Structural Safety Analysis of Bow-Doors

Transcription

1 THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING Structural Safety Analysis of Bow-Doors ULF KARLSSON Department of Naval Architecture and Ocean Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2004

2 Structural Safety Analysis of Bow-Doors ULF KARLSSON ULF KARLSSON, 2004 ISSN CHA / NAV / R-04 / 0090 Department of Naval Architecture and Ocean Engineering Chalmers University of Technology SE Göteborg Sweden Telephone +46 (0) Printed by Chalmers Reproservice Göteborg, Sweden 2004

3 Structural Safety Analysis of Bow-Doors Ulf Karlsson Department of Naval Architecture and Ocean Engineering Chalmers University of Technology Abstract Bow-doors are one of the most vulnerable parts on a RoRo-vessel. At the same time, they are necessary for the profitability of today s RoRo-vessels. The minimum design rules for bow-door systems, stated in the International Association of Classification Societies (IACS) S8, constitute the basis for the class rules. In this work, the rules of IACS S8 have been assessed regarding the structural integrity for bow-doors of the clam door type. The work is based on previous damages, compared to the design rules, so as to find weaknesses in the rules. Damages were mapped and weak points in the rules as well as in the design and construction stages were identified. For the found problem areas part analyses were performed. Fractures and cracks dominate as the cause of damage for bow-door systems. In most of these cases cracks in welds are involved. Fatigue proved to be a major cause for the development of fractures and cracks. Assuming the bow-door as a rigid-body and distributing the forces equally on each support, as prescribed by IACS S8, proved to be an approach that leads to inadequate results. The deformation of the door causes the loads on the supports to differ significantly and the real pressure distribution subjects the lower supports to much higher loads then the upper ones. It is common with gaps between the supports in the doors and the corresponding ones in the hull. These gaps will considerably affect the load distribution on the supports. In certain situations, bow propellers may excite bow-door arms into resonance cycling. This would give rise to a large number of high stress cycles and might even cause fatigue failure. Keywords: bow-door; IACS S8; damage; analysis; safety; arm; structure; ship design; fatigue. I

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5 Acknowledgments Many people, companies and organisations have contributed with information and work in this project, both people in the project group, reference group one [1], and from outside, reference group two [2]. Great thanks for all of your support, information and advice. As you all know, without you it would not have been possible to fulfil this project. Especially, I would like to thank my supervisor Anders Ulfvarson for his knowledgeable advice, guidance and always positive and good mood. Erland Johnson and Gunnar Kjell at the Swedish National Testing Institute, SP, for their measuring and analysis work of the bow-door arm. Jan-Ove Carlsson and Pontus Dhalström at MacGREGOR RoRo-Division and Olle Thomsson at Lloyd s Register, for all the time and effort they put into this project. I would also like to thank Vinnova and the Swedish Maritime Administration for funding this project and Swedish Maritime Administration for their participation in the project. To all at the Department of Naval Architecture and Ocean Engineering, it has really been fun; even if the workload has sometimes been heavy, the good atmosphere makes one cope. II

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7 Table of Contents Abstract.. I Acknowledgements II Table of contents... III 1. Introduction Background Aim of the Project Limitations Method Pre-Study of Bow-Door Systems Why Bow-Doors Bow-Door Systems Design Procedures Design Rules with Comments Incidents and Damages to Bow-Door Systems Structural Damage Analysis of Bow-Doors Introduction Method General Damage Distribution Damage Distribution on the Main Parts Arms Severity of the Damage Cases 21 5 Identified Problems - Items to Analyse 22 6 Fractures and Cracks Fatigue or Overloads High- or Low-Cycle Fatigue Load Distribution on Bow-Door Supports Introduction Rules Required Strength of the Supports according to IACS S Required Strength according to the Finite-Element Calculation Application of a Measured Pressure Event Stress and Fatigue Analysis of a Bow-Door Arm Introduction Real Arm Verses the Model Load Cases Bow Propellers Fatigue Analysis Conclusions Recommendations Rule-Based Recommendations Design-Based Recommendations Further Work References III

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9 1. Introduction 1.1 Background Accidents with many casualties due to failure of bow-door systems and several additional incidents, are the cause for concern of the safety of bow-door systems. The problem in these cases has been visors and bow-doors that, for some reason, have been open or opened at sea. This project is limited to bow-door systems of the clam door type, partly because no visors have been constructed since the loss of M/S Estonia, at least not in Europe, and partly to limit the size of the project in order to be able to conduct analysis with enough depth. 1.2 Aim of the Project The main task of this project is to increase the safety of RoRo-ferries. The main purpose is to investigate if the design rules of today are good enough for the ships of tomorrow. The purpose is also to propose changes in the rules and design methods if needed. The classification societies design rules for bow-door systems are based on the International Association of Classification Societies, IACS, S8 rules, which also constitutes the minimum requirements. Hence, the IACS S8 has been the analysed rules in this project. The aim is also to find methods for analysis of this kind. 1.3 Limitations The descriptions and information provided in this report are limited to RoRo-passenger ships with bow openings of the bow-blade type, or clam doors, i.e., two doors that are either attached to the hull via arms and open horizontally outwards, or two doors that are directly attached to the hull with hinges and open down and outwards. It does not consider the inner watertight door, the surveillance system or the crew s management of the system. 1

