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Conceptual Study of a Fast Landing Craft Unit Part 1 - Design MIKAEL RAZOLA [email protected] 070-7104074 TORVALD HVISTENDAHL [email protected] 070-4856392 Master Thesis KTH Centre for Naval Architecture Stockholm, Sweden, February 2010 KTH Centre for Naval Architecture

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Page 1: Conceptual Study of a Fast Landing Craft Unit/Menu... · Conceptual Study of a Fast Landing Craft Unit Part 1 - Design MIKAEL RAZOLA razola@kth.se 070-7104074 TORVALD HVISTENDAHL

Conceptual Study of a Fast Landing Craft Unit

Part 1 - Design

MIKAEL RAZOLA [email protected] 070-7104074

TORVALD HVISTENDAHL [email protected]

070-4856392

Master Thesis KTH Centre for Naval Architecture

Stockholm, Sweden, February 2010

KTH Centre for Naval Architecture

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ABSTRACT The purpose of this thesis is to develop an initial design of a new Landing Craft Unit (LCU). A LCU is a specialized craft that is used to transport heavy vehicles such as battle tanks from a capital platform at sea to the shore. The capital platforms, within NATO called Landing Platform Docks (LPD), embark and disembark the LCUs over a well deck at the stern. A majority of the LCUs in use today have insufficient performance, which highly limits the tactics of an amphibious operation.

This study presents the initial design of a completely new Fast Landing Craft Utility (FLCU). The craft is a catamaran with two lightweight demihulls connected by a transversally and vertically movable cross structure carrying a cargo platform. The movable cross structure makes it possible for the craft to reduce its draught and adjust its beam. These are two key advantages since the craft can disembark in shallow water and fit into a variety LPDs. The FLCU measures 20 m over all and is designed to carry one battle tank with a weight of 62 tonnes. Fully loaded, the craft can maintain 20 knots in sea state 3 and in unloaded condition up to 30 knots. It is designed for autonomous control.

One of the key aims of this thesis is to provide a material that shows the feasibility of the proposed craft. This was done using a number of different analysis techniques, such as hydrodynamic and hydrostatic calculations to prove the seaworthiness and performance of the craft. The scantlings are determined using the DNV rules for classification of High Speed, Light Craft and Naval Surface Craft in combination with more detailed FEM analysis to further secure the feasibility of the concept.

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PREFACE The work of this thesis was carried out during the autumn of 2009 and the winter of 2010 at the SSPA office in Stockholm. The task was commissioned by SSPA and carried out under the supervision of David Eckerdal, consultant at SSPA and Doctor Anders Rosén researcher at the Royal Institute of Technology in Stockholm. We would like to send our appreciation and special thanks to: Jesper Lodenius, David Eckerdal, Hans Liljenberg and Ulf Mansnérus at SSPA, for their input and support during the work at SSPA. Anders Rosén at KTH, for his encouragement and support.

Stockholm February 2010

Mikael Razola Torvald Hvistendahl

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1. TABLE OF CONTENTS Abstract ...........................................................................................................................................................................3  Preface.............................................................................................................................................................................4  1.   Table of contents..................................................................................................................................................5  2.   Introduction ..........................................................................................................................................................6  

2.1.   Purpose .........................................................................................................................................................6  2.2.   Method..........................................................................................................................................................6  2.3.   Report ...........................................................................................................................................................7  2.4.   Vision ............................................................................................................................................................7  

3.   Specification of requirements.............................................................................................................................9  3.1.   Cargo Vehicles .............................................................................................................................................9  3.2.   Landing Platform Dock, LPD ................................................................................................................10  3.3.   Conclusions................................................................................................................................................11  

4.   Concept................................................................................................................................................................12  4.1.   Benchmarking ............................................................................................................................................12  4.2.   The Concept of FLCU.............................................................................................................................12  

5.   Initial design ........................................................................................................................................................15  5.1.   General arrangement ................................................................................................................................15  5.2.   Hull structural design................................................................................................................................17  5.3.   Cargo Control System (CCS) design ......................................................................................................18  5.4.   Platform Structural design .......................................................................................................................19  5.5.   Propulsion and maneuvering...................................................................................................................20  5.6.   Hydrostatics and stability .........................................................................................................................20  5.7.   Seakeeping ..................................................................................................................................................21  

6.   Conclusions .........................................................................................................................................................22  7.   Future work.........................................................................................................................................................24  

7.1.   Hydromechanics........................................................................................................................................24  7.2.   Structure......................................................................................................................................................24  7.3.   Systems........................................................................................................................................................24  

8.   References ...........................................................................................................................................................25

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2. INTRODUCTION Large assault ships mainly carry out the transport of troops and heavy vehicles, such as tanks to an area of operation close to a shore. They are within NATO called Landing Platform Docks (LPD) and have the capability to disembark and embark landing crafts over a well deck. The well deck is a floodable deck in the stern of the ship. By flooding the deck the ship can lower itself and dock/launch different landing craft units (LCUs), which carry out the transports from the LPD to the shore. The landing crafts have traditionally been fairly simple constructions mainly built to manage the transportation in calm sea state, over short distances and in a completely secured area. Further more; most of them are constructed to land at standard NATO beaches, which limits the possibility to attack by surprise since their possible landing areas are highly predictable. Today the tactics of amphibious operations has changed, making the need for technical improvements of the landing crafts fairly urgent. Among the demands are better action radius, higher speeds and a more autonomous behaviour of the LCU. Despite the apparent need to meet these demands, the development of new technical LCU solutions is still only in its cradle.

2.1. PURPOSE The purpose of this thesis is to make a conceptual study and initial design of a new FLCU, Fast Landing Craft Utility. The analysis incorporates; hull geometry, general arrangement, weight analysis, hydrostatic analysis, hydrodynamic analysis, structural arrangement and finally dimensioning of the structure.

