HIGH LEVEL REVIEW HELIDECK AND ACCOMMODATION Helideck and accommodation ... · PDF fileHIGH LEVEL REVIEW HELIDECK AND ACCOMMODATION Helideck and accommodation facilities on offshore

HIGH LEVEL REVIEW HELIDECK AND ACCOMMODATION

Helideck and accommodation facilities on offshore

platforms for wind farms TenneT

Public version Report No.: 130112-NLLD-R1, Rev. A-Public

Date: 9 June 2015

DNV GL – Report No. 130112-NLLD-R1, Rev. A-Public – www.dnvgl.com Page i

Project name: High level review helideck and accommodation DNV GL Energy

R&S/RES

Utrechtseweg 310,

6812 AR Arnhem

The Netherlands

Tel: +31 26 356 9111

Report title: Helideck and accommodation facilities on offshore

platforms for wind farms

Customer: TenneT, Utrechtseweg 310, 6812 Arnhem

Contact person: Harry van der Heijden

Date of issue: 9 June 2015

Project No.: 130112

Report No.: 130112-NLLD-R1, Rev. A-Public

Applicable contract(s) governing the provision of this Report:

Objective:

High level review to assess the advantages and disadvantages of a helideck and accommodation facilities on an offshore substation platform

Prepared by: Verified by: Approved by:

Eeke Mast

Senior Consultant

Robin Redfern

Project Engineer

Hans Cleijne

Head of Section

Frenando Sevilla

Offshore wind engineer

[Name]

[title]

Erika Echavarria

Engineer

[Name]

[title]

Copyright © DNV GL 2014. All rights reserved. This publication or parts thereof may not be copied, reproduced or transmitted in any

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are trademarks of DNV GL AS. The content of this publication shall be kept confidential by the customer, unless otherwise agreed in

writing. Reference to part of this publication which may lead to misinterpretation is prohibited.

DNV GL Distribution: Keywords:

☒ Unrestricted distribution (internal and external) Offshore platform, helideck, helipad,

accommodation, offshore wind ☐ Unrestricted distribution within DNV GL

☐ Limited distribution within DNV GL after 3 years

☐ No distribution (confidential)

☐ Secret

Rev. No. Date Reason for Issue Prepared by Verified by Approved by

A 9-6-2015 report after mid-term memo - FINAL Erika Echavarria,

Fernando Sevilla,

Robin Redfern,

Eeke Mast,

Robin Redfern,

Eeke Mast

Hans Cleijne

DNV GL – Report No. 130112-NLLD-R1, Rev. A-Public – www.dnvgl.com Page ii

Table of contents

1 EXECUTIVE SUMMARY ..................................................................................................... 3

1.1 Background 3

1.2 Substation Maintenance Requirements 3

1.3 Helideck considerations 3

1.4 Accommodation considerations 5

1.5 Conclusions and recommendations 6

2 INTRODUCTION .............................................................................................................. 7

2.1 Background 7

2.2 Aim and approach 8

3 CURRENT APPLICATION ON OFFSHORE PLATFORMS ........................................................... 9

3.1 Inventory of offshore platforms in North-West Europe 9

3.2 Trends from the platform inventory 11

4 QUALITATIVE DESCRIPTION OF THE MAINTENANCE NEEDS ............................................... 13

4.1 Preventive and corrective maintenance of offshore substations 13

4.2 Major Component Replacement 14

4.3 Availability at Offshore Substations 14

5 REVIEW OF ACCESS SYSTEMS........................................................................................ 16

5.1 Introduction 16

5.2 Transferring Systems for personnel and small components 16

5.3 Onshore-based Marine Access 19

5.4 Offshore-based Marine Access 22

5.5 Other vessels 22

6 HELIDECK ON AN OFFSHORE PLATFORM ......................................................................... 24

6.1 Helicopters at offshore wind projects 24

6.2 Regulations and requirements 26

6.3 Helidecks and winching platforms 28

6.4 Summary of Helicopter Logistics 29

6.5 Estimations workability helicopter versus vessel 30

7 OFFSHORE ACCOMMODATION FACILITIES ....................................................................... 33

7.1 Offshore accommodation for offshore wind 33

7.2 Regulations and requirements 37

7.3 Summary of Offshore Accommodation 38

8 CONCLUSIONS AND RECOMMENDATIONS ........................................................................ 40

8.1 Background and Overview of Existing Projects 40

8.2 Substation Maintenance Requirements 40

8.3 Helidecks and Heli-hoist Platforms 40

8.4 Offshore Accommodation 42

9 REFERENCES ................................................................................................................ 44

1 EXECUTIVE SUMMARY

1.1 Background

The Dutch government has stated its intention to start five tender rounds in five wind farm zones in

2015-2019. They have also appointed TenneT as the offshore TSO. TenneT will be responsible for

installing the offshore high voltage stations to export the generated electricity to shore. TenneT intends

to construct 5 identical offshore substation platforms, each with up to 700MW of capacity up to 38km

from the nearest coast. These substations are substantially larger in power capacity than any existing

offshore (AC) substations.

This study is a high level, qualitative review to assess the advantages and disadvantages of a helideck

and accommodation facilities on an offshore substation platform by identifying possible barriers and/or

opportunities of such facilities.

This study is a qualitative review, not a quantitative review: it will not include an economic comparison

of the perceived impacts of different access methodologies. The aim is to help TenneT conclude whether

quantitative investigation is likely to be worthwhile.

1.2 Substation Maintenance Requirements

DNV GL estimates an average of 10 to 30 days of scheduled maintenance per year to be required for a

substation of this scale. Considering these relatively low scheduled maintenance requirements and that

the shortest maintenance interval required is expected to be monthly or more, it is clear that the

addition of a helideck or accommodation facilities to the offshore substations would not provide

significant benefits to the operation of the substation platforms themselves.

Assuming good levels of redundancy are implemented within the configuration of the substation power

and SCADA systems, most failures will not incur production losses and therefore the repair or

replacement of such components can often be carried out as scheduled maintenance or scheduled access

to the substation, reducing or even eliminating the benefit provided by quick transfer of technicians by

helicopter.

A major failure on the offshore substation, as occurred at the Nysted Offshore substation in 2007, will

require specialist technicians, vessels and replacement components and therefore is constrained much

more by mobilisation of the necessary resources than rapid deployment of technicians from offshore

based accommodation or by helicopter transfer.

1.3 Helideck considerations

1.3.1 Trends from the platform and helideck inventory

There are no clear trends for the number of projects featuring helidecks or heli-hoist platforms with

distance from shore. However, there is a strong trend with respect to project capacity, with all projects

greater than 400MW and the majority of projects above 300MW featuring either a helideck or a heli-hoist

platform on the associated offshore substation(s).

Currently, helicopters are in regular use for turbine O&M purposes at the Horns Rev Project in Denmark,

Alpha Ventus, Global Tech 1 and Borkum Phase 1 (when commissioned) in Germany as well as Greater

Gabbard in the UK. Additionally, contracts are in place at a number of other projects for provision of a

helicopter for emergency search and rescue services. It is not clear whether projects which feature

helidecks utilise these for maintenance of the offshore substation, although such use is anticipated.

DNV GL – Report No. 130112-NLLD-R1, Rev. A-Public – www.dnvgl.com Page 4

1.3.2 Helicopter logistics for offshore wind farms

The distances from shore of the 5 proposed TenneT substations are well within the operational range of

commonly used twin-engine helicopters, such as the Airbus EC135, provided the helicopter landing site is

not located far inland.

The substantial size of offshore substations makes them well suited to helidecks or heli-hoist platforms.

In most cases it is believed that a helideck on the offshore substation is largely intended to facilitate

helicopter access to the turbines and to support emergency response procedures as opposed to solely

providing access to the substation platform itself. Helicopter access to the turbines is performed by

hoisting and for many aircraft, payload is limited for undertaking such heli-hoist operations due to a

requirement to be able to maintain hover in the event of one engine failing. On this basis, some

operators are known to land on the platform helideck to temporarily drop off technicians on the

substation helideck prior to performing heli-hoist operations. Therefore, the greatest benefit of a helideck

is likely to be in support of helicopter logistics at the wider wind farm project. The benefits of a heli-hoist

platform will be largely limited to O&M of the substation platform itself..

Projects utilising helicopters for the O&M of wind turbines are understood to have found them to be cost

effective. They provide fast response to small repairs or diagnosis works where large parts or tools are

not required.

1.3.3 Helicopter requirements

Recent incidents in the North Sea oil and gas industry have led to recent changes in regulation stated by

the British CAA with respect to the sea states in which helicopters may be deployed. This has now limited

the use of helicopters by the lesser of sea-state 6 or the certified ditching performance of the helicopter,

understood to be the sea state in which the aircraft may remain floating upright in the water.

Due to European cooperation concerning regulations and guidelines on aviation, the restrictions on

offshore helicopter flights as stated by the CAA could be taken into account for the Dutch airspace as

well. This would mean that, apart from the requirements of flying under the meteorological limitations

imposed due to adherence to visual flight rules, restrictions could also be placed on the helicopter-

specific sea state permitted for safe offshore flight as they are in place now in the UK. For example, the

Airbus EC135 is understood to be limited to sea-state 4, comparable to the access limitations from

specialist marine access systems such as the Ampelmann in conjunction with large vessels. This would

greatly reduce the greatest benefit of helicopters to offshore wind projects: their insensitivity to sea

states.

From a regulatory perspective, the addition of a helideck is not restricted. The structure should be

carefully located on the superstructure in the design phase. The implications of adding a helideck to a

normally unmanned platform and the associated maintenance burden should be noted.

According to helicopter landing areas regulation also adhered in the Netherlands (CAP 437, wherever

practicable, helicopter hoisting should not be employed as the standard method for transfer of personnel,

suggesting that a helideck is the only option if regular access by helicopter is to be adopted.

Helidecks, and to some extent heli-hoist platforms, are aircraft specific and must be designed for the

dimensions and loading requirements of the aircraft with which they shall be used. Helidecks (and heli-

hoist platforms) require maintaining and certification with particular emphasis on fire fighting, visual aids

and surface friction. This may require frequent maintenance visits to remove guano and check

equipment.


For large substations, such as those proposed by TenneT, the additional cost of a helideck (anticipated to

be in the region of €1M to €2M) is likely to be dwarfed by the overall cost of the platform and therefore

may be justifiable on a percentage cost basis to retain future flexibility and improve the safety case. A

heli-hoist platform is expected to be in the region of €200k - €500k.

In all cases the use of a helicopter for O&M purposes at an offshore wind farm is heavily subject to the

design risk assessment conducted by the developer of the project and associated advisors.

1.4 Accommodation considerations

1.4.1 Accommodation inventory

Only 3 offshore wind projects to date feature offshore accommodation and with the exception of Horns

Rev II, these are located more than 70km from the coast. To date only Global Tech 1 is known to include

permanent accommodation on the offshore substation. Thjs reinforces the assumption that distance from

O&M port is a primary driver for offshore accommodation facilities.

1.4.2 Offshore Accommodation facilities requirements

The reduced cost through combining the offshore substation and accommodation module on one

platform is anticipated to be counteracted by the increased design challenges of ensuring safety of all

personnel against fire and electrical faults, minimising long-term exposure to electromagnetic fields and

ensuring access for maintenance to the heavy major electrical components.

