ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V009T00A001. doi:10.1115/OMAE2017-NS9.

This online compilation of papers from the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Anchors and Pipelines

2017;():V009T10A001. doi:10.1115/OMAE2017-61046.

This paper investigates the effect of soil drainage on the uplift resistance of buried pipelines, and their susceptibility to upheaval buckling. The uplift resistance of buried pipelines is considered through analytical and numerical predictions for both drained and undrained conditions. Combinations of soil strength parameters for typical soils are estimated based on common correlations. For certain ranges of typical normally consolidated soil conditions, particularly those with high critical state friction angles, the drained uplift resistance may be lower than the undrained resistance. This observation is important because in typical practice only drained or undrained behaviour is considered depending on the general type of soil backfill used. In this case, the critical or minimum uplift resistance may be overlooked. Further, the changing undrained uplift mechanism between shallow and deep conditions is investigated. It is found that the common approach of considering the minimum of either a local (flow around) or global (vertical slip plane) failure can overestimate the uplift resistance in normally consolidated soils.

Commentary by Dr. Valentin Fuster
2017;():V009T10A002. doi:10.1115/OMAE2017-61348.

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated, such as 360-degree rotation of the anchor arm, gravity installation of anchors with high soil strain rate, and keying and diving (or penetration) of anchors. As a very important component of the installation or mooring system, anchor line connects the anchor and the anchor handling vessel (AHV) or floating moored platform. With moving of the AHV or platform, anchor line produces a space movement, and forms a reverse catenary shape and even a three-dimensional profile in the soil. Numerical analysis on the behaviors of anchor lines and deepwater anchors requires techniques that can deal with large strains and deformations of the soil, track changes in soil strength due to soil deformation, strain rate and strain softening effects, appropriately describe anchor-soil friction, and construct structures with connector elements to conform to their characteristics. Being an effective tool of large deformation finite element analysis, the coupled Eulerian-Lagrangian (CEL) method is advantageous in handling geotechnical problems with large deformations, where a traditional Lagrangian analysis is coupled with an Eulerian phase of material advection. This paper gives an overview of several key techniques in the CEL analysis of comprehensive behaviors of deepwater anchors, including construction of the embedded anchor line and the anchor line in the water, installation of gravity installed anchors (GIAs), keying or diving of drag anchors and GIAs, and implementation of the omni-directional arm of GIAs. Numerical probe tests and comparative studies are also presented to examine the robustness and accuracy of the proposed techniques. The aim of this paper is to provide a numerical framework to analyze the comprehensive behaviors of anchor lines and deepwater anchors.

Commentary by Dr. Valentin Fuster
2017;():V009T10A003. doi:10.1115/OMAE2017-61570.

The OMNI-Max anchors, which are used as foundations for mooring deep-water offshore facilities, are raised recent years for their dynamically installation. ANSYS CFX 17.0 is a computational fluid dynamic (CFD) program, capable of simulating the dynamically installation process of the OMNI-Max anchor. In the simulation, soft clay with linearly increasing shear strength is modeled as Eulerian fluid material. The clay is subjected to high shear strain rate during the dynamical installation procedure, hence the H-B model is proposed as it is applicable to a wide range of shear strain rate. Different anchor impact velocity levels are modeled to investigate their effects on the anchor final penetration depths. To improve the anchor impact velocity and final penetration depth, a booster, which is retrievable and renewable, is attached to the tail of the anchor. The results demonstrate that the anchor would achieve deeper penetration depth with the increase in impact velocity. Also the anchor with a booster could reach a deeper penetration depth than that of the single anchor owing to the increase of the anchor total energy.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Buckets, Suction Caissons and Skirted Foundations

2017;():V009T10A004. doi:10.1115/OMAE2017-61378.

In this paper the installation procedure of suction caissons is investigated by means of coupled seepage large deformation analysis performed with finite element methods. The modelling techniques employed to enable simulations of the penetration of a caisson into the soil under offshore conditions, i. e. several tens of meters below the water level. The numerical model includes a u-p-formulation, which is used to calculate the excess pore pressures and effective stresses from the total stresses. The Coupled-Eulerian-Lagrangian (CEL) approach available in conjunction with the Abaqus/Explicit solver is used. The calculation results are compared to centrifuge tests that were carried out recently at the Centre for Offshore Foundation Systems (COFS). This sheds light on the potential and the limitations of the presented numerical techniques. This paper concludes with a brief discussion of alternative numerical approaches that could be capable of the simulation of caisson installation.

Commentary by Dr. Valentin Fuster
2017;():V009T10A005. doi:10.1115/OMAE2017-61503.

The Sapinhoá and Lula North-East fields, 300km offshore Rio de Janeiro, Brazil are the two pilot fields of the Pre-salt development strategy. Field architecture consists of satellite wells connected to a spread moored FPSO in each of the fields. Through a design competition Petrobras and Partners selected the de-coupled riser system developed by Subsea 7. In each field two BSRs (Buoy Supporting Risers) anchored at 250m below waterline support the rigid risers on one side and flexible flowlines running to the FPSO on the other. Each BSR displaces close to 10,000t of water and provides a nominal net up thrust of 3,250t. This Paper highlights the engineering challenges and the solution developed for the large-scale foundation anchors that support these massive BSRs in the harsh environment of the Santos Basin in 2140m water depth.

Vertically below each corner of the BSR tank an 8m diameter by 18m penetration suction anchor houses the receptacles for the pair of tethers. Tension in the tethers was tuned to optimise the system stiffness (to minimise lateral BSR excursion orbits and avoid clash of risers and FPSO mooring lines) while requiring minimum anchor capacity. Four ballast modules of 150t each sit on top of each suction anchor to provide the remainder of the required uplift resistance. Soil conditions across both fields consist of soft silty clay. A geotechnical FE model of the suction anchor in Abaqus was used to evaluate the interaction of the structure, surrounding soil and trapped water beneath the top cap. The model was used to develop the complete load-displacement curves of the system during the undrained design current events.

A consolidation FE model using the Soft Soil model in PLAXIS showed that the hybrid anchor/ballast system under sustained uplift loading is stable throughout the design life for the level of sustained design load.

Although the LRFD verification format of suction anchors and gravity anchors are well covered in the design codes, there seems to be a gap in the coverage of this particular type of hybrid anchor. The governing condition was found to be the long term drained pull-out capacity under sustained loading with the assumption of slowly leaking suction port and air evacuation port at the top.

Commentary by Dr. Valentin Fuster
2017;():V009T10A006. doi:10.1115/OMAE2017-62043.

The design of suction caissons for offshore wind turbines is generally performed deterministically, using load and resistance factors taken from relevant codes and standards. This approach has been widely accepted for the geotechnical design of suction caissons in well-characterised soil conditions. However, soil layering and properties often vary considerably from location to location within an offshore wind farm. Furthermore, the installation process can also cause changes in the soil state in the vicinity of the skirts, which will affect noticeably the response of the suction caisson during operation. While this can be considered implicitly in the geotechnical design by means of assumptions, the inherent uncertainty of the soil state will impact the overall performance of the structure considerably. On the other hand, an optimized design for the system requires an accurate prediction of the foundation stiffness.

The authors present a reliability-based framework for the assessment of foundation stiffness, taking into account the most important input parameters and their expected variability. The framework can be applied both in foundation design as well as during actual installation in order to provide immediate feedback to permit adjustment or mitigation before the installation is finalized. Using a reliability-based approach in design allows an assessment of the probability of reaching the limiting design criteria and to quantify the related risk.

The proposed reliability-based framework can be applied to other design aspects. This is exemplified using the example of coupled foundation installation and capacity assessment. The authors further discuss how the framework can be extended to more complex design procedures.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Fluid-Soil-Structure Interaction

2017;():V009T10A007. doi:10.1115/OMAE2017-61263.

Seabed consolidation state could be seen as the initial simulation condition for numerically simulating wave-induced seabed response. In this study, based on a three dimensional numerical model, effects of initial consolidation state on the 3D wave-induced unsaturated seabed response around mono-pile were investigated. By model application, the consolidation state of seabed around the pile foundation was described. Two common calculation approaches (seabed consolidation is considered or not) for wave-induced seabed response were compared by describing the distributions pattern of soil effective stresses and pore pressures around the pile. Significance of the consolidation state on seabed dynamic response against distances to pile was also carefully addressed. Numerical simulations indicated (1) the initial consolidation significantly increases the vertical effective normal stress in the vicinity of pile, (2) effects of the initial seabed consolidation on the wave-induced seabed response decrease with the increasing distance to the pile. This study suggests the initial consolidation should be considered in assessing the seabed stability for the design of the mono-pile foundation.

Topics: Waves , Seabed
Commentary by Dr. Valentin Fuster
2017;():V009T10A008. doi:10.1115/OMAE2017-61416.

The wave-structure-seabed interaction (WSSI) around circular rubble-mound breakwater head is investigated using a three-dimensional (3D) numerical scheme. The result reveals that the presence of breakwater has strong effect on wave motion and seabed response. The turbulence induced by the breakwater head gives rise to extensive pore pressure around the breakwater head, which could further lead to liquefaction or scour and might eventually result in breakwater failure.

Topics: Waves , Breakwaters , Soil
Commentary by Dr. Valentin Fuster
2017;():V009T10A009. doi:10.1115/OMAE2017-61640.

In order to prevent the future risk of soil and structural failures, it is essential to evaluate the dynamic seabed soil behaviors in the vicinity of the offshore foundations under dynamic wave loadings. Three-dimensional (3D) numerical analysis is conducted on the interaction between waves, seabed soil and a gravity-based wind turbine foundation. An OpenFOAM based numerical code developed by Tang [1]for wave-structure-seabed interaction is applied. The nonlinear waves are modeled by solving the Navier-Stokes equations for incompressible flow. The dynamic structural response of the foundation is computed using a linear elasticity solver. The transient responses of the seabed are solved by an anisotropic poro-elastic soil solver. The dynamic interaction between different physical domains is implemented by boundary condition coupling and updating in the integrated FVM based framework.

