Impact driving of large piles/monopiles – contributions from pile driving monitoring Rui Silvano, Senior Project Engineer Good morning. Cathie Associates is a specialist consultant working uniquely in offshore geoscience and geotechnics. With our background in O&G we now have approximately 60% of our revenues from renewable energy projects. Worked on nearly 50% of German and UK projects, and nearly all French developments.

3rd Party review Expert witness What do we do? Deliver practical, safe, cost-effective engineering solutions Survey design, management and supervision Foundation geotechnical design Construction support Jack up stability Cable routing Trenching 3rd Party review Expert witness Pile driving monitoring Data integration and interpretation

Impact driving of large piles/monopiles – contributions from PDM Is the transmitted energy as high as we believe? Soil resistance increase with time after driving in sand: What is dynamic pile testing telling us? How does it compare with recently published trends? An insight into noise generation through numerical modeling. Can we reduce the noise at the source?

Large anvil and/or tapered follower Sometimes submerged Impact driving - 1. Energy losses Large anvil and/or tapered follower Sometimes submerged IHC anvil and follower for 7m OD Large anvil Tapered follower Menck hammer cut-away

Measured energy losses Impact driving - 1. Energy losses Measured energy losses End of drive – 60-85% logged energy Pile OD: 2.5m ENTHRU from dynamic pile tests Different installation contractors Different PDM contractors Different hammers Different sites Underwater at end of drive

Measured energy losses Impact driving - 1. Energy losses Measured energy losses Enthru energy ranging from 70-100% logged energy BUT Don’t start oversizing the hammer - SRD may be lower than we think as diameter increases! Correct tapered

Possible sources of energy loss Impact driving - 1. Energy losses Possible sources of energy loss EXAMPLE SHOWN FOR UNDERWATER DRIVING NOT TO SCALE Source Energy loss Imperfect air cushion and water escape Up to 50% if full Large anvil – mass, seating? ? Bending in conical section? Few %? Water pressures induced by downward movement of conical section + radial expansion (inside and outside) 5-10%

Axial pile resistance verification German context (BSH, EC7,DIN1054) Impact driving - 2. Soil resistance after driving Axial pile resistance verification German context (BSH, EC7,DIN1054) Ultimate axial pile resistance shall be determined or confirmed by pile load testing Minimum 10% of locations and covering different geotechnical categories (2 per cat.) Currently not required for Monopiles – Still useful to check pile driveability and driving stresses Dynamic pile testing accepted (ample experience with static and dynamic tests in cohesionless soils) If design method assumes some gain in pile resistance after driving due to ageing or setup (e.g. ICP) -> Pile re-strike testing must be performed to verify the achieved resistance after a certain time End-of-Driving (EOD) normally not enough -> Beginning of Re-Strike (BOR) required Pre-piling vs Post-piling !! Very different impacts in project

Designer dilemma (EOD vs BOR) Impact driving - 2. Soil resistance after driving Designer dilemma (EOD vs BOR) ICP full method assumptions: EOD shaft resistance approximately 60-80% of calculated ICP resistance achieved approximately 10 days after pile installation Rimoy et al. (2015) - Géotechnique Extended open database Zhejiang University/ Imperial College London (ZJU-ICL) - Yang et al. (2015) – trend shifted to right (longer time required)

Re-strike test results – German Bight Impact driving - 2. Soil resistance after driving Re-strike test results – German Bight (Sand) Database is growing rapidly Setup can be relied upon Pile OD = 2.1-2.5m 6 different sites Different installation contractors Different PDM contractors Different hammers

Re-strike test results – German Bight Impact driving - 2. Soil resistance after driving Re-strike test results – German Bight 25% increase of total resistance per log-cycle (e.g. 1.25 after 10 days) appears to be a cautious estimate Limited toe resistance increase -> setup factor on shaft resistance is larger Rimoy and Jardine (2015) attempt to include D effect is not in-line with field measurements 𝛼 𝑡𝑜𝑡𝑎𝑙 =1+0.25 𝑙𝑜𝑔 10 𝑇