Much of the work is targeted to the U.K royal navy and their future needs. The reasons for this are several. The British Royal Navy has a clear need for new landing craft technology. Their main platforms are of NATO standard, meaning that a craft compatible with British Royal Navy can be expected to be useable within other navies. This was also one of the main requests, which together with the specification of requirements were set up by the job initiator Swedish maritime consulting company, SSPA Stockholm.

2.2. METHOD The work follows the fairly canonical design spiral common in ship design. It consists of a set of main activities, which many times are depending on each other. The work stretches over these activities starting at rough estimations, improving for each loop, ending up in a detailed design. A schematic picture of the spiral fitted to the activities in this work can be seen in Figure 1.

Figure 1. The design spiral

Used methods will be more thoroughly described under each chapter in the report. It is also important to stress that this work only covers the initial design, which means that not all laps in the spiral to a final design will be carried out. For example was the hydrostatics of the craft evaluated many times during the design process but only the final results will be explicitly presented.

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It is also important to emphasize that some parts of the final system were not designed in detail. The decision of which details to be more thoroughly investigated, was mainly based on showing the feasibility of the concept.

2.3. REPORT The report consists of two parts; Part 1 - Design and Part 2 – Analysis. The Design part will present the background, the specification of requirements, the concept development and the initial design of the FLCU and summarize the results from the work on a general level. The part of analysis will provide more comprehensive presentations of methods and results.

2.4. VISION The base for this work is a vision of improving landing operations. One part of this is naturally the craft and the technical system it represents. Another important part of this is the progress of tactical development, which can be achieved by new technology. These two parts are strongly connected and depended on each other in the context of making landing operations more efficient. The overall goal with the new LCU is to make landing operations more efficient and provide a platform, which enables tactical development.

2.4.1. Technical vision A key to more time-efficient landing operations is the performance of the FLCU. This can be gained in many ways, but some parts are naturally of more importance. The following list presents the main areas identified as crucial to the new concept:

• Speed • Seakeeping • Compatibility (cargo, beach, LPD et.c.) • Controllability • Simplicity

The possibility to reach higher speed even in bad sea state is fundamental to increase the overall transport capacity and to enable operations over larger distances. This also connect to the request for compatibility since a craft with a good speed register and seakeeping properties can operate in a larger variety of coastal areas around the world. By compatibility is also meant that the craft must have the possibility to carry different types of vehicles and land them on a range of beach types.

One of the trends in modern warfare is that more crafts, vehicles and aeroplanes are designed for autonomous operations. The most recent example is probably the UAV (Unmanned Aerial Vehicle), which is used for recognisance in high-risk area. The intention is that the new FLCU is completely autonomous or could be controlled by the personnel situated in the cargo, for example a tank.

The final word simplicity refers to the overall design ambition that should be applied. The FLCU will often operate, under extreme conditions, which will make the systems fairly exposed. To ease maintenance and operations simplicity is a key factor to the usefulness of the craft.

2.4.2. Tactical vision The most obvious scenarios for the use of a FLCU are during military operations in coastal areas. One should however not neglect that the craft could also be of great use under other circumstances such as humanitarian operations. Although this study will focus on the technical part i.e. the construction and development of the FLCU, it is still important to at least on a visionary way look at how the FLCU could be used.

One conceivable non-military scenario could be natural disasters in coastal regions. During the last years, natural disasters like flooding, hurricanes and earthquakes have showed to pass frequently. Many of them occurred in developing countries, causing enormous needs for rapid aid and assistance. The most resent is the earthquake at Haiti January 12th 2010. In one hit most of the buildings in capital Port au Prince were destroyed and so was the harbour. Since the airport had reduced capacity to land airplanes, a main obstacle was how to get resources and aid in to the area of disaster. In an early stage before the harbour could be used some of these transports were carried out by LPDs equipped with LCUs. USA, France and

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the Netherlands sent units like this. The earthquake in Haiti is thus a perfect example of how LCUs can be of great importance in humanitarian operations when rapid transports from ships to a shore are needed.

Another scenario at the border between plain military operations and humanitarian operations is peacekeeping operations. The map in Figure 2 shows the actual status of such ongoing operations within the UN January 2010 [8].

Figure 2. Ongoing peace keeping operations within the UN organisation [8]

More than 50 % of the operations are carried out in costal regions with hardly none or very poor infrastructure. Aside from the plain peace keeping task, one important mission for the force is also to support and secure transports and the rebuilding work of the country. Especially in an early stage, where harbours might be unusable or have insufficient security, LCUs are a good solution for the transport of troops, vehicles and resources.

When it comes to plain landing operations, many people probably associate them with the D-day in Normandy at the end of World War II. Without going further into that episode, one can draw the conclusion that such landing operations belong in history. In modern warfare such large casualties would be unacceptable and the huge number of troops needed to succeed would be a very inefficient use of resources. Nevertheless it is still of great significance to get a beachhead for large military operations. The most recent example of this is the American, British and Polish invasion of Iraq 2003. During one night the peninsula of Al-Faw was secured by an amphibious force and then served as a beachhead.

According to Ulf Mansnérus consultant at SSPA, a typical landing operation of today could be two amphibious companies shipped to a shore within the 6-hour darkness over a distance up to 30 NM. The fairly long distance is to reduce the vulnerability of the high value platforms such as a LPD. It is also desired that the LCUs can reach higher speeds to be able to launch an attack by surprise.

