Power & Propulsion Meeting the concept

Transcription

1 Power & Propulsion Meeting the concept Thomas Couch, BEng (Hons) CEng CMarEng MIMarEST Principal Naval Engineer, BMT Defence Services Ltd. Jeremy Fisher, MSc BEng (Hons) MIMarEST Principal Naval Engineer, BMT Defence Services Ltd. SYNOPSIS A Hybrid Power and Propulsion system offers the greatest flexibility between power generation sources and propulsion prime movers, low annual fuel consumption and also avoids the running of the prime movers at continuous low loads. BMT have developed knowledge of how a hybrid concept is developed in the context of a typical naval auxiliary vessel and this paper will explore recent experience. This paper will principally address how the concept design evolves to meet the concept of operations for the platform to deliver the operational need. The various aspects of propulsion and electrical system integration processes and capabilities are addressed including examples of where innovation has been used in machinery and systems to meet variations in operating profile and usage demands for vessels that provide worldwide logistical support. The boundaries of Hybrid Power and Propulsion system are explored in the context of variable propulsion and electrical loads including the impacts of operating concepts. The authors present the conclusion that where Hybrid Power and Propulsion system are used there are a number of factors driven by the concept of operations that can influence the final solution but through suitable design management process the solution can remain robust and meet the customer requirements. INTRODUCTION In the context of the vessels considered in this paper, namely Naval Auxiliary Vessels, a Hybrid Power and Propulsion system offers the greatest flexibility between power generation sources and propulsion prime movers, low annual fuel consumption and also avoids the running of the prime movers at continuous low loads. As explored further in the paper Hybrid Drives for Naval Vessels (Ref. 3) BMT has found this to be the most suitable arrangement to meet the requirements where low lifecycle costs are as important as the maximum sustained speed. In this paper the term Concept refers to the Concept of Operations which is also sometimes termed as the Operating Philosophy or Operating Concept. The focus of this paper is meeting the Concept by which the authors mean how the design is developed to meet the Operating Concept. BACKGROUND The BMT DSL Project Lifecycle as illustrated in Figure 1 aligns with the Concept, Assessment, Design, Manufacture, In-Service, Disposal (CADMID) cycle used by UK MoD and other nations. The maturity levels shown align to those described in MAP (Ref. 1). Author s Biography Thomas Couch is a Principal Naval Engineer within BMT Defence Services. After graduating in Marine Systems Technology from the University of Plymouth in 1997 he joined BMT Defence Services in 1998 engaged on various engineering tasks prior to being seconded to the Aircraft Carrier Alliance in In 2008 he joined Thales Naval Ltd taking on the role of Diesel Generator Integration Engineer, before becoming Technical Lead for Power Generation in In 2012 he re-joined BMT Defence Services focusing on the delivery of the Power and Propulsion system designs for the UK MoD MARS Fleet Tanker and the Norwegian Logistics and Support Vessel. Jeremy Fisher is a Principal Naval Engineer and the Deputy Head of Naval Engineering within BMT Defence Services. After graduating in Mechanical Engineering from the University of the West England in Bristol in 2003 he joined BMT Defence Services in July 2003 and worked on various projects prior to joining the Aircraft Carrier Alliance in August He joined Thales Naval Ltd in October 2008 as the Lead Engineer for Steering and Stabiliser Systems and in August 2010 became the Propulsion and Manoeuvring Technical Lead responsible for the Shaftline & Propeller Systems, Steering Systems and Stabiliser System. In 2012 he re-joined BMT Defence Services focusing on the bid design for the Norwegian Logistics and Support Vessel and continuing with the project to manage the delivery of the Electrical system design.

2 Figure 1 - Project Lifecycle The processes and activities described in this paper address how the Power and Propulsion system design is evolved through the Basic Design stage of the Design Cycle described above. A key completion milestone for BMT DSL when conducting platform design is acceptance of the Basic Design. Following this, the shipyard will typically undertake a majority of the detailed design, build and support final trials and acceptance of the vessel prior to entering service. The focus of this paper is a Hybrid Power and Propulsion system which as currently specified can also be referred to as COmbined Diesel electric Or Diesel (CODLOD). This is illustrated in Figure 2 below. Figure 2 - Hybrid Power and Propulsion System POWER & PROPULSION SYSTEM DESIGN DEVELOPMENT Following the Concept Design phase, and dependent on contracting arrangements, projects are generally split into a Preliminary Phase and a Basic Design phase. The following section provides an overview of what these different phases mean in terms of the development of the Power and Propulsion System, and gives some specific examples of some of the issues considered.

