Design Methodology for Cross-flodding Connects on Naval Vessels

UnclassifiedThis document is supplied by QinetiQ for USCG Engineering Logistics Center under Contract No. DTCG40-03-P-40387. Design methodology for crossflooding connects on Naval vessels Mr A Peters; Mr M Galloway QinetiQ/FST/CR033339/1.0 January 2004 Requests for wider use or release must be sought from: Intellectual Property Division QinetiQ Ltd Cody Technology Park Farnborough Hampshire GU14 0LX Copyright © USCG Engineering Logistics Center [2004] Unclassified Unclassified Administration page Customer Information Customer reference number Project title Customer Organisation Customer contact Contract number Date due January 2004 DTCG40-03-P-40387 Design methodology for cross-flooding connects on Naval vessels USCG - Engineering Services Mr P Minnick Principal author A J Peters BEng CEng MRINA QinetiQ Haslar Haslar road Gosport Hants PO12 2AG Authorised by Name Post Signature Date of issue January 2004 Dr M R Renilson Technical Manager, Hydrodynamics +44 (0) 2392 335217 [email protected] Record of changes Issue 1.0 Date January 2004 Detail of Changes Page 2 QinetiQ/FST/CR033339/1.0 Unclassified Unclassified Abstract The scope of this project was to develop a methodology for the design of crossconnects on Naval vessels. As a demonstration of the state-of-the-art design philosophy and the use of time-domain simulation for design, an alternative crossconnect arrangement for the USCG’s 270-ft (Corvette-sized) WMEC ‘Famous’ Class ship (CG270) was performed. After using the methodology and the time-domain program to design the new system the performances of both the current and alternative cross-flooding arrangements were compared in relation to current IMO and USCG criteria. The study was conducted using the FREDYN program in order to compare the time-domain analysis of the current and alternative cross-flood systems. The existing method to statically calculate, by hand the time to cross-flood is examined and the merits and shortcomings of the two cross-connect arrangements are then discussed. The design and analysis of the cross-flooding ducts were performed using a nonlinear time-domain ship motion program called FREDYN. The use of a time-domain code allows cross-flooding ducts to be modelled to take account of the vessel motion and transient flow after the damage. This allows the effectiveness of the crossflooding ducts and the time taken to cross-flood to be assessed in a seaway. The worst damage case under the current USCG criteria was selected as the test case incorporating the tanks containing a cross-flooding system. For each damage scenario a set of thirty-minute simulations were performed with the vessel in a deep seagoing condition for a matrix of speed, heading sea state conditions. The simulations were repeated with and without the existing cross-flooding activated and with the new system activated. In the damage cases tested, the cross-flooding system was shown to improve the “after damage” performance of the vessel. In most of the cases tested the damage list angle was reduced by up to 8 degrees. The effectiveness of the new design of crossflooding ducts was demonstrated. QinetiQ/FST/CR033339/1.0 Page 3 Unclassified Unclassified Blank page Page 4 QinetiQ/FST/CR033339/1.0 Unclassified This work was funded by USCG ELC under customer reference DTCG40-02-Q-41363 QinetiQ assignment code 300411 0001. The analysis of the cross-flooding ducts was performed using a non-linear timedomain ship motion program called FREDYN.0 Page 5 Unclassified . by hand. After using the methodology and the time-domain program to design the new system the performances of both the current and alternative cross-flooding arrangements were compared in relation to current IMO and USCG criteria. The existing method to statically calculate. The simulations were repeated with and without the existing cross-flooding system activated and with the new system activated. an alternative crossconnect arrangement for the USCG’s 270-ft (Corvette-sized) WMEC ‘Famous’ Class ship (CG270) was performed. As a demonstration of the state-of-the-art design philosophy and the use of time-domain simulation for design. The effectiveness of the new design of cross-flooding ducts was demonstrated. QinetiQ/FST/CR033339/1. The FREDYN hullform was provided by the customer with a pertinent set of drawings of the vessel.Unclassified Executive summary The scope of this project was to develop a methodology for the design of crossconnects on Naval vessels. For each damage scenario sets of thirty-minute simulations were performed with the vessel in a deep seagoing condition for a matrix of speed. In most of the cases tested the damage list angle was reduced by up to 8 degrees. The worst damage case under the current criteria was selected as the test case incorporating the compartments containing a cross-flooding system. In the damage cases tested the cross-flooding was shown to improve the after damage performance of the vessel. the time to cross-flood is examined and the merits and shortcomings of the two cross-connect arrangements are then discussed. which was used to calculate hydrostatics and static stability calculations. heading and sea state conditions. Compartment definition in FREDYN is limited to orthogonal plane definitions of the compartment boundaries with the exception of the hull. The study was conducted using the FREDYN program in order to compare the time-domain analysis of the current and alternative cross-flood systems. The use of a time-domain code allows the ducts to be modelled to take account of the vessel motion and transient flow after the damage is initiated. A 3D solid static stability model was created using the PARAMARINE tool. This allows the effectiveness of the cross-flooding ducts and the time taken to cross-flood in a seaway to be assessed. 0 Unclassified .Unclassified Blank page Page 6 QinetiQ/FST/CR033339/1. 2 Damage extents 4.5 Effects of through life growth USCG 270-ft WMEC cutter 4.3 CG270 model generation 4.1 Criteria for cross-flood systems 2.1 FREDYN and PARAMARINE model 4.3 Duct routing and sizing 3.5 Alternative duct design 4.1 Run Set 1 .2 Current cross-flooding systems 2.7 Run selection 4.4 Cross-flooding modelling in FREDYN 4.Variation with ship loading condition 6.2 Run Set 2 .8 Ship condition Simulations Discussion 6.5 Results of parametric variation 2 3 5 7 10 10 11 13 13 14 17 19 19 21 23 23 24 25 25 25 26 30 31 31 32 33 34 35 35 36 37 38 38 3 4 5 6 QinetiQ/FST/CR033339/1.2 Cross-flooding design 3.6 Matrix of tests 4.Variation with position on wave at damage onset 6.3 Current static based hand calculations FREDYN and cross-flooding methodology 3.0 Page 7 Unclassified .4 Duct positioning 3.Variation with ship speed and heading 6.1 FREDYN and the extreme motions of damaged ship 3.Unclassified List of contents Administration page Abstract Executive summary List of contents List of figures List of tables 1 2 Introduction Review of cross-flooding systems 2.4 Run Set 4 .3 Run Set 3 .Variation with sea state 6. 0 kts .0 kts .sea state 5 .deep condition USCG duct .beam seas .beam seas .beam seas .light condition QinetiQ duct .7 kts .deep condition USCG duct .sea state 4 .USCG duct Heading vs time to cross-flood .beam seas .7 kts .sea state 5 .sea state 5 .no duct Heading vs RMS roll angle .0 kts .beam seas .Unclassified 7 8 9 10 Conclusions Recommendations References Tables Table 1: Figures Figure 1: Figure 2: Figure 3: Figure 4: Figure 5: Figure 6: Figure 7: Figure 8: Figure 9: Figure 10: Figure 11: Figure 12: Figure 13: Figure 14: Figure 15: Figure 16: Figure 17: Figure 18: Figure 19: Figure 20: Figure 21: Figure 22: Figure 23: Figure 24: Figure 25: Figure 26: Figure 27: Figure 28: Figure 29: Figure 30: Figure 31: Figure 32: Figure 33: 40 41 42 Sea state wave heights and period 43 11 No duct .QinetiQ duct Heading vs time to cross-flood.7 kts .beam seas .0 Unclassified .0 kts .sea state 4 .sea state 4 .7 kts .7 kts .deep condition QinetiQ duct .deep condition USCG duct .light condition No duct .sea state 5 .7 kts .beam seas .beam seas .deep condition Heading vs transient roll angle in sea state 5 .USCG duct Heading vs transient roll angle .sea state 4 .deep condition No duct .sea state 5 .beam seas .0 kts .deep condition 44 44 44 45 45 45 46 46 46 47 47 47 48 48 48 49 49 49 50 50 50 51 51 51 52 52 52 53 53 53 54 54 54 Page 8 QinetiQ/FST/CR033339/1.0 kts .sea state 4 .QinetiQ duct Heading vs RMS roll angle .7 kts .sea state 5 .QinetiQ duct Heading vs time to cross-flood at 12 kts before damage 0 kts after damage Heading vs time to cross-flood at 7 kts before damage 0 kts after damage Heading vs time to cross-flood at 7 kts before damage 7 kts after damage No duct .beam seas .0 kts .no duct Heading vs mean list angle .no duct Heading vs transient roll angle .USCG duct Heading vs RMS roll angle .0 kts .7 kts .light condition USCG duct .deep condition Heading vs transient roll angle at 12 kts before damage 0 kts after damage Heading vs transient roll angle at 7 kts before damage 0 kts after damage Heading vs transient roll angle at 7 kts before damage 7 kts after damage Heading vs RMS roll angle at 12 kts before damage 0 kts after damage Heading vs RMS roll angle at 7 kts before damage 0 kts after damage Heading vs RMS roll angle at 7 kts before damage 7 kts after damage Heading vs transient roll angle .beam seas .deep condition QinetiQ duct .deep condition QinetiQ duct .beam seas .7 kts .sea state 4 .deep condition Heading vs transient roll angle in sea state 4 .0 kts .deep condition Heading vs transient roll angle in sea state 3 . light condition Heading vs time to cross-flood .QinetiQ duct Heading vs time to cross-flood .USCG duct Heading vs RMS roll angle .USCG duct Heading vs transient roll angle .opening towards 0 kts Sea state vs transient roll angle at beam seas .light condition Heading vs transient roll angle in sea state 4 .in sea state 3 Heading vs