from waves and current @ +30° from wave.
- With wind @ +45° from waves and current @ -30° from waves.
+/- 1s of Tp will be the sensitivity case to investigate the effect to the mooring system due to changes in Tp. It will be tested only for the cases that lead to maximum offset for both 100% and 10% Loaded Conditions.
Wind Spectrum Sensitivity
Sensitivity analysis on the worst case for each of the 100% and 10% Loaded Conditions will be performed using different wind spectrum. This is to compare against the same cases where another wind spectrum is considered.
fpso_spread_mooring_analysis_guide_for_benign_condition.pdf |