The dynamic wave pressure on the structure and the seabed, the elastic responses of the structure and the changes of the pore pressure, shear stress and seepage flow structure in the seabed are investigated. Highest wave-induced shear stress along the foundation is predicted by solving the deformable structure model. For the seabed soil in the vicinity of the foundation, it is found that the presence of the foundation affects the soil responses by amplifying the wave induced shearing effect on the underlying seabed. Vertical distributions of the pore pressure in the seabed beneath the foundation are investigated with different angles relative to the wave propagation direction. A parametric study of isotropic and anisotropic soil permeability is performed and demonstrates that for the simulated soil in this work, the consideration of the anisotropic permeability is suggested.

Commentary by Dr. Valentin Fuster
2017;():V009T10A010. doi:10.1115/OMAE2017-62484.

In coastal and offshore structures, the predominant forces leading to lateral movements are mainly due to waves, currents, winds, berthing forces, mooring forces and lateral earth pressure due to unstable slope as a result of dredging or siltation etc. Due to relative movement between the piles and the soil, the load transfer to pile is a complex soil interaction problem. It is a two way problem and should be solved by structure-soil model with appropriate load displacement characteristics of both the structure and the soil. Pile-soil interaction analysis is carried out by numerical methods based on iterative techniques of equilibrium of forces and moments, based on relative stiffness of pile-soil system. Conventionally API guidelines and Vesic equation are used to analyze the laterally loaded piles. The study of laterally loaded pile in active soil wedge requires a proper assessment of soil structure interaction phenomenon involving the interaction between pile surface and the surrounding soil. The instability of soil wedge can occur due to self-weight, surcharge load, dredging, siltation and earthquake force. The soil structure interaction problem of piles located in active soil wedge has rarely been approached. Laterally loaded piles are analyzed by methods derived from the classical beam on elastic foundation mode in which the soil support is approximated by a series of independent elastic spring. The soil spring constants estimated from API guidelines and Vesic equations are not suitable for piles located in active soil wedge. Hence in this paper, a numerical study is carried out for a berthing structure in dense sand using PLAXIS 3D and STAAD Pro, in order to study the behaviour of piles in active soil wedge.

Topics: Soil
Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Pile Foundations

2017;():V009T10A011. doi:10.1115/OMAE2017-61423.

Offshore Wind Turbines (OWT) are slender structures with sensitive dynamics, strongly influenced by the soil-structure interaction. The structure is subjected to cyclic and dynamic loads with frequencies close to the first natural frequency of the offshore wind turbine. To avoid any resonance phenomenon, a precise evaluation of the initial first natural frequency of the wind turbine is essential. The present work deals with the evaluation of the natural frequency of an OWT’s scaled model with monopile foundation. The main factor influencing the natural frequency is the soil-structure interaction which needs to be assessed precisely. To do so, a simple method presented by [Adhikari and Bhattacharya, 2012] assimilates the offshore wind turbine as an Euler-Bernoulli beam on a flexible foundation with lateral and rotational springs. The key factor in the evaluation of the natural frequency is the value of the stiffness of these springs. In this way, this paper presents a method combining experimental measurements and a finite element model on Abaqus which allows a precise evaluation of the stiffness of the springs. The proposed method is compared to the existing methods used to evaluate the soil’s stiffness (such as [Eurocode 8, 2003]). The suggested method gives a fine evaluation of the response of the structure with a mean deviation below 1%, compared to the average errors obtained for the previous methods ranging from 6.6 to 17.4%.

Commentary by Dr. Valentin Fuster
2017;():V009T10A012. doi:10.1115/OMAE2017-61600.

This paper presents and discusses a new web based application for analysing piled foundations subject to lateral loads; a common problem in civil engineering. Details of the algorithm used to form and solve the non-linear system of equations are provided, along with details of the application’s deployment on Amazon Web Services cloud infrastructure. The application is particularly convenient to use as it does not require the installation of any software. The responsive web based interface scales according the screen size of the user’s device and can therefore be run on PC, tablet or smart phone.The application has been developed with both industry and the research community in mind. A particularly convenient method for specifying user defined soil springs is available. This feature is illustrated in this paper with an example that also validates the numerical implementation.

Commentary by Dr. Valentin Fuster
2017;():V009T10A013. doi:10.1115/OMAE2017-61603.

This paper describes an automated algorithm for determining the length and diameter of monopile foundations subject to lateral loads with the aim of minimising the pile weight, whilst satisfying both ultimate and serviceability limit states. The algorithm works by wrapping an optimisation routine around a finite element p - y model for laterally loaded piles. The objective function is expressed as a function representing the pile volume, while the ultimate limit state and serviceability limit states are expressed as optimisation constraints. The approach was found to be accurate and near instantaneous when compared to manual design procedures and may improve design outcomes and reduce design time and costs.

Topics: Stress , Design
Commentary by Dr. Valentin Fuster
2017;():V009T10A014. doi:10.1115/OMAE2017-62250.

As part of the national off-shore wind power development project by the government of Korea, the very first off-shore wind turbine utilizing tri-pod suction buckets for its sub-surface foundation has been successfully completed. This off-shore wind turbine with a capacity of 3MW has been designed, constructed, and installed in late 2016. It is located approximately 200 meters offshore with a water depth of approximately 10 meters. Sub-surface soil consists of interbedded clay and sand layers. Details of the design, construction, and installation of this wind turbine with tripod suction bucket foundation system are described and discussed.

Commentary by Dr. Valentin Fuster
2017;():V009T10A015. doi:10.1115/OMAE2017-62469.

Monopiles are at present the most widespread foundation type for offshore wind turbines (OWTs), due to their simplicity and economic convenience. The current trend towards increasingly powerful OWTs in deeper waters is challenging the existing procedures for geotechnical design, requiring accurate assessment of transient soil-monopile interaction and, specifically, of the associated modal frequencies.

In this work, advanced 3D finite element (FE) modelling is applied to the dynamic analysis of soil-monopile-OWT systems under environmental service loads. Numerical results are presented to point out the interplay of soil non-linearity and cyclic hydro-mechanical (HM) coupling, and its impact on transient response of the system at increasing load magnitude. It is shown how the lesson learned from advanced modelling may directly inspire simplified, yet effective, spring models for the engineering dynamic analysis of OWTs.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Seabed Processes

2017;():V009T10A016. doi:10.1115/OMAE2017-61130.

This paper reports the deformation behavior of silica and carbonate sands under a strip foundation subjected to uniaxial vertical load. Small-scale physical modelling tests of a strip surface foundation under vertical load were conducted in a geotechnical centrifuge and Particle Image Velocimetry/Digital Image Correlation (PIV/DIC) was used to analyze images of an exposed plane of the model beneath the foundation to visualize the failure mechanisms. The observed mechanisms are interpreted in conjunction with load-settlement response and cone penetrometer resistance profiles. The failure mechanisms are illustrated through normalized vertical and horizontal displacement fields and shear and volumetric strain fields derived from the PIV analysis. Different soil deformation mechanisms and load-settlement responses were observed in the different sands. Soil resistance profiles measured using a miniature cone penetrometer do not correlate with the measured foundation bearing resistance and an interpretation of particle shape effect is introduced to explain the differing behaviors. The results presented improve understanding of the different responses in carbonate sands and silica sand beneath a shallow foundation under vertical load.

Commentary by Dr. Valentin Fuster
2017;():V009T10A017. doi:10.1115/OMAE2017-61299.

Offshore structures are founded on submerged foundations. The excavation of submerged foundations in the sea bed is a difficult task to accomplish when it comes to the decommissioning of these offshore structures. The extraction resistance is a lot higher than the pressure acting on the structure due to hydrostatic pressure, earth pressure and its self-weight. Once the extraction begins, a negative pore water pressure is created until inflowing pore water compensates this negative pore water pressure. This depression is hindering the extraction of the submerged foundation. Additionally, the resistance is dependent on the embedment depth of the structure, the soil properties as well as the extraction velocity, which influences the dimension of the negative pore water pressure. The numerical investigation of this dynamic problem is a limitation for continuum based approaches like the Finite Element Method (FEM) due to the occurring large deformations. These results from the soil bed failing under the movement of the structure and hence starting to flow. Additionally, in order to estimate the created depression, the investigation of the water-soil-interaction is crucial, as the change of the pore water pressure plays a significant role. Therefore, it is necessary to analyze the behavior of the soil particles and the pore water pressure. In order to do this, a coupled Euler-Lagrange approach, namely the combination of Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM), is used. In these simulations on one hand, the liquid phase, e.g. the water, is considered as a continuum, while on the other hand, for the solid phase, e.g. the soil, a particle representation is chosen. Hence, it is possible to compute the particle-particle — as well as the fluid-particle-interactions. The calculations are carried out with the open source software package CFDEMcoupling®, which combines the discrete element code LIGGGHTS® with CFD solvers based on OpenFOAM®. This paper introduces the coupled CFD-DEM approach to simulate the extraction of a submerged plate in the soil bed. In this work, the soil grains are idealized by spherical particles of different diameters. In order to consider effects of dilatancy and contractancy in the soil bed, different relative densities are investigated. Additionally, a variation of the extraction velocity of the plate is carried out to examine the dependence on the creation of negative pore water pressure. For each case, the extraction resistance is calculated. The flow velocity and the pressure distribution in the vicinity of the structure are analyzed.

Commentary by Dr. Valentin Fuster
2017;():V009T10A018. doi:10.1115/OMAE2017-61496.

