cable details and grid connection statement - forewind - · pdf file4.3 inter-array subsea...

August 2013

Cable Details and Grid

Connection Statement

Pursuant to Regulation 6(1)(b)(i) of the Infrastructure Planning

(Applications: Prescribed Forms and Procedure) Regulations 2009

Application reference: 7.2

DOGGER BANK CREYKE BECK

This page is intentionally blank


Title:

Cable Details and Grid Connection Statement

Contract No. (if applicable)

Onshore / Offshore

Document Number:

F-ELC-SP-001

Issue No:

3

Issue Date:

11-08-13

Status:

Issued for 1st. Technical Review

Issued for 2nd. Technical Review

Issued for PEI3

Issued for Application

Prepared by: Fabio Spinato

Checked by: Melissa Read

Approved by:

David Flood

Signature / Approval meeting

David Flood

Approval Date:

13/01/2013

Revision History

Date Issue No. Remarks / Reason for Issue Author Checked Approved

25/07/13 1 Internal review MR FS 08/08/13

08/08/13 2 Final for approval MR DF 11/08/13

13/08/13 3 SMT sign off MR SMT 13/08/13


Contents

1 Introduction and summary ............................................................................................. 1

2 Project overview ............................................................................................................ 3

2.1 Background ......................................................................................................... 3

2.2 Project description .............................................................................................. 3

2.3 Site selection ....................................................................................................... 3

2.4 Operation ............................................................................................................ 4

3 Proposed cable routes .................................................................................................. 5

3.1 Cable route ......................................................................................................... 5

3.2 Onshore converter station and substation location ............................................. 7

4 Offshore electrical system ............................................................................................. 9

4.1 Overview ............................................................................................................. 9

4.2 Wind turbine generators .................................................................................... 10

4.3 Inter-array subsea cables ................................................................................. 10

4.4 Offshore collector platform ................................................................................ 11

4.5 Inter-platform subsea cables ............................................................................. 12

4.6 Offshore converter platform .............................................................................. 13

4.7 Export system HVDC subsea cables ................................................................ 13

5 Offshore cable installation ........................................................................................... 15

5.1 Pre-installation works ........................................................................................ 15

5.2 Offshore cable installation methods .................................................................. 15

5.3 Crossing of existing infrastructures ................................................................... 17

6 Onshore electrical system ........................................................................................... 18

6.1 Overview ........................................................................................................... 18

6.2 Landfall transition joint bay................................................................................ 18

6.3 Export system HVDC underground cables........................................................ 18

6.4 Onshore converter substation ........................................................................... 19

6.5 Export system HVAC cables ............................................................................. 20

6.6 Connection works at Creyke Beck substation ................................................... 21

7 Onshore electrical system installation ......................................................................... 23

7.1 Onshore HVDC cable corridor .......................................................................... 23

7.2 Onshore cable installation methods .................................................................. 25

7.3 Onshore crossings ............................................................................................ 26

7.4 Onshore construction compounds .................................................................... 26

8 Conclusion .................................................................................................................. 28


Table of Figures

Figure 1: The offshore export cable corridor ................................................................. 5

Figure 2: The onshore export cable corridor ................................................................. 7

Figure 3: Visualisation of proposed converter station siting .......................................... 8

Figure 4: A representation of the offshore electrical system ......................................... 9

Figure 5: An example inter-array layout connecting the collector substation .............. 10

Figure 6: Example of 33kV, three-core, XLPE insulated, armoured subsea cable ...... 11

Figure 7: Typical offshore collector platform design. .................................................. 12

Figure 8: Example of 245kV, three-core, XLPE insulated, armoured, subsea cable ... 13

Figure 9: Example of a subsea HVDC cable ............................................................... 14

Figure 10: Typical HVDC, underground cable. ............................................................ 19

Figure 11: An illustrative converter substation layout ................................................. 20

Figure 12: A 400kV XLPE insulated underground copper conductor cable ................. 21

Figure 13: Indicative onshore cable route working widths based on two illustrative

construction options for an HVDC section ................................................................... 24

Figure 14: Indicative HDD construction compound (launch pit) .................................. 27

Figure 15: Indicative HDD construction compound (receiving pit) ............................... 28


F-ELC-SP-001 Page 1 © 2013 Forewind

1 Introduction and summary

1.1 Introduction

1.1.1 This Cable Details and Grid Connection statement is provided by Forewind Limited in

relation to the Dogger Bank Creyke Beck offshore wind farms.

