December 2003Heidrun TLP Model Tests - Summary1 HEIDRUN TLP MODEL TESTS Summary of results (Pictures...

December 2003 Heidrun TLP Model Tests - Summary 1

HEIDRUN TLP MODEL TESTSSummary of results

(Pictures and figures are partly prepared by Marintek personel)

Sverre Haver, Statoil, December 2003

Content:

Background for and aim of model test

Model and instrumentation

Environmental conditions

Phase 1 – Examples, results and conclusions

Phase 2 – Examples, results and conclusions

Comparison with original design values


Background and AimMore severe accidental wave conditions (i.e. wave conditions corresponding

to an annual probability of 10-4) are predicted. Significant wave height increased from 18.6m to 20m.(ULS sea state conditions (10-2) are more or less as before, increased from15.5m to 16m.)

With updated accidental wave conditions, the topside structure (in particular the MSBs) is much more exposed to wave impacts than accounted for in original design.

Due to these impacts, increased structural response is expected in lower hull, in main support beams (MSBs), in tethers, and in tether connections to hull and bottom foundation. Life boats may also be exposed to wave impacts.

The aim of the model test is to establish the background for predicting reliable estimates for the 10-4 response in all structural elements mentioned above.

Phase 1 should identify the critical 10-4 environmental condition and ensure that a proper instrumentation was carried out. Phase 2 should ensure that the inherent randomness of the extremes was accounted for by executing a large number of realizations for the critical environmental condition.


Heidrun TLP


Test Model


LOCATION OF AIRGAP AND FORCE SENSORS

In Phase 2, the lifeboatforce sensor was movedto a lifeboat closer to centerline.Some airgap sensorswere also moved


SENSORS FOR MEASURING DECK IMPACTS


ENVIRONMENTAL CONTOUR LINES FOR SEA STATE CHARACTERISTICS Hs and Tp

HEIDRUN SITE


Contour lines for individual wave parameters, H and T


Example from Test 2111Regular waves with height 20m and period 11s


Example from Test 2750Regular waves with height 35m and period 16.5s


MAX TETHER LOAD FOR REGULAR WAVESNote: Given height and period are target values, actual values may differ somewhat, especially for the height.

TETHER LOAD (MN) IN H10 FOR REGULAR WAVES 270 DEG.

Target wave period (s)11 12 13 15 16.5 18

10 102.7Target 20 165.7 173.8 156.8 113.3 108 116.6wave 30 299.3 157.5 126.8 150height 35 174.4 162(m) 40 184.7 159.9

TETHER LOAD (MN) IN H10 FOR REGULAR WAVES 315 DEG.

Target wave period (s)11 12 13 15 16.5 18

10 113.2 115.2 110.5 107.9 106.1 107.9Target 20 174.5 182.7 184.5 125.9 116.2 121.6wave 30 232.5 165.6 131.8 148.3height 35 180.9 201.2(m) 40 185.6 167.5


OBSERVATIONS FROM REGULAR WAVE TESTS

The most critical individual waves are steep waves with a rather large crest height.

Too much weight should not be given to the total tether forces shown in table on previous page – in particularly not for the highest waves. When repeating such extreme waves, the offset and therefore the set-down will be very much amplified. The mean tension will be significantly overestimated (25-30%) as compared to the mean tension in a severe storm. Focus should be on the dynamic tension. However, this quantity is also somewhat conservative since an overestimation of the set-down will make topside more exposed to wave impacts.

The criticality of steep waves may seem to be in conflict with the selected extreme sea state, hs = 20m and tp = 20s (irregular sea state). However, this sea state is likely to be of a combined nature, i.e. a considerable amount of the energy is concentrated around a period band much shorter than 20s, see figure next page.


WAVE SPECTRUM OF CRITICAL ALS SEA STATE

Note combined nature of sea state (swell + wind sea)


Example from Test 3010Irregular sea states with hs=12.3m and tp=11s


Example from Test 3060Irregular sea state with hs=17.5m and tp=15.5s


RESULTS OF IRREGULAR SEA STATE TESTS PHASE 1

The next figures show:

The 3-hour maximum tether tensions for the most exposed corner are shown for all tests.