time to cross-flood .0 kts Sea state vs time to cross-flood .no duct Heading vs transient roll angle .deep condition Heading vs RMS roll angle in sea state 5 .USCG duct Heading vs time to cross-flood .0 Page 9 Unclassified .0 kts Sea state vs RMS roll angle at beam seas .no duct Heading vs RMS roll angle .light condition Heading vs transient roll angle in sea state 5 .opening towards .no duct Heading vs mean list angle .7-7 kts Sea state vs time to cross-flood .light condition Heading vs RMS roll angle in sea state 4 .12-0 kts Sea state vs RMS roll angle at beam seas .deep condition Heading vs RMS roll angle in sea state 3 .QinetiQ duct Heading vs RMS roll angle in sea state 3 .no duct Sea state vs transient roll angle at beam seas .in sea state 4 Heading vs time to cross-flood .opening towards 12-0 kts Sea state vs transient roll angle at beam seas .7-7 kts Sea state vs transient roll angle at beam seas .light condition Heading vs transient roll angle .opening towards .12-0 kts Sea state vs time to cross-flood .USCG duct 55 55 55 56 56 56 57 57 57 58 58 58 59 59 59 60 60 60 61 61 61 62 62 62 63 63 63 64 64 64 65 65 66 67 Initial distribution list Report documentation page QinetiQ/FST/CR033339/1.QinetiQ duct Heading vs RMS roll angle .deep condition Heading vs RMS roll angle in sea state 4 .light condition Heading vs RMS roll angle in sea state 5 .opening towards 7-7 kts Sea state vs RMS roll angle at beam seas .in sea state 5 Sea state vs transient roll angle at beam seas .Unclassified Figure 34: Figure 35: Figure 36: Figure 37: Figure 38: Figure 39: Figure 40: Figure 41: Figure 42: Figure 43: Figure 44: Figure 45: Figure 46: Figure 47: Figure 48: Figure 49: Figure 50: Figure 51: Figure 52: Figure 53: Figure 54: Figure 55: Figure 56: Figure 57: Figure 58: Figure 59: figure 60: Figure 61: Figure 62: Figure 63: Figure 64: Figure 65: Heading vs transient roll angle in sea state 3 .opening towards . Unclassified List of figures Figure 2-1: Straight duct .plan view Figure 4-1: Damage zone and cross-flooding duct location Figure 4-2: PARAMARINE hullform Figure 4-3: Stern of CG270 Figure 4-4: Damage region Figure 4-5: Cross-flooding tanks and existing duct Figure 4-6: Alternative QinetiQ cross-flood design (red) List of tables Table 4-1: Damage region Table 4-2: Run list Table 5-1: Ship conditions Table 6-1: Results from parametric variation 15 15 16 16 16 25 27 28 28 29 31 26 32 34 38-39 Page 10 QinetiQ/FST/CR033339/1.0 Unclassified .cross-section Figure 2-4: Double duct .cross-section Figure 2-3: Double bottom tank .cross-section Figure 2-2: Inverted U type duct .cross-section Figure 2-5: Double duct . The program was written by MARIN with steerage and guidance from the CRN committee. a Co-operative Research Navies (CRN) committee was established with the aim of producing intact dynamic stability criteria for naval vessels. DRDC. and can predict the vessel’s behaviour in waves. The study was conducted using the FREDYN program in order to compare the timedomain analysis of the current and alternative systems to the design guidance given in the report “Cross-flooding of a Frigate Sized Vessel”. however. FREDYN. March 2001). FREDYN. freeboard. in moderate to extreme seas. The scope of this project was to develop a methodology using the state-of-the-art design philosophy and the FREDYN program for the design of cross-connects on naval vessels. This required an extensive sensitivity study of a number of design parameters. rather than simple pseudo-static analysis. excitation forces and rigid-body dynamics. To investigate the effect on the vessel of the two cross-flooding duct designs a matrix of runs was completed with FREDYN to assess the performance of the vessel. This permits investigations into the dynamics of damaged vessels in realistic environments. As this would be impractical at full scale and both costly and time consuming in terms of model experiments. The criteria were required to ensure that new vessels were safe. a time-domain prediction code. the committee needed to test in-service and new ship designs. The merits and shortcomings of the IMO hand calculation and the proposed method are discussed as well as the two cross-connect arrangements.Unclassified 1 1.4 QinetiQ/FST/CR033339/1. while avoiding the high build and life cycle costs associated with over-engineering. dynamic stability and range of positive stability. France. (Peters. was developed. QinetiQ. been well understood. Static stability based hand calculation using the IMO guidelines for the performance of the cross-flooding ducts can be performed quite easily using data from standard static stability software. The use of the FREDYN program enables the performance of the cross-flooding ducts to be analysed in a seaway and so allowing the time taken to cross-flood to be calculated more accurately.2 1. unlike the currently available frequency-domain programs. Both the current and alternative cross-flooding arrangements were tested in a large matrix of scenarios and also assessed in relation to current IMO and USCG criteria. the US Coast Guard. The latest version of FREDYN can model vessels with damaged compartments and cross-flooding ducts. during and after flooding can be examined.0 Unclassified . until recently. Canadian. this does not take account of the vessel motions or transient flow after damage. The effectiveness in a seaway of cross-flood (cross-connect) arrangements fitted to naval ships has not. MARIN and NSWCCD. As an example to demonstrate the methodology an alternative cross-connect arrangement was designed for the USCG’s 270-ft (Corvette-sized) WMEC ‘Famous’ Class ship (CG270).3 1. GM. The other advantage of the simulation is that the performance of the vessel before. which is the current practice. Netherlands. is able to take account of the non-linearities associated with the drag forces. including KG. The CRN committee comprises representatives from the Australian. For Page 11 1.1 Introduction In 1990. To derive the dynamic stability criteria. UK and US MOD/DoDs. to assess their relative safety and probability of capsize. Unclassified each simulation. 1.0 Unclassified . statistics of the run parameters were plotted to show the time taken to cross-flood and the resulting vessel behaviour. Page 12 QinetiQ/FST/CR033339/1.5 This work was funded by United States Coast Guard customer reference DTCG40-02Q-41363 QinetiQ assignment code 350753 0001. The maximum angle of heel after flooding but before equalisation shall be less than 15 degrees. QinetiQ/FST/CR033339/1. the origins date back to data and information gathered over many years. thus allowing the time taken to achieve cross-flooded equilibrium to be assessed. which has been adopted by many Navies around the world including the USCG and UKMOD. Following this incident a review of stability assessment was undertaken. The criteria goes on to state that for the unsymmetrical case that the angle of heel after equalisation has completed should be less than 12 degrees for two or more compartment damage.1 Criteria for cross-flood systems The 1960 SOLAS conference first laid out the requirements for cross-flooding systems where a maximum time for cross-flooding was defined as 15 minutes. which resulted in new stability criteria for US Navy ships (Sarchin and Goldberg. This covers the intact and damaged stability criteria. which struck vessels of USN Pacific Fleet causing the loss of 790 men and three destroyers (see Calhoun. 2001). This limit was probably based on evidence from the time and the need to set an achievable standard. 2. large-amplitude motion dynamics play an important role in the capsize behaviour of a frigate in waves and to the performance of the cross-flooding arrangement fitted. 1962). Where it is necessary to correct large angles of heel the means adopted shall. As with many static-based stability criteria adopted around the world. The use of a time-domain simulation program enables the performance of the cross-flooding ducts to be analysed in a seaway. However.Unclassified 2 Review of cross-flooding systems Longitudinal subdivision is common practice in ship design. Static stability analysis of the effectiveness of the cross-flooding ducts can be performed using standard static stability software. The effectiveness of cross-flood arrangements fitted to naval ships in a seaway has until recently been little known. Where cross-flooding fitting is required the time to equalisation shall not exceed 15 min. Unsymmetrical flooding is to be kept to a minimum consistent with efficient arrangements. This criterion for cross-flooding is stated as follows: 5. This internal arrangement can introduce asymmetric flooding in damage cases which can be resolved in many ways involving improving general stability with solid ballast or with liquid loading restrictions.0 Page 13 Unclassified . These fittings shall be acceptable to the Administration. Suitable information concerning the use of the cross-flooding fitting shall be supplied to the master of the ship. where practical. but where controls to cross-flooding fittings are provided they shall be operable from above the bulkhead deck. This criteria is now included in the current regulations as Regulation 8 (5) in Chapter 2 Part B of the International Convention for the Safety of life at sea (SOLAS. Cross-flooding systems are regarded as a possible solution to this problem. 1981). but this does not take account of the vessel motions or transient flow after damage. This applies especially to the great Pacific Typhoon of December 1944. be self-acting. • Area A1/A2 greater than or equal to 1. It does not specify any time constraints for this. which is divided into criteria for damage stability for both sideprotected and non-protected vessels. and there is no one design suitable for all situations.2 USCG criteria From the current USCG Design and Construction Standard (DCS) SWBS 079 on the use of "Cross-Connection of Tanks" it states "cross-connection of tanks should only be employed where other alternatives have been evaluated and are deemed impracticable”. Cross-flooding time shall not exceed five minutes Prior to cross-flooding the following criteria shall be met: • Heel shall not exceed 20 degrees. Page 14 QinetiQ/FST/CR033339/1. which states that the maximum list shall not exceed 20 degrees and that arrangements exist for rapidly reducing the list to less than 5 degrees.2 Current cross-flooding systems The following duct types have been seen fitted to both commercial and naval vessels in recent years. 