1.4.3 Platform maintenance

Accommodation for the maintenance of solely the substation is not justified due to the relatively minimal

anticipated maintenance requirements. Therefore any offshore accommodation module would need to be

primarily intended for use by the wind farm.

The proposed TenneT platforms are located comparatively close to shore (<40km) and therefore, unless

a suitable O&M port is much further away, there is no strong requirement for offshore accommodation in

order to maintain the substation platform or adjacent wind farms. Offshore accommodation becomes

more economically attractive with distance from O&M port and wind farm project size, with most projects

further than about 30NM to 40NM (55km – 75km) from port expected to be reliant upon offshore

accommodation to avoid excessive travel times, low productivity due to sea sickness and fatigue and an

on-site parts and consumables store.

1.4.4 Fixed versus floating accommodation

Offshore accommodation can take two basic forms, either a fixed platform or floating accommodation

with a variety of different vessels available. Crucially, fixed platform accommodation reduces transfer

time and therefore also the likelihood of sea sickness, but it does not itself increase the sea states in

which transfers to turbines can be achieved. For this reason, current industry trends suggest that the

market is moving more towards floating accommodation configured to provide the dual purpose of

accommodating technicians and providing direct, safe access to offshore structures in higher sea-states

than could normally be achieved by traditional work boats. A further benefit of floating accommodation is

likely to be the potential to operate at night, due to the intrinsically safer “walk to work” approach

enabled by the use of a specialist access system such as the Ampelmann or similar systems.


1.5 Conclusions and recommendations

1.5.1 Helideck

For the maintenance of the platform itself, the helideck will have limited advantages, especially in the

case of stricter sea state regulation for helicopters.

The primary reason for considering a helideck on an offshore substation is therefore the support of O&M

logistics at the wind farm. For instance technicians could be dropped off at the platform to reduce the

payload and make hovering for a heli-hoist to the turbines possible.

Therefore the case for installing a helideck is not clear-cut and is likely to be heavily driven by

possibilities to use the helideck for the maintenance of the surrounding wind farms and detailed safety

reviews.

For large substations, such as those proposed by TenneT, the additional cost of a helipad is anticipated

to be in the region of €1M to €2M and therefore may be justifiable on a percentage cost basis to retain

future flexibility.

1.5.2 Accommodation facilities

From the results of this review, DNV GL believes that the costs and other constraints associated with the

installation of an accommodation platform, either on the same structure as the offshore substation or as

an independent structure, are unlikely to justify the benefits at the proposed TenneT project sites.

Instead, the adoption of a strategy where the majority of scheduled maintenance at all 5 proposed

platforms is performed as part of an annual maintenance campaign, for which the chartering of an OSV

or similar vessel providing access and accommodation will likely prove more cost effective. Alternatively

a similar solution may be adopted at each platform independently by collaborating with the associated

wind farm owner for the purposes of the annual scheduled maintenance campaign, nominally to be

performed during summer months for improved access and minimal loss of production.


2 INTRODUCTION

2.1 Background

The Dutch government has stated its intention to start five tender rounds in five wind farm zones in

2015-2019. They have also stated their intention to appoint TenneT as the offshore TSO. TenneT will

then be responsible for installing the offshore high voltage stations to export the generated electricity to

shore.

The schedule for the five tender rounds and the areas are summarised in Table ‎2-1 and graphically

shown in Figure ‎2-1. For the transformer station for these wind farm zones, TenneT plans a standardised

design for all five platforms. Each platform will have a capacity of 700 MW and the connecting inter-array

cables will be 66 kV.

Tender Year Wind farm zone Capacity [MW] Distance to Coast [km] Distance to port [km]

2015 Borssele Wind Farm Zone 1 700 30 65

2016 Borssele Wind Farm Zone 2 700 38 65

2017 South Holland coast Wind Farm Zone 1 700 26 35

2018 South Holland coast Wind Farm Zone 2 700 26 35

2019 North Holland coast Wind Farm Zone 700 25 30

Table ‎2-1: Short description of the planned 5 tender rounds.

Figure ‎2-1: Existing wind farms and assigned wind farm zones for the upcoming tenders.


2.2 Aim and approach

This study is a high level review to assess the advantages and disadvantages of a helideck and

accommodation facilities on an offshore substation platform. This high level review will assess the

advantages and disadvantages by identifying possible barriers or opportunities for the application of a

helideck or accommodation facilities on the platforms.

This shall be performed under the following methodology:

Reviewing existing and under-construction offshore substation platforms (Section ‎3);

Describing the maintenance needs of an offshore platform (Section ‎4);

Inventory of existing access systems (Section ‎5

Review of helicopter logistics, regulations and the associated use of helidecks and heli-hoist

platforms at offshore wind projects (Section ‎6 );

Review of the logistics, regulations and requirements for offshore accommodation (Section‎7);

and

Conclusions of the above findings for the proposed TenneT Platforms (Section ‎8).

This qualitative assessment aims to help TenneT to make a more informed decision on whether inclusion

of a helideck and/or accommodation on the platform could form an opportunity for TenneT.

This study is a qualitative review, not a quantitative review: it will not include an economic comparison

of the perceived impacts of different access methodologies. The aim is to help TenneT conclude whether

quantitative investigation is likely to be worthwhile.


3 CURRENT APPLICATION ON OFFSHORE PLATFORMS

3.1 Inventory of offshore platforms in North-West Europe

DNV GL has examined the current application of a helideck and accommodation facilities for wind farms

in North-West Europe. This includes offshore wind farms in Belgium, Denmark, Germany, United

Kingdom and The Netherlands. Only wind farms that are currently commissioned or under construction

have been included. Note: All information in this table is based on public domain information and

therefore may be subject to inaccuracies. Table ‎3-1 presents the results. Figure ‎3-1 to Figure ‎3-3

present the results graphically.

Figure ‎3-1: Number of projects with accommodation, categorised in terms of

distance to coast and five-year commissioning interval.

Figure ‎3-2: Number of projects with a heli-hoist or helideck, categorised in terms of

distance to coast and five-year commissioning interval.

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Name Country No of offshore

substations Year


Capacity [MW]

Helideck or Heli-hoist

Accommodation facilities

Blyth United Kingdom 0 2000 1 4 N/A N/A Horns Rev 1 Denmark 1 2002 18 160 Helideck No

Nysted Denmark 1 2003 11 166 No No

Scroby Sands United Kingdom 0 2004 4 60 N/A N/A

North Hoyle United Kingdom 0 2004 9 60 N/A N/A

Kentish Flats United Kingdom 0 2005 9 90 N/A N/A

Barrow United Kingdom 1 2006 9 90 No No

Burbo Bank United Kingdom 0 2007 9 90 N/A N/A

OWEZ Netherlands 0 2007 10 108 N/A N/A

Beatrice United Kingdom 0 2007 22 10 N/A N/A

Prinses Amaliapark Netherlands 1 2008 23 120 No No

ThorntonBank 1 Belgium 0 2008 27 30 N/A N/A

Lynn and Inner Dowsing United Kingdom 0 2009 6 194 N/A N/A

Rhyl Flats United Kingdom 0 2009 9 90 N/A N/A

Belwind 1 Belgium 1 2009 46 165 No No

Gunfleet Sands United Kingdom 1 2010 7 173 No No

Rodsand II Denmark 1 2010 9 207 No No

Robin Rigg United Kingdom 2 2010 11 180 No No

Thanet United Kingdom 1 2010 15 300 No No

Horns Rev 2 Denmark 1 2010 32 209 Helideck Separate (24)

Alpha Ventus Germany 1 2010 56 60 Helideck No

Baltic 1 Germany 1 2011 19 48 No No

Walney 1 United Kingdom 1 2011 20 184 No No

Ormonde United Kingdom 1 2012 11 150 No No

Sherringham Shoal United Kingdom 2 2012 20 317 No No

Walney 2 United Kingdom 2 2012 20 184 No No

Teeside United Kingdom 0 2013 2 62 N/A N/A

Lincs United Kingdom 1 2013 9 270 No No

Anholt Denmark 1 2013 23 400 Helideck No

London Array United Kingdom 2 2013 28 630 Heli-hoist No

Thornton Bank 3 Belgium 0 2013 28 110.7 N/A N/A

Thornton Bank 2 Belgium 1 2013 28 184.5 Helideck No

Greater Gabbard United Kingdom 2 2013 37 504 Helideck No

Bard Offshore 1 Germany 1 2013 102 400 Helideck No

West of Duddon Sands United Kingdom 1 2014 19 389 Heli-hoist No

Riffgat Germany 1 2014 19 108 Helideck No

Northwind Belgium 1 2014 37 216 No No

Belwind demo Belgium 0 2014 45 6 N/A N/A

Meerwind Ost/Sud Germany 1 2014 56 288 Helideck No

Nordsee Ost Germany 1 2014 57 295 Heli-hoist No

Global Tech 1 Germany 1 2014 109 400 Helideck Yes (34)

Westermost Rough United Kingdom 1 2015 11 210 Heli-hoist No

Humber Gateway United Kingdom 1 2015 11 219 No No

Gwynt Y Mor United Kingdom 2 2015 19 576 Heli-hoist No

Luchterduinen Netherlands 1 2015 23 129 No No

Butendiek Germany 1 2015 31 288 Helideck No

Baltic 2 Germany 1 2015 39 288 No No

Amrumbank West Germany 1 2015 44 288 Helideck No

Borkum Riffgrund I Germany 1 2015 44 312 Helideck No

Borkum Phase 1 Germany 1 2015 54 200 Helideck No

Dan Tysk Germany 1 2015 70 288 Helideck Separate (50)

Horns Rev III Denmark 1 2017 30 400 Helideck No

Gemini Netherlands 2 2017 85 600 Helideck No

Note: All the information in this table is ased on public domain information and therefore may be subject to inaccuracies.

Table ‎3-1: List of offshore wind farm and helideck and accommodation facilities.


Figure ‎3-3: Number of projects with a heli-hoist or helideck, categorised in terms of

project capacity intervals.

3.2 Trends from the platform inventory

3.2.1 Accommodation

The only substation platform identified with accommodation facilities on the same structure is at the

Global Tech 1 project in the German North Sea. This wind farm is located more than 100 km off the

coast, substantially further than the proposed TenneT platforms which are located less than 40 km from

the nearest land.

Two further wind farms, Horns Rev II and Dan Tysk, were identified with separate accommodation

platforms located adjacent to the substation platform. Dan Tysk is another far-shore project, located

approximately 70km from the nearest land and is due to be fully commissioned this year. The

accommodation platform for Horns Rev II was the first accommodation platform for offshore wind. This

project is considered something of an outlier from the perspective of accommodation facilities, since the

wind farm is located only 32 km off the coast. It is connected to the transformer station with a walkway

(see Figure ‎7-1). It is understood by DNV GL that access between the accommodation platform and the

shore is conducted by helicopter or vessel, but vessels alone are used for the subsequent shuttling of

technicians to the turbines, implying that transfers are still subject to metocean access constraints.

