Enhancing tug safety through internationally harmonised stability regulations

Transcription

1 Enhancing tug safety through internationally harmonised stability regulations BTA Safety Seminar Edinburgh, 9 th November 2017 Gijsbert de Jong Bureau Veritas Marine & Offshore Marine Marketing & Sales Director

2 Safety context: stability is key! 2

3 Technological context: innovation in design Ever larger merchant ships and offshore units require increasingly sophisticated and powerful tugs New design concepts focus on improving performance and safety of ship assist and escort operations Improved seakeeping performance for terminal operations in exposed waters Emission control & energy efficiency Fuel choice: dual fuel, LNG/CNG, bio fuel, hydrogen? Power distribution: hybrid (diesel-electric), all electric? Hybrid power generation: IC engines, batteries/capacitors, fuel cells? 3

4 Regulatory context: need for harmonisation Lack of international standards for tugs Considerable differences in requirements between classification societies Rules not up-to-date with latest technical developments No international stability requirements safety-critical operation! Wide variety of statutory safety equipment requirements for tugs < 500 GT BV, LR and ABS worked together on harmonisation of classification requirements for tugs within scope of SafeTug Joint Industry Project (JIP) R&D as basis for drafting technical requirements Input and feedback from JIP partners BV further developed guidelines in cooperation with industry partners NI617 (July 2014), 2014 Push to include NI617 towing and escort stability criteria in amendments to 2008 Intact Stability Code (through French IMO delegation) 4

5 NI617: basic principles & scope of requirements Basic principles No one size fits all approach: harbour tug offshore salvage tug Keep it practical, but stay open minded towards innovation Industry involvement through feedback loops with SafeTug JIP partners Technical requirements are function of the selected class notation(s) Definition of design loads for towing, escorting and pushing Towing and escort stability Towing and escort equipment Safety matrix for fire safety, life saving appliances, radio installations and navigation equipment for non-convention tugs Anchor equipment Interaction between tug and assisted ship 5

6 Background: classic tug girting Tug-induced tripping (self-tripping) Tendency of tug to overturn itself under influence of heeling moment created by opposing towline pull and steering forces (propellers) Tow-induced tripping (tow-tripping) Tendency for tow to veer off and create unexpected large transverse force/heeling moment Self-tripping generally considered as governing on modern tugs due to increased propulsion power Focus of harmonised stability rules But, real-life incidents often related to tow-tripping Usually small and relatively old tugs 6

7 Background: stability during escort service Escorting is considered to include active (emergency) steering, braking and otherwise controlling of escorted ship by (escort) tug operating in indirect towing mode Typical speed range escorted ship: 6 to 10 kn Thrust applied to indirectly generate towline force, as thrust vector not parallel to towline Quasi-static equilibrium reached between forces and moments arising from: Hydrodynamic lift and drag forces acting on hull and appendices of the tug advancing through the water at a drift angle relative to the flow Thrust vector (azimuth thrusters) Towline force ( deliverable ) 7

8 Background: stability during escort service Escorting is associated with considerable heeling angle High transverse forces, heeling angles and speed through water are characteristic of normal operation of escort tugs,... as opposed to accidental situation on ship assist tugs! Stability criteria need to reflect increased risk associated with escorting and are therefore more stringent than normal towing stability criteria F SD Seal Powered indirect mode 8

9 IMO developments Starting point MSC 86 (Nov 2010) with (unplanned) output on development of amendments to Part B of the 2008 Intact Stability (IS) Code in order to include criteria for towing and anchor handling operations (aftermath of Bourbon Doplhin capsize accident) Pre-2014 work undertaken by IMO s SLF and DE Sub-committees Since 2014 brought together in SDC Sub-committee BV active engagement started with SLF 55 Correspondence Group (Feb 2013) and continued until the inclusion of the harmonised towing and escort stability criteria in the final proposal at SDC 3 (Feb 2016), which was adopted by MSC 97 (Nov 2016) 9

10 Regulatory considerations Towing and escort stability criteria will be included in part B, chapter 2 Recommended design criteria for certain types of ships, which now includes tugs escort tugs Common practice for flag states to refer to recommended criteria to supplement mandatory general criteria included in part A Guidance to national administrations for domestic regulations Formal entry into force on 1 January 2020 (IMO four-year cycle policy for introduction of new amendments) BV already applies the harmonised stability requirements on a case-by-case basis and plans to implement the new IMO into the classification rules in July

11 Scope of application & quick release requirements Stability criteria apply to ships with keel laying date on or after 1 January 2020 engaged in harbour towing, coastal or ocean-going towing and escort operations and to ships converted to carry out towing operations after this date Self-tripping criterion based on BV NI617 (with extended application for transverse offset of towing point) Tow-tripping criterion based on existing national regulations Escort criteria based on BV NI617 Updated mandatory requirement (part A) that vessel engaged in towing operations should be provided with means for quick release of towline ( means of last resort ) IACS working on unified requirements for towing winch emergency release systems (driven by loss of Flying Phantom) 11

12 IMO towing stability Self-tripping criterion Energy balance approach based on areas: (Righting energy) A (Heeling energy) B Heeling lever generated by maximum available transverse thrust and corresponding opposing towline pull: HL= BP C T h cosφ r sinφ g C T = 0.50 for conventional tugs C T = 0.9/(1+d/L LL ) for ASD, RSD, ATD & VWT; minimum value 0.50 or 0.70 depending on case d = longitudinal distance between towing point and thrust point (combined heel and yaw effect) r = transverse offset of towing point, if applicable Weathertight openings required to be open during operation to be considered as down-flooding points for stability calculations: ER vents! C T d aft h aft d fwd h fwd 12