1975). 2. It then states that where cross-connection of tanks is utilised.1. The non-protected criteria relate to the 270-ft cutter that is the class used in this investigation. Even if the tank layout is a similar shape and in a similar position to an existing design the size of the required ducts is still unknown.1 DDS 079-1 criteria The US Navy stability criteria are documented in the Design Data Sheet (DDS) 079-1 (US Navy.Unclassified 2. A brief description of some of the main styles of cross-connection duct designs that are commonly used is given below. the following applies: • • • The cross-flooding system shall prevent transference of liquids from one tank to the other during normal rolling of the ship.1. The DDS 079-1 states that an angle of less than 15 degrees is required after damage for operational requirements. with some of their advantages and disadvantages. 2. Different types of cross-flooding arrangements are suitable for different tank positions and damage scenarios. There is no mention of cross-flood systems except for in the side-protected vessels.4.0 Unclassified . As long as the top of the U Tube remains permanently below the damaged waterline the fluids will cross-flood QinetiQ/FST/CR033339/1. This system is often used where normally empty compartments are causing the asymmetry. which is in-line with the IMO guidelines for passenger ships.cross-section The “inverted U” type duct is often used to connect tanks together in a similar manner to the straight duct but with the reduction of the possibility of the fluids mixing during the ship’s normal motions due to its shape.0 Page 15 Unclassified . which consists of a straight pipe connecting the bottom of two wing tanks together. which stops the tanks mixing fluids in normal service. This creates a totally passive system. “Inverted U” type duct Tank 2 Tank 1 Figure 2-2: Inverted U type duct .cross-section This is the simplest of all cross-flooding designs. This is not desirable and can cause stability issues. This system is less suitable when it connects two tanks. which states that cross-flooding systems should be self-starting where possible. as this system allows easy transfer of liquids. and contamination if the tanks contain different fluids. A variation of this system has a valve in the centre of the pipe.Unclassified Straight duct Comp 2 Comp 1 Figure 2-1: Straight duct . The down side to a valve is that it has to be manually opened following damage which means there is an additional risk and time delay to the cross-flooding operation due to the required human action. or at least minimises.Unclassified successfully after damage. Double duct arrangement Tank 2 Tank 1 Tank 2 Tank 1 Figure 2-4: Double duct .cross-section The pipe arrangement above is a similar system that is often used to connect double bottom tanks together. This design. The disadvantage of this system is that double the amount of pipe is required. If the top of the pipe emerges from the water then the duct will not function.0 Unclassified . There are both disadvantages and advantages to this type of system. The downside to this system is that one tank cannot be pressed full while the other is empty. Tank 2 Tank 1 Figure 2-3: Double bottom tank . The design of this type of duct requires careful design to ensure that the duct will function after damage. The layout of the pipes prevents. the tanks will cross-flood continuously if the duct stays below the damage waterline. Page 16 QinetiQ/FST/CR033339/1. means that the tanks are prevented from mixing liquids in the intact state. After the damage event.cross-section Figure 2-5: Double duct . because of its low position in the ship.plan view The double duct system is another type of arrangement that has been used in recent years. the mixing of the tanks. which it is likely to be the case due to its low position in the ship. but depending on the damage waterline it is possible for both pipes to flood which decreases the time to cross-flood and incorporates some element of redundancy in the system. which is the flow reduction factor that is based on calculations for flow in pipes. This is also true for the time-domain computational method as the calculation of the flow reduction factor is also required. QinetiQ/FST/CR033339/1. It was shown by Peters (2001) to be possible with care that suitable values could be produced for the total friction coefficient and used successfully in the time-domain simulations. This formulation does not take account of the transient roll or the motion of the vessel during the cross-flooding process. 1961). The shape and length of the pipe is also taken into account through the inclusion of a total pipe friction coefficient. This formulation then provides a simple answer to the time to cross-flood based on static calculations. but actual final positioning and sizing of the duct is critical to the final performance and requires careful evaluation. It is also not always straightforward for the designer to select the water height head to use in these calculations. This formulation takes account of the static water head at the start and end point of the cross-flooding and the amount of water to cross-flood. Using a time-domain program like FREDYN allows the parameters that effect the cross-flooding performance to be assessed both independently or together.Unclassified These previous used arrangements show some of the basic designs for cross-flooding systems. Care must be taken in the calculation of the ‘f’ term. 2.3 Current static based hand calculations The current practice for a designer assessing the performance of cross-flooding systems involves using an approximate formula that was derived by Dr Ing Gino Solda in 1961 (Solda. The formula is as follows: 1− Hf Ho 2gHo 2W To = sf 1 Hf 1− Ho (1) To W s f g Ho Hf = = = = = = = Time to cross-flood Total volume of water for equalisation Cross-sectional area of cross-flooding pipe Flow reduction factor for the duct Acceleration due to gravity Head of fluid before equalisation Final head of water (after complete equalisation) This formulation is suitable for calculating an initial figure for the time to cross-flood at the early stages of design.0 Page 17 Unclassified . Unclassified In comparison to the tank experiments (Peters.0 Unclassified . In the example in this report. it was shown possible in calm water to achieve predictions using this static formula with about 10% error. This meant that an accurate full hydrostatic computer model of the damaged ship was required to get accurate water head data to achieve good results using the formulation above in calm water. It was easy to achieve 30 to 40% differences in the time to cross-flood in comparison to the results from the experiments and time-domain simulations due to inaccuracies in the prediction of the pressure heads. 2001). An additional problem occurred with double duct arrangements due to predicting the contribution by the second duct which was difficult to determine. Relatively small changes in the pressure heads were found to change the time to cross-flood significantly. often under predicting the time to cross-flood. with little additional effort a full detailed investigation of the crossflooding can be made. using just the damage drafts to approximate the damage water heads again caused differences of the same magnitude. Page 18 QinetiQ/FST/CR033339/1. By exporting the geometry to a program like FREDYN. where all physical factors are considered. Both the viscous forces and the potential forces are added to complete the physical model.0 Page 19 Unclassified . In reality the vessel would be rolling and pitching about in waves causing the pressure heads to continually change thereby effecting how the cross-flooding system operates.1. 3.1 FREDYN and the extreme motions of damaged ship The FREDYN program was written by MARIN with steerage and guidance from the CRN committee. Unlike the currently available frequency-domain programs. Nonlinearities have to be considered as they arise from: • • • • Effect of large angles on excitation forces.Unclassified 3 FREDYN and cross-flooding methodology The current practice for any design of a cross-flood system involves basic static analysis with the calculations using the assumption of calm water. x . The theory for predicting the large amplitude motions with FREDYN has been described by McTaggart and De Kat (2000) and by Van ‘t Veer and De Kat (2000). using this approach classical rigid-body dynamics can be used to derive the equations of motion. The derivation of the equations of motions for a ship subjected to flooding through one or more damage openings is based on the conservation of linear and angular momentum for six coupled degrees of freedom. The recent advances in time-domain simulation now allows a damage simulation to be conducted in 6 degrees of freedom with cross-flooding and down-flooding included. t ) v (2) QinetiQ/FST/CR033339/1. and Integration of wave induced pressure up to free surface. FREDYN was designed to enable the simulation of motion of an intact steered ship in wind and waves (MARIN. This allows the transient behaviour and motions after damage to be taken into account when evaluating the cross-flooding system. wave orbital velocities and wind. Drag forces associated with hull motions. Rigid body dynamics with large angles.1 Time-varying mass In time-domain simulations it is necessary to integrate first-order equations of the form (see De Kat and Peters. 