10 2. Method This licentiate thesis is a summery report based on four part-, or project reports. These are: - Pre-Study of Bow-Door Systems CHA / NAV / R-04 / Structural Damage Analysis of Bow-Doors CHA / NAV / R-04 / Stress and Fatigue Analysis of a Bow-Door Arm CHA / NAV / R-04 / Supports of Bow-Doors according to IACS S8 CHA / NAV / R-04 / 0089 The reports are all registered at the department of Naval Architecture and Ocean Engineering, Chalmers University of Technology, Göteborg, Sweden. The results from the analyses are based on verifiable facts. The research material, or the chosen bow-door system, have a common design and are representative for most of the bowdoor designs of this type. All bow-door systems are, however, designed according to the same basic rules, comprised within the IACS S8 rules. The research method used consists of six steps, as described in diagram Pre-study of bow-door systems 2. Structural damage analysis of bow-doors 3. Identifying problems Items to analyse 4. Part-analysis of identified problems 5. Validation 6. Recommendations Diagram 2.1. Schematic diagram of the used research method. 2

11 Step 1 The purpose of Step 1 is to gather information of different types of bow-door designs, the design method and the design rules they are to comply too. It also describes some incidents and damages that have previously occurred to bow-door systems. This information constitutes the base for the continuation of this project. The information was gathered through interviews with representatives of companies designing bow-door systems, ships crews and other people knowledgeable in this area [1, 2], the study of different types of bow-door systems, damage reports [3, 4], and of the IACS S8 rules [5], which the class rules are based on. Step 1 consists of Chapter 3, Pre-Study of Bow-Door Systems. Step 2 In this Step, we try to find structural weaknesses in these kinds of systems, based on previous damages that have occurred. The result will give information of which parts that demand further investigation. The goal with the damage analysis has been to investigate what kind of structural damage that has occurred on bow-door systems and surrounding hull structure, where the damages have occurred and how common they are. All relevant ships with bow-doors in a specific classification society are part of the analysis. Step 2 consists of Chapter 4, Structural Damage Analysis of Bow-Door Systems. Step 3 From Steps 1 and 2, items demanding further analysis were identified. Five items were found. Step 4 Of the five items found in need of analysis, four have been analysed. These are: - Fractures and Cracks (Chapter 6) - Load Distribution on Bow-Door Supports (Chapter 7) - Gaps between the Plates of Supports (Section 7.4.3) - Stress and Fatigue Analysis of a Bow-Door Arm (Chapter 8) One item remains to be analysed: - The rules for the hull structure around the doors Step 5 In Step 5, the results from the part-analyses were validated with regard to the IACS S8 rules and to design aspects. Step 6 Finally, both rule-based and design-based recommendations are made regarding changes of the active rules and regarding designing issues. 3

12 3. Pre-Study of Bow-Door Systems 3.1 Why Bow-Doors The best bow-door is the one that does not exist Christer Schoug, Stena RoRo One of the most vulnerable areas of RoRo-vessels is the bow-door [2]. However, bow-doors are essential to the profitability of this type of ship. The main reason is the large cost of having a ship in port. The limiting factor regarding time-in-harbour is loading and unloading from a RoRo deck. The best solution today for fast loading and unloading is to let the vehicles drive through the ship. To make this possible, there needs to be a bow-door and an aft ramp. Wide lanes through the bow are also important, as this makes it easier and faster to drive trailers in and out from the ship. The alternative to bow-doors is having very wide vessels in which vehicles can turn around. This would work on ships trading on longer routes, where the ship stays in harbour for a longer time and where loading and unloading is not the limiting time factor. If the vessel is to go on a shorter route, the time-in-harbour is generally short, and loading/unloading becomes the limiting factor. Another reason to mount bow-doors on vessels, even if they are going to trade on a longer route, is to make the vessels all-round, to be able to traffic more routes. The second-hand value is also considered, as it is easier to sell a ferry with bow-doors than without. The cost of mounting bow-doors on a ship afterwards, without the ship being prepared for it, is about 50% higher than to install them during construction of the ship. The customer s wish of fast unloading is also an issue to consider. 4

13 3.2 Bow-Door Systems Bow-doors can be divided into two types, visor and bow-blades. From here on, bow-blades will be called bow-doors. Photo 3.1. Visor Visors Visor systems consist of one door, the visor, which is hinged to the hull by two hinges, positioned on the weather deck. The visor opens upwards in a rotating track around these hinges. Since the hinges are normally positioned aft of the visor, the sea forces are directed in the opening direction of the visor and the locking devices can therefore be subjected to heavy loads. Visors have not been installed on new civil ferries, at least not in Europe [1], since MV Estonia lost her visor and sank. Although many ferries still have visors, they have not been designed for a long time and are therefore not part in this analysis. Bow-Doors There are two types of bow-doors, one where the doors are hinged to the hull by arms and one where the doors are hinged directly to the hull via hinges. The system with the doors attached to arms is the most common [1]. The main reason for using bow-doors instead of visors is that the sea forces are in the closing direction of the bow-doors. Another reason is that visors cannot be mounted on ships with a high bow, or where the opening does not reach up to weather deck. Photo 3.2. Bow-doors with arms. The advantages of having bow-doors with arms are that the doors open horizontally outwards. Thereby the doors can be mounted on any ship, regardless of its bow shape. The disadvantages are that it is an expensive and complicated system with more parts that have to interact and can fail. The doors are not as stiff as visors and become more easily deformed and the arms and hinges are very sensitive to vibrations in an open position [1]. The advantage with bow-doors directly attached to the hull is the more stable attachment in the open position. The design is also cheaper and simpler, with fewer parts that can fail. A disadvantage is that the doors cannot be used on ships that have a pronounced flare, since the doors then would go into the water, or ice, when opened [1]. Photo 3.3. Bow-doors directly hinged to the hull. 5

