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Q&A with Steven Lee - Technical Insight of the Pentamaran

Lead Naval Architect, Steven Lee, from BMT Specialised Ship Design explains the origins of the Pentamaran, and shares his technical insights to how this tested design offers unique capabilities for Autonomous operations.

2 June 2020

Defence and Security Specialised Ship Design

Q&A with Steven Lee - Technical Insight of the Pentamaran

BMT in Specialised Ship Design | Pentamaran

1. What is the history of the Pentamaran design which leads into the development of the USV R&D project?

The Pentamaran concept was developed by Nigel Gee and Associates (NGA) around 1997, prior to becoming a member of BMT. Following extensive research and development over intervening years BMT are now further developing this technical platform to wider markets which include defence operations with Unmanned Surface Vehicles (USVs).

The Pentamaran platform was born out of research within the high-speed freight market. Around 1995 Nigel Gee and Associates (NGA) was approached to investigate the feasibility of transporting 13,000 tonnes of cargo, at 30 knots with not more than 30MW of power. Investigation into long, slender mono-hulls revealed that it was possible to achieve the target, but only at length beam ratios which were beyond that needed to maintain stability requirements. This led to the investigation of using a multihull solution, principally a stabilised mono-hull, traditionally recognised as a trimaran.

Hydrodynamically a stabilised mono-hull platform allows the optimisation of the central hull to be undertaken without the stability constraints of a conventional mono-hull. On such a platform stability can then be ‘added’, and indeed finely tuned with the sizing of the sponsons (also frequently termed ‘outriggers’ or ‘amas’). In this regard, the stabilised mono-hull platform can have its stability characteristics (both intact stability and roll stiffness) more closely engineered and tuned than that of a conventional mono-hull, or other multihull solution.

However, it is not just intact stability and roll stiffness characteristics that are a driver to the design of trimaran sponsons. Damage stability characteristics play a significant factor. Typically, damaged stability requirements will increase the length of the sponsons required to meet post-damage criteria. This increase in sponson size carries a resistance penalty.

The solution developed by NGA to avoid this resistance penalty was to have two sponsons per side, with only one set on either side being immersed in the static upright condition. Such a configuration allows for far shorter and lower immersion sponsons to be used to achieve the intact stability and roll stiffness characteristics desired, whilst the forward sponsons provide the reserve of buoyancy needed at higher heel angles or in damaged conditions, without any associated drag penalty.

With the use of five hulls this design solution was termed the Pentamaran. A significant number of designs have been developed and model tested, from smaller (65m 60 knot) high speed fast ferries, large (300m 50 knot) trans-oceanic fast freight ships and high-speed sea-lift naval logistics ships and fast combatants. The Pentamaran platform is also the subject of numerous published technical papers in the public domain and international patents.

Throughout the late 1990’s and early 2000’s there was a significant focus on modal shift in transportation networks, and the use of high-speed shipping to rival less time sensitive cargo that would be carried by air freight. A number of designs were developed for large high-speed transoceanic freight ships including extensive model testing programmes in the ocean basin at Marintek in Trondheim. Concurrent to this use of the Pentamaran for large fast ferry applications delivered further innovation where the lower propulsive power required was offset not by offering greater speed, but allowing design solutions constructed of steel as opposed to aluminium and running traditional medium speed engines. These designs offered both greater robustness in service, reduced susceptibility to the phenomenon of parametric rolling which trimarans suffer from, increased deck area and ease of docking in comparison to trimarans as well as lower capital and operating costs.

With a shift in world economic and naval focus post September 2001, the Pentamaran platform was investigated both by the UK and US defence establishments for application to future fast combatant programmes as well as high speed sealift and support.

Over the last 18 months BMT have been reviewing the emerging requirements for long range / high endurance USVs which indicate that the focus on performance should be in the slow to mid speed range. This significantly reduces the propulsion power requirements and enables vessels to operate for extended durations and is evident that developing a highly efficient hull form will have a significant impact on propulsion selection and capability. Historical testing indicates that when operating at low Froude numbers the Pentamaran is capable of providing lower resistance compared to typical mono-hulls and catamarans. Studies indicate that a 40m Pentamaran USV could require up to 30% less power than a typical mono-hull sized to provide a similar capability at a steady operating speed. Reductions of this magnitude can have a significant impact on through life and build costs for fuel consumption, machinery selection, maintainability to name a few.

2. What historical Pentamaran data has been utilised in the development of this USV design?

The USV design draws data from different sectors which BMT provide production design information for not just the defence sector. BMT have developed numerous innovations across multiple sectors and in order to progress we need to cross fertilise this data to improve efficiencies within our design. BMT are at the forefront of vessel design in the defence, commercial, offshore and yacht sectors. Excellent sea keeping characteristics and high efficiency are defining features of our hull forms. BMT has a proven track record in designing a wide range of hulls form type including Extreme Semi Swath Catamarans, Swaths, Catamarans, Mono-hull and Planning Craft to name a few. BMT draws from this plethora of information to evolve and develop highly efficient hull forms for the future.