3 Compared to the Concept Design phase the overall aim of the Preliminary Design phase is to provide an increased level of definition of the Power and Propulsion system and identified associated systems to demonstrate broad compliance against customer requirements, de-risking key areas, and allowing further cost estimating to be undertaken. Generally this phase is undertaken in support of the bidding process. The Preliminary Design stage is critical for the Power and Propulsion design area to validate and mature the Power and Propulsion system arrangement, key components, and associated operating philosophy (normal and alternative line-ups) against the specific customer requirements and standards. At this stage it is important to get the design right by avoiding changes resulting from misinterpretation of customer requirements. This can occur as a result of plain mis-understanding of key terms (e.g. Load Centre). Like most key aspects of a ship design if a change is required to resolve problems in later design stages this can disproportionally impact programme, cost and ship integration challenges. Therefore relevant uncertainty allowances and margins should be applied and the maturity of the design should be assessed and recorded. The Basic Design phase generally takes place following project award. The overall aim of the Basic Design phase is to provide an increased level of definition of the propulsion system and identified associated systems to demonstrate compliance against contracted customer requirements, and derisking key areas. One of the key aspects of this increased level of definition is the increase in the level of interaction with equipment suppliers and the shipyard in order to de-risk the integration and operation of the Power and Propulsion arrangement. Dependent on the contracting arrangement this could also include detailed interaction with the customers and their ship operators in order to make sure that their operating principals are considered within the design process and to ease their acceptance of the design solution. Figure 3 illustrates the relationship between the key tasks and activity work streams of analysis, physical/functional integration and procurement activities in terms of residual risk during the three phases. As illustrated the key aim is to consistently reduce the risk at each stage. In order to achieve this risk reduction profile it is essential to effectively identify and manage the tasking and resources. It is important to note that although each design phase is distinct, each phase is setup and managed in order to allow seamless transfer of design intent, design decision rationales and design artefacts between phases, especially if there is a pause period between any phases. Figure 3 - Design Phase Residual Risk Spiral

4 POWER & PROPULSION SYSTEM SELECTION The Power and Propulsion System selection process re-evaluates the design options considered at the Concept Design stage and identifies the key performance of the chosen design. The aim of the assessment is to provide a comparison of the design options considered and give confidence that the selected solution remains valid. Hence, it is undertaken throughout preliminary design only and subsequently superseded by specific performance calculations undertaken during Basic Design associated to achieving compliance with contract requirements. Propulsion Option Analysis Activities The propulsion option analysis activities within BMT DSL typically comprise usage of Ptool (Ref. 2) and other in-house calculation tools that draw on a library of equipment data and parametric estimating tools. The starting point to the analysis is the supply of key input data, namely: Customer Requirements; Operating Profile; Hullform Resistance and Powering estimates; Electrical Load Analysis. The process by which different Power and Propulsion System options are considered and evaluated is described in a previous BMT paper entitled Hybrid Drives for Naval Auxiliary Vessels (Ref. 3). The key outputs of the propulsion option analysis completed within Ptool are a series of graphs similar to those shown in Figure 4 and Figure 5. Figure 4 illustrates for a given operating profile the corresponding cumulative fuel consumption costs for 3 propulsion design cases showing whole life cost is lowest for the Hybrid design. Figure 5 illustrates the hourly fuel consumption rate for 3 propulsion design cases at different vessel speeds. Figure 4 Whole Life Cost Comparison Figure 5 Hourly Fuel Consumption Rate