The paper investigates the behavior of buried offshore pipelines overlying active reverse faults using the finite element (FE) tool Abaqus. In the FE analyses, the pipeline is modeled with 3D shell elements and the soil with continuum elements. Equivalent boundary conditions are imposed at both pipeline ends to account for the elastic response in the far-field away from the fault. Nonlinear materials, nonlinear interactions, and nonlinear geometries are adopted.The Mohr-coulomb constitutive model with strain-softening is used to model the soil behavior and true stress-strain properties are incorporated to model the response of the pipeline steel material. The effects of soil properties, pipeline diameter, diameter to thickness ratio, and fault displacement are investigated. The results focused on analyzing the pipeline deformations, the axial strains, and the buckling behavior of the pipeline with increasing vertical bedrock displacement. Critical compressive strains are calculated and compared with the DNV code provisions.

Commentary by Dr. Valentin Fuster
2017;():V009T10A019. doi:10.1115/OMAE2017-62365.

Available formulae for the time scale of scour below a submarine pipeline are so far mainly restricted to waves-only and currents-only conditions with normal incidence and no burial of the pipeline. This paper presents experimental results on temporal and spatial developments of local scour below a partially buried pipeline under combined waves and currents with oblique incident angle. There are also some tests with waves-only and current-only conditions for validation purpose. All tests were conducted under live-bed conditions. The maximum Shields parameter is used to formulate the nondimensional time scale of local scour under combined waves and current conditions. The effects of flow incident angle and embedment depth on the time scale of scour were investigated. The results show that the time scale increases with the increase of incident angle and also increases with the increase of the embedment depth for combined waves and current conditions. The effective shields parameter is applied to account for the effect of the oblique incident angle based on the fact that both the normal component and the axial components of the flow contribute to the scour process when the currents or waves approach the pipeline with an oblique angle. It is found that the previously proposed empirical formula for time scale of scour for waves-only and current-only conditions, is also applicable to the combined waves and current conditions, if the maximum Shields parameter is used as the governed parameter in quantifying the time scale.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Seabed Properties

2017;():V009T10A020. doi:10.1115/OMAE2017-61695.

Steel catenary risers (SCRs) are subjected to fatigue in the touchdown zone (TDZ) where the pipe interacts with the seabed. In this zone the seabed is subjected to intermittent episodes of cyclic loading and reconsolidation during long-term operation. Cyclic loading, reconsolidation and maintained load can cause variations in the soil strength and stiffness, which has a significant influence on the fatigue life of the riser in the TDZ.

The weakening effect of cyclic loading on soil strength is well recognized throughout design practice, and methodologies for determining the cyclic ‘fatigue’ of clay during undrained cyclic loading are well established (e.g. Andersen et al. 1988; Andersen 2015). However, traditional undrained assessments neglect the effects of drainage and consolidation that inevitably occur in pipe-seabed interaction during long-term operational stages, and can lead to changes in stiffness by a factor of up to 5 or 10. This overlooked effect of consolidation on soil resistance and stiffness can be very important for SCR fatigue analysis.

In this paper, a new analytical framework considering these effects has been used to analyze vertical pipe-seabed interaction. This framework is developed using a critical-state concept with effective stresses, and by discretizing the soil domain as a one-dimensional column of soil elements. The model can accurately capture the changing soil resistance and stiffness to account for the effects of remoulding, reconsolidation and maintained load. The framework is used to back-analysis the pipe-soil interaction response during small and large amplitude vertical cycles. The simulation prediction compares well with the measured results from the laboratory (Aubeny et al., 2008), and can accurately capture the observed changes in stiffness of up to a factor of 5.

Topics: Stress , Pipes , Soil , Seabed , Accounting
Commentary by Dr. Valentin Fuster
2017;():V009T10A021. doi:10.1115/OMAE2017-61933.

This work addresses the experimental study of the forces exerted during the extraction of sediment samples using push corers. The study aims to measure push and pull forces under different deployment conditions for corer speed and coring depth using sand, sandy silt and silt as sediment. To guarantee a repeatable automated process, a KUKA KR6 robot manipulator was used to extract the sample. The forces were measured using a bidirectional S-type load cell. The required data is extracted from the robot’s internal variables log and from the load cell’s data acquisition system. The raw data is processed to develop simplified models for the forces using linear regression which are further analyzed and tested. Finally, the results obtained are discussed in the context of core sampling and practical conclusions are drawn from the experiments.

Commentary by Dr. Valentin Fuster
2017;():V009T10A022. doi:10.1115/OMAE2017-62517.

A torpedo base is a type of conductor casing that embeds into the seabed mainly by free fall using its own weight as driving energy. One of the advantages of this concept is to install the conductor casing before the dynamic positioned (DP) drillship arrival at the location. This reduces the time of the well drilling leading to significant cost saving. However, the need to withstand the challenging loads of the ultra-deep water scenarios pushed the typical torpedo base design to its limit and, consequently, modifications to its original geometry and more accurate design models are needed. Therefore, in this work, a new torpedo base, designed to sustain high axial loads in very soft clays, is analyzed with a three-dimensional finite element (FE) model. This model accounts for the setup-effects of the soil with the use of a previously proposed analytical approach to estimate the stress state of the soil at any time after the installation of the base. The results obtained indicate that the axial holding capacity of the base varies along time. The holding capacity increase rapidly at the beginning of the installation, but this rate reduces after the first days. Depending on soil characteristics, full axial capacity may be reached more than one year after the installation of the base. Moreover, the use of more than four fins welded to the shaft of the conductor casing modifies the shear zone along the base, but does not contribute to a significant increase in the axial holding capacity.

Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Design Codes for Planning of Marine Operations

2017;():V009T12A001. doi:10.1115/OMAE2017-61252.

This paper presents a numerical model intended to simulate the mooring load and the dynamic response of a moored structure in drifting ice. The mooring lines were explicitly modelled by using a generic cable model with a set of constraint equations providing desired structural properties such as the axial, bending and torsional stiffness. The 6 degrees-of-freedom (DOF) rigid body motions of the structure were simulated by considering its interactions with the mooring lines and the drifting ice. In this simulation, a fragmented ice field of broken ice pieces can be considered under the effects of current and wave. The ice-ice and ice-structure interaction forces were calculated based on a viscoelastic-plastic rheological model. The hydrodynamic forces acting on the floating structure, mooring line and drifting ice were simplified and calculated appropriately. The present study, in general, demonstrates the potential of developing a full numerical model for the coupled analysis of a moored structure in a broken ice field with current and wave.

Topics: Waves , Ice , Mooring
Commentary by Dr. Valentin Fuster
2017;():V009T12A002. doi:10.1115/OMAE2017-62313.

The first edition of the DNV Offshore Standard “Design of Floating Wind Turbine Structures”, DNV-OS-J103, was published in June 2013. The standard represented a condensation of all relevant requirements for floaters in existing DNV standards for the offshore oil and gas industry which were considered relevant also for offshore floating structures for support of wind turbines, supplemented by necessary adaptation to the wind turbine application.

As part of the harmonization of the DNV GL codes for the wind turbine industry after the merger between Det Norske Veritas (DNV) and Germanischer Lloyd (GL) in the autumn of 2013, DNV GL currently plans to publish a revision of DNV-OS-J103 in 2017, to become identified as DNVGL-ST-0119.

The new revision is intended to reflect the experience gained since 2013 as well as the current trends within the industry.

Commentary by Dr. Valentin Fuster
2017;():V009T12A003. doi:10.1115/OMAE2017-62426.

This paper presents the current state of regulations, guidelines and the engineering in the Norwegian aquaculture industry. The statistics of fish escapes is evaluated and the need for further developments of the regulations, in planned revisions, of the Norwegian standard, are laid. Simplified case studies are shown to present the main forces acting on fish farms.

Commentary by Dr. Valentin Fuster
2017;():V009T12A004. doi:10.1115/OMAE2017-62461.

The abundance of consistent high strength winds off the world’s coastlines and the close proximity to dense population centers has led to development of innovative marine structures to support wind turbines to capture this energy resource. Off the US coast, 60% of the offshore wind lies in deep water (greater than 60m) where the development of Floating Offshore Wind Turbine (FOWT) hull technology will likely be required in lieu of fixed bottom technology such as jacket structures. The United States National Renewable Energy Laboratory (NREL) and the offshore wind community commonly refer to 60m as the transition point between fixed bottom structures and floating structures due to economic reasons. Floating wind turbines deployed in the harsh offshore marine environment require the use of materials that are cost-effective, corrosion resistant, require little maintenance and are highly durable. This has led the University of Maine to develop a concrete hull technology called VolturnUS for full-scale 6MW FOWTs. In this work, experimental testing was conducted to verify the performance of the concrete under operational, serviceability, and extreme loading conditions as required by the American Bureau of Shipping Guide for Building and Classing Floating Offshore Wind Turbines. The testing included structural testing sub-components of the hull and served as experimental verification of American Bureau of Shipping (ABS) concrete design methodology which is currently approved and being used to design the first commercial scale FOWTs in the United States. Two 6MW wind turbines supported on VolturnUS concrete hulls will be used for the New England Aqua Ventus I project. The project is planned to be deployed and connected to the grid by 2019 in the Northeast U.S. and is funded by the US Department of Energy.

Commentary by Dr. Valentin Fuster
2017;():V009T12A005. doi:10.1115/OMAE2017-62516.

Fatigue design standards for offshore structures became needed with development of offshore structures in harsh environments like the North Sea during the 1970s. The Ultimate Limit State had earlier been the most significant design requirement for similar platforms in the Mexican Gulf being less utilized with respect fatigue.

The need for fatigue design of ship structures became increased as more high strength steel was being used in these structures during the 1970s. The analysis method for long term loading and assessment of fatigue capacity has been improved over the years and this has also resulted in need for new and revised design standards.

New types of structures and structural components have been developed like tension leg platforms and support structures for wind turbines. These structures are subjected to significant dynamic loading such that fatigue design becomes the main issue and relevant fatigue design standards are needed.

