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Power & Propulsion – Meeting the Concept

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.

21 March 2017

Defence and Security Specialised Ship Design Shipping - Technical Services

Power & Propulsion – Meeting the Concept

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.

Design Development 

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. 

System Selection 

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:

  • Customer Requirements
  • Speed-time Operating Profile
  • Hullform Resistance and Powering estimates
  • Electrical Load Analysis (ELA).

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:

  • The P&P Operating Philosophy which helps the P&P stakeholders (i.e. the operator, the client, the shipbuilder and equipment 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 P&P system.  It provides descriptions of the ship's activities, a description of the whole system, the electrical system and the top level control philosophy both in normal operating scenarios, as well as any key degraded modes. It also includes a description of a system configuration in relation to the ship modes of operation and the transitions between those modes.
  • Interface Scope Diagrams which comprise high level block diagrams which define 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. They are vital in de-risking and establishing any equipment scope gaps which may impact integration scope.
  • Transition Diagrams capture the dynamic behaviour and the 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. Behind the diagrams lie a significant number of known operating boundary conditions which need to be met during each transition. 
  • Arrangement Drawings – BMT develops the key arrangement drawings associated with the P&P arrangement and associated systems to incrementally mature and de-risk the P&P definition. 

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:

  • Motor Cruise - low speed cruise using the hybrid machines in power take in (motor) mode.
  • Diesel Cruise – medium-speed cruise mode using the main engines for propulsion power and the hybrid machines in the power take off (generator) mode and/or diesel generators for electrical generation.
  • Full Speed – fastest achievable speed where all propulsion power is delivered by the main engines and the DG-sets provide all electrical power.
  • RAS - propulsion power is provided by the main engines and electrical power is provided by a combination of DG-sets & hybrid machines.
  • Manoeuvring - propulsion power is provided by the main engines and electrical power is provided by a combination of diesel generators & hybrid machines. Bow thrusters are also energised.
  • Restricted Waterways – propulsion power is provided by the main engines and electrical power is provided by DG-sets.

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

2.‘Ptool - Fast Performance Modelling of Marine Power & Propulsion Systems’, J Buckingham, All Electric Ship 2000, Paris, May 2010.


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