One conceivable scenario could be a LPD similar to the Albion class heading to a safe distance of about 30 NM from a shore in an area of operation. It is carrying totally six FLCUs standing ready on the well deck. Three FLCUs take one battle tank each and three FLCUs take two battle vehicle 9040C each. They are put into sea at almost the same time due to the roll-on roll-off capacity. They are then navigated into the shore by the onboard automatic navigation system. The trip is monitored from the vehicles over an interface to the FLCU. The vehicles use their own sensors but can also get complementary information like overall tactical plots or radar pictures from the LPD or other information sources. Communication can also be done between the crafts, which mean that they can share information and operate together in case of threats. After the vehicles have been disembarked, the FLCUs can either go back to the LPD or go to a safe standby position and just wait for a pick up signal from the vehicles.

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3. SPECIFICATION OF REQUIREMENTS The vision together with main particulars of the mother ship and the main cargo can be concretized in a specification of requirements. In this case the mother ship was set to the LPD (Landing Platform Dock) of UK Albion class. For the main cargo the SSPA request was to use the battle tank 122 and the battle vehicle 9040 C. Even though none of them are UK systems, they are still representative for the heaviest type of vehicles that the FLCU can be expected to transport.

3.1. CARGO VEHICLES The vehicles described in the following section are the primary and secondary cargo to be transported by the LCU. Besides these vehicles it is also desirable for the craft to be able to transport miscellaneous cargo that sometimes require ballistic protection.

The primary cargo for the Landing Craft Utility is the Swedish tank 122, also known as the Leopard 2 produced in Germany. The tank shall be able to use its sensors while being transported. It is also preferable for it to be able to use its weapon systems in transit. Table 1 describes the main particulars of the tank 122 [5].

Table 1. Main particulars of battle tank 122.

Length, including turret, [m] 9.97 Length, only coach, [m] 7.72 Width, [m] 3.78 Height, [m] 3.12 Vertical centre of gravity, [m] 1.30 Longitudinal centre of gravity, [m] 4.00 Transverse centre of gravity, [m] 1.89 Weight, fully loaded, [m] 62.0

The tank is equipped with one 120 mm cannon, two 7.62 mm machine guns and a battery of smoke launchers. For communication the tank is equipped with the RA 180 Radio, the LTS 90 and the TCCS Tank Command and Control System (LSS in Swedish). It utilises a passive IR-system for visual images in reduced sight conditions. The tank exerts a ground pressure of 9.4 N/cm2 when at rest and has a band width of 635 mm. The tank can wade in up to 1.2 m deep waters; which of course is a significant restriction when it comes to disembarking the vehicle.

Figure 3. Battle tank 122.

The secondary cargo for the LCU is the battle vehicle 9040C. It is equipped with a 360° rotating turret tower and is operated by a crew of 3. The main particulars of the 9040C can be found in Table 2 [6].

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Table 2. Main particulars of the battle vehicle 9040C.

Length, [m] 7 Width, [m] 3.4 Height, with and without antennas, [m] 2.71 (4.17) Weight, [m] 28

The vehicle can wade in up to 1.4 m deep waters and has a band width of 533 mm.

Figure 4. Battle vehicle 9040C on mission in Liberia.

The vehicle is armed with a 40 mm automatic cannon, a 7.62 mm machine gun, six smoke launchers and two flare launchers. Since the weight of the 9040C is just short of half that of the battle tank 122, it could be kept in mind that two of these vehicles could be transported at the same time with a cargo weight that is less or the same as for the primary cargo Tank 122.

3.2. LANDING PLATFORM DOCK, LPD The British Royal Navy has recently updated their assault capacity with a completely new concept for amphibious operations. The concept is based on two large LPDs and 10 LCUs. They are able to embark, transport, deploy and recover by air and surface means, troops with their equipment, vehicles and miscellaneous cargo forming part of an Amphibious Assault Force. The first of the ships, HMS Albion was commissioned in 2003 and has since then been used in a wide range of naval operations. The displacement of Albion is 18500 tonnes with a draught of 7.1 m. It measures 176 m in length overall and 25.6 m over the beam.

Each LPD carries 4 LCUs and a further of 4 LCVPs (Landing Craft Vehicles Personnel) at a time and can be loaded from a well deck, stern gate and/or smaller side ramps. The ship can operate two Merlin sized helicopters and is equipped with a modern combat information system as well as communication systems which also make it suitable to carry out the lead in larger naval operations (Figure 5).

Figure 5. The HMS Albinion and the well deck [1].

The well deck is 60 m in length, 14.8 m in breath and has a draught of 1.7 m. This sets dimensional constraints on the LCU. Currently a wooden barrier divides the well deck longitudinally, creating two

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separate loading lanes, each with a breath of 7.4 m and a length of 60 m. The air clearance of the well deck at loading condition has been estimated from photographs to 8 m. These dimensions are critical since the proposed design should fit in and use this space efficiently.

3.3. CONCLUSIONS The well deck must be used in an efficient way i.e. maximising the available space. Either the craft is 7.4 m or 14.8 m wide. These sizes use the full breadth of either the separate loading lanes or the full width of the cargo hold. Since the barrier between the lanes is not supposed to be taken away in a close future and some ships only have one loading lane, 7.4 m of maximum beam remains as the single realistic option. Regarding lengths, everything evenly divided to 60 m is possible (10, 15, 20, 30, 60 m). However 10 m is too short and 60 m would give bad redundancy with few crafts. Hence the options 15, 20 and 30 m remains as realistic options.

Reviewing the vision together with the selected cargo and LPD, the performance requirements of the FLCU is developed. The new craft shall do at least 20 knots, preferably 30 knots, when fully loaded and have a range of at least 100 NM. Naturally the craft must be able to stand fully loaded on its keel on both the well deck and a sand bottom. The complete specification of requirement is shown in the following table.

