RELIABILITY
OF MOORING SYSTEMS FOR FLOATING PRODUCTION SYSTEMS
OBJECTIVE: Mooring systems for floating production systems, including the
mooring lines and anchors, are currently designed on the basis of individual
components. The most heavily loaded line and anchor are checked under
extreme loading conditions with the system of lines intact and with one
line removed. However, the performance of the floating production system
depends more directly on the performance of the system of lines and anchors
rather than on the performance of a single line or anchor.
This project will assess and compare the component and system reliabilities
for the mooring system of a deepwater Spar. Results will allow evaluation
and possible improvement of the design codes for FPS mooring lines and
anchors.
APPROACH: The reliability of mooring and foundation components and the
overall mooring system for deepwater Spars are being analyzed and compared
as follows:
1. Designs for a representative Spar and mooring system for several depths
in the Gulf of Mexico were selected for study.
2. Models describing the performance of individual components and for the
mooring system are being used in the analysis. The performance models include
models describing (1)the time-varying loads on individual mooring lines
and foundations during hurricane and loop current events, and (2) the time-varying
capacities of the mooring lines and foundations. Existing models that represent
the current state of practice are generally being adopted, although some
modifications have been necessary to suit the purposes of this study.
3. Uncertainties in the performance models, including those due to (1)
temporal variations in loads, (2) spatial variations in seafloor properties,
and (3) limited data sets to calibrate the load and capacity models are
being identified and quantified.
4. These performance models are being used to assess and compare the component
and system reliabilities and develop measures of system performance, such
as robustness and redundancy. These measures will also be compared to those
for conventional steel jackets for reference..
DEPLOYMENT
OF RESULTS: Results will be documented in a final report and
published in journals and conference proceedings. Results should provide
helpful reference material for industry to use in evaluating and possibly
improving existing design codes.
ANTICIPATED
PROJECT DURATION: 3 years
PROJECT
PLAN FOR YEAR 3 (2004-2005):
Previous
Results: In the first year, we adapted a hydrodynamic model
to calculate time-varying mooring line loads on a Spar, and identified
and
modeled Spar designs for 3000-ft water depth with wire rope moorings
and 6000-ft and 10000-ft water depths with polyester moorings. The
model compared
favorably to model tests for hurricane and loop current environments,
and was used to evaluate the distribution of maximum lifetime loads.
We also
developed a model for the foundation pullout under time-varying load.
In the second year, we analyzed the reliability for components in the
system (lines and foundations) and for the system itself considering hurricane
events for the 3000-ft water depth. The hydrodynamic model was modified
to analyze performance when a foundation fails (the model accounts for
the residual capacity in the line due to the weight of the anchor) versus
when a line breaks (the model assumes no residual capacity). Results are
presented in a de-coupled format so that the response of the mooring system
is expressed in terms of metocean variables (wave heights, wave periods,
and current and wind profiles), hydrodynamic variables that affect the
loads (drag coefficients, added mass coefficients and VIV coefficients),
and design variables that affect the system capacity (mooring line strength,
connection strength, and foundation capacity). This allows the reliability
to be easily assessed and results to be extended to different metocean
conditions and different design capacities (such as different factors of
safety in Working Stress Design).
Scope
of Work In the third year, the study will focus on finalizing the
reliability analyses through:
• including loop current design events
• extending the analyses to the 6000 and 10000-ft water depths
• conducting sensitivity studies
• engaging an industry review team to transfer the results to practice
Anticipated Results Expected results include:
• comparison of individual component reliabilities (e.g., line breakage
versus foundation failure)
• comparison of the system reliability to reliabilities of the weakest
component (redundancy)
• the impact on system and component reliabilities due to
o
water depth
o mooring line configuration
o metocean conditions
o factors of safety
Results will be documented in a final report, and should provide
helpful reference material for industry to use in evaluating
and possibly
improving existing design codes.
PRINCIPAL
INVESTIGATORS & OTHERS INVOLVED
IN PROJECT:
PI(s): Jun Zhang and Bob Gilbert
OTRC PROJECT STATUS REPORT
Date: December 2005
Project
Name: Reliability of Mooring Systems for Floating Production
Systems
MMS
Project: 423 TO Numbers: 18203/35990
PI: Jun
Zhang and Bob Gilbert
COTR: S.