2002). 2002): =v x  = f (v . and can predict the vessel’s resulting behaviour in waves. The approach is a physical one. excitation forces and rigid body dynamics. 3. The latest version of FREDYN can model vessels with damaged compartments and cross-flooding ducts. The fluid inside the ship is considered as a free particle with concentrated mass. FREDYN is able to take account of the non-linearities associated with the drag forces. which appear when expressing the conservation of momentum in a shipfixed co-ordinate system.I z z . The summation signs in the RHS represent the sum of all external force contributions. Using the above approach and generalised 6x6 mass matrices yields the following equations of motion for a damaged vessel in the ship-fixed co-ordinate system: Fx Fy Fz Mx My Mz ( m 0 + m f ).0 .0 )q r ( I x x . this equation can be written: dm F −v dv dt = = v dt m + m' (5) which is of the form required for numerical integration. [a(∞)] is the added mass matrix that is part of the linear radiation forces (the convolution integrals are part of the force terms in the RHS).0 )p q ( [ M 0 ] + [a ( ∞ )] + [ M f ] ) . The “additional terms” in the RHS of the equations of motion stem from cross products.v Gr) ( m 0 + m f ).u G q )  f ]. potential th flow and viscous fluid forces.I y y . including the effect of damage fluid. time-dependent mass and inertia terms associated with the floodwater.( w G q . One of the exciting force contributions that is treated "exactly" stems from the hydrostatic and dynamic wave pressure. and from the motion of the fluid relative to the ship.( u Gr . including non-zero off-diagonal terms. [Mf] is the 6x6 matrix containing all ship-acceleration related.0 Unclassified . xG = (6) + a d d itio n a l te rm s The matrix [M0] is the generalised 6x6 mass matrix of the intact ship.[M ( I z z . This represents the Froude-Krylov force.I x x .0 . The equations are derived by taking the time derivative of momentum to give: F= d dv dm ( mv) = m + v dt dt dt (3) The second term on the RHS may be moved to the LHS and combined with the force vector: F −v dm dv = (m + m ') dt dt (4) The term m’ is the mass that varies with time due to the floodwater. The equations of motion are solved using a 4 order Runge-Kutta scheme. Page 20 QinetiQ/FST/CR033339/1.0 )p r ( I y y .0 .Unclassified The effect of time varying mass associated with the flooding is treated as follows. x G + .( v Gp . while m is the actual physical mass of the body. After rearranging.w Gp ) ( m 0 + m f ). Q. or Van 't Veer and De Kat. 3. 3. This analysis is applied to each damage opening or holes between two compartments. the velocity through a damage opening can be calculated. In the case of irregular waves. This coefficient accounts for a combination of several effects (such as friction losses).1.1. 2002. airflow and compression effects are modelled using the appropriate gas laws. 2002) demonstrated that the simulation program FREDYN effectively modelled the motions of a ship with cross-flooding systems operational. A simple yet practical approach is to assume that the water level of the floodwater inside any compartment remains horizontal (earth-fixed) at all times.2 3. the flooding model is based on the Bernoulli equation (see De Kat and Peters. the following empirical formulation is used: Q = Cd v 2 A (7) where A is the area of the opening and Cd is the discharge coefficient.2. the fluid mass inside a shipboard compartment is known at each time step.0 Page 21 Unclassified . 2000).1 Water ingress and fluid loading Hydraulic flow To estimate the flow rates of water entering a compartment. These points were derived during the “Cross-flooding of Frigate Sized Vessels” project conducted in March 2001 for the USCG and the UKMOD (Peters. To obtain the total discharge through an opening. Comparison of previous cross-flooding validation between a FREDYN prediction and an experiment of a damaged Leander class frigate operating in a seaway (De Kat and Peters.2 Cross-flooding design Using either the current static hand calculations or using the time-domain program the following points give some guidance for consideration while designing a crossflooding system.2 Quasi-dynamic fluid loading Based on the computed inflow and outflow of fluid through all openings.Unclassified which is obtained by pressure integration over the instantaneous wetted surface of the hull at each time step. QinetiQ/FST/CR033339/1.1. Linear wave theory is used to describe the sea surface and wave kinematics. In addition. Based on the difference in pressure head. It assumes stationary flow conditions and no loss of energy due to friction or increased turbulence. This will account for a large part of the non-linearities that affect the ship response. the model makes use of linear superposition of sinusoidal components with random phasing. Cross-flooding ducts are modelled in a similar way to this but account is taken of the friction in the pipe.2. 3. And v2 is fluid velocity. This implies that the damage fluid causes a vertical force (due to gravity) to act on the ship and that any sloshing effects are neglected. which is the case for the current and alternative arrangements. Page 22 QinetiQ/FST/CR033339/1. When a cross-flooding system involves human intervention or additional machinery or pumps then the time to operate is increased. The first main point that should be noted is that cross-flooding reduces the reserve of buoyancy on the intact side of the ship. This is also undesirable if the ship sustains asymmetric damage elsewhere then the part full fuel tanks can drain freely under gravity into the tank on the lower side. void spaces or cofferdams. the position should be carefully assessed. in an extreme case. This will result in immediate and continual flooding to the point of equilibrium. FREDYN simulations (Peters 2001) have highlighted the case where a wider ship. with a duct opening in the centre of the side tanks. The inverted U duct and double type duct would not be affected to such an extent due to their design. If the compartment will not press full when damaged then a system along the lines of the straight or inverted U duct is recommended. Design studies and model experiments at Haslar have shown that even the initial transient roll is fractionally reduced with cross-flooding systems that initiate immediately after damage. then cross-flooding will not initiate until the duct submerges below the waterline. During the design of cross-flooding systems the design should be made passive or automatic where possible. A system requiring activation will not begin operation during the critical seconds immediately following damage. This reduces problems associated with additional free-surface effects and cross-flooding effectiveness. The inverted duct has the advantage that its shape resists mixing. stopping completely any flow into the far side tank. It must be fitted to ensure that during the initial rolling after damage all the openings stay below the water surface. in a totally passive manner. If the duct rises above the waterline. as is the risk of the system not operating effectively. The straight duct system is ideal for connecting two empty tanks. the highest part of the duct must be formed in a way so that none of the duct rises above the waterline any time after damage. Cross flooding could. but if it is used to connect two fuel tanks there is likely to be constant mixing of tanks. substantially reducing stability and lowering downflooding points closer to the waterline. to a certain extent. When straight duct systems are fitted low in a ship that has a high beam. especially with the double type duct. The straight duct option will allow tank mixing in the intact state unless a valve is fitted. remain pressed full even during rolling. All parts of the duct route should be below damaged waterlines at all times if the inverted U type duct is used. if possible.Unclassified 2001).0 Unclassified . This should be investigated to ascertain whether cross-flooding would firstly be beneficial or not. for example during the transient roll. Where possible. as the duct openings are likely to be well below the waterline throughout the damage event. thus degrading stability. cross-flooding systems should be fitted in regions where the compartments will fill completely after damage and. rolled after damage to an angle so as to raise the end of the duct above the water. cause a ship to sink further and reduce waterplane area. Ducts of 0. This is only recommended in the extreme circumstances to stop the tanks mixing. Any valve should be fitted near the centre of the duct to reduce the possibility of it not operating after damage. If cross-flooding is to connect two tanks together then the tanks should contain the same type of contents. It is recommended that the tops of the ducts should be positioned so that they do not rise above the water surface. increasing the time to cross-flood. transient and damage phases to ensure immediate and effective cross-flooding. and preferably be as large as practically possible for the compartment. so reducing the mixing of tank fluids. particularly in the transient roll. as the system then only floods through one duct.25 m.Unclassified The double type duct system has shown to be very effective in certain conditions and less effective in others. Ducting should be the shortest possible length and contain as few bends and valves as possible to reduce frictional losses. Duct sizes should be physically or computationally modelled to assess any potential free surface or stability problems during the cross-flooding stage. which is close to the size used in frigate sized vessels. because when the compartment does not fill the duct often emerges above the waterline. Other considerations are to be made to the pipe run to reduce the risk of damage to the cross-flooding arrangement during the damage event. The duct diameter used in that experiment scaled to a 0. It is recommended that this type of system should only be fitted in compartments that will be pressed full after damage.28 m diameter duct at full scale. stopping the cross-flooding.4 Duct positioning The pipe openings and pipe runs should be positioned so they remain below the water line during the intact. The design to stop or restrain the mixing of tanks must not reduce the effect of the ducts if damage is sustained. even in small tanks. Current designs often have openings at the top of the tanks or a curved connection to restrain cross-flooding. The remote opening of the valve should be possible from several locations on the ship.4 m diameter are suggested as suitable for frigate type vessels as they showed a rapid cross-flooding in the double type ducts case. It is suggested that the duct diameter should not be lower than 0.0 Page 23 Unclassified . 