Table 3. Specification of requirements

Class: Requirement: Specific figure (if applicable): 30 m in length 7.4 m in beam 1.7 m in draught fully loaded

1. Dimensions: not be greater than:

8 m in air draft from keel to highest point

Have roll on roll off capability Be equipped with an electronic interface towards the cargo

10 m in length 3.8 m in width 3.2 m in height

Primary cargo, battle tank Leopard 2 with dimensions of:

62 tonnes in weight 7 m in length 3.4 m in width 2.7 m in height

2. Cargo Capacity:

Secondary cargo, 2 battle vehicle 9040 C with dimensions:

28 tonnes in weight

be operational in Sea state 3 3. Seakeeping: survive in sea state 4

be able to land at a variety of shorelines including the NATO standard shoreline be able to stand on a well deck fully loaded be able to stand on a sand bottom fully loaded speed fully loaded: 20 knots + speed unloaded: 30 knots +

4. Performance:

range: 100 NM +

allow the cargo to use its sensors during transportation give ballistic protection to fragile cargo

5. Tactics

allow the cargo to act with its weapon systems during transportation 6. Structure be designed according to the DNV Rules for Classification of High Speed Light

Craft and Naval Surface Craft 7. Remaining be designed with simplicity and economics in mind

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4. CONCEPT From the specification of requirements some points can be identified as critical to the concept. These are the geometry of the craft, the performance (speed, seakeeping and hydrostatics), structural strength and mass estimation. Naturally the aim of this study is also to produce an attractive concept more efficient than the solutions used today. Hence a benchmarking study was done to analyze the performance of other concepts and get inspiration from their technical solutions.

4.1. BENCHMARKING The landing craft units in use today and being developed can be divided into the following categories; the conventional types, the hovercraft types, the catamaran concepts and the surface effect types. The different types with a corresponding representative example are shown in Figure 6.

Figure 6. LCU categories from top left; Conventional type LCU mk10 (UK) [2], Hoover craft type LCAC (USA) [4],

Catamaran type L-CAT [3] (France), SES type PACSCAT (UK) [7].

The four types of categories represent a spread of technical solutions each with certain advantages and disadvantages. The box shaped conventional type has the benefit of simplicity and robustness but are on the other hand very slow and have poor sea keeping characteristics. For the hovercraft, speed is not an issue. The LCAC can make up to 40 knots and has the benefit of being independent to draught. However, a hovercraft is a more complex and vulnerable system that also cost a lot to run. The SES (Surface Effect Ship) PACSCAT is a combination of a conventional LCU and a hovercraft. It has better speed resources than the conventional craft and is more robust although it still suffers from fairly poor seakeeping properties.

The most innovative concept representing something entirely new is the catamaran concept, L-CAT. It is multihull with a submersible loading platform in the middle. When docking or approaching the landing area the platform is lowered down, changing the submerged hull so that the craft operates more like a traditional LCU. During transportation the platform is raised and the craft operates as a catamaran. The advantage of this is combining the low draught characteristics of the traditional LCUs with the low resistance slender hull shape of the catamaran for increased speed and seakeeping performance. The disadvantage is a fairly large beam, which makes it incompatible with many LPDs. A more thorough description of the different crafts can be found in part 2 of this report.

4.2. THE CONCEPT OF FLCU Initially a general brainstorming was carried out to identify and classify some possible directions/classes of solutions. From that, one main direction was chosen for further investigation in a second stage. The

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outcome of this final stage was an idea regarding the design of the craft in general terms and principal function. Following the evaluation of the different possible solutions to the transportation problem the choice fell on a multihull solution. This decision was based on the assumptions that:

• The concept has good potential to reach higher speed • High speed can be maintained even in difficult sea state • Good seakeeping properties • The cargo handling can be solved in several ways for example the L-Cat principle • Can easily stand on a flat well deck without the extra support, necessary for v-shaped hulls

Due to the geometric limitations set by the LPD in which the craft shall operate, there are a finite number of different main dimensions that can be used in order to optimize the platform with respect to the available space. The LPD has 2 separate loading lanes, each 7.4 m wide and 60 m long. One important criterion is that the craft should fit in the existing LPD without modification, such as removing the centre barrier.

The first approach was to design a craft similar to the L-Cat but with a smaller beam which would make it compatible with LPD loading lanes. This option could be dismissed at an early stage. Such a solution would be very difficult to design due to small margins in width. The final solution is instead based on the L-cat principle with the addition that the FLCU can adjust its beam. Hence it will fit in to the LPD and still have the advantage of draught reduction capacity. The principal function of the craft can be visualised with three different modes. They are shown together with a structure of the problems to solve in Figure 7.

Figure 7. Schematic picture of the different modes and the structure of the problems to solve

It is obvious from the figure above that the transformation between the three modes is the main challenge to solve. However, trying to answer that question without showing the feasibility of the other levels would be a very inefficient way to work and also mean that the craft in the end would risk failing some fundamental criterion. Following the structure shown in above figure the first lap in the design spiral was carried out on a very rough basis. The concept passed all levels and a mechanical solution to the transformations was developed. This ended up in a concept serving as basis for the final analyze work. The methods and results being used are more thoroughly treated in part II of this report.

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The final concept is a state of the art solution that will enable substantial development of landing operations. The key to its superior properties is a catamaran hull connected by a highly specialized cross structure. This cross structure consists of a platform carrying the cargo and a Cargo Control System (CCS), which serves as connection between platform and hulls and also enables the movement between the different modes. A synoptic picture of the FLCU is shown in Figure 8.

Figure 8. Overview of the FLCU

The CCS is based on 4 L-shaped lever arms. By initiating a rotation in the hull connection, the platform will be lowered and simultaneously move forward. This will give the platform a lowered position slightly beneath the demihull baseline and almost 2 m forward of the bow. This gives certain advantages such as the platform taking most of the beaching impact, enabling that the hulls can be made less robust than normally stipulated. Large hydraulic cylinders produce the movement. A locking system that locks the lever arms during transportation and beach landing mode is also installed.