5 Further propulsion option analysis undertaken include customer requirements for towing, selfberthing, and vessel-vessel electrical supplies which have been shown to drive an increase in the rating of the prime movers and main generators. Further Considerations Self-berthing Considering the example of self berthing, the platform is required to have the ability to achieve a defined minimum lateral speed with zero rate of turn (this can only be minimised in practice), in order to perform self berthing. In order to meet this requirement the self-berthing would be undertaken with both propellers driven by the main diesel engines, electrical power generated by the main diesel generators, and the Bow Thruster(s) in use. The rudders are assumed to be of the high-lift type to provide the necessary performance for such manoeuvring. Lateral movement can be via the use of the thrusters in combination with the propulsion system and independent rudder operation, which will deliver a varying power in order to give the correct heading angle. The main diesel engine on one shaft provides continuous power to achieve lateral force at the stern in combination with the rudder. The other main diesel engine and shaft provides high power and short duration thrust in the opposing direction to counteract the turning moment. A self berthing simulation study is typically undertaken in order to ensure that the platform can achieve the berthing requirements, as well as illustrating the main engine, shaft line and thruster activity. It should be noted that the main diesel engine load acceptance and load rejection rates are considered during such analysis as this drives the rate of change of power. Sensitivity Analysis The platform operating profile is developed based on the information in the customer requirements documentation or suitable assumptions based on historical data can be used as an interim measure. Figure 6 shows the variation in customer defined operating profiles for three similar auxiliary vessels. Figure 6 - Operating Profiles The comparison shows the different operating profiles that end users might adopt, for example Ship 1 spending a considerable proportion of her time at 17 knots, compared to Ship 2 which spends an equivalent amount of time operating in the 5 6 knot range. Although in this case the 3 vessels could be considered to be similar in generic ship type the variations in operating profile between Ship 1 and Ship 2 reduces the margin by which the Hybrid solution provides the lowest whole life cost solution; noting that operating for long durations at high transit speeds lends itself to a more simplistic diesel mechanical solution. This further supports the need to develop a common understanding and interpretation of the wider requirements; for example replenishment at sea (RAS) and its associated electrical power consumption, and propulsion engine loading. In this case it is only through consideration of the RAS propulsion and electrical loads that the selection remains valid, due to the inherent increase in ships electrical load during this period which swings the balance of

6 electrical/propulsion load further towards the electrical generation and the flexible electrical power generation nature of the hybrid system. As the design develops, the master equipment list is matured with catalogue or candidate supplier data which is used to populate the electrical load analysis. Sufficient margins should be included in the main diesel generator and hybrid machine ratings to ensure the risk of late changes to the design is minimised. Typically a design margin of 15% on the nameplate rating of the equipment is the starting point for the electrical load analysis with the aim to reduce this margin, by increasing the maturity of the data, to 5% on completion of basic design. Physical and Functional Integration The Physical/Functional activities are undertaken as required to provide the required level of definition to de-risk the integration of the Power and Propulsion scope into the vessel. As applicable the precise level of involvement within the Physical/Function work stream should be agreed with the shipyard however it is normal for BMT DSL to take ownership of this work stream during the Basic Design phase and transitioning it to the yard during the Detailed Design phase. In order to support this work stream the following key activities are undertaken: Interface Scope Diagrams - These comprise high level block diagrams which are essential in the definition, review and agreement of the interfaces between components of the relevant equipment and systems, as well as defining and agreeing the scope of supply for the components with suppliers. Vital in de-risking and establishing any equipment scope gaps which may impact integration scope; Power and Propulsion Operating Philosophy A document intended to help the operator, the client, the shipbuilder and suppliers to gain an understanding of the Design Intent of both normal and key reversionary operation at the whole system level covering the main components of the propulsion System as well as electrical generation and distribution down to main distributed equipment power level (e.g. 440V). It is important to note that the high level Operating Philosophy document is only intended to provide an overview of the Operating Philosophy in order to maintain the Design Intent. Unless necessary for the definition of design intent it is not intended during Basic Design to go into the details of the operation of individual items of equipment either in their normal or reversionary modes of operation or provide a definition of the inter equipment/ipms control philosophy. The safety management aspect of the design is addressed through separate processes but it is noted that this may drive the modes of operation for systems/ equipment; Arrangement Drawings - The incremental maturing of the definition of the key arrangement drawings associated with the Power and Propulsion arrangement and associated systems is essential in de-risking the vessel design. A key aspect in maturing these work streams is early identification and structured engagement with key project stakeholders. This is essential in order to support the system and equipment integration activities and further development of the high level Operating Philosophy document. This engagement can be achieved via the facilitation of a series of meetings as follows: Propulsion System Integration/Interface (PSI) Meeting BMT DSL Lead meeting, with attendance from the Shipyard, customer, propulsion and relevant electrical system equipment suppliers. Aims to review and progress the physical and functional integration of the Propulsion System; Operating Philosophy Meetings BMT DSL Lead with attendance from the shipyard, customer, control system integrator and suppliers as required. The aim of the Operating Philosophy meeting is to define and agree the Operating Philosophy for the vessel and at a high level its systems with the Users and the control system integrators. Experience from recent projects has proven how important the early definition of the Power and Propulsion Operating Philosophy is to communicating the operating intent of the vessels propulsion system. It also serves to gain agreement with the various stakeholders and drive the development of a number of key dependencies for example equipment selection, bunker tanks (volume and location), ancillary system requirements. This is important for any vessel, but is even more important for hybrid vessel where there is a considerable increase in the level of interaction between the various components of the system due to the number and complexity of propulsion and generation operating modes, as well

