General Report for TC209
Offshore Geotechnics
Pisano, Federico; Gavin, Kenneth
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2017
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Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul
2017
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Pisano, F., & Gavin, K. (2017). General Report for TC209: Offshore Geotechnics. In Proceedings of the
19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017
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General Report for TC209
Offshore Geotechnics
Rapport Général du TC209
Géotechnique Marine
Pisanò F., Gavin K.G.
Department of Geoscience and Engineering, Delft University of Technology, Delft, The Netherlands
ABSTRACT : The present report overviews the 22 contributions submitted to the session on offshore geotechnics held by Technical Committee (TC) 209 at the 19th International Conference on Soil Mechanics and Geotechnical Engineering (ICSMGE). Following a
general discussion on the nature (fundamental or applied) of the papers included in this session, some topical highlights are presented. The content of 2017 TC209 session confirms the close collaboration between researchers and industry in this area as the topics and focus directly address the current needs of the offshore industry. In particular, the growing interest around offshore wind developments is clear with numerous contributions on the performance of foundation systems during installation and operation. Other applications to more traditional oil and gas and coastal/nearshore geo-engineering are also addressed, as well as a few fundamental studies on difficult soil conditions, in-situ testing and novel numerical methods for large deformation problems.
RÉSUMÉ : Ce rapport présente une vue d’ensemble des 22 contributions soumises dans la session dédiée à la géotechnique offshore et organisée par le Comité Technique (TC) 209 lors du 19e Congrès International de Mécanique des Sols et de Géotechnique (ICSMGE). Après une discussion générale sur la nature (fondamentale ou appliquée) des papiers soumis dans cette session, quelques questions d’actualité sont présentées. Le contenu de la session TC209 de 2017 confirme l’étroite collaboration entre les chercheurs et l’industrie dans ce domaine puisque les sujets abordés traitent directement des besoins actuels de l’industrie offshore. En particulier, l’intérêt grandissant pour les développements liés à l’éolien offshore est clair avec un nombre important de contributions concernant les performances des systèmes de fondation (essentiellement monopieux) en cours d’installation et d’opération. D’autres applications de géo-ingénierie plus traditionnelle, principalement liées aux industries pétrolière et côtière/nearshore, sont également abordées, ainsi que quelques études fondamentales sur les conditions de sol difficiles, les essais in-situ et de nouvelles méthodes numériques pour les problèmes de grandes déformations.
KEYWORDS: offshore geotechnics, offshore wind, oil and gas, nearshore works
1 INTRODUCTION
The session organised by TC209 offers an opportunity to identify, and reflect on, modern research trends in offshore geotechnical engineering. Over the past decades, this discipline has promoted significant advances in many areas of geotechnical engineering, ranging from site investigation to laboratory soil testing and from foundation design to marine geohazard assessment. Considering the continued need for cost optimisation, the offshore industry demands the solution of fundamental geo-problems associated with e.g. difficult soil conditions, cyclic loading and large soil deformations.
The highlights from the 22 contributions collected in this session have been arranged in the following themes:
1. fundamental studies 2. offshore wind 3. oil and gas 4. nearshore works
While the first “fundamental” area includes general works not restricted to any offshore sector, all other papers in themes 2 to 4 were associated to these areas by the individual paper authors themselves. Such a classification is in fact quite artificial, yet instrumental to a preliminary synopsis of all research efforts. When deemed necessary, the wider applicability of certain research findings will be elaborated by the reporters.
Figure 1 depicts the distribution of all session papers over the above thematic areas. The focus on offshore wind topics stands out clearly with 10 papers (45.5% of the total) in the area. Conversely, far less attention is received by more traditional oil and gas and nearshore applications (3 papers each, 13.6%),
although all fundamental studies (6 papers, 27.3%) have potential impact across the themes. Within the theme offshore wind, most papers address research questions related to the performance of foundation systems that are also relevant to oil and gas developments. The thematic overview in Figure 1 seems consistent with the oil and gas world crisis and the contemporary rush to renewables, reflected in developments of commercial projects and R&D initiatives involving massive investments from both public and private investors.
Figure 1. Distribution of session papers over different thematic areas.
Considering the whole session from a methodological standpoint, Figure 2 shows that both experimental and numerical approaches are being explored to address offshore geotechnical problems. Studies exclusively based on numerical analysis (45.5%) outnumber experimental works (31.8%), while complete integration of experiments and simulations is only achieved in 22.7% of all cases. Understandably, limitations in budget and facilities might often hinder experimental activities, whereas the increased availability of numerical simulation
packages is nowadays impacting research trends in both academia and industry.