3. as there will definitely be some mixing with passive crossflooding systems. reducing the transient peak. 3.3 Duct routing and sizing It has been shown by Peters (2001) that the effect of increasing the cross-sectional area of the duct is proportional to the decrease in roll angle during the first 4 roll oscillations. this system floods through both ducts allowing for a rapid cross -flooding. QinetiQ/FST/CR033339/1. It has been shown that when the compartment is pressed full and below the waterline. Ducts should be designed so as to reduce the potential sloshing between tanks. including the bridge. An option may involve a manual valve to be fitted to stop mixing if the vessel operates in scenarios that cause large roll angles. 0 Unclassified .Unclassified For bottom and side tanks. stopping cross-flooding where it may not do so in an earlier condition. so as to offer some protection to the ducts during the damage incident. that follow closely the shell plating. the ducts would still operate effectively. The condition later in life may result in. It is recommended that a minimum of two ducts are fitted to a compartment to incorporate an element of redundancy so that cross-flooding will still occur (at a slower rate) if one gets blocked through damage. although in cases tested often there is flooding through both ducts so increasing the rate of cross-flooding. Double type ducts. the ducts should be as far as possible from the shell plating to ensure the duct itself does not get bent or blocked during damage. This is to ensure that as KG and displacement grow. part of the duct rising above the waterline. for example. should be positioned preferably at each end of the compartment near the bulkheads.5 Effects of through life growth The chosen design of cross-flooding system should be analysed at a range of expected through life conditions for the vessel. Page 24 QinetiQ/FST/CR033339/1. 3. The double type duct has the disadvantage that it requires the complexity of two pipes to be fitted. where the risk of collision damage is highest. The general approach is to identify the damage zones and compartments. 4. 4.0 Page 25 Unclassified . generate the appropriate computer models to facilitate the analysis.Unclassified 4 USCG 270-ft WMEC cutter To demonstrate the methodology and use of FREDYN in the design and guidance for cross-flooding designs the USCG 270-ft cutter was chosen as an example case for the study.2 Damage extents The damage zones and their extents were provided by the USCG. Figure 4-1: Damage zone and cross-flooding duct location QinetiQ/FST/CR033339/1. A detail description of the process follows. The damage zones were selected based on the static stability studies where the damaged list angles were shown to be the worst. The 270-ft cutter has an existing cross-flooding pipe between the Port and Starboard Ballast tanks 4-03-1-W and tank 4-103-1-W. The damage zone extents and associated cross-flooding for the damage case are shown in Table 4-1 and their location in Figure 4-1.1 FREDYN and PARAMARINE model The modelling approach taken for this task followed Haslar standard practice developed during previous FREDYN cross-flooding studies. verify these models against benchmark data and finally run a matrix of FREDYN simulations. The hull definition was generated from a surface fit of curve geometry data provided by the USCG ELC in the form of “270wmec. which gives confidence in the algorithms and equations used. DB-1 Platform DB DB DB DB DB TT 1 Platform 1 Platform-Main Deck 1 Platform Main Deck Main Deck Main Deck Main Deck st st st Table 4-1: Damage region 4. this surface was then Page 26 QinetiQ/FST/CR033339/1.5 165 165 169 169 165 151.5 165 165 103 103 124 165 From 145 120. The transom was treated as a separate surface fit due to rapid change in curve direction.hul” file containing offset data. This work was performed on the behalf of the UKMOD. Stbd Stbd Stbd Stbd Stbd Symm. Stbd Symm. some manual fairing was performed to remove any inflexion points. Symm. Symm. The surface fit operation in PARAMARINE automatically provides a good match to the curve data. PARAMARINE was chosen as the software for which the static stability model would be produced. Stbd Stbd Symm.Unclassified Damage Space Clean Ballast Seachest Engine Space CPP Pump Oil Oily Waste Diesel Oil Service Clean Ballast Aft Void Dry Provisions Engineers Ctrl Room Elevator Crew Accommodation Electronic Store Uptakes Galley and Mess CPO Accommodation Description 4-103-1-W 4-162-1-F 4-165-1-F 4-165-3-F 4-169-1-W 4-169-0-A 3-152-0-E 3-165-1-Q 1-103-3-A DB DB st Deck Frames From 103 117 103 161. it also served as a benchmark test to validate the FREDYN model.0 Unclassified . The transom surface was later “sewn” to the main hull surface. a basic static stability model and the FREDYN dynamic stability model. QinetiQ (Haslar) has rigorously tested and validated PARAMARINE against pure mathematical models. which is the static modelling package used by the UK Ministry of Defence (UKMOD). The Graphics Research Corporation (GRC) developed PARAMARINE.3 CG270 model generation Two computer models of the USCG 270-ft WMEC were required to perform FREDYN simulations.5 165 165 169 169 186 186 186 165 169 186 124 124 165 186 Extent Stbd Stbd Symm. however. A static stability model was required to provide the basic hydrostatic inputs for FREDYN. The internal arrangement was generated from the general arrangement drawings and frame sections provided by the USCG ELC. The subdivision breakdown is displayed in Figure 4-4.0 Page 27 Unclassified . For the purpose of this task only the proposed damage section of the vessel has been fully subdivided. therefore some compartments may have a combination of rooms and corridors. It should be noted that this PARAMARINE model does not detail every room and passageway in each watertight compartment. Figure 4-2: PARAMARINE hullform QinetiQ/FST/CR033339/1.Unclassified used to develop a solid hull definition Figure 4-2. Each section in the damage zone was subdivided into its watertight compartments and tanks. The transom is highlighted in Figure 4-3. The cross-flooding duct was modelled using PARAMARINE to help visualise its complex shape. however.Unclassified Figure 4-3: Stern of CG270 Figure 4-4: Damage region The USCG 270-ft WMEC is fitted with a cross-flooding system connecting clean ballast tank 4-103-1-W with clean ballast tank 4-103-2-W.0 Unclassified . the duct was not used in the subsequent static stability analysis. The cross-flooding arrangement is shown in Figure 4-5. Page 28 QinetiQ/FST/CR033339/1. shaft. The model validation was conducted in accordance with current practice for UK Navy computer models to Sea Systems Publication Number 24 (SSP24) Stability of Surface Ships (REFERENCE SSP24). therefore the PARAMARINE design was required to be modified so that the validation parameters would be comparable. and the vertical centre of gravity to be within 1%. The internal compartments were also subject to validation to SSP24 standards where data existed. All subsequent calculations performed used the modified PARAMARINE design. There are no criteria set for compartment longitudinal and transverse centre of gravity.9 m apart. The standards require the compartment volumes to be within 2%. as these are deemed less important. tanks. At each stage in the development of the FREDYN model a number of checks and comparisons were performed against the PARAMARINE design and the data supplied by USCG ELC. therefore a 1% criteria for the transverse centre of gravity was introduced.Unclassified Figure 4-5: Cross-flooding tanks with existing duct Finally point buoyancies were added to reproduce the additional buoyancy gained from the appendages (e. propeller.0 Page 29 Unclassified .. e. boss and hub). QinetiQ/FST/CR033339/1. The FREDYN hullform was provided by the USCG ELC in the form of a FREDYN “cda” file. The very basic hullform validation was conducted. The FREDYN model does not take into account the additional buoyancy gained by the vessel’s appendages.g. As this study involves asymmetrical damage the transverse centre of gravity of the compartments is of great importance. as the geometry of the hullform for FREDYN was sent from the USCG. This was the only alteration made to the PARAMARINE design.g. The hull definition is based on twenty-one sections approximately 3. Page 30 QinetiQ/FST/CR033339/1. y and z position of the openings of the duct are defined and so is the cross-sectional area of the pipe. The x. The coefficients were calculated based on the standard flow-in-pipe analysis. 