Although the number of 3 offshore accommodation platforms is insufficient to draw firm conclusions, the

trends in Figure ‎3-1 agree with DNV GL expectations, that accommodation platforms are mostly only

justifiable at far-shore projects. Clearly Horns Rev II provides an interesting exception to this rule and it

is not clear to DNV GL why the Dong Energy chose to adopt this strategy for the project. For this project,

Ramboll [‎5] remarks that:

“On Horns Rev 2 the operation is carried out by two operators one for the substation and one for

the wind turbines & accommodation. The accommodation platform was intended to save

transport cost and time due to higher maintenance activity on the wind turbines than expected.

On Horns Rev 2 the personal is shuttled between onshore and substation/wind turbines by boat

which is the primary means of transportation. The maintenance activity has subsequently

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dropped due to higher experience and improved parts, therefor the need for the accommodation

has diminished. “

It should be noted that emergency accommodation, or a “refuge room”, is required on all offshore

substations and therefore such facilities have been specifically excluded from this analysis.

3.2.2 Helideck

According to the ‘Mijnbouwwet’, offshore platforms in the Dutch EEZ in the Oil & Gas sector have to have

a helideck, unless exemption has been granted by the Minister. However, these Oil & Gas platforms are

quite different from the usually unmanned offshore wind transformer stations.

When looking at the results of the inventory in Figure ‎3-2, the presence of an offshore helideck or heli-

hoist platform appears to have a weak correlation with distance from coast. This is in line with

expectations, since whilst proximity to shore is certainly a contributor, the preferences of the wind farm

developer and the severity of the metocean climate, as well as national trends, appear to be stronger

drivers. For example all German wind farms in the North Sea have a helideck or heli-hoist platform,

whereas in the UK, only Greater Gabbard has a helideck. Four others have a heli-hoist platform.

Figure ‎3-3, shows a stronger correlation with project capacity, with all offshore substations above

400MW featuring either helidecks or heli-hoist platforms, and the vast majority above 300MW as well.

This is likely to be attributable to providing as much flexibility for future maintenance strategies and

campaigns in these larger projects, particularly as the high overall cost of larger substation platforms will

tend to dwarf the additional costs due to helidecks.


4 QUALITATIVE DESCRIPTION OF THE MAINTENANCE NEEDS

4.1 Preventive and corrective maintenance of offshore substations

The preventive and corrective maintenance activities of an offshore substation can be divided into three

main categories:

Non-Intrusive scheduled maintenance: This maintenance category comprises any task

which is pre-planned and which could be performed without affecting production of the wind

farm. These scheduled works are often conducted on a seasonal basis, with the bulk of work

being carried out in the summer to maximise the probability of access to the offshore platform.

However, some minor tasks may be required on a more frequent basis throughout the year.

Intrusive scheduled maintenance: This maintenance category comprises any task which is

pre-planned and which requires the equipment to be temporarily stopped for maintenance work

to be undertaken. If no redundancy is allowed, this activity will limit the production of the plant.

For this reason, these scheduled works are often conducted on a seasonal basis, with the bulk of

work being carried out in the summer to maximise the probability of access to the offshore

platform and minimise lost production.

Unscheduled maintenance (failure repairs): Any unplanned maintenance activities resulting

from a failure of a system, sub-system, or component fall within this group. The level of

corrective action, and the impact of the unscheduled maintenance upon the substation

availability, depends on the severity of the failure and the extent of any redundancy. Minor

equipment failures could be repaired without incurring production losses, while major failure

events can have a greater impact on the availability of the project for long periods depending

upon the location and equipment involved.

TenneT has provided DNV GL with its estimations of scheduled maintenance needs for standard AC

platforms on a Monthly, 3-Months and 6-Months frequencies bases. These estimations have been

estimated from the maintenance needs of the Helwind Beta HVDC substation as provided by Siemens in

its “Material Handling-Component Inspection and Maintenance Overview” report [‎1]. DNV GL has

performed a brief review of these data by comparing the requirements estimated by TenneT against the

Helwind Beta reported requirements (with HVDC-specific equipment removed) and against DNV GL’s

balance of plant maintenance database, as sourced from a variety of public domain sources (see

Appendix).

To perform this brief review, DNV GL has undertaken the following steps:

1. Quick scan and selection of the equipment requirements for AC substations from the “Material

Handling-Component Inspection and Maintenance Overview” report prepared by Siemens [‎1].

2. Classification of the maintenance requirements into the following categories: Mechanical, Piping,

Electrical, Instrumentation, HVAC, Fire Protection Systems & Rescue, Structural, Architectural

and Outfitting (as classified by TenneT in its assumptions).

3. Estimation of total hours per maintenance category of the envisaged maintenance requirements

for the AC substation for the full lifetime of the project.

4. Comparison against TenneT’s estimations and DNV GL’s typical assumptions.


Based on the high level review described above, DNV GL considers TenneT’s estimated maintenance

requirements to be in line with good practice and with DNV GL’s generic and typical assumptions. It must

be noted that TenneT only supplied estimations for Monthly, 3-Monthly and 6-Monthly maintenance

intervals, whereas the data provided by Siemens in [‎1] also gave maintenance requirements for

additional frequencies, including yearly, 2-yearly etc. Therefore DNV GL considers that TenneT’s

estimates from the Siemens data are appropriate when combined with the effort included under these

additional maintenance frequencies.

DNV GL’s database of maintenance requirements for offshore substations has been divided into the three

main categories described above and is reproduced in Appendix A. From these assumptions, DNV GL

estimates that a typical, large AC offshore substation with 2 or more offshore transformers will require

on average 10 to 30 days per year of scheduled maintenance (including maintenance of the SCADA

systems, communications systems and MV switchgear, typically operated and maintained by wind farm

operators). All data in Appendix A are based on a variety of public domain sources as well as DNV GL

experience and whilst these values are representative of a variety of projects, actual scheduled and

unscheduled tasks should always be carried out in line with Original Equipment Manufacturer (OEM)

guidelines.

4.2 Major Component Replacement

Major components within offshore substations, such as transformers, reactors and gas insulated

switchgear, are unlikely to fail once installed correctly. Nevertheless change-out procedures for these

items are considered a high priority as failure of these components can result in significant lost power

production capacity. Therefore, it is critical to mitigate this risk with suitable preparations and efficient

procedures, including the careful location of any additional structures, such as helidecks or

accommodation modules.

Given that transformers and reactors are likely to be the most difficult major component to both procure

and change-out and their relevance to transmit the energy of the wind farm, this sub-section discusses

some important issues relating to transformer change-out strategy as well as serving as an indicative

illustration of a major offshore substation component. Furthermore, other major offshore substation

components will typically have relatively similar change-out procedures to the transformer (e.g.

disconnecting, lifting, re-commissioning, etc.) albeit with lower weights to be lifted.

Offshore substation transformers are substantially heavier than most of the major components, with

indicative weights in the order of 200 - 250 tonnes to lift for a high voltage AC transformer. Change-out

of offshore substation transformers require a large crane vessel which lifts the failed component out of

the roof of the substation and exchanges it for the replacement component. Substation design will have

a significant impact on these operations. The time required for these large crane vessels to remain on-

site may be relatively short, in the order of 1 – 8 days; however the total time for the repair can extend

substantially beyond this due to all the preparation and re-commissioning works as well as a large

dependence on vessel, parts and equipment lead times and weather delays. A well-documented

transformer replacement at the Nysted Offshore Wind Project led to a 4.5 month outage and even this

duration was the result of a fortuitous prompt availability of both a heavy-lift vessel and a spare

transformer winding [‎2].

4.3 Availability at Offshore Substations

DNV GL has performed a variety of modelling studies in the past to estimate the average annual

availability of balance of plant for wind farm projects. The studies suggest that the average annual


energy availability of a single HVAC offshore substation (assuming a 2 offshore transformers

configuration to allow for some redundancy) range between 99.2% and 99.7%.

As detailed in Appendix A, losses in production incurred by the offshore substation are mainly related

with the required scheduled maintenance of the offshore transformers and with potential minor and

major failures in the transformers and their cooling systems.

Different sensitivity analyses performed by DNV GL have demonstrated that the availability of offshore

substation platforms is heavily dependent on the following parameters:

Redundancy levels: The main equipment which could incur production losses is the offshore

transformer. Despite their generally good reliability, major incidents have occurred in the past,

such as the Nysted Offshore Wind Farm transformer major failure in 2007 which caused a total

project outage of approximately 4.5 months. Due to the significant cost impact that such

outages could represent, redundancy of equipment and system configuration is of vital

importance. Redundancy could be implemented by: utilisation of multiple transformers operating

at 100% or less or their rated power, interconnection between offshore substations (when

applicable) and installation of more than one export cable. It is important to note that failures of

export cables are estimated to be the single greatest contributor to project unavailability after

the wind turbines. However, this review is focused on the offshore substation components and

therefore excludes all array and export cabling.

Stocking strategy: Due to the highly customized nature of offshore transformers, the lead time

required to source a spare could range from a few months to a year. For this reason, operators

have started to assess the benefits of stocking a spare major transformer. Stocking of a

transformers will reduce production losses substantially in the event of a failure and some

assessments previously performed by DNV GL suggest that the potential production losses could

justify the capital expenditure required for stocking one transformer.

Scheduled intrusive maintenance of the offshore transformers: As this activity requires

the shutdown of the transformer and hence generates losses in production, an efficient and well

planned scheduled maintenance regime will help to minimise production losses.


5 REVIEW OF ACCESS SYSTEMS

5.1 Introduction

The following sub-sections describe the access systems used to transport personnel and components to

or from an offshore platform including a review of access systems used to transfer personnel or small

parts and components between the vessel and the offshore platform, and vice versa. The last section

focuses on access systems for change out of major components such as heavy lift vessels and cranes.

The access methods, vessels, and concepts detailed in this section are in various stages of development.

Despite a wide range of emerging approaches, most currently operational projects have adopted the

standard access method of stepping from a marine vessel directly to the access ladder. However,

despite the good safety record that has been maintained to date, it is evident that there is scope for

improvement, both in terms of the accessibility of offshore structures and in the safety of the access

methods adopted to achieve such access. The market is responding to the potential risks of current

access methods and the reduction of revenue that results from poor accessibility, while the slow

evolutionary improvement in work boat access is becoming interspersed with the adoption of more

revolutionary solutions, as some projects start to embrace helicopters, offshore-based working, SWATH

vessels, and sophisticated access systems. As projects are situated farther from port and in more

onerous conditions, these trends are likely to continue, with developers seeking to identify approaches

which best suit their projects in terms of both direct costs and project revenue.

This section comprises publically available information from manufacturers’ websites and promotional

material, as well as appropriate conference papers and DNV GL experience. Note that the capability of

the vessels given in the following sections, stated in terms of significant wave height (Hs), is purely

indicative based upon supplier information and, where available, industry experience. The limiting factor

for access capability is primarily wave height, but factors such as current, wind speed, wave period,

water depth, localized wave effects, wave direction, ice, and visibility are also important parameters.

5.2 Transferring Systems for personnel and small components

5.2.1 Transferring Personnel

There are a few options for transferring personnel to the offshore platform. These are:

The “step-over” approach, with technicians stepping across from the bow of the vessel directly to

the platform access ladder (boat landing).

Heave-compensated gangway between the vessel and the platform or the boat landing. Some

large vessels with dynamic positioning capabilities, such as “floatels” or “motherships” (see also

Section ‎7.1.2) may use specialist gangways to dock directly with the platform at the base of the

tower via a gate in the railings.

