Deadline VI Appendix 7 Description of Export Cable Protection … · Figure 1 Illustrative example of rock berm protection for unburied export cables 1.2.6. Rock burial is one of

November 2014

Deadline VI Appendix 7 Description of Export Cable Protection Parameters

DOGGER BANK TEESSIDE A & B

F-EXL-DVI-002 Appendix 7 © 2014 Forewind Page ii

Document Title Dogger Bank Teesside A & B

Deadline VI Appendix 7

Description of Export Cable Protection Parameters

Forewind Document Reference F-EXL-DVI-002 Appendix 7

Issue Number 1

Date November 2014

Drafted by Edward Ross

Approved by Mark Legerton

Date / initials approval ML 19-Nov-2014


F-EXL-DV-003 Appendix 8 © 2014 Forewind Page iii

Contents

1. Introduction ................................................................................................................... 1

1.1. Introduction ......................................................................................................... 1

1.2. Remedial Cable Protection ................................................................................. 1

2. Export Cable Indicative Rock Berm Design .................................................................. 3

2.1. Rock Cable Protection Design Considerations ................................................... 3

2.2. Rock Berm Design Considerations ..................................................................... 3

2.3. Indicative Rock Berm Design .............................................................................. 4

3. Export Route Remedial Protection Quantities ............................................................... 6

3.1. Export Cable Corridor Burial Feasibility .............................................................. 6

3.2. Remedial Cable Protection Quantity ................................................................... 9


F-EXL-DVI-002 Appendix 7 Page 1 © 2014 Forewind

1. Introduction

1.1. Introduction

1.1.1. In the Issue Specific Hearing of 11-12 November 2014, the Examining Authority

requested more detail from Forewind on how the worst case parameters were

developed for the use of remedial cable protection for the High Voltage Direct

Current (HVDC) export cables from the offshore converter station to the cable

landfall on the Teesside coast. This request was further referenced in the

Hearing Action Points List 2 – point 2.10. Forewind’s considered response to

this specific Action Point is detailed in this document.

1.2. Remedial Cable Protection

1.2.1. Burial is the preferred protection technique for the Dogger Bank Teesside A & B

HVDC export cables, as it typically provides the best protection, at the lowest

cost, in the shortest time. The offshore cables will, therefore, be buried wherever

it is feasible and economic to do so, with additional or alternative remedial cable

protection measures applied only if necessary.

1.2.2. Typically, the most common drivers for the installation of remedial cable

protection are adverse geotechnical conditions and the proximity of other

structures or assets. The presence of hard rock or other challenging seabed

conditions, such as rapidly changing sediment strata types or buried boulders,

can prohibit the burial of cables. Additionally, due to safety considerations, it is

not always possible to operate cable burial equipment in close proximity to

offshore structures. As a result, some form of remedial cable protection will be

required in the vicinity of the structure. The remedial cable protection method

and detailed design chosen will reflect the level of risk to which the cable is

exposed, e.g. the expected levels of scouring, seabed mobility, fishing activity,

and anchoring.

1.2.3. The Dogger Bank Teesside A & B application describes a range of potential

remedial cable protection parameters and technology options. Typical remedial

cable protection measures include one or a combination of the following options:

Rock or gravel burial;

Concrete mattresses;

Flow energy dissipation devices;

Protective aprons or coverings and;

Bagged solutions.

1.2.4. Section 3.10 (Remedial Cable Protection) of Chapter 5 Project Description of the

ES (ref 6.5) provides a fuller description of the various forms of remedial cable

protection types.

1.2.5. Protection of the HVDC export cables by rock burial (also known as rock

placement or rock dumping) has been identified as the worst case remedial



cable protection for seabed footprint due to the width of rock berm that may be

required. The methodology used in developing this worst case based upon rock

burial is described within the following sections.

Figure 1 Illustrative example of rock berm protection for unburied export cables

1.2.6. Rock burial is one of most technically robust and commonly used remedial cable

protection techniques and involves installation of a rock ‘berm’ over the cable.

