IMO REVIEW OF THE INTACT STABILITY CODE. Sample calculations using a wind criterion. Submitted by Germany. Resolution A.749 (18) and MSC.

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1 INTERNATIONAL MARITIME ORGANIZATION E IMO SU-COMMITTEE ON STAILITY AND LOAD LINES AND ON FISING VESSELS SAFETY 46t session Agenda item 6 SLF 46/6/8 3 July 2003 Original: ENGLIS REVIEW OF TE INTACT STAILITY CODE Sample calculations using a wind criterion Submitted by Germany Executive summary: Action to be taken: Paragrap 3 Related documents: ackground SUMMARY In te context of discussions on performance based criteria, Germany proposes a wind criterion wic was developed in te context of te discussions on te Weater Criterion contained in te IS Code. Sample calculations yield tat tis kind of wind criterion offers a more detailed calculation metodology of wind eeling levers mainly based on wind tunnel measurements. Resolution A.749 (18) and MSC.75(69) 1 Germany wises to draw te Sub-Committee s attention to te experience gained wit sample calculations for a wind criterion as sown in te annex. For tese calculations sip types wit large windage areas were cosen in order to compare te GM limiting values required by bot te weater criterion and te proposed wind criterion. Te coefficients used in tis context ave been taken from various wind tunnel measurements. For calculating te eeling moment caused by wind tree alternatives are proposed, wereas te sample calculations temselves ave been carried out wit te most conservative values stated in square brackets. 2 From te comparison of te result, it can be sown tat te required GM-values are in general of te same magnitude for bot te Weater Criterion and te proposed wind criterion. Tus te proposed wind criterion may be seen as equivalent to te Weater Criterion in terms of setting safety margins, owever, due to te nature of tis kind of performance-based criteria tere is no need for adaptation to specific sip types. Suc criteria will also address te performance of future sip designs. Action requested of te Sub-Committee 3 Te Sub-Committee is invited to consider te information as per te annex and take action as appropriate. *** For reasons of economy, tis document is printed in a limited number. Delegates are kindly asked to bring teir copies to meetings and not to request additional copies.

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3 1 Introduction TEST CALCULATIONS USING A WIND CRITERION forces and moments are relevant to te safety of intact sips, in particular for sips wit large windage areas. Witin te current IS-Code [2] wind action is considered witin te Weater Criterion. ut problems in application arise due to te calculation procedures - te empirical formulae do not reflect pysics sufficiently well - and due to te coupling to te roll motion. Wile te pysical penomena wind pressure and roll resonance are calculated via independent empirical formulae, te joint assessment via te lever arm curve causes a coupling in te assessment, suc tat wit te current formulation:.1 at times it is not possible to calculate a limiting GM according to te Weater Criterion only; and.2 for some sips wit large windage areas but very long periods of roll (e.g., large Containersips) te Weater Criterion is irrelevant despite te large windage area. Reflecting te problems wic arise from purely empirically formulated criteria as sip design develop it was decided, at te last SLF meeting, tat a cange towards "performance based" formulations is intended for future developments. Wit respect to wind forces and moments acting on sips a lot of knowledge and data is internationally available. Results from wind tunnel test and numerical simulations sow tat te points of attack traditionally assumed in te centre of te area often vary substantially in eigt for different sip types Tis paper aims at sowing te potential of a wind criterion for te assessment of sip stability. 2 General set-up Te sip is subject to a steady, [uniform] wind and a wind gust bot from a direction [perpendicular] to te sip s centerline at [zero] speed. A four step approac is adopted:.1 Te environmental conditions (steady wind and gusts) are defined..2 Te resulting wind forces and point of attack (above te waterline) are estimated..3 Te sip s reaction is calculated..4 Te sip s performance is assessed. Te aim of tis four step approac is to allow for a performance-based and transparent formulation as well as making an easy excange of calculation and/or measurement metods possible.

4 Page 2 3 Environmental conditions For tese test calculations te following environmental conditions are assumed:.1 Steady wind of [26] m/s wit a [uniform] inflow profile..2 A wind gust of [36] m/s wit a [uniform] inflow profile. 4 force and point of attack Te wind side forces (lateral force) onto te sip are calculated using te following relation: F ρ air 2 = C y u Alat [ N] (1) 2 Were ρ air is te density of te air (to be taken as [1.3 kg/m³]), u is te wind speed (steady or gust respectively as defined in section 0), and A lat is te lateral-plane area of te individual sip above te water line. C y is te force coefficient for te side force. Furtermore, te eigt of te point of attack above te waterline needs to be determined. If specific wind tunnel measurements or oter directly obtained data are not available, te following minimum standard values are to be used: and for < 0.85: for 0.85: C y = [1.0] (2) = [ = [1.05] centera ] centera A lat wit = (3) L oa wit L oa being te sip s over all lengt, being te breadt of te sip and cetera being te vertical distance between te centre of te lateral area and te waterline. Werever appropriate and especially wen unusual vessels are to be assessed, like e.g. vessels wit pronounced or diverse superstructures (e.g., drilling vessels), C y and sould be determined via eiter direct wind tunnel measurements or using appropriate data from measurements of vessels of comparison (e.g., lendermann [1] publised collected data from wind tunnel tests for several types of sips). 5 eeling moment Te wind eeling moment is determined as te product of te wind side force and te total eeling lever arm, wic is te vertical distance between te points of attack of te wind force and its reaction force on te underwater ull, respectively: M = F *( Water + ) [Nm] (4)

