How deep does an anchor penetrate the seafloor?

Transcription

1 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-1 How deep does an anchor penetrate the seafloor? Christian Maushake Federal Waterways Engineering and Research Institute Coastal Department (BAW-DH) Hamburg, Germany Contact Address Website BAW-DH Wedeler Landstraße 157 D Hamburg Germany

2 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-2 How deep does an anchor penetrate the seafloor? Christian Maushake Federal Waterways Engineering and Research Institute Coastal Department (BAW-DH), Hamburg, Germany How deep does an anchor penetrate the seafloor?. This question has been explored by a team of scientists and technicians as well as the crews of three involved ships in the German Bight. One HHP AC 14 and one Hall type anchor have been dropped and dragged in a series of 18 trials following an exact specified procedure. The behaviour of the anchor, especially the impact on the seafloor and the maxiumum penetration depth have been documented and surveyed before, during and after the anchor trials. For this different hydroacoustic methods like Sediment Echosounder (SES) and Sonarsystems (MBES, SSS), videodocumentation and the measurement of pressure and pulling forces have been implemented. Motivation for this large scale fieldwork is the shore connection of wind-energy parks by sea cables in the German Bight. The actual regulations prescribe a burial depth of 3m for sea cables in shipping channels. The reason for these increased requirements is the risk potential which is seen by anchor manouvres in emergency cases and disasters. On the other hand the realization of a burial depth of 3m is very ambitous from a technical point of view as well as very cost intensive. Therfore the approving authority (Generaldirektion Wasserstraßen und Schifffahrt, GDWS) and the transmission system operator Tennet Offshore GmbH agreed upon this investigations to determine the real penetration depths of anchors into the seafloor. The results of the tests potentially shall support the improvement of the regulations for the burial depth of sea cables in shipping channels. Next to Tennet the Federal Maritime Hydrographic Agency (BSH) the dutch research institute Deltares and the Federal Waterways Engineering and Research Institute (BAW) have been involved in the field work, the documentation and the scientific evaluation of the results. Figure above: Offshore support vessel ESVAGT CONNECTOR during anchor drag Figure left: Background: survey vessels WEGA (BSH) and GUARDIAN Foreground: HHP AC14 anchor on deck ESVAGT CONNECTOR

3 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-3 How deep does an anchor penetrate the seafloor? Anchor tests in the German Bight Christian Maushake Federal Waterways Engineering and Research Institute Coastal Department (BAW-DH) Hamburg, Germany The Northsea an undisturbed wideness? The seas are moving from traffic routes, fishing and recreational areas to an economic ressource as an energy reserve AWZ / EEZ W a t e r s T e r r i t o r i a l Offshore windparks in operation under construction approved planned blocked 2

4 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-4 Offshore wind some facts Target figures (according to BSH annual report 211) 126 licensing procedures for Offshore windfarms are being processed by the BSH 875 wind turbines 25-3 MW power by 23 Reality (end of 211 / actual) 47 (~95) ~175 (~47) Individual wind turbines MW power 3 Grid connection the challenge Far behind the targets example: Riffgat starting without grid connection Lacking of a systematic regulatory model Offshore network plan by BSH (introduced 212) Offshore network plan jointly developed with other authorities (e.g. GDWS) and the transmission system operator (TenneT) some issues Environmental Legal Technical o Liabillities (e.g. ammunition) o Burial depth of seacables 4

5 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-5 prescribed burial depth of sea cables 1.5 m outside shipping channels 3. m inside Complex in terms of Costs (increasing dramatically with every dm) Burial depth of seacables Regulations and risks Technology (in areas with difficult soil conditions) Reason for the increased requirements in shipping channels: risk potential which is seen by anchor maneuvers in emergency cases and disasters Sea cable Shipping channel 1.5m 1.5m 3m agreed upon investigations to determine the real penetration depths of anchors into the seafloor 5 Test anchors Hall ~11.7 to. ~ 364 cm ~ 28 cm HHP AC14 ~8.3 to. ~ 364 cm ~ 196 cm ~ 197 cm Up to 294 m length / 8 DWT ~ 28 cm 6

6 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-6 Vessels and tasks Guardian Esvagt Connector Wega Survey vessel: ROV, MBES ROV inspection during anchor pulls Post-pull MBES surveys Offshore Tug: Anchorhandling Anchor handling and pulling (Bollard pull max.17 to.) Survey vessel: SSS, SES (MBES) Pre-pull survey (SSS / SES) Post-pull survey (SES,SSS) 7 Test sites 1 km UG BSH NORD Loosely layered finesand UG BSH SÜD Medium dense sand with some silt UG Densely packed Sand with stiff clay layers Kabeltrassen Terschelling - German Bight Norderney UG survey box 3 test sites with different soil conditions reflecting the interaction between anchor and seabed 3 test sites 2 anchors (Hall, AC14) 3 pulls each anchor 18 pulls 8