1.1.2 Dogger Bank Creyke Beck comprises two proposed offshore wind farms to be

located within the Dogger Bank Zone in the North Sea. As the proposal is an offshore

generating station of more than 100 megawatts, under the Planning Act 2008 it is

classed as a Nationally Significant Infrastructure Project. Therefore, in order to gain

development consent and a deemed Marine Licence to construct and operate the

wind farms it is necessary to apply to the Major Applications and Plans Directorate of

the Planning Inspectorate for a Development Consent Order (DCO).

1.1.3 This statement accompanies the final application and is submitted alongside a draft

DCO, an Environmental Statement, a Consultation Report and other statutory and

non-statutory documents.

1.2 The scope of this document

1.1.4 This Cable Details and Grid Connection Statement provides details of the electrical

system which connects the wind turbines to the national electricity export system.

This comprises an offshore electrical system and an onshore electrical system. The

offshore system extends from each individual wind turbine generator, via collector

and converter platforms, to the cable landfall where the offshore cables are jointed to

the onshore cables in a landfall transition joint bay. The onshore electrical system

extends from this bay to the connection point at the existing National Grid Electricity

Transmission (NGET) 400kV Creyke Beck substation near Cottingham in the East

Riding of Yorkshire (hereafter referred to as the „Creyke Beck substation‟). The

entirety of the onshore cable route is provided through underground cables with no

overhead lines required.

1.1.5 The export system will be realised through high voltage direct current (HVDC)

technology, which requires a converter station at both ends of the export system for

the conversion between HVDC and high voltage alternating current (HVAC). The

converter modules will be mounted on specific platforms for offshore locations and in

dedicated converter buildings (converter substations) for onshore locations.

1.1.6 The application for development consent contains all of the above electrical grid

connection works required for Dogger Bank Creyke Beck. A requirement under

Regulation 6(1)(b)(i) of the Infrastructure Planning (Applications: Prescribed Forms

and Procedures) Regulations 2009 (the APFP Regulations) is that an application for

an offshore generating station must provide details of the proposed route and method



of installation of any cable with the application. In addition Regulation 5(2)(q) allows

for the submission of any other documents which are considered necessary to

support the application; it is considered the grid connection details fall within this

category.

1.1.7 This statement sets out the location, type and proposed installation methods for the

electrical system. Further detail on the project description and cable installation

methods can be found in Chapter 5 of the Environmental Statement.



2 Project overview

2.1 Background

2.1.1 In January 2010, following a competitive tender process, The Crown Estate awarded

Forewind Limited (Forewind) the exclusive development rights for „Zone 3 Dogger

Bank‟; the largest of the Round 3 offshore wind farm zones. The Dogger Bank Zone

comprises an area of 8,660km2, and is located in the North Sea between 125km and

290km off the coast of Yorkshire. Forewind is a consortium comprising four leading

international energy companies (RWE, SSE, Statkraft and Statoil).

2.1.2 Dogger Bank Creyke Beck is the first application within the zone to be submitted to

the Planning Inspectorate for consent. The application comprises two offshore wind

farms; Dogger Bank Creyke Beck A and Dogger Bank Creyke Beck B, along with the

associated development necessary to enable connection to the national grid.

2.2 Project description

2.2.1 Dogger Bank Creyke Beck is located within Tranche A of the Dogger Bank Zone,

which lies over 125 kilometres (km) off the coast of Yorkshire. The connection point

to the electricity transmission network is proposed to be the existing Creyke Beck

substation. Further information on the location and design of the wind farms, the

cable corridor and substation is set out in the Environmental Statement. In addition

Part 1 of Schedule 1 of the draft Development Consent Order (DCO) describes the

works for which development consent is being sought.

2.2.2 The total maximum installed power of Dogger Bank Creyke Beck is proposed to be

2,400 megawatts (MW) (2.4 gigawatts (GW)). However, the electrical system is

designed to deliver up to 1,000 MW per project at the connection point. The purpose

of providing a higher installed capacity is to take account of several factors that

reduce the actual power being delivered to the network. These may include: wake

effect, wind turbine and electrical system availability, electrical losses and the

average expected capacity factor. The total connected power (also known as

transmission entry capacity) for the two projects is therefore 2,000 MW which will not

be exceeded.

2.3 Site selection

2.3.1 The selection of the cable corridor, connection point and landfall has been subject to

wide-ranging consultation with National Grid, statutory consultees and the local

community over the past few years. In addition, rigorous environmental assessment

has been undertaken and any likely significant effects on the environment have been

taken into account in the final site selection.



2.3.2 Therefore, the route presented at this stage is the result of an iterative process taking

into account and balancing many constraints.

2.4 Operation

2.4.1 Generators have a choice around how the electrical assets to connect to the

transmission network are constructed. Generators can either construct the network

themselves or can instead opt for an Offshore Transmission Owner (OFTO) to do so.

