12 - KTF 2008 DNV Alpha Factor
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Transcript of 12 - KTF 2008 DNV Alpha Factor
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DNV -FactorKTF - Subsea Lifting Operations, Stavanger 2008
Per . Alvr2008-12-03
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Version Slide 203 December 2008
Content
What is new?- History- DNV-RP-H102- Update
background- Comparing
Application- Guidance- Example Case
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Version Slide 303 December 2008
- Factor Currently in the RulesPurpose: Take into account uncertainty in the WFBasis: DNMI report DSO265/Lund-95/15325 (4/5-95)
Rule requirement given by:Table that shows as a function of TR and (design) Hs (TR = TPOP + TC)
OR
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Version Slide 403 December 2008
DNV-RP-H102 - - Factor This is an Alpha factor intended used for REMOVAL of offshore platforms. However, it has since 2004 been used for many installation projects as well. The differences are:
Based on TPOP and not TR 10% increase allowed for weather
forecast level A
10% increase allowed for a reliablemonitoring system
Alpha factors for < 12 hours split in two groups; 0-6 hours and 6-12 hours
In addition a somewhat reduced criteria for (un-restricted) operations between 72 hours and 120 hours are possible.
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Version Slide 503 December 2008
- Factor JIP
Alpha factor study from 1995 based on data from only two locations and one forecast provider. The update is based on more locations and 3 providers:
General impression that quality of forecasting services increased
Monitoring possibilities/techniques improved
The alpha factor is important for safety and cost of offshore operations It should be as reliable as possible!
Main reasons for update required:
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Version Slide 603 December 2008
New - Factors Alpha factors based on the new study shown in the table below Further post-processing of the data required Special considerations for forecast periods > 72 hrs
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Version Slide 703 December 2008
Comparing - Factors
Compared with:
Rules for Planning and Execution of Marine Operations
Reference Period Factor WF level A Monitoring
DNV-RP-H102 TPOP Equal +10% +10%
Rule Update T +5% +5%
+ for short operations
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Version Slide 803 December 2008
Content
What is new?- History- DNV-RP-H102- Update
background- Comparing
Application- Guidance- Example Case
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Version Slide 903 December 2008
Step 1 Define OPLIM
The OPLIM (design criteria, CD) shall never be taken greater than the minimum of:- The environmental design criteria.- Maximum wind and waves for safe working- or
transfer conditions for personnel.- Equipment (e.g. ROV and cranes) specified weather
restrictions. - Limiting weather conditions of diving system (if any).- Any limitations identified, e.g. in HAZID/HAZOP,
based on operational experience with involved vessel(s) etc.
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Version Slide 1003 December 2008
Step 2 Define the Operation Period
Marine operations with a reference period (TR) less than 72 hours may be defined as weather restricted.
TR is in the VMO-Rules used both to define the required weather window and as basis for selection of the alpha factor.
TR = TPOP + TC- TR - Operation reference period- TPOP - Planned operation period- TC - Estimated maximum contingency time.
The operation shall only be considered completed when the object is in a safe condition.
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Version Slide 1103 December 2008
Step 2 Operation Period (Slide 2)
Defined start of operation in relation to the issuance of weather forecast not clearly defines neither in the VMO-Rules nor in DNV-RP-H102.
If TPOP is used as input for selecting the Alpha factor TPOPshall be defined from the issuance of the last weather forecast.
Also TPOP (-factor) for operations that could be interrupted should be defined as the time between weather forecasts(e.g. normally 12 hours) + the planned time required to bring the object into a safe condition.
In order to clarify a new expression T will be introduced in the Rule update.
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Version Slide 1203 December 2008
Step 3 Find and CO The defined TR and the design wave heigth, CD, (OPLIM) are
used as input to find the Alpha factor in Table 3.1 (Pt.1 Ch.2 in VMO-Rules)
The operational criteria - CO - is in the VMO Rules defined as CO = x CD.
In DNV-RP-H102 the forecasted (monitored) operational criteria - CO - is defined as CO = x OPLIM.
An operation could be divided in sub operations for which different and CD (OPLIM) could be defined.
Updated Rules will use the same terminology as in DNV-RP-H102 for this.
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Version Slide 1303 December 2008
Application - Example Case
Description
In the North Sea a sub sea structure is transported by a barge and will be lifted of the barge and lowered to the bottom by a crane vessel. The operation consists of the sub-operations shown (next slide) with planned time periods and design Hs (defined by design calculations or maximum safe sea condition for personnel working according to the Offshore manager) in the below table. It is not considered acceptable to reverse the operation after the cutting of seafastening has commenced.
The last weather forecast is issued 0600 hours by two independent weather forecasters. (Level A weather forecast may be assumed). 0800 hours the barge is in position and transfer of personnel to the barge commences. No special evaluation of the required contingency time has been done, but the operation is considered as rather standard. A contingency time (TC) equal to TPOP could hence be assumed.
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Version Slide 1403 December 2008
Example Case Schedule
Sub-operation Time (TPOP) Hs Reason
1 Transfer personnel to barge, cutting of seafastening and prepare for lift 7 hours 2.5 mPersonnel limitation
2 Lift-off & structure through splash zone. 1 hours 2.0 m Calculations
3 Lower structure to above bottom and positioning 4 hours 3.0 m Calculations
4 Setting, confirm position and disconnect 2 hours 2.5 m CalculationsTOTAL Operation Period 14 hours
Operation in North Sea Schedule in Table No contingency time study provided Design wave in Table with reason (pers. or calc.) stated Not reversible operation
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Version Slide 1503 December 2008
Example Case -factor
Sub-op. TR /1/ -factor /1/ Hs /1/ TPOP /2/ -factor /2/ Hs /2/1 Hs=2.5 1.0 2.52 Hs=2.04 Hs=2.5
What is the -factor and limiting (forecasted) operational wave according to /1/ and /2/ for sub-operations 1, 2 and 4 ?
Operational Design Wave Height [m]Period [hours] 1
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Version Slide 1603 December 2008
Example Case Weather Forecast
0600 hours to 2400 hours : 1.5m
2400 hours to 0600 hours : 1.5m increasing to 2.0m
0600 hours to 1200 hours : 2.0m
1200 hours to 1800 hours : 2.0m increasing to 4.0m
Question
Assuming a weather meeting 0750hours and that the book is followed will it be found acceptable to start the operation according to /1/? And what about according to /2/?
Both weather forecasters have issued the following weather forecasts for Hs:
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Version Slide 1703 December 2008
Example Case Weather Forecast (2)
0
0.5
1
1.5
2
2.5
3
3.5
4
TR in hours
W
a
v
e
H
e
i
g
h
t
(
H
s
)
i
n
m
0800 1200 1600 2000 2400 0400 0800 1200 Hours
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 TR /1/ & /2/
0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 TPOP /2/
Weather F.
Design Hs
Lim WF Hs /2/
Lim WF Hs /1/
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Version Slide 1803 December 2008
DNV -FactorContent - Factor Currently in the RulesDNV-RP-H102 - - Factor - Factor JIP New - FactorsComparing - Factors ContentStep 1 Define OPLIM Step 2 Define the Operation Period Step 2 Operation Period (Slide 2)Step 3 Find and COApplication - Example CaseExample Case ScheduleExample Case -factorExample Case Weather ForecastExample Case Weather Forecast (2)Lysbildenummer 18