IACS URS11 defines the dimensioning wave load for ship design, but what does it mean from a statistical point of view?

Transcription

1 IACS URS11 defines the dimensioning wave load for ship design, but what does it mean from a statistical point of view? Seamocs meeting in Malta Gaute Storhaug, DNV Maritime 19th of March 2009

2 Overview What is the purpose of ship design rules? What is IACS URS11? What is REC. 34? Which assumptions do REC. 34 include and are there any potential for improvement? How does model and full scale tests compare with IACS URS11? Conclusions Slide 2

3 What is the purpose of ship design rules? Ensure safe ship and safe operation ship fit for purpose! - Avoid damages to hull - Ensure sufficient stability and floatability - Ensure sufficient fire protection and fire fighting capability - Ensure safe navigation - Avoid engine and propulsion failure -... Typical hull damages are: - Fatigue failure (cracks) - Dents (impacts, collision and grounding) - Corrosion - Yielding/permanent deformations (local and/or global overloading) and - Buckling collapse (local and/or global overloading) Global collapse and yielding can be caused by extreme wave loads and/or static overloading Slide 3

4 I.e. we do not want this to happen! Slide 4

5 I.e. we do not want this to happen! Overloading confirmed by DNV s calculations Slide 5

6 What is IACS UR S11? IACS = International Association of Classification Societies - Including Det Norske Veritas (DNV) UR = Unified Requirements S = Strength of Ships 11 = Longitudinal strength standard (12 pages) free download of this standard IACS UR S11 gives requirements to design wave load and global strength of seagoing steel ships above 90m - To ensure that they do not break in two - Records show that this is good enough (in most cases) and maybe too conservative in some cases Slide 6

7 What is IACS UR S11? In a sea state the hull girder is exposed to a wave bending moment, Mw There is also a static still water bending moment due to cargo loading! Vessel in sagging Moment distribution O.M. Faltinsen Mw = M C L 2 B Cb x 10-3 (knm)... sagging Mw = M C L 2 B (Cb + 0,7) x 10-3 (knm)... hogging Slide 7

8 What is REC. 34? REC. 34 is IACS Recommendation for Standard Wave Data - to be used for calculations of wave bending moments - It also gives recommendations of assumptions to be used for calculating these bending moments Issued first in 1992 Elzbieta et al improved REC. 34, resulting in a major revision in The assumptions used in calculations give bending moments that compare reasonably well with IACS URS11 Slide 8

9 Which assumptions do REC. 34 include? Assumption 1: North Atlantic is assumed used as design basis It represents the worst area A ship should be able to operate anywhere, hence use the worst area OMAE2006: KNMI/ERA-40 But!!! Slide 9

10 Which assumptions do REC. 34 include? cont. We should consider what is acceptable risks (probability of failure x consequence) and this should reflect - Where ships actually operate today and - Where ships will operate in the near future, say 40 years, reflecting both Panama Channel widening, Arctic passage and climate change E.g. container vessels barely operate in the North Atlantic today North Atlantic 14% Small vessels East Asia-Europe 25% largest vessels North Pacific 42% Slide 10

11 Which assumptions do REC. 34 include? cont. The scatter diagram is related to significant uncertainties 16.5m Hs Wave data based on old ship observations, but adjusted and improved. New tools are used to route around storms We can not use new satellite data directly: e.g. 100year 26.5m Hs The captains are not crazy! Searoutes Ltd. OMAE2006: KNMI/ERA-40 Slide 11

12 Which assumptions do REC. 34 include? cont. Assumption 2: All wave headings (0-360 ) can be ass umed to have an equal probability of occurrence and cos 2 directional wave spreading should be used - i.e. the moment contribution from different directions is statistically smeared out - Consequence e.g.: M Example from North Atlantic operation - More head/stern seas and less beam seas! 0 + M M 2 0 i.e. much less than in pure head sea N Vessel Heading W E The important thing is however which heading we have in high sea states - This means that the heading profile should reflect routing and captains actions, which are significant and depends on the wave height - We can not expect that the ship is designed to meet 16.5m in head sea!!! S Slide 12

13 Which assumptions do REC. 34 include? cont. Example from North Pacific operation - When the significant wave height is above 6m, the vessels have stern or stern quartering seas in almost 80% of the time! Slide 13