Lifted from the vessel to the offshore platform in a basket-type arrangement. This typically

requires a large vessel and crane arrangement rated for man-riding.

Helicopter landing or hoisting personnel and small parts. For this a helideck or a hoisting

platform is required.

More information about transfer systems is given below in Section‎5.2.3.


5.2.2 Transferring components

Davit cranes are the principal method for lifting smaller objects between a vessel and the platform and

the weight limit of such lifts will depend upon both the safe working load of the davit and the current sea

conditions. Typically, davit cranes are limited to somewhere between 100 – 2,500 kg depending upon

the specifications of the particular crane adopted, but the true safe working load will also depend on the

dynamic amplification factor applied to account for wave-induced motions and such as transverse and

snatch loads.

The maximum size of parts, tools and consumables that may be transported by workboats is usually

governed more by the lifting capacity of the davit (or other crane) on the offshore structures than by the

deck capacity of the work boat, although work boats are not suitable for very large components due to

the limitations in deck strength and anchor points for sea lashings.

5.2.3 Transfer systems

Table ‎5-1 shortly describes different systems commercially used or under development for transferring

personnel and small components from a vessel to an offshore structure.

Table ‎5-1: Summary of main transferring systems

Type Description

OAS (Offshore Access System)

Deployable gangway Development Stage: Trailing on

offshore wind farm / Deployed in offshore oil and gas.

OAS was developed by Offshore Solutions, which operates as a subsidiary of Ampelmann Operations B.V. since November 22, 2013. The OAS system comprises a pedestal crane with a telescopic gangway of up to 21m long, fitted to the deck of a vessel. The gangway is hydraulically operated and heave compensated. It operates dynamically until the clamped or elephant’s foot connection is made, at which point the control

system is stopped and the structure is free to move passively in

all six degrees of freedom as required to accommodate differences in motion between the vessel and offshore structure. Only then are transfers made. The company claims that OAS can connect in sea states significantly higher than 3m Hs when mounted mid-ships.

Ampelmann

Motion compensated gangway Development Stage: Commercially

available (support of both installation and O&M activities

in the offshore wind industry).

The Ampelmann is an inverted Stewart Platform, an assembly of

hydraulically- or electrically-actuated rams operating in six degrees of freedom. The Ampelmann is designed to be fixed to the deck of a large vessel (ideally more than 70m LOA). A control system monitors the real-time motion of various accelerometers positioned on the Ampelmann platform and vessel, and uses these measurements to compensate for the motion of the vessel and create a steady base for personnel and equipment transfer.

The transfer is made across a telescopic gangway attached to the

platform (up to 25m long). No connection is made between the gangway and offshore structure, relying solely on the heave-compensation and vessel DP systems to maintain minimal differential motion between the two. This has the added benefit of requiring little or no modification to foundation designs for the

Ampelmann system to be used. Ampelmann claims that when installed on a 25m vessel, the system can fully compensate for the vessel motion in Hs of up to 1.5m; when installed on a larger 50m vessel the system can compensate in up to 2m Hs; and in larger vessels up to 2.5m.


Personal Transfer System (PTS)

Type: Crew lifting system Development Stage: Final testing

This system is being developed by Personnel Transfer System

GmbH. The PTS comprises a remote controlled crane installed on each turbine (or substation) which lifts the technicians, one at a time, from the vessel to the working deck of the structure. It also compensates for the motion of the vessel, and the crane control system allows for an automatic transfer of personnel. According to the designers, the limit for safe operation is 500 kg in sea-states up to 3 m Hs, and 800 kg in sea-states up to 1.5 m,

allowing it to be used for transfer of large components to the turbine working deck. The PTS has passed the prototype phase and is understood to be ready for final testing offshore. During November/December 2007 and January 2008 PTS was tested at a harbour site in Hamburg, Germany. The system developer states that the system can be used in sea-states characterized by significant wave heights of up to 3 m if used in conjunction with a

large vessel.

FROG and OWAS systems

Type: Personnel transfer pod Development Stage: In use in the

oil and gas industry and under development for the offshore

wind industry.

This comprises a buoyant personnel transfer capsule which is

transported using a standard deck crane on a larger vessel. It can transfer up to nine personnel, with light equipment and tools, per lift, and is designed to protect crews against any vertical and lateral impacts which might occur during transfer. These systems are used for vessel-to-vessel and vessel-to-installation transfers and have accrued significant use in the oil and gas industry. Reflex Marine is now proposing a smaller, lighter personnel

transfer capsule that can be easily stowed on a workboat, combined with a specially-built turbine crane. This system, known as the Offshore Wind Access System (OWAS), is a simpler utilization of the FROG specifically for offshore wind purposes. An advantage of this system is that it can be used in conjunction with small work boats.

Maxccess Transfer System

Type: Platform with motion compensation

Development Stage: Commercially available

Developed by UK-based OSBIT Power Limited, Maxccess has been chosen for use at the Sheringham Shoal Wind Farm on the UK East Coast, following a series of sea trials, including at Statoil’s Hywind demonstration floating wind turbine in Norway. Maxccess

is a device which may be mounted to the foredeck of most work boats. It clamps onto either of the vertical tubular spars of the boat landing and allows the vessel to roll, pitch and yaw freely, while preventing vertical and horizontal bow motion. The connection is created without the need for active compensation or complex control software. A small, stepped gangway then provides direct access to the ladder.

The Windlift

Type: Suspended Platform Development Stage: Prototype

The Windlift system, developed by Fassmer, is a height-adjustable platform for access to offshore wind turbines from

small, floating vessels. Personnel and equipment are transferred

to the platform at vessel-deck level. The platform, which is fitted around the turbine foundation, is then hoisted to the working deck level, avoiding the need for technicians to climb external ladders.

OWAS

FROG


5.3 Onshore-based Marine Access

5.3.1 Vessel Access

A wide range of conventional and specialist vessels are available to provide frequent personnel

transportation and access to offshore wind farm developments from an onshore location (e.g. O&M port).

These vessels vary in capacity, speed, and significant wave height (Hs) transferring capabilities and

include:

Quick response vessels (e.g. Rigid Inflatable Boats (RIB));

Work boats (traditional catamarans); and

Small Water-plane Area Twin Hull vessels (SWATH vessels).

A brief review of these vessels is provided in Table ‎5-2.

Examples of onshore-based access vessels are given in Table ‎5-3 and Table ‎5-4.

Autobrow

Type: Work Boat and bridge system Development Stage: Design

Designed by Ad Hoc Marine Designs, developed by Otso Ltd, and

supported by work boat supplier South Boats, the Autobrow is an actively compensated gangway system designed to be lightweight, reliable and flexible, which can be retrofitted to existing vessels with no requirement for vessel or boat landing modifications. Vertical active compensation from a hydraulic system is intended to remove the effects of heave and pitch while passive mechanisms are understood to allow the gangway to

compensate for roll to a limited extent.

BMT & Houlder Turbine Access

System (TAS) Mark II

Type: Bridge system Development Stage: Prototype

This transfer system is a development of the award-winning TAS® system, developed by Houlder with BMT Nigel Gee. The device can be fitted to small vessels without the need for dynamic

positioning capabilities. TAS is understood to utilize a system of

damped rollers and active compensation to reduce the differential motion between the vessel and offshore structure. In this manner, reliance on the standard friction grip between the vessel and boat landing is minimised.


Table ‎5-2: Summary of onshore-based marine vessels for offshore structure access

Vessel Advantages Disadvantages Hs Limit (1)

Quick Response

Vessel

(i.e. Rigid Inflatable Boats (RIB)

Provide fast access to the site

Widely available in the market

More fuel efficient than most work boats

Potential for use as daughter craft

Unsuitable for transit over large distances

Unsuitable for transit in onerous conditions

Unsuitable for transferring spare components and consumables larger than ~50 kg

~0.75m-1.25m

Workboat

(Aluminum or Composite

Catamarans)

Operational experience at most offshore wind projects to date

Can lease vessel on long-term

basis

Widely available in the market

Large work boats can accommodate lifting equipment

Potential to accommodate some access systems

May also operate from fixed offshore bases, floatels or

motherships if these are fitted with boat landings.

Personnel facilities and comfort make it unsuitable for journeys longer than ~2 hours.

0.6 - 1.75 m (1)

Small Water-

plane Area Twin Hull (SWATH) Vessel

Vessels already in use for

commercial and military applications, including at the Bard 1 offshore wind project.

More stable vessel may facilitate

personnel transfer in more onerous conditions

Passenger comfort during transit

improved compared to mono-hulls / catamarans

Can accommodate medium-size spare parts

Potential to accommodate some access systems

Expensive

Large vessel draft

1.0 – 2.0

m (1)

1. Assuming standard turbine access, involving crew stepping from vessel bow to turbine ladder while vessel is driven against turbine, unless otherwise

stated. These correspond to a DNV GL estimate based on manufacturer or supplier claims and/or operational experience. These limitations may vary

with wave period, current, wind, or other access criteria.


Table ‎5-3: Examples of onshore-based access vessels to offshore structures (Windcat)

Windcat Variant MK I MK II MK III MK IV

Length Overall [m] 15 16 18 27

Beam [m] 6.1 6.1 6.1 9.0

Draft [m] 1.9 1.9 1.9 1.7

Classification MCA 2 MCA 2 MCA 2 DNV + 1A1 HSLC

Max Speed [knots] 25 28 28 31

Cruising Speed [knots]

21 25 25 26

Propulsion Propellers / Jets Propellers Propellers Propellers

Accommodation

Seating for 12,

shower, toilet, cooking

Seating for 12,

shower, toilet, cooking

Seating for 12, shower, toilet, cooking, TV,

computer facilities

Seating for 45,

toilet, TV, computer facilities

Deck Crane 2 ton SWL 2 ton SWL 2 ton SWL 2 ton SWL

Figure ‎5-1: The Windcat MKIII.

5.3.2 Helicopter Access

Technicians, small components, and equipment can be hoisted onto the offshore platform or transferred

directly from the helicopter, if a helideck is available.

Table ‎5-5 summarises pros and cons of the helicopter use for O&M in offshore wind farms. Please refer

to Section ‎6.1 for more information about regulations and safety.

Some of the helicopters used to transport technicians to offshore structures include: the Bell 206 L series,

the Sikorsky S76 series, and the Airbus EC135 series. These helicopters typically hold 4 to 13

passengers and are suited to daytime flights over short distances. Their main characteristics are

presented in Table ‎5-6.


Table ‎5-5: Advantages and disadvantages of helicopter use

Advantages Disadvantages Constraints

Operational experience

Can lease helicopter on long or short term basis

Can operate in any sea conditions (not withstanding regulatory

limitations

Short transit times

Suitable for far offshore projects

Fits well with dispersed nature of offshore wind projects

Expensive

Requires winching area or helideck

Only suitable for diagnosis, small component repairs and minor

faults

Possible regulatory / consenting restrictions

Wind speed limit ~40 knots;

visibility > ~4 km

Table ‎5-6: Helicopter specifications

Specification Bell 206L4 EC135 Sikorsky S76D

Passenger capacity1 1 + 6 PAX 1 + 4-6 PAX 1 + 12/13

Length [m] 12.92 12.16 16.0

Height [m] 3.32 3.51 4.4

Rotor diameter [m] 11.28 10.2 13.4

Cruising speed [km/h] 185-225 254 Up to 287

Range [km]2 Up to 600 Up to 600 Up to 800

1 Passenger capacities are approximate and are dependent on aircraft configuration 2 Helicopter range is impacted heavily by payload, wind speed and other factors such as location of alternative emergency landing sites.