If they construct the assets themselves, then the generator must transfer the assets

to an OFTO post-construction and installation. OFTOs are selected on a competitive

basis through a tender process run by Ofgem, the GB energy regulator.

2.4.2 At this application stage it has not yet been decided on how the electrical assets will

be constructed, so details of ownership cannot be provided within this document.



3 Proposed cable routes

3.1 Cable route

3.1.1 The offshore export cables for Dogger Creyke Beck will be installed within one 2km

wide corridor as shown in Figure 1. A 2km wide corridor is considered necessary to

accommodate the maximum number of cables for both projects and provide flexibility

to microsite around any obstacles (such as wrecks) or features (such as biogenic

reef) that require avoidance. The Order Limits also includes an area 750 metres

either side of this cable corridor in which temporary construction activities may take

place, such as the use of anchor spreads for installation vessels. The export cable

corridor will extend predominantly approximately south-west from the south-western

corner of the development zone, to come ashore at the cable landfall zone on the

Holderness coastline, in the East Riding of Yorkshire. Each project will have two

HVDC power cables in a pair. The maximum total export cable length for high voltage

direct current (HVDC) cables, from the offshore platform to the landfall, is 420km for

Dogger Bank Creyke Beck A and 378km for Dogger Bank Creyke Beck B.

Figure 1: The offshore export cable corridor



3.1.2 In relation to Dogger Bank Creyke Beck B, the final exit point of the HVDC export

cable from the array areas has yet to be determined, so two options are presented in

the DCO; Work No. 2BA exits to the west and Work No. 2BC exits to the south

passing through Dogger Bank Creyke Beck A. Although considerable work has been

carried out on each of the routes presented in the Environmental Statement, flexibility

is needed as a balance between environmental and technical considerations will

need to be struck in choosing the most appropriate exit point once post-consent

design optimisation work has been carried out and the location of the converter

substation established. There is a provision in the draft Development Consent Order

that only one of these options can be constructed, not both.

3.1.3 The landfall for the offshore export cables is along the northern section of the

Holderness coastline, between Skipsea and Fraisthorpe, and the location is an area

approximately 1km wide located to the north of the village of Ulrome. At the landfall

the offshore HVDC export cables from both Dogger Bank Creyke Beck A and B will

be connected to the onshore HVDC cables within a landfall transition joint bay.

3.1.4 The onshore export cable route will consist of an approximately 30km long HVDC

section from the landfall transition joint bay to the onshore converter substation,

followed by an approximately 2km long HVAC section of cable to the Creyke Beck

substation which provides the connection point to the UK transmission network.



Figure 2: The onshore export cable corridor

3.2 Onshore converter station and substation location

3.2.1 The development consent order includes a provision for two onshore converter

stations, one for each of the Creyke Beck projects, along with the works required to

connect to the existing National Grid substation at Creyke Beck.

3.2.2 Having determined the connection point would be the Creyke Beck substation, the

most appropriate location for the converter stations was subsequently determined. A

key factor in the site selection process was proximity to the connection point in order

to minimise the landscape and visual effects associated with introducing new

electricity infrastructure to the environment. The area selected is an agricultural area

located approximately 3km north-north west of the substation on the northern side of

the A1079. The indicative layout and expected dimensions of the converter

substations are illustrated in Figure 3.

3.2.3 Each converter station will convert HVDC export power to 400kV HVAC prior to

connection to the Creyke Beck substation.



Figure 3: Visualisation of proposed converter station siting



4 Offshore electrical system

4.1 Overview

4.1.1 The offshore electrical system is comprised of the following elements:

wind turbine generators positioned in arrays;

inter-array subsea cables, connecting the wind turbines to the offshore collector

platform(s). The inter-array cable system is rated at nominal voltage in the

range of 33kV to 72.5kV;

HVAC offshore collector platforms, where step-up transformers increase the

voltage to between 132kV and 400kV;

the inter-platform subsea cables transferring the generated power from the

collector platforms to the offshore converter platforms;

the offshore converter platforms for the conversion of the generated power to

HVDC. The nominal HVDC voltage will be up to ± 550kV; and,

the offshore HVDC export system, comprised of HVDC subsea cables.

4.1.2 An indicative connection schematic for the offshore electrical system can be found in

Figure 4. The development consent order provides for up to 400 wind turbines, up to

eight offshore collector platforms and up to two offshore converter platforms in total,

with the final number pending detailed electrical design.

Figure 4: A representation of the offshore electrical system



4.2 Wind turbine generators

4.2.1 Up to 400 three-bladed, horizontal axis wind turbine generators will be installed at

Dogger Bank Creyke Beck. Through the movement of turbine blades in the wind, a

generator is turned which transforms kinetic energy into AC electricity.

