HEIDRUN TLP MODEL TESTS Summary of results (Pictures and figures are partly prepared by Marintek personel)

Transcription

1 HEIDRUN TLP MODEL TESTS Summary of results (Pictures and figures are partly prepared by Marintek personel) Sverre Haver, Statoil, December Content: Background for and aim of model test Model and instrumentation Environmental conditions Phase Examples, results and conclusions Phase Examples, results and conclusions Comparison with original design values December Heidrun TLP Model Tests - Summary Background and Aim More severe accidental wave conditions (i.e. wave conditions corresponding to an annual probability of - ) are predicted. Significant wave height increased from 8.6m to m. (ULS sea state conditions ( - ) are more or less as before, increased from 5.5m to 6m.) 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 - response in all structural elements mentioned above. Phase should identify the critical - environmental condition and ensure that a proper instrumentation was carried out. Phase should ensure that the inherent randomness of the extremes was accounted for by executing a large number of realizations for the critical environmental condition. December Heidrun TLP Model Tests - Summary

2 Heidrun TLP December Heidrun TLP Model Tests - Summary Test Model December Heidrun TLP Model Tests - Summary

3 LOCATION OF AIRGAP AND FORCE SENSORS In Phase, the lifeboat force sensor was moved to a lifeboat closer to centerline. Some airgap sensors were also moved December Heidrun TLP Model Tests - Summary 5 SENSORS FOR MEASURING DECK IMPACTS December Heidrun TLP Model Tests - Summary 6

4 ENVIRONMENTAL CONTOUR LINES FOR SEA STATE CHARACTERISTICS H s and T p HEIDRUN SITE December Heidrun TLP Model Tests - Summary 7 Contour lines for individual wave parameters, H and T December Heidrun TLP Model Tests - Summary 8

5 Example from Test Regular waves with height m and period s December Heidrun TLP Model Tests - Summary 9 Example from Test 75 Regular waves with height 5m and period 6.5s December Heidrun TLP Model Tests - Summary 5

6 MAX TETHER LOAD FOR REGULAR WAVES Note: Given height and period are target values, actual values may differ somewhat, especially for the height. TETHER LOAD (MN) IN H FOR REGULAR WAVES 7 DEG. Target wave period (s) Target wave height (m) TETHER LOAD (MN) IN H FOR REGULAR WAVES 5 DEG. Target wave period (s) Target wave height (m) December Heidrun TLP Model Tests - Summary 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 setdown will be very much amplified. The mean tension will be significantly overestimated (5-%) 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, h s = m and t p = s (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 s, see figure next page. December Heidrun TLP Model Tests - Summary 6

7 WAVE SPECTRUM OF CRITICAL ALS SEA STATE Note combined nature of sea state (swell + wind sea) December Heidrun TLP Model Tests - Summary Example from Test Irregular sea states with h s =.m and t p =s December Heidrun TLP Model Tests - Summary 7

8 Example from Test 6 Irregular sea state with h s =7.5m and t p =5.5s December Heidrun TLP Model Tests - Summary 5 RESULTS OF IRREGULAR SEA STATE TESTS PHASE The next figures show: The -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 = 5m (xxx = 7deg. and xxx = 5deg.): hs=.m, tp=s:,,,, hs=.5m, tp=s:,,,5,,,,5 hs=7.5m, tp=5.5s: 6,7,8,9, 6,7,8,9 hs=8.8m, tp=7s:,,,,,,, hs=9.8m, tp=8.5s: 5,6,7,8, 5,6,7,8 hs=m, tp=s:,,,,,,, hs=9m, tp=s: 5,6, 5,6, : Repeating to check repeatability.,,: Repeating with wind/current induced offset: m,.5m, m : Repeating with offset m, 5: Repeating with offset m : Repeating with no deck.,,: Repeating with offset: m,.5m, m : Repeating with offset m. December Heidrun TLP Model Tests - Summary 6 8

9 MAX TETHER TENSION PHASE TESTS 7deg. December Heidrun TLP Model Tests - Summary 7 MAX TETHER TENSION PHASE TESTS 5deg. December Heidrun TLP Model Tests - Summary 8 9

