System Based Design of Offshore Support Vessels Stein Ove Erikstad1 and Kai Levander2

ABSTRACT In this paper, we will present how the System Based Design (SBD) method can be applied in the design of offshore support vessels (OSV’s). The SBD method was first presented at IMDC in Kobe in 1991, and has since been successfully applied in the development of a large number of ship designs, in particular cruise ships and ferries. The adaptation towards OSV’s includes the development of appropriate breakdown structures for vessel main functions, weights, areas and volumes. Further, a number of existing designs have been analyzed to provide experience-based data on a detailed functional level. Using SBD, the functional design of the vessel can be developed to a high degree of detail without premature commitment to specific overall dimensions, layout and arrangements. SBD can also provide a foundation for modular design. Combined with a 3D visual sketching tool, this method can support the generation of several alternative vessel configurations fast and with a much reduced design effort.

KEY WORDS Offshore Support Vessel; Design;

INTRODUCTION The System Based Design method was first presented at IMDC in Kobe in 1991. In the 20 years following, this method has been successfully applied in the development of a large number of ship designs, in particular cruise ships and ferries. At NTNU we have included SBD for cargo vessels in the teaching of Marine Design since 1995. In Norway ship design and shipbuilding are mainly related to offshore vessels and a government funded R&D project called SHIP-4C was established 2010 between NTNU, DNV and STX OSV. System based design should be based on an appropriate functional breakdown structure that captures the nature of service type vessels, at the same time is valid across different types of OSV’s. Correspondingly, breakdown structures for weights, areas and volumes have been defined. Further, a number of existing designs have been analyzed to provide experience-based data on a detailed functional level. In this paper, we will present the main results from the research work and the experiences we have made so far in applying the System Based Design for offshore vessels. In particular, we will focus on how this methodology supports a process where the functional design of the vessel can be developed to a relatively high degree of detail, while at the same time avoiding a premature commitment to specific overall dimensions, layout and arrangements. Further, we will discuss how this may provide a foundation for a modular design platform. Combined with a 3D visual sketching tool, this method can support the efficient generation of alternative vessel configurations in the conceptual design stage. This project includes the development of a SBD-model for offshore vessels and collection of area, volume and weight data from existing vessels.

1 2

Norwegian University of Science and Technology (NTNU), Norway SeaKey Naval Architecture, Finland

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SHIP DESIGN PROCESS The most common way to describe the ship design has been by a spiral model, capturing the sequential and iterative nature of the process. The task structure is “select dimensions - evaluate capacity and performance - redesign”. This model easily locks the naval architect to his first assumption, and the focus in the design process will be to patch and repair this single design concept rather than to generate and evaluate alternative designs. An approach that better supports innovation and creativity is needed. The design should start from the mission specified for the ship. The mission statement settles tasks, capacity and performance expected by the owner or operator. As a consequence, the design task structure changes to “define systems and functions – estimate size and weight- select dimensions –check performance”. This approach “straightens” the design spiral and reduces the number of loops needed to find a technically feasible and economically preferable solution. The step-by-step process of the SBD process can be summarized as follows: 



  

Customer requirements - Mission statement o Task, capacity, performance demands, range and endurance o Rules, regulations and preferences o Operating conditions, like wind, waves, currents, ice Functional requirements - Initial sizing of the ship o Based on capacity, where the areas and volumes needed for cargo spaces and task related equipment defines the size of the vessel o Based on weight, where the cargo weight and the weight of task related equipment and of the ship itself defines the size of the vessel Form - Parametric exploration o Variation of main dimensions, hull form and lay out of spaces on board to satisfy the demands for both capacity and weight Engineering synthesis o Calculating and optimising ship performance, speed, endurance and safety Evaluation of the design o Calculating building cost and operation economics

Figure 1: The System Based Design process

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MAIN TYPES OF OFFSHORE SUPPORT VESSELS Though we consider the SBD approach presented in this paper to be generally applicable across most common types of offshore support vessels, the actual implementation of the method has been for the three main OSV types, namely Platform Support Vessels (PSV), Anchor Handling, Tug, Supply vessels (AHTS) and Offshore Construction Vessels (OCV). In the following, we will shortly describe each of these three types:

PSV Platform Support Vessel PSV´s operate from a shore base carrying supplies to drillships, offshore construction vessels and production platforms. The vessels normally have an open cargo deck aft, with storage tanks for liquid and dry bulk cargo below the deck. Good manoeuvrability and dynamic positioning is needed for keeping the vessel close to the platforms during the unloading of the supplies.