Buffington
Estimated
Completion Date: September, 2005
Project Description:
The performance of the floating production
system depends more directly on the performance of the system of mooring
lines and anchors rather than
on the performance of a single line or anchor. To assess and study the
component and system reliabilities for the mooring system, a “theme” Spar
and its mooring system is used for the reliability analysis. The theme
has three variations: a semi-taut, steel wire mooring system in 3,000 feet
of water; a taut polyester mooring system in 6,000 feet of water; and a
taut polyester mooring system in 10,000 feet of water. The results of this
project may be used for the evaluation and possible improvement of the
design codes for FPS mooring lines and anchors.
Progress:
1. The reliability analysis for the theme spar under hurricane loading
conditions was completed for all three water depths. Both component and
system reliabilities were addressed in this analysis. The major conclusions
based on this work thus far are (1) mooring systems for permanent productions
systems have a high level of reliability relative to generally accepted
industry targets; (2) the reliability for the taut systems is higher than
that for the semi-taut system due to the relatively small contribution
of environmental loading versus pre-tension for the taut systems; and (3)
reliability levels are not necessarily consistent among components with
the lines and chains being more likely to fail than the anchors.
2. A workshop was held with industry on February 15, 2005. The purpose
of the workshop was to present results to date and to solicit industry
feedback on the work. The workshop was well attended with more than 20
participants and the feedback was positive.
3. Papers were written for the 2005 OMAE conference and the 2005 International
Symposium on Frontiers in Offshore Geotechnics.
4. The response and reliability analyses for the loop current loading conditions
has been completed, and the reliabilities associated with hurricanes and
loop currents have been compared.
The draft of the final report has been completeed and sent to MMS for
review.
Reports & Publications:
Gilbert, R. B., Choi, Y. J., Dangyach,
S. and Najjar, S. S. (2005), “Reliability-Based
Design Considerations for Deepwater Mooring System Foundations,” Proceedings,
Frontiers in Offshore Geotechnics, Perth, Western Australia, in press.
Yu Ding, B. Theckum purath, Jun Zhang,
R. B. Gilbert, S. Dangyach and Y.J. Choi (2005), “Reliability of Mooring Systems for A Spar”,
Proceedings of OMAE 2005, paper No. 67290.
Gilbert, R. B., Choi, Y. J. and Dangyach,
S. (2004), “Reliability
Analysis for Spar Foundations – Component and System Effects,” Draft
OTRC Report.
Dangyach, S. (2004), “Reliability Analysis for FPS Mooring Systems,” M.S.
Thesis, Department of Civil Engineering, The University of Texas at Austin.
VanShaar, S. (2002), “Dynamic Analysis for Suction Caissons and
Geologic Model for Makassar Strait,” M.S. Thesis, Department of Civil
Engineering, The University of Texas at Austin.
Yu Ding, Minsuk Kim, Xiaohong Chen and
Jun Zhang (2003) ‘A Numerical
Code (COUPLE6D) for Coupled Dynamic Analysis of Moored Offshore Structures’,
Proceedings of International symposium on deep-water mooring system, Houston,
TX, 2003, p168-182.
Return to top
OTRC PROJECT STATUS REPORT
Date: May, 5, 2005
Project
Name: Reliability of Mooring Systems for Floating Production Systems
MMS
Project: 423 TO Numbers: 18203/35990
PI: Jun Zhang and Bob Gilbert
COTR: A. Konczvald
Estimated
Completion Date: September, 2005
Project Description:
The
performance of the floating production system depends more directly on
the performance of the system of mooring
lines and anchors rather than
on the performance of a single line or anchor. To assess and study the
component and system reliabilities for the mooring system, a “theme” Spar
and its mooring system is used for the reliability analysis. The theme
has three variations: a semi-taut, steel wire mooring system in 3,000 feet
of water; a taut polyester mooring system in 6,000 feet of water; and a
taut polyester mooring system in 10,000 feet of water. The results of this
project may be used for the evaluation and possible improvement of the
design codes for FPS mooring lines and anchors.