Figure 2. Approach-based classification of session papers.
Table 1 details the experimental and numerical methodologies adopted in all 22 papers. With regard to experimental approaches, the session confirms the traditional preference in offshore engineering for field tests and centrifuge modelling (O’Loughlin, 2015), very often made possible by joint industry projects (JIPs). Whilst 1-g tests can provide very valuable data, particularly on mechanisms, scale-effects can deter their easy application to field problems. Therefore, increased financial resources are often necessary to perform field and centrifuge testing.
Table 1. Experimental and numerical methodologies adopted in all session papers (mixed experimental-numerical works are counted twice for classification purposes).
Approach Methodology # papers
Experimental (12 papers)
soil laboratory testing 2
1g small-scale testing 2
centrifuge small-scale testing 4
field testing 4 Numerical (15 papers) standard FE 11 LDFE and MPM 3 DEM 1
Results from numerical analyses appear in 15 out of 22 papers, with the majority implementing standard Finite Element (FE) calculations. While discrete simulations – e.g. through the Discrete Element Method (DEM) – are applied to a limited number of offshore applications, continuum-based analyses of large deformation processes are gaining increasing popularity for the study of complex penetration/installation problems (Wang et al., 2015). Although large deformation FE (LDFE) methods have been applied for quite some time (Hu and Randolph, 1998), the current progress in this area seems dominated by the latest developments of the Material Point Method (MPM).
2 FUNDAMENTAL STUDIES
The papers discussed in this section are considered fundamental contributions to the sub-areas here termed difficult soils and in-situ testing, development of numerical methods and capacity equations for suction-installed units.
2.1 Difficult soils and in-situ testing
The work by Nakata deals with the characterisation of sand-coral mixtures, which impose challenging geotechnical conditions for some Japanese projects (Figure 3-left). The results from minimum-density and angle of repose tests are discussed to explore the influence of the coral gravel content on
the void ratio and, in turn, the average friction angle. In keeping with DEM simulations, the transition from sand to pure-coral shear strength seems to take place over gravel contents in the range of 20-50% (Figure 3-right).
0 10 2 0 3 0 4 0 5 0 6 0 20 25 30 35 40 45 50 A n g le o f re p o s e ( d e g ) C or al gr av el ( 1 00 %) Te st r e sult Cor al gr a ve l c on te n t (% ) C hiib ish i sa nd ( 0% ) DEM r e sult GX G1
Figure 3. (left) Coral particles in a coral gravel soil; (right) mixture angle of repose at varying coral gravel content – from Nakata (2017).
Up-to-date reflections on the interpretation of the piezoprobe in-situ test in soft clay are provided by Hernandez-Martinez et al. The authors shed new light on the determination of in-situ pore-water pressures (possibly not hydrostatic) and horizontal consolidation coefficient (ch) from dissipation tests. The
methods from the literature by Tortesson (1977), Levadoux and Baligh (1986) and Houlsby and Teh (1991) are compared with real measurements at both onshore and offshore sites. Considering all uncertainties, the methods suggested by Tortesson and Houlsby and Teh appeared more reliable than the Baligh and Levadoux method, which tended to overestimate ch
from 2 to 4 times.
2.2 Development of numerical methods
The papers by Reinaldo et al. and Brinkgreve et al. are two meaningful examples of the research efforts being devoted to the implementation of the MPM in geotechnical engineering. As for offshore applications, the current focus is on the simulation of installation processes for piles and suction units, soil-pipeline interaction, submarine landslides and possible impacts on subsea structures. Apparently, the applicability of the MPM is not restricted to any particular offshore sector, nor even to offshore problems. The proceedings of the recent 1st International Conference on the Material Point Method (MPM 2017, Delft – Netherlands) give a comprehensive picture of the state of the art within the geotechnical community (Rohe et al., 2017).
To date, the main challenges faced by the MPM community are (not exhaustively) related to (i) high computational costs, (ii) numerical accuracy and (iii) difficult validation.
The significance of point (i) is reflected by the fact that both Reinaldo et al. and Brinkgreve et al. report on 2D calculation examples (Figures 4-5). As pursued by Brinkgreve et al., resorting to implicit time integration is deemed beneficial for practical applicability.
