3DTD’s Best Practice Guide

The first stage of developing route options should start with procuring the latest OS map of a study area. The OS Map tile allows for the early design and dimensioning of route options to later be developed using a format appropriate for development into detailed design.

It is useful to note at this stage of project development, the difference between main spine length (large diameter bulk export pipe) and branch lengths (smaller diameter pipe, which typically serves a single development/consumer), as they impact both costs and available technical solutions. Additionally, initial expansion and consent strategies can also be introduced/annotated at this stage. However it should be noted that without a detailed appraisal of the buried environment these designs are concept only.

Whilst informative, all too often this level of initial concept design, actually represents the final level of risk management/design development where many Heat Network project designs go to “all risk” construction tenders.

The main reason for this practice is a lack of commercial awareness regarding the impact this approach has on costs/risks; together with insufficient project guidance regarding the substantial civil and coordination risks/costs associated with undertaking major/complex projects within dense city centre environments, All too often, commencing a civil project with insufficient route proving results in excessive additional costs for the client during tender and construction, as the Contractor is provided with insufficient information to appraise their risks against, and routes materially change in construction.

Industry Analysis

As a result, typical industry costs are rising, and can often deem feasible schemes, commercially unviable because of factoring in “all risk” rates, instead of reducing risks through detailed designs. Additionally, the difference between contract cost and end cost to the client is rising as an alarming rate, and this is predominantly due to a lack of de-risking of the buried environment and early route proving/development.

Very few industries enter into construction contracts with such an inadequate level of design/information, and the following stages in this guide therefore aim to set out more appropriate measures which can be used to significantly improve the accuracy of the design and pre-construction process.

Heat Network routes are complicated as they seek to install large assets within already congested environments. Heat Networks are formed by welding large diameter, typically 12m length, rigid steel pipes (flow and return). Their size, weight, and rigid/inflexible nature therefore requires a significant volume of clear space to be identified in advance of excavation, as their route designs cannot reactively/rapidly change direction due to a network’s thermal expansion.

As buried heat networks typically exist within dense urban areas, where large volumes of pre-existing utilities, ducts and chambers are located, planning the installation of such large pipes is essential. The alternative options are both costly and can impose significant long-term quality impacts.

Commercially it is important to understand and minimise the impact of in construction utility clashes as; depth of excavation, and unproductive sites (awaiting design changes/consents) are the greatest costs associated with installing Heat Networks. This is why identifying and mapping utility density at an early stage, begins the process of managing/reducing the client’s installation costs.

By obtaining the statutory utility drawings (Gas, Water, Electricity, Drainage and Data), these essential records can be overlaid against the OS Map and route options to identify areas of pre-existing congestion, and highlight safety risks (such as excavating around Medium/High Pressure Gas Mains, large chambers and HV infrastructure).

Statutory utility drawings also contain far more information than just their outline location. An experienced assessment of their content is key to identifying risks which can also lead to significant costs and delays. Details such as material type, age, size, duct configuration and chamber sizes/invert levels can be interpreted at this stage, all of which can significantly impact construction costs/safety.

The Desk Top Study stage is also essential for gaining a deeper insight into the unique historical
landscape which each buried network is to be installed.

Heat Networks are most feasible and predominantly exist in dense mixed-use city centre/urban areas, where civil projects are often impacted by the buried risks resulting from the historical layout of a town or city, its legacy transport infrastructure, and the industry which took place there decades ago.

The historical OS Map review process is effective and produces annotated drawings, capturing:

• Legacy Foundations/Tunnels/
  Bridges.
• Historical Waterways/ Canal Paths.
• Historical Rail/Tram infrastructure.
• Changed Street Layouts.
• Land Use/Contamination.
• Church/ Burial Grounds.
• Historical Energy
• Infrastructure
• Local Stakeholder Knowledge and records.

By understanding an area’s historical structural landscape at an early stage, the client significantly reduces its project risks, prior to tender.

The desk top study stage is only the start of identifying/managing legacy risks and should also be complemented by local knowledge.

This process is highly effective at identifying where further investigations/actions may be required, and ensuring that any dialogue with local Stakeholders is informed and tailored to the unique landscape and histories of each city.

Heat Networks often interface with a variety of Major Infrastructure Crossings, which can include;

• Rail/Tram Lines
• Major Highways and Junctions
• Motorways
• Bridges
• Rivers/Canals
• Tunnels

Understanding the commercial and technical requirements associated with their consent processes is essential to both programme and cost management.

The differing; solutions, parties, costs and timescales associated with assessing and obtaining consents at major infrastructure crossings vary significantly, as do their risks.
3D Technical Design therefore recommend the production of concept designs and method statements, in order for the proposed solutions to be clearly understood by all parties involved.

Throughout each stage of the Advanced Feasibility Process; all buried, site and stakeholder risks should be noted within a Technical Risk Register, which later becomes the base of the design HAZID, (Hazard Identification).

Each risk should be assessed for potential commercial impact, and the effect of its recommended control measures updated to assist in prioritising risks and funding for further investigations .

To enable the detailed design of a district energy network, modelling the layout and depth of the chambers, ducts and utilities, which surround and intersect the network’s route options, is crucial.

