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Research Papers: Ocean Renewable Energy

Assessment of Allowable Sea States During Installation of Offshore Wind Turbine Monopiles With Shallow Penetration in the Seabed

[+] Author and Article Information
Lin Li

Centre for Ships and Ocean Structures (CeSOS),
Centre for Autonomous Marine Operations and Systems (AMOS),
Department of Marine Technology,
Norwegian University of Science and Technology (NTNU),
Trondheim NO-7491, Norway
e-mail: lin.li@ntnu.no

Wilson Guachamin Acero, Zhen Gao, Torgeir Moan

Centre for Ships and Ocean Structures (CeSOS),
Centre for Autonomous Marine Operations and Systems (AMOS),
Department of Marine Technology,
Norwegian University of Science and Technology (NTNU),
Trondheim NO-7491, Norway

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received December 21, 2015; final manuscript received April 20, 2016; published online June 1, 2016. Assoc. Editor: Yi-Hsiang Yu.

J. Offshore Mech. Arct. Eng 138(4), 041902 (Jun 01, 2016) (17 pages) Paper No: OMAE-15-1128; doi: 10.1115/1.4033562 History: Received December 21, 2015; Revised April 20, 2016

Installation of offshore wind turbines (OWTs) requires careful planning to reduce costs and minimize associated risks. The purpose of this paper is to present a method for assessing the allowable sea states for the initial hammering process (shallow penetrations in the seabed) of a monopile (MP) using a heavy lift floating vessel (HLV) for use in the planning of the operation. This method combines the commonly used installation procedure and the time-domain simulations of the sequential installation activities. The purpose of the time-domain simulation is to quantitatively study the system dynamic responses to identify critical events that may jeopardize the installation and the corresponding limiting response parameters. Based on the allowable limits and the characteristic values of the limiting response parameters, a methodology to find the allowable sea states is proposed. Case studies are presented to show the application of the methodology. The numerical model of the dynamic HLV–MP system includes the coupling between HLV and MP via a gripper device, and soil–MP interaction at different MP penetration depths. It is found that the limiting parameters are the gripper force and the inclination of the MP. The systematic approach proposed herein is general and applies to other marine operations.

Copyright © 2016 by ASME
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Figures

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Fig. 1

System setup for the MP hammering process

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Fig. 5

Numerical models for the soil–MP interactions

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Fig. 6

Typical soil reaction moment versus MP inclination due to cyclic loading with period of 6 s

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Fig. 8

Standard deviations of MP inclinations and contact forces on one hydraulic cylinder at different MP penetration depths (pene) and wave conditions from 3-hr time-domain simulations (Hs = 1.5 m, Dir = 150 deg)

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Fig. 3

Methodology to find the allowable sea states for the initial hammering process

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Fig. 14

p–y curves for different soil types

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Fig. 2

Flowchart of the MP hammering procedure

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Fig. 4

Elastic model of the gripper contact elements

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Fig. 7

Standard deviations of HLV motions at different MP penetration depths (pene) and wave conditions from 3-hr time-domain simulations (Hs = 1.5 m, Dir = 150 deg)

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Fig. 9

Response spectra of the gripper surge motion for two different wave peak periods at different MP penetration depths (pene): (a) Hs = 1.5 m, Tp = 5 s, Dir = 150 deg and (b) Hs = 1.5 m, Tp = 10 s, Dir = 150 deg

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Fig. 20

Correction forces at different penetration depths (pene) and initial mean inclinations

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Fig. 10

Response spectra of the gripper sway motion for two different wave peak periods at different MP penetration depths (pene): (a) Hs = 1.5 m, Tp = 5 s, Dir = 150 deg and (b) Hs = 1.5 m, Tp = 10 s, Dir = 150 deg

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Fig. 11

Response spectra of the HLV yaw motion for two different wave peak periods at different MP penetration depths (pene): (a) Hs = 1.5 m, Tp = 5 s, Dir = 150 deg and (b) Hs = 1.5 m, Tp = 10 s, Dir = 150 deg

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Fig. 12

Response spectra of the hydraulic cylinder force for two different wave peak period conditions at different MP penetration depths (pene): (a) Hs = 1.5 m, Tp = 5 s, Dir = 150 deg and (b) Hs = 1.5 m, Tp = 10 s, Dir = 150 deg

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Fig. 13

STD of hydraulic cylinder contact forces at different MP penetration depths (pene) when using a jack-up vessel (Hs = 1.5 m, Dir = 150 deg)

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Fig. 16

Mean upcrossing rate of the hydraulic cylinder force using 20 samples. Legends: time-domain simulation (*), curve fitting (–), empirical 95% confidence band (CI – – – ), smooth confidence band (CIextr · · · ): (a) Hs = 1.5 m, Tp = 6 s, pene = 4 m and (b) Hs = 1.5 m, Tp = 10 s, pene = 6 m

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Fig. 17

Extreme cylinder forces in 3 hrs at different penetration depths (pene): (a) Hs = 0.8 m, Dir = 150 deg and (b) Hs = 1.5 m, Dir = 150 deg

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Fig. 18

Illustration of the phases between hammering activities: (a) system mean position before hammering, (b) system position after hammering, (c) system equilibrium mean position after hammering, and (d) system mean position after correction

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Fig. 19

Responses of the HLV–MP coupled system after hammering and during the correction phase (corresponds to Figs. 18(b) to 18(d))

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Fig. 21

Maximum MP inclination over 10 min at different penetration depths (pene): (a) Hs = 0.8 m, Dir = 150 deg and (b) Hs = 1.5 m, Dir = 150 deg

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Fig. 22

Case studies: (a) case 1, (b) case 2, (c) case 3, and (d) case 4

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Fig. 15

Extreme cylinder force in 3 hrs versus MP maximum inclination in 10 min for different sea states and soil properties at different penetrations: (a) Hs = 2 m, Tp = 6 s and (b) Hs = 1.6 m, Tp = 8 s

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Fig. 23

Allowable sea states for MP initial hammering operation for typical HLV headings

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