12 - KTF 2008 DNV Alpha Factor

DNV α-Factor

KTF - Subsea Lifting Operations, Stavanger 2008

Per Ø. Alvær2008-12-03

Version Slide 203 December 2008

Content

What is new?- History- DNV-RP-H102- Update

background- Comparing

Application- Guidance- Example Case


α - Factor – Currently in the Rules

Purpose: Take into account uncertainty in the WFBasis: DNMI report DSO265/Lund-95/15325 (4/5-95)

Rule requirement given by:Table that shows α as a function of TR and (design) Hs (TR = TPOP + TC)

OR


DNV-RP-H102 -α - 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.


α - 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:


’New’ α - 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


Comparing α - 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 Tα ≈+5% +5% + for short

operations


Content

What is new?- History- DNV-RP-H102- Update

background- Comparing

Application- Guidance- Example Case


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.


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.


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 (α-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 Tα will be introduced in the Rule update.


Step 3 – Find α 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 = α x CD.

In DNV-RP-H102 the forecasted (monitored) operational criteria - CO - is defined as CO = α x OPLIM.

An operation could be divided in sub operations for which different α and CD (OPLIM) could be defined.

Updated Rules will use the same terminology as in DNV-RP-H102 for this.


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.


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 m Personnel

limitation2 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


Example Case – α-factor

Sub-op. TR /1/ α-factor /1/ Hs /1/ TPOP /2/ α-factor /2/ Hs /2/1 Hs=2.5 1.0 2.52 Hs=2.04 Hs=2.5

What is the α-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<Hs≤ 2 2<Hs≤ 4 Hs > 4

TR < 12 0.68 0.76 0.80

TR < 24 0.63 0.71 0.75

TR < 48 0.56 0.64 0.67

TR < 72 0.51 0.59 0.63

TPOP + TC= 14 + 14

= 28 h.0.56 1.10.64 1.6 0.78 2.0

0.75 1.50.84 2.12+7 = 9

9+1 = 1010+6 = 16


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:


Example Case – Weather Forecast (2)

0

0.5

1

1.5

2

2.5

3

3.5

4

TR in hours

Wav

e H

eigh

t (H

s) in

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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12 - KTF 2008 DNV Alpha Factor

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