Therefore these values are indicative at best and such flight envelope discussions should be held with helicopter operators or specialists.

5.4 Offshore-based Marine Access

Descriptions and examples of the main approaches to offshore-based working are described in Section ‎7

and summarised in Table ‎5-7.

5.5 Other vessels

There are emerging concepts to support O&M of offshore structures. These include vessels, transfer

systems, launch and recovery systems.

Some examples of new O&M specialised vessels are:

The Wind Server Vessel from Fjellstrand, the design is intended to increase the stability of the

bow of the vessel when stationary, making it ideal for transferring engineers to turbines.

Nauti-Craft from Nauti-Craft Pty Ltd., it allows the hulls to conform to the ocean’s surface while

maintaining the stability of the deck for crew transfers.

Substation cranage may have the capability to lift certain components (e.g. lightweight switch gear bays)

between the platform and a vessel. Most medium sized vessels used in the wind industry for transporting

spares could accommodate these components; examples include Anchor Handling Tugs and Offshore

Support Vessels (OSV).


Table ‎5-7: Summary of offshore-based marine vessels for offshore structure access

Vessel Advantages Disadvantages Hs Limit (1)

Floatel

(Converted ferry or small cruise ship)

Large

Plenty of luxury accommodation

Comfortable and stable

Can remain offshore for extended periods

Often used during wind farm

commissioning works where large numbers of technicians are required.

Expensive capital and running costs

Large vessel draft

No direct access to offshore structures

Dependent on smaller vessels

for transfers

N/A

Offshore Support Vessel (OSV), with a basic gangway (2)

(Large work boats)

Large number available within oil and gas sector

Large vessel may facilitate personnel transfer in more onerous conditions

Can remain offshore for extended periods

Potential to accommodate access system (e.g. heave compensated gangway)

Dynamic positioning


Large vessel draft

1.0 – 1.5 m (3)

Mothership, with a basic gangway(2)

Custom-made / adapted vessels with most of the equipment required for offshore wind O&M services

More stable vessel may facilitate personnel transfer in more

onerous conditions

Equipped with daughter craft (e.g. Quick Response Vessels or small Work Boats) to be deployed in benign conditions for rapid access to multiple structures.

Can stay offshore for extended periods

Can accommodate medium size spare parts

Can accommodate access system (e.g. heave

compensated gangway)

Adaptable from vessels in commercial use / concept


Large vessel draft

Limited experience with launch

and recovery systems for regular deployment of daughter

craft. Some experience with davit launched systems particularly for emergency or search and rescue applications

1.0 – 1.5 m (3)

1. Assuming standard turbine access, involving crew stepping from vessel bow to turbine ladder while vessel is driven against turbine, unless otherwise stated.

2. See Section ‎5.2.3 for more information on gangways and other transfer systems.

3. DNV GL estimate based on manufacturer or supplier claims and/or operational experience. Does not account for wave period, current, wind, or other access criteria. Values assume no specialist access system is installed


6 HELIDECK ON AN OFFSHORE PLATFORM

6.1 Helicopters at offshore wind projects

6.1.1 Industry experience to date

Helicopters have been used for many decades for accessing offshore structures in both civil and military

capacities. However, their regular use in the offshore wind sector is limited to relatively few projects,

partly reflecting the relatively early stage of the industry, but also wider concern regarding the

associated safety risks. Currently, helicopters are in regular use at the Horns Rev Project in Denmark,

Alpha Ventus, Global Tech 1 and Borkum Phase 1 (when commissioned) in Germany and Greater

Gabbard in the UK. Additionally, contracts are in place at a number of other projects for provision of a

helicopter for emergency search and rescue services.

The real benefit of accessing offshore structures via helicopter is the inherent insensitivity to wave

conditions coupled with the high transit speeds at which they operate, though lack of motion-induced

sickness is another factor not to be underestimated. Other meteorological conditions, primarily poor

visibility and low cloud base, may restrict operating windows under visual flight rules, but often these

occur during relatively benign periods when access can be made by boat.

6.1.1.1 Helicopters for Accessing Wind Turbines

For wind farms, the good accessibility and quick response time offered by helicopters fits well with the

relatively high-frequency, low effort failures which form a large proportion of wind turbine downtime,

leaving vessels to attend to the less frequent, larger failures as well as the scheduled maintenance

burden. This benefit is further complemented by the ability of a small helicopter to attend multiple,

dispersed structures quickly.

For access to turbines, wind farm operators are initially met with increased operating costs due to the

inclusion of full-time helicopter charter. However, modelling results and industry experience to date

indicate that this increase in cost is often lower than the increase in revenue due to reduced downtime

as a result of the lower exposure to weather risk and speedier transit.

6.1.1.2 Helicopters for Accessing Offshore Substations

Repairs and diagnosis works on offshore substations may be aided by helicopter access, particularly

since the operations and maintenance of the platforms is typically provided by parties not associated

with the wind farm and therefore may lack full time technicians or a dedicated O&M facility, port or

vessels due to the comparatively low maintenance demand. Furthermore, some of the work regularly

undertaken on substation platforms requires specialist or third party personnel and therefore access

delays due to onerous conditions can incur additional costs by keeping these contractors on standby.

The greater equipment redundancy and lower unscheduled maintenance requirements typically seen at

offshore substation platforms are unlikely to justify the costs of a dedicated helicopter for substation

O&M. However, it may be possible to negotiate call-off agreements with helicopter operators to provide

guaranteed access to a helicopter at relatively short notice, thereby avoiding the cost of a dedicated

aircraft whilst retaining the flexibility to charter one at short notice. Clearly the success of such an

approach will depend heavily on ensuring availability of a helicopter at short notice during onerous

weather conditions, when other offshore operators may also be requiring the aircraft.


6.1.2 Future, far shore projects

To date, frequent, far-shore helicopter operations have largely been confined to the offshore oil and gas

industry. However, far-shore wind projects are very different in their requirements to those of oil and

gas. Instead of transporting large groups of technicians to single platforms, offshore wind demands

crews of between 2 and 5 technicians to be transported to many separate structures and occasionally

more to the offshore substation. For relatively near-shore projects the dispersed nature of wind farms is

well catered for by frequent trips with relatively small aircraft such as the Airbus EC135. As projects

move further from land, however, this simple “shuttling” approach becomes less attractive, since

increased transit times require larger helicopters with greater endurance (flying time) and the benefits

from rapid response are reduced, whilst incident risk increases.

Boat access to turbines is still a necessity even when helicopters are available at a project. Indeed, the

rapid, wave-resistant benefits of a helicopter are complimented well by the bulk carrying capacity and

insensitivity to visibility provided by work boats. Therefore, as projects move further offshore the slow

transit times offered by floating vessels will result in a radically different approach to turbine access,

whether helicopters are included in a strategy or not. The manner in which helicopters might assist with

access to far-shore sites is therefore unavoidably interlinked with the chosen approach to floating access.

On this basis, severity of wave climate is the most significant driver for selecting helicopter access, with

distance from shore, and therefore transit time, a secondary factor (except in terms of limits of

endurance for some aircraft).

A likely outcome of far-shore developments will be the requirement for technicians to live offshore in

either fixed or floating accommodation, much like the conventional approach to offshore oil and gas

production. This would open up two potential roles for helicopters:

1. Transporting crews between the onshore and offshore bases during shift changeovers and,

2. Shuttling crews between the offshore base and the turbines.

Clearly the former of these is suited to larger helicopters with substantial technician carrying capacity

and endurance between refuelling, such as the Sikorski S-92 or equivalent, as used for similar

operations at existing oil and gas installations. The second role is much the same as that already used at

near-shore projects, with one important exception; the aircraft must be stationed and refuelled offshore,

placing a number of significant additional logistical and safety requirements on the offshore base. The

use of helicopters to fill these roles mitigates against the impact of sea-state on access to turbines,

which is likely to provide a greater benefit to the project in such exposed, far-shore locations.

Whilst some helicopters are capable of landing on a floating structure, most require the vessel to be

underway and are heavily constrained by vessel motion and therefore sea state. For these reasons,

helicopters in conjunction with floating offshore accommodation (e.g. Offshore Supply Vessels, Floatels

or Motherships) are considered relatively unlikely for day-to-day activities. However, according to

Siemens [‎3], helidecks are likely to feature on their new fleet of Service Operation Vessels (SOVs), with

the intention of using these for regular access to turbines. The type of aircraft and associated operational

parameters and restrictions for operating helicopters in conjunction with these vessels are not known. As

discussed in Section ‎7.3, floating offshore accommodation appears to be emerging as the more favoured

approach to offshore based O&M due to the dual-purpose capability of both accommodation and the

provision of transfers direct to structures. On this basis, whilst feasible for fixed-platform offshore-based

O&M strategies, the role of helicopters for day-to-day O&M tasks at far shore projects is not expected to

be common.


6.2 Regulations and requirements

6.2.1 Regulation helideck and helicopter operations

Part of the Dutch EEZ is located underneath British airspace, but none of the five offshore wind farms

zones analysed as part of this study are located in this area. Only Dutch regulations need therefore to be

taken into account. This said, a European approach is considered in aviation and regulatory bodies may

take ‘foreign’ guidelines into account such as those of the UK Civil Aviation Authority (CAA).

Possible TenneT helidecks will fall under the Dutch Aviation Act1, as is generally applicable to helidecks in

the Dutch North Sea. The competent authority here is the State. In the connected ‘Regulations on the

safe use of airports and other sites’ 2, all helidecks in the North Sea have to comply to the ICAO

Appendix 14 Vol. II (heliports), regarding their construction, design, equipment and use, apart from

some exceptions (mostly for helidecks built before a certain time). The Commissie MER also refers to JAR

OPS 3 subpart E and CAA guideline CAP 764 as relevant guidelines for helidecks.

The Beleidsnota Noordzee 2011-2015 refers the 5 NM obstacle-free zone for helicopter traffic zones (as

described in JAR-OPS 3). At a height of 1500 ft, 5 NM from the platform, the approach will be set in. In

case of a failed approach while flying on instruments, the helicopter requires time to climb again and

restart the approach, and therefore an area of 5 NM should be obstacle-free around the helideck. It is

however stated in the Beleidsnota that in some cases operations can deviate from this, as flying on

visual flight rules (VFR) is also a safe manner of flight, although it introduces some restrictions on the

weather conditions. Some existing wind farms utilising helicopters for regular access are understood to

operate around VFR and therefore, whilst restricted by particular visibility-related weather conditions,

the substation helideck may be located within the wind farm field.

This approach therefore requires the following conditions to be met:

Visibility of 4000 m or more

Cloud base above 600 ft (though somewhat project dependent)

Daylight operating hours.

Recently, the CAA changes their guidelines due to the low survivability of helicopter accidents. In its CAA

CAP 1145 report, it is stated that from 1 September 2014, no offshore flights should be made if the

significant wave height of the sea over which the flight will be conducted to or from an offshore location

exceeds the certificated ditching performance of the helicopter. For offshore wind farms fairly small

helicopters are used, increasing the limiting effect of the restriction3. For example, on a website4 it is

stated that this would restrict flight movements of an Airbus EC155 to sea state 45, which is exceeded in

the North Sea around 27.7% of the time.