The minimum air gap are given for a number of positions. (Zero air gap is defined as the airgap when the wave crest reach cellar deck level.)

Test numbers, wind/current offset = 15m (3xxx = 270deg. and 4xxx = 315deg.):

hs=12.3m, tp=11s: 3001,3002,3020, 4000,4011 hs=14.5m, tp=13s: 3020,3030,3040,3051, 4022,4030,4040,4050hs=17.5m, tp=15.5s: 3060,3070,3080,3090, 4060,4070,4080,4090hs=18.8m, tp=17s: 3110,3120,3130,3140, 4110,4120,4130,4140hs=19.8m, tp=18.5s: 3150,3160,3170,3180, 4150,4160,4170,4180hs=20m, tp=20s: 3210,3220,3231,3240, 4210,4221,4231,4240hs=19m, tp=22s: 3250,3260, 4250,4260

3241, 3242: Repeating 3240 to check repeatability.3310, 3320,3330: Repeating 3240 with wind/current induced offset: 10m, 22.5m, 30m3340: Repeating 3140 with offset 30m, 3510: Repeating 3010 with offset 0m3410: Repeating 3240 with no deck.4310, 4324,4333: Repeating 4240 with offset: 10m, 22.5m, 30m4341: Repeating 4140 with offset 30m.


MAX TETHER TENSION PHASE 1 TESTS – 270deg.


MAX TETHER TENSION PHASE 1 TESTS – 315deg.


AIRGAP AT FRONT OF MSB_WEST – 270deg.

Wave crest reaches MSB for an airgap less than 8.5m


AIRGAP IN FRONT OF MSB_EAST – 270 deg.

Wave crest reaches MSB for an airgap less than 8.5m


AIRGAP AT TWO LIFE BOAT POSITION-270 deg.

Wave crest reaches life boat if airgap less than about 8m

NORTH WEST CORNER

NORTH BETWEEN MSBs


AIRGAP AT TWO LIFE BOAT POSITIONS – 315 deg.

NORTH WEST CORNER

NORTH BETWEEN MSBs


AIRGAP AT FRONT OF MSB-WEST – 315 deg.

Wave crest reaches MSB for an airgap of less than 8.5m


CONCLUSIONS OF PHASE 1

The most critical ”10-4 sea state” is the sea state defined by hs=20m and tp=20s. Large tether loads are also observed for a sea state defined by hs=18.8m and tp=17s.

The largest deck impact loads are observed for beam sea direction. A comparable tether load level is observed for both the diagonal and the beam sea direction. This because fewer tethers (4) have to carry the most of the dynamic load for the diagonal direction. (In original design the diagonal direction was found to be the most critical direction because deck impacts were less pronounced.)

In Phase 2 a large number (20) of runs with different random seeds of the critical sea state are carried out for both beam sea and diagonal sea. For the most extreme events, the tests are repeated without the deck structure in order to isolate deck impact induced ringing from classical ringing.


HORISONTAL DECK IMPACT VERSUS UNDISTURBED CREST HEIGHTPhase 1 and Phase 2 results 270deg.

0

50

100

150

200

250

20 21 22 23 24 25 26

Wave4_Cal crest (m)

Ho

r. d

eck

imp

act

load

(M

N)

Wave4_Cal: Undisturbed incoming crest height at nominal position of MSB_west


VERTICAL VERSUS HORISONTAL IMPACT LOAD

Phase 1 and Phase 2 results, 270 deg.

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250

Maximum hor. deck load (MN)

Max

imu

m v

er.

dec

k lo

ad

(MN

)


HISTOGRAM DECK IMPACT LOADS3-hour maximum of each test with hs=20m and tp = 20s, 24 obs.