5 Page 3 Were Water is determined using te following formulae, if specific measurements or oter direct data are not available: for < 0.85: for 0.85: Water Water = [ = [1.05] centeru ] centeru A ulat wit = (5) L oa A ulat being te lateral-plane area of te individual sip below water, L oa being te sip s over all lengt, being te breadt of te sip and ceteru being te vertical distance between te centre of te underwater lateral area and te waterline. Were appropriate and especially wen unusual sips are to be assessed, like e.g. multi-ull vessels or sips wit large appendages, direct measurements sould be used alternatively to determine Water, e.g., via model experiments in water or air. For te standard procedure M is constant for all angles of eel. In cases were appropriate directly determined data is available for different angles of eel, tis may be used accordingly. 6 Stability assessment Te resulting eeling angles due to M are calculated based on te static calm water lever arm curve. In case of steady wind te resulting static angle of eel ϕ SW sould not exceed [10] degrees or less in cases were te andling of te sip is substantially influenced by eel. In case of gust action te angle of eel ϕ WG is determined were te area below te wind eeling lever curve [equals] te area below te rigting lever curve (illustrated in figure 1). ϕ WG must be attainable witin te range of positive rigting levers and unprotected openings sould not be immersed at tis angle. Figure 1: Determination of ϕ GW

6 Page 4 7 Application examples Te following table sows application results for a large variety of sips, comparing te resulting GM values to te values according to te current Weater Criterion. Listed are te sip type, te lengt over all L oa, te beam, te draugt T, te windage area A lat, te GM req according to te current Weater Criterion, and te GM req according to tis wind criterion, as well as te static angle of eel ϕ SW and te angle of eel in case of gust action ϕ WG according to te proposed conservative standard evaluation. Sip type L oa T A lat GM acc. Weater Criterion GM acc. Criterion [m] [m] [m] [m²] [m] [m] [ ] [ ] Cruise Sip Cruise Sip Cruise Sip Cruise Sip Pax-Ferry RoPax-Ferry RoPax-Ferry RoPax-Ferry RoPax-Ferry RoPax-Ferry RoPax-Ferry RoPax-Ferry RoRo-Sip RoRo-Sip RoRo-Sip RoRo-Sip RoRo-Sip Car-Carrier Containersip < It can be seen, tat te required GM-values are in general of te same magnitude according to bot criteria. For some sips (especially some of te larger cruise vessels and RoPax-Ferries) te Weater Criterion is more stringent, in oter examples it is te wind criterion. 8 Conclusion, furter developments and validation Te examples sow te potential a wind criterion offers for te assessment of sip stability. Wen evaluating tese results more closely, it sould be noted tat te Weater Criterion was developed to represent sips wit large windage areas on average, wile tis proposed wind criterion uses conservative formulae for te standard assessment. Te wind criterion enables an easy straigt forward use of direct measured or calculated values alternatively wic is in line wit te intended cange towards performance based formulations. In order to assess and validate te criterion and its potential impact on today s sips in detail it is necessary to also take into account oter stability criteria. For example wen evaluating te results for te 320 m containersip it is found tat te Weater Criterion, despite te large windage area, is not te relevant intact stability criterion. If no furter stability rules were to be applied tis sip tus could be sailed wit te minimum GM of 0.15m, and tus at te first glance ϕ SW ϕ WG

7 Page 5 te wind criterion seems to be comparatively stringent. ut in tis case it is important to know tat te actual GM limiting curve of tis vessel requests an even iger GM-value tan according to te wind criterion due to damage stability requirements. Future developments and te validation process furtermore need to:.1 define te environmental conditions in more detail (e.g., te duration of te gust) and ceck te applicability and significance of te wind speeds (steady and gust) wit respect to teir probability of occurrence;.2 develop furter te conservative formulations to obtain te standard values: - te side force coefficient: C y (formula 1), te current value was determined based on lendermann [1] and is illustrated in figure 3; - te calculation of te eeling moment lever arm components above and below te water line: and Water (formulae 3 and 5 respectively). Te current formulation for was derived based on [1] and is illustrated in figure 2. Te formula 5 for Water could not be based on a sufficient bandwidt of direct measurements so far and tus must be seen as very preliminary, reflecting only te general trend; and.3 assess te criteria for te stability assessment wit respect to significance and practicability. 9 References [1] W. lendermann. Loading of Sips Collected Data from Tunnel Tests in Uniform Flow. IFS erict 574, Institut für Sciffbau der Universität amburg, [2] International Maritime Organization IMO. Code on Intact Stability for all types of sips covered by IMO instruments. Resolution A.749(18) as amended by resolution MSC.75(69), IMO, Figure 2: Figure 3: C y