Fatigue assessment of fixed offshore structures in the North Sea has been standard practice since the 1970s. Fatigue of floating platforms became a requirement after the accident with the Alexander Kielland platform in 1980. Later new types of structures have been installed in the North Sea such as tension leg platforms and floating production ships where fatigue has been an important part of the design. A further challenge with respect to fatigue came with the development of support structures for wind turbines to be installed in the sea.

This paper gives an overview of the development of fatigue design standards for marine structures over the last 40 years. This includes the significance of refined calculation of long term stress range distribution, calculation of hot spot stress, size effect and effect of mean stress effect on fatigue design of ship structures.

Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Fatigue Analysis

2017;():V009T12A006. doi:10.1115/OMAE2017-61811.

A bridge between platforms needs to operate safely and continuously over its lifecycle. This paper focuses on the fatigue assessment of the bridge pin connection due to relative movements between platforms. A nonlinear time domain stochastic fatigue analysis of the pin connection in a bridge in the North Sea using a combined model of the jacket platforms and the interconnecting bridge is presented. The fatigue life is compared to the fatigue life from a linear frequency domain stochastic analysis.

The facility has been in operation for more than 40 years and the operator requested an update of the inspection plans for the bridge. An RBI analysis has been done according to [1] based on fatigue results from wind gusts and relative movements. Regarding the fatigue assessment due to relative movements there are uncertainties related to selection of the friction coefficient. It was assessed that a friction coefficient of 0.4 is slightly conservative in this case. The fatigue life of the pin was calculated based on a linear frequency domain stochastic analysis, assuming that the bridge was fixed at both ends and this was considered reasonable conservative for fatigue estimation. Efforts have been made in the study presented here to assess the conservatism through a nonlinear time domain stochastic fatigue analysis. The sliding connections of the bridge are simulated by nonlinear springs. The effects of assuming different friction coefficients and different nonlinear spring models for a certain friction coefficient on the fatigue damage of the pin are investigated by a sensitivity study. The fatigue lives of the pin thus computed for a series of short-term sea states for the different assumptions for the friction coefficient and the nonlinear spring model are then compared to the result from a corresponding frequency domain approach.

Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Innovative Marine Structures or Installation Procedures

2017;():V009T12A007. doi:10.1115/OMAE2017-61189.

The paper will look into the hydrodynamic loads and responses on the proposed Submerged Floating Tube Bridge (SFTB) through the Digernessund by the Norwegian Public Roads Administration (Statens vegvesen, NPRA). The aim is to show how different hydrodynamics aspects during the prelimiary design can be simply addressed under the given environmental conditions. Different SFTB systems are introduced as the first step. A simplified method based on modal analysis is introduced and implemented for evaluation of the motions and stress, bending moments along the bridge. Firstly, a 2D Boundary Element Method (BEM) solver is developed and verified, which is further used for solving the hydrodynamics coefficients of different bridge cross sections. The 3D hydrodynamic coefficients of pontoons are solved by the commercial software AQWA. The analysis procedure of the simplified method for the global SFTB responses is presented. The Eigen periods of the Bjørnefjord SFTB is re-calculated by the present model as a first validation of the implementation. The loads and responses of the bridge under given wave conditions are then estimated. The evaluation of the possibility of vortex induced vibrations of the current SFTB design is given.

Commentary by Dr. Valentin Fuster
2017;():V009T12A008. doi:10.1115/OMAE2017-62292.

In present work, a novel concept by combing a monopile wind turbine and a heave-type wave energy converter has been proposed, that is referred as the ‘MWWC’ (Mono-WT-WEC Combination) system herein. Concept feasibility study has been carried out by doing coupled aerodynamic and hydrodynamic numerical simulation in the time domain. Aerodynamic loads and output wind power of the NREL 5MW wind turbine are determined by the NREL Aerodyn code, based on BEM method. Hydrodynamic loads of the mono-pile and the WEC are calculated by the AQWA code, which is available for modeling multi-body systems including both mechanical and hydrodynamic couplings between the mono-pile and the WEC. Firstly, the effect of different power-take-off (PTO) parameters and wave periods on the performance of the WEC’s wave energy production under typical wave cases has been investigate, and a preliminary optimal value for the PTO’s damping stiffness has been proposed; secondly, the dynamic characteristic of the MWWC system has been investigate using coupled wind-wave loads analysis under typical operational sea cases. Finally, the extreme responses of the MWWC system have been obtained for its ULS design, and the potential challenging areas of the MWWC system has been highlighted.

Commentary by Dr. Valentin Fuster
2017;():V009T12A009. doi:10.1115/OMAE2017-62314.

Common industrial practice for designing floating wind turbines is to set an operational limit for the tower-top axial acceleration, normally in the range of 0.2–0.3g, which is typically understood to be related to the safety of turbine components. This paper investigates the rationality of the tower-top acceleration limit by evaluating the correlation between acceleration and drivetrain responses. A 5 MW reference drivetrain is selected and modelled on a spar-type floating wind turbine in 320 m water depth. A range of environmental conditions are selected based on the long-term distribution of wind speed, significant wave height, and peak period from hindcast data for the Northern North Sea. For each condition, global analysis using an aero-hydro-servo-elastic tool is carried out for six one-hour realizations. The global analysis results provide useful information on their own — regarding the correlation between environmental condition and tower top acceleration, and correlation between tower top acceleration and other responses of interest — which are used as input in a decoupled analysis approach. The load effects and motions from the global analysis are applied on a detailed drivetrain model in a multi-body system (MBS) analysis tool. The local responses on bearings are then obtained from MBS analysis and post-processed for the correlation study. Although the maximum acceleration provides a good indication of the wave-induced loads, it is not seen to be a good predictor for significant fatigue damage on the main bearings in this case.

Commentary by Dr. Valentin Fuster
2017;():V009T12A010. doi:10.1115/OMAE2017-62534.

Based on the three dimensional potential theory and finite element method (FEM), this paper presented a method for time-domain hydroelastic analysis of a floating bridge in inhomogeneous waves. A floating bridge in both regular and irregular waves, is taken as a numerical example. This method is firstly validated by the comparisons of the results between frequency domain method and presented time domain method under regular wave condition. Then the hydroeleastic responses of the floating bridge in waves with spatially varying significant wave height/peak period are presented, with the purpose to illustrate the feasibility of the proposed method. The primary results at this stage indicate that the inhomogeneity of the waves might affect the structure dynamic responses of the floating bridge in waves.

Commentary by Dr. Valentin Fuster
2017;():V009T12A011. doi:10.1115/OMAE2017-62617.

Many of the various proposed wave-energy converter (WEC) units are immersed oscillating bodies, which, in the primary conversion stage, collect input power as the product of two oscillating factors, a velocity and wave-induced force. The latter factor is vulnerable to destructive wave interference, unless the extension of each WEC unit is sufficiently small. Two simple, elementary-mathematical, inequalities express two kinds of upper bounds for the wave power that may be absorbed by an oscillating immersed body. The first upper bound, published in the mid 1970s, is well-known, in contrast to the second one, Budal’s upper bound, which was derived a few years later, and which takes the WEC’s hull volume into consideration. Combining the two different upper bounds and considering also a typical wave climate, we may conclude that for a WEC array plant deployed in the North Atlantic, each point-absorber WEC unit volume should typically be about 300 cubic metre, and its primary-converted power take-off (PTO) capacity should be in the range of 50 to 300 kW. These heaving WEC units, being monopole wave radiators, may have a much higher PTO-capacity-to-immersed-hull-wet-surface ratio than any other type of WEC unit, such as those using dipole-mode (e.g. surge- or pitch-mode) radiation. For large-scale utilization of wave energy, arrays of WEC units are required.

Commentary by Dr. Valentin Fuster
2017;():V009T12A012. doi:10.1115/OMAE2017-62659.

Technology and competence from offshore structures, which were developed to serve in the harsh environment of the North Sea, have played a major role in developing technology for extreme fjord crossings in Norway. The Høgsfjord Project, and later the E39 Fjord Crossings Project, have taken advantage of the existing competence and experience from offshore structures. To a great extent, it has been a question of adapting offshore technology and combining it with advanced bridge technology to new types of structures. These new bridges are in many respects very different from offshore structures, mainly due to their length, slenderness, long resonance periods and complex dynamic behavior.

Professor Torgeir Moan has been heavily involved in the technology development for the fjord crossings. First, in the Høgsfjord submerged floating tunnel project, and now in the Fjord Crossings project with the bold aim of making the E39 Coastal Highway along the west-coast of Norway free of ferries. He has always showed genuine interest in the challenges, and have always been willing to help identifying and solving the problems. His contribution has been vital to the outcome of the work.

Commentary by Dr. Valentin Fuster
2017;():V009T12A013. doi:10.1115/OMAE2017-62663.

Intensive research and development of Mega-float was implemented by Technological Research Association of Mega-Float (TRAM) in late 1990s. Mega-float is a concept of a pontoon type VLFS installed in protected calm water typically large bay, and the concept was promoted by TRAM after relatively long concept development phase in Japan.

This paper reviews the research and development of VLFS in Japan. The research and development of Mega-float is briefly reviewed firstly and two research and developments after Mega-float are reviewed.

Concerning development of Oil-Gas development, logistic hub concept has been developed by J-DeEP, a consortium comprised of major Japanese heavy industries, shipping company, ClassNK and NMRI, and proposed for presalt oil field development offshore Brazil.

Another research is pontoon type coal storage and offloading facility to be installed offshore South Sumatra, Indonesia. The coal storage and offloading facility was investigated by JIP comprised of JMU, ClassNK, Osaka University and University of Tokyo.

Commentary by Dr. Valentin Fuster
2017;():V009T12A014. doi:10.1115/OMAE2017-62680.