2001). The first duct is input with the tank opening defined on the one end and the virtual tank defined on the other end. A second duct then is defined between the virtual tank and the crossflooding tank. Again the coefficients are defined for that part of the duct.14 as the duct was modelled in two parts to account for the height rise of the pipe. The coefficient for the alternative design was 1. This method accounts for the size and length as well as the shape of the duct. Pipes that have a higher part to stop undesired cross-flooding when the tanks are intact require additional modelling. at the position of the highest point of the duct a very small ‘virtual’ tank is created close to the size of the duct cross-section. To model the existing cross-flood pipe as currently fitted to the 270-ft cutter involves modelling the duct in three parts. The existing duct coefficients are higher due to the number of bends in the pipe run.65 and 2. In selecting a coefficient extreme care should be taken as this can greatly affect the flow both in terms of flow rate and time to achieve equalisation. This ensures that the vertical path of the water in the pipe is taken account of and that the flooding will occur as the real duct.4 Cross-flooding modelling in FREDYN Defining an accurate cross-flooding system in FREDYN involves careful detailing to ensure realistic modelling. For straight pipes or pipes where the duct openings are at the highest point of the pipe run this is straightforward. The coefficients calculated for the current duct design were 5. 4. The flow coefficient for the pipe is derived from using standard values mentioned above based on dimensions and shape of the duct. Firstly.74 for each duct. This was shown to be sufficiently accurate when used in FREDYN when compared with experiment data (Peters.0 Unclassified .Unclassified In order to accurately model the cross-flooding ducts a flow coefficient is required to incorporate the friction effects in the duct. A sensitivity study was completed on the coefficients selected for each duct to ensure valid calculations with small changes to the coefficients. This flow coefficient is required to ensure that the ducts flow in a realistic manner. The coefficients in this duct then take into account the length and bends in this part of the duct. Ship speed was also included to investigate if this improved the situation for the ship after damage. The ducts are also shorter than the current duct design.0 Page 31 Unclassified .6 Matrix of tests To fully investigate the performance of the cross-flooding arrangements a number of simulations were required. if available. which could affect the performance of the ship following damage and hence how it may affect the crossflooding. as this was deemed suitable for this tank layout.Unclassified 4. QinetiQ/FST/CR033339/1. 4. which indicates the position relative to the current duct. the ducts have a slight curvature to them so that restrictions to the flow within each pipe is kept to a minimum. A list was compiled of parameters. The matrix was selected to not only thoroughly investigate the ducts but. This pipe system joins the top of each tank to the bottom of the other. and is shown in Figure 4-6.5 Alternative duct design Figure 4-6: Alternative QinetiQ cross-flood design (red) As an example. The diameter of the two ducts was smaller than that of the current duct as it was expected that both ducts would cross-flood. Due to the position inside the tank. The results form the time-domain analysis highlight where improvements to both designs could be made. and how it may affect the vessel’s behaviour. to provide some guidance to the operator on heading and speed selection after damage. an alternative cross-flooding design was created for the 270-ft cutter using the FREDYN program and the guidelines listed in chapter 3. These were to be included in a matrix of runs to assess the current and alternative duct performance. The chosen design was based on the double duct design. as would be demonstrated in the tests. The performance before fine-tuning was demonstrated in the matrix of runs. This was then examined during a large matrix of runs alongside the current cross-flooding arrangement. The zero speed/no cross-flooding case has been used as a baseline case to identify where the situation is improved. Run Set 2. was selected to assess the effect of sea state on the performance of the cross-flooding systems. Each set of runs in the above table concentrates on a particular part of the matrix with the number of runs set so data trends can be deduced. The third set. In this set. Run Set 3.0 Unclassified . Repeat runs were also conducted at different Page 32 QinetiQ/FST/CR033339/1. The selection tested two different ship conditions at three headings and in three sea conditions to allow the performance to be assessed. with the current design and with an alternative design with the ship at different speeds and orientation to the waves while the other variables were kept constant. The aim of Run Set 4 was to evaluate the effect of the ship’s position on the wave to establish how orientation effects the initial damage transient responses for the noncross-flooding and cross-flooding cases. This demonstrated how the cross-flooding performance is affected in different sea conditions as the ship motions increased. was selected to assess the effect of ship loading condition on the performance of three cross-flooding systems. Run Set 1. This allowed an assessment of the effect of heading and speed on the performance of each of the cross-flooding systems to be made. aimed to assess the performance without cross-flooding.Unclassified Parameter/Matrix Ship Condition Cross-flood systems Speeds Headings Sea Conditions Damage Occurrence Repeats TOTAL Run Set 1 1 3 3 8 1 1 1 63 Run Set 2 2 3 1 3 3 1 1 54 Run Set 3 1 3 3 2 5 1 1 90 Run Set 4 1 2 1 1 1 3 3 18 Table 4-2: Run list 4. The second set. the non-cross-flooding situation. the current design and an alternative design were tested in a range of wave conditions at three speeds in beam seas (damage opening towards and away from the waves). The first set of runs in the table. which was agreed excessive to meet the aims of this project. It also identified the issues that occur at slow forward speed following damage.7 Run selection To conduct the entire run combinations for all the initial variables that were selected would have resulted in a requirement for over 8000 simulations. 8% to give a list angle of 17. To create a suitable condition for the tests the two ballast tanks were emptied. Standard permeabilities were used except the stores were lowered in permeability to 60% representative of a full store. The engine room was also reduced slightly in permeability to 75%. The diesel oil service tanks were also lowered to 25% full. in a trough. The KG was then lowered by 3 inches to give a list angle close to 19 degrees. the two ballast tanks that crossflood are both pressed full.0 Page 33 Unclassified . which caused a list angle greater than required. In this condition.8 Ship condition To select a ship condition to test. For Run Set 3 a second condition was required which was basically a minimum operating condition. The current loading conditions of this class did not provide an interesting case as all of the damage criteria were met and the static list angles were less than 15 degrees. the main condition for the 270-ft cutter class B deep condition was initially selected. The KG was then raised by 1.Unclassified points in the wave realisation (damage initiating on top of a wave crest.5 degrees. This condition is sufficiently different to the deep condition to investigate the effect that ship condition has on the performance of the cross-flooding systems. Consequently. This gave a more suitable condition in which to test crossflooding designs. 4. QinetiQ/FST/CR033339/1. or in a quiescent location between larger wave groups) to investigate how that effected the ship behaviour. minor modifications were made so that the final list angle after damage was increased to just over the current 15 degrees criteria limit (USCG and SOLAS). Page 34 QinetiQ/FST/CR033339/1.42 Gmfluid (m) 0.72 Draft AP (m) 4. To define the ship condition in FREDYN a full loading case is not required.20 3.Unclassified 5 5.64 Table 5-1: Ship conditions The simulations were performed as defined in the test matrix above.41 5. The vessel in these simulations then drifted to take up whichever heading it naturally wanted to the waves. All of the compared runs are conducted in the same wave time history and with damage occurring at the same point of time so that the performance of the ducts can be directly compared. The tanks that contain fluid prior to being damaged are entered into the FREDYN load case so that the correct flooding will occur after damage. The zero speed runs had yaw fixed in the simulation so that they could not change heading so the true effect of heading on the vessel performance could be realised. In FREDYN the vessel displacement is calculated from the draft marks input from data provided by PARAMARINE. as this currently cannot be done automatically with FREDYN. Ship Condition Deep (1) Light (2) Displacement (Tonnes) 1875 1716 Kgfluid (m) 5. At the point of damage the RPM was set to zero (in the required runs) manually.0 Unclassified . For this the overall ship conditions used are outlined in 5-1.64 0.1 Simulations The simulations were conducted mostly in the deep modified condition described in section 4. although due to the vessel size the main tanks were all modelled and loaded accordingly. with the tanks loaded separately.38 Draft FP (m) 4.8.34 4. This makes these runs more realistic but more difficult to directly compare after the initial performance. At the forward speed cases the vessel was started at the correct speed and control made by the autopilot. The main statistics were gathered from each run and collated in a spreadsheet. QinetiQ/FST/CR033339/1. These runs were not conducted with fixed headings so the vessel was free to take up whatever heading after the revs were stopped when the damage occurred. Each simulation run produced large data files containing the vessel’s motion and the damage water levels in each of the flooding compartments. Plots 10 through 12 show the RMS motion for the three duct arrangements at the 3 speeds tested. In all but head seas the USCG duct reduces the transient roll by 5 to 7 degrees and the QinetiQ duct by 1 to 2 degrees more. 12 kts prior to damage and 0 kts after. These plots are shown in Figures 1 through 65. The three plots show the speed variations. 