Offshore wind turbines are subjected to combined static and cyclic loads due to its self weight, wind, current and waves. For the design of support structures, a point of concern is whether the highly varying loads may cause cyclic degradation of the soil leading to a permanent undesired pile settlement and tilting for the wind turbine. In particular during a severe storm, the large cyclic loads are being more critical as the wind and waves are typically from a single direction. The DTU 10MW wind turbine supported by a jacket at 33 m water depth is considered in this study, where the piles are axially loaded in order to bear the moment under wind and wave actions. This paper investigates the cyclic loads using traditional linear irregular waves and fully nonlinear irregular waves realized from the wave solver Ocean-Wave3D previously validated until near-breaking wave conditions. This study shows that the nonlinear irregular waves introduce more extreme cyclic loads, which result in significantly larger pile settlement than using linear wave realizations. For the case in this study, linear wave theory underestimates pile settlement at least 30% compared to nonlinear wave realizations.

It is now widely accepted that, due to significant economies of scale, the levelized cost of energy in offshore wind industry decreases as the turbine size and rated power increases. For offshore wind turbines, fixed and floating foundations can be quite complimentary when sites span a large water depth range. This paper presents the new WindFloat semisubmersible design supporting a 10MW generic wind turbine made by DTU [1]. This study evaluates the initial global performance of the WindFloat 10MW hull. In addition, RAO and frequency domain accelerations at the nacelle are presented. A comparison of the OpenFAST model, that we plan to use in all the future analysis, will be conducted against the benchmarked OrcaFAST model used by PPI and validated against the WindFloat prototype [2].

Fixed-bottom offshore wind turbines (OWTs) are typically located in shallow to intermediate water depth, where waves are likely to break. Support structure designs for such turbines must account for loads due to breaking waves, but predictions from breaking wave models often disagree with each other and with observed behavior. This variability indicates the need for a better understanding of each model’s strengths and limitations, especially for different ocean conditions. This work evaluates and improves the accuracy of common breaking wave criteria through comparison to Computational Fluid Dynamics (CFD) simulations of breaking waves. The simulated ocean conditions are representative of potential U.S. East Coast offshore wind energy development sites, but the discussion of model accuracy and limitations can be applied to any location with similar ocean conditions. The waves are simulated using CONVERGE, a commercial CFD software that uses a Volume of Fluid (VOF) approach and includes adaptive mesh refinement at the evolving air-water interface. First, the CFD model is validated against experimental data for shoaling and breaking wave surface elevations. Second, 2D simulations of breaking waves are compared to widely-used breaking wave limits (McCowan, Miche, and Goda) for different combinations of wave height, wavelength, water depth, and seafloor slope. Based on these comparisons, the accuracy and limitations of each breaking limit model are discussed. General usage guidelines are then recommended.

Offshore structures operating at sea are severely affected by slamming pressure. This slamming pressure significantly shortens the design life of offshore structures including offshore wind turbines and results in personal and material damage. This slamming load has to be fully considered in the design phase of the structure, and therefore good quality of experimental results should be supported. In Korea, offshore wind turbines have excellent conditions geographically. In the West sea, the construction of the jacket-type wind turbine is advantageous because the water depth is low, and in the East sea, which is relatively deep, the floating wind turbine is possible; thus, it is easy to generate a large-scale wind farm. For offshore wind turbines, several technical aspects should be considered. Among them, the damage of the structure caused by the slamming load should be studied. In the case of FOWTs operating in large waves, slamming can cause structural damage, and repeated slamming loads reduce the structural strength and shorten the design life. The slamming load should be calculated by applying the maximum peak, its width, duration and the dynamic load according to the natural period of the structure, and the importance of structural design. The results of the experiments can be used to determine the structural design, and the slamming load can be estimated to provide the design techniques offshore wind turbines as the design variables. In this study, we investigated experimentally the elastic effects of a cylindrical model on the slamming load characteristics by free drop test at height 1.0m and 1.7m.