Figure 4. (left) MPM and FEM regions for a DDMP simulation of pile driving; (right) vertical stress distributions during pile driving from MPM and DDMP calculations – from Brinkgreve et al. (2017).
Solutions to improve local accuracy – especially in terms of stress-strain fields – are also considered by Brinkgreve et al., such as the use of the so-called Dual Domain Material Point
(DDMP) method (Figure 4). The method features combined MPM and FE regions and seems capable of reducing the typical stress oscillations produced by the standard MPM.
Reinaldo et al. address point (iii) by comparing the results from the 2D GIMP (Generalized Interpolation Material Point) method and the Small Strain Path Method (SSPM) for the undrained analysis of wall installation in homogeneous clay. The encouraging comparison in terms of lateral soil displacements (Figure 5-right) represents a step forward against the common difficulties in validating MPM results – benchmark results involving large soil deformations are indeed quite rare.
Figure 5. (left) Horizontal displacement contours after wall driving; (right) comparison between GIMP and SSPM horizontal displacement distributions – from Reinaldo et al. (2017).
2.3 Capacity equations for suction-installed units
The findings by Park and Park and Choi at el. could have actually been discussed in Sections 3 and/or 4, even though the authors are not specific as to the application area. Both contributions concern the capacity of suction-installed units, in the former case under compressive vertical loading, in the latter under inclined tension (suction anchors).
Park and Park exploit a set of 320 FE analyses to derive a bearing capacity formula for suction buckets on sand overlying clay. The study relies on standard plasticity modelling of soil behaviour (Mohr-Coulomb/Tresca models for sand/clay, respectively), and includes the parametric analysis of relevant geometrical/mechanical factors (bucket aspect ratio, depth of the clay layer, sand friction/dilatancy, clay undrained strength). The outcome is a spreadsheet-friendly bearing capacity formula built upon the new results and the previous studies by Hung and Kim (2012) and Park et al. (2016).
The paper by Choi at el. has a similar goal and provides a “ready-to-use” capacity formula for suction anchors in soft clay subjected to horizontal-tension (HV) loading. The calibrated parameters for the HV capacity envelope derive from numerical upper-bound plasticity calculations (Aubeny et al., 2003) performed at varying aspect ratio (3<L/D<6) and clay strength profile. The proposed equation is verified with respect to three NGI designs based on semi-3D FE calculations (NGI, 1997). The authors propose their new design equation as an effective tool for the early phases of a project, though to be used cautiously for aspect ratios and strength profiles out of the range considered.
3 OFFSHORE WIND
The importance of offshore wind research is evident from the continuation of the scientific debate held four years earlier at the 18th ICSMGE (Jewell, 2013). With 10 contributions from Northern Europe and Eastern Asia belonging to this subsection, the huge development of the offshore wind industry in these geographical areas is clear. Despite the vast range of the discipline, the focus of the 2017 session is all on the analysis and design of foundation systems. Indeed, the costs for
foundation design and construction can easily rise to 30-40% of the total project budget, where there is ample room for fundamental advances beyond the short-term needs of the industry (see the research agenda released by the European Academy of Wind Energy – van Kuik et al., 2016). The highlights from all offshore wind papers are presented after grouping into different foundation types, namely monopiles, suction buckets and gravity base foundations.
3.1 Monopiles
Monopiles are by far the most common foundation solution in offshore wind projects, due to their competitive fabrication and installation costs. Despite the experience available on the design of offshore piles for the oil and gas sector, the large diameter monopiles used for wind turbines have demanded – and still do – substantial review of existing design approaches (Doherty and Gavin, 2012). This is urging significant investments and recently completed research programmes, such as PISA, will allow design optimization – see e.g. Zdravkovic et al. (2015) and Byrne et al. (2017).
The papers reviewed in this subsection are representative of the main frontier topics currently debated (Arany et al., 2017). In particular, the following classification of all paper subjects is considered for the sake of clarity:
i. installation methods and effects on soil conditions ii. lateral capacity and stiffness
iii. response to loading cycles and dynamic behaviour.