13 IMO towing stability Tow-tripping criterion Heel angle check of tug dragged along transversely by assisted ship: (Equil. angle) j e < (Down-flooding angle) j f Heeling lever generated by transverse drag resistance at 5 kn speed through water: HL= C 1 C 2 γ V2 A p h cosφ r sinφ+c 3 d 2 g C 1, C 2 = coefficients related to lateral traction corrected for heel angle C 3 = coefficient accounting for distance from center of lateral projected area A P to waterline as fraction of draught related to heel angle h = vertical distance from waterline to towing point d = mean draught r = transverse offset of towing point, if applicable 13

14 Clarification transverse offset of towing point Self-tripping heeling arm positively influenced by transverse offset of towing point (improved stability behavior) 14

15 Clarification transverse offset of towing point Several towing systems developed to create this effect, but careful verification of working principle required!! Case of double fairlead: r 2 0 is correct, r 1 is not correct 15

16 Clarification transverse offset of towing point Several towing systems developed to create this effect, but careful verification of working principle required!! Case of vertically movable towing hook: r 2 is correct, r 1 is false 16

17 Comparison self-tripping & tow-tripping Generally self-tripping more stringent than tow-tripping High propulsion power makes modern tugs more vulnerable to self-tripping Tow-tripping criterion raises questions w.r.t. relevance (safety?), applicability (modern tug designs?) and assumptions (5 kn speed sufficient?) Recommendation to IMO: reconsider tow-tripping criterion on basis of energy balance, with updated coefficients and speed 17

18 IMO towing stability Escort criteria Area ratio requirements to ensure safe margin against capsizing: A 1.25B C 1.40D Heeling arm to be taken as constant Proven concept (over 125 escort tugs classed by BV!) Additional equilibrium heel angle criterion: j e 15 Operational criterion related to workability and crew Based on experience feedback from tug masters (SafeTug JIP) In line with IMO anchor handling stability regulations comfort Escort simulations acceptable 18

19 NI617 Escort performance assessment Full scale trials or model testing Computer simulation (e.g. CFD based) Requirement include presentation of basic assumptions and underlying theoretical models, as well as submission of a validation report (comparison with results of model tests/full scale trials) Calculations for each loading condition over applicable range of escort speeds, maximising steering/braking/towline force and heeling moment/angle Results to be presented in butterfly/tug assist diagrams showing envelope of (steady state) combinations of steering and braking forces, covering applicable escort speed range Possibility for methodology AiP Additional requirement for operating information to be provided on board, including design limits and specific instructions (ref. Sec 2, [3.7]) 19

20 Escort operating limits failure scenarios 20

21 Escort operating limits failure scenarios 21

22 Escort operating limits draft proposals 22

23 New kids on the block 23

24 Application of towing stability criteria Configurations whereby propulsion units are distributed longitudinally along the tug, e.g. RotorTug, RAVE tug & EDDY Tug Two approaches: 1. Consider total transverse towline force as sum of towline forces generated by aft and forward propulsion units in accordance with self-tripping criterion separately, assuming that the aft and forward units can be considered as independent 2. In case the towing point is located between aft and forward propulsion units it is possible to generate a stable equilibrium (effectively towing in transverse direction) and equilibrium of forces and moments (in horizontal plane) should be applied in addition to approach 1 Maximizing thrust at one location and demanding horizontal plane moment equilibrium around towing point under consideration yields applied thrust at other location 24

25 Application example Tug with two thrusters forward and one thruster aft All thrusters equal power (F thrust,fwd = 2 F thrust,aft ) All towing points at centre line (towing point T 1 on foredeck, towing point T 2 above aft thruster, towing point T 3 between thrusters) Heeling lever approach 1 (any towing point T i ): HL φ,ti = F Thrust,aft Heeling lever approach 2 (T 3 ): h 1 + l i,aft L i,aft + 2 h LL 1 + l i,fwd L i,fwd cos φ LL g HL φ,ti = F Thrust,aft h i,aft + l 3,aft l 3,fwd h i,fwd g cos φ 25

26 Conclusion IMO MSC 97 (Nov 2016) adopted amendments to 2008 IS Code, including towing and escort stability requirements, largely based on BV tug guidelines (NI617, 2014) Enhanced safety in tug design and operation Level playing field for the industry Long awaited comprehensive internationally harmonised regulatory framework for tug stability that is technically consistent, pragmatic in application and open to innovation Entry into force on 1 January 2020, but earlier application by BV New regulations workable, but room for improvement Tow-tripping criterion has limited value (validity, speed) Improved escort operator guidance with traffic light system (scenario based) Extended application to innovative new designs; tailored approach needed for more out-of-the-box designs 26

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28 ANNEX New BV class notations and rules for tugs July 2017

29 New set of BV class notations Service notations tug (design bollard pull = [T BP /9.81] t) escort tug (design bollard pull = [T BP /9.81] t, design maximum steering force = [T Y,MAX /9.81] t, design maximum braking force = [T X,MAX /9.81] t, design maximum escort speed = [V MAX ] kn) Operating area notations operating within 5 miles from shore operating 4h from a place of refuge escort service limited to non-exposed waters Note: waters considered as non-exposed where wind fetch is not more than 6 nautical miles and significant wave height is not more than 1.0 m Example tug (design bollard pull = 70 t) operating 4h from a place of refuge 29

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