There are several challenges facing the design of mooring system of floating offshore wind turbines (FOWTs), including installation costs, stability of lightweight minimalistic platforms, and shallow water depths (50–300m). For station keeping of FOWTs, a proper mooring system is required in order to maintain the translational motions in surge and sway and the rotational motions in yaw of the platform within an adequate range. A combination of light pre-tension, shallow water depth and large platform motions in response to a survival storm condition can result in snap-type impact events on mooring lines, thus increasing the line tension dramatically. In this paper, we present a new snap load criterion applicable to a catenary mooring system and compare it with Det Norske Veritas’ criterion for marine operations. As a case study, we examine the extreme tension on a catenary mooring system of a semi-submersible FOWT exposed to a 100-year storm condition. The software OrcaFlex was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic loads for FOWT analyses. Snap-type impact events were observed in the numerical simulations and were characterized by two criteria. Tension maxima were fitted using composite Weibull distributions (CWDs) and comparisons of probability exceedance were made for the two different snap load criteria.

The U.S. East Coast has the potential for offshore wind deployment and contribute to the energy demands of the area. A safe and cost-effective development of offshore wind farms requires detailed and accurate meteorological and oceanographic (MetOcean) analysis and, thus, a wave and hydrodynamic database has been produced along the U.S. East Coast. The high-resolution wave and hydrodynamic models are calibrated and validated against various NOAA measurements along the U.S. East Coast. The wave model open lateral boundaries are forced by spectral data from a global wave model. Wind forcing for both models were taken from CFSR wind fields, and the hydrodynamic model was also forced by atmospheric pressure field. Spatially and temporally-varying water level and depth-averaged currents were used to force the wave model. The hydrodynamic model was forced by tidal elevations at the boundaries. The model’s resolution varies from 15 km along the offshore boundaries to approximately 3 km near the coastline. Wave spectra resolution was of 25 frequencies and 16 directions. New wave model features are explored, including the effect of atmospheric stability on wave growth, the air-sea density ratio and the current impact on the wave growth rate. These processes are now included during the calibration process of the wave model. Hydrodynamic model results show good agreement with surface elevation measurements, with no bias and small errors. Results of the wave model show overall good results, although some overestimation of the extreme events is noticeable for the studied cases. The high-quality wave and hydrodynamic models developed within this work will serve as a MetOcean database for developers or alternatively as forcing for establishing local high-resolution (100s m resolution) models, i.e. for complex bathymetry at specific wind farm sites. Further work includes longer-term validations, data assimilation and assessment of other processes such as bottom friction and their impact on the estimation of extreme waves.

As the offshore wind industry moves toward deeper water, with larger turbines and correspondingly larger monopile foundations, nonlinear loads from steep waves may become more important for the ULS design. Nonlinear numerical wave tanks (NWTs) for generating wave kinematics, to be used as input to i.e. Morison’s equation, have been applied by several research groups, but further validation of the obtained wave kinematics is needed. Furthermore, the load models for larger diameters also need to be evaluated. The present work first compares the wave elevation results from existing two-dimensional (2D) NWT tools. The nonlinear wave elevation is compared to experimental results from model tests of two regular waves. Several methods of wave generation are considered. The conclusion from the study is that the selected codes represent physics well, even though we see individual differences, which are discussed. Differences between codes are not necessarily only due to differences in the applied theory, but also modeling of wave flap, numerical beach etc. Load (and response) estimates from using the NWT kinematics combined with the Morison load model are compared to experimental results, two sets of CFD simulations, and to simpler load models. Alternative methods of selecting the load coefficients for the simplified load models are also discussed.

General Electric, the National Renewable Energy Laboratory (NREL), the University of Massachusetts Amherst (UMass), and Glosten have recently completed a US Department of Energy (DOE)-funded research program to study technologies for mitigating loads on floating offshore wind turbines through the use of advanced turbine controls and tuned mass dampers (TMDs). The analysis was based upon the Glosten PelaStar tension leg platform (TLP) with GE Haliade 150 turbine, a system developed in a previous front end engineering design (FEED) study funded by the Energy Technology Institute (ETI) in the UK. The platform was designed for the WaveHub wave energy research site, with a mean water depth of 59-m. Loads were analyzed by running time-domain simulations in four 50-year return period (50-YRP) ultimate load state (ULS) conditions and 77 fatigue load state (FLS) environmental conditions. In 50-YRP conditions advanced controls are not active. The influence of TMDs on ULS loads have been reported previously (Park et al. [2]). In FLS conditions advanced controls and TMDs afford dramatic reductions in fatigue damage, offering the potential of significant savings in tower structural requirements. Simulations in turbine idling conditions were run in OrcaFlex , and simulations in operating conditions were run in FAST . Simulations were run with a baseline turbine controller, representative of the current state of the art, and an advanced controller developed by NREL to use collective and individual blade pitch control to maintain rotor speed and reduce tower loads. UMass developed a number of TMD types, with varying system configurations, including passive nonlinear dampers and semi-actively controlled dampers with an inverse velocity groundhook control algorithm. Loads and accelerations in FLS conditions were evaluated on the basis of damage equivalent loads (DELs), and fatigue damage was computed by Miner’s summations of stress cycles at the tower base. To study sensitivity to water depth, loads were analyzed at both the 59-m WaveHub depth and a more commercially realistic depth of 100 m. TMDs reduce fatigue damage at the tower-column interface flange by up to 52% in 59-m water depth and up to 28% in 100 m water depth. Advanced controls reduce fatigue damage at the tower-column flange by up to 22% in 59-m water depth and up to 40% in 100 m water depth. The most effective load-mitigation strategy is combining advanced controls with TMDs. This strategy affords a 71% reduction in fatigue damage in both 59-m and 100-m water depths.