Installation. The total costs of a foundation system relate largely to installation, particularly for large turbines and deep water depths. Monopiles are commonly driven into the soil through impact hammering, that is by subjecting the pile head to hammer blows of given energy content. It is crucial to find optimal combinations of number of blows and related energy, as the former governs full penetration, while high values of the latter will result in excessive underwater noise and possibly pile damage. The study by Anusic et al. presents a comparison between standard driving (30-40 blows per minute) and HiLo, the recent piling concept proposed in 2013 by IHC (60 blows per minute). Specifically, HiLo targets noise reduction through blows at lower energy, while time delay is avoided by increasing the hammering frequency. Based on driving records from a wind farm in the German Bight, the authors present evidence showing the satisfactory performance of the HiLo method, allowing for lower energy and noise level with comparable piling time (Figure 6-left).
Pile
Zone 3 Zone 2
Zone 1 Berlin Sand
Figure 6. (left) Standard vs HiLo driving: comparison of hammer energy vs effective piling time – from Anusic et al. (2017); (right) – FE model with different post-driving relative density zones – from Labenski and Moormann (2017).
Another important issue concerns the high degree of uncertainty about installation effects on the soil around the monopile, in
turn affecting the lateral response. Albeit novel methods for pile driving simulations will certainly impact this research area (see Section 2.2), practical analyses with input from experimental observations are still the most viable option. An example is the simplified approach proposed by Labenski and Moormann, who set up a 3D FE soil-pile model with different relative density zones around the pile (Figure 6-right) – depending on the installation method (impact or vibratory driving). Experimental observations from scaled model tests are used to set realistic density values, and it is shown how a decrease in relative density does not necessarily imply a softer response. The lateral behaviour of the monopile seems to be governed by the overall combination of variations (either increases or decreases) in relative density in the soil mass.
Lateral capacity and stiffness. Li et al. consider for large diameter monopiles in clay the fundamental problem of defining the lateral failure mechanism under undrained conditions. The authors use a 3D FE model validated against centrifuge test results to conclude that soil failure occurs in the form of a reversed cone with circular plan section. While this finding contradicts current design assumptions, the numerical analyses reveal the substantial independence of the mechanism shape on pile size, load eccentricity and clay properties.
Yu and Leung address the influence of cyclic loading on the lateral stiffness of free-head monopiles in clay. Based on cyclic centrifuge tests at imposed displacement amplitude, it is shown that the degradation of lateral stiffness is mainly caused by soil plastic straining and remoulding (Figure 7-left). In particular, a direct relationship is observed in displacement-controlled tests between the post-cyclic-to-intact lateral stiffness ratio and the corresponding ratio in terms of undrained clay strength. This quantitative observation is then exploited for FE modelling purposes, and a simplified derivation of cyclic-equivalent soil stiffness is proposed and validated against centrifuge results (Figure 7-right). Even though displacement-controlled tests may not capture real vibration conditions, this work is an appreciable effort to quantify cyclic stiffness degradation through an objective index, namely the undrained strength degradation ratio. -0.4 -0.2 0.0 0.2 0.4 -300 0 300 600 1%D Initial stiffness Monotontic
Final cycles of repeated loading
P ( k N ) y (m) Post-cyclic stiffness 5%D -0.4 -0.2 0.0 0.2 0.4 -300 0 300 600 1% 2.5% 5% 10% 20%D Remolded stiffness P ( k N ) y (m) Monotonic test
Final cycles from repeated loading Remolded reaction force FEM with initial strength FEM with remolded strength
Load amplitude
Figure 7. Monotonic vs post-cyclic lateral load-displacement curves: (left) experimental results; (right) comparison between experiments and numerical simulations – from Li et al. (2017).
Response to many loading cycles and dynamic behaviour. There are areas in which overwhelming design uncertainties are still to be overcome, such as in the analysis of monopile under multiple loading cycles and/or dynamic conditions. The fundamental mechanisms driving the accumulation of pile displacements and rotations are not yet fully understood, with obvious impact on the reliability of existing prediction methods (Arany et al., 2017). Much room seems available for new fundamental studies that should aim to clarify the role of all contributing factors (e.g. soil type, combination/amplitude/duration of cyclic loading, water drainage, etc.). Niemann et al. contribute to this issue by presenting the results of centrifuge tests on a monopile in sand subjected to both 1-way and 2-way lateral cyclic loading (500 cycles for each test). In particular, 1-way cyclic
tests show the influence of the load amplitude Hamp on the
accumulation of pile head displacements, as well as on the changes in bending moments and the reduction of subgrade reaction with increasing number of cycles. While displacement and moment data are in agreement with the study by Rosquoet et al. (2007), the Extended Strain Wedge Model (ESWM – Tasan, 2011) is found to be a promising simple tool for predicting the cyclic response of monopiles to numerous loading cycles.