Progress:
During the past 6 months, the following progress has been made:
- The reliability analysis for the theme spar under hurricane loading
conditions was completed for all three water depths. Both component and
system reliabilities were addressed in this analysis. The major conclusions
based on this work thus far are (1) mooring systems for permanent productions
systems have a high level of reliability relative to generally accepted
industry targets; (2) the reliability for the taut systems is higher than
that for the semi-taut system due to the relatively small contribution
of environmental loading versus pre-tension for the taut systems; and (3)
reliability levels are not necessarily consistent among components with
the lines and chains being more likely to fail than the anchors.
- A workshop was held with industry on February 15, 2005. The purpose
of the workshop was to present results to date and to solicit industry
feedback on the work. The workshop was well attended with more than 20
industry participants and the feedback was positive.
- Papers were written for the 2005 OMAE conference and the 2005 International
Symposium on Frontiers in Offshore Geotechnics.
- The reliability analysis for the loop current loading conditions was
formulated and a series of simulations with the hydrodynamic model are
underway to complete this analysis.
Reports & Publications:
Gilbert,
R. B., Choi, Y. J., Dangyach, S. and Najjar, S. S. (2005), “Reliability-Based
Design Considerations for Deepwater Mooring System Foundations,” Proceedings,
Frontiers in Offshore Geotechnics, Perth, Western Australia, in press.
Yu
Ding, B. Theckum purath, Jun Zhang, R. B. Gilbert, S. Dangyach and Y.J.
Choi (2005), “Reliability of Mooring Systems for A Spar”,
Proceedings of OMAE 2005, paper No. 67290.
Gilbert,
R. B., Choi, Y. J. and Dangyach, S. (2004), “Reliability
Analysis for Spar Foundations – Component and System Effects,” Draft
OTRC Report.
Dangyach,
S. (2004), “Reliability Analysis for FPS Mooring Systems,” M.S.
Thesis, Department of Civil Engineering, The University of Texas at Austin.
VanShaar,
S. (2002), “Dynamic Analysis for Suction Caissons and
Geologic Model for Makassar Strait,” M.S. Thesis, Department of Civil
Engineering, The University of Texas at Austin.
Yu
Ding, Minsuk Kim, Xiaohong Chen and Jun Zhang (2003) ‘A Numerical
Code (COUPLE6D) for Coupled Dynamic Analysis of Moored Offshore Structures’,
Proceedings of International symposium on deep-water mooring system, Houston,
TX, 2003, p168-182.
Return
to top
OTRC PROJECT STATUS REPORT
Date: December 2004
Project
Title: Reliability of Mooring Systems for Floating Production
Systems
MMS
Project: 423 TO Numbers: 18203/35990
PI: Jun Zhang and Bob Gilbert
COTR: A. Konczvald
Estimated
Completion Date: September, 2005
Project Description:
The
performance of the floating production system depends more directly
on the performance of the system of mooring
lines and anchors rather
than on the performance of a single line or anchor. To assess and study
the component and system reliabilities for the mooring system, a “theme” Spar
and its mooring system is used for the reliability analysis. The results
of this project may be used for the evaluation and possible improvement
of the design codes for FPS mooring lines and anchors.
Progress:
1) Time varying tensions and tangents of mooring lines of the ‘theme’ Spar
were computed as a function of the significant wave height (Hs) and peak
spectral period (Tp) describing the metocean conditions during a storm.
Based on an analysis of the time-record for each combination of Hs and
Tp, parameters describing an extreme value distribution for the maximum
tension load in a 3-hour storm event have been determined. This information
is then combined with a probabilistic description of the metocean conditions
to evaluate the reliability of the individual mooring lines and foundations.
2) A model was developed to predict the pull-out capacity for a suction
caisson as a function of pull-out distance. This model integrates the
acceleration of the caisson under a time-varying load to determine velocities
and displacements and is based on bounding cases for the soil behavior.
It was calibrated with model test data from a related OTRC project (Rauch
and Olson).
3) A component reliability analysis has been conducted to evaluate and
compare the annual and lifetime reliability of individual lines and foundations
during a storm event. One significant conclusion is that the foundation
is generally less likely to fail than the line itself, due to the use
of higher safety factors in conventional foundation design practice and
due to the presence of a lower bound on the foundation capacity. The
lower bound capacity, which corresponds approximately to the remolded
strength of the soil, has a significant effect on the reliability and
can be verified on a project-specific basis when the caissons are installed.