4.2.2 Offshore wind turbine technology is evolving rapidly and it is anticipated that turbines

rated at approximately 4MW to 10MW will be available within the timescales of

construction.

4.3 Inter-array subsea cables

4.3.1 The inter-array electrical system connects the wind turbines to the collector platforms.

The wind turbines are connected in arrays depending on the turbine size and wind

farm layout.

4.3.2 Figure 5 shows an indicative inter-array layout.

Figure 5: An example inter-array layout connecting the collector substation (shown as a rectangle at the centre)

4.3.3 The inter-array electrical system will likely be made of three-core conductor, XLPE

insulated armoured cables suitable for submarine installation. An example of such a

cable is shown in Figure 6.



Figure 6: Example of 33kV, three-core, XLPE insulated, armoured subsea cable

4.3.4 It is anticipated that, in order to optimise the economy of the system, cables of

different sizes will be used throughout the system.

4.3.5 The use of copper conductor cables is the industry standard for submarine

application. Copper is more resilient to the marine environment and a heavier weight

allows an easier installation. Submarine cables are protected against mechanical

damage by one or more armour layers, which are typically made of stranded steel

wires. The XLPE insulation is also industry standard practice, as XLPE is a

substantially inert compound that minimises the environmental impact of the cable.

4.4 Offshore collector platform

4.4.1 Up to eight offshore collector platforms (four each for Dogger Bank Creyke Beck A

and B) will be constructed, which will receive power from the wind turbines via the

inter-array cable systems. Transformers will be located on the collector platforms to

increase the voltage of the AC power received from the wind turbine generators. This

aims to minimise transmission losses so that the electricity can be efficiently

transmitted to the offshore converter platform.

4.4.2 Figure 7 shows an example of an offshore collector platform. Converter platforms

(see below) are of similar design, albeit with larger dimensions.



Figure 7: Typical offshore collector platform design. Source: Seagreen Wind Energy

4.5 Inter-platform subsea cables

4.5.1 The power is transferred from the collector platforms to the converter platforms

through inter-platform cables which operate at a higher voltage than the inter-array

cables. The structure of the inter-platform subsea cables is substantially similar to the

structure of inter-array cables, however one major difference is the thickness of the

insulating layer required as a result of the higher voltage. Single core cables in trefoil

could also be considered for the inter-platform circuits.

4.5.2 The inter-platform cables will therefore typically be HVAC, one- or three-core, XLPE

insulated, armoured subsea cable. Figure 8 shows an example of a three-core cable.



Figure 8: Example of 245kV, three-core, XLPE insulated, armoured, subsea cable

4.6 Offshore converter platform

4.6.1 The electricity generated will be transmitted to shore using HVDC technology, which

provides significant technical advantages and power loss reduction over long

distance when compared to HVAC transmission.

4.6.2 Therefore an offshore converter platform is required for each project to convert the

power generated by the wind farm and collected by the offshore collector platforms

from Alternating Current (AC) to Direct Current (DC). This power is then exported to

the landfall via the HVDC subsea cables.

4.7 Export system HVDC subsea cables

4.7.1 The offshore HVDC export cables provide for the transmission of electricity between

the offshore converter platform and the transition bays at the onshore landfall point.

The purpose of the transition bays is to connect the offshore HVDC export cables to

the onshore HVDC cables.

4.7.2 The cables of the offshore export system are likely to be copper conductor, single-

core, XLPE insulated, armoured cables, suitable for submarine installation. The

structure of a XLPE insulated HVDC cable is very similar to the structure of more

conventional HVAC cables. The main difference concerns the XLPE compound



which is specific for HVDC applications. Mass impregnated cables could also be

considered, pending detail design.

Figure 9: Example of a subsea HVDC cable



5 Offshore cable installation

5.1 Pre-installation works

5.1.1 The preferred installation method for all the offshore cables is through direct burial

into the seabed. The objective is the protection of the cable by installing it at a

sufficient depth, which must be compatible with the risks anticipated for each stretch

of cable route. Where the cable burial is unfeasible or inadequate, additional and/or

alternative protective measures will be considered.

5.1.2 The preferred cable routes, as identified within the limits of the development consent

order, would be surveyed via pre-construction geophysical and geotechnical surveys.

This would also locate any obstacles that may obstruct cable laying (e.g. rocks,

wrecks, metal objects, unexploded ordnance) and inform the installation method to

be used.