Figure 2: Platform Supply Vessel

AHTS Anchor Handling, Tug, Supply These vessels are used for placing platform anchors in the right positions, recovering anchors and relocating them if needed. In deep water the weight of long chains demand high bollard pull capacity from the vessel, but also increasing pulling force from the anchor handling winches. Towing of platforms and drilling rigs also demand a powerful machinery and high pulling force.

Figure 3: Anchor Handling, Tug, Supply Vessel

OSCV Offshore Construction Vessel Offshore construction vessels are used for building and maintaining platforms, well heads, under-water pumping units, pipelines and power cables. They have a large open work deck with heavy cranes, moon pools, pipe storage and cable carousels. Often they also have diving equipment and remote operated underwater vehicles (ROV). Accommodation facilities are needed not only for the ship crew but also for the construction work force. The OSCV:s often have a helicopter landing platform for the exchange of personel onboard.

Figure 4: Offshore Construction Vessel

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OSV SIZE AND CAPACITY Compared with many cargo ships offshore support vessels are rather small in size and have low cargo capacity. But these vessels normally operate close to their shore base and frequent trips are more important than high cargo capacity. The capability to perform their mission in heavy weather is also very important and the vessels must have good sea keeping and manoeuvring performance. In fig 4.1 and 2 deadweight and installed machinery power are shown for the three main types of OSV’s. In PSV’s the main power demand is for propulsion, but also for thrusters in station keeping during off-loading at the platforms. AH&T’s need propulsion power to generate high bollard pull, but also auxiliary power for the winches. Also OSCV’s have high auxiliary power demand for cranes and other construction equipment. Offshore Vessels 12 000

DWT at max draught [ton]

10 000 OSCV 8 000

6 000 PSV

4 000

AHTS

2 000

0 0

2 000

4 000

6 000

8 000

10 000 GT

12 000

14 000

16 000

18 000

20 000

16 000

18 000

20 000

Figure 5: OSV vessel deadweight

Offshore Vessels 30 000

AHTS

25 000

Installed Power [kW]

OSCV 20 000

15 000

PSV 10 000

5 000

0

0

2 000

4 000

6 000

8 000

10 000

12 000

14 000

GT

Figure 6: Installed power

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GENERIC FUNCTION BREAKDOWN STRUCTURE FOR OFFSHORE SUPPORT VESSELS In SBD the functions of the vessel is divided into two main categories, payload systems and ship systems. Payload systems are directly related to the “money making potential” and consist of cargo spaces, cargo handling equipment and spaces needed for cargo treatment onboard. The ship functions include the systems needed to carrying the payload safely from port to port.

Task Related Systems

Ship Systems

OSV Systems

In OSV we have renamed the payload systems to “Task Related Systems” describing the major offshore support tasks of cargo transport, anchor handling & towing and offshore construction (Figure 7). In the Norwegian offshore business the SFI group system is used for specifications, weight calculation and cost estimation (see Appendix 1). To facilitate data collection and also the use of the SBD-model we wanted to stay as close as possible to the SFI-grouping. But like many other shipbuilding group systems SFI does not distinguish between tasks related systems and ship systems, which we consider a benefit in the SBD-design theory. There is a separate sub-group “Equipment for cargo”, but anchor handling & towing must be picked out from sub-group “Ship equipment” and offshore construction systems are included in both “Equipment for cargo” and in “Ship equipment”. There are some other minor differences, like wood covering of cargo decks, helicopter platforms and lifesaving.