4) A methodology has been developed to efficiently perform a system reliability
analysis in order to evaluate the level of redundancy in the station-keeping
system under storm loading conditions. Preliminary results indicate that
the probability of system (at least two lines) failure is less than half
that for a component, depending on the cause of a damage to the first
line.
5) The mooring lines in 3,000 ft water case are the traditional combination
of steel chain-wire-chain, while those in 6,000 and 10,000 ft water cases
are the integration of steel chain-polyester rope-steel chain. The numerical
simulation of a polyester mooring system is conducted using extended
CABLE3D based on the large elongation formulation. It is shown that the
restraining force of the mooring system in 10,000 ft water is reduced
by 16~20 % when the elongation of polyester ropes is accounted.
Work continues to focus on studying how these results vary with the design
factors of safety, water depth, and loading conditions (extreme wave
versus extreme current).
Reports & Publications:
Dangyach,
S. (2004), “Reliability Analysis for Spar Mooring Systems,” M.S.
Thesis, Department of Civil Engineering, The University of Texas at Austin,
93 pp.
Gilbert,
R. B., Choi, Y. J., Dangyach, S. and Zhang, J. (2005), “Reliability-Based
Design Considerations for Deepwater Mooring System Foundations,” Proceedings,
Frontiers in Offshore Geotechnics, Perth, Western Australia.
Yu
Ding, Minsuk Kim, Xiaohong Chen and Jun Zhang (2003) ‘A Numerical
Code (COUPLE6D) for Coupled Dynamic Analysis of Moored Offshore Structures’,
Proceedings of International symposium on deep-water mooring system,
Houston, TX, 2003, p168-182.
Return
to top
OTRC
PROJECT STATUS REPORT
Date: June 2004
Project
Name: Reliability of Mooring Systems for Floating Production
Systems
Project
Number: 423 Task Order: 18023
Principal
Investigators: Jun Zhang (A&M) and Bob Gilbert (UT)
Estimated
Completion Date: September, 2005
Project
Description:
The
performance of the floating production system depends more directly on
the performance of the system of mooring lines and anchors rather than
on the performance of a single line or anchor. To assess and study the
component and system reliabilities for the mooring system, a Spar and
its mooring system is used for the reliability analysis. The results
of this project may be used for the evaluation and possible improvement
of the design codes for FPS mooring lines and anchors.
Progress:
1)
The Spar and mooring system being used for the reliability analyses in
this project was adapted from a Spar and mooring system design that has
been studied and model tested in DeepStar projects.
2) A state-of-art numerical code capable of simulating the dynamic response
of a floating structure and its mooring/tendon/riser system in the time domain,
COUPLE6D, is being used to calculate the (6 degrees of freedom) motions of
the Spar and tensions in its mooring lines at their fairleads. The predicted
results agree satisfactorily with the model test data. Time varying tensions
and tangents of mooring lines are being computed as a function of the significant
wave height (Hs) and peak spectral period (Tp) describing the metocean conditions
during a storm.
3) Based on analyses of the time-record for each combination of Hs and Tp,
parameters describing an extreme value distribution for the maximum tension
load in a 3-hour storm event have been determined. This information is then
combined with a probabilistic description of the metocean conditions to evaluate
the reliability of the individual mooring lines and foundation elements.
4) A model was developed to predict the pull-out capacity for a suction caisson
as a function of pull-out distance. This model integrates the acceleration
of the caisson under a time-varying load to determine velocities and displacements
and is based on bounding cases for the soil behavior. It was calibrated with
model test data from a related OTRC project (Rauch and Olson).
5) A component reliability analysis has been conducted to evaluate and compare
the annual and lifetime reliability of individual lines and foundations during
a storm event. One significant conclusion is that the foundation is generally
less likely to fail than the line itself, due to the use of higher safety factors
in conventional foundation design practice and due to the presence of a lower
bound on the foundation capacity. The lower bound capacity, which corresponds
approximately to the remolded strength of the soil, has a significant effect
on the reliability and can be verified on a project-specific basis when the
caissons are installed.
