Submersible Ship Nuclear Replacement – SSN(R)

Background and requirement

Submersible Ship Nuclear Replacement (SSNR) programme was in concept phase looking at the future needs of the UK within the underwater battlespace. It comprised multiple projects of which a submarine will play a role in that force-mix. The SSN(R) team was created to progress the design of this submarine and investigate the various options available to replace Astute once it leaves service.

An Outline Business Case (OBC) was required to secure funding and enable progression into Programme Design & Definition (PDD) phase. The key milestone required to enter PDD was the selection of a Primary Propulsion Plant, this would set hull diameter and allow long lead items to be ordered.

TP Group was tasked with providing cost inputs to Primary Plant down-selection, ensuring capability was achieved at lowest possible cost and all OBC evidence requirements were met. The decision panel required eight costed options detailing the cost of design and build, focussing on key drivers and cost maturity.


TP Group provided support to multiple aspects of the SSN(R) programme including:

  • Work Breakdown Structure – development of WBS to ensure all costs are captured and enable consistency of comparison across options.
  • Cost Modelling – developing a cost model to accurately model programme costs and risks in support of primary plant down-select.
  • Cost Estimating – developing parametric and bottom-up estimates to inform decision makers on the key cost drivers that differentiate technology options.
  • Cost Analysis – sensitivity and what-if analysis to examine the impact of assumption changes on programme cost, focussing on changes in production drumbeat, platform numbers, and delivery dates.
  • Cost Assurance – providing independent cost assurance and advice on assumptions, model structure, data collation, and estimating basis.
  • Maturity Assessment – analysis of underlying data to ascertain overall cost maturity, enabling targeted effort in weakest areas.
  • COEIA – undertaking an investment appraisal of all plant options to inform down-select, using net present value and break-even analysis to ascertain value for money (VFM) option.

The key tenet of our approach was focusing on establishing agreed assumptions and increased data maturity. This ensured all estimates were defendable and transparent when scrutinised, with consistent data maturity across the work breakdown structure (WBS).


The SSN(R) programme now has a clearly defined WBS and set of assumptions, this will be invaluable in the future providing an accurate cost baseline from which the programme can be managed, helping to achieve the cost stability desired by the Submarine Delivery Agency (SDA).

Using the evidence-based programme costs provided by TP Group, the project team successfully down-selected a primary plant enabling OBC1 to gain approval, providing three years of funding to mature boat design. To ensure this work continues the customer awarded TP Group additional tasks to update cost estimates as the programme matures.

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