7 as because in general a single supplier may not be awarded the entire scope of hybrid system supply, which results in an increase in the level of integration complexity. The system operating philosophy is captured within the Operating Philosophy Document. This document aims to provide descriptions of the ship's activities, a description of the whole power and propulsion system, the electrical system and the top level control philosophy both in normal operating scenarios as well as any key degraded modes. It should also include a description of a system configuration in relation to the ship modes of operation and the transitions between those modes. The document is fully version controlled and should be generated at an appropriate level during the Concept and Preliminary phases with buy-in from internal stakeholders. During the Basic Design phase it is essential that all stakeholders (internal and external) are fully engaged, and that incremental review and formal approval is achieved. In order to understand the document it is useful to outline the key sections. The following provides an outline of what is included in the key technical sections: Ship s Designated Activities; o A description of the role of the ship and the activities it will undertake. This information will likely be taken from a platform level CONcept of OPerations (CONOPs) document provided by the customer. As applicable the CONOPs should be read in the context of any other relevant contract documentation, paying close attention to any identified document hierarchy; o Although these activities may not be in the form of direct quantified requirements, they can be used to help develop propulsion system configuration options which will produce a capability in-line with the customer's expectations; Power and Propulsion System Architecture Overview; o To include a description of the P&P system and a system architecture diagrammatic; Power and Propulsion System Control Philosophy; o To include a high level description of the vessel control system, including any specific Power and Propulsion system control details; Ship Activities and System Configurations; o This section highlights the modes of operation for the vessel and how these relate to the Power and Propulsion system configurations. It aims to tie together the Ship s designated activities with the possible P&P system configurations; Automatically Configured Modes; o This includes a description of the standard propulsion system operating modes and equipment line-ups which can be automatically configured; o The modes themselves are defined depending on the propulsion system design and the customer requirements. This section should include a description of each mode in terms of the configuration of propulsion equipment and the electrical system line-up, as well as a schematic representation of the mode showing what equipment, switchboard, transformers, etc. are in operation; Transitions between Automatically Configured Modes; o This should include graphical representation of the status of all key equipment's during the transitions; Manually Configured Modes; o This should include descriptions of the P&P system configurations which could be utilised, but are not automatically configured. Instead, they are configured by the manual selection of propulsion equipment, via IPMS; Other Activities; o Activities which in themselves are not recognised as defined modes, but are envisaged to impact/influence Power and Propulsion Operation and require definition, for example self-berthing, crash stop, blackout recovery, electrical load shedding philosophy. KEY TECHNICAL DEVELOPMENT ACTIVITIES As identified within the Operating Philosophy definition above there are a number of key technical areas which require definition and maturing during the design phase. Using experience gained from

8 active design and build projects this section provides some additional information on some of the key areas. Transitions between Automatically Configured Modes The efficient running of any vessel is vital when considering its operating philosophy however, as discussed previously in this paper the operation of the hybrid system for the vessel type identified provides some additional challenges in order to define the most appropriate operating arrangement for each mode of operation. It should be noted that the most appropriate arrangement may range from being the most efficient arrangement (for example for long distance transit) to the most robust operating arrangement (for example when operating in restricted waters, or in RAS configuration). The following provides examples of some of the key automatically configured modes which are available (it should be noted that other manually configured modes are also available): Motor Cruise - low speed cruise using the hybrid machines in power take in (motor) mode; Diesel Cruise - specific cruise mode using the main diesel engines for propulsion power and the hybrid machines in the power take off (generator) mode and/or main diesel generators for electrical generation; Full Speed - the upper end of the Diesel Cruise mode where all propulsion power is delivered by the main diesel engines and the main diesel generators provide all electrical power; RAS - propulsion power is provided by the main diesel engines and electrical power is provided by a combination of main diesel generators & hybrid machines; Manoeuvring - propulsion power is provided by the main diesel engines and electrical power is provided by a combination of main diesel generators & hybrid machines. Bow thrusters are also energised; Restricted Waterways propulsion power is provided by the main diesel engines and electrical power is provided by a single main diesel generator. The vessel design intent is that at a push of a button it should be possible for the ship s control system to automatically change the propulsion and electrical generation and distribution system hardware configuration, as well as control system settings to achieve the new mode with no manual intervention. The complexity of this re-configuration is dependent on which mode is being changed from and to. From the perspective of the development of these transitions it is essential to fully engage with the impacted equipment suppliers in order to understand the capability of their equipment and gain agreement of the operational intent. This is achieved primarily via the Operating Philosophy and Propulsion System Integration meetings discussed earlier and recorded in a series of Transition Diagrams. The Transition Diagrams aim to capture the dynamic behaviour and status of all of the key components within the vessels propulsion and electrical generation and distribution system hardware through the transition from one automatic mode to another in as simplified a way as possible. Behind the diagrams lie a significant number of known operating boundary conditions which need to be met during each transition. Figure 7 illustrates a generic transition diagram for a hybrid propulsion system automatically transitioning from Motor Cruise mode to Diesel Cruise mode.