The CAP1145 also states that all passengers should be seated next to a push-out emergency window

unless special personal protection equipment is used or the helicopter is equipped with side-floating

emergency floating systems.

1 Wet Luchtvaart, Article 8.1

2 Regels Veilig Gebruik Burgerluchthavens en andere terreinen

3 The CAP1145 also states that all passengers should be seated next to an exit, which is less restricting for these smaller helicopters as most

passengers are seated next to a window. 4 http://aerossurance.com/helicopters/helicopter-ditching-limitations/

5 For sea state 4, significant wave height is 1.25-2.5 m.


Notitie vliegveiligheid6 states that for visual ‘detachment’ of the surrounding obstacles, it is best to place

the platform with a helideck on the edge of a wind farm, not in the middle.

For Helicopter Main Routes a minimal obstacle clearance at 1000 ft is required. The 6-10MW wind

turbines to be expected in the regarded timeframe of 2015-2023 will have a tip height of below 1000 ft

and will therefore not be an issue for Helicopter Main Routes under this guidance.

6.2.2 Unmanned platforms with a helideck

Some specific issues have to be taken into account in the case of a helideck on an unmanned platform.

CAP437 Standards for offshore helicopter landing areas of the Civil Aviation Authority in the UK mentions

that guano and associated bird debris have shown to be a considerable issue for normally unmanned

installations. It can cause degradation of visual aids and can destroy the required friction surface of the

deck if allowed to remain. In the case of an unmanned installation, permanent removal is more difficult

and continuous monitoring of the state of the visual aids and friction surface has to be performed, and

effective preventive measures have to be taken.

Ramboll notes that such preventive measures are considered by their topsides design department for the

5 TenneT substations. A system designed to scare seagulls has proven to be very effective directly

reflecting operator maintenance scheduling.

Such considerations also have to be taken concerning firefighting equipment. As manual control of for

instance foam is not possible, CAP437 recommends automatically activated systems such as nozzles in

the helideck that can effectively cover the entire landing area in foam.

CAP437 also discourages long term storage of aviation fuel on a normally unmanned installation, as the

quality of unused fuel stored offshore deteriorates and fuel quality should be checked every 6 months to

ensure flight safety. Fuel storage also introduces a fire hazard.

6.2.3 Safety

Safety is naturally a preeminent issue contemplated by developers when determining their approach to

turbine access and is exacerbated with distance from shore due to the increased response times of land-

based search and rescue teams as well as the more onerous conditions and greater flying time resulting

in higher risk exposure. In the UK, offshore helicopter accidents in the oil and gas sector in recent years

has raised concern amongst developers and operators and hence, whilst technically favourable in many

cases, the results from risk assessments during project development often lead to avoidance of

helicopters for routine activities. Meanwhile in Germany helicopter use is being widely considered by

developers, particularly in light of the generally greater distance from shore and the often onerous sea

conditions.

Whilst a helicopter is often viewed as a safety risk to an offshore project, they can also be seen as a

dedicated emergency response solution, if suitably specified and equipped for these purposes. Some

operators have agreements with helicopter operators, not for day to day access, but instead for the

provision of search and rescue services. The large area of the North Sea over which oil and gas platforms

are installed has led BP to come up with their “Jigsaw” concept, comprised of onshore and offshore

based helicopters as well as multiple vessels all aimed at minimising rescue times. Whilst such large-

scale solutions are unlikely in the offshore wind sector in the immediate future, it may be possible for

6 Notitie vliegveiligheid in relatie tot offshore windparken, 1 December 2008.


operators to include a helicopter nominally allocated to O&M activity in their emergency response plans,

although such dual-purpose arrangements are likely to be hard to put in practice due to the requirement

for rapid response of appropriately equipped aircraft to an emergency situation.

6.2.4 Placement of the helideck

According to the International Civil Aviation Authority (ICAO) Annex 14 Vol II, no obstacles should

extend past the heliport level height in the immediate vicinity of the helideck and there should be a 180

degrees obstacle free arc with the origin at the centre of the final approach and take-off area. Within this

180 degree section, the vertical descending gradient should have a ratio of 5 vertical units to 1

horizontal unit. The helideck should also be placed as far from the disturbed platform as possible.

In the CAA helideck design considerations paper7 it is noted that the easiest way to ensure this is to

locate the helideck in a corner of the platform with maximum overhang. The helideck should be located

at a height above surrounding structures to avoid turbulence downwind. An air gap between the helideck

and the superstructure underneath is required for beneficial flow over helideck. Without the air gap, the

wind conditions above the helideck are likely to be severe (see Figure ‎6-1).

Figure ‎6-1: Air gap, Source CAA helideck design considerations –

environmental effects (CAA paper 2008/03)

6.3 Helidecks and winching platforms

The size of current offshore turbines precludes an economically viable helicopter landing platform

(helideck or helipad) on the nacelle and therefore access to turbines is restricted to heli-hoist operations

(HHO) onto winching platforms (where installed). Meanwhile broader structures such as oil and gas

platforms or offshore substations are more easily adapted to have helidecks, which are generally

favoured from a safety perspective. Helidecks (and with respect to nearby obstacles, heli-hoist platforms)

are aircraft specific, with each helideck designed for the dimensions and loading associated with a

particular model of helicopter.

At some existing offshore wind projects the helideck on the offshore substation is a critical aspect of the

wind turbine access strategy. This is due to a legal requirement to perform HHO under a single-engine

redundancy condition, in other words the aircraft must be able to remain in hover in the event of a

complete failure of one engine. Due to the dependency on aircraft payload to fulfil this requirement, it is

7 CAA helideck design considerations – environmental effects, CAA paper 2008/03.


sometimes necessary to temporarily land on the offshore substation (or other structure with a helideck)

and drop off a crew of technicians prior to performing the first HHO. These technicians are then collected

and the second HHO may be performed. The same process is adopted in reverse for the return journey.

For this reason, helidecks on offshore substations or other nearby structures can perform a useful role

for the transfer of technicians to turbine structures. However the extent of this is clearly heavily

dependent upon:

The wind farm operator’s choice of access strategy (including associated risk assessment)

The aircraft deployed at the project

The number of technicians per crew

The distance from the heliport

The distance from an alternative emergency landing location

Some offshore substations feature heli-hoist platforms. It should be noted that according to CAP 437,

wherever practicable, helicopter hoisting should not be employed as the standard method for transfer of

personnel. Instead a helideck should be used to reduce the time spent hovering. It will therefore not

provide improved accessibility to the substation itself. CAP 437 also states that when the heli-hoist

platform is located above accommodation spaces, these spaces need to be cleared during the hoisting

operation.

As discussed in Section ‎4, the substation maintenance requirements are considered to be sufficiently low,

and largely centred around less time-critical, periodic maintenance, that the benefits of helicopter access

for O&M activities on the substation are expected to be relatively minimal for most sites, particularly if

located in benign or moderate metocean climates. However, inclusion of a heli-hoist platform may

provide a more cost-efficient solution than a helideck to emergency response. From an emergency

response perspective, the benefits of a helideck or heli-hoist platform would need to be assessed as part

of a detailed design risk assessment process, including input from relevant helicopter operators.

6.4 Summary of Helicopter Logistics

From a regulatory perspective, the addition of a helideck is not restricted. The structure should be

carefully located on the superstructure in the design phase. The implications of adding a helideck to a

normally unmanned platform and the associated maintenance burden should be noted.

Due to European cooperation concerning regulations and guidelines on aviation, the restrictions on

offshore helicopter flights as stated by the CAA could be taken into account for the Dutch EEZ as well.

This would mean that, apart from the requirements of flying under the meteorological limitations

imposed due to adherence to visual flight rules, restrictions are also placed on the helicopter-specific sea

state permitted for safe offshore flight.

The distances from shore of the 5 proposed TenneT substations are well within the operational range of

commonly used twin-engine helicopters, such as the Airbus EC135, provided the helicopter landing site is

not located far inland.

The substantial size of offshore substations makes them well suited to helidecks or heli-hoist platforms.

The greatest benefit of a helideck is likely to be in support of helicopter logistics at the wider wind farm

project, whilst benefits of a heli-hoist platform will be largely limited to O&M of the substation platform

itself. Both the wind farm and the offshore substation may benefit from a helideck due to the improved


safety case, although any benefit to the emergency response plan will be heavily dependent upon

matching the helideck design to the type of aircraft to be used for emergency response purposes.

The greater size, weight-bearing characteristics and location over the edge of the substation will

inevitably result in a much higher cost for a helideck, than a heli-hoist platform. Costs associated with

the fabrication and installation of a helideck are anticipated to be around €1M - €2M, with a heli-hoist

platform expected to be in the region of €200k - €500k. Nevertheless, with respect to the wider

substation and associated structure, neither option is expected to be a significant contributor to the

overall CapEx.

6.5 Estimations workability helicopter versus vessel

In section‎ 5, the operational limits of the different options for access were discussed. If these operational

limits are combined with climate data for the Dutch part of the North Sea, a rough estimation can be

made for the workability and specifically the difference in workability for the different types of vessels

and helicopters.

A moderate North Sea climate data set has been chosen to make this comparison. In this data set wind

speeds, visibility and Hs are given hourly over a 7-year period. In Figure ‎6-2, the workability of a vessel

versus its significant wave height Hs operational limit is shown for this moderate North Sea climate as an

example. Workability is defined here as the percentage of the time the operational limits are not

exceeded. From the example in Figure ‎6-2, the workability of a vessel with an Hs limit of 2 m is 70.9%.

Figure ‎6-2: Accessibility as the percentage of the time the significant wave

height limit on the x-axis is not exceeded.

In Table ‎6-1 the operational limits are listed from the main access possibilities. The helicopter access is

separated into two cases:

Helicopter: Operational limits based on visibility, wind speeds and daylight operation, following

current regulation.

Helicopter restricted: Operational limits based on current UK regulations, with the added limit of

the sea state limit following the ditching performance as stated by the CAA (see section ‎6.2.1).

0%

20%

40%

60%

80%

100%

0 2 4 6 8

Ve

sse

l acc

ess

ibili

ty [

%]

Significant wave height limit vessel [m]


Access Operational limits Technicians Parts limits

Work boat Hs < 0.6 – 1.75 mm Max 12 0.1 - 2.5 t

Offshore Support

Vessel with motion

compensated

access system

Hs < 2 – 3 m Use of knuckleboom

crane on vessel, crane

on substation. Limit

around 5 t.

Helicopter Visibility > 4000 m, wind

speeds < 21 m/s, daylight

Around 4 Around 200-300 kg

Helicopter

restricted

As above, with ditching

performance accounted: Hs

<2.5 m

Around 4 Around 200-300 kg

Table ‎6-1: Main access options and operational limits.

Combining the Moderate North Sea climate with the operational limits from Table ‎6-1 gives a rough

estimation of the difference in accessibility by a helicopter or a vessel. It should be noted that it has

been assumed that the daylight hours is a fixed number of 13 hours per day; there is no seasonal

change.