Using the volume of the platform when it is lowered the draught of the craft can be reduced significantly. This allows the FLCU to get closer to the shore and gaining a tactical advantage.

In the connection between the CCS and the platform a mechanical movement device is fitted. The device can drag and push the lever arms along a tube shaped beam, which enables that the hulls can be pushed together and thus reduce the beam of the FLCU so that it will fit in to the LPD loading lanes.

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5. INITIAL DESIGN Following the conceptual design a more detailed general and structural arrangement was developed in conjunction with developing the lines of the hulls. The FLCU general arrangement can be divided into four interconnected parts; the hulls, the platform, the Cargo Control System (CCS) and the propulsion & manoeuvring systems. The overall performance of the FLCU is largely dependent on the interaction between the above-mentioned systems and thus all these are developed simultaneously. The calculations and analysis behind the results presented here may be found in the second part of this report.

5.1. GENERAL ARRANGEMENT

Developing the hull shape of the craft is a matter of balancing a number of different properties such as resistance, hydrostatics, seakeeping, manufacturing and structural demands. The final shape has narrow bow sections to reduce motions and a negative stem in order to give the craft wave piercing properties. Since one of the beams, handling the platform elevation, will be connected close to the bow, the sections are concave to increase the area of attachment. The prismatic coefficient is 0.7 and the block coefficient 0.65 according to the recommendations of Hans Liljengren at SSPA. It is desirable to make the hulls more slender, but impossible due to the minimum displacement requirements.

As previously described the LPD sets strict constraints to the main dimensions of the FLCU. The consequence of having the hulls beneath the platform in docking mode is that no reduction of draught can be gained as in the beach-landing mode. Thus the maximum draught must not be more than 1.7 m fully loaded with the platform raised.

In light condition the draft will be reduced from 1.7 m to 0.9 m. This will increase the possible speed from 20 to more than 30 knots, which means that the ship will operate in the planning region. Thus the bottom is designed with a chine following the approximated water line in light condition (Figure 9).

Figure 9. Body plan of the FLCU.

The length has a great influence on many design parameters such as resistance, seakeeping and hydrostatics. It does also directly affect the overall efficiency of the total system since it determines how many FLCUs could be transported inside of the LPD. The final length between perpendiculars of 18.5 m makes it possible to dock three FLCUs in one lane at the same time. This would increase the possible number of tanks during transportation at sea from today’s 4 to 6 units. The displacement is 115.6 tones at full load and 53.7 tones in light condition. A detailed mass analysis can be found in the second part of this report.

The height of the freeboard is determined according to DNV by the reserve buoyancy of the hull above the waterline up to the bulkhead deck at the maximum operational displacement of the vessel. In this case this enclosed volume should provide a reserve buoyancy of 100 % of the crafts volume displacement at full load. The clearance between the water surface and the platform bottom is also taken into consideration when determining freeboard height. The main particulars are displayed together with a principal description of the different transportation modes in Figure 10. The tank depicted in the figure is the primary cargo, battle tank 122.

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Figure 10. Main particulars and the three different modes, docking mode (top), transportation mode (middle) and beach landing

mode (bottom).

The engine is mounted on two main girders in between bulkhead 2 and 3. From the gearbox a shaft goes through bulkhead 2 back to the water-jet unit. The fuel tanks are located between bulkhead number 3 and 4 in the bottom of the hull. Their centers of gravity coincide with the ships longitudinal centre of gravity; hence the fuel level does not affect the trim of the craft. Each fuel tank can hold up to 1.75 m3 of diesel.

Since the centre of gravity changes when the platform moves from transportation mode to landing mode trim tanks must be installed. They are fitted on each side of the water-jet at the stern, between bulkhead 1 and bulkhead 2. Each ballast tank can hold 1.25 m3 of water. The general arrangement of the craft is displayed in Figure 11, where the platform is highlighted in green, tanks in purple and the CCS-system in red. The bulkheads are numbered from 1 (transom) – 5 (bow).

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Figure 11. General arrangement drawing of the FLCU.

Deck 1 house the systems needed to operate the cargo control system, such as hydraulics, bearings and brakes.

5.2. HULL STRUCTURAL DESIGN One part of this study is to evaluate two material concepts against each other in the context of a Fast Landing Craft Utility application. The selected material systems are aluminium and carbon sandwich. Aluminium has been the preferred material, not only for lightweight high-speed catamarans but also for crafts such as the fast Combat Boat 90H. The aluminium concept in the CB 90H has, according to Ulf Mansnérus at SSPA, proven to be extremely durable and successful. There are however more advanced materials, such as carbon fibre, which offers advantages such as reduced weight. The carbon composite material concept has recently been successfully used in application such as the CB 90E and the Visby class corvette. In this study a standard modulus fibre, the T700 (E<265 GPa) is chosen to provide a good balance between strength and stiffness as well as being thoroughly tested in the CB 90E and Visby class corvette.

The scantlings are determined using the DNV rules for classification of High Speed, Light Craft and Naval Surface Craft (2006) [9]. Since the platform is designed to handle the beaching loads, there is no need to include these loads when determining the scantlings for the hulls. However, in order to make the carbon concept more durable to the wear and tear of standing on the well deck and beaching, the sandwich turns into a single skin laminate at the keel. The bottom is also coated with a urethane elastomer to improve the tolerance to small damages from for example rocks.

The final concept is designed using the carbon composite material system. There are two primary reasons for this: a possible structural weight reduction of 47 % and, since the sandwich panel itself is a stiffened plate the final structure is less cluttered compared to the aluminium option. The final structural weight of the carbon craft is 7.3 tonnes compared to 13.7 tonnes for the aluminium craft.