14 Door Arm Hinges Hinges y z x The direction of the reaction forces from the supports and stoppers. Symbols Figure 3.4. Bow-door system with arms. Cleating/Locking device Support Stopper Guide Icebreaking cylinder An outer bow-door system generally consists of the following components: (Components in italic are shown in figure 3.4 and/or photo 3.5) 1. Two doors 2. Manoeuvring arms with hinges or just hinges 3. Hydraulic manoeuvring cylinders 4. Cleating/locking devices between the doors (2-3 devices) 5. Cleating/locking devices between each door and the hull (2-3 devices per door) 6. Rubber seal 7. Supports (5-7 per door) 8. Stoppers (3-4 devices) 9. Guide (1-2 per door) 10. Icebreaking cylinder (1-2 per door) 11. Locking for open position Bow-door systems are hydraulically manoeuvred. The manoeuvring is usually fully automatic and governed by a PLC (Programmable Logical Controller). Sensors, usually magnetic and contact sensors, survey the system and the bow area is illuminated for surveillance with a camera. To prevent ice formation, the bow area is heated. On the tank top a drainage pump and a water level alarm are installed. 6

15 Surveillance camera Ramp Heaters 7 Photo 3.5. Stitch image of a bow-door system with arms (inside view). Doors The two doors are to have at least the same strength as the surrounding bow-hull, according to IACS S8.4 [5], and the rules for the door structure are the same as for the foreship structure. Manoeuvring arm Each door is attached to an arm that carries the door during opening and closing and in an open position. The arms have the shape of a box beam since they must carry heavy loads in the form of a bending moment and torsion. They are attached to the doors at approximately where the centre of gravity of the doors is. The door is attached to the arm with two hinges, of which only one hinge carries vertical loads, and the arm is attached to the hull, also with two hinges, of which only one hinge carries vertical loads. Hydraulic manoeuvring cylinder The arm, and thereby the door, is manoeuvred by a hydraulic cylinder. Usually the hydraulic cylinder is attached to the side of the hull and at the rear end of the arm. 7

16 Cleating- and locking devices Cleating devices The purpose of the cleating devices is, at closing, to cleat the doors to each other and to the hull (support to support), compressing the rubber seal between the doors and the hull to keep the bow-doors weather-tight. A hydraulic cylinder manoeuvres each cleating device. Locking devices The task of the locking devices is to lock the doors in a cleated position and to prevent the doors from opening. They are not to transfer any sea loads, this being the task of the supports. They are usually hydraulically manoeuvred, but manually locking devices exist. The cleating and locking devices are positioned between the doors (2-3 devices) in order to cleat and lock the doors to each other and around the edge of the doors (2-3 devices per door) in order to cleat and lock the doors to the hull. There are different types of cleating and locking devices. One type is a combined cleating/locking device. It first cleats the doors with a hydraulically manoeuvred hook pulling in an eye-plate and then locks them by putting the mechanical connection over the centre and manually by sticking a rod through the mechanical connection. In another design, the cleating and locking device are separate devices. The cleating devices first cleat the doors with a hydraulic hook pulling in an eye-plate. The locking devices then push a hydraulic rod through an eye-plate, which stays locked by gravity. Rubber seal Along the edge of each door is a rubber seal. In the hull and in one of the doors is a coaming in which the rubber seal is compressed at closing. The purpose is to keep the bow-doors weather-tight. There are support edges on each side of the rubber seal to prevent it from becoming too compressed, and to shelter it from water pressure and ice. Supports The doors open outwards and the forces from the sea are therefore in the closing direction. These forces are transferred from the doors into the hull via supports on the doors and the corresponding supports in the hull. Each door has 5-7 supports, positioned around the door frames. The supports transfer forces in different directions, depending on their position. The supports mainly consist of thick steel plates welded to the doors and hull at structurally strong points and strengthened with brackets etc. Stoppers Between the doors, there are stoppers with the purpose of transferring loads between the doors. The stoppers consist of plates welded on structurally strong points on the doors. Guide The task of the guides is to guide the doors at opening and closing. There can be one or two guides mounted in the front and upper part of the bow-hull and/or on the tank top. The guides consist of guide rails in the hull and guide-wheels on the doors, which follow the rails and guide the doors during opening and closing. 8