14 Which assumptions do REC. 34 include? cont. Assumption 3: Use zero speed. - The speed is in particular important for nonlinear response effects In high sea states the vessel speed is reduced due to increased resistance, but what if you have a container vessel with hp? - Large container vessels may maintain 10 knots anyway, but good documentation is lacking. - Monitoring/vessel tracking relative to storms is needed (AIS, LRIT + hind cast) The speed should be correlated with the sea state and heading profile From a 300m bulk carrier (in North Atlantic) In addition to wave height wave period is also important as well as propeller racing... Slide 14

15 Which assumptions do REC. 34 include? cont. Assumption 4: Pierson-Moskowitz wave spectrum is used. - This represents fully developed sea The consequence of alternatives are not much considered, e.g. - Nonlinear wave profile may be important (e.g. Draupner wave) - Two peaked spectra and bi-directional seas may occur. Steep wave Slide 15 Ingo Drummen

16 How does model and full scale tests compare with IACS URS11? First let us look at the moment distribution along the vessel from calculations - Example from some container vessels (for illustration purposes; not the whole story) - May exceed the IACS URS11 both aft, midship, but not in the forward part! kn 5 kn 10 kn 15 kn Relative URS Similar for the static still water moment distribution (from various loading conditions) The static and dynamic moments are today added, but the simultaneous behaviour should be considered means that the procedure has some safety margin today. - Not much documented here Slide 16

17 How does model and full scale tests compare with IACS URS11? Cont. Then we have the problem with whipping from full scale measurements Hogging IACS URS11 85% increase 4400TEU Only! 6m significant wave height Bow impact IACS URS11 Sagging 95% increase Slide 17

18 How does model and full scale tests compare with IACS URS11? Cont. Then we have the problem with whipping from model tests - Summary of various sea states (and peak periods) up to 9m significant wave height - All realistic assumptions 1.4 NOT OK NOT OK OK IACS URS Full scale in 6m Hs Full scale measurements confirm this in high forward speed in close to head sea Sag Hog Sag Hog Sag Hog Aft Midship Fore W ith whipping W ithout whipping Slide 18

19 How does model and full scale tests compare with IACS URS11? Cont. Whipping tends to increase for larger ships with more flexible hulls, more bow flare and higher speeds - The operational experience with these ships is limited! - They operate on calm trades (today) E.g. >10 000TEU only from 2006 Slide 19

20 Conclusions In order to achieve optimum cost effective, environmental friendly and reliable future design the dimensioning wave loading should be based on: 1. Scatter diagram based on recent high quality wave data including typhoons 2. Routing to avoid storms as well as captains actions in storms 3. Distribution of ships in different trades (today and in the near future) 4. Heading profile and speed profile versus sea states 5. Statistically combined still water and dynamic (wave) loading 6. Hydro elastic effects such as whipping 7. Good understanding of the sensitivity for various assumptions made Much of this is multidisciplinary challenges involving a great deal of statistics The success of such improvements depends on long term research and good cooperation. - The challenges are also none technical! Slide 20

21 Thank you for your attention Note! This presentation reflects the opinion of the author and not necessarily DNV Slide 21

22 Abstract Ships vibrate due to waves, and these wave induced vibrations can not easily be avoided by moderate changes to the hull lines. The waves may cause the whole hull girder to vibrate due to whipping (transient response), which increase the extreme loading. Recently this has also become an industry concern. Modern hull monitoring systems in combination with model tests are the best tools to answer the key questions: How important is the wave induced vibrations, and does it have to be included in design? The presentation will review the basis of IACS URS11 wave load requirements, and compare these with measured results. Measurements have been carried out on a container vessels operating in the North Atlantic. An elastic model has also been tested in a towing tank. Results are obtained at quarter lengths and amidships. The full scale measurements and model test show that IACS URS11 rule loads may be exceeded in far less than extreme sea states, in particularly amidships and in the aft ship. The IACS UR S11 may need revision for container ship design. MAIB s report based on the investigation of the MSC Napoli incident (vessel broke in two) also recommends increased requirements for container ship design and further research into the effect of whipping. Slide 22