6) A methodology has been developed to efficiently perform a system reliability
analysis in order to evaluate the level of redundancy in the station-keeping
system under storm loading conditions. Preliminary results indicate that the
probability of system (at least two lines) failure is about an order of magnitude
smaller than that for a component, depending on the cause of damage to the
first line.
7) A similar approach is being developed to describe the line loads as a function
of the current speed and the profile of current with depth for a loop current
event.
Work
during the next 6 months will focus on studying how these results vary
with the design factors of safety, water depth, and loading conditions
(extreme wave versus extreme current).
Reports & Publications:
Gilbert,
R. B., Choi, Y. J. and Dangyach, S. (2004), “Reliability Analysis
for Spar Foundations – Component and System Effects,” Draft
OTRC Report.
Yu
Ding, Minsuk Kim, Xiaohong Chen and Jun Zhang (2003) ‘A Numerical
Code (COUPLE6D) for Coupled Dynamic Analysis of Moored Offshore Structures’,
Proceedings of International symposium on deep-water mooring system,
Houston, TX, 2003, p168-182.
Return
to top
OTRC
PROJECT STATUS REPORT
Date: December 2003
Project
Name: Reliability of Mooring Systems for Floating Production
Systems
TEES
Project Number: 32558-5888S MMS Task Order: 18023 MMS
Project Number: 423
Principal
Investigators: Jun Zhang (A&M) and Bob Gilbert (UT)
Estimated
Completion Date: August, 2004
Project
Description:
The
performance of the floating production system depends more directly on
the performance of the system of mooring lines and anchors rather than
on the performance of a single line or anchor. To assess and study the
component and system reliabilities for the mooring system, a “theme” Spar
and its mooring system is used for the reliability analysis. The results
of this project may be used for the evaluation and possible improvement
of the design codes for FPS mooring lines and anchors.
Progress:
To
study the motions of a Spar and tensions in its mooring lines when one
of its anchor caisson was pulled out, the numerical scheme for computing
the dynamics of mooring lines has been extended to allow for the tension
at the anchor end to be described as a function of the velocity of the
caisson movement, weight of the caisson and the soil behavior.
Transverse forces induced by currents have been included based upon a modified
Morrison equation and calibrated lift force coefficient and Strouhal number.
A component reliability analysis has been conducted to evaluate and compare
the annual and lifetime reliability of individual lines and foundations during
a storm event. One significant conclusion is that the foundation is generally
less likely to fail than the line itself, due to the use of higher safety factors
in conventional foundation design practice and due to the presence of a lower
bound on the foundation capacity. The lower bound capacity, which corresponds
approximately to the remolded strength of the soil, has a significant effect
on the reliability and can be verified on a project-specific basis when the
caissons are installed.
A methodology has been developed to efficiently perform a system reliability
analysis in order to evaluate the level of redundancy in the station-keeping
system under storm loading conditions. Preliminary results indicate that the
probability of system (at least two lines) failure is about an order of magnitude
smaller than that for a component.
Work
during the next 6 months will focus on studying how these results vary
with the design factors of safety and the water depth.
Reports & Publications:
Yu
Ding, Minsuk Kim, Xiaohong Chen and Jun Zhang (2003) ‘A Numerical
Code (COUPLE6D) for Coupled Dynamic Analysis of Moored Offshore Structures’,
Proceedings of International symposium on deep-water mooring system,
Houston, TX, 2003, p168-182.
Return
to top
OTRC
PROJECT STATUS REPORT
Date: June,
2003
Project
Name: Reliability of Mooring Systems for Floating Production
Systems
Project
Number: 32558-5888S Task Order: 18023
Principal
Investigators: Jun Zhang (A&M) and Bob Gilbert (UT)
Estimated
Completion Date: 8/31/03
Project
Description:
The
performance of the floating production system depends more directly on
the performance of the system of mooring lines and anchors rather than
on the performance of a single line or anchor. To assess and study the
component and system reliabilities for the mooring system, a “theme” Spar
and its mooring system is used for the reliability analysis. The results
of this project may be used for the evaluation and possible improvement
of the design codes for FPS mooring lines and anchors.
Progress:
1)
A Spar tested extensively in MARIN’s wave basin has been chosen as the ‘theme’ spar
and its mooring system consisting of 14 individual mooring lines is adapted
as an example for the reliability analysis.