All the papers discussed so far analyse monopiles in relation to static loading conditions (no inertial effects) and pre-established water drainage conditions (either drained or undrained). An exception is the work of Pisanò et al., where an integrated 3D FE soil-monopile-turbine model is set up through computational procedures originally developed for seismic geotechnical applications (McKenna, 1997). The numerical analyses include dynamic conditions, hydro-mechanical coupling and advanced plasticity modelling of cyclic sand behaviour. Accordingly, the response of a 5 MW wind turbine to lateral wind/wave loading is simulated in the time-domain, while pore pressure build-up/dissipation and soil plastification around the monopile (Figure 8-left) are naturally reproduced. The proposed modelling approach is suitable to investigate non-linear soil effects in relation to the main natural frequency, a major structural design drivers. The dynamic load-displacement response of the monopile head (Figure 8-right) clarifies that the global behaviour under dynamic/cyclic loading depends on the unloading-reloading stiffness of the foundation and its evolution during the loading history. It is further shown that increasing loading amplitudes can determine non-negligible variations in natural frequency, though in patterns not easy to capture through traditional p-y analyses.
Figure 8. (left) Example of cyclic soil stress path from hydro-mechanical dynamic FE analyses; (right) dynamic load-displacement response of the monopile head at increasing loading amplitude – from Pisanò et al. (2017).
3.2 Suction buckets
The discussion on suction bucket foundations should include all geotechnical issues being discussed for monopiles. In addition, different geometrical configurations are to be considered, depending on whether suction units are deployed as single (monopods) or compound (tri/quadri-pods) foundations. In the latter case – especially relevant to waters deeper than 30 m – each bucket experiences horizontal-moment loading combined with alternating vertical tension and compression (push-pull mechanism). This wider range of loading conditions requires the solution of the same capacity/serviceability issues tackled for pure lateral loading, especially in light of the dearth of research completed to date. The discussion about suction buckets at the present geo-offshore session is limited to single bucket configurations (monopods) and monotonic loading conditions.
Deb and Singh present a study on the capacity of suction caissons in dense sand subjected to lateral loads of varying eccentricity. The results of 3D FE analyses on a 12 m diameter
caisson show how the lateral loads for ultimate capacity (ultimate limit state) and 0.5° rotation at the bucket lid (serviceability limit state) are affected by the overturning moment, the embedded foundation length and the vertical dead load. The authors discuss the harsher operational conditions induced by combined horizontal-moment loading, and provide examples of lateral load-moment interaction diagrams associated with 0.5° rotation at the bucket lid. Although these analyses are meant for preliminary design, it seems prudent to be careful about displacement/rotation values from analyses based on perfectly plastic soil models and monotonic loading.
Similarly, Bagheri et al. analyse the response of monopods to eccentric laterals loads at different aspect ratios and soil conditions (medium dense and dense sand). In this case, 3D FE simulations are performed in combination with a strain-hardening soil model, which is expected to reproduce more accurately the monotonic pre-failure response – force-displacement and moment-rotation curves. The authors extend the approach of Bagheri et al. by extrapolating analytical (power law) formulas for the initial evaluation of capacity and displacements/rotations under horizontal-moment loading. 3.3 Gravity base foundations (GBFs)
A range of GBF concepts that withstand lateral loads through sliding resistance are being developed for the offshore wind sector, where ensuring sufficient sliding capacity whilst optimising the GBF shape and weight is a major concern in design (Figure 9). Steenfelt presents a practical study of sliding risk, particularly focussing on the H/V < 0.4 criterion set by the Eurocode 7. Based on previous field test results and theoretical considerations, it is concluded that, in case of concrete GBFs on clay tills, the H/V < 0.4 requirement is mostly superfluous. Indeed, while drainage is normally promoted by the gravel bed interface between the GBF and the underlying fine-grained soil, the sliding risk can be minimised via proper preparation of the concrete-gravel interface.
Figure 9. (left) GBFs for offshore wind turbines at the sites Thornton, Belgium and (right) Rødsand 2, Denmark – from Steenfelt (2017).