5.1.3 If an obstruction is located it would be assessed and an appropriate strategy would

be established to remove or avoid the obstruction. Typically a Pre Lay Grapnel Run

(PLGR) and Remote Operated Vehicle (ROV) survey would be conducted to clear

the obstruction. Where the obstacle is suspected to be unexploded ordinance,

specialist mitigation would be employed to either avoid or make safe the obstruction.

The geophysical surveys would also serve to identify the location of sand waves

along the cable route so that an assessment can be made as to whether such

features can be avoided or, if not, what level of seabed preparation (such as pre-lay

sweeping) is required to ensure an appropriate burial depth is achieved in stable (i.e.

non mobile) seabed conditions.

5.1.4 Prior to cable installation, cable burial trials may be conducted in advance of the main

installation programme to ensure that the chosen equipment would be suitable for the

ground conditions encountered and that an appropriate burial depth can be achieved.

If undertaken, any such trial may involve trials of lengths of up to 1km in each of the

soil types likely to be encountered along the export cable route.

5.2 Offshore cable installation methods

5.2.1 There are several different methods available for the installation of submarine cables,

the most common of which are:

simultaneous lay and burial using, for example, a cable plough;

post lay burial using, for example, a jetting Remote Operated Vehicle (ROV);

and,

simultaneous lay burial / post lay burial with a combination of plough, jet

trencher and/or mechanical trencher.



5.2.2 The final decision on installation method would be made on completion of the pre-

construction geotechnical site investigation surveys. However, it is likely that for the

inter-array cables of Dogger Bank Creyke Beck a combination of ploughing, jetting

and trenching will be considered. This approach is considered to take into account

the inevitable variation in sedimentary conditions along the cable routes.

5.2.3 Cable burial ploughs cut through the seabed, lifting the soil from the trench. Cable

ploughs are designed to cut a narrow trench, with a slot of material temporarily

supported before falling back over the trenched cable. The advantage of this method

is that burial can be achieved as the cable is laid, thus minimising risk to the cable

before the completion of the installation works. However, the number of vessels

which can carry out this method and that have the required cable carrying capacity

for “heavy” power cable is limited. The performance of a plough and the depth of

burial which can be achieved are a function of plough geometry and seabed

conditions, with dense or stiff sediments providing the greatest challenge.

5.2.4 Where seabed conditions are predominantly soft sediment material it may be

considered appropriate to bury the array cables with a Directionally Positioned (DP)

vessel post installation. Under this process the cable would be laid on the seabed

first and a Remote Operated Vehicle (ROV) fitted with high-pressure water jets would

be subsequently positioned above the cable. The jets fluidise a narrow trench into

which the cable sinks under its own weight. The jetted sediments settle back into the

trench and with typical tidal conditions the trench coverage would be reinstated over

several tidal cycles.

5.2.5 In locations where seabed conditions comprise very stiff soils (typically over 100kPa)

and/or bedrock, ploughing and jetting techniques may not be appropriate for cable

burial. One approach for installing cables in very stiff or hard seabeds would be to

use mechanical trenchers which can either be used to simultaneously bury the cable

as it is laid or in a “post lay” mode where the cable is laid by one vessel and burial is

achieved by another vessel following on behind. Simultaneous lay and burial of the

cable tends to be preferred since this reduces risk to the cable from exposure.

However, if a post lay burial solution is used then typically the length of time of

exposure would only be a few hours (depending on the exact arrangements). During

this time any unburied lengths of cable would probably be protected using a guard

vessel.

5.2.6 It should be noted that simultaneous lay and burial can also be achieved by

ploughing in stiff materials to 140KPa and above (e.g. chalk) by use of specially

designed “rock ripping” ploughs as well as certain types of “standard” subsea plough

or mechanical trencher.

5.2.7 Full details of the cable installation works and vessels used are set out in Chapter 5

of the Environmental Statement.



5.3 Crossing of existing infrastructures

5.3.1 Where existing subsea services cross the offshore cable routes alternative and/or

additional protection and installation methods will be required. The draft DCO

provides the power to carry out cable crossings of existing cables and pipelines

within the Order Limits. The number of crossings associated with the inter-array

cabling will be determined following the confirmation of the final layout post-consent.

However, discussions with the relevant cable and pipeline operators are underway. It

is currently anticipated crossing agreements will be needed with:

Tata Communications – in relation to the TATA North Europe

telecommunications cable;

BT Subsea / Cable and Wireless Worldwide – in relation to the UK-Germany 6

telecommunications cable

BT – in relation to the UK-Denmark 4 telecommunications cable

Gassco – in relation to the Langeled gas pipeline; and,

Shell UK – in relation to the Shearwater to Bacton (SEAL) gas pipeline.