The rapid expansion of world population, the exhausting of inland resources and the requirement of sustainable development of world economy have strengthened the efforts of mankind to increase the capability of resource exploitation and space utilization in the ocean. Very Large Floating Structures (VLFS) are among those marine structures that have attracted long lasting attention in ocean utilization for several decades. The applications of different size VLFS as floating piers, floating airports, floating hotels, floating fuel facilities and even floating cities have triggered extensive researches. Several projects including the conceptual design and construction of VLFS have been launched, for instance, Mega-Float in Tokyo Bay, Floating oil storage bases in Kamigoto and Shirashima islands, Floating emergency rescue bases in Yokohoma, Floating performance stage in Singapore, Large floating bridge in Norway, Mobile offshore base (MOB) in USA and Multi-purpose floating base near islands in China.

Due to much larger dimensions, relatively smaller global rigidities and lower natural frequencies than an ordinary ship, a VLFS has apparent flexible body responses rather than rigid body motions in waves. Hence hydroelastic analyses are of great importance in design and safety assessment of a VLFS. Extensive researches have been carried out during the past decades in the development of prediction methods of hydroelastic responses of VLFSs. However, most publications in this field were for VLFSs in open sea. If a VLFS is deployed near islands and reefs in complicated geographical environment, the wave conditions, wave loads and the hydroelastic responses of a VLFS will be quite different than in open sea.

In this paper the three-dimensional hydroelasticity theories that have been widely used in the analysis of a VLFS in deep or shallow open sea with constant water depth are briefly introduced. Based on these theories the numerical approaches of hydroelastic analyses of a VLFS near island and reefs in shallow sea, developed recently by CSSRC, are described. Some important technical problems, including description of wave environment, design scheme, connectors between modules, hydroelastic responses, coupled responses with mooring system and safety analysis of a VLFS deployed near islands and reefs are also discussed.

Commentary by Dr. Valentin Fuster
2017;():V009T12A015. doi:10.1115/OMAE2017-62691.

Energy, fisheries and transport infrastructures are increasingly being established offshore. Facilities such as offshore wind farms may occupy large areas and compete with other users of the maritime space. Accordingly, offshore platforms that can combine many functions within the same infrastructure could offer significant benefits. This applies to economy, optimization of spatial planning and minimization of the impact on the environment.

In the present paper, some proposed innovative designs for multi-use offshore platforms are described. The technical, economical and environmental feasibility of designing, installing, operating, servicing and maintaining such platforms are discussed. The relevant platforms under consideration are targeted towards ocean renewable energy (in particular offshore wind), aquaculture and related transport maritime services.

Innovative designs for multi-use offshore platforms that intend to allow optimal coupling of the various activities and services are highlighted. Issues such as safe and efficient installation, operation, maintenance and monitoring are also briefly discussed in the paper.

Commentary by Dr. Valentin Fuster
2017;():V009T12A016. doi:10.1115/OMAE2017-62698.

Long floating bridges supported by pontoons with span-widths between 100m and 200m are discrete hydro-elastic structures with many critical eigenmodes. The response of the bridge girder is dominated by vertical eigenmodes and coupled horizontal modes (lateral) and rotational modes (about the longitudinal axis of the bridge girder). In this paper it is focused on design principles to reduce the response with regards to these eigenmodes.

It is shown for a floating bridge with 200m span-width that by inserting a bottom flange the vertical eigenmodes can be lifted out of wind driven wave regime. It is also shown that selecting a pontoon length that give cancellation of excitation forces is beneficial, and that the geometrical shaping of the pontoon can be efficient to decrease the bridge response.

Commentary by Dr. Valentin Fuster
2017;():V009T12A017. doi:10.1115/OMAE2017-62711.

Size effects are extremely important to understand and accounting for their inclusion in fatigue tests is imperative in order to produce meaningful results. This can however be problematic as in dealing with very large offshore structural details and sections, testing to destruction can involve extremely large loads and gripping arrangements that can be cost prohibitive. It is therefore important to be able to simplify test specimens so that the local detail is retained without losing important size influences.

This paper describes an investigation into whether or not extracted specimens from actual Offshore Wind Monopile Sections are in fact the best choice for fatigue testing. The results are analysed and a recommendation is made for future test programmes involving offshore wind monopile and tower applications.

Commentary by Dr. Valentin Fuster
2017;():V009T12A018. doi:10.1115/OMAE2017-62714.

This paper describe the development of floating bridge projects along the coast of Norway. Over the decades, the need to replace existing ferry connections with fixed linkes has beed rapidly increasing. However, the Norwegian nature with its deep and wide fjords, together with low traffic numbers have given the engineers big challenges in designing cost effective bridges for many of the crossings. One solution that was developed in the late 80s and early 90s by a combination of offshore technology and bridge technology was the end-anchored floating bridge concept. So far, two bridges of this design have been completed, the Bergsøysund Bridge and the Nordhordland Bridge. Prof. Moan was a key contributer in the developing work, enabling a safe design of the bridges. As the remaining ferry crossings are even deeper and wider, there is a need to further develop this concept, as well as developing new concepts, so that we are able to replace even further ferries with fixed links. The paper will look back at the two already buildt floating bridges, as well as the floating bridge concepts that might be used to cross the Bjørnafjord, on the west coast of Norway.

Commentary by Dr. Valentin Fuster
2017;():V009T12A019. doi:10.1115/OMAE2017-62718.

In April 2014 a series of 30:1 scaled shake table test of the NREL-5MW monopile offshore wind turbine were conducted. Significant whirling motions were observed during the test [1], which were seldom reported for civil engineering and offshore engineering structures. This study is thereby devoted to revealing the mechanism of the whirling motion of a typical tall monoplie offshore wind turbine.

The original shake table model is simplified into a uniform cantilever beam with a large lumped mass on the top. Two nonlinear coupled integro-differential equations of motion containing cubic nonlinearities due to curvature and inertia are used and solved by both analytical and numerical approaches. The ElCentroExp random base excitation and lateral harmonic base excitations with different amplitudes and frequencies are considered in the analysis to fathom the instability mechanism.

The analysis results show that, for harmonic base excitations with frequency within a specific range, when the amplitude of base excitation and the initial perturbation exceed a certain threshold, stable whirling motion response will be triggered and the motion of shake table model will be controlled by whirling. However, the analysis results also demonstrate that, subjected to the ElCentroExp base excitation or its equivalent lateral harmonic base excitations, neither analytical nor numerical approach can produce whirling motion response, regardless of the damping ratio. The LM model will always show planar motion response. This differs significantly from the observation of shake table model test.

Commentary by Dr. Valentin Fuster
2017;():V009T12A020. doi:10.1115/OMAE2017-62719.

The hydrodynamic analysis of a multiple pontoon-type floating bridge (FB) interacting with oblique waves in water of finite arbitrary depth can be performed, at least in principle, using a general-purpose radiation-diffraction code. The fluid is assumed to be inviscid, and the flow can be considered as incompressible and irrotational, and the velocity potentials are calculated by boundary element method. To study the influences of the water depth and the gap between bodies on the hydrodynamic properties of the pontoons, RAOs (Response Amplitude Operators) of modes of each pontoon versus the wave frequencies are calculated and presented. The results show that the RAOs of pitch modes of different pontoons have differences in high frequency in heading waves, and those RAOs differences of heave of different pontoons in heading and oblique waves are small. Furthermore, all the others results nearly match into one. The influences of width of the gap between pontoons to RAOs are small, but the water depth has obvious influence on RAOs. In addition, the motions of FB are simulated and compared in different sea conditions which are represented by the JONSWAP spectrum. The results show that not only the peak wave periods but also the significant wave heights have obvious influences on the motions of the FB.

Topics: Waves , Water
Commentary by Dr. Valentin Fuster
2017;():V009T12A021. doi:10.1115/OMAE2017-62720.

The Norwegian Public Roads Administration is running a project ‘Ferry free coastal route E39’ which includes replacing ferry crossings by bridges or tunnels across eight fjords in Western Norway. Since most of the fjords are wide and deep, traditional fixed links are not possible to be constructed. Therefore, floating bridge and tunnel concepts are proposed for the fjord-crossing project. Because floating bridges and tunnels have many structural components close to the water surface, a critical concern of accidental ship collision loads is then raised. Considering the large displacement and high speed of the passing ships, the interactions between the bridge structure and the ship bow can be significant should collision occurs. It is therefore important to carefully evaluate bridge response subjected to ship collision loads in the design process. This paper presents a case study of ship collision analysis of the floating bridge concept for Bjørnafjorden. Two possible collision scenarios, i.e. ship-pontoon and ship-girder collisions, are considered. First, local structural deformation and damage are numerically investigated by detailed finite element models using LS-DYNA. Second, bridge global response under ship collision loads are simulated in USFOS. By combining the local and global analyses, a comprehensive overview of bridge response under ship collision load can be obtained.

Commentary by Dr. Valentin Fuster
2017;():V009T12A022. doi:10.1115/OMAE2017-62721.

Floating suspension bridges, one of several new designs to make it possible to cross deep and wide fjords, consist of three spans and supported by two tension leg platforms and two fixed traditional concrete pylons. Geometric nonlinearities, nonlinear aerodynamic and hydrodynamic forces and nonlinear mooring systems can become of high importance. Time domain methods are commonly applied when nonlinearities need to be considered. The main challenge in time domain simulation of the floating suspension bridge is the modelling of frequency-dependent aerodynamic self-excited forces and hydrodynamic radiation forces. This paper shows how rational functions fitted to aerodynamic derivatives and hydrodynamic added mass and potential damping can be converted to state space models to transform the frequency-dependent forces to time-domain. A user element is implemented in the software ABAQUS to be able to include the self-excited forces in the dynamic analysis. The element is developed as a one node element that is included in the nodes along the girder and the tension leg platforms. The responses of the floating suspension bridge under turbulent wind forces and first-order wave excitation forces are calculated in a comprehensive case study and compared with results obtained using a multi-mode frequency domain approach to illustrate the performance of the presented time-domain methodology.

Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Inspection, Monitoring, Maintenance and Repair

2017;():V009T12A023. doi:10.1115/OMAE2017-61467.

Based on the rain-flow counting technique, a frequency domain method is developed for calculating the fatigue damages caused by the bimodal Gaussian loads. Firstly, by considering the reduction effect of low frequency loads on high frequency ones, the amplitudes of the small rain-flow cycles are obtained. Secondly, the amplitudes of the large rain-flow cycles are determined by means of Turkstra’s rule of load combination. Moreover, based on the numerical solutions developed, the two analytical formulas for the damage estimates of small and large cycles are developed. Both numerical and analytical solutions are benchmarked against the rain-flow damage estimates and compared with the existing ones. The numerical analyses show that the damages estimated by the new method are close to the rain-flow damages.

Topics: Fatigue damage
Commentary by Dr. Valentin Fuster
2017;():V009T12A024. doi:10.1115/OMAE2017-61751.

The objective of the present study is to analyse and identify the most suitable corrosion degradation model, fitted with real corrosion depth measurement data sets and to reproduce the corroded steel plate surface as a function of time and spatial distribution using advanced statistical methods. An approach for properly identifying the best fitted model to real corrosion depth measurement data sets is employed. A sequence dependent data analysis is performed based on the fast Fourier transform, which is used as an input for a random field modelling of the corroded steel plate surfaces.

Commentary by Dr. Valentin Fuster
2017;():V009T12A025. doi:10.1115/OMAE2017-62422.

In structural integrity management, it is essential to know the fatigue crack growth potential. The lessons learned from use of refined fatigue analyses, fracture mechanics and probabilistic methods for platforms in-service are presented. For ageing offshore units of semi-submersible design, the inspection history of more than 20 000 NDT inspections and detection of close to 1000 fatigue cracks, are used in this study. These experience data are used to assess the potential for Non-conservative estimate for the fatigue crack growth potential.

Commentary by Dr. Valentin Fuster
2017;():V009T12A026. doi:10.1115/OMAE2017-62425.

For marine platforms, assessing the structural resilience in a corroded condition is vital for both design and maintenance practices. With the development of computational and experimental methods for structural analysis, the accuracy of the structural response prediction relies on a better understanding of the material degradation process. However, a realistic estimate of corrosion is inherently a complex undertaking. Corrosion of even a single form can often involve multiple stages, each of which has different steps across several geometric scales; corrosion systems are often multi-layered and involve geometric complexities; the mechanical factors (stress/strain distributions) could affect the corrosion initiation and kinetics. These complexities have resulted in scientific barriers to the advancement of a corrosion prognosis that forecasts damage accumulation, as well as a computational realization of the corrosion-structural analysis. This paper reviews the numerical and experimental work that the authors have done, including the development of nonlinear finite element models to assess the behavior of damaged steel ship structures, full-field experimental verifications, application of the mechano-electrochemical theory and in situ tensile-corrosion tests. It is intended that the outcome of this research will be the establishment of a systematic multi-scale multi-physics experimental and numerical protocol for predicting aged structural resilience.

Topics: Corrosion
Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Modelling and Analysis of Marine Operations

2017;():V009T12A027. doi:10.1115/OMAE2017-62003.

This paper deals with a nonlinear model predictive control (NMPC) scheme for a winch servo motor to overcome the sudden peak tension in the lifting wire caused by a lumped-mass payload at the beginning of a lifting off or a lowering operation. The crane-wire-payload system is modeled in 3 degrees of freedom with the Newton-Euler approach. Direct multiple shooting and real-time iteration (RTI) scheme are employed to provide feedback control input to the winch servo. Simulations are implemented with MATLAB and CaSADi toolkit. By well tuning the weighting matrices, the NMPC controller can reduce the snatch loads in the lifting wire and the winch loads simultaneously. A comparative study with a PID controller is conducted to verify its performance.

Commentary by Dr. Valentin Fuster
2017;():V009T12A028. doi:10.1115/OMAE2017-62034.

Jack-up barges are commonly used for marine operations in the offshore oil and gas, and offshore wind industries. A critical phase within the marine operation activities is the positioning of the jack-up legs onto the seabed. During this process, large impact velocities and forces may arise from the barge’s heave, roll and pitch motions, and structural damage of the legs can occur. This paper numerically investigates the effect of a flopper stopper (FS) on the motion responses of a jack-up barge from the offshore wind industry. The FS is known as a passive roll compensation device. It is suspended from the side of the barge by means of wire ropes and cantilever beams. A simple geometry of an FS is proposed, and the working principle introduced. For the loading condition before the leg-soil impact occurs, global dynamic analyses of the coupled system are conducted. Characteristic values of impact velocities are used to establish the jack-up operational limits in terms of the significant wave height and peak period. By comparing the operational limits for the barge with and without FS, it is found that FS should be placed on the weather side. At beam seas, the current FS can lead to a maximum increase in the operational wave height limit of 35%, whereas for the other wave headings, it may not be beneficial to use FS.

Commentary by Dr. Valentin Fuster
2017;():V009T12A029. doi:10.1115/OMAE2017-62307.

Marine subsea operations are performed by highly specialized ships, referred to as Inspection, Maintenance, and Repair (IMR) and Offshore Construction Vessels (OCV). Although the ships and their on-board equipment are designed to operate in harsh environmental conditions, the current practice often is to terminate operations when a rigid and conservative weather limitation is reached, often specified in terms of the significant wave height as the exclusive criterion. Such general limitations do not account for vessel specific motion behavior. Since the offshore industry is aiming for all-year-round safe subsea operations, there is a strong interest amongst ship designers, owners and operators to establish vessel and task specific criteria. The project Vessel Performance within the Norwegian Centre for Research-based Innovation on Marine Operations (SFI MOVE), is developing response-based procedures, that are leading to case-specific operational ranges. This approach enables the full exploitation of vessel performance capabilities for safe and efficient offshore operations. Two methods with different complexity levels are proposed. Firstly, on the higher level, detailed operability analysis for a fleet and sea area of interest are performed by means of numerical tools.

This level can be used to obtain detailed results for existing ships, but the procedure can also be applied as guidance in the design stage. Secondly, on the lower level, generic diagrams can be used to estimate and compare the operational performance of different vessels based on fact sheet parameters. This is especially relevant for decision making processes where a detailed study cannot be performed.

Commentary by Dr. Valentin Fuster
2017;():V009T12A030. doi:10.1115/OMAE2017-62342.

Current methods for installation of offshore wind turbines are all sensitive to the weather conditions and the present cost level of offshore wind power is more than twice the cost of land-based units, increasing with water depth. This paper presents numerical simulations of a novel experimental gripper design to reduce the environmental effects applied to a catamaran type of vessel during wind turbine installation. In SFI MOVE project in NTNU Aalesund, our team proposed a novel wind turbine installation process. A new catamaran vessel will carry pre-assembled wind turbines to the installation location. Two new designed grippers on the deck will make a lifting operation to install the wind turbine onto the turbine foundation. Three prismatic grippers with several rolling contact points at the end are attached in an arc at the catamaran’s aft, designed to grasp the turbine foundation in order to make a connection between the two in the horizontal plane. This paper will only emphasize the contact responses between the turbine foundation and the three grippers during the wind turbine installation process. Numerical simulations are carried out using the virtual prototyping framework Vicosim which is developed by NTNU Aalesund. The simulation results show validation of a key part of the proposed new wind turbine installation idea.

Commentary by Dr. Valentin Fuster
2017;():V009T12A031. doi:10.1115/OMAE2017-62447.

Increasing demands on economy in the offshore industry and extension to North Atlantic all-year operations bring along a requirement for more accurate numerical simulations of marine installation operations. These simulations will assess the necessary vessel and crane capacity and determine the limiting sea state for the installation. An essential input to such analysis is hydrodynamic coefficients for the structures to be installed. Together with the vessel capacity, the limiting sea-state will, in many cases be dependent on the coefficients used in the simulations. A realistic estimate of the coefficients for the structure in question will therefore have directly influence on the cost of the operation. Coefficients for hydrodynamic added mass and damping for simple geometries are given in standard textbooks and in the “Recommended practice for modelling and analysis of marine operations”, DNVGL-RP-H103. For more complex structures and structure parts on the other hand, there is a lack of published data. MARINTEK and NTNU have through the last decades performed model testing of different structural details as well as complete subsea structures. During the research program MOVE, started in 2015 by NTNU, MARINTEK and SINTEF, a considerable amount of model test data is collected and compared to obtain general trends and connections suitable for engineering estimates of coefficients to be used in numerical simulations. Suction anchors are widely used for permanent mooring of floating production and storage vessels, and also for foundation of subsea structures. In several cases use of suction anchors have replaced piling for foundation of offshore wind turbines and jacket platforms. The anchors can be exposed to high hydrodynamic forces during the installation. This paper presents hydrodynamic coefficients in heave for suction anchors with different degree of perforation of the top plate and different height/diameter relations. The results range from anchors with no perforation to around 18% perforation.

Topics: Suction
Commentary by Dr. Valentin Fuster
2017;():V009T12A032. doi:10.1115/OMAE2017-62499.