Figure ‎6-3: Rough estimation of the extra accessibility of a helicopter

Figure ‎6-3 shows the results of this rough estimation. One can see that there is a significant accessibility

difference between the helicopter and a workboat limited to an Hs of 0.6 – 1.75 m, with an improvement

of approximately 15% and 22% for the restricted and unrestricted cases respectively at a work boat Hs

limit of 1.5m. For the larger Offshore Support Vessels with an access system, this difference diminishes

to 4-8%. If the stricter rules stated by the British CAA for British airspace will apply in the future in

Dutch airspace, it is not expected that a helicopter will still give a higher accessibility verses the special-

purpose Offshore Support Vessels.

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

0 1 2 3 4 5 6

Esti

mat

ed

imp

rove

me

nt

in a

cce

ss w

ith

re

spe

ct t

o w

ork

bo

at [

%]

Significant wave height limit vessel [m]

Helicopter

Restrictedhelicopter


Clearly this difference in accessibility is only particularly relevant for the unscheduled maintenance, since

scheduled maintenance, by definition, is less critical and should not incur downtime due to waiting on

weather. Rapid response to a failure is important since downtime is likely to lead to a compensation

claim which could be around 1 million € per day. A key issue for the offshore wind industry is that

generally access is least likely during high-wind periods when production potential is high, effectively

amplifying the resultant lost production.

It should be noted that this high-level estimation only states an estimation of the percentage of time

certain operational limits are exceeded; it does not evaluate an estimation of the period this exceedance

will last (persistence). Therefore it should be viewed purely as indicative of the benefits of the alternative

solutions.


7 OFFSHORE ACCOMMODATION FACILITIES

7.1 Offshore accommodation for offshore wind

Offshore-based operations are likely to be essential for far-shore projects due to the considerable transit

times associated with regular access to these sites and associated wellbeing of technicians (sea sickness,

fatigue, etc.). The offshore oil and gas industry has laid the foundations for this approach with the

regular use of both fixed and floating offshore accommodation at many well sites. However, the

demands from offshore wind projects are very different from those of offshore oil and gas platforms,

especially in regard to the dispersed nature of wind turbines, where teams of 2-5 technicians and

replacement parts and consumables may be required on multiple structures, as opposed to 20+

technicians on one large installation.

It is extremely unlikely that the maintenance demands of the offshore substation will be sufficient to

justify offshore-based working for the substation alone and therefore offshore accommodation would

need to provide benefit to the surrounding wind farm(s) as well to make economic sense. Therefore,

even when technicians are living in the vicinity of the project, the issue of accessing turbines remains.

Furthermore, particularly with fixed offshore accommodation, vessels (and conceivably helicopters) are

required to shuttle technicians and parts between the offshore base and the turbines. This can either be

achieved by vessels (and helicopter) arriving at the platform from a shore-based location at the start of

the shift or by installing some form of launch and recovery system for deploying and retrieving vessels

on and off the base, a technology for which there is minimal precedence for the purposes of day-to-day

use.

Descriptions of the main approaches to offshore-based working are presented in the sub-sections below.

7.1.1 Fixed platforms

One solution to the issue of sizeable transit times when accessing far-shore projects is the use of a fixed

offshore accommodation platform on which technicians live for typically 2 – 4 weeks at a time before

being replaced at a shift change. Such platforms are relatively well understood structures within the oil

and gas industry and typically use either monopile or jacket foundation concepts. There may also be the

opportunity to add accommodation to offshore substation(s) associated with an offshore wind project,

thereby minimizing the total number of separate structures and associated cost. However, this can lead

to issues relating to the safety of living in close proximity to high-voltage equipment due to exposure to

EMF limits and increased risk of fire which could drive the need for separate accommodation, as well as

the ease of crane access to large equipment (e.g. transformers) for maintenance purposes.

As identified in Table ‎3-1, Horns Rev 2 in Denmark and Global Tech 1 and Dan Tysk (once fully

commissioned) in Germany are currently the only offshore wind projects to be using bespoke fixed

offshore accommodation for the purposes of regular O&M activities. At Horns Rev 2 the accommodation

platform has been located immediately adjacent to the offshore substation, such that boat landings, a

helideck and some emergency equipment may be common to both structures. Similarly the

accommodation module at Global Tech 1 is located on the offshore substation, reducing the number of

independent structures. Work boats are understood to transit to the offshore platforms from an onshore

base, arriving offshore at the start of the shift and thereby maximizing the time available to technicians

for working on the turbines.


Figure ‎7-1: Accommodation Platform (right) and Substation (left) at Horns Rev 2.

(Source: energinet.dk)

It is understood that the helideck on the offshore substation at Horns Rev 2 is used for conveying

technicians and small parts between shore and the offshore platforms, but access to turbines at Horns

Rev 2 is achieved by vessel. As discussed in Section ‎6.1.2, such an in-field helicopter “shuttle” service

would provide the additional merit of limited impact of sea-state conditions, but is likely to require

considerable additional equipment on the offshore platform for the purposes of re-fuelling, maintenance,

sheltering, fire-fighting and weather monitoring.

An alternative approach to fixed offshore accommodation is to use a jack-up vessel as an

accommodation platform. This has the benefit of keeping a jack-up vessel on site in case of the need for

major component exchange operations, but requires ownership of a jack-up solely for the use of that

particular project or a sharing agreement with neighbouring projects, both of which can be costly

solutions.

7.1.2 Floating offshore accommodation

Offshore accommodation vessels are widely used in offshore oil and gas as well as offshore construction

projects and have developed a wide variety of alternative names and characteristics. Three alternative

approaches to offshore-based accommodation and access are described in more detail as below.

7.1.2.1 Floatels

Floatels are one name given to vessels intended solely for the accommodation of a significant number of

personnel. Often converted ferries of cruise ships, these feature luxury, hotel-like accommodation and

are kept at anchor within the vicinity of a project as required while retaining the ability to sail to port

under their own power for crew changes, restocking or sheltering from storms. They typically feature a

boat landing in one or more locations on the hull to facilitate transfer of technicians to and from the work

boats.


Source: DNV GL

Figure ‎6-5: Floatel Wind Ambition used at London Array Phase 1

DNV GL is unaware of any floatels on long-term contracts for O&M purposes at a particular offshore wind

project. However, it is not uncommon for such vessels to be relied on during construction and

commissioning works as well as for concerted maintenance campaigns.

7.1.2.2 Offshore Support Vessels

These vessels are considerably larger than traditional work boats though they do not typically

accommodate as many technicians as the larger floatels. OSVs are designed to operate in harsh climates

and to stay at sea for periods of a week or more. As such, they are fitted with personnel living quarters,

kitchen, bathrooms, and entertainment facilities. Such vessels are typically upwards of ~50 m in length,

and will require Dynamic Positioning capability with redundancy (DP2 or DP3) to operate in proximity to

wind turbines or other offshore structures. The major differentiator from floatels is that OSVs are

normally equipped with access systems that facilitate personnel and part transfer to the turbines.

Chevalier Floatels is a Dutch-based company with 10 years of experience in the floatel industry.

Currently, Chevalier Floatels has developed two state-of-the-art vessels for the offshore wind and oil and

gas industries, the DP Galyna (shown in Figure ‎7-3) and the Gezina. The DP Galyna is 68 m in length

and includes an Ampelmann heave compensated gangway with a reliable DP2 system which transfers the

technicians to the wind turbines or platforms. Accommodation is provided for up to 55 passengers

(including 16 crew members). Boat landings for work boats or other support vessels are mounted to the

hull of such ships can be used to increase their versatility.

The Wagenborg Walk to Work vessel is equipped with an Ampelmann system for crew transfer up to a

wave height of 2.5m and has the possibility to be additionally fitted with a helideck. It can accommodate

40 technicians and parts. Recently the NAM/Shell UK has chosen the Wagenborg Walk to Work vessel for

servicing their 52 platforms in the Southern North Sea, mainly due to the newly imposed restrictions on

sea state deployment for offshore helicopter flights of the CAA (see Section ‎6.2) and the advantage of

moving personnel together with the required parts for maintenance [‎3].


Figure ‎7-3: Chevalier DP Galyna Concept with Ampelmann Access System

(Source : http://www.cfbv.com)

Figure ‎7-4: The Wagenborg walk to work vessel. (Source: www.wagenborg.com)

Other, recently commissioned OSVs include two Esvagt vessels recently constructed for Siemens in

support of two of their German Offshore Wind projects, with another in the pipeline.

7.1.2.3 Motherships

As opposed to floatels, which are solely designed for accommodation of a large number of passengers

and to OSVs which are equipped with specific tools required for the offshore industry, motherships are

designed specifically for the deployment of smaller vessels or “daughter crafts”. To achieve this, a launch

and recovery system is required to allow the daughter crafts to be deployed and retrieved as required. In

the simplest case this takes the form of a deck crane lowering these daughter crafts over the leeward

side of the mothership with a quick-release mechanism to allow each daughter craft to move off safely

on the water. Such mothership vessels are typically upwards of ~70 m in length, and are therefore able

to accommodate a large number of passengers.

This approach fits well with the dispersed nature of offshore wind projects, since multiple crews can be

deployed at any given moment. However, historically this approach has had limited value to other

offshore industries and hence is currently an emerging technology, with minimal experience to date. The

principles of deploying daughter crafts are, however, well tested within the military and emergency

rescue sectors, as indicated by the image in Figure ‎7-5, wherein a daughter craft is launched from an

offshore support vessel.

http://www.cfbv.com/vessels.html

http://www.wagenborg.com.3.cdn.iwink.nl/uploads/fancybox/1e919107-dffd-4b54-b1a8-7b32c8a0b773/2801431319.jpg


Source: DNV GL

Figure ‎7-5: Launch of a daughter craft from an offshore support vessel

In more onerous conditions where safe deployment or transfers using daughter crafts is not feasible,

transfers might also be undertaken using specialist access systems such as an Ampelmann system

installed on the motherships. However, as with an OSV, a fully redundant dynamic positioning (DP2)

system would be an essential prerequisite to this.

The Damen Walk to Work vessel is an example of a crossover to the mothership approach and the

offshore support vessel. It carries a daughter craft for access to the turbines, and it employs a motion-

compensated platform as well for the direct transfer of personnel. In its product sheet, it claims a 3.1m

significant wave height limit for transfer. The 90m long vessel can accommodate 40 passengers as well

as its 20-person crew.

Figure ‎7-6: The Damen walk to work vessel. (Source: www.damen.com)

7.2 Regulations and requirements

In the previous permit procedure for offshore wind farms in the Dutch EEZ (all permits under this system

have now been revoked), the transformer station was part of the wind farm in the application under the


Water Act8. For the guidelines to platform accommodation facilities, DNV GL has looked at DNV-OS-J201,

which has been used for certification for several offshore wind transformer platforms.

In these guidelines, it is stated that the platform should be divided into areas according to their hazards

and the type of activities. The accommodation facilities should be separated from the other areas and be

located as far as possible from hazards, making the risk as low as reasonably possible. On the Global

Tech 1 platform, the only transformer station found in offshore wind projects with on board

accommodation, the accommodation facilities are located on the upper deck, separated from the other

areas.

All platforms have to have minimum provisions, including protection from weather, vibration, noise and

strong electromagnetic fields, an emergency toilet, emergency rations of water and food, sleeping bags

and desk space for working with computers. All platforms have to have a temporary refuge area in the

case of an emergency, with adequate communication facilities.