2) COUPLE, a state-of-art numerical code capable of simulating a floating structure
dynamically interacting its mooring/tendon/riser system in the time domain,
was used to calculate the (6 degrees of freedom) motions of the ‘theme’ Spar
and the tensions of its mooring lines at their fairleads. In two extreme cases,
a 100-year hurricane storm and Loop currents, the numerical results are found
to be in satisfactory agreement with the corresponding laboratory measurements.
3) Time varying tensions and tangents were computed for the theme spar as a
function of the significant wave height (Hs) and peak spectral period (Tp)
describing the metocean conditions during a storm. Based on an analysis of
the time-record for each combination of Hs and Tp, parameters describing an
extreme value distribution for the maximum tension load in a 3-hour storm event
have been determined. This information can then be combined with a probabilistic
description of the metocean conditions to evaluate the reliability of the individual
mooring lines and foundations. A similar approach is underway to describe the
line loads as a function of the current speed and the profile of current with
depth for a loop current event.
4) A model was developed to predict the pull-out capacity for a suction caisson
as a function of pull-out distance. This model integrates the acceleration
of the caisson under a time-varying load to determine velocities and displacements
and is based on bounding cases for the soil behavior. It was calibrated with
model test data from a related OTRC project (Rauch and Olson).
5) A preliminary reliability analysis has been conducted to evaluate and compare
the annual and lifetime reliability of individual lines and foundations during
a storm event. Initial results indicate that there is more design conservatism
in the foundation element than in the mooring line, and that a line failure
is likely to have more impact on overall integrity or reliability or capacity
of the mooring/foundation system than the failure (pullout) of a foundation
element.
Reports & Publications:
VanShaar,
S. (2002), “Dynamic Analysis for Suction Caissons and Geologic Model
for Makassar Strait,” M.S. Thesis, Department of Civil Engineering, The
University of Texas at Austin.
Two
conference papers respectively regarding the reliability analysis of
mooring lines and the whole mooring-anchor system will be submitted this
summer.
Return
to top
OTRC
PROJECT STATUS REPORT
Date: Nov
15, 2002.
Project Name: Reliability
of Mooring Systems for Floating Production Systems
Project Number: 32558-5888S
Task Order: 18023
Principal Investigators: Jun
Zhang (A&M) and Bob Gilbert (UT)
Estimated Completion
Date: Aug. 31, 2003
Project Description:
The
performance of the floating production system depends more directly on
the performance of the system of mooring lines and anchors rather than
on the performance of a single line or anchor. To assess and study the
component and system reliabilities for the mooring system, a “theme” Spar
and its mooring system is used for the reliability analysis. The results
of this project may be used for the evaluation and possible improvement
of the design codes for FPS mooring lines and anchors.
Progress:
1)
A Spar tested extensively in MARIN’s 3D wave basin has been chosen as
the ‘theme’ spar and its mooring system consisting of 16 individual mooring
lines is adapted as an example for the reliability analysis.
2) COUPLE, a state-of-art numerical code capable of simulating a floating structure
dynamically interacting its mooring/tendon/riser system in the time domain,
is used to calculate the (6 degrees of freedom) motions of the ‘theme’ Spar
and the tensions of its mooring lines at their fairleads. In two extreme cases,
a 100-year hurricane storm and Loop currents, the numerical results are found
to be in satisfactory agreement with the corresponding laboratory measurements.
3) Having reached satisfactory agreement between the measurements and simulations,
we then computed the tensions and tangents of all 16 mooring lines as a function
of time at their connections with the anchor, which are used for the reliability
analysis of caisson anchor and the whole mooring system.
4) The probability density functions for extreme tension of each individual
mooring line were computed respectively for twelve different sea states and
two different scenarios of the mooring system: an intact system and a system
with one mooring line broken. Related results are being used to compute the
failure probability of the Spar’s mooring/foundation system in a period of
25-year service.
5) A model was developed to predict the pull-out capacity for a suction caisson
as a function of pull-out distance. This model integrates the acceleration
of the caisson under a time-varying load to determine velocities and displacements
and is based on bounding cases for the soil behavior. It was calibrated with
model test data from a related OTRC project (Rauch and Olson).