17 Icebreaking cylinders There are one or two icebreaking cylinders for each door, positioned on the tank top and/or on the front top of the doors. Their purpose is to help push out the doors, especially when they are frozen stuck. Locking for open position In a fully open position, doors are locked by one hydraulically manoeuvred rod, or hook, each. These are positioned on the side of the hull and lock through an eye-plate on the arm. Manoeuvring sequence Below is an example describing a typical manoeuvring sequence during opening and closing. Opening the doors 1. Locking pins are manually pulled out from the cleating/locking devices; 2. Hydraulic pressure is turned on; 3. The cleating/locking devices between the doors and the hull are unhooked; 4. The cleating/locking devices between the doors are unhooked; 5. The icebreaking cylinders push the doors out; 6. The manoeuvring cylinders push on the arms and open the doors; 7. In a fully open position the doors are locked with the locking for open position ; 8. The ramp is lowered; 9. The hydraulic pressure is turned off. Closing the doors 1. Hydraulic pressure is turned on; 2. The ramp is raised; 3. The locking for open position unlocks; 4. The manoeuvring cylinders pull the arms and close the doors; 5. The cleating/locking devices between the doors cleat the doors towards each other, compressing the rubber seal, and finally lock the doors in a cleated position; 6. The cleating/locking devices between the doors and the hull cleat the doors against the hull, compressing the rubber seal, and finally lock the doors in a cleated position; 7. Hydraulic pressure is turned off; 8. Locking pins are manually pushed into the cleating/locking devices. 9

18 3.3 Design Procedures Procedures The request for bow-door systems comes late in the design process of a ship [1]. Often, the bow structure has already been determined. This means that the designs most often cannot be optimal. Before constructing a bow-door system, the design companies receives data of the bow shape in the form of a body plan and the ramp main dimensions, i.e., width, length, and the free height through the bow. From this data, they decide the shape and the size of the bow-doors and the location in the bow. They then design the locking arrangement, supports, manoeuvring equipment, surveillance system etc. Fittings of the equipment are adjusted after the bulkheads, reinforcements in the hull and the space available in co-operation with the shipyard. Information and drawings of the system are sent to the shipyard, including information about how large forces that are to be transferred from each support, hinge and cleating/locking device. The shipyard then has the responsibility for seeing to it that these forces are transferred correctly into the hull through meeting webs, stringers, brackets etc. The design companies follow the class rules very much as they are written, for which the IACS S8 rules serve as a base. They do not add any safety margins of their own. This makes the correctness of these rules important. Designing When calculating the loads on each support, the projected areas are divided into segments, one for each support, with the lines drawn up between the supports. The design external force for each projected area, calculated according to IACS S8.3.1a, is divided between the area segments and constitutes the calculated force on the corresponding support. The redundancy is calculated in a way where one support at a time can be removed, at which the stress on the other supports must not exceed allowable values according to IACS S8 (class) rules. Figure 3.6. Projected area divided into segments, longitudinal-direction. The allowable play between the supports in the hull and in the doors and between locking pins and eye-plates are not to exceed 3mm according to the IACS S8 rules. This is however, already at the construction stage, difficult to achieve due to initial deformation caused by the lower precision on constructions of this size. 10

19 Some design companies always carry out a finite-element analysis of doors and of the arms and their hinges; sometimes even of other items, for instance the fore bottom support. Other design companies do not perform a finite-element analysis, except when they design the steel structure for the bow-doors. There are many design solutions to bow-door systems, among other things for how loads transfer into the hull. This can differ greatly between ships. In the designs, the ramp in a folded position usually constitutes the watertight bulkhead. The steel material used is generally AH-36. DH-36 is chosen when thicker steel is to be used, which is a softer and more ductile material and thus less fragile and gives a better margin for brittle fracture. Experience and continuous improvements is the method used for the development of bowdoor systems and elimination of critical points. Responsibility The design companies offer one year s guarantee for the bow-door system. The design companies have full responsibility for the bow-door systems they deliver, even if the classification society in question has approved them. Sometimes a second party, for instance shipyards in China or Korea, delivers the construction. This splitting of the design/construction results in a less reliable construction and the question of responsibility becomes unclear. Feature bow-door systems The opinion of the future bow-doors are somewhat divided. Some design companies believe that the speed of Ro-Pax ships in the future will increase and that the bow shapes thereby will become sharper. The radius, or the roundness, of the bow will thereby decrease. They also think that the bow opening will be narrower with only one lane, due to the sharper bow. Two lanes would make the bow-doors very long. Others believe that future bow-doors will be wider and longer and made for two lanes through the bow. They do not think that the speed of ships in the future will be higher than today s high-speed crafts with a speed of up to knots. They agree about the fact that lightweight materials will not be used in the future, but that the materials they use today will also be used in the future. They think that the bow-doors attached directly to the hull will be less common in the future. Faster ships have larger stem angels, or sharper noses, and these kinds of doors cannot be mounted on sharp noses, as they would go into the water on opening. 11