In the case of accommodation facilities, the spaces should have adequate ventilation, heating and

lighting and protection from vibration, noise, electromagnetic fields, fumes and inclement weather. The

fire requirements are stricter, reflected in the requirements on walls and flammability, as persons needs

to have enough time to escape in case of emergency. For such manned installations, a sprinkler system

will be required. Hazardous activities, such as refuelling of a helicopter, should be planned away from the

accommodation.

7.3 Summary of Offshore Accommodation

Offshore accommodation becomes more economically attractive with distance from O&M port and wind

farm project size, with most projects further than about 30NM to 40NM (55km – 75km) from port

expected to be reliant upon offshore accommodation to avoid excessive travel times and low productivity

due to sea sickness and fatigue. Such offshore accommodation further reduces inefficiencies by providing

an on-site parts and consumables store. Whilst it is extremely unlikely that the maintenance burden of

an offshore AC platform would justify dedicated offshore accommodation, when coupled with the

requirements of the surrounding wind farm(s), the benefits of reduced transit time may start to justify

the large CapEx implications.

Offshore accommodation can take two basic forms, either a fixed platform or floating accommodation

with a variety of different vessels available. Crucially, fixed platform accommodation reduces transfer

time and therefore also the likelihood of sea sickness, but it does not itself increase the sea states in

which transfers to turbines can be achieved. For this reason, current industry trends suggest that the

market is moving more towards floating accommodation configured to provide the dual purpose of

accommodating technicians and providing direct, safe access to offshore structures in higher sea-states

than could normally be achieved by traditional work boats. A further benefit of floating accommodation is

likely to be the potential to operate at night, due to the intrinsically safer “walk to work” approach

enabled by the use of a specialist access system such as the Ampelmann or similar.

To date only Global Tech 1 is known to include permanent accommodation on the offshore substation.

Whilst this solution can present a substantial cost saving due to the reduced number of individual

structures, the safety risks from fire and electro-magnetic field exposure as well as the difficulty of

overhead access for replacing major electrical components can make this a tough engineering challenge

and requires careful consideration as part of a detailed risk assessment process.

8 Waterwet


Accommodation modules are required to be separate from the other areas, with risks as low as

reasonable possible. This requirement can be seen translated in the Global Tech 1 station, where the

accommodation constitutes the entire upper deck of the structure. In case of major component

replacement (e.g. transformer), this entire upper deck may have to be lifted off.


8 CONCLUSIONS AND RECOMMENDATIONS

8.1 Background and Overview of Existing Projects

TenneT are seeking to construct 5 offshore substation platforms, each with up to 700MW of

capacity up to 38km from the nearest coast. These substations are substantially larger in power

capacity per platform than any existing offshore AC substations.

The benefits for the inclusion of a helideck, heli-hoist platform or accommodation module have

been reviewed within this study, both in terms of benefit to the substations themselves and the

wider benefit to the offshore wind farms they support.

A review of existing and under-construction projects has been undertaken to determine any

trends in helideck or accommodation provision.

8.2 Substation Maintenance Requirements

DNV GL estimates an average of 10 to 30 days of scheduled maintenance per year to be required

for a substation of this scale. Considering these relatively low scheduled maintenance

requirements and that the shortest frequency required is expected to be monthly or less, it is

clear that the addition of a helideck or accommodation facilities to the offshore substations would

not provide significant benefits to the operation of the substation platforms themselves.

Assuming good levels of redundancy are implemented within the configuration of the substation

power and SCADA systems, most failures will not incur production losses and therefore the repair

or replacement of such components can often be carried out as scheduled maintenance or

scheduled access to the substation, reducing or even eliminating the benefit provided by quick

transfer of technicians by helicopter.

A major failure on the offshore substation, as occurred at the Nysted Offshore substation in 2007,

will require specialist technicians, vessels and replacement components and therefore is

constrained much more by mobilisation of the necessary resources than rapid deployment of

technicians from offshore based accommodation or by helicopter transfer.

8.3 Helidecks and Heli-hoist Platforms

8.3.1 Considerations

Currently, helicopters are in regular use for turbine O&M purposes at the Horns Rev Project in

Denmark, Alpha Ventus, Global Tech 1 and Borkum Phase 1 (when commissioned) in Germany

as well as Greater Gabbard in the UK. Additionally, contracts are in place at a number of other

projects for provision of a helicopter for emergency search and rescue services. It is not clear

whether projects which feature helidecks utilise these for maintenance of the offshore substation,

although such use is anticipated.

Projects utilising helicopters for the O&M of wind turbines are understood to have found them to

be cost effective.

Helicopters provide fast response to small repairs or diagnosis works where large parts or tools

are not required. Their greatest benefit to offshore wind projects has historically been their

insensitivity to sea states.

Recent incidents in the North Sea oil and gas industry have led to recent changes in regulation

with respect to the sea states in which helicopters may be deployed. This has now become a


much more aircraft-specific factor, limited by the lesser of sea-state 6 or the certified ditching

performance of the helicopter, understood to be the sea state in which the aircraft may remain

floating upright in the water. For example, the Airbus EC135 is understood to be limited to sea-

state 4, comparable to the access limitations from specialist marine access systems such as the

Ampelmann in conjunction with large vessels.

In most cases it is believed that a helideck on the offshore substation is largely intended to

facilitate helicopter access to the turbines and to support emergency response procedures as

opposed to solely providing access to the substation platform itself. For many aircraft, payload is

limited for undertaking heli-hoist operations due to a requirement to be able to maintain hover in

the event of one engine failing. On this basis, some operators are known to temporarily drop off

technicians on the substation helideck prior to performing heli-hoist operations. In this case, the

platform acts as central helideck, not limited by this payload restriction.

According to CAP 437, wherever practicable, helicopter hoisting should not be employed as the

standard method for transfer of personnel, suggesting that a helideck is the only option if regular

access by helicopter is to be adopted. For turbines a helideck is clearly not a viable option and

therefore heli-hoist platforms provide the only solution for helicopter access.

Helidecks, and to some extent heli-hoist platforms, are aircraft specific and must be designed for

the dimensions and loading requirements of the aircraft with which they shall be used.

Helidecks (and heli-hoist platforms) require maintaining and certification with particular

emphasis on fire fighting, visual aids and surface friction. This may require frequent maintenance

visits to remove guano and check equipment.

In all cases the use of a helicopter for O&M purposes at an offshore wind farm is heavily subject

to the design risk assessment conducted by the developer of the project and associated advisors.

To date, helicopters have been much more widely considered by developers in Germany than in

the UK.

There are no clear trends for the number of projects featuring helidecks or heli-hoist platforms

with distance from shore. However, there is a strong trend with respect to project capacity, with

all projects greater than 400MW and the majority of projects above 300MW featuring either a

helideck or a heli-hoist platform on the associated offshore substation(s).

For the maintenance of the platform itself, the helideck will give an additional accessibility of

roughly 4-8%. However, if stricter sea state regulations apply in the Netherlands as they are

now enforced for British airspace, this additional accessibility advantage will be minimal.

Due to the dependency upon wind farm developer operating preferences, an alternative option to

committing to the installation of a helideck at this stage may be to design the structure to

facilitate a helideck, but omit the helideck itself. In this fashion the platform would be designed

to take a helideck, with necessary hard points and load bearing capacity but the fabrication and

installation of the helideck itself can be excluded from cost estimates. This way flexibility is

retained to fit a helideck at a future date either as part of onshore fabrication, or conceivably

once installed offshore. Clearly some contractual arrangement for the provision of a helideck

would need to be agreed with the wind farm developer in this event. However, the additional

costs to ensure the primary steelwork can accommodate the added loads due to a helideck will

depend heavily upon the overall structural design of the platform and may limit the benefits of

this approach.


8.3.2 Conclusions and recommendations

For the maintenance of the platform itself, the helideck will have limited advantages, especially

in the case of stricter sea state regulation for helicopters.

The primary reason for considering a helideck on an offshore substation is therefore the support

of O&M logistics at the wind farm. For instance technicians could be dropped off at the platform

to reduce the payload and make hovering for a heli-hoist to the turbines possible.

Therefore the case for installing a helipad is not clear-cut and is likely to be heavily driven by

wind farm operators and detailed safety reviews.

For large substations, such as those proposed by TenneT, the additional cost of a helipad is

anticipated to be in the region of €1M to €2M and therefore may be justifiable on a percentage

cost basis to retain future flexibility.

8.4 Offshore Accommodation

8.4.1 Considerations

Only 3 offshore wind projects to date feature offshore accommodation and with the exception of

Horns Rev II, these are located more than 70km from the coast, reinforcing the assumption that

distance from O&M port is a primary driver.

The proposed TenneT platforms are located comparatively close to shore and therefore, unless a

suitable O&M port is much further away, there is no strong requirement for offshore

accommodation in order to maintain the substation platform or adjacent wind farms.

Accommodation for the maintenance of solely the substation is not justified due to the relatively

minimal anticipated maintenance requirements. Therefore any offshore accommodation module

would need to be primarily intended for use by the wind farm.

Emerging trends suggest that the industry is pursuing floating offshore accommodation in

preference to fixed accommodation.

Inclusion of an accommodation module on the same structure as the offshore substation has

only been adopted at Global Tech 1 to date.

The reduced cost through combining the offshore substation and accommodation module on one

platform is anticipated to be counteracted by the increased design challenges of ensuring safety

of all personnel against fire and electrical faults, minimising long-term exposure to

electromagnetic fields and ensuring access for maintenance to the heavy major electrical

components.

8.4.2 Conclusions and recommendations

From the results of this review, DNV GL believes that the costs and other constraints associated

with the installation of an accommodation platform, either on the same structure as the offshore

substation or as an independent structure, are unlikely to justify the benefits at the proposed

TenneT project sites.


Instead, the adoption of a strategy where the majority of scheduled maintenance at all 5

proposed platforms is performed as part of an annual maintenance campaign, for which the

chartering of an OSV or similar vessel providing access and accommodation will likely prove

more cost effective. Alternatively a similar solution may be adopted at each platform

independently by collaborating with the associated wind farm owner for the purposes of the

annual scheduled maintenance campaign, nominally to be performed during summer months for

improved access and minimal loss of production.


9 REFERENCES

1. “Material Handling, Component Inspection and Maintenance Overview. Study Reports, Offshore

Platform – HelWin Beta”, prepared for TenneT by Siemens, Ref: 110193-21-LS-M0002 , 22-01-

2015.

2. N. Andersen, J. Marcussen, E.Jacobsen, S. B. Nielsen, Experience gained by a major transformer

failure at the offshore platform of the Nysted Offshore Wind Farm, Presented at 2008 Wind

Integration Conference in Madrid, Spain.

3. “Siemens’ new service operation vessels are making waves in the industry”, ref:

http://www.energy.siemens.com/hq/en/energy-topics/energy-stories/sov.htm

4. Interview Johan Adriaanse, Director Operations at Wagenborg Offshore, 23 April 2015.

5. Email Nicolai Gullitz, Marine Engineer at Ramboll, 1 June 2015.

http://www.energy.siemens.com/hq/en/energy-topics/energy-stories/sov.htm


APPENDIX A DNV GL DATABASE OF OFFSHORE SUBSTATION

MAINTENANCE REQUIREMENTS

CONFIDENTIAL

About DNV GL Driven by our purpose of safeguarding life, property and the environment, DNV GL enables organizations

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