Engineering design codes specify a variety of different relationships to quantify vertical variations in wind speed, gust factor and turbulence intensity. These are required to support applications including assessment of wind resource, operability and engineering design. Differences between the available relationships lead to undesirable uncertainty in all stages of an offshore wind project. Reducing these uncertainties will become increasingly important as wind energy is harnessed in deeper waters and at lower costs. Installation of a traditional met mast is not an option in deep water. Reliable measurement of the local wind, gust and turbulence profiles from floating LiDAR can be challenging. Fortunately, alternative data sources can provide improved characterisation of winds at offshore locations. Numerical modelling of wind in the lower few hundred metres of the atmosphere is generally much simpler at remote deepwater locations than over complex onshore terrain. The sophistication, resolution and reliability of such models is advancing rapidly. Mesoscale models can now allow nesting of large scale conditions to horizontal scales less than one kilometre. Models can also provide many decades of wind data, a major advantage over the site specific measurements gathered to support a wind energy development. Model data are also immediately available at the start of a project at relatively low cost. At offshore locations these models can be validated and calibrated, just above the sea surface, using well established satellite wind products. Reliable long term statistics of near surface wind can be used to quantify winds at the higher elevations applicable to wind turbines using the wide range of existing standard profile relationships. Reduced uncertainty in these profile relationships will be of considerable benefit to the wider use of satellite and model data sources in the wind energy industry. This paper describes a new assessment of various industry standard wind profile relationships, using a range of available met mast datasets and numerical models.

This work investigates the implementation of a novel, NASA-developed Fluid Harmonic Absorber (FHA) technology to mitigate platform motions and structural loads that can lead to lighter platforms, increased turbine performance, and ultimately, a lower LCOE. The novel damping strategy takes advantage of existing water ballast in the VolturnUS semi-submersible platform to achieve significant performance gains with minimal additional equipment and complexity. NREL’s FOWT software FAST is modified to include the primary features of the FHA technology. A study of the University of Maine-developed VolturnUS semi-submersible FOWT augmented with FHA technology is undertaken to quantify global performance of the system. When compared to the baseline technology, numerical simulations of a redesigned platform utilizing the FHA dampers indicate a reduction of 15.8% in hull structural material. Finally, the improvements in LCOE resulting from this mass reduction are assessed to demonstrate the advantages of NASA’s FHA technology for FOWT applications.

The typical offshore wind turbine generator (WTG) currently being installed worldwide produces 6 to 8 megawatts of electrical power and stands more than 100 meters above the ocean surface. The next generation turbines will produce 12 megawatts or more. In the summer of 2016 five of these turbines were installed in the coastal waters of Rhode Island. They are representative of the latest in a constantly evolving series of WTGs. As manufacturers continue to develop more powerful turbines, larger and larger specialized vessels will be needed to lift the components at assembly sites offshore. The process these vessels use to build America’s offshore windfarms will need to be different from the approach normally used at sites outside the U.S. This is due in part to regulatory restrictions imposed by the Merchant Marine Act of 1920, which is more commonly known as the Jones Act. As it pertains to offshore windfarm construction, the Jones Act prohibits foreign-flagged vessels, (vessels built outside the United States) from moving cargo between two U.S. ports. Under the law, an offshore windfarm within territorial waters is considered a U.S. port. Therefore, employing a foreign built windfarm construction vessel to transport components from a nearby port to the construction site, as it typically does, violates the Jones Act. Although the Jones Act might appear to be a deterrent to the development of offshore wind in the U.S., it is not. The developers of the Rhode Island’s Block Island Wind Farm (BIWF) used a combination of U.S. and foreign-flagged vessels to construct BIWF, while remaining in compliance with the law. This paper reviews the approach used at Block Island and discusses how it can be applied to future projects. An analysis of alternatives for various vessel fleet combinations that can be used for this purpose is provided. The analysis is based on results produced by a throughput and scheduling model developed by the authors. The model leverages the experience gained during the construction of BIWF, and was validated with data collected during the project. The alternatives include scenarios utilizing existing vessels modified for windfarm construction service, foreign-flagged windfarm construction vessels, and new vessels specifically built for the U.S. market. The paper concludes with a review of the relative efficiencies of various fleet configurations while undertaking the construction of a notional windfarm located off the East Coast of the U.S.