5.3.2 Alternative protection measures include concrete mattressing, fronded concrete

mattressing, rock dumping, bridging or the positioning of gravel bags.

5.3.3 There is no single universally accepted crossing design that would be applicable in all

situations. Designs would vary with the seabed properties at the particular location.

Each crossing would have a range of features possibly unique to that location, based

on:

the physical properties of the crossing, for example the cable size and weight,

bend radius and armouring;

protection requirements relative to the hazard profile, including depth of burial or

extent of mattress/rock cover;

the physical properties and protection status of the crossed product;

seabed properties at the crossing point, for example substrate type, morphology

and stability (presence of mobile bedforms); and,

any constraints placed by the crossed party, for instance location and burial

determination standards, maintenance clearance zone, plough approach limits

and notification zone.

The components most commonly used to protect telecommunication cables

would be flexible mattresses and graded rock. These components may be used

exclusively or in combination.



6 Onshore electrical system

6.1 Overview

The onshore electrical system is comprised of the following elements:

landfall transition joint bay;

the onshore HVDC cables;

the onshore converter substations;

the onshore HVAC export system connecting the converter substation to the

connection works at the Creyke Beck substation; and,

the connection works required at the existing Creyke Beck substation.

6.2 Landfall transition joint bay

6.2.1 Offshore HVDC cables will end at the landfall transition joint bay, where they are

connected to the onshore HVDC cables. The transition bay location has been

carefully chosen to be a convenient distance inland from shore, in particular taking

into account local coastal erosion which is a feature of the selected landfall area.

6.2.2 The landfall transition joint bay is buried underground and provides the following

functions:

providing a suitable environment to the cable joint elements;

providing a reliable anchoring point to the offshore cables;

allowing the landfall directional drilling operations a suitable angle of attack;

and,

creating a pulling ramp for the offshore cable installation (if not separated from

the transition bay).

6.2.3 The pit will have a concrete plinth on the bottom to provide the cable joints with an

anchoring point. Approximate maximum pit dimensions are 1.5m deep, 4m wide and

up to 12m long but dimensions may vary depending on the final electrical design.

6.3 Export system HVDC underground cables

6.3.1 The cables of the onshore export system will likely be single-core, XLPE or mass

impregnated insulation cables, suitable for underground installation. Either copper or

aluminium conductors can be used for the onshore HVDC cable, pending the final

electrical design. It is likely aluminium cable will be the preferred option due to them

being lighter and allowing easier installation and handling. An example of

underground, HVDC cable is illustrated in Figure 10.



Figure 10: Typical HVDC, underground cable.

6.3.2 Since the risk of mechanical damage to underground cables onshore is lower than

for submarine cables, it is industry practice to install unarmoured cables.

6.4 Onshore converter substation

6.4.1 The development consent order includes provision for up to two 1,000MW onshore

converter substations, with a total rating of 2,000MW, for the conversion of the wind

farm output from HVDC to HVAC at the connection nominal voltage (400kV). The

HVDC cables which transmit electricity along the onshore cable route terminate at

the converter substation. The connection to the onshore connection point at the

Creyke Beck substation is made through HVAC underground cables.

6.4.2 Each converter station will include:

a valve hall – a large building housing electronic and electro-mechanical

equipment to convert the power from DC to AC;

an ancillary building containing a control room, equipment rooms, operator

facilities and storerooms;

outdoor equipment areas containing rigid aluminium DC and AC current

connectors, transformers, filtering systems, switchgear, measuring and ancillary

electrical systems, cable sealing ends and air-conditioning equipment; and

areas for car parking and internal roads.



Figure 11 illustrates a converter substation like those proposed for Dogger Bank

Creyke Beck.

Figure 11: An illustrative converter substation layout (Courtesy of Siemens)

6.4.3 The footprint of such a design based on data from the major suppliers and allowing

for roads, drainage and construction area is approximately 2 ha per converter station.

An area of up to 1 ha adjacent to each station would be required for construction and

laydown areas. Landscaping would be undertaken as required and it is envisaged

that this may require an additional area of up to 2 ha. The converter stations would

be serviced by a tarmac road of up to 6m in width. Therefore, the total land take for

the two converter stations including permanent landscaping and temporary

construction areas is approximately 10 ha.

6.5 Export system HVAC cables

6.5.1 HVAC cables will transmit electricity at 400kV between the converter station and the

point of connection. Underground HVAC cables are likely to be conventional XLPE

aluminium or copper cables rated at 400kV nominal voltage; the construction and

installation of such cables are industry consolidated practices. As it can be seen in

Figure 12, illustrating the split view of a 400kV, the structure of HVAC cables is not

dissimilar from the structure of conventional medium voltage cables.