Underwater noise Impact driving - 3. Underwater noise German regulations: < 190dB peak sound level at 750m < 160dB SEL (sound exposure level) at 750m Monopile foundation sizes are increasing (>8m diameter) Increased driving resistance Increased hammer size Increased noise levels due to hammer energy Koschinski & Lüdemann, 2014

Underwater noise Impact driving - 3. Underwater noise Peak sound pressure 𝐿 𝑝𝑒𝑎𝑘 =20𝑙𝑜𝑔 𝑃 𝑝𝑒𝑎𝑘 𝑃 0 [dB], where P0 = 1mPa. Used to consider the peak sound level underwater noise Sound exposure level (SEL): 𝑆𝐸𝐿=10𝑙𝑜𝑔 1 𝑇 0 𝑇 1 𝑇 2 𝑃 𝑡 2 𝑃 0 2 𝑑𝑡 [dB], where T0 = 1s. Used to consider the exposure level over the duration of a single blow, or multiple blows by accumulation 200kPa=226dB 50kPa=214dB

Noise source simulation Impact driving - 3. Underwater noise Noise source simulation Pile driving model setup Pile diameter 8m OD x 100mm WT SRD assessment Wave equation modelling Hammer selection – MHU 2100 (2100kJ) Output force – time histories at pile top

Noise source simulation Impact driving - 3. Underwater noise Noise source simulation Water (acoustic medium) Sand (elastic medium) 28m 40m 64m 30m Axisymmetric FEA model Monopile Diameter 8m OD x 100mm WT Total length 64m Embedded length 32m Soil Elastic (plastic interface) Menck MHU 2100 hammer 95% efficiency

Noise simulation – water pressure Impact driving - 3. Underwater noise Noise simulation – water pressure

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 0ms

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 4ms

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 6ms

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 8ms Compression (Mach) wave travels down first followed by the more intense “tension” wave as the pile wall springs back under the force generated by the “tension” wave on the inside of the pile

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 12ms

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 16ms

Dynamic water pressure – outside pile Impact driving - 3. Underwater noise Dynamic water pressure – outside pile T = 20ms

Pressure wave inside and outside Impact driving - 3. Underwater noise Pressure wave inside and outside Pressures (noise) higher inside and frequency content is different Inside Outside At 6m, cavitation would limit negative pressures to about 160kPa Higher suction pressures would actually cause cavitation and so the peak pressures would be lower (limited to water depth + 10m, i.e. about 100kPa at surface)

Pressure wave – softened hammer Effect of hammer signature Impact driving - 3. Underwater noise Pressure wave – softened hammer Effect of hammer signature Cushioned blow reduces peak pressure and changes frequency content Softened blow, MHU2100 Control point at 18m depth, 1m away from the pile

Pressure wave – frequency content Effect of hammer signature Impact driving - 3. Underwater noise Pressure wave – frequency content Effect of hammer signature Frequency peaks (90Hz, 150Hz) associated with tension and compression waves Reduction in wave peak at 150Hz for cushioned case Softened case Shorter impulse results in higher frequency content. Higher peak is tension but would be reduced if cavitation would be considered Control point at 18m depth, 1m away from the pile

Impact driving of large piles/ monopiles – contributions from pile driving monitoring Conclusions Energy transmitted Not as high as contractors believe Not all the reasons are understood Could lead to incorrect back analysis of SRD methods for large piles Soil resistance increase after driving in sand Growing database -> setup can be relied upon Recent published trends are not in-line with field measurements Noise generation Sensitive to nature of hammer blow Potential for reducing peak pressure and changing frequency content of noise Vibration… not to be forgotten

Ongoing R&D - Lateral stiffness of vibro-driven monopiles Abaqus model to study the influence of degraded soil close to the pile on monopile lateral stiffness Influence of soil stiffness and width of degraded soil (reference case driven monopile) Next step: calibrate against field measurements