Shown in the figure above is the indicative rock berm profile used for calculating

the worst case parameters contained within the ES. Rock protection can be

deployed from specialist ships or barges using techniques such as side casting,

where rocks are pushed overboard with lateral hydraulic slides and discharged

onto the seabed. Alternatives include fall-pipe systems; where rocks are fed into

a funnel at the top of a fall-pipe and discharged at a controlled rate as guided by

sensors at the base of the pipe – which can also be remotely steerable.



2. Export Cable Indicative Rock Berm Design

2.1. Rock Cable Protection Design Considerations

2.1.1. As discussed in the response to ExA [1] 6.12, the HVDC export cables have

been identified as key risk areas for the Dogger Bank Teesside A & B projects;

the HVDC cables are a key long-lead component, suitable installation and repair

vessels are a global supply chain bottleneck and the installed cables are a

single point of failure for the whole project, with both cables required to be fully

operational for any power to be exported.

2.1.2. In developing the design of a rock berm consideration has to be given to the

hazards posed to the cables. These can include environmental hazards, such as

wave and current exerting hydraulic forces on the objects on the seabed, and

manmade hazards such as dropped objects, fishing, a vessel negligently

dragging an anchor whilst underway, or a drifting anchored vessel (due to bad

weather or power loss). An analysis undertaken by BT of the causes of

submarine telecommunication faults around UK waters between 2007 and 2010

identified 39% of faults resulted from fishing activities and 36% resulted from

anchors. A copy of this study has been provided in response to ExQ [2] 3.3.

2.1.3. Whilst the BT study identified that fishing activities resulted in slightly more of the

submarine telecommunication faults, Forewind developed its rock berm consent

envelope for the HVDC export cable based upon interaction with anchors. Any

form of interaction between cables an anchor is more likely to lead to a cable

failure, due to the high amount of forces caused by a dropped or dragged

anchor. There is also a possibility that in a single incident a vessel, whilst

underway, could drag an anchor across the HVDC export cables for both the

Dogger Bank Teesside A & B projects. The loss of both projects at the same

time could have an impact on the UK transmission network.

2.2. Rock Berm Design Considerations

2.2.1. Given the extremely low probability of an anchor being dropped directly on top of

a section of cable, the common industry practice is for rock berms to be

designed assuming the anchor will be dragged over it. In the event of a dragging

anchor, a rock berm should initiate an outbalancing force on the anchor and

anchor chain, resulting in breakout of the anchor, allowing it to travel over the

berm.

2.2.2. In addition to the interaction mechanism between the anchor and the rock berm

the following factors would also be considered:

Anchor Type (Standard, HHP, etc.);

Anchor Dimensions (fluke length, shank length, etc.);

Anchor penetration depth;

Seabed Characteristics; and

Materials used in the construction of the rock berm

2.2.3. It is not possible to design a rock berm that is optimal for every possible

combination of the factors listed above. Instead, a risk based approach,



considering the factors, would be undertaken during the detailed design phases

of the projects. This work would be informed by further geotechnical surveys

(cone penetration tests, boreholes, etc.), conducted along the cable corridors to

complement and ground-truth the geophysical survey data collected so far by

Forewind and by further work looking at the marine traffic in the vicinity of the

HVDC export cable corridor. It is likely that the profile of the rock berm will vary

along the HVDC export cable corridor to take account of local conditions. For

example, areas in which hard seabed conditions are present the rock berm may

be reduced due to the shallower anchor penetration depth. Conversely the rock

berm may be bigger in areas where larger vessels frequently operate due to the

larger dimensions of their associated anchors.

2.3. Indicative Rock Berm Design

2.3.1. For the ES, Forewind developed the indicative rock berm profile, shown

previously, for the HVDC export cables to allow a maximum footprint and

volume of remedial cable protection to be assessed. A typical 3 tonne stockless

anchor, shown in the figure below, was used as the basis for this rock berm

design. This anchor size is widely used and is commonly carried by ferries and

medium sized commercial vessels.