Figure 12: A 400kV XLPE insulated underground copper conductor cable

6.5.2 The HVAC cables installation is expected to be through direct burial in open

trenching. For both ordinary installation and obstacle crossing the techniques used

are identical to the ones considered for the HVDC cables.

6.6 Connection works at Creyke Beck substation

6.6.1 Creyke Beck 400kV substation was initially identified by National Grid as the onshore

connection point for the projects as part of the Grid Connection Application process,

following initial option assessment work carried out jointly by National Grid and

Forewind.

6.6.2 Following acceptance of the grid connection offer and in parallel with the

development phase, National Grid and Forewind have jointly undertaken further

detailed assessment of options to identify the optimum onshore interface point(s) for

the projects. When developing options to connect new generation to the

Transmission System, National Grid must ensure that the Transmission System

continues to comply with relevant obligations in respect of security and quality of

supply. Also, National Grid must ensure that the connection offered meets the

requirements of Forewind where applicable. The assessment of options has

therefore included environmental considerations, land availability, technical

requirements, economic considerations, and programme and timing requirements.

6.6.3 The detailed option assessment work has confirmed the initial conclusion that Creyke

Beck was and remains the optimum onshore connection point for the projects

6.6.4 Works will be required at the National Grid substation at Creyke Beck, part of which

is leased to Northern Power Grid, to allow connection of Dogger Bank Creyke Beck

offshore wind farm. Consenting, constructing, operating and maintenance and

decommissioning of some of these works will be the responsibility of National Grid,

with the balance being the responsibility of Forewind.



6.6.5 The works within Creyke Beck substation will require new switchgear connection

bays comprising typically new busbars and busbar supports, circuit breakers,

disconnectors and earth switches, measuring transformers and associated ancillary

equipment. These works are referred to as „works to connect actions‟ and have been

considered in the environmental impact assessment work for Dogger Bank Creyke

Beck.

6.6.6 The preferred areas for connection works are identified within Work No. 9A and 9B

on the Works Plans (application reference: 2.4.2). However, as the precise location is

uncertain, Work No. 9C is also included within the Order Limits as an alternative

connection point.

6.6.7 The National Grid connection may require consent under the provisions of the Town

and Country Planning Act (TCPA) 1990, for which the determining authority is East

Riding of Yorkshire Council. Should planning permission be required, it is anticipated

that the application will be lodged by National Grid in 2014 or 2015.



7 Onshore electrical system installation

7.1 Onshore HVDC cable corridor

7.1.1 The majority of the proposed onshore cable route passes through agricultural land,

which at the time of the surveys was primarily being used for arable purposes. For

the two projects Dogger Bank Creyke Beck, the maximum total width for two adjacent

cable systems during installation is 36m for the HVDC cables over a distance of

approximately 30km and 38m for the HVAC cables over a distance of approximately

2km, including cable trench, haul roads, fencing and temporary topsoil and subsoil

storage areas.

7.1.2 The 36m corridor swathe is based on the following indicative working widths,

although other breakdowns of the working width are possible following detailed

design:

two cable trenches of nominal width 1m;

a 2m zone on either side of the trench to allow for battering of the trench and

maintaining a safe distance between the edge of the trench and the stored

subsoil or haul road;

a 0.5m wide French drain to one side of each haul road (included within the 2m

separation zone from the trench);

two 6m wide haul roads (one per contract), each to allow two-way construction

traffic flows;

a 6.5m zone for each circuit trench for topsoil and subsoil storage; and

a 1m separation between each haul road.

7.1.3 The corridor swathe is identified in Figure 13.



Figure 13: Indicative onshore cable route working widths based on two illustrative construction options for an

HVDC section (courtesy of Ramboll)

7.1.4 Where major horizontal directional drilling (HDD) is necessary, for example to bury

underneath key features such as the River Hull, the railway line or main roads, the

below ground sections of the cable route may be up to 53m wide for a major HDD, up

to 70m in the case of the HDD below Figham Pastures, or up to 250m wide for the

landfall HDD, due to the varying depth and distances of the HDDs.

7.1.5 Due to the nature of the ground conditions along the length of the cable route and the

potential issues surrounding surface water run-off and groundwater, it is envisaged

that a haul road will be required for a significant portion of the cable route. The haul

road will be formed using methodologies such as hardcore material or proprietary

bog mats.

7.1.6 The exact requirements for the built-up haul road will be dependent on a number of

factors such as the time of year that the works are being undertaken and the detailed

knowledge of subsoil and groundwater regime obtained from the ground

investigation.