20 3.4 Design Rules with Comments In this part, the design rules have been analysed and comments have been made based on discussions with a reference group [1] General IACS (International Association of Classification Societies) was founded in 1968 with the aim of securing as much uniformity as possible between societies. IACS contributes to maritime safety and regulations through technical support, compliance verification and research and development. More than 90 % of the world s cargo-carrying tonnage is covered by the classification design, construction and through-life compliance rules and standards set by the ten Member Societies and two Associates of IACS. The IACS rules constitute a basis, or provide minimum requirements, for the rules of the classification societies. The rules of IACS UR S8, Rev. 3 Nov. 2003, [5] Bow-doors and inner doors concerns the arrangement, strength and securing of bow-doors. The IACS UR S8 was approved by IACS members to take effect from 1982 and applies to new ships Comments on the IACS S8 rules The design companies follow the class rules very much as they are written, for which the IACS S8 rules serve as a basis, and do not add any safety margins of their own. This makes the correctness of these rules important. Regarding the IACS S8 rules, some parts might be a bit vague, as seen below: S8.3 Design loads S8.3.1 Bow-doors A design pressure serves as the basis for the dimensioning of the bow-doors, defined by the rule S8.3.1 a. S8.3.1 a. The design external pressure P e, in kn/m 2, to be considered for the scantlings of primary members, securing and supporting devices of bow-doors is not to be less than the pressure normally used by the Society nor than: P e = 2.75λC H ( tan α)(0.4v sin β + 0.6L 0.5 ) 2 Where: V L λ contractual ship s speed, in knots, ship s length, in m, but need not to be taken greater than 200 metres, coefficient depending on the area where the ship is intended to be operated: λ=1 for seagoing ships, λ=0.8 for ships operated in coastal waters, λ= 0.5 for ships operated in sheltered waters, 12

21 C H = L for L < 80 m 1 for L 80 m α β flare angle, defined as the angle between a vertical line and the tangent to the side shell plating, measured in a vertical plane normal to the horizontal tangent to the shell plating at the point on the bow-door, l/2 aft of the steam line on the plane h/2 above the bottom of the door, as shown in Figure 3.7. entry angle, defined as the angle between a longitudinal line parallel to the centreline and the tangent to the shell plating in a horizontal plane, measured at the same point as α. Figure 3.7. Definition of α and β. Comment: - the flare and entry angles, α and β, used in the formula for the external pressure P e are considered only at a specific point, but vary along the bow differently on different ships. S8.3.1 b The design external forces, in kn, considered for the scantlings of securing and supporting devices of bow-doors are not to be less than: F x = P e A x F y = P e A y F z = P e A z Where: A x, A y and A z is the projected area of the door from the respective direction. For bow-doors, including bulwark, of unusual form or proportions, e.g. ships with a rounded nose and large stem angles, the areas and angles used for determination of the design values of external forces may require to be specially considered. Comment: - There is no definition of rounded nose and large stem angles. 13

22 S8.6. Securing and supporting of bow-doors S8.6.1 General In rule S8.6.1, it says: Bow-doors are to be fitted with adequate means of securing and supporting so as to be commensurate with the strength and stiffness of the surrounding structure. Comment: - This rule is impossible to comply with. S8.6.2 Scantlings S8.6.2 c: For side-opening doors the reaction forces applied on the effective securing and supporting devices assuming the door as a rigid-body are determined for the following combination of external loads acting simultaneously together with the self weight of the door: Case 1 F x, F y and F z acting on both doors, Case F x and 0.7 F z acting on both doors and 0.7 F y acting on each door separately, Where F x, F y and F z are applied at the centroid of projected areas. Comments: - The doors are assumed to be a rigid-body, which they are not. - The design external forces are applied at the centroid of the projected areas and the reaction forces can be distributed uniformly on the effective securing and supporting devices, irrespective of their position. In reality, the lower parts of the doors, and their supports and securing devices, will be subjected to much larger forces than the upper parts of the door. Rule S8.6.2 e says: The distribution of the reaction forces acting on the securing and supporting devices may require to be supported by direct calculations taking into account the flexibility of the hull structure and the actual position and stiffness of the supports. Comment: - There is no definition of when direct calculations are required Others There are no special rules for the bow-hull structure depending on the size of the doors. If the speed of ships increases in the future, the ships would become more slender. This would require longer bow-doors to be able to have the same lane-width through the bow. The rules for the hull structure around the doors might then be inadequate. 14

23 3.5 Incidents and Damages to Bow-Door Systems From damage reports, incidents [3, 4] and interviews with knowledgeable people [1][2], some conclusions can be made as follows: If doors open at sea in heavy weather, there will be little time left to close them before the waves rip them off. Even if the direction of the waves is from the ship s stern, the pitching and stamping motions will probably cause the doors to fall off in a quite short time. This means that one of the most important issues is to prevent the doors from opening at sea. It is apparent that any bow-door can be wrecked in heavy sea if the ship s speed is not adjusted to the conditions. The rules for bow-door designs obviously presume that a ship s speed is adjusted to the conditions. Common Damages It is common that cracks develop in the structure, at the attachments for the arm hinges, cleating and locking devices and supports (mostly lower supports), etc. Many of the cracks have developed due to poor detail design and construction, stiffened points and bad welding. Sometimes supports in the hull are pushed in, often a bottom support, by their corresponding support in the door, causing a hole where water can enter and flood the bow-door area. Bow-door supports are often not cleated steel to steel, or with a play of maximum 3mm according to IACS rules, against the corresponding supports in the hull. Play in the linking system for cleating and locking devices and worn eye-plates for locking pistons is quite common. Heavy weather has caused indents to shell plates in doors and in hulls and sometimes even damaged locking devices and structure. The rubber seal often breaks due to ice, movements of the doors and water pressure. This represents no danger and the rubber seal is more less a consumption product. 15