Figure 2 Dimensions of a typical 3 tonne anchor

2.3.2. At this stage it is not possible to perform detailed design calculations for the rock

berm, as there is insufficient information available. However, a number of broad

guidelines have been developed by marine contractors to provide preliminary

dimensions when designing a rock berm. The equations within guidelines,

shown below, allowed Forewind to develop an indicative rock berm profile for

the 3 tonne anchor.

𝐵𝑡𝑜𝑝 = 2 × 𝑆ℎ𝑎𝑛𝑘 𝐿𝑒𝑛𝑔𝑡ℎ

𝐵𝑏𝑜𝑡𝑡𝑜𝑚 = 𝑂𝐷𝑐𝑎𝑏𝑙𝑒 + 10 × 𝐹𝑙𝑢𝑘𝑒 𝐿𝑒𝑛𝑔𝑡ℎ

2.3.3. Where by:

Btop is the breadth at the top of the rock berm [m]

Bbottom is the breadth at the base of the rock berm [m]



ODcable is the outer diameter of the cable [m]

2.3.4. Using these guidelines the indicative breadth at the base of the rock berm was

estimated to be 15.2m.

2.3.5. During the detailed design phase of each project further engineering analysis,

including model tests, would be undertaken to identify the suitable rock berm

dimensions for the specific location and the associated risks identified.



3. Export Route Remedial Protection Quantities

3.1. Export Cable Corridor Burial Feasibility

3.1.1. In addition to developing an indicative profile for rock berms placed over the

HVDC export cables, Forewind performed a burial assessment along the export

cable corridor. Whilst the final selection of cable burial and remedial protection

methods will be made during the detailed design phases of the Dogger Bank

Teesside A & B projects, and require additional geotechnical surveys (cone

penetration tests, boreholes, etc.), to have been conducted along the cable

corridors, the geophysical surveys undertaken by Forewind were sufficient to

allow a feasibility study to be undertaken for the purposes of informing the ES.

This feasibility study allowed Forewind to identify those areas along the export

cable corridor potentially with problematic conditions, such as hard rock, where

burial may not be feasible and some form of remedial protection will be required.

3.1.2. The Dogger Bank Teesside A & B export cable corridor seabed is characterised

by a combination of bedrock outcrops, gravel, sand and clays. This is illustrated

within the map supplied in response to ExQ [2] 3.1 part 1 which shows the

surface geology for Dogger Bank Teesside A & B export cable corridor. The

rock outcrops primarily occur towards the landwards portion of the cable route,

whilst clays primarily occur within the Dogger Bank zone itself. The presence of

boulders is generally associated with the bedrock outcrops and clays.

3.1.3. The subsurface geology, strata occurring at depths of greater than 30cm, will

exert a significant influence on the installation of the HVDC export cables.

Throughout the export cable corridor it is anticipated there will be sections of

sub-cropping rock that will make cable burial more challenging and may require

the use of remedial cable protection. Shown in the figures below is a section of

export cable corridor from the map supplied with ExQ [2] 3.1 part 1 and the

same section overlain with the anticipated subsurface geology.

Figure 2 HVDC export cable corridor seabed surface geology, approximately 27 to 47km from landfall.



Figure 3 HVDC export cable corridor seabed subsurface geology, approximately 27 to 47km from landfall.

3.1.4. The anticipated depth of the sub-cropping bedrock varies along the export cable

corridor. Where the sub-cropping bedrock occurs at depths exceeding 3m, it is

considered to have no impact on cable installation as this is deeper than the

anticipated maximum cable burial depth. The target cable burial depth will be

specified during the detailed design phase of the projects and will likely vary

along the route based on consideration of a number of factors, including;

seabed geology, physical processes and risk posed by and to other marine

users.