Development of remote energy requires large pipeline networks to be placed in more challenging environments such as offshore in deeper waters or on land in Arctic or near-Arctic locations. Pipeline installed and operated in such regions may be subjected to large plastic strains. Engineering critical assessments (ECA) are commonly carried out during design, installation and operation of offshore pipelines to determine acceptable flaw sizes in pipeline girth welds. A number of fracture mechanics-based procedures are available for ECA of pipeline girth welds. Most of these methods are primarily stress-based assessments and are therefore not directly applicable to cases where the displacement-/strain-controlled loading generates large amounts of plastic deformation. For such cases, strain-based fracture assessment for pipeline/girth welds should be carried out instead. However, limited guidance on strain-based assessment is available in the current codes and standards used primarily by the oil and gas industries. This paper reviews the existing strain-based fracture assessment methods, and reports the results of preliminary studies performed to compare the methods reviewed with the available full-scale pipe test data.

Deep-water tendon and riser systems are often subjected to severe fatigue loading from waves, currents and vessel movements. The girth welds between successive lengths of pipe or at pipe terminations represent fatigue-critical features where failure would be catastrophic. Hence, validation fatigue testing by full scale pipes of the most critical welds are often performed to ensure adequate quality and/or to document a better S-N curves than those available in standards today like DNVGL-RP-C203 [1] and BS7608 [2]. To better understand the fatigue performance with respect to identify trends, dependencies and critical features that influence the fatigue performance, a JIP on Fatigue of Girth Welds were initiated in 2011. Two phases have been conducted and a total of 1700 full scale one sided girth welds, mostly run by Stress Engineering, have been statistically analyzed. The test data has been interrogated to investigate the effect of as-welded condition, OD ground, OD/ID ground, un-reeled pipe, reeled pipe, thickness and effect of misalignment. Based on these analyses, new S-N curves for risers and pipelines have been included in DNVGL-RP-C203 for non-reeled girth welds. This paper presents the findings and trends from the JIP work which has been the rationale for the updates of girth welds in section 2.10 in DNVGL-RP-C203 2016 edition.

The use of synthetic fiber ropes for subsea installation is extending, as the offshore industry explores deeper waters, but there are few data available to evaluate the lifetime of these materials. In a previous OMAE presentation the authors described results from the first phase (2010–2013) of a JIP aiming to understand the mechanisms controlling the long term behavior of HMPE fibre ropes [1]. This presentation will describe the results from the second phase of this study (2014–2018) in which predictive models have been developed and applied to a range of improved braided rope materials. Two modeling approaches will be discussed, an empirical method based on residual strength after cycling, and a numerical approach using finite element software specifically adapted to fibre materials [2]. An extensive test program, which has generated a database of CBOS (cyclic bend over sheave) results for various grades of HMPE and different constructions, will be described. Comparisons have been made with steel wire handling lines in order to quantify the benefits of fibre ropes for these deepwater applications.

Oil and Gas industry in the last decades has increased the use and need of heavy wall thickness line pipes, in particular for onshore / offshore high pressures and high temperatures (HP/HT) and offshore deep water / ultra-deep water applications. The paper presents the results achieved by Tenaris on seamless line pipes in grades X65/X70, according to API 5L / ISO 3183, with wall thickness in a range from 40 to 60 mm and diameter between 6 5/8” and 16”, produced by hot rolling process followed by quenching and tempering. Such line pipes are able to withstand very demanding conditions, like sour environment, very high pressure and wide temperature range. In this publication, the main outcomes of laboratory testing activities on the mentioned materials will be presented as part of heavy wall line pipe qualification. For this purpose, a special testing program, including mechanical and corrosion tests, has been executed. Material demonstrated an excellent behaviour, exhibiting both mechanical, toughness and stress corrosion properties suitable for the envisaged harsh applications.