New products are often developed to provide specific requirement needs which may have different capability focus’ and will utilise data from different market areas to create the optimal solution. A Pentamaran hull form developed/optimised for slow to medium speed operation will utilise different data to that for high speed operation. With the decades of in-house historical information available, BMT are able to develop solutions to meet differing requirements.

3. How mature is the design? What kind of development testing has BMT carried out?

The Pentamaran hull form is considered to be a mature design which has undergone extensive resistance, seakeeping and structural measurement tests to determine global loading in severe conditions at model scale. The current USV concept utilises this test data to determine the feasibility of a USV with a Pentamaran. The USV design is at concept level with high level weights, powering and arrangement design carried out to assess the feasibility for such a craft. BMT have set nominal performance requirements/capabilities as a starting point to determine the principle particulars of the craft in order to make a comparison to alternative hull forms such as mono-hulls, catamarans and trimarans.  The concepts designed to date look at integrating both Commercial off the Shelf (COTS) and high Technology Readiness Level (TRL) equipment to develop a solution, with the primary focus on hull form development, efficient propulsion system and redundancy.

A conclusive set of requirements would help to evolve the design further, but at this stage, it is clear that the Pentamaran hull form can offer significant benefits to future designs which may require long range / high endurance requirements, whether these are crewed vessels or USVs.

4. How does autonomy impact system designs?

When looking at autonomous operations, removing the human operator requirements into the design fundamentally changes the design philosophy that impacts not just the configuration of the power systems but also the entire below decks arrangement. Without the ability to have someone to fix a piece of equipment, the systems need to incorporate multiple levels of redundancy. One of the primary areas to focus on was the propulsion system. The propulsion system needs to be both robust and highly efficient across the operational speed range in order to minimise power requirements and fuel consumption.

To provide high efficiency BMT have chosen to use a Controllable Pitch Propeller CPP arrangement with multiple independent shaft lines. In addition to this, each shaft line is to be driven by 2 engines via a 2 input gear box. For a 40m vessel, this equates to 3 shaft lines with 6 propulsion motors.  A 6 engines propulsion solution offers ‘graceful’ degradation, providing the best possible efficiency across a broad range of operating speeds and a high level of redundancy to ensure the vessel is capable of getting back to a service point.

To further improve redundancy, 3 engines are proposed to be located in one machinery space and 3 in another. Alternatively, BMT have reviewed utilising electric motors which are fed from a common electrical supply. This system allows multiple electrical generator units to be utilised and supply the power to the electric motors from a position located above the main weather deck, reducing the likelihood of flooding. For normal loitering speeds, only one generator would be required to provide the power which means a single unit would be operating more efficiently compared to multiple diesel propulsion engines. For cruise speeds, 3 out of 6 generators would operate and for high speed, all generators would operate. BMT have undertaken numerous diesel electric and full electric production designs for ferries, luxury yachts and are working on solutions for the offshore energy market and is where cross sector fertilisation is crucial to developing a workable solution.

Installing a number of smaller units not only provides improved redundancy, it also aids in maintenance due to the smaller size of equipment. Other system areas are also fundamental to mission success and would include duplication to provide resilience, but this is to be investigated and developed during the next phase of the design spiral.

5. What size vessels have BMT considered, and to provide what autonomous capability?

 The Pentamaran hull form is proven in model tests up to 280m with classification level design packages complete for some of these designs. For the current USV designs, BMT have reviewed a 40m and 62m design which are to provide over 40 days and 60 days endurance capability based on an operational profile such as: 75% Loiter & 25% Cruise / Sprint. The slow loiter speed is essential to maximising the endurance capability for vessels which may need to remain on station for long periods of time or cover large areas, such as surveillance patrol craft, survey vessels or floating Forward Operating Bases (FOBs). Recharging facilities and small energy stores could also potentially increase the on station duration further.  Larger craft will be able to provide increased endurance and USVs which might operate a specific route from A-B could easily be developed in line with the crewed vessel services which the Pentamaran was initially developed for.

6. What applications have BMT considered for the Pentamaran USV design?

There is a very wide range of missions the Pentamaran could be used for because of the range of payloads, sensors, and effectors it can carry. Long endurance, deep water, intelligence, surveillance, and reconnaissance (ISR) and enduring communications node are both well suited to this platform which can remain on station, be discreet and have a low human cost if lost. It can also provide the ability to facilitate distributed physical effectors networks. Here are some potential Defence and Security (D&S) applications:

  • Support continuous at sea deterrent
  • Deployable Early warning systems
  • Support sea-lane clearance by using offboard vehicles
  • Coastal defence
  • Advanced force protection
  • Transfer of equipment from ship to land/land to ship
  • Collect and process passive/active EM spectrum ISTAR data
  • Launch/recover unmanned air vehicles

Outside of the D&S sector, the Pentamaran provides a highly efficient hull form which could provide long endurance capabilities to any USV or crewed craft. For example survey / oceanographic operations requiring a vessel to survey a large area of interest, Search and Rescue which would enable the craft to operate 24/7 with rotating observation crews onshore, Used as a forward operating base for smaller UXVs, providing refuel capability and extending the range capacity for smaller UXVs. Where used as a FOB a small number of crew could be onboard providing maintenance, refuel/recharge facility to other UXVs or smaller crewed vessels as a stepping stone to a fully automated capability.

 

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