The study by Seo et al. concerns a less common type of pile supported GBF. The piles are provided to enhance the bearing capacity and stiffness of the soft seabed soils. A 3D FE model based on Tresca soil plasticity is first validated against centrifuge test results; then, the same model is used for a parametric study on the influence of horizontal loading direction, eccentricity (moment-to-horizontal force ratio) and pile length. The FE results conclude that the horizontal load-displacement response of the whole GBF is only slightly affected by the load direction and the pile length, while significant (and expected) influence of the eccentricity is evident. The bending response of the supporting piles is conversely quite sensitive to all aforementioned parameters. In particular, the maximum bending moment decreases at larger pile lengths, due to the overall lower rotation that the upper mat experiences when supported by long piles (that develop more efficient push-pull behaviour).
4 OIL AND GAS
As mentioned in the introduction, the session reflects decreasing interest in oil and gas applications – especially when compared to previous offshore geotechnical events (see e.g. Meyer, 2015). This might be the consequence not only of lower research budgets, but also of the maturity already achieved regarding certain traditional subjects. Indeed, the 3 papers assigned to this subsection bring up new interesting research topics.
After exploiting offshore oil and gas fields for decades, the industry must now face the challenges of decommissioning existing structures and foundations. In this respect, the contribution by Gaudin et al. tackles the problem of assessing the uplift resistance of subsea foundations in clay, and specifically the suction force developing at the soil-foundation interface. Based on centrifuge testing and coupled hydro-mechanical FE analyses, the authors conclude that the undrained uplift resistance can be estimated from compressive bearing factors and operative shear strength values accounting for the loading history. The use of suction flaps (Figure 10-left), perforated foundations or eccentric tension loads seem to provide promising countermeasures against suction generation during uplift.
.
Figure 10. (left) Foundation model with suction flap to reduce uplift resistance – from Gaudin et al. (2017); (right) spudcan reinstallation near existing footprint – from Jun et al. (2017).
An area of continued interest in oil and gas geotechnics has always been the analysis of spudcan footings during installation and operations. In the past decades, substantial efforts have been devoted to improve the geotechnical input to the site specific assessment of jack-up rigs, including – among other issues – the evaluation of spudcan penetration, capacity, fixity and related punch-through risk. The present version of the ISO standard 19905-1 (ISO, 2012) collects the achievements produced by years of industry-academy cooperation. Expected developments on spudcan-related research will concern the analysis of spudcan touch-down and extraction, interaction with buried structures, reinstallation near existing footprints, etc. The preliminary work by Jun et al. tackles the last item using a combination of 1g small-scale tests and 3D LDFE simulations. In particular, a novel spudcan shape, featuring holes and underside profiles, is proposed to ease footprint-spudcan interaction in clay (Figure 10-right).
The deep water activities still ongoing in the Gulf of Mexico continue to motivate geotechnical research on the interaction between soft clays and suction anchors – in fundamental analogy with the contents in Section 2.3. The work by Olin and Ovando confirms through FE analyses the main factors affecting the undrained horizontal-vertical capacity of suction piles, such as interface adhesion, aspect ratio, padeye location and load inclination (Andersen et al., 2005).
5 NEARSHORE WORKS
The 3 papers grouped within this subsection are at a glance quite diverse and specific, thus hard to relate to general research threads in nearshore geotechnics.
Hou et al. aim to reduce the construction costs of offshore cofferdams by exploiting cheap underwater soft soils subjected to cement injection (stabilisation), bagging and final solidification. The shear strength of bagged soils is expectedly found to depend on the curing time, as well as on the properties of the natural soil, the water content and the cement percentage – here deemed sufficient when as much as 8%. The authors conclude their study by proposing a construction practice for offshore cofferdams, and commenting on the (improvable) settlement performance after construction.
Kasama et al. performed 1g geo-hydraulic physical modelling to study the breakwaters of the Kamaishi Harbor and support resilient design against tsunamis. In particular, different configurations of the block reinforcement on the breakwater foundation are investigated in terms of weight, open ratio and layout pattern. The authors propose a formula to calculate the weight of block reinforcement ensuring stability under tsunami-induced seepage and overflow. It is also observed that the damage of the breakwater is minimised when cylindrical blocks with height-to-radius H/R=3/4 are placed in a triangulate layout. The work of Albert et al. recalls the nearshore disaster that took place on January 13th 2012, when the Costa Concordia cruise vessel shipwrecked close to the Giglio island (about 15 km off the coast of Tuscany, Italy). The paper describes the design of a hold-back system installed to prevent wreck sliding during winter storms and parbuckling operations. The hold-back system was formed by steel caissons anchored to the rocky seabed and connected to the wreck through steel slings (Figure 11-left). To ensure sufficient capacity under the remarkable design pulling load of 8 MN, preliminary load tests on single tendons were performed in the field (Figure 11-right). The authors elaborate on the experimental confirmation of relevant design assumptions, and especially on the avoidance of progressive failure due to viscous effects and cyclic loading.