7.2 Onshore cable installation methods

7.2.1 The onshore underground cables will be installed in one of the following ways:

laid directly in to the ground via open trenches and backfilled;

laid into ducts in the ground and backfilled; or

laid by Horizontal Directional Drilling (HDD).

Open trench installation

7.2.2 The onshore route for Dogger Bank Creyke Beck stretches for a considerable portion

over farmland. The open trench installation is therefore the preferred installation

method.

7.2.3 An ordinary two cables trench is about 1.5m deep and about 1m wide at the trench

base (1.5m for the HVAC sections). For arable land installation it is practice to install

the cables at no less than 1.2m underneath the surface to prevent damage from third

parties, in particular, deep ploughing and drain excavation.

7.2.4 The trench bottom must be smooth so rocks, boulders and any other item that may

mechanically damage the cables are removed. Power cable trenches are backfilled

with stabilised material to provide adequate thermal conductivity and drainage. The

trench top layer, i.e. the top 200-300mm, is backfilled with native material and the

original surface reinstated as required.

7.2.5 Warning tapes and protective concrete or hard plastic tiles are placed in the ground

above the cables.

Ducted installation

7.2.6 For some of the cable route sections the cable could be installed in ducts. Ducts

increase the cable mechanical protection and/or increase the installation flexibility.

The use of ducts on the project will be dependent on the phasing of the projects,

whether two circuits are being installed together, the availability of cables and the

strategy adopted by the installation contractor(s) for the projects.

7.2.7 Ducts can be installed with all the associated civil works, prior to the installation of the

cables, which can reduce the footprint and length of time required for construction for

each section. However ducting can increase the thermal resistivity, therefore

reducing the cable rating. CBS or other stabilised material can be used in duct

installation in order to increase the rate of heat dissipation and increase the cable

rating. Ducting requires that the cables would have to be installed through regular

jointing bays by nose pull methods.

7.2.8 There are constraints associated with siting of jointing bays (e.g. they cannot be

placed in floodplains) and there is a limit to cable pulling tension and therefore a limit

to the length of cable and cable bending radius that can be pulled.



7.2.9 Where the cable route crosses roads and some watercourses and utilities, ducting is

the preferred method of installation in order to decrease the disruption to these routes

during construction and for maintenance during the operational phase.

7.3 Onshore crossings

7.3.1 Where the route crosses significant obstacles such as large watercourses, utilities,

railways or major roads where open trenching is not practical, the preferred method

of cable installation will be by horizontal directional drilling (HDD). HDD is a

steerable, trenchless method of installing underground pipes, ducts and cables in a

shallow arc along a prescribed bore path by using a surface launched drilling rig.

HDD is a well-established method for cable installation where the more conventional

trench installation is either unfeasible, or undesirable for environmental reasons.

7.3.2 Minor crossings will be realised by alternative methods including hand or digger

excavation, if required. Each case will be addressed by considering the most suitable

crossing technique.

7.4 Onshore construction compounds

7.4.1 Temporary construction compounds will be required at various points and stages of

the onshore works, in particular:

one landfall works construction compound for each project;

up to three primary compounds for cable installation for each project;

up to six intermediate compounds for cable installation for each project; and

HDD construction compounds.

7.4.2 The compounds will typically be comprised of the following main elements:

cable laydown area;

portakabin with offices and comfort rooms;

lock-up storage area;

car park;

machinery and lifting equipment area; and,

utility area.

7.4.3 Landfall and HDD construction compounds will also include an HDD equipment area

and the area for the launch pit. Figure 14 illustrates an example of an HDD

construction compound, which also requires a compound at the receiving end.



7.4.4 The transition bay at the landfall should be included in the landfall construction

compound.

Figure 14: Indicative HDD construction compound (launch pit)



Figure 15: Indicative HDD construction compound (receiving pit)

7.4.5 Primary compounds will be located in the proximity of junctions, usually „A‟ roads for

ease of access, whereas secondary compounds will be distributed along the cable

route, as required by the installation plan.

8 Conclusion

8.1.1 This document provides an overview of the electrical grid connection works which will

be required for Dogger Bank Creyke Beck, as included within the application for a

development consent order. A requirement under Regulation 6(1)(b)(i) of the

Infrastructure Planning (Applications: Prescribed Forms and Procedures) Regulations

2009 (the APFP Regulations) is that an application for an offshore generating station

must provide details of the proposed route and method of installation of any cable

with the application. This statement has provided details on both the offshore and

onshore electrical system, providing details of the location, type and proposed

installation methods. Additional detail can also be found in Chapter 5 of the

Environmental Statement submitted with the application.

cable details and grid connection statement - forewind - · pdf file4.3 inter-array subsea...

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