This paper describes a study aimed at finding and demonstrating a feasible method to reduce the uncertainties in calculation of dynamic forces and limiting sea states for installation of protection covers produced from glass fiber reinforced polyester (GRP). Uncertainties arise in the choice of hydrodynamic coefficients and the applied analysis method e.g. the Simplified Method, as suggested in DNV-OS-H206, versus time-domain simulations. The maximum limiting sea state for water entry and lowering through the splash zone has been assessed stepwise by use of alternative methods. Firstly, the hydrodynamic force coefficients for a fully submerged, selected GRP cover were estimated manually, by use of simplified data in DNVGL-“Recommended practice for modelling and analysis of marine operations”, DNVGL-RP-H103. The estimated hydrodynamic added mass was compared with the potential theory solution obtained by use of WAMIT. WAMIT calculations are also performed to obtain added mass and potential damping for the cover with different draughts at the selected installation angle. Viscous damping and added mass will be dependent on amplitude of oscillation and is studied by CFD simulations. A fully submerged cover is oscillated harmonically with different amplitudes at a selected period. The obtained added mass and damping coefficients were used in a numerical model including installation ship, lifting gear and GRP cover, in the non-linear time domain simulation program SIMO. The lowering through the splash zone were finally performed in some selected wave conditions to illustrate how a realistic limiting sea-state for the lowering through the splash zone may be estimated.

Topics: Seas
Commentary by Dr. Valentin Fuster
2017;():V009T12A033. doi:10.1115/OMAE2017-62649.

In order to compute fatigue damage during offshore transports it is necessary to assume a description of the sea states encountered during the voyage. In recent years, it has become a common approach to apply directional long-term scatter diagrams for the transportation route, taking into account vessel speed, course and time of year for the departure.

An important contribution to the transportation fatigue damage is usually the wave induced inertia load. For ship shaped vessels additional viscous damping needs to be included in order to estimate correct roll response. However, since viscous roll damping is non-linear, correct estimation of fatigue damage can only be obtained by computing partial damage for all individual sea states in the scatter diagram. This becomes very time-consuming and is usually not done.

Instead, the roll damping level is tuned to match typical mean sea states in the scatter diagram. The roll damping will then be too low for higher sea states and too large for smaller sea states. When choosing the roll damping level, the aim should be to obtain an overall error in transportation fatigue damage which is minimized.

This paper describes a method to estimate a representative viscous roll damping level for transportation fatigue analyses.

Commentary by Dr. Valentin Fuster
2017;():V009T12A034. doi:10.1115/OMAE2017-62716.

Operations where a flexible pipe, umbilical or cable is loaded out from an on-shore spool to the hold of an installation vessel can cause a build-up of torsion. In unfavourable cases, the torsion has been known to cause spiralling or various forms of damage to the tensile armour.

This paper presents a vocabulary for the description of torsion. It then gives a short review of design codes, enumerates known failure modes (some of which have not always been identified as torsion related), and discusses the mechanisms of torsion generation, with an emphasis on the effect of internal friction. It concludes with some ideas on torsion prevention.

Topics: Cables , Torsion , Pipes , Vessels
Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Reliability Analysis of Marine Structures and Operations

2017;():V009T12A035. doi:10.1115/OMAE2017-62294.

Safety of marine structures is one of the most important problems to be concerned by all stakeholders. Prof. Torgeir Moan of Marine Technology at the Norwegian University of Science and Technology has dedicated almost all his academic life in investigating various issues in this field and he and his colleagues and students have made significant contributions to the analysis methods. He is certainly deserved to be dedicated a special symposium in honor of his contribution. The present author was greatly influenced by him and worked for the similar issues. In this paper, the author will re-discuss the issue of the safety of marine structures from a philosophical point of view based on his own experience. The purpose of this presentation is to identify the key factors on safety of marine structures and to suggest the potential treatment methods for these factors. It is expected that this discussion is of some value for the future researches in this field of safety of marine structures which was greatly focused by Prof. Torgeir Moan.

Commentary by Dr. Valentin Fuster
2017;():V009T12A036. doi:10.1115/OMAE2017-62708.

Dynamically positioned (DP) vessels are widely used in marine operations in the offshore oil & gas industry. The risk caused by DP vessels’ position loss is not negligible, and managing DP safety in marine operations is indispensable. This paper presents an overall framework for DP safety management in offshore marine operations. It consists of the following three key steps, i.e. 1) Identification of position loss scenarios, 2) Risk analysis in terms of position loss frequency and consequences, and risk evaluation, 3) Mitigation of risk via measures to eliminate the risk, or to reduce the likelihood of position loss, as well as to mitigate the consequence in marine operations given DP vessel position losses. Case studies from DP shuttle tankers and DP mobile offshore drilling units are presented to illustrate key principles in each of the above three steps. Quantitative risk analysis and evaluation approach is highlighted, and the associated challenges are described. Recommendations to further improve DP safety management in offshore marine operations are proposed.

Topics: Safety , Vessels
Commentary by Dr. Valentin Fuster
2017;():V009T12A037. doi:10.1115/OMAE2017-62709.

Loads for the purpose of structural design are often based on estimated extreme values of time-varying loads based on limited amounts of data. Uncertainty in the estimation of the design loads inevitably leads to uncertainty in the resultant levels of structural reliability. In this paper, uncertainty is assessed for estimates of extreme wind loads calculated using statistical methods based on the average conditional exceedance rate (ACER), fitting of a Gumbel distribution and Peaks-Over-Threshold (POT). The ACER method gave the best results, but all the methods gave results which would normally be considered to be sufficiently accurate for engineering applications. However, for structures designed on the basis of the estimated values of V100 or V500, the uncertainty in the estimated design loads produced very uncertain probabilities of failure with a significant increase in their expected value. It is concluded that the uncertain distribution of the probabilities of failure must be taken into account when evaluating structural safety and a ‘fiducial confidence function’ is proposed for this purpose.

Topics: Reliability
Commentary by Dr. Valentin Fuster
2017;():V009T12A038. doi:10.1115/OMAE2017-62712.

Safety regulation and standards have developed intensively since the startup of the petroleum industry in Norway in the early 1970’ties. Prescriptive standards where used in the early days. In the 1980’ties risk analysis and the 10−4 criteria were introduced, along with accidental loads and the progressive collapse limit state. Since 2002 barriers have been a general part of the safety regulation in Norway. This paper aims at reviewing the development of safety regulation of structures and marine systems in Norway, present the barrier regulation as it is described in the Norwegian regulations, and finally evaluate the relevance of the barrier regulation for structures and marine systems.

Commentary by Dr. Valentin Fuster
2017;():V009T12A039. doi:10.1115/OMAE2017-62713.

Reliability analysis and probabilistic models for wind turbines are considered with special focus on structural components and application for reliability-based calibration of partial safety factors. The main design load cases to be considered in design of wind turbine components are presented including the effects of the control system and possible faults due to failure of electrical / mechanical components. Considerations are presented on the target reliability level for wind turbine structural components. Application is shown for reliability-based calibrations of partial safety factors for extreme and fatigue limit states are presented. Operation & Maintenance planning often follows corrective and preventive strategies based on information from condition monitoring and structural health monitoring systems. A reliability- and risk-based approach is presented where a life-cycle approach is used. An example with wind turbine blades is considered using the NORCOWE reference wind farm.

Commentary by Dr. Valentin Fuster
2017;():V009T12A040. doi:10.1115/OMAE2017-62715.

The present paper is predominantly a conceptual contribution with an appraisal of major developments in risk informed structural integrity management for offshore installations together with a discussion of their merits and the challenges which still lie ahead. Starting point is taken in a selected overview of research and development contributions which have formed the basis for Risk Based Inspection Planning (RBI) as we know it today. Thereafter an outline of the methodical basis for risk informed structural systems integrity management, i.e. the Bayesian decision analysis is provided in summary. The main focus is here directed on RBI for offshore facilities subject to fatigue damages. New ideas and methodical frameworks in the area of robustness and resilience modeling of structural systems are then introduced, and it is outlined how these may adequately be utilized to enhance Structural Integrity Management (SIM). Finally, the concept of Value of Information analysis (VoI) from the Bayesian pre-posterior decision analysis is proposed, as an overarching methodical platform for the planning and optimization of structural Health Monitoring (SHM) activities in the context of SIM at both operational and strategic levels.

Topics: Risk
Commentary by Dr. Valentin Fuster
2017;():V009T12A041. doi:10.1115/OMAE2017-62717.

Structural Reliability Analysis (SRA) is a useful tool in structural engineering. Uncertainty in input parameters and model uncertainties in the analysis predictions are explicitly modelled by random variables. With this methodology, the uncertainties involved are handled in a consistent and transparent way. Compared to a deterministic analysis, SRA provides improved insight in how the various uncertainties involved influence the results. The main results from SRA is the calculated probability of structural failure, but other useful results such as uncertainty importance factors and design points being the most likely combination of all variables at failure represent helpful information.

The present paper illustrates some the features using SRA for two different types of application. The first application is the use of SRA as a tool for code calibration and the second shows the application of SRA to a problem where common practice is likely to be rather conservative and therefore leading to unacceptable results, but where the degree of conservatism is not known.

Two examples are chosen to illustrate code calibration; i.e. hull girder ultimate limit state (ULS) for tankers and ULS for mooring design in the ULS for floating offshore vessels. Code calibration involves both SRA and design analysis following the code. It is shown how the design analysis can be modified in order to better reflect a chosen target reliability level across a selected set of test cases representative for what the code should cover.

Fatigue of subsea wellhead systems is selected as an example of a special case when application of existing rules may lead to unsatisfactory results which are likely to be rather conservative. It is shown how results can be presented in terms of the accumulated probability of fatigue failure as a function of time. This may be a more suitable basis for decision making than a calculated fatigue life from a standard analysis. It is also illustrated how importance factors from the SRA can be used as guidance on how to prioritize effort in order to improve prediction of the fatigue damage.

The present paper is not intended to be detailed in all input and analysis methodology, but draw the attention towards the possibilities and benefits of applying SRA in structural engineering, where the examples are used to illustrate this potential.

Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Stochastic Dynamic Response Analysis of Marine Structures

2017;():V009T12A042. doi:10.1115/OMAE2017-61184.