A full description of the methods and results of the scantling calculations is found in the second part of this report.

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5.3. CARGO CONTROL SYSTEM (CCS) DESIGN The movement of the platform is carried out by a hydraulic system. The system is in Figure 11 marked with red color. Each hull has two different CCS stations, the aft and the bow station. The aft station is situated at bulkhead number 3 and the bow station at bulkhead number 5. The bow station is controlled by one lift and two locking systems and the aft station is controlled by two of each. The aft station CCS is shown in Figure 12.

Figure 12. Aft station CCS, side, top and 3D view.

The CCS consists of an L-shaped lever arm welded together with a tube beam, named the hull tube beam. This beam is connected to two grease lubricated slide bearings, one on each side of the hull. Both bearings are laminated together with the bulkhead, the cargo deck and the side structure. Two discs are welded on the tube beam. A hydraulic cylinder connected in between the two discs produces the movement. The platform can be locked in the upper and lower position by a locking device pushing a plug into a cut-out in the discs. The cylinders and locking device are powered from a hydraulic pump fitted to the gearbox of each main engine.

To transform from transportation mode to docking mode and vice verse, another hydraulic system is used. This system is still on the conceptual stage and thus not fully dimensioned. The principle function is shown in Figure 13.

Figure 13. Principal sketch of transverse movement system, bottom, side and 3-D view

Lever arm Connection to platform

Locking device

Hydraulic cylinders

Platform tube beam

Gear wheel

Rail

Space to house hydraulics

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The central part in this system is two gear wheels which are mounted at each side of the lever arms. The gear wheels run on a gear track in the platform tube beam. This enables the lever arm to slide along the platform tube beam and hence adjust the width of the FLCU. In the end position, the gear tracks are elongated around the platform tube beam to allow for the 90 degree rotation when lowering the platform. The gear wheels are powered by hydraulic engines which in term are powered from the engine transmission. None of the hydraulic systems are drawn in the previous figure, but there is space to install necessary equipment such as engines, pipes, shafts, bearings and control devices.

The unconventional design with only four attachment zones between the hulls makes the CCS one of the most critical areas of the craft. A conventional catamaran usually has a large and flat cross structure, permanently connected to the hulls with a number of elements all contributing to the global stiffness. To dimension and study the load transfer between the different parts of the CCS a global FEM model was developed in ABAQUS. The results from this calculation showed that it is possible to construct the load carrying parts of the CCS with reasonable dimensions. Constructed in high tensile steel, the mass of one lever arm, hull beam included, is approximately 2 tonnes.

The FEM model was also used to study the global stiffness of the FLCU. For the worst considered case the difference in displacement between the bow and the stern is 400 mm in the vertical direction. All procedures, methods and results are more thoroughly described in part 2 of this report.

5.4. PLATFORM STRUCTURAL DESIGN The platform is essentially a large beam simply supported at point 1 and 2 (Figure 14). The width of the platform is determined by the primary cargo, which has a vehicle width of 3.8 m. In addition to this a minimum manoeuvring margin of 0.3 m on each side is needed according to Christer Nedin at FMV. Hence the total minimum width of the platform is 4.4 m. The length of the platform is largely dependent on the cargo handling and the ability to transfer vehicles between two crafts in the well deck. The overall length is 19 m and this gives that a flap with a minimum length of 1 m needs to be fitted at the bow of the platform. This flap will not only serve as a bridge connecting two crafts in the well deck, but also help the vehicles to disembark safely at the shore.

Figure 14. General and structural arrangement of platform

The platform is constructed using aluminium and steel. This material concept is chosen for several reasons. The platform will be subjected to extreme loads when landing and standing at the shore, due to this the centre girder is constructed by steel. The platform also needs to be tough due to the abrasion and local loads imposed by the cargo, well deck and shore. Aluminium is used wherever possible to reduce weight.

The cargo platform is designed using the specification of requirements, determined by cargo loads, sea loads, buoyancy and operation. The minimum plating thickness and stiffener sectional modulus due to

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cargo loads on the platform was determined using DNV HSLC & NSC. A detailed investigation of the required scantlings of the platform was performed using FEM calculations, this allows for weight reduction and consideration of global stiffness and strength, which is of utmost importance for the operability of the craft. This is also necessary due to the unconventional nature of the craft and the boundary conditions applied on the platform.

The direct calculations together with the DNV scantlings provided the initial scantlings of the platform. By using these two methods it is proven that the platform can be constructed to a total weight of 13 ton.

As designed, the platform can handle both the primary and the secondary cargo, it provides a conceptual mean to load and unload cargo at the shore or LPD. It is as lightweight as possible with the means and time available; however weight can be saved with further studies. It provides a reserve buoyancy of 60 ton, thus reducing draft when landing to less than 1 meter. The design allows the platform to be raised and lowered and it fulfils the relevant requirements of the DNV HSLC & NSC rules.

5.5. PROPULSION AND MANEUVERING The multihull configuration offers an advantage when it comes to propulsive resistance. Due to the slender nature of the hulls the free surface disturbance is less than for a conventional monohull and thus the wave-making resistance is reduced.

The resistance is analyzed using a slender body method as implemented in the Maxsurf Hullspeed software [12]. This method calculates the wave-making resistance of the craft. In addition to this the frictional resistance, the wind resistance and the added wave resistance is also added. The final total resistance at full load and at 20 knots is determined to be 115 kN. These results together with the efficiency of the water-jet propulsion system, ranging from 50-60 % depending on the speed, provide the total required engine power.