10 AIRGAP AT FRONT OF MSB_WEST 7deg. Wave crest reaches MSB for an airgap less than 8.5m December Heidrun TLP Model Tests - Summary 9 AIRGAP IN FRONT OF MSB_EAST 7 deg. Wave crest reaches MSB for an airgap less than 8.5m December Heidrun TLP Model Tests - Summary

11 AIRGAP AT TWO LIFE BOAT POSITION-7 deg. Wave crest reaches life boat if airgap less than about 8m NORTH WEST CORNER NORTH BETWEEN MSBs December Heidrun TLP Model Tests - Summary AIRGAP AT TWO LIFE BOAT POSITIONS 5 deg. NORTH WEST CORNER NORTH BETWEEN MSBs December Heidrun TLP Model Tests - Summary

12 AIRGAP AT FRONT OF MSB-WEST 5 deg. Wave crest reaches MSB for an airgap of less than 8.5m December Heidrun TLP Model Tests - Summary CONCLUSIONS OF PHASE The most critical - sea state is the sea state defined by h s =m and t p =s. Large tether loads are also observed for a sea state defined by h s =8.8m and t p =7s. 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 () 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 a large number () 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. December Heidrun TLP Model Tests - Summary

13 HORISONTAL DECK IMPACT VERSUS UNDISTURBED CREST HEIGHT Phase and Phase results 7deg. 5 Hor. deck impact load (MN) Wave_Cal crest (m) Wave_Cal: Undisturbed incoming crest height at nominal position of MSB_west December Heidrun TLP Model Tests - Summary 5 VERTICAL VERSUS HORISONTAL IMPACT LOAD Phase and Phase results, 7 deg. Maximum ver. deck load (MN) Maximum hor. deck load (MN) December Heidrun TLP Model Tests - Summary 6

14 HISTOGRAM DECK IMPACT LOADS -hour maximum of each test with hs=m and tp = s, obs. No. of obs. 5 5 Histogram - -hour max horisontal deck impact load Impact Load (MN) Histogram - -hour max vertical deck impact load FIGURES CLEARLY DEMONSTRATE THE VERY LARGE SCATTER FROM REALIZATION TO REALIZATION. THIS IS THE REASON FOR THE NEED OF A RATHER LARGE NUMBER OF -HOUR REALIZATIONS OF THE CRITICAL SEA STATE, HERE REALIZATIONS ARE SELECTED. IF THERE HAD BE NO SCATTER FROM REALIZATION TO REALIZATION, WE COULD HAVE ADOPTED THE MEAN -HOUR MAX OF THE - SEA STATE AS AN ADEQUATE ESTIMATE OF THE - IMPACT LOAD. DUE TO THE SCATTER, LOADS LARGER THAN THE MEAN MAY BE OBSERVED FOR LOWER SEA STATES. IN ORDER TO ACCOUNT FOR THIS, A HIGHER FRACTILE HAS TO BE SELECTED. 5 HERE THE 9% FRACTILE IS SELECTED No. of Obs. Impact Load (MN) December Heidrun TLP Model Tests - Summary 7 -HOUR EXTREME VALUE DISTRIBUTION FOR DECK IMPACTS FYMAX_7_Fase + ( obs.) Momenttilpassing 9% FRACTILE:.5 -ln-lnp(x<x)) Data -min Gumbel -min NB! NB! When selecting design impacts, more weight is given to upper tail than what standard fitting suggests, i.e. the design values given below are based on an eye fit to upper tail. T (MN) ALS DESIGN IMPACTS: FY_MAX = 5MN FZ_MAX = MN Both MSBs has to be checked for the horizonal impact load. The vertical impact load is mainly caused by wave-deck (not MSB) and has to be implemented in a conservative way. -ln-lnp(x<x)) FZMAX_7_Fase + ( obs.) Momenttilpassing Data -min Gumbel -min T (MN) December Heidrun TLP Model Tests - Summary 8