PAS 128 is an industry recognised specification for underground utility detection, and the deployment of Electromagnetic Location (EML) technologies, Ground Penetrating Radar (GPR) and 3D post-processing in accordance with PAS128 enables the production of a 3D utility model. When complemented by a Topographical Survey (including chambers and above ground positions), these surveys are the most appropriate methods of detecting the buried environment, prior to verification measures (including trial holes and trench excavation.

Therefore, following the Advanced Feasibility stage, and once a preferred route is agreed, the next recommended stage is to produce a underground utility survey extent drawing against which; topographical, EML and GPR surveys are tendered.

Depending on the terrain, there are differing methods for undertaking GPR, each with high but differing degrees of accuracy(MP1-4).

Equally, there are differing Quality Levels which can be achieved through EML/GPR/Post Processing, and understanding the limitations/accuracy of the survey results, is as important as understanding their benefits.

When developing a Heat Network’s detailed design using GPR Survey data, the first stage is the production of an accurate and informed route in Plan (GA Design).

The GA route should; show weld and chamber positions, be informed by the local environment (Topographical Survey), and target a path clear of services. Additionally, curved pipes which allow the route’s direction to change in a controlled manner, can be designed. Curved pipes minimise expansion stresses, as they reduce the impact /need for short degree bends, and mitred welds during installation.

Expansion loops/offsets can also be accurately planned in advance around buried services and existing chambers. Furthermore, the hydraulic model can also be refined, and accurate expansion calculations signed off by the pipe manufacturer.

The detailed GA route also enables improved; traffic management planning, building entry positioning, phasing, and multi utility coordination.

At this stage, soft market tenders are recommended, which inform the market of the quality driven approach being taken, and improve budget accuracy. Reasonable assumptions can also be made about the route’s average depth, and the high risk utilities/risks accurately prioritised for verification (trial holes).

It is important for any client to note that upfront identification and management of such risks is necessary under CDM 2015. In particular, following this process improves HAZID identification and improves the accuracy of temporary works designs.

The 3D pipe modelling stage is where real innovation, quality and value is embedded into the network design, and is achieved by undulating the network depths to account for, and avoid, intersecting utilities. District Heating pipe can naturally deflect by up to 300mm over a 12m length, and a combination of this and the use of curved pipes allows the design of the network route to undulate between/below services in a controlled manner.

This process significantly minimises excavation; depths, disruption and costs, and demonstrates improved project control to all stakeholders.

It is important to highlight that, whilst this process significantly reduces the risks of buried conflicts, there will always remain a level of in-construction change, due to the nature of the buried environment. The 3D Modelled GPR process however, significantly reduces instances of in construction design change, whilst also enabling remote redesign to be deployed (following receipt of the as-surveyed excavated environment).

Following the development of the 3D pipe route in CAD, sectional drawings (produced using the 3D Model) set out recommended trench lines and depths. These further inform and improve the tender process through providing target depths of excavation (the most significant cost factor when installing heat networks).

Additionally, sectional drawings enable parties to make informed decisions regarding sleeves/thrust bore solutions and other pre-servicing techniques which carry significant risks, unless accurately modelled and proven in advance .

Sectional drawings represent the greatest level of planning available to de-risk a route. They allow a contractor to appraise risks and accurately plan, programme and tender the works.

Risks are annotated within the drawings, including stating the level of accuracy (QL Level) each utility has been detected to at the GPR/EML stage.
3D Technical Design recommend 3D modelling is produced within a BIM II environment, and that model is regularly updated as the project is delivered. This approach has seen Heat Network designs and installations successfully coordinated amongst other utilities in the most dense environments, to extremely tight tolerances.

Commercially; the cost of this approach is far from inhibitive. Whilst advanced, it is actually cheaper (% of CAPEX) when compared to plantroom models.

Typically, all in rates for; Feasibility, GPR/EML and the production of Sectional Drawings/ HAZID schedules are between 3-5% of CAPEX. This approach not only significantly reduces contract costs (by up to 33%), it delivers significant control and justifies lower contingency costs, when compared to an un-proved network route.

Prior to and throughout the Advanced Feasibility and Detailed Design stages, a detailed programme should be maintained. The programme should demonstrate key client milestones, and the timescales associated with critical path activities such as the obtaining of Statutory Utility Drawings and undertaking and GPR surveys.

The programme must not be rigid, but instead reflect the iterative nature of how a Heat Network design and delivery process develops. It should be updated regularly with information such as; third party consent processes and known construction restrictions/opportunities.
An effective design programme demonstrates control to key stakeholders, and can enable the early agreement of works phasing in critical areas.

Whilst this de-risking process is advanced, and can significantly reduce construction costs, a reasonable provision and plan should still remain for in construction risks/changes.

This provision should reflect the project’s risks which remain after the sectional drawings are completed, and also the contracting strategy.

In a deep buried environment, routes cannot be 100% proven prior to excavation. However, the process which 3D Technical Design Ltd. recommends (particularly in high risk/dense City Centre environments) is leading the industry in both reducing the costs of installing heat networks, and informing clients at an appropriate stage about project risk.