The minimum required engine power according to these calculations is 1080 kW, but in order to gain some margin and following the advice from Rolls Roys and Hans Liljenberg at SSPA, a slightly higher power is chosen. The selected engine is the MTU 12 V 2000 M93 with a rated power of 1340 kW. The total weight of one engine, gearbox and couplings is 3.986 tonnes. The MTU engine has a fuel consumption of 341 litres/hour at the rated power. According to the specification of requirements the craft has to be able to make a round trip to the shore without refuelling. This is a total distance of 60 NM. The craft travels at 20 knots from the LPD to the shore fully loaded, but once unloaded it does at least 30 knots back from the shore. This gives that the total time for a single round-trip is 2.5 hours. This gives a total fuel consumption of 1705 litres for both engines, which with a density for diesel of 0.85 kg/m3 gives a total mass for the fuel of 1.45 tonnes.

The FLCU is propelled by two A56 Kamewa water-jet aggregates. The water-jet system controls are powered from a hydraulic pump fitted to the engine transmission. The weight of one aggregate is 2.23 tonnes.

The evaluation of manoeuvring properties was restricted to a common stability analysis. Due to the significant difference in weight between loaded and unloaded condition the course stability of the craft will vary from stable in loaded condition to unstable in unloaded condition. Hence, the craft must be fitted with course stabilizing equipment.

5.6. HYDROSTATICS AND STABILITY The key to the operability of the FLCU is the hydrostatic and hydrodynamic properties of the craft. Special concerns for this particular craft are the change in trim due to the movement of the platform and the stability in the fully loaded docking mode.

The FLCU was evaluated against the IMO HSC2000 criteria for four load cases: loaded/unloaded in docking mode and transportation mode respectively [11]. All of these load cases fulfil the requirements. The most critical case is when the craft is in docking mode, fully loaded with the hulls in inboard position.

The second matter for analysis is the floating position in the different modes. The craft is designed to have a level trim when it is fully loaded in transportation mode. An effect of the platform movement from raised to lowered position is that the centre of gravity changes position longitudinally and vertically. For

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the beaching load case this must then be compensated with ballast water in the stern of each hull, so that the craft has a level trim when approaching the beach.

5.7. SEAKEEPING An assessment of the seakeeping characteristics of the FLCU is made with the use of the linear strip-theory. It is concluded that this is not the optimal tool for this analysis, but it is the only method available at this time. The purpose of the analysis is to show the feasibility of the craft with respect to the specification of requirements. There are two especially limiting criteria for the FLCU: slamming on the cross structure bottom and emergence of the water-jet intake pipe. In addition to these criteria the FLCU is also evaluated against the NORDFORSK criteria for small fast crafts [ref].

The analysis is done in head and beam seas using the Maxsurf Seakeeper program [10]. When modelling a catamaran in Seakeeper only one hull is considered. This has proved to give reliable results for coupled heave and pitch motions. It is also possible to model uncoupled roll motions, the method that Seakeeper uses is to consider one hull but the real separation between the demihulls are used to calculate the actual metacentric height. The roll motion is modelled as an alternative heaving of the two demihulls. The added mass and damping for the roll motion is then the same as for the added mass and damping of the heave motion of one hull.

The FLCU is, according to the specification of requirements to be able to operate at full speed in Sea-State 3. This means a significant wave height between 0.5-1.25 m. Using a zero crossing period of 6 second, a mean value of the period of the most common wave in the North Atlantic and the Baltic Sea, the results are encouraging. The craft is analysed at 5 – 20 knots, but the upper limit of validity for the strip method is estimated at 13 knots, or Froudes number 0.5.

The craft passes most of the limiting criteria. The maximum vertical acceleration at the centre of gravity at 13 knots is 15.4 m/s2. This is slightly higher than the accelerations stipulate by DNV at 10.1 m/s2. The craft has no problem fulfilling the NORDFORSK criteria, except for the roll motion criteria, where it fails with a small margin.

The interesting results are for the head seas case. It can be determined with a sufficient accuracy that the platform clearance is enough to fulfil the seakeeping criteria, at least at 13 knots.

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6. CONCLUSIONS The purpose of this thesis is to deliver a conceptual design of a new type of landing craft unit. The craft is to serve the landing operations as typically performed from landing platform docks by the British Navy. It is to offer an advantage to existing designs as well as an improved alternative to the designs currently being developed today. The work started from scratch, reviewing the current landing crafts and those being developed. This work, together with input from the job initiator, SSPA, resulted in a specification of requirements for the new type of LCU. The primary cargo was identified and the system in which the craft would operate was mapped. From these limitations and demands a concept evaluation was performed resulting in the development of a catamaran craft.

The catamaran configuration was judged to have the most potential with respect to speed, seakeeping and cargo handling. Employing a completely new concept of transverse moving demihulls fulfils the constraints on the craft footprint set by the landing platform dock. Further, in order to make the beaching manoeuvre as efficient as possible as well as giving the craft an advantage of low draught, the cargo platform is abatable. This allows the draught of the craft to reduce from 1.7 m in docking and transportation mode, to only 0.9 m in beaching mode.

The final conceptual design is 18.5 m between perpendiculars, the fully loaded displacement is estimated to 115.6 tonnes and the maximum speed at full load is 20 knots. The craft has a docking breadth of 7.4 m increasing to 11.4 m in transportation mode. This increases stability and reduces resistance.

One of the primary aims of this thesis is to confirm the feasibility of the proposed craft. In relation to the specification of requirements as stated in Table 3, each class of requirements is commented on in Table 4 with the purpose of reviewing how, and to what extension the requirements are fulfilled.

