Geotechnical Design of Embedded Mooring Systems: Analysis of Chain-Soil Interactions’ Effect on Suction Anchor Design
Translated title
Geotechnical Design of Embedded Mooring Systems
Authors
Houmann, Stefan Rysgaard ; Pedersen, Troels Juul ; Jensen, Christian Lykke
Term
4. term
Education
Publication year
2023
Submitted on
2023-06-09
Pages
151
Abstract
Dette speciale handler om, hvordan man dimensionerer de havbundsankre og kæder, der holder flydende vindmøller på plads. Fokus er på sugeankre—store stålcylindre, der installeres ved at skabe undertryk—og på fortøjningskæder, som er delvist nedgravet i havbunden. Tre praktiske spørgsmål styrer arbejdet: hvordan jord og en indlejret kæde påvirker hinanden, hvordan man modellerer et ankers udtrækskapacitet (modstanden mod at blive trukket op), og hvordan man optimerer det samlede kæde‑anker‑system. For at forstå kæde‑jord‑interaktionen blev tre metoder fra litteraturen sammenlignet (Neubecker & Randolph, 1995; Lee m.fl., 2014; Mortensen, 2015). Mortensens tilgang gav de mest rimelige estimater af kædens form i jorden, hvordan jorden reducerer den last, kæden overfører, og den deraf følgende stigning i lastvinklen på ankeret. Ankrenes ydeevne blev derefter analyseret med finitte element‑analyser (FEM) i PLAXIS 3D under forskellige lasttilfælde. To modelleringsvalg viste sig vigtige: ankerets egenvægt bør medtages, fordi den påvirker deformationsfeltet i jorden, og kraftkontrolleret belastning gav rimelige resultater sammenlignet med deformationskontrolleret belastning. Til sidst blev der gennemført en surrogatbaseret optimering, som kombinerede Mortensens kæde‑jord‑model med FEM‑resultaterne for at minimere materialeforbruget. Studiet viser, at det er mere effektivt at øge skørtlængden (den cylindriske vægs længde) på sugeankeret end at øge diameteren, og at en effektiv placering af padeye (fastgørelsesøjet) ligger mellem 0 og 0,25 gange skørtlængden. Resultaterne giver praktisk vejledning i at designe sugeankrede fortøjninger, der er stærkere, mere driftssikre og mere omkostningseffektive til flydende vindmøller.
This thesis explores how to design the seabed anchors and chains that keep floating wind turbines in place. It focuses on suction anchors—large steel cylinders installed by suction—and on mooring chains that are partly buried in the seabed. Three practical questions guided the work: how soil and an embedded chain interact, how to model an anchor’s pull‑out capacity (its resistance to being pulled out), and how to optimise the combined chain–anchor system. To understand chain–soil interaction, three published methods were compared (Neubecker & Randolph, 1995; Lee et al., 2014; Mortensen, 2015). Mortensen’s approach gave the most reasonable estimates of the chain shape in the soil, how the soil reduces the load carried by the chain, and the resulting increase in the loading angle on the anchor. Anchor performance was then analysed with finite element analyses (FEA) in PLAXIS 3D under different loading conditions. Two modelling choices were found to matter: the anchor’s self‑weight should be included because it affects the ground deformation field, and applying loads in a force‑controlled way produced reasonable results compared with displacement‑controlled loading. Finally, a surrogate‑based optimisation combined Mortensen’s chain–soil model with the FEA results to minimise material use. The study indicates that increasing the skirt length of the suction anchor is more effective than increasing its diameter, and that an effective padeye (line attachment) position lies between 0 and 0.25 times the skirt length. These findings provide practical guidance for designing suction‑anchored moorings that are stronger, more reliable, and more cost‑efficient for floating wind turbines.
[This summary has been rewritten with the help of AI based on the project's original abstract]
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