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DAY 2 TECHNICAL PROGRAMME
Sequential Downtime Analysis as a Tool for
Planning Operations in Harsh Weather Locations
Ed Ballard, Orwell Offshore Ltd
PRESENTATION OVERVIEW
• Introduction to concept and motivation for using Sequential Downtime Analysis
(SDA) and comparison with conventional methods
- Example Case
-
Definition of Task Sequence (installation at field in North Sea)
Metocean data used
Results of analyses
Implications for planning of operations
- Conclusions
3
SEQUENTIAL DOWNTIME ANALYSIS
• Estimation of the likely duration of offshore operations is of interest for the
planning, costing and specification of the equipment to be used.
4
SEQUENTIAL DOWNTIME ANALYSIS
• Conventionally, expected operability has been assesses using metocean data in the
form of:
– Scatter diagrams, exceedance curves
5
SEQUENTIAL DOWNTIME ANALYSIS
• Conventionally, expected operability has been assesses using metocean data in the
form of:
– Scatter diagrams, exceedance curves
– Monthly Means
– Non-exceedance persistence tables
6
SEQUENTIAL DOWNTIME ANALYSIS
- Conventional methods cannot account for:
- Fluctuations in metocean conditions over short intervals.
- Cumulative effect of delays associated with individual tasks.
- SDA particularly suitable for:
- Lifting operations, for which vessel motions are critical.
- Operations involving personnel and/or cargo transfer.
-
Based on Monte-Carlo Simulations
- Large number of simulations performed, accounting for variation in
duration of multiple tasks in addition to metocean variation.
- Method allows the probability distribution for the duration of a
complex job sequence accounting for likely metocean variation to be
determined.
7
DEFINITION OF TASK SEQUENCE - DURATIONS
• Each task in the job sequence defined in terms of:
– Minimum duration
– Likely delay (duration variation) on top of minimum duration
Task
Number
1
2
3
4
5
6
7
Task
Description
MWA-GB Install
Reset
Riser1 Install
Reset
Riser2 Install
Reset
Riser3 Install
Average Duration
Duration Variation
36
6
6
1
24
3
6
1
24
3
6
1
24
3
8
DEFINITION OF TASK SEQUENCE - DURATIONS
• Each task in the job sequence defined in terms of:
– Minimum duration
– Likely delay (duration variation) on top of minimum duration
– Large number of task durations conforming to selected statistical description randomly
generated.
Task
Number
1
2
3
4
5
6
7
Task
Description
MWA-GB Install
Reset
Riser1 Install
Reset
Riser2 Install
Reset
Riser3 Install
Average Duration
Duration Variation
36
6
6
1
24
3
6
1
24
3
6
1
24
3
9
DEFINITION OF TASK SEQUENCE - DURATIONS
• Individual random length tasks combined to form a large
number of job sequences for use in Monte Carlo simulations.
Task
Number
1
2
3
4
5
6
7
Task
Description
MWA-GB Install
Reset
Riser1 Install
Reset
Riser2 Install
Reset
Riser3 Install
Delays
Zero Delay
50th Percentile (P50)
95th Percentile (P95)
Average Duration
Duration Variation
36
6
6
1
24
3
6
1
24
3
6
1
24
3
Duration
Hours
Days
126.0
5.25
139.9
5.83
149.2
6.21
• No metocean effects yet considered.
10
DEFINITION OF TASK SEQUENCE – LIMITING METOCEAN CRITERIA
• The complete definition of the tasks making up job sequence requires the setting
of limiting metocean criteria:
Hs [m]
– Limiting sea-states for operations from floating structures will have dependence on wave period.
– Define Wave Height vs Peak Period contour using detailed analyses.
– Wind and current limits constant values.
3.0 -3.5
FAIL FAIL FAIL FAIL FAIL FAIL FAIL
2.5 - 3.0
FAIL FAIL FAIL FAIL FAIL FAIL FAIL FAIL
2.0 - 2.5
FAIL FAIL FAIL FAIL PASS PASS PASS PASS
1.5 - 2.0
FAIL FAIL FAIL PASS PASS PASS PASS PASS PASS
1.0 - 1.5
PASS PASS PASS PASS PASS PASS PASS PASS PASS
0.5 - 1.0
PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS
0.0 - 0.5 PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS
3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 6.0-6.5 6.5-7.0 7.0-7.5 7.5-8.0 8.0-8.5 8.5-9.0
Tp [s]
11
METOCEAN DATA
• Hindcast or recorded time histories of metocean data are required.
• Multiple sources, some public (NOAA WaveWatch III)
12
MONTE CARLO SIMULATIONS
• Monte Carlo simulations are performed using a large number of realisations of the
job sequence.
• For each step in the metocean time series, all job sequence realisations are run.
• The results of the multiple simulations allow the expected duration of the job
sequence if started on a particular date to be determined.
13
MONTE CARLO SIMULATIONS
• Durations to complete for all days in 2005.
14
TIMING OF OPERATIONS
• The results of the simulations can be used to determine likely durations of job
sequence for all months.
• This can allow selection of optimum month for the job if there is flexibility in its
scheduling.
15
SELECTION OF EQUIPMENT USED FOR OPERATIONS
Hs [m]
• If the operation must be performed at a particular time of the year, the results of
an SDA analysis can be used to optimise the equipment used.
• For instance, an alternative vessel could be chartered, which increases limiting Hs
for critical lifts by 0.5m over full Hs/Tp contour.
3.0 -3.5
FAIL FAIL FAIL FAIL FAIL FAIL FAIL
2.5 - 3.0
FAIL FAIL FAIL FAIL FAIL FAIL FAIL FAIL
2.0 - 2.5
FAIL FAIL FAIL FAIL PASS PASS PASS PASS
1.5 - 2.0
FAIL FAIL FAIL PASS PASS PASS PASS PASS PASS
1.0 - 1.5
PASS PASS PASS PASS PASS PASS PASS PASS PASS
0.5 - 1.0
PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS
0.0 - 0.5 PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS PASS
3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 6.0-6.5 6.5-7.0 7.0-7.5 7.5-8.0 8.0-8.5 8.5-9.0
Tp [s]
16
SELECTION OF EQUIPMENT USED FOR OPERATIONS
• Effect of increasing the limiting Hs contour can be significant.
17
SELECTION OF EQUIPMENT USED FOR OPERATIONS
• SDA analysis results allow assessment to be made of comparative costs.
• For example, it can be used to determine whether it is better financially to charter
the more capable vessel.
Number
Required
Installation Vessel
1
Barge
1
Tug
2
Total Cost Per Day
Vessel
Cost Per Day [USD]
Original Spread
Revised Spread
150 000
200 000
5 000
5 000
25 000
25 000
205 000
255 000
18
SELECTION OF EQUIPMENT USED FOR OPERATIONS
• August – Likely worst cost for original spread almost same as
revised spread median cost.
• October – Worst case for revised spread only slightly higher
than original spread.
19
SUMMARY AND CONCLUSIONS
• SDA method has been used to demonstrate how the duration
of a job sequence made up of a series of tasks can be
estimated.
• The estimated duration accounts for:
– Delays associated with individual tasks
– Delays associated with exceedance of metocean limits
• The results of the SDA analyses presented have been used to
demonstrate how it can be used as planning tool for either:
– Selection of the optimum time of year for particular equipment;
– Selection of optimum equipment for installation at a particular time of
year.
20
Thank You
21

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