21 March 2017
A well-designed hybrid power and propulsion (P&P) system offers significant operational flexibility between the power generation sources and the propulsion prime movers to achieve low annual fuel consumption and low engine running hours, together with the opportunity for fewer prime movers onboard. Such a design also reduces incidences of prime mover running at continuous low loads.
BMT has identified an efficient hybrid propulsion concept which has been developed and defined for implementation into two naval auxiliary vessel designs. For such ships, BMT has found the hybrid design to be the most suitable arrangement to meet the requirement for low lifecycle costs, without sacrificing the ability to achieve maximum sustained speed requirement. Ref.1.
The propulsion concept of operations studies consider how the system is used in a range of modes, both for normal steaming and for Replenishment-At-Sea (RAS), as well as for reversionary operations where one or more equipment are out-of-service.
The BMT Hybrid Power and Propulsion system design can also be referred to as COmbined Diesel eLectric Or Diesel (CODLOD). The equipment arrangement is illustrated below.
Following the Concept Design phase, and dependent on contracting arrangements, ship design projects are generally split into a Preliminary/Feasibility Phase, a Basic Design phase and a Detailed Design & Build Phase.
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 P&P design area to be validated with good maturity of the P&P system arrangement, key components, and associated operating philosophy (normal and alternative line-ups) and performance against the specific customer requirements and standards. Uncertainty allowances and margins are applied and the maturity of the design is assessed and recorded. The bidders would have sent preliminary procurement specifications to key equipment suppliers to build up their cost model.
Following contract award, the Basic Design phase provides a level of P&P definition which is sufficient to allow it to be assessed by a classification society and designated regulatory bodies such as the UK MCA. This definition is to demonstrate compliance against contracted customer requirements, to de-risk key areas and to allow mature equipment specifications to be defined and sent to potential equipment suppliers.
The Design Phase Residual Risk Spiral shown above 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. The key aim is to consistently reduce the risk at each stage.
The selection process re-evaluates the P&P options, i.e. mechanical drive, all electric drive and a range of hybrid solutions, considered at the Concept Design stage. The chosen design has the best balance of performance, cost and supportability.
The assessment provides a comparison of the design options considered and confirms 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.
Propulsion Option Analysis Activities
The P&P option analysis activities typically comprise the analysis of a range of potential P&P solutions to identify their performance, machinery set-ups and fuel consumption etc. BMT employs a number of analysis tools for such purposes depending on the level of information provided and the detailed required. These analysis tools (Ref. [i]) draw on a library of equipment data and employ parametric estimating tools, where firm data is not yet available. The starting point to the analysis is the supply of key input data, namely:
The process by which different P&P options are considered and evaluated is described in Ref. 2. The key outputs of the analysis are a series of comparison graphs similar to the one shown below which illustrates the hourly fuel consumption rate for three propulsion design cases at different vessel speeds.
The customer requirements for manoeuvring especially, towing, self-berthing are also analysed. Electrical system behaviours are also assessed especially between vessel-vessel electrical supplies which may drive an increase in the rating of the prime movers and generators.
Physical and Functional Integration
Modelling and analysis activities are undertaken to provide the required level of definition to de-risk the physical and functional integration of the P&P into the vessel. The precise level of definition is agreed with the shipyard however, it is normal for BMT to take ownership of these activities during the Basic Design phase and then transition them to the yard during the Detailed Design phase.
BMT creates the following key items:
Early identification and structured engagement with all the key project stakeholders is essential to support the system and equipment integration activities and the further development of the high level Operating Philosophy document. Whilst important for any vessel, it is even more important for a hybrid design 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. Where a single supplier is not awarded the entire scope of hybrid system supply, this may also result in an increase in the level of integration complexity.
Transitions Between Automatically Configured Modes
The operation of a hybrid system provides some additional challenges in order to define the most appropriate operating arrangement for each mode of operation. 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 when operating in restricted waters, or in RAS configuration).
For hybrid propulsion, the key automatically configured modes are:
The ship’s control system automatically changes from one mode to another at a push of a button. The complexity of this re-configuration is dependent on the two modes and to understand and define these transitions, BMT fully engages with the equipment suppliers in order to understand the full envelope capability of their equipment and to gain agreement of the operational intent.
During Basic Design, BMT creates a P&P Operating Philosophy and employs a range of tools and methods to bring stakeholders together to achieve a successful complex P&P designs such as a hybrid propulsion solution. The output from such tools provide clear pictorial views of the system behaviours which allow informed insight and discussion to take place. This reduces risk and ensures that the design solution meets all the relevant requirements and constraints in an effective manner.
1. ‘Hybrid Drives For Naval Auxiliary Vessels’, J Buckingham, BMT Defence Service, October 2013. (available at www.bmtdsl.co.uk/downloads/conference-papers)
2.‘Ptool - Fast Performance Modelling of Marine Power & Propulsion Systems’, J Buckingham, All Electric Ship 2000, Paris, May 2010.
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