6.1 Run Set 1 . Figures 7 through 9 show the transient roll angle at the different headings for the 3 duct options (no duct. Figures 1 through 3 show an example of the roll angle traces for the 3 duct options (no duct.Unclassified 6 Discussion The dynamic stability simulations were performed as described in Chapter 5. The vessel often tended towards beam seas after forward speed was lost. At the forward speed both the cross-flooded cases have lower RMS roll after damage compared to the non-cross-flooding case and are at a lower mean heel angle. Analysis of the time traces allows an easier understanding of how the variables affect the performance of the vessel. These six plots show the how the speed variation effects the roll motion after damage. USCG duct and QinetiQ Duct designs). USCG duct and QinetiQ Duct designs) in a sea state 5 at 7 kts prior to damage and 0 kts after. The roll motion once the vessel has cross-flooded was compared using RMS motion about the mean heel angle. 7 kts prior to damage 0 kts after and 7 kts before and after.0 Page 35 Unclassified . For the purpose of this report not all of the simulations are discussed in detail. Figures 4 through 6 show the same plots but with the vessel at 7 kts after damage. This allowed plotting to be conducted to show trends in each of the run sets of data. The first point that can be seen in these three plots is that the no-duct case generally has a higher transient at most headings and all speeds. From Figures 7 through 9 it is generally shown that the lowest transients occur in head sea and increase with the worst between beam and stern quartering seas.Variation with ship speed and heading Run Set 1 includes runs at 8 headings and 3 speed variations to identify the effect on the vessel and cross-flooding after damage. The lowest motions for all ducts was at the 7 kts before and 7 kts afterwards due to the stabilisers working efficiently to reduce the roll motions. This suggests that both the current USCG duct and the QinetiQ duct are operating quickly enough to affect the initial transient roll. The transient roll angle in this report refers to the first large roll excursion that occurs as the vessel floods. All three plots show a general reduction in the RMS roll motion after cross-flooding has completed in comparison with the no-duct case. A selection of time traces of certain runs are also included to demonstrate the performance of the vessel after damage that can be created using this method. Variation with ship loading condition In Run Set 2 the objective was to identify how the ship condition may change the performance of the ducts in typical wave conditions. The second reason is due to the double duct system flooding through both ducts when all of the ducts are below the water surface. Comparing the plots of the transients in the sea state 3 shows transient rolls angles of between 22 and 25 degrees for both ship conditions while the ducts appear to make little difference initially on this heel angle. As expected the transient roll and RMS roll (after flooding completed) increases with sea state. the pattern is similar with the duct case but with lower transient roll angles.32 m diameter of the current duct. which appears to be the case with this vessel as the ducts are low in the vessel. Each plot shows the performance of the vessel in sea states 3. Figures 40 through 42 show the same plots for the current USCG duct. Figures 37 through 39 show the transient roll. The pipe run in the QinetiQ system only contains one slight bend and the pipe run is also shorter. with the QinetiQ duct producing the lowest transient rolls in the sea state 5 at both conditions and at all the three headings. The two ship conditions were tested in sea states 3. which is smaller than the 0. The time to cross-flood can be seen to vary between 200 to 220 seconds (10%) due to heading and speed change in the same waves. 6. The same pattern can be seen with the transient roll and the RMS roll increased with the higher sea states. This shows an example of time history behaviour of the vessel with different ship conditions. Figure 21 shows that QinetiQ duct also varies by 14 seconds but cross-flooding completes in 100 to 114 seconds. RMS roll and mean list angle respectively for the no-duct case in the deep condition. At all the headings both of the ducts show reduced transient rolls due to the rapid flooding performance of the ducts. The diameter of the QinetiQ duct is 0. which is close to twice the rate of the current USCG duct. 4 and 5 at beam seas (damage opening towards and away from the waves) as well as in head seas at zero speed.2 Run Set 2 .Unclassified Figures 13 through 21 also show the variation for each of the duct cases plotted for each speed to show how the speed affects the performance. Figures 37 through 39. Figures 31 through 33 show the transient roll response for the three duct variations in the deep condition in the three sea states.25 m.0 Unclassified . The differences occur due to two factors. Figure 18 shows how the time to cross-flood with the USCG Duct is effected by the heading and speed. Page 36 QinetiQ/FST/CR033339/1. 4 and 5. Figures 25 through 27 show the plots of roll against time for the vessel in a light condition in sea state 4. Similarly Figures 34 through 36 show the same plots but for a light condition. This ensures a much less restricted flow than the current design. In comparison to the no-duct case. Figures 28 through 30 show the same run condition but with the vessel in the deep condition. The condition 2 results indicate greater transient rolls in the three sea states tested when compared with the deep condition data. The effect of the opening towards or away from the showed similar motions and the time to cross-flood was the same. In the sea state 4 and 5 it shows that the ducts do start to reduce the transient roll. which is both long and has many tight bends. Figures 22 through 24 also highlights this with plots of the time to cross-flood of the two ducts at the three speeds and the different headings. The QinetiQ duct produces the least difference in transient roll with the angle consistently below 25 degrees in all the sea states. At 12 kts before the damage and 0 kts after damage little difference is shown with the cross-flooding cases but the no-duct case is improved. The time to cross-flood for the USCG duct can be seen to increase slightly as the sea state increases (seen clearer in Figure 30). Figures 61 through 63 show the time to cross-flood as a function of sea state for the different speeds and headings. Both the duct cases show that the transient roll does not increase as much with the duct operating.3 Run Set 3 . Figures 43 through 45.0 Page 37 Unclassified . showing that the stabiliser fins are operating effectively. with variation less than 20 seconds. a similar pattern can be seen as seen to that from the USCG duct.Variation with sea state Run Set 3 was to extend the number of sea states and speeds in beam seas to identify performance trends. with the least difference shown in the higher sea states. Like the USCG duct the time to cross-flood is effected by sea condition with the time to cross-flood increased by 10 seconds in the sea state 5 conditions. 6. The two conditions clearly show only a small difference in the time to cross-flood. These plots show that in both conditions the duct cases reduce the RMS roll after flooding as well as reducing the mean heel angle.Unclassified With the QinetiQ duct. as previously discussed. at both conditions and the 3 sea states. The three figures show that there is some variation in cross-flooding time at the different sea conditions. which operate effectively at the forward speed and improve the roll performance after damage. This is due to the active fins. The RMS roll of the QinetiQ duct is very similar to that of the USCG duct case. the QinetiQ duct cross-floods quicker. The light condition is shown in Figures 49 through 51. The RMS roll for the three cases are shown in Figures 46 through 48 for the three seas states in the deep condition. The transient heel angle does not vary as much as in the other two cases with the angle consistently below 25 degrees. The heading does not show much effect on the time to cross-flood of the QinetiQ duct as much as seen with the USCG duct. Figures 52 through 54 show the time to cross-flood of the USCG and QinetiQ ducts. Figures 58 through 60 show the comparison in the RMS roll response for the different duct arrangements in the sea states and at the three speeds. Figures 55 through 57 show the transient roll for the three cases in a number of sea states up to an extreme sea state 9 that causes capsize in the noduct case. with head seas been the worst in each sea state for the time taken to cross-flood. The 7 kts before and after damage again show the lowest RMS for all of the three cases. The duct cases have consistent lower RMS roll after damage. For the 7 kts before and the 7 kts afterwards there is also a reduction in roll in the no-duct case that closes on the performance of the two duct cases. so producing a slightly different RMS value after cross-flooding. The sea state 3 and 4 are very similar with a time to cross-flood of 101 seconds to cross-flood. This shows that the ducts continue to operate in even QinetiQ/FST/CR033339/1. The zero-speed cases show the increase in transient roll with sea state. Lowest heading for RMS roll is 180 degrees. RMS roll and time to cross-flood all increase slightly as the loading condition moves towards light ship.Unclassified the most severe sea states. Page 38 QinetiQ/FST/CR033339/1. The runs in sets 1 through 3 were all run and damaged at the same point in the same wave realisation so that better direct comparisons could be made between the runs. 6. 