The paper studies the path following of a ship sailing in restricted waters based on an output feedback control, which consists of a state feedback control law and an extended updated-gain high-gain observer. According to the separation principle, the state feedback control and the extended updated-gain high-gain observer are designed separately. The state feedback control law is designed based on a robust guaranteed cost control method assuming that system states are measurable. Sufficient conditions are given for the control based on a linear uncertain system. The extended updated-gain high-gain observer, whose gains are updated according to the nonlinear functions of available evaluation errors, is used to reconstruct system states. Then the output feedback control is obtained by replacing states value in the state feedback control law with its estimation yielded by the state observer. Numerical simulations confirm the effectiveness of the proposed control method for the path following of a ship sailing in restricted waters.

Extensive water entry experiments were performed to identify the influence of elasticity on impact-induced loads acting of flat bottom structures. Two bodies were tested. One body was fitted with stiffened, rigid bottom plating; the other body, with thin elastic plating. Bottom pressures and forces acting on the flat bottom plating and impact-induced elastic bottom strains were measured. High-speed videos of water entry sequences were evaluated to investigate hydroelastic effects on air trapping.

Potential liquefaction of some cargoes (Nickel ore, iron ore, ...) is a major risk for the maritime industry. The difficulties to simulate accurately the behaviour of these materials as well as their interaction with a bulk carrier model leaded us to use a non-Newtonian highly viscous fluid to model a liquefied ore. An experimental approach is presented in this paper. Roll responses of a ship model as well as details on the internal free surface behaviours are investigated for different loading conditions: solid cargo, fresh water and viscous fluids.

Vertical pipes are extensively applied on offshore petroleum operations, such as subsea wellbore drilling, wellbore re-entry, installations of equipment on the sea bottom, hydrocarbon production as the conductor for oil and gas from the wellhead at the sea bottom, among other applications. Recent oil and gas field discoveries in ultra deepwater brought an increase of the amount of produced and processed petroleum onboard of floating production system. Consequently, the process plant demands more cooling capacity. One of the alternatives is to elevate cold sea water. The present study aims to improve the understanding of the behavior of a vertical pipe forced to move at its upper end in a water tank. Displacements along the pipe length are measured, and main features of the behavior of the pipe model with its bottom end free are observed. Motions imposed at the vertical pipe intent to represent motions of the surface from the floating platform or ship, due to waves, currents and wind. Pipe model vibrations are analyzed, and results allowed improvements for the input in numerical model simulations to Orcaflex software. Developments of the analysis procedure and evaluations of the experiment allow more reliable design of riser operations with its bottom end free in actual sea operations. In the present work, a physical model with a pipe of small diameter has been developed for the laboratory experiment, and an optical measurement system is used to measure the pipe model displacement in the water tank. Motions are imposed at the top termination by a mechanical device. Experiment is described in details and comparative analyses between experiment and numerical simulations were carried out. Results from numerical simulations as well as from the experiment brought important contributions for description and understanding of the vertical pipe behavior.

Internal waves in the ocean occur in stably stratified fluids when a water parcel is vertically displaced by some external forcing and is restored by buoyancy forces. A specific case of such internal waves is internal tides and their associated currents. These currents can be significant in areas where internal waves degenerate into nonlinear solitary waves, known as solitons. Solitons are potentially hazardous for offshore engineering constructions, such as oil/gas pipelines and floating platforms. The most efficient mechanism of soliton generation is the tidal energy conversion from barotropic to baroclinic component over large-scale oceanic bottom obstructions (shelf breaks, seamounts, canyons and ridges). In this paper, a methodology is provided to compute diagnostics and prognostics for soliton generation and propagation, including the associated currents. The methodology comprises a diagnostic tool which, through the use of a set of theoretical and empirical formulations, selects areas where solitons are likely to occur. These theoretical and empirical formulations include the computation of the integral body force (1), the linear wave theory to compute the phase speed and the empirical model proposed by (2). After the selection procedure, the tool provides initial and boundary conditions for non-hydrostatic numerical models. The numerical models run in 2D-V configuration (vertical slices) with horizontal and vertical resolutions ranging from 50 to 200 m and 5 to 10 m, respectively. Examples are provided for an open ocean location over the Mascarene Plateau in the Indian Ocean. Validation of diagnostics and prognostics are provided against ADCP and satellite data.