Cargo Spaces

Anchor Handling and Towing

Dry cargo decks Liquid and dry bulk cargo Cargo handling equipment Winches and reels Rope and chain storage Handling equipment

Offshore Construction

Lifting equipment Construction equipment Diving equipment Spaces in accommodation

Ship Structure

Hull Forecastle Deckhouse

Ship Outfitting

Offshore operation support Ship equipment Rescue and Fire fighting Crew and client spaces

Accommodation Service spaces Technical spaces in accom.

Machinery

Machinery main components Machinery systems Ship systems

Tanks and Voids

Fuel and Lube Oil Water and Sewage Ballast and Void

Figure 7: The function structure for OSV’s is divided into task related systems and ship systems In SBD areas and volumes demanded in the vessel to accommodate all systems are first calculated, independent of preselected main dimensions, hull lines or standard layouts. Thus, SBD is like a checklist that reminds the designer of all the factors that affect the design and record his choices. It gives the possibility to compare the selections with statistical data derived from existing, successful designs. The result is a complete system description for the new ship, including the volumes and areas needed onboard to fulfill the mission. This gives the total volume of the vessel and the Gross Tonnage can be calculated. Based on these data a first estimate of weight and building cost can be made. The next step in the design process is to select main dimensions and define the form. By variation of the main dimensions the space and weight in the selected design is matched to the system description.

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SHIP SYSTEMS Ship outfitting In ship outfitting space are located for tunnel and retractable thrusters, steering gear and mooring equipment. In the SFI –grouping also garbage handling, incinerator plant and deck stores are listed here. SBD adds life boats and other outdoor deck equipment to this subgroup.

Accommodation Facilities for crew and clients, like cabins, common spaces, stairs and corridors are outlined here. The areas needed are calculated from cabin sizes and square meters per person. Service spaces are based on area per persons. Technical spaces include AC rooms, lifts and electrical substations. Most of the accommodation spaces are located in the deckhouse.

Machinery Machinery covers the SFI groups “Machinery main components”, “Machinery systems” and “Ship systems”. The size of machinery spaces are estimated based on the total installed power.

Tanks for ship consumable Storage tank capacity for fuel and lubrication oil for the machinery is calculated based on the specified range in the mission statement. The SBD-model can estimate additional space needed if LNG is used as fuel. Fresh water and sewage storage capacity is based on endurance days. Ballast water tanks can be used also for transporting drill water to the platforms. Many OSV’s have passive anti-roll tanks to reduce roll motion when operating in rough seas. Offshore construction vessels with big cranes need extra ballast tanks to reduce heeling when handling heavy loads.

Figure 8: Machinery proposal (www.wartsila.com)

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USING EXPERIENCE-BASED DATA IN SBD Using the functional breakdown structure as a backbone, the adaptation of SBD towards offshore support vessels require the collection of data from existing vessels to serve as a link between the individual functional requirements and the required space and area for each function. For some functions, data from various OSV types can be used interchangeably, while for other functions there are substantial differences among vessel types. In the SBD OSV project, we gathered detailed design data for a number of STX vessels, both from platform supply vessels, anchor handlers and offshore construction vessels. The data collection process comprised the following main steps: 1.

Gathering vessel main characteristics, including deadweight, installed power, number of crew and passenger, major equipment specs (anchor handling winches, offshore crane, cargo handling equipment etc.).

2.

Collecting tonnages and volumes from the tonnage measurement book. This provided information about gross tonnage (GT), hull volume and superstructure volume of the vessel. For confirming the gross tonnage, these data were checked towards vessel classification data in DNV Exchange. When there is discrepancy about GT between the tonnage calculation book and DNV published values, the numbers are taken from DNV Exchange. These numbers are used as benchmarks for checking the level of error of space calculation.

3.

Inspecting the general arrangement drawings to find the deck heights which will be used for volume calculation. The tank plan of the vessel was used for generating data for tank capacities.

4.

Measuring the areas for different rooms/spaces/equipment using the AutoCAD drawings (Figure 8). These are subsequently tabulated into specific groups in the spread sheet according to their deck level. The area measurement is done on deck-by-deck basis, making it possible to know how much space is occupied by a certain equipment/cabin on different deck levels. For instance, machinery rooms are usually extended over two deck levels and from the spread sheet, it is easy to find out the area or volume on each deck level occupied by the machinery room.