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain nonlinear exciting forces in a seastate with significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The method is validated against experimental measurements with good agreement. The higher order moments of the nonlinear load are evaluated from simulated force records and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. The identification of the inertia and drag coefficients of a generalized nonlinear wave load model trained against experiments using Support Vector Machine learning algorithms is discussed.

Commentary by Dr. Valentin Fuster
2017;():V009T12A043. doi:10.1115/OMAE2017-61444.

This paper compares two approaches for the estimation of long-term response of wave load effects on offshore structures. These approaches are applied to estimate the extreme value of the cross section interaction ratio of a tubular component of the bracing system of a semisubmersible platform. The tubular component is subjected to axial loads and bending moments due to static loads and wave effects. The iteration ratio in the ultimate limit state is defined by applying design criteria derived from API RP-2A LRFD [6]. The approaches are also applied to estimate the long-term response of a single degree of freedom system due to wave actions.

The first approach is based on the proposals of Videiro and Moan [3]. The results of the first approach are compared with a new model of long-term response estimation, based on the up-crossing rate distribution of the response process.

Commentary by Dr. Valentin Fuster
2017;():V009T12A044. doi:10.1115/OMAE2017-61520.

The development of reliable fish farm structures for open seas becomes more and more important. One of the challenges is to design a robust structure to withstand the harsh offshore environmental loads. This paper investigates a semi-submersible type offshore fish farm system for open seas. This system consists of a semi-submersible support structure with pontoons and braces, a catenary mooring system and net cages. The support structure is designed to be rigid to resist severe offshore conditions. A preliminary hydrodynamic and response analysis is carried out for this concept. The linear hydrodynamic properties using different composite models with panel and Morison elements are computed. Based on the hydrodynamic analysis, linearised frequency-domain and coupled time-domain analysis are performed to predict the extreme motions of the support structure and the extreme tensions in the mooring lines. The results indicate that the frequency-domain method underestimates the extreme responses, and the couplings between the structure and the mooring system need to be considered in the time-domain. Responses using various hydrodynamic models are also compared to evaluate the influences of the viscous effects from the pontoons and the nets of this fish farm concept.

Commentary by Dr. Valentin Fuster
2017;():V009T12A045. doi:10.1115/OMAE2017-61802.

In this paper, a floating bridge concept is proposed. This bridge concept comprises a two oppositely curves in plan, which enables the cancellation of the axial forces at the bridge as one arch will be under compression while the other arch is in tension due to environmental forces acting in one direction. The road deck is carried by truss structures that are kept above the water by several elliptical cylindrical pontoons. To reduce drag load, the cross sectional area facing the current is reduced as much as possible, while the buoyancy is kept the same based on the initial weight estimation. Initial design consideration and methodology of a double curved floating bridge is presented, and a numerical model is established for analyzing this concept. Hydrodynamic and structural dynamic aspects are included in the numerical model. Parametric study of the bridge structural rigidity is performed to investigate the effect to the responses. White noise, regular and irregular wave simulations are carried out to investigate the dynamic responses of the floating bridge under different conditions.

Commentary by Dr. Valentin Fuster
2017;():V009T12A046. doi:10.1115/OMAE2017-62336.

A series of seakeeping simulations accounting for the wave-induced vibration is performed on three large container ships with different sizes. Time series of bodily motions, accelerations and stress due to vertical bending moment are calculated for the three ships navigating in a short-term sea state. Ship forward speed is varied from 0 knot to 20knots to investigate the sensitivity of the hydroelastic response to the change of the speed. Statistical analysis is made over the time series results, and the results are compared in terms of significant value. The uncertainty of the wave-induced vibration with respect to the ship speed is evaluated for the respective ships. It is found out that the increase rate of pitch motion, accelerations and stress to the increase of the ships’ forward speed is different from each other. It is further observed that the acceleration and vertical bending moment increase is less prominent for the largest ship.

Commentary by Dr. Valentin Fuster
2017;():V009T12A047. doi:10.1115/OMAE2017-62467.

West Africa Sea, which is located in the tropical area, has special environmental conditions. Relatively mild sea state and long period swell as the main component of waves lead to significant high energy in low frequency region. Frequently there will be a special sea condition which contains long period swell and local storm wave. Research in this paper mainly focuses on motion and force characteristics of offloading system (contains offloading line, CALM buoy and its mooring system) under the special West Africa environmental conditions. Radiation/diffraction theory is adopted to calculate potential force acting on CALM buoy. The viscous drag force is considered by special disk-type Morison elements which are arranged according to certain rules. Physical model tests of truncated offloading system are carried out to verify the numerical model. The motion and force characters of offloading system under West Africa swell and wind sea are studied in both numerical method and model tests method. CALM buoy motion characteristics in specific environmental condition is analyzed. Relationships between CALM buoy motions and flexible structure (mooring line or offloading lines) tension forces are also researched.

Topics: Seas
Commentary by Dr. Valentin Fuster
2017;():V009T12A048. doi:10.1115/OMAE2017-62561.

The WindFloat prototype is a semisubmersible type foundation supporting a 2 MW, 3 bladed, horizontal axis Vestas V-80 turbine. The 8-year project is near its completion. After 3 years of planning, engineering and fabrication, the prototype was installed in 2011 in the northern Portugal Atlantic waters. Following 5 years of operations and electricity production, the unit was decommissioned in the summer of 2016. This paper retraces the prototype project going back to the early objectives, focusing on its 5-year performance and lessons learned. The overall assessment of the impact of the prototype on the incoming pre-commercial projects is discussed. Some emphasis is placed on both the decommissioning of the unit and the economics of the project, as these have not yet been published.

Commentary by Dr. Valentin Fuster
2017;():V009T12A049. doi:10.1115/OMAE2017-62578.

In this paper, numerical modelling and analysis of a hybrid-spar floating wind turbine is presented. The hybrid-spar consists of steel at the upper part and the precast prestressed concrete (PC) at the lower part. Such a configuration is referred to as a hybrid-spar in this paper. The hybrid spar was successfully installed offshore of Kabashima Island, Goto city, Nagasaki prefecture, Japan on October 18, 2013 (see OMAE2015-41544 [1] for details). In this paper, some details on numerical modelling of the hybrid-spar for design load analysis are presented. Then, the validation of the numerical analysis model is presented for a full-scale hybrid-spar model with 2-MW wind turbine.

Commentary by Dr. Valentin Fuster
2017;():V009T12A050. doi:10.1115/OMAE2017-62710.

An overview of the development of the Hywind concept is presented with focus on numerical analysis tools, model and full scale experiments, and control systems.

Several analysis tools have been developed and matured over several years, and verified against model and full scale experiments.

Special purpose control systems have been developed for the Hywind concept for the purpose of stabilization of floater motions as well as motion control.

Commentary by Dr. Valentin Fuster

Torgeir Moan Honoring Symposium: Validation of Simulation Models

2017;():V009T12A051. doi:10.1115/OMAE2017-62175.

In this research, real-time hybrid testing of floating wind turbines is carried out at model-scale. The semi-submersible, triangular platform is built to support two vertical-axis wind turbines (VAWTs). On account of incongruous scaling issues between the aerodynamics and the hydrodynamics, the wind turbines are not constructed at the same scale. Instead, remote-controlled (RC) plane motors and propellers are used as actuators to mimic only the tangential forces on the wind turbine blades, which are attached to the physical model. On a VAWT, the tangential force is proportional to the torque on the turbine, which contributes to the power production. A control algorithm is implemented using the wind turbine generators to optimize the platform heading and hence, the theoretical power absorbed by the wind turbines. The computer fluid dynamics simulations of two-dimensional counter-rotating turbines is briefly discussed. The results from the simulations are discussed in the context of existing onshore experimental data. This experimental approach only seeks to recreate the aerodynamic force which contributes to the power production. In doing so, the generator control algorithm can be validated. The advantages and drawbacks of this technique are discussed, including the need for low inertia actuators, which can quickly respond to control signals.

Commentary by Dr. Valentin Fuster
2017;():V009T12A052. doi:10.1115/OMAE2017-62598.

In this work, an alternative method is proposed for validation of ship handling models using onboard monitoring data of normal ship operations. If validation data can be collected during ship service, one can obtain a large number of “repetitions” without dedicated sea trials that are expensive. Although the accuracy of each sample will probably be less than in dedicated trials, it might be compensated by the fact that many samples will be available. As a case study, the results of simulation of the research vessel of the Norwegian University of Science and Technology (NTNU), Gunnerus, using MARINTEK’s vessel simulator (VeSim) are presented and evaluated by certain maneuvers identified from Gunnerus onboard measurements. To validate a ship simulator, it is shown that it might be better to investigate the agreement between simulation and full-scale measurements during operations similar to the ones that shall be simulated later, than just validating against standardized maneuvers such as turning circle and zig-zag tests.

Commentary by Dr. Valentin Fuster
2017;():V009T12A053. doi:10.1115/OMAE2017-62601.

A framework is described in which a manoeuvring simulator can be built up from model tests. It is applied to a modern LNG powered RORO ferry, the M/F Landegode, with model tests and computational results being used to make full-scale predictions. These predictions are tested against full-scale manoeuvring performance measurements, and are shown to be of a high quality.

Commentary by Dr. Valentin Fuster
2017;():V009T12A054. doi:10.1115/OMAE2017-62666.

With the steady growth of the number of Dynamic Positioning (DP) vessels, increasingly complex designs and operations, and a decreasing number of experienced DP operators, effective operational risk management tools are key for safer and more efficient operations. One of the key aspects when looking at the operational risks is the estimation of the vessel position and heading after the worst case single failure and in the transient period after the failure has occurred. The aim of this paper is to provide insight about the use of comprehensive dynamic operability analyses performed by time-domain simulations for understanding the vessel performance and limitations, in turn providing valuable and reliable input to operational risk assessment and planning. This paper presents also a comparison with results from full-scale trials.

Commentary by Dr. Valentin Fuster

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