24 4. Structural Damage Analysis of Bow-Doors 4.1 Introduction In this damage analysis, we try to find weaknesses in bow-door systems based on previous damages that have occurred. The result will give information on what parts in the system that needs to be investigated more closely. The goal has been to investigate what kind of structural damage that occurs on bow-door systems and the surrounding hull structure, where the damages occur and how common they are. All relevant ships with bow-doors in a specific classification society are part of the analysis. There were 38 ships. The age varies between 2-36 years, with an average age of 14.7 years. The ships were built during the years Together they have 557 years of service. For these ships, a total of 141 reported damages were found concerning structural damage on bow-door systems and the surrounding hull structure. In the analysis, all registered reports are included. No distinction has been made between severe and less severe damages, since this information has in most cases not been possible to distinguish in the reports. It was, in many cases, not possible to conclude if the doors are hinged to arms or directly hinged to the hull. Both types are therefore represented here without distinction. The rules for bow-door designs, IACS S8 [5], was established in 1982 and revised in The bow-door systems in the analysis are therefore designed according to different rules. This has not been considered in the analysis. Older damage reports are missing in the data-base. Many reports do not provide information about where the damages have occurred, in which detail or what kind of damage it is. Other sources of error are that some damages have been repaired without being reported to the classification society. Damages found by surveyors are sometimes not reported if the crew repairs them while the surveyor is still present and can approve the repair. For this Damage Analysis, the following limitations have been applied: - Ships with visors are excluded, i.e., only ships with two doors opening horizontally outwards, are part of the analysis. - Ships less than 90 m in length are excluded. - Ships that operate in too calm or sheltered waters are excluded. Included here are all short-distance ferries, defined as ferries with equal ends. - Damages that have occurred due to contact or human errors are excluded. - Damaged rubber seals are not included in the analysis, since this is normal wear and tear damage. - Faults that only require adjustments are not part of the analysis since, these are included in normal maintenance. 16

25 4.2 Method Structural damages on the ships bow-door systems were searched for in the classification society s database. The search period covered all of their lifetime. Information on the ships was gathered in a table showing when the ship was built, how old it was at the different damage occasions, which parts that had been damaged, what kind of damages there were and where the damages had occurred. The main structural parts in a bow-door system are: 1. Door structure 2. Bow-hull structure 3. Cleating/Locking devices 4. Hinges 5. Arms (if present) 6. (Supports) Since it in the damage reports not has been possible to specifically distinguish damage on supports, these probably appear as damages on doors or hulls; they are not included in the analysis. A summarizing of the damages for each of these main parts was made. This shows in what kind of detail the damages had occurred, the number of damages, the type and location of the damages and at which ship age they occurred. 17

26 4.3 General Damage Distribution Damaged ships Age Ship Diagram 4.1. Damages versus ship ages. Diagram 4.1 shows the 38 ships included in the analysis on the horizontal axis, each represented by a pillar, and the age of the ships on the vertical axis. Pillars in grey represent the ships for which no damage reports were found. The pillars in black represent the ships for which damage reports were found. The number above each black pillar shows the number of damages found for each respective ship. As can be seen in the diagram, the number of damages found on the ships increases with the age of the ships, as expected. Undamaged Ships 47% (8.2 years) Damaged Ships 53% (20.5 years) Of the total of 38 ships in the analysis, 20, or 53%, had reports of structural damage on the bow-door system and/or the surrounding hull structure. The average age of the 38 ships was 14.7 years. For the 20 ships with damage reports, the average age was 20.5 years, which can be compared to 8.2 years for the 18 ships without any damages reported. Diagram 4.2. Damaged and undamaged ships. 18

27 4.3.2 Damaged Main Parts A total of 141 damages were found and these were distributed on the main parts as follows: Door 47 damages Hull 25 Cleating/Locking 39 Hinge 28 Arm 2 In Diagram 4.3 the percentage distribution of the damages is shown together with the average age of the ships when the damages occurred. ARM 1% (15.0 years) HINGE 20% (19.6 years) CLEATING/ LOCKING 28% (13.9 years) DOOR 33% (18.3 years) HULL 18% (15.9 years) Diagram 4.3. Percentage distribution of the damages on the Main Parts. The distribution of the 141 damages on the main parts shows that damages on doors are the most common with 33% of the cases, closely followed by damages on the cleating/locking devices with 28%. Hinges and hull have fewer damages, 20% and 18%, respectively. Arm damages were only reported twice. The average age of the ships, when the damages occurred on the different parts, varies between years. 19