5.

For rooms/equipment spaces whose volume cannot be directly measured from the area occupied by it (e.g. various equipment, machinery space); the volume needs to be manually calculated by looking into profile plans as well as deck plans. Information from specification booklet can also be used for this purpose.

Offshore vessels are outfitting intensive, making it difficult to measure the exact size and shape of the various equipment modules from the general arrangement drawings. Thus, measurement of equipment spaces will typically be approximate, and do require substantial knowledge of offshore vessel design and arrangements. In this project, the spaces were measured from general arrangement drawings in AutoCAD. In a 2D drawing, areas can be easily detected, but when it comes to volumes, 2D information becomes difficult to interpret correctly. This is especially true for equipment spaces, enclosed hull spaces where the walls/hull plating are curved, as well as for volumes for propeller, shafts, and thrusters. Also, making a distinction between tank capacities for ship operation versus task related operation is in some cases difficult. Tank capacities are usually given as a whole in tank capacity charts. If task related capacities are not separately specified in the vessel specification book, all tank capacities are categorized in ship operation.

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Figure 9: Measuring areas and volumes for the OSV database from an AutoCAD drawing

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SYSTEM SUMMARY The system summary summarizes the total space demand for the vessel. The gross volume is calculated in cubic meters [SI units] and converted to Gross Tonnage (Figure 9). Also deck areas are important in OSV design with task related equipment located both on indoor and outdoor decks. For technical spaces located outside the engine room, like machinery shops and stores or AC rooms the deck area demand is needed. The system summary also shows the distribution of space on board for all specified tasks and for the required ship systems (Figure 10). SPACE ALLOCATION Cargo Spaces Anchor Handling & Towing Offshore Construction TOTAL TASK RELATED SPACES

m²/DWT m³/DWT 0,22 0,36 0,25 1,24 0,47 m2/GA

Offshore Operation Support Ship Equipment Rescue and Fire Fighting

1,60

Area m² 930 1 030 0 1 960

Volume m³ 1 530 5 210 0 6 740

0 622 58

0 1 840 223

680

2 060

602 293 106 296 147 138 20 1 600

1 746 840 311 901 437 414 58 4 700

m3/GV

0,12 0,011

0,08 0,009

TOTAL SHIP OUTFITTING

Crew and Client Cabins Common Spaces Stairs Ship Service Catering Hotel Service Construction related spaces in accommodation FURNISHED SPACES Technical spaces in the accommodation TOTAL ACCOMMODATION

Machinery Main Components Machinery and Ship Systems Engine casing, air intakes and funnel TOTAL MACHINERY

m²/person m³/person 20,1 58,2 9,8 28,0 3,5 10,4 9,9 30,0 4,9 14,6 4,6 13,8 0,7 1,9 53,3 156,7 8,2

32,6

246

977

62

189

1 850

5 680

m²/kW 0,03

m³/kW 0,16

-

0,06

464 160 285

2 627 566 881

0,06

0,25

909

4 070

-

5 100

m³/kW TANKS AND VOID

0,32

GROSS AREA & VOLUME GROSS TONNAGE

5 400

23 700 GT 6 800

Figure 9: System summary for an AHTS vessel

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Figure 10: Space distribution for an AHTS vessel

WEIGHT ESTIMATION AND BUILDING COST The first weight and building cost estimate can be done based on the system summary. In the concept design phase it is sufficient to use only the 6 main groups for the lightweight estimate (Figure 11). The main dimensions have not been selected at this stage in the SBD process and cannot be used for the estimates. Task related items are best calculated “piece by piece” because they differ from each OSV type. Structure weight of the hull is calculated based on the hull volume and separately for the deckhouse including the forecastle. Ship Equipment is based on gross volume and Accommodation outfitting based on the accommodation area. For Machinery the installed power is used and for Ship Systems the gross volume. To give the necessary accuracy weight data from built OSV’s has been tabulated following the SBD-grouping.