9 MDE Start t=25s +10s idle MDE Clutch-In t=15s MDE/MDG Load Transfer t=5 10s PTI Clutch-Out t=2s PTO Clutch-In t=15s MDE/MDG/HM Load Transfer t=5 10s 100% 600rpm MDE brought to idle at zero load. Engine runs at idle for 10s MDE clutched in at zero load. 390rpm MDE 50% 300rpm RPM MDE ramps up as propulsive power is transferred between HM in PTI & MDE. 100% PTI Rating Equivalent 0kW 0rpm MDE Power MDE load increases as HM PTO load is increased. Hybrid Machine 100% 1200rpm 50% 600rpm 0kW 0rpm RPM HM speed set to give PTI speed equal to MDE idle speed 100% PTI Rating PTI 0kW PTO Drive power of PTI is reduced to zero. HM is driven by MDE at the end of the transition. PTI clutch is disengaged when drive load is zero 0% When PTI clutch is disengaged, PTO is engaged (though HM remains de-energised) PTO takes up SEL 100% MDG provides SEL and power for propulsion via HM in PTI mode. MDG Power MDG ramps down generation load to provide the SEL only. MDG maintains power generation capacity for SEL. MDG load reduces as HM PTO takes up SEL MDG 50% 0.5SEL Datum (Approx value) 0% Propeller Pitch Full Zero Reverse Pitch Position Pitch reduced to keep similar thrust as HM speed is increased Propeller pitch varies to maintain constant thrust throughout transient propeller shaft speed variations during the clutch transfer process 100% Propeller/ shaftline speed 50% 0% RPM Shaftline speed varies with HM speed Further Remarks Time to complete power transfer is dependent on the MDG load rejection rate and the MDE ramp rate. Rate of transfer is defined by the slowest of these two. Time to complete power transfer is dependent on the MDG load rejection rate and the MDE ramp rate. Rate of transfer is defined by the slowest of these two. Following the load transfer, system now in Diesel Cruise mode. Shaft speed and pitch will now follow combinator curve according to lever demand Figure 7 - Transition Diagram from Motor Cruise to Diesel Cruise Modes The Transition Diagram illustrates the complexity which must be considered when making the transition in terms of the various load transfers, compatibility of rotational components engaging and disengaging as well as the physical performance limitations of the engines, and electrical power systems to accept load. Without defining of these transitions as early as possible within the design phase it is not possible to validate the capability of the propulsion system to achieve the design intent, or provide the control system integrator with the required understanding. Diesel Motor Cruise as a Reversionary Mode The hybrid propulsion system arrangement provides the capability to manually configure the propulsion system in what is known as Diesel Motor Cruise mode. In this mode one shaftline acts in Motor Cruise Mode, whilst the other acts in Diesel Cruise Mode. I.e. one propeller is driven by the Hybrid Machine (HM) in Power Take In (PTI) mode, whilst the other is powered by a Main Diesel Engine (MDE). As well as driving the propeller, this MDE also powers the associated HM in Power Take Off (PTO) mode. This power developed in PTO mode is then sufficient to support the ship s normal electrical load, and provide power to the other HM in PTI mode. The propulsion system configuration described is illustrated in Figure 8.