15 DECK IMPACT AND TETHER RESPONSE Example December Heidrun TLP Model Tests - Summary 9 TETHER TENSION VERSUS DECK IMPACTS WAVE DIRECTION: 7 DEG. Estimated ALS impact 75 Stagkraft H (MN) Hor. dekklast (MN) For a horisontal impact load of 5MN (ALS-impact), the corresponding tether force is likely to be between 5 and 5MN. Estimated ALS impact 75 5 For a vertical impact load of MN (ALS-impact), the corresponding tether force is expected to be between and 5MN. Stagkraft H (MN) Vert. dekklast (MN) December Heidrun TLP Model Tests - Summary 5

16 HISTOGRAM -HOUR MAX TETHER LOAD No. of obs. 6 5 Histogram -hour max tether load, H Tether Load (MN) Large scatter is observed, Consequently a higher fractile has to be selected as the proper estimate of - tether load. (See comments on slide no. 7.) Histogram -hour max tether load, H 5 No. of obs Tether Load (MN) December Heidrun TLP Model Tests - Summary HISTOGRAM -HOUR MIN. TETHER LOAD Histogram -hour min tether load, H No. of obs Tether Load (MN) Histogram -hour min tether load, H No. of obs Tether Load (MN) December Heidrun TLP Model Tests - Summary 6

17 -HOUR EXTREME VALUE DISTRIBUTION TETHER LOAD ( TETHERS) -ln-lnp(x<x)) H_7_Fase + Momenttilpassing Data -hour max Gumbel T (MN) NB!NB! MORE WEIGHT IS GIVEN TO UPPER TAIL WHEN CHARACTERISTIC VALUES ARE SELECTED, SEE ALSO SLIDE No 7. 9% FRACTILE ASSUMED TO ACCOUNT PROPERLY FOR SCATTER AROUND THE MEAN. (See slide no.7) 9% FRACTILE:.5 ALS TETHER LOAD, AUTUMN : TETHERS: 5MN PR. TETHER: 6.5MN ALS ORIGINAL DESIGN: -ln-lnp(x<x)) - H_7_Fase + Momenttilpassing Data -hour max Gumbel PR. TETHER: 5.MN T (MN) December Heidrun TLP Model Tests - Summary EXAMPLE : EFFECT OF DECK IMPACT ON TETHER LOADING December Heidrun TLP Model Tests - Summary 7

18 BUDGET ALS MAX TETHER LOAD (The last column will be updated as the results from Phase is properly analysed.) SOURCE WAVE SLAMMING INDUCED RINGING (INCL. AS STATIC LOAD IN ORIGINAL DESIGN) DYNAMIC LOAD, EXCL. RINGING ORIGINAL DESIGN.6 MN 7MN MODEL TEST (PRELIMINARY APPROXIMATE NUMBERS EXCEPT THE SUM) -MN 8MN CLASSICAL RINGING MN MN (*) SUM DYNAMIC 7.5MN 6MN SUM TOTAL 5.5MN 6MN (Preliminary) (*) Based on tests with and without deck, at present no reason to change original ringing estimate. Further data analysis will be carried out. A CONSIDERABLE INCREASE IN THE DYNAMIC LOAD IS ESTIMATED December Heidrun TLP Model Tests - Summary 5 CONCLUSIONS OF TESTS - TOPSIDE IMPACT LOADS ARE ESTIMATED TO BE: 5MN HORISONTAL AND MN VERTICAL. ALS DYNAMIC TETHER LOAD (NON-YIELDING FULL SCALE MODEL ) FROM IRREGULAR WAVE TESTS (H S =m and T P =s): 6MN DYNAMIC TETHER LOAD (NON-YIELDING FULL SCALE MODEL) FROM REGULAR WAVE TEST (ALS WAVE H=m and T=s): MN (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 6MN WHEN NO YIELDING IS ACCOUNTED FOR. AS A MEASURE OF A REASONABLE MARGIN MN DYNAMIC TETHER LOAD IS SUGGESTED. December Heidrun TLP Model Tests - Summary 6 8