7 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-7 1 km UG BSH NORD Loosely layered finesand Anchor trial procedure Pre - pull - survey Side scan sonar and Sediment Echosounder survey on every test site Soil conditions, detection of obstacles, finalization of drop positions UG BSH SÜD Medium dense sand with some silt Terschelling - German Bight UG Densely packed Sand with stiff clay layers UG survey box Anchor pulls Move offshore tug to drop position Dropping anchor ROV video check of anchor position and alignment Anchor pull up to 8 to. (load cell) or anchor break out ROV Video check of final position Recover anchor Kabeltrassen Norderney Post - pull - survey SSS, MBES and SES survey of anchor track 9 SES Pre pull survey () BSH Nord BSH Süd Norderney 1

8 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-8 Combined SSS / SES survey of anchor tracks (Post pull survey) 2 1 drop end 1 SSS 2 SES anchortracks 11 Detection of anchor penetration depth as performed with SES processing software ISE Digitized seafloor Digitalisierter Seeboden Anchortrack Ankerspur N2 Anchortack Ankerspur N1 Manuelle Nach-Digitalisierung im Bereich der Ankerspuren (pink) Manually Re-Digitization on the anchor tracks (pink) Level Niveau of des un-disturbed ungestörten Seebodens seefloor Digitize seafloor ISE automatically Identify anchor track(s) Re-Digitize the level of un-disturbed seafloor in the zone influenced by the anchor ISE manually Overwrite seafloor level ISE automatically N2 Anchor penetration depth (depth of influence) N1 Detect depth of influence (anchor penetration depth) ISE target picker 12

9 6 th Workshop Seabed Acoustics, Rostock, November 14/15, P3-9 Anchortrack (SSS) BSH Nord BSH Süd 57 Penetration depth Δz (m) Norderney BSH Nord Pulling force (t) Pull 1 SES - Echoplots ID: 591 Δz :.88 Pull 2 ID: 51 Δz :.56 ID: 54 Δz :.11 ID: 52 Δz :.38 ID: 58 Δz :.72 ID: 59 Δz :.23 ID: 56 Δz :.56 ID: 53 Δz :.16 ID: 55 Δz :.13 ID: 57 Δz :.29 area Nord Pos N5 Typ Hall Length 87 m Max. pull 58 t Max. Δz.88 m BSH Nord BSH Süd [m] Anchortrack (SSS) Penetration depth Δz (m) Norderney Pulling force (t) Pull 1 Pull 2 Pull 3 SES - Echoplots ID: 15 Δz :.25 ID: 111 Δz:.28 ID: 14 Δz :.1 ID: 11 Δz :.21 ID: 13 Δz :.26 ID: 19 Δz :.25 ID: 12 Δz :.28 ID: 17 Δz :.17 ID: 11 Δz :.7 ID: 16 Δz :.33 area Pos V1 Typ AC14 Length 17 m Max. pull 73 t Max. Δz.33 m

10 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-1 Anchortrack (SSS) BSH Nord BSH Süd 61 Norderney Penetration depth Δz (m) Pulling force (t) SES - Echoplots area Pos Typ Length Max. pull Max. Δz V6 Hall 26 m 8 t.51 m ID: 65 Δz :.15 ID: 64 Δz :.41 ID: 63 Δz :.51 ID: 62 Δz :.23 ID: 61 Δz :.29 Ankerzugversuche BSH - Nord Ankerspuren Nord AC14 Hall 2 Ankerzüge! N1 1. Zug 2. Zug 1. Zug 2. Zug 2 Ankerzüge! N2 N3 1. Zug 2. Zug 2 Ankerzüge! N5 Anmerkung: Länge des 1. und des 2. Zuges aus Ankerspur abgeschätzt 1. Zug 2. Zug 2 Ankerzüge! N6 16

11 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-11 Ankerzugversuche BSH - Süd Ankerspuren AC14 Hall S1 S4 S2 S5 S3 S6 17 Ankerzugversuche Ankerspuren AC14 Hall 1. Zug 2. Zug 3. Zug 3 Ankerzüge! V1 V2 V3 V4 V5 V6 18

12 6 th Workshop Seabed Acoustics, Rostock, November 14/15, 213 P3-12 Anchor test Summary of all anchor pulls UG BSH NORD UG BSH SÜD UG 1 km tracklength Max. Position Ankertyp pull Max. Δ z [m] [t] [m] N1 AC N2 Hall N3 AC N5 Hall N6 Hall S1 AC S2 AC S3 AC S4 Hall S5 Hall S6 Hall V1 AC V2 Hall V3 AC V4 Hall V5 AC V6 Hall For all anchor tests a maximum depth of influence of not more then 1. m could be reported, accounting for all potential errors. In the (critical) traffic seperation zone () this value this value does not exeed.8 m. 19 Conclusion SES is a powerfull tool to detect small buried structures like anchor tracks No anchor penetration deeper than 1m could be observed (accounting for potential errors) One of the best documented large scale anchor trials ever have been reported Results have been accepted by the approving authorities German offshore network plan will be updated based on the results and the expertise of Deltares and BAW reduction of burial depth saving of Millions for electricity consumers 2