Figure 11. (left) Sketch of Costa Concordia wreck and hold-back system for removal; (right) pulling field test set-up – from Albert et al. (2017).
6 CONCLUDING REMARKS
The papers submitted to the TC209 session confirm the intense research activities around offshore geotechnical applications, promoting both fundamental developments and engineering design. This report provides the main highlights from all 22 papers, and sets out to relate them to current research trends and knowledge gaps. Overall, the offshore wind arena seems to attract the present interest of most geo-offshore experts, with contributions concerning different foundation systems and related design issues. Despite many remarkable achievements, significant open questions remain about the analysis and optimisation of installation processes, as well as the performance of soil-foundation-turbine systems under cyclic loading and dynamic conditions – not only in relation to offhore wind projects. These subjects are expected to dominate offshore research for many years ahead, as the set-up and use of foundation systems evolves continually in response to industry trends.
7 REFERENCES
7.1 Papers submitted to the session
Albert L., Lambrughi A., Spinelli D., Staffini F. 2017. Costa Concordia wreck removal project – active anchorages for the foundations of the hold back system. In Proceedings of the 19th ICSMGE, Seoul.
Anusic I., Eiksund G.R., Meissl S., Liingaard M.A. 2017. Effects of new technique of large diameter monopile installation in the North Sea. In Proceedings of the 19th ICSMGE, Seoul.
Bagheri P., Son S.W., Kim J.M. 2017. Evaluation of the bearing capacity of suction bucket foundations used for offshore wind turbine using finite element modelling. In Proceedings of the 19th
ICSMGE, Seoul.
Brinkgreve R., Burg M., Liim L.J., Andreykiv A. 2017. On the practical use of the Material Point Method for offshore geotechnical applications. In Proceedings of the 19th ICSMGE, Seoul.
Choi Y.J., Schroder K., Lacasse S. Comparison of analytical and numerical design of suction anchors in deepwater clays. In
Proceedings of the 19th ICSMGE, Seoul.
Deb T.K., Singh B. 2017. Numerical modelling of bucket foundations in dense sand supporting offshore wind turbines. In Proceedings of
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Gaudin C., Li X., Tian Y., Cassidy M.J. 2017. About the uplift resistance of subsea structures. In Proceedings of the 19th ICSMGE,
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Hou J., Liu A., Liu W. 2017. Offshore cofferdam construction technology with bagged soil solidification. In Proceedings of the
19th ICSMGE, Seoul.
Jian Y., Leung C.F. 2017. Behavior of free-head monopile in clay. In
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Jun M., Kim Y., Hossain M.S., Cassidy M.J. 2017. Physical and numerical modelling of novel spudcans for easing footprint-spudcan interaction issues. In Proceedings of the 19th ICSMGE,
Seoul.
Kasama K., Zen K., Hirasawa M. 2017. Stability evaluation of the block reinforcement for tsunami resilient breakwater. In
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Labenski J., Moormann C. 2017. Numerical simulation of the lateral bearing behaviour of open steel pipe piles with regard to their installation method. In Proceedings of the 19th ICSMGE, Seoul.
Li W., Luo R. Yang M. 2017. Numerical study on laterally loaded monopiles in clay: soil failure model. In Proceedings of the 19th
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Nakata Y. 2017. Void ratio and angle of repose for coral gravel sand mixture. In Proceedings of the 19th ICSMGE, Seoul.
Niemann C., Tian Y., O’Loughlin C., Cassidy M.K. 2017. Response of piles under cyclic lateral loading – centrifuge tests. In Proceedings
of the 19th ICSMGE, Seoul.
Olin M., Ovando E. 2017. Behavior of suction piles under undrained loading up to failure. In Proceedings of the 19th ICSMGE, Seoul.
Park D., Park. J. 2017. Capacity of bucket foundation on sand over clay layers. In Proceedings of the 19th ICSMGE, Seoul.
Pisanò F., Corciulo S., Zanoli O. 2017. Role of advanced soil modelling in the dynamic analysis of offshore wind turbines. In Proceedings
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