Anchor Penetration Trial in the German Bight to Optimise Cable Burial Depth ICPC Plenary – Dubai, U.A.E | | A. Drews, M. Petzold, D. Sinclair

TenneT is Europe´s first cross-border grid operator for electricity. With approximately kilometres of (Extra) High Voltage lines and 36 million end users in the Netherlands and Germany we rank among the top five grid operators in Europe. Our focus is to develop a north-west European energy market and to integrate renewable energy. Taking power further The energy turn around depends on a successful grid expansion.

TenneT‘s tasks offshore Anchor Penetration Trial | ICPC Plenary -Responsible for the grid connection of offshore wind farms in the German sector of North Sea -HVAC and HVDC grid connection -~ 1500 km of HVAC or HVDC subsea power cables installed, contracted or tendered (without interconnectors) -~ preplanings for another 1800 km of cables

Anchor Penetration Trial | ICPC Plenary Burial Depth Requirements in Germany for Power Cables Export Cables & Innterconnetor Offshore (from ~10 m depth contour) - 1,5 m DoB outside traffic separation zones - 3 m DoB inside traffic separation zones - to avoid external threats For comparison: Waddensea: 1,5 m DoB Nearshore: 1,5 – 5 m DoB - in dependence of sediment mobility -No individual risk based assessment of burial depth for cables required -Assessment of likelyhood of damage by anchors required -Statistical approach vrs. single case

Anchor Penetration Trial | ICPC Plenary Motivation Challenges of 3 m DoB -3 m is no industry standard, no garantee reaching 3 m -seabed conditions – stiff clays in parts of the routes -no dredging due to environmental constraints -remidial trenching exposes cable to a higher risk of damage during installation -survey – higher measurement uncertainties -repair – longer times for recover and new reburial challenges Advantages of a reduced burial depth -meets industry standards in cable burial -larger market to deploy burial tools -time reduction in cable laying operations -time reduction for cable recover in case of a repair, easier reburial -less disturbance for ship traffic

Participants in the offshore operation – Anchor Penetration Trial | ICPC Plenary -management of offshore operation -anchor pull -sonar and video by ROV support -operation of sediment echo sounder -scientific support -operation of FS WEGA -operation of side scan sonar -scientific support -documentation of trials -scientific support

Chosing anchors by determination of a representative ship Anchor Penetration Trial | ICPC Plenary -Traffic analysis to determine a „design ship“ -Cargo Ship with dwt -> Chose of an Hall anchor of 11.7 t according to GL tables on correlation on anchor mass and dead-weight of ships FractileLenght (m) dwt (t)TEUTmax (m) 50 %< ,6 75 %< ,8 80 %< ,8 90 %< ,5 95 %< ,5

Anchor Penetration Trial | ICPC Plenary Anchors with chain 11,7 t Hall Anchor & 8,4 t AC 14 HHP ~60 m anchor chain (81 mm Gr. III) with a weight of ~2 t m steal wire ~364 cm ~ 322 cm ~280 cm holding capacity – Hall: 4-6 times its mass-weight – 470 kN to 690 kN, AC 14: 8-11 times ist mass-weight – 620 kN to 950 kN ~289 cm

Test Sites – Overview & Soil properties Anchor Penetration Trial | ICPC Plenary VTG: dense sand underlayed with stiff clays layers water depth: 25 m BSH Nord: loose layered fine sand water depth: 35 m BSH South: medium dense to dense sand with silt water depth: 28 m Test sites (soil densities) are representative for all TenneT cable routes.

Ships and their functions (1) Anchor Penetration Trial | ICPC Plenary Esvagt Connector -pulls the anchor -bollard pull of 107 t -Water pressure sensors on anchors -in service for TenneT Guardian -Multibeam -ROV with Video and Sonar -in service for TenneT

Ships and their functions (2) Anchor Penetration Trial | ICPC Plenary FS WEGA -Sediment Echosounder -Side Scan Sonar -operated by BSH and in service for BAW Karen M Guard Vessel for the trials in the traffic separation zone - in service for TenneT

Trial Program conducted (17 positions, 21 pulls) Date GMT+2 Test Start E Start N :12:00 N1-AC :48:00 N2-Hall :32:00 N3-AC :19:00 S1-AC :20:00 S2-AC :22:00 S3-AC :30:00 V1-AC :24:00 V3-AC :19:00 V5-AC :32:00 V2-Hall :55:00 V4-Hall :24:00 V6-Hall :45:00 S4-Hall :48:00 S5-Hall :54:00 S6-Hall :12:00 N5-Hall :23:00 N6-Hall Anchor Penetration Trial | ICPC Plenary Coordinates from GPS Log Esvagt Connector

Trial program – Pre-Survey Seabed properties on trial areas are known by surveys covering the test side or data from wind farms close by Anchor Penetration Trial | ICPC Plenary Offshore Pre-Survey: - Side Scan Sonar and Sediment Echosounder on every test site to - assess soil conditions - detect obstacles - determine position for anchor drop Example here: VTG test side

Execution of Trials close to real anchoring - Anchor was lowered to ~10 m above seabed and dropped ̵ also no free fall in a real case – chain, leads and water column give resistance to a free fall – (pre-calculations to assess the deviation from reality) - Investigation of anchor position and chain by sonar and video ̵ worst case -> highest penetration depth can be reached with a straight anchor chain and a well positioned anchor horizontal on the seabed ̵ alignment of anchor and chain by ship movement ̵ in a real case anchor droppes into a random position on the seabed - Start of pull by forward move of the ship „Esvagt Connector“ - pull until breakout of anchor - pulling forces limited to 80 t to recover the anchor Anchor Penetration Trial | ICPC Plenary

Trial Program – Post Pull Survey Anchor Penetration Trial | ICPC Plenary -Side Scan Sonar, Multibeam, and Sediment Echo Sounder survey of anchor tracks -Video on final position of anchor AC-14Soll PositionDrop PositionFinal Position Pull Distance in Meter Force in Metric Tons UTM EastUTM NorthUTM EastUTM NorthUTM EastUTM North V ,00max 78 Multibeam Record Example: VTG Position 3 | AC 14 anchor chain

Side Scan Sonar - VTG Position 3 | AC Anchor Penetration Trial | ICPC Plenary m m drop position final position - SSS used to plan SES lines

Sediment Echo Sounder - VTG Pos. 3 | AC Anchor Penetration Trial | ICPC Plenary SSS Max. penetration depth measured by SES: 0.19 m

Video - VTG Position 3 | AC Anchor Penetration Trial | ICPC Plenary - anchor fully covered with a clay load on base after pull

Penetration Depth recorded Anchor Penetration Trial | ICPC Plenary - pulling force complies and exceeds the holding power of the anchors –> therefore, results are realistic and comparable to a real anchor event!

Trial Results Measured Results - anchor penetration depth is less 1 m for both anchor types (including all measurement errors) - anchor resists on seabed after drop Additional calculations and interpretations - addition of geometry error gives a theoretical maximum penetration depth of 1,2 m - the fluke tip to shank distance may be used as an estimate for the penetration depth (1.0m and 1.2m for AC14 and Hall respectively) - extrapolation to a 29 t anchor gives a theoretical penetration depth of 1,6 m DoB of 1,5 m is agreed with the German authorities also for the TSS Anchor Penetration Trial | ICPC Plenary

Public Availablity of Results Reports and Videos will be made public available on the SCUK web page or send me an with your request: Anchor Penetration Trial | ICPC Plenary