28 4.4 Damage Distribution on the Main Parts Diagrams 4.4 a,b,c,d shows the percentage distribution of the damages in the different details for the main parts. Seal chanel 11% (18.3 years) Frame 33% ( 23.1 years) Stiffener 33% (22.7 years) Bracket 11% Shell plate (6.3 yeras) 11% (21.0 years) Shell plate 8% (22.0 years) Bracket 42% (17.0 years) Stiffner 50% (8.8 years) Diagram 4.4 a. Door On 13 ships, or on 34% of the total number of ships, a total of 47 damages were found in relation to doors. Diagram 4.4 b. Hull For the hull, a total of 25 damages were found on 6 ships, or on 16% of the ships. Pins/ Bearings 36% (19.4 years) Attachment/ Bracket 64% (17.0 years) Locking pin/eye 37% (17.8 years) Attachment/ Bracket 63% (14.9 years) Diagram 4.4 c. Cleating/Locking device In total, 39 damages were found on 12 ships (32%) related to cleating/locking devices. Diagram 4.4 d. Hinge For hinges, 28 damages were found on 10 ships, corresponding to 26% of the ships. General for all main parts is that fractures and cracks dominate as the cause of damage. Most common are fractures and cracks in stiffeners, brackets, attachments and in the door and hull frame. Fractures and cracks were the cause in 70% of all damage cases and in most of these cases, cracks in welds were involved. Other found damages were; buckled, deformed, torn and worn. For the door and the hull, most damages (80-90%) occurred in the aft parts. For the hull, there were three times more damages in the upper parts than in the lower parts. For the doors the damages between the upper parts and the lower parts were in equal numbers. Damages in the cleating/locking devices occurred in 5 out of 6 occasions in the lower-placed devices. Damages for the cleating/locking devices located at the centre were 4 times more common than damages in the outer devices. Hinges placed low had 2.6 times more damages then hinges placed high. 20

29 4.5 Arms Only two cases of damage in arms were found. These were found on the same ship and on the same occasion (See Section 4.6). As mentioned before, it has not been possible to establish how many of the ships that have their doors hinged to arms. An estimate is that approximately 40-45% of the ships in this analysis have this arrangement. It would then mean that one out of 16 ships with arms have suffered damages in an arm. 4.6 Severity of the Damage Cases Most of the damage occasions in this analysis were of minor severity and represented no danger for the ship, the passengers or the crew, except in two cases. In one case, after a voyage in heavy weather, one of the bow-doors on a vessel was difficult to close. The reason was found to be a large crack in the port-side arm. The crack had progressed from the fore upper part of the arm, started in what seemed to be a bad weld, and progressed in the upper plate and down on the sides. A small crack was also found in the transition between the eye for the upper pin in the hinge and the top plate of the arm. Photo 4.5. Crack in a bow-door arm. It was found that if the arm had cracked off completely, the bow-door would probably have been lost at sea, as the arm was a part of the locking arrangement. The redundancy regarding the locking arrangements for keeping the doors in a closed position was in this case inadequate. In the second case, a vessel in extreme heavy weather suffered damage to the collision bulkhead in the region of the bow-door s lower hinge horizontal bracket. The damage resulted in flooding of the area between the outer bow clam doors and the inner watertight ramp through ingress of water via the clam door s top horizontal joint. In view of the water ingress and associated electrical equipment problems, the voyage was terminated and the vessel returned to port. 21

30 5. Identified Problems - Items to Analyse From the previous chapters the following five items were found requiring further analysis: 1. Fractures and Cracks (Chapter 6) In Chapter 4, Structural Damage Analysis of Bow-Doors, fractures and cracks were found to dominate as the cause of damage. As such, it is vital to find the cause of the development of cracks in order to be able to prevent or reduce them. 2. Load Distribution on Bow-Door Supports (Chapter 7) From Part 3.5, Incidents and damages, it was found that supports are sometimes pushed in, especially bottom supports. According to the rules, the design external forces are to be applied at the centroid of the projected areas and the doors are assumed to be rigid bodies. The reaction forces can then be distributed uniformly on the effective securing and supporting devices, irrespective of where they are positioned. In reality, the door flexibility will alter the force distribution on the supports and a real pressure distribution will subject the lower parts of the doors, and their supports, to much larger forces than the upper parts and their supports. The design rules for the dimensioning of the supports needs to be analysed regarding the influence of the hull flexibility and the real pressure distribution. 3. Gaps between the plates of supports (Chapter 7.4.3) Bow-door supports are often not cleated steel to steel, or with a play of maximum 3mm according to IACS S8.6.1 a, against the corresponding supports in the hull. This will affect the load distribution on the supports. 4. Stress and Fatigue Analysis of a Bow-Door Arm (Chapter 8) In one case, described in Section 4.6, Severity of the Damage Cases, a large crack was found in a bow-door arm. If the arm had cracked off completely, the bow-door would probably have been lost at sea, as the arm was a part of the locking arrangement. To investigate if arms can be a part of the supporting- and locking arrangement, an analysis of a bow-door arm has been made. 5. The rules for the hull structure around the doors. There are no special rules for the bow-hull structure depending on the size of the doors. If the speed of ships increases in the future, the ships will become more slender. This would require longer bow-doors to be able to have the same lane-width through the bow. The rules for the hull structure around the doors might then be inadequate. This ought to be analysed. Of these, items 1, 2, 3, 4 are analysed below. Items 5 remain to be analysed. 22