Lightweight LWT

Cargo storage & handling Anchor handling & towing Offshore construction Diving and ROV

Structure

Main hull Deckhouse, forecastle

Ship Equipment Accommodation

Machinery

Machinery main-components Machinery system

Ship Systems

Deadweight DWT

Offshore Support Vessels

Task related equipment

Task related

Dry & liquid cargo Ropes & chains Construction supplies

Supplies

Crew Provision and stores

Bunker

Heavy fuel oil Marine diesel oil Lube oil

Water

Fresh water Sewage in holding tanks Ballast & heeling water

Figure 11: Lightweight and deadweight main groups

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Figure 12: Weight distribution

Figure 13: Cost distribution

GEOMETRIC DEFINITION AND WEIGHT BALANCE Statistics for OSV help selecting suitable main dimensions for the first iteration of the geometric definition for the vessel. The displacement from the weight calculation is used to calculate hull form parameters. In this way the weight balance will always be correct. The height of double bottom and the location of decks are selected and areas and volumes calculated. Decks in the hull can have large openings around engines in the machinery spaces or for dry bulk tanks, chain lockers and moon pools. The actual areas left for the specified systems are estimated as percentage of the total area for each deck. Above the main deck both open decks and enclosed or covered spaces must be calculated. Volumes of enclosed and covered spaces are included in the Gross Tonnage. The available deck areas and volumes in the hull and deckhouse are compared with the space demand in the system summary and if needed the geometry can be adjusted (Figure 14).

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Figure 14: Geometric definition for an AHTS vessel 35,0 30,0 25,0 20,0 15,0

10,0 5,0 0,0

-10

0

10

20

30

40

50

60

70

80

90

100

Figure 15: Geometric long-ship chart from Excel

Main Dimensions for OSV Statistics from previously built vessels is a good starting point for selecting suitable main dimensions for the OSV (Fig 16). The naval architect must consider any special features planned for this ship. The charts for the main dimensions do not show much difference between PSV, AHTS or OCV, except for draught. The demand for high bollard pull for anchor handling or towing is best achieved by large propellers and therefor more draught than in other OSV’s.

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Figure 16: Main dimensions for OSVs

40,0

STABILITY CHECK

35,0

The intact stability is calculated for the max draught condition using the same weight groups for the lightweight and the deadweight as in the weight calculation. The center of gravity for each group is estimated in relation to the depth of the hull to main deck to facilitate easy comparison with statistical data from built vessels. A simple geometric midship chart of the vessel is of great help in estimating the center of gravity for the different weight items (Fig 29). This chart is automatically generated in the Excel spreadsheet when the geometric definition is performed. Hydrostatic properties, like KB and BM are based on data from typical OSV hull forms

34,20

30,0

29,20 26,10

25,0

24,10 21,20

20,0

18,30 15,40

15,0

12,50

KM KG

10,0

9,50 Tmax 5,60

5,0

1,50 0,0 -20,0

-15,0

-10,0

-5,0

0,0

5,0

10,0

15,0

20,0

Figure 17: Geometric mid-ship chart from Excel .

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DEVELOPING A MODULAR CONCEPT SKETCH IN 3-D The functional breakdown structure represents a product architecture for the vessel, which can be used as a backbone for a modular design platform. For most of the functions, one or a set of corresponding module may be defined. Each module can be scaled according to the area and space requirements being developed as part of the SBD model. The modules can then be arranged by using templates, defining how these modules connect. This will support a quick, automated sketching of the design solution (Vestbøstad, 2011). The template states where a module should be positioned, while the breadth and height is automatically scaled based on the main characteristics of the vessel. Then the length is scaled to satisfy the volume demand. As an example, the winch module be placed in front of the deck module and made as wide and high as possible within the constraints and then scaled by length. Some examples of the graphical user interface are given in Figure 18. Here, the design information has been taken directly from the SBD process to generate the 3D-sketches to the right based on a library of modules, scaled and positioned in three dimensions. The prototype model is implemented in the tool Google SketchUp. The 3D model plays a complementary role to the SBD model, providing a visual feedback to the designer on the design decisions made. The rendering of the model has by purpose been made sketchy, avoiding attention to modeling details, and instead drawing the attention towards the overall conceptual solution.