FIGURES CLEARLY DEMONSTRATETHE VERY LARGE SCATTER FROM REALIZATION TO REALIZATION. THIS IS THE REASON FOR THE NEEDOF A RATHER LARGE NUMBER OF 3-HOUR REALIZATIONS OF THE CRITICAL SEA STATE, HERE 24 REALIZATIONS ARE SELECTED.

IF THERE HAD BE ”NO” SCATTER FROM REALIZATION TO REALIZATION, WE COULD HAVE ADOPTED THE MEAN 3-HOUR MAX OF THE 10-4 SEA STATE AS AN ADEQUATE ESTIMATE OF THE10-4 IMPACT LOAD.

DUE TO THE SCATTER, LOADS LARGER THAN THE MEAN MAY BE OBSERVEDFOR LOWER SEA STATES. IN ORDER TOACCOUNT FOR THIS, A HIGHER FRACTILE HAS TO BE SELECTED.

HERE THE 90% FRACTILE IS SELECTED.

Histogram - 3-hour max horisontal deck impact load

0

1

2

3

4

5

5 25 45 65 85 105

125

145

165

185

Impact Load (MN)

No

. o

f o

bs.

Histogram - 3-hour max vertical deck impact load

0

1

2

3

4

5 15 25 35 45 55 65 75 85 95 105

115

125

135

145

155

165

Impact Load (MN)

No

. o

f O

bs.


3-HOUR EXTREME VALUE DISTRIBUTION FOR DECK IMPACTS

FYMAX_270_Fase 1+2 (24 obs.)Momenttilpassing

-2

-1

0

1

2

3

4

0 20 40 60 80 100 120 140 160 180 200 220

T (MN)

-ln

-ln

P(X

<X

))

Data 20-min

Gumbel 20-min

FZMAX_270_Fase 1+2 (24 obs.)Momenttilpassing

-2

-1

0

1

2

3

4

0 20 40 60 80 100 120 140 160 180 200 220

T (MN)

-ln

-ln

P(X

<X

))

Data 20-min

Gumbel 20-min

90% FRACTILE: 2.25

ALS DESIGN IMPACTS:

FY_MAX = 150MNFZ_MAX = 130MN

NB! NB!When selecting design impacts,more weight is given to upper tail than what standard fittingsuggests, i.e. the design valuesgiven below are based on an ”eye fit” to upper tail.

Both MSBs has to be checked for thehorizonal impact load. The verticalimpact load is mainly caused by wave-deck (not MSB) and has to beimplemented in a conservative way.


DECK IMPACT AND TETHER RESPONSE

Example 2


TETHER TENSION VERSUS DECK IMPACTS

WAVE DIRECTION: 270 DEG.

100

125

150

175

200

225

250

275

0 50 100 150 200 250 300

Hor. dekklast (MN)

Sta

gkr

aft

H10

(M

N)

100

125

150

175

200

225

250

275

0 50 100 150 200 250

Vert. dekklast (MN)

Sta

gkr

aft

H10

(M

N)

For a horisontal impact load of 150MN(ALS-impact), the corresponding tetherforce is likely to be between 225 and 250MN.

For a vertical impact load of 130MN (ALS-impact), the correspondingtether force is expected to be between 210 and 250MN.

Estimated ALS impact

Estimated ALS impact


HISTOGRAM 3-HOUR MAX TETHER LOAD

Large scatter is observed, Consequently a higher fractilehas to be selected as the proper estimate of 10-4

tether load.

(See comments on slide no. 27.)

Histogram 3-hour max tether load, H10

0

1

2

3

4

5

6

145 155 165 175 185 195 205 215 225 235 245 255 265 275

Tether Load (MN)

No

. o

f o

bs.

Histogram 3-hour max tether load, H20

0

1

2

3

4

5

145 155 165 175 185 195 205 215 225 235 245 255 265 275

Tether Load (MN)

No

. o

f o

bs.


HISTOGRAM 3-HOUR MIN. TETHER LOAD

Histogram 3-hour min tether load, H10

0123456789

-5 5 15 25 35 45 55 65

Tether Load (MN)

No

. o

f o

bs.