3.1.5. The feasibility study identified that approximately 30% of the Dogger Bank

Teesside A and 31% of the Dogger Bank Teesside B cable corridor may

encounter challenging geotechnical conditions. These conditions are primarily

hard sediments encompassing areas of outcropping and sub-cropping rock and

to a lesser extent stiff clays. These hard sediments do not rule out cable burial

but some installation techniques, described within Section 3.9 (Offshore Cable

Installation and Removal) of Chapter 5 of the ES (ref 6.5), will not be suitable.

For example, jetting, injecting high pressurised water into the seabed to fluidise

a trench, is only suitable for sands and low density clays and would be incapable

of installing a cable in the presence of rock. However, mechanical trenching

installation methods are typically designed and utilised for installing cables

within harder sediment types.

3.1.6. To generate a worst case for the ES, Forewind has assumed that it would be

necessary to use remedial cable protection in all areas of the export cable route

identified as being potentially challenging for cable installation. Whilst this is a

conservative approach, given the potential to overcome these challenges with

the selection of installation tools, it was felt appropriate for the purposes of the

ES. A conservative approach was taken as at this stage only geophysical data

was available. Until additional geotechnical surveys are undertaken (cone

penetration tests, boreholes, etc.) the precise physical characteristics of the soils

cannot be known, which s is critical to understanding which burial or protection

techniques will be suitable



3.1.7. Forewind also considers that this level of conservatism is appropriate as it was

not possible for the feasibility study to access all the possible constraints on

cable installation. The requirement for remedial cable protection is most

commonly a result of adverse geotechnical conditions and the proximity of other

structures or assets, however, other factors such as seabed topography, current

strength, seabed mobility, etc. can all constrain cable burial.

3.1.8. For example, the seabed topography can have a significant effect on cable

burial, with steeper slopes being particularly challenging or impossible for some

installation tools. In general tools mounted on tracked ROV’s are incapable of

working on slopes steeper than 12° to 15°. The stability of the slope is also a

consideration, as tracked ROV’s require sufficient traction for their tools to

operate effectively. It may be possible to microsite cable burial around areas of

difficult topography or it may be necessary to use some form of remedial cable

protection.

3.1.9. The strategy for challenging areas of seabed topography and other cable

installation constraints will be defined during the detailed design phases of the

Dogger Bank Teesside A & B projects.



3.2. Remedial Cable Protection Quantity

3.2.1. Using the parameters discussed in the previous sections it was possible to

develop a worst case usage of remedial cable protection for the HVDC export

cables to be assessed as part of the ES using the following assumptions:

Up to 30% of the Teesside A HVDC export cables may require remedial

cable protection;

Up to 31% of the Teesside A HVDC export cables may require remedial

cable protection;

Rock burial is the chosen method of remedial cable protection;

An indicative breadth of 15.2m for the rock berm; and

An indicative cross-sectional area of 14.8m2 for the rock berm.

3.2.2. Based upon these assumptions worst case footprints and volumes of remedial

cable protection were produced for each of the Dogger Bank Teesside A & B

projects. These values are surmised in the table below.

Table 1 HVDC export cables indicative remedial cable protection values.

Parameter Teesside Project A Teesside Project B

Maximum total offshore HVDC export cable length

(from offshore platform to landfall) per project (km) 573.2 484.4

Maximum export cable protection footprint area per

project (km2)

2.57 2.31

Maximum export cable protection volume per project

(m3)

2,496,785 2,242,473

3.2.3. In addition to the use of remedial cable protection for unburied sections of the

HVDC export cable, similar methods and materials may be used when it is

necessary to cross existing pipelines and submarine telecommunication cables.

At a crossing point it will be necessary to bring the HVDC export cable to the

surface of the seabed so it can be laid across the existing asset. Section 3.10

(Offshore Cable & Pipeline Crossings) of Chapter 5 of the ES (ref 6.5) provides

further details on the requirements for cable crossings.

Documents

Deadline VI Appendix 7 Description of Export Cable Protection … · Figure 1 Illustrative example of rock berm protection for unburied export cables 1.2.6. Rock burial is one of