Figure 18: 3-D model in Google SketchUp, showing alternative vessel configurations based on different templates, all having the same areas and volumes (Vestbøstad, 2011)

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CONCLUSIONS In this paper we have presented the adaptation of the System Based Design method to support the design of offshore support vessels. This development has taken place in close collaboration with STX OSV, who has provided most of the detail design data from existing vessels that is required for the function-to-form mapping, mainly in terms of required areas and volumes. In addition, STX OSV has provided a real testing context for the validation of the model. In addition to the collection and processing of OSV-specific design statistics data, the adaption has mainly been concerned with developing a correct and useful functional breakdown structure. This structure should address the specific characteristics of service vessels, where the division between ship functions and “payload” functions is different, and to some extent less evident, than what is the case for more traditional transport vessels. In addition, the functional breakdown structure should be valid across most common types of OSVs, thus supporting the reuse of design experience data and system solutions for shared functions. The proposed functional breakdown structure inherits core aspects both from the SBD functional structure used for transport vessels and from the SFI structure commonly used across the Norwegian shipbuilding community. The resulting design model provides a sound platform for the efficient development of the conceptual design solutions for main types of OSVs. The method reuse design data from existing designs in a structured and controlled way. Not by using an existing design as a starting point, as is a common practice today, but rather exploiting existing design data indirectly in terms of providing area and space requirements for core vessel functions. The methodology presented here can also be used to develop a set of vessel designs with different equipment configuration and capabilities. These vessels can then be evaluated towards alternative contract scenarios to find the capability level that maximizes lifecycle revenue (Erikstad, et al., 2011). The functional breakdown structure also provides a sound foundation for modular product architecture, serving to identify and scale a set of geometric properties that corresponds to the vessel’s key functions. The subsequent arrangement of these “system space” reservations into a complete design may either be on a free form basis, or by using one or several templates that encapsulates both spatial and logical design configuration rules. This approach has much in common with both the design building block approach (Andrews et al, 2003), as well as the arrangement generation approach advocated by (van Oers et al, 2011). At the same time, at least in its present state, it is a simpler and less formal approach, where the main purpose is to visualize the conceptual solution both to the designer and the prospective customer. The “sketchiness” of the 3D model is in itself a feature rather than a limitation, helping to focus attention towards the conceptual design solution as such (also referred to as “style”) rather than technical engineering details. The work presented here will also be adapted towards forming the foundation for the teaching related to OSV design at the Department of Marine Technology at NTNU, serving as an extension to the existing learning material related to using SBD for transport vessels, cruise vessels and ferries.

ACKNOWLEDGEMENTS We would like to thank the Ship4C project for financing this work, Kjetil Øverås and Henning Borgen for valuable discussions and input during the development of the model, and Øyvind Vestbøstad for the contribution related to the 3D model.

REFERENCES LEVANDER, K., “System Based Passenger Ship Design”, Proceedings/IMSDC 91, Kobe 1991 LEVANDER, K., “Innovative Ship Design – Can innovative ships be designed in a methodological way”, Proceedings/IMSDC 03, Athens 2003 ERIKSTAD, S.O., S. SOLEM and K. FAGERHOLT, 2011, "A Ship Design and Deployment Model for Non-Transport Vessels", Ship Technology Research, vol 58, no 3, september 2011, pp 132-141 ANDREWS, D. J. (2003). "A Creative Approach to Ship Architecture." International Journal of Maritime Engineering 15

OERS, B. v. (2011). A Packing Approach for the Early Stage Design of Service Vessels. Department of Marine & Transport Technology. Delft, TU Delft. PhD: 305 VESTBØSTAD, Ø. (2011) – “System Based Ship Design for Offshore Vessels”, MSc Thesis, NTNU

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Appendix: Norwegian SFI grouping system

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