31 6. Fractures and Cracks 6.1 Fatigue or Overloads In Chapter 4, Structural Damage Analysis of Bow-Doors, fractures and cracks were the cause, or partly the cause, of 99 damages, or in 70% of all 141 damages. In order to investigate what normally causes the fractures and cracks to develop, if they are due to occasional overloads, fatigue, or both, all the fractures and cracks found in the analysis are represented in Diagram 6.1, showing the damage distribution over the ship s age. Number of Fractures/Cracks Ship Age (Years) Diagram 6.1. Distribution of Fracture/ Crack damages over the ships age. Diagram 6.1, however, gives a distorted picture, since the average age of the ships in the analysis is 14.7 years, i.e., about half of the ships have not reached an age of 14.7 years. The number of damages for the lower ship ages in the diagram are therefore overrepresented, since more ships have reached this age, compared to the number of damages for higher ship ages that fewer ships have reached. The number of damages in the diagram will in reality be increasingly higher with the age of the ships. Number of Fractures/Cracks Ship Age (Years) In order to compensate for the misrepresentation we give the older damages a higher weight. The abstraction is to consider all ships to have reached an age of 30 years and re-calculate the numbers of damages from the relations we have in diagram 6.1. The more representative damage distribution is shown in Diagram 6.2. Diagram 6.2. Modified distribution of Fracture/Crack damages over the ships age. For example, we have 3 damages at a ship age of 25 years (diagram 6.1). In Diagram 4.1, we find that 8 ships have reached this age while 30 ships have not. To find the relevant proportional value, we apply the relation between the 3 damages and the 8 ships to all 38 ships: 3 x 38 = 14 damages 8 This is the relevant proportional value for the number of damages at age 25, considering all ships to have reached this age. As can be seen in Diagram 6.2, the number of damages has increased with the ships age and are, especially for higher ship ages, much higher then in Diagram

32 Summing up all damages for the first 10 years of age, for the age between years and between years, the diagram will be as follows: FRACTURES / CRACKS Number of Fractures/Cracks Ship Age (Years) Diagram 6.3. Modified distribution of Fracture/Crack damages over the ships age (10-year periods). The first 10-year period shows somewhat more damages than the second, year period. One possible explanation could be initial faults in the construction, such as bad welds, construction errors or even poor design. These kinds of damages will be discovered and repaired in the ship s early years. The number of damages would subsequently decline, as it does in the next period between years. For the last 10-year period there are several times more damages found than for the earlier two periods. A filled line has been drawn through the columns in the diagram, illustrating how the damages would occur. If the cause of the fractures and cracks were due to random overloads, the distribution of the damages would follow a straight increasing line, as the dotted line in the diagram shows, or at least look something like that. The solid line, however, does not follow this line at all. Instead, the line goes up in the first period, down in the second and increases thereafter rapidly in the last period. This suggests fatigue to be a major cause for the development of fractures and cracks. It is of course difficult, from this analysis, to say how large a share fatigue has, or how large a share overloads has, concerning the development of fractures and cracks. Most certainly both fatigue and overloads are to blame, probably combined in many cases, but we can definitely conclude that fatigue is a major cause of the development of fractures and cracks in bow-door systems. 24

33 6.2 High- or Low-Cycle Fatigue In this Section, we will try to analyse what the main cause of the development of cracks in the main parts is, if it is due to high or low-cycle fatigue. Here fatigue is referred to as crack growth. This does not mean that there will be a fatigue failure during the ship s lifetime, but merely that the stress ranges are large enough to grow cracks. Fatigue occurs in spots where the stress ranges are high, so called hot spots. These hot spots are normally found at sharp edges and corners, often in welds. When fatigue in the following is mentioned, it is assumed to be in spots like these. Many small stress ranges over a long time can give rise to high-cycle fatigue, i.e., the crack growth for each cycle is small and a large number of cycles are needed to cause a fatigue failure. For our material, several million cycles are needed generally. Large stress ranges can give rise to low-cycle fatigue, i.e., the crack growth for each cycle is larger than for small stress ranges and fatigue failure occurs already within 100,000 cycles for our material. In bow-door systems, both small and large stress ranges occur giving rise to both high and low-cycle fatigue. The following events subject bow-door systems to loads: Waves Flare slamming Opening/closing Loading/unloading Thrusters Waves The loads that the waves give rise to in a bow-door system, as well as in other ship hull structures, are mainly due to the bending and twisting of the ship when moving in the waves. A ship meets approximately 100,000,000 waves during its lifetime. If we calculate with a ship s lifetime of 30 years, the ship will encounter approximately 3,000,000 waves a year. This will induce a large number of stress cycles. These stress cycles will, however, only give rise to small stress ranges [1] and can thereby only cause high-cycle fatigue in some areas. Very large waves can cause large stress ranges but the number of these are small during a ship s lifetime. Flare slamming Flare slamming occurs rarely. Occasionally a ship is subjected to flare slamming, but when this happens, the ship s speed and/or heading is changed to avoid it. A RoRo-ferry is in average subjected to approximately 100 flare slamming occasions a year [2]. Flare slamming can gives rise to direct overloads, i.e., yielding of the material, and large stress ranges (lowcycle fatigue) [1]. 25