6. The transient is more greatly affected by the motion at the point of damage rather than the point that it occurs on the wave. 7 kts Before-0 kts After Highest 12 kts Before-0 kts After transient roll 7 kts Before-7 kts After 1 Ship Heading Every 45 degrees Time to cross-flood about 220 sec. showing that the transient roll was affected only slightly effected by the size and steepness of the irregular wave where damage occurred. With the point of damage occurring with the ship on top of the wave the lowest transient rolls were produced. The results are shown in Figures 64 and 65.Variation with position on wave at damage onset Run Set 4 was completed to investigate the effect of ship position and point of damage on a wave to the transient behaviour. which randomly changes the waves. Highest transient roll 1 Ship Heading Every 45 degrees 1 Ship Heading Every 45 degrees Highest RMS Lower RMS roll roll 2 Ship Loading Condition Deep Condition Light Condition Time to cross-flood about 110 sec. Transient roll. Lower RMS Cross-flooded produces roll better RMS Roll at all headings . There was also only 2 to 3 degrees difference between seed numbers. however.5 Results of parametric variation Overview of the results from the parametric variation study Alternative QinetiQ Duct Run Set 1 Parameter Ship Speed Variation No Duct USCG Duct Comment on Variation to Parameter The fin stabilisers are effective in reducing motions if the ship can maintain speed after damage as compared to dead in the water (assuming they are still operational).for No duct the 90 degree seas are worse transient and RMS than the 270 seas.0 Unclassified . Time to cross-flood varies by up to 10%. The different speeds made very little difference to the time both the cross-flooding designs took to complete cross-flooding. For all intents and purposes.4 Run Set 4 . but in most cases it made little or no difference. the effect is minimal. Lowest 90 and 270 largest transient roll variation in transient roll and RMS roll 180 lowest transient generally . on the wave slope or on the wave crest. Lowest Transient roll and RMS roll transient roll increase with increasing and RMS roll SS and the spread between systems increases with SS. SS. Time to cross-flood varies little with opening towards and away from the waves.r.t. Time to cross-flood is basically constant w. Table 6-1: Results from parametric variation QinetiQ/FST/CR033339/1.Unclassified Run Set 2 Parameter Ship Loading Condition Variation Damage Opening towards the Sea Damage Opening away from the Sea SS3 to SS9 No Duct USCG Duct Alternative QinetiQ Duct Comment on Variation to Parameter 3 Sea State Highest transient roll and RMS roll 3 Sea State SS3 to SS10 4 Sea State 4 Wave Position Damage Opening towards the Sea Damage Opening away from the Sea Trough Wave Slope Wave Crest Time to cross-flood varies little with opening towards and away from the waves. No runs Very little variation in roll response between damage initiation in trough.0 Page 39 Unclassified . Very little variation in time to cross-flood at different speeds and sea states. The cases with the ship continuing on at 7 kts after damage showed an improvement in the transient and RMS motions after damage. for the duct and no-duct cases. The transient roll was often seen to be worse in head seas than in beam seas. This is due to the anti-roll stabilisers remaining effective and reducing the roll even after damage. which also passes the current criteria. Previously. the initial large transient rolls were also reduced in comparison to the no-duct case. the time to cross-flood is less than 15 minutes. The transient roll after damage depended more on the vessel’s response to the wave itself rather than the point on the wave where the damage occurred. The duct design using the new methodology and time-domain simulation showed a time for cross-flooding almost half that of the current design (see Figure 6). From IMO. the method to determine time to cross-flood utilised purely static calculations. The run plan as presented above has shown to provide a suitable test matrix in which to evaluate the performance of existing and new designs. The timedomain simulations give a better insight into how the cross-flooding ducts operate at sea and their effect on the vessel during and after damage. In the situation of the ship with speed prior to the damage and zero speed afterwards. Due to the rapid flooding. there appeared to be little or no difference as compared to the zero-speed case. This time-domain analysis allowed the performance of the cross-flooding to be assessed and the time taken to cross-flood to be calculated with the vessel motion taken into account. unlike the current design that incorporates multiple tight bends. as the speed was quickly lost and the control of heading was lost. Page 40 QinetiQ/FST/CR033339/1. An exploration of different damage initiation times within the same seaway showed variations of only 4 degrees in the transient roll angle. which assume calm water for the entire cross-flooding process. The pipe run of the redesigned duct has only a sight curvature to it thereby allowing as free flow as possible. Both the cross-flooding system developed using the guidance in this report and the existing system fitted on the 270-ft cutter pass the current criteria examined in this report. which decreased the time to complete cross-flooding. probably due to the position of the wave trough at the point of damage. Once past the transient roll.Unclassified 7 Conclusions Dynamic stability cross-flooding simulations allowed the effectiveness of the crossflooding ducts to be thoroughly investigated.0 Unclassified . Two ducts were used in this system and in nearly every case both ducts contributed to the counter flooding. where the USCG defines 5 minutes to cross-flood. These static calculations are suitable for initial sizing of ducts and give an initial insight into the performance of the crossflooding but do not provide the complete picture of the performance. the 7 kts into head seas case resulted in the lowest RMS roll motion after damage. though the pipe diameters were less than that of the current design. This ensures that the performance meets the requirements in wide selection of scenarios. The heel angle in the conditions tested also reduced the mean heel after damage to the order of 10 degrees. Even though the existing ducts were not as effective as the new duct design created using the described methodology.0 Page 41 Unclassified .Unclassified 8 Recommendations It has been shown that following this methodology and using a suitable time-domain code that an effective cross-flooding arrangement can be designed to ensure an effective operation in all conditions and including all transient effects from the onset of damage through the point of equilibrium. this analysis was conducted for more unstable conditions than the vessel currently operates at. The current duct cross-flooded between 200 and 220 seconds in all of the runs tested. Due to modifications to the loading conditions for the 270-ft WMEC class. QinetiQ/FST/CR033339/1. the existing duct system was seen to improve the vessel performance after damage over the no-duct case. It is therefore recommended that the guidelines and test plan described in this report should be adopted when considering fitting or replacing cross-flooding systems in the future. which is within the current guidance. It is recommended that these ducts be kept operational as they have been shown to improve the mean damage list angle of the vessel where the initial mean roll angle without crossflooding is less than 20 degrees. Naval Institute Press. [2] [3] [4] [5] [6] [7] [8] [9] [10] SOLDA. MARIN. “Capsize Risk of Intact Frigates in Irregular Seas”.0”. UK Ministry of Defence. US Navy. March 2001 . USN (ret. 2000. 2002.0 Unclassified .The Third Fleet and the Pacific Storm of December.L. and DE KAT. Stability Standards for Surface Ships. “Equalisation of Unsymmetrical Flooding” . 2001.O. A. currently Naval Sea Systems Command. Crete.Stability and Buoyancy of US Naval Surface Ships.J. A.). G. T. and GOLDBERG. Transactions SNAME. “Cross-flooding of Frigate Sized Vessels”. “Experimental and Numerical Investigation on Progressive Flooding and Sloshing in Complex Compartment Geometries”. Washington. “Stability of Surface Ships Part 1 . Part 1. 1944”. [12] VAN ’t VEER.O. International Maritime Organisation. “FREDYN User’s Manual Version 9. Feb. SOLAS Consolidated Edition. “Stability and Buoyancy Criteria for US Naval Surface Ships”. Part 1. 1 August 1975. Transactions SNAME. L. DC. 1981.Unclassified 9 References [1] CALHOUN. A. 2002. Naval Ship Engineering Center. Stability Standards for Surface Ships.Commercialin-Confidence .O. London. R. 1962. PETERS. STAB 2000. UK Ministry of Defence. DE KAT. “Model Experiments an Simulations of a damaged Frigate”. Maryland. 305-321.Transactions of the Royal Institution of Naval Architects. “Typhoon: The Other Enemy . Capt C. Conventional Ships.S. J. [11] US Navy. K. 2000. pp. Launceston. Conventional Ships.Conventional Ships”. 2001. Raymond. DDS 079-1. 2000. Vol. J. Page 42 QinetiQ/FST/CR033339/1.H.J. 1961.USCG and the UKMOD. and PETERS. MOD Defence Standard 02-109 (NES 109). MARIN. Sea Systems Publication No 24. McTAGGART. Proceedings of the IMAM 2001 Congress. Tasmania. 2000. SARCHIN. Annapolis. and DE KAT. J. Design Data Sheet . MOD. th Proceedings of the 7 International Conference on Stability for Ships and Ocean Vehicles. Unclassified 10 Tables Sea State Wave Height and Period Sea State Sea State Three (SS3) Sea State Four (SS4) Sea State Five (SS5) Sea State Six (SS6) Sea State Nine(SS9) Table 1: Sea state wave heights and period Wave Height (m) 0.88 3.5 8.4 20 QinetiQ/FST/CR033339/1.00 20.8 9.7 12.1 Modal Wave Period (s) 7.25 5.88 1.0 Page 43 Unclassified .