6) Simulated time-histories of the line load from Step 4) were input to the
model for suction caisson capacity to investigate the loading characteristics
(such as magnitude, number, and durations of peaks) required to fail the suction
caisson (defined as complete pull-out).
Reports & Publications:
VanShaar,
S. (2002), “Dynamic Analysis for Suction Caissons and Geologic Model
for Makassar Strait,” M.S. Thesis, Department of Civil Engineering, The
University of Texas at Austin.
Two
conference papers respectively regarding the reliability analysis of
mooring lines and the whole mooring-anchor system will be submitted next
year
Return
to top
OTRC
PROJECT STATUS REPORT
Date: July
2002.
Project
Name: Reliability of Mooring Systems for Floating Production
Systems
Project
Number: TEES 32558/5888S & MMS 18203
Principal
Investigators: Jun Zhang (TAMU) and Bob Gilbert (UT Austin)
Estimated
Completion Date: (Starting Sept. 1,2001) Aug. 31, 2003
Project
Description:
The
reliability of mooring systems for floating production systems is often
characterized by studying the reliabilities of the mooring components
and foundation components separately. This project is investigating the
overall reliability of the mooring and foundation system. A “theme” Spar
and its mooring and foundation components are being used to study and
assess both the system and component reliabilities. The results of this
project may be used for the evaluation and possible improvement of the
design codes for FPS mooring lines and anchors.
Progress:
1)
A Spar tested extensively in MARIN’s 3D wave basin by has been chosen
as the ‘theme’ spar and its mooring system consisting of 16 individual
mooring lines is adapted as an example for the reliability analysis.
2) COUPLE, a state-of-art numerical code capable of simulating a floating structure
dynamically interacting its mooring/tendon/riser system in the time domain,
is used to calculate the (6 degrees of freedom) motions of the ‘theme’ Spar
and the tensions of its mooring lines at their fairleads. In two extreme cases,
a 100-year hurricane storm and Loop currents, the numerical results are found
to be in satisfactory agreement with the corresponding laboratory measurements.
3) With the satisfactory agreement between the measurements and simulations
in mind, we then computed the tensions and tangents of all 16 mooring lines
as a function of time at their connections with the anchor, which will be used
for the reliability analysis of caisson anchor and the whole mooring system.
The probability density functions respectively for extreme tension and the
fatigue loads of each individual mooring line in 25-year service period are
being computed.
4) A model to predict suction caisson pullout versus time during a storm or
current event has been developed, and is being applied to the load histories
for the hurricane and Loop Current.
Reports & Publications:
Van
Shaar, S. (2002), “Investigation of Effects of Temporal Variations in
Loads and Spatial Variations in Soil Properties on Suction Caisson Performance”,
M.S. Thesis, Department of Civil Engineering, The University of Texas
at Austin, to be completed August, 2002.
Two
conference papers respectively regarding the reliability analysis of
mooring lines and the whole mooring-anchor system will be submitted next
year.
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OTRC
PROJECT STATUS REPORT
Date: January
30, 2002
Project
Name: Reliability of Mooring Systems for Floating Production
Systems
Task
Order: 18203 Project Number: 5888S
Principal
Investigators: Jun Zhang (TAMU) and R.M Gilbert (U Texas)
Estimated
Completion Date: August 2002
Project
Description:
The
performance of the floating production system depends more directly on
the performance of the system of mooring lines and foundation anchors
rather than on the performance of a single line or anchor. This study
is assessing the component and system reliabilities for the combined
mooring-foundation of system deepwater floating production systems. The
potential impact could lead to the evaluation and possible improvement
of the design codes for FPS mooring lines and anchors.
Progress:
1)
A Spar that is representative of those being used today in the Gulf of
Mexico has been selected for the study.
2) This problem in multi-disciplinary. Ongoing discussions between PI’s are
seeking to establish a mutual understanding of the two different disciplines – fluid-structure
and structure-foundation responses.
3) The mooring line forces applied to the caisson anchor are being computed.
Model-test data for a Spar has been obtained to calibrate model of line loading
versus time.
4) Developed model to predict anchor pullout versus time during a storm or
Loop Current event.
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