Histogram 3-hour min tether load, H20

0

1

2

3

4

5

6

7

8

-5 5 15 25 35 45 55 65

Tether Load (MN)

No

. o

f o

bs.


3-HOUR EXTREME VALUE DISTRIBUTION TETHER LOAD (4 TETHERS)

90% FRACTILE: 2.25

ALS TETHER LOAD, AUTUMN 2003:

4 TETHERS: 250MNPR. TETHER: 62.5MN

ALS ORIGINAL DESIGN:

PR. TETHER: 52.2MN

H10_270_Fase 1+2Momenttilpassing

-2

-1

0

1

2

3

4

100 120 140 160 180 200 220 240 260 280 300

T (MN)

-ln

-ln

P(X

<X

))

Data 3-hour max

Gumbel

H20_270_Fase 1+2Momenttilpassing

-2

-1

0

1

2

3

4

100 120 140 160 180 200 220 240 260 280 300

T (MN)

-ln

-ln

P(X

<X

))

Data 3-hour max

Gumbel

NB!NB!MORE WEIGHT IS GIVENTO UPPER TAIL WHEN CHARACTERISTIC VALUESARE SELECTED, SEE ALSO SLIDE No 27.

90% FRACTILE ASSUMED TO ACCOUNT PROPERLYFOR SCATTER AROUND THE MEAN. (See slide no.27)


EXAMPLE 1: EFFECT OF DECK IMPACT ON TETHER LOADING


BUDGET ALS MAX TETHER LOAD(The last column will be updated as the results from Phase 2

is properly analysed.)

SOURCE ORIGINAL DESIGN

MODEL TEST 2003(PRELIMINARY APPROXIMATE NUMBERS EXCEPT THE SUM)

WAVE SLAMMING INDUCED RINGING(INCL. AS STATIC LOAD IN ORIGINAL DESIGN)

0.6 MN 20-30MN

DYNAMIC LOAD, EXCL. RINGING 7MN 8MN

CLASSICAL RINGING 20MN 20MN (*)

SUM DYNAMIC 27.5MN 36MN

SUM TOTAL 52.5MN 62MN (Preliminary)

A CONSIDERABLE INCREASE IN THE DYNAMIC LOAD IS ESTIMATED

(*) Based on tests with and without deck, at present no reason to change original ringing estimate. Further data analysis will be carried out.


CONCLUSIONS OF TESTS

10-4 TOPSIDE IMPACT LOADS ARE ESTIMATED TO BE: 150MN HORISONTAL AND 130MN VERTICAL.

ALS DYNAMIC TETHER LOAD (NON-YIELDING FULL SCALE MODEL ) FROM IRREGULAR WAVE TESTS (HS=20m and TP=20s): 36MN

DYNAMIC TETHER LOAD (NON-YIELDING FULL SCALE MODEL) FROM REGULAR WAVE TEST (ALS WAVE H=32m and T=13s): 42MN (THIS LOAD IS RATHER CONSERVATIVE SINCE THE REGULAR WAVE TEST CAUSE A VERY LARGE MEAN OFFSET AND, CONSEQUENTLY, A LARGE MEAN SET-DOWN.)

SUGGESTED ALS ROBUSTNESS CRITERIUM: THE NUMERICAL PLATFORM RESPONSE ANALYSES – ACCOUNTING FOR YIELDING AND SLACK TETHERS OF REAL FULL SCALE MODEL - SHALL ENSURE THAT THE HEIDRUN TLP WILL- WITH A REASONABLE MARGIN - WITHSTAND THE EXTERNAL FORCES CAUSING A DYNAMIC TETHER LOAD OF 36MN WHEN NO YIELDING IS ACCOUNTED FOR. AS A MEASURE OF A REASONABLE MARGIN 40MN DYNAMIC TETHER LOAD IS SUGGESTED.

December 2003Heidrun TLP Model Tests - Summary1 HEIDRUN TLP MODEL TESTS Summary of results (Pictures...

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