10 CP Propeller CP Propeller FWD STBD Propulsion Equipment Main Gearbox Main Gearbox AFT PORT Propulsion Equipment Hybrid Machine (in PTO) Hybrid Machine (in PTI) Main DE Main DE Main DE Main DG Power HM VSD HM VSD Power Main DG Figure 8 - Diesel Motor Cruise Configuration This mode is not proposed as a normal operating mode, as although it offers a very low number of prime movers being utilised, the propulsion and power management implications associated with operating vessel shaft lines at different speeds and propeller pitches can result in some risk to the system stability. It is therefore proposed as only being achievable by manual configuration in the event of machinery not being available for example. Hybrid Machine as a Main Generating Set In general, Classification Societies do not mandate that hybrid machines be considered as a main (primary) source of power when operating in power-take-off (PTO) mode. Instead the onus is on the designer/ shipbuilder to determine and present the level of reliance placed on the hybrid machines in the Electrical Load Analysis. In practice they would not be part of the Power and Propulsion system if they were not going to be used noting that the basis of selecting a Hybrid Power and Propulsion system is flexibility between power generation sources and propulsion prime movers and the ability to maintain the optimum loading on the equipment. Therefore by default they should be considered a main (primary) source of power. In order to achieve an acceptable solution from a platform concept of operation viewpoint (i.e. to avoid the requirement for PTO parallel operation with a DG), it is necessary to specify a protection system that uses time based discrimination and a transient overload capacity in the hybrid machine and convertor sizing. The convertor sizing should exceed the PTO rating of the hybrid machine by a margin considered to accommodate any throw over load that cannot be shed by the power management system in the event of a fault on the system. When increasing the convertor sizing to accommodate an increased electrical load the impact on the mechanical elements of propulsion system such as transient clutch rating and any torque limits on the PTO shaft of the gearbox should be considered. Design Optimisation for Recoverability One of the fundamental design features of recent design development projects undertaken by BMT has been consideration of achieving compliance with Lloyds Propulsion and Steering Machinery Redundancy Separate (PSMR*) or DNV-GL Redundant Propulsion and Separate notations respectively. Broadly speaking both notations share a common aim of guaranteeing that the vessel propulsion and steering system is of redundant design such that the availability of at least 50% of propulsive power and 50% steering capability can be restored after single component failure before loss of steering speed, as well as any failure within certain prescribed limits caused by fire or flooding.

11 For the hybrid arrangement as outlined previously this is principally achieved by the provision of two completely independent propulsion systems and two independent steering systems with their associated ancillaries, power supplies and control systems. Although at a simplistic level the provision of two completely independent shaft lines appears relatively easy to achieve and prove there is a significant amount of effort required to make sure that the required design rules are put in place and adhered to in order to prevent the risk of support or control equipment rendering the arrangement unacceptable. For example a seemingly innocuous connection to an electrical supply from an inappropriate source, or where a header tank for a stern tube bearing which is required to be located above a certain height can, if not managed correctly, become located in a common fire incident area with the opposite shaft line tank and thus resulting in a common mode failure rendering the ship immobile due to bearing overheating Experience has shown that unless the fundamental notation requirements are established from the outset of the vessel design it can take a significant effort to correct any inadequacies, and prove capability. CONCLUSIONS This paper discussed only a small selection of the observations and changes in approach that BMT have developed relating to platform design projects. The intent of such work is to reduce risk and ensure that the design solution developed is resolved thoroughly against all the relevant requirements and constraints in an effective manner. BMT have identified that a fundamental consideration which is of particular importance for Hybrid Power and Propulsion systems is the Power and Propulsion Operating Philosophy. Firstly the operating philosophy helps ensure that all stakeholders gain a common understanding of the overall requirements that drive the design solution. Secondly it helps all stakeholders understand the implications of meeting such requirements in the design from concept, to preliminary and through basic design stages. In turn, this helps ensure the Power and Propulsion solution remains valid and that unjustifiable or excessive 'corner of the envelope' requirements are not able to drive the design solution unduly. ACKNOWLEDGEMENTS The authors would like to thank their BMT colleagues for their input, as well as the companies and organisations which have been involved in past and current projects including; UK Ministry of Defence (UK MOD), RFA, NDLO, Wärtsilä, GE, Servowatch, RENK, Schottel. REFERENCES 1. MAP Maritime System Maturity levels for Ships Issue 7, Naval Technical Publications Policy Committee, December Fast Performance Modelling of Marine Power & Propulsion Systems, J Buckingham, BMT DSL, May Hybrid Drives For Naval Auxiliary Vessels, J Buckingham, BMT DSL, October (2. & 3. available at