Table 4. Review of specification of requirements

Class Criterion: Comment: Critical factors 1. Dimensions Fulfilled The craft dimensions are within the

requirement set by the LPD and cargo

Mass estimation, which affects the draught in docking mode

2.Cargo capacity Fulfilled The craft can transport the primary and secondary cargo and the design of the deck allow for other vehicles to transported as well

Structural design of the cargo platform

3. Seakeeping Partially fulfilled

Due to the validity range and limitations of the method being used, only an indication of the seakeeping properties at certain headings can be obtained

Clearance between water surface and platform bottom, wave induced loads on the cross structure, transverse stability in docking mode

4. Performance Fulfilled Several methods have been used to evaluate hull resistance indicating that the selected propulsion system will be sufficient to reach required speed

Hull resistance, resistance due to waves, wind and manoeuvring properties

5. Tactics Partially fulfilled

Deck design allows for cargo to use sensors and weapons. No solution for ballistic protection is yet designed.

Keep the deck free from interfering equipment such as a bridge or large antennas

6. Structure Partially Fulfilled

The whole structure is designed according to DNV HSLC & NSC rules.

Introduction of loads in cross structure connections and robustness

7. Remaining Fulfilled Even though some parts of the crafts systems is complex in itself. The basic design and the general solutions are designed with simplicity in mind.

Control systems

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As Table 4 shows, the majority of the classes are fulfilled and the concept feasibility is concluded to be confirmed. The main reason that some of them are judged as partially fulfilled is due to the need for more thorough analysis before moving on in the design process. It is also worth to note that all results in the conducted investigation indicates that the craft will fulfil the specification of requirements and thus providing a new, entirely different concept for landing operations.

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7. FUTURE WORK The conclusions of this work indicate that the proposed craft is feasible, but requires further studies. These studies can be divided into three categories: hydromechanics, structure and systems. The future work contained in these categories is outlined below.

7.1. HYDROMECHANICS The craft has been analyzed with respect to hydrostatics, resistance and seakeeping. The areas that are in need of further investigation are the seakeeping and resistance. The methods used in this thesis only serves as an indication of the performance of the craft.

The linear strip theory used in the seakeeping analysis is limited to head seas and a simplified roll motion analysis only. It is also limited to low speed. The seakeeping characteristics of the craft are shown to be sufficient within the limits of the linear theory, and the 2.5-D strip theory is suggested for further evaluations of the craft. This would be the next logical step before commencing with model trials for the final verification.

The resistance of the craft is determined using the slender body method and the next step would be to use model trials to validate the resistance in the fully loaded condition as well as obtain results in the light condition. The final result of this analysis has a large effect on the selection of propulsion machinery and in the end the weight of the craft. It is therefore vital to validate the calculations using model trials.

7.2. STRUCTURE The structure scantlings have been determined using the DNV High Speed Light Craft & Naval Surface Craft rules according to the specification of requirements. Direct calculations are used to validate the rule-based calculations as well as analyse areas where the rule formulations are deemed unusable or not suitable to a full extent. These areas are primarily the cargo platform and the cargo control system. These analyses confirm the feasibility of the concept, but there is a need for additional calculations in the next design step. They also show that there is room for weight reduction, especially in the cargo platform.

Questions are also raised regarding the robustness criteria formulated in the DNV rules, especially in a landing craft application. A method to determine minimum laminate thicknesses in areas subjected to high local loads such as beaching must be developed.

Further investigation of the cargo control system and especially the interaction between the hull, CCS and platform must be performed. These parts are simplified and treated more or less separately in this study. The first step in this further study would be to construct the CCS system in higher detail. This would mean e.g. selecting the bearings, dimensioning of the structure that will introduce the crossbeam loads in the hull structure and determine the scantlings for the structure that supports the hydraulic cylinders and the brakes.

7.3. SYSTEMS The scope of this thesis does not include a detailed analysis of the different control systems onboard, such as systems for autonomous navigation and manoeuvring systems. In a further study this would have to be dealt with, perhaps resulting in slight changes to the appearance of the craft. In the proposed design there is for example no superstructure, there is however room to fit one if necessary. The hydraulic cargo control systems have been dimensioned on an approximate level and should provide some guidance to the required size of the system. This works have also provided weights critical to the hydrostatic and hydrodynamic calculations.

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8. REFERENCES 1. Royal Navy website about the Albion class, http://www.royalnavy.mod.uk/operations-and-

support/surface-fleet/assault-ships/albion-class/ , Available 2009-09-21 2. German company Schottel information about propulsion,

http://www.schottel.de/uploads/PDF1176992341.pdf, available 2009-09-21 3. Information about the French navy, http://www.netmarine.net/f/bat/sabre/caracter.htm,

available in 2009-09-21(in French only) 4. Global Security information on the Zubr LCAC,

http://www.globalsecurity.org/military/world/russia/1232_2-specs.htm, available 2009-09-21 5. Försvarsmakten information on battle tank 122, http://www.mil.se/sv/Materiel-och-

teknik/Fordon/Stridsvagn-122/, available 2009-09-21 6. Försvarsmakten information on battle vehicle 9040C, http://www.mil.se/sv/Materiel-och-

teknik/Fordon/Stridsfordon-9040C/, available 2009-09-21 7. Marinelog article about British FLC PACSCAT,

http://www.marinelog.com/DOCS/NEWSMMIX/2009oct00121.html , available 2010-01-21 8. UN peacekeeping operations January 2010,

http://www.un.org/en/peacekeeping/currentops.shtml, available 2010-01-25 9. DNV rules for classification of High Speed Craft & Naval Surface Craft, January 2006 10. Maxsurf Seakeeper Manual version 11.1, Formation Design Systems Pty Ltd 1998. 11. 2000 HSC Code, IMO, London 2001, ISBN 92-801-5122 12. Maxsurf Hullspeed Manual, Formation Design Systems Pty Ltd 1998.