Extracting modal parameters of offshore structures under ambient excitation is crucial for the structural health monitoring. In this paper, a procedure combining Natural Excitation Technique (NExT) and Polyreference Complex Exponential (PRCE) method to identify modal parameters of an offshore jacket platform under ice excitation is presented. First, a numerical study is conducted to verify the method using displacement response under simulated ice load. The selection of reference channel is found to play an important role in modal parameter identified result. Then the measured response acceleration signals obtained from a sea testing were analyzed applying the proposed method. Modal frequencies and damping ratios as well as corresponding mode shapes identified from different segments of the whole response time history are in good agreement, which validates the proposed methodology.
As the most common kind of offshore structures, steel jacket-type platforms have been widely used in offshore oil and gas exploitation. These platforms are subjected to various kinds of environmental loadings. During an offshore structure’s service life, structural damage caused by environmental loads would be continuously accumulated. The damage may result in a significant change in the modal properties of the platform, such as natural frequencies, damping ratios and mode shapes. Thus, the modal parameter estimation, which is the process of determining modal parameters such as natural frequency and damping ratio from test data (Maia et al., 1997), of an offshore platform structure, is the essential step for modal-based damage detection. The dynamic testing of offshore platform is performed in the field, and the most practical means of exciting the structure is ambient test which is defined as the excitation experienced by a structure under its normal operating conditions (Farrar et al, 1999). The offshore platforms are subject to ambient excitation from sources such as wind, waves, ice, etc. The input ambient excitation is often unmeasurable. Therefore, the modal parameters of these structures must be identified by output-only modal identification methods.
The study of extracting modal parameters of structures from response testing data has a long history (James III et al., 1995). There have been plenty of different approaches, including both frequency domain such as peak-picking from Power Spectral Density (PSD) functions and time domain methods such as Ibrahim Time Domain (ITD) method, the complex exponential method (Prony’s method) and Eigensystem Realization Algorithm (ERA) and so on. Most of the current modal identification techniques have been based upon the measured data being the frequency response function (FRF) or the equivalent impulse response function (IRF) (Maia et al., 1997; Ewins, 2000). However, ambient excitation does not lend itself to FRF calculations because the input excitation cannot be quantified (James III et al., 1995).
Ploughing is one of the most common methods for burying subsea pipelines or cables for protection against trawls, on-bottom stability, for better thermal insulation or for meeting the legislative requirement. A ploughability assessment is undertaken during design and planning stages to evaluate suitability a plough for required burial depth and to determine the duration of offshore ploughing operation. This paper presents the insight into ploughability assessment, state-of-the-art methodology for the assessment and highlights some critical limitations that shall be undertook by the designers and engineers. An accurate ploughability assessment undertaken with full knowledge of its limitations would reduce project risks and could lead to millions of dollars in cost savings.
Subsea pipelines and cables are instrumental for energy and communications sectors. There are thousands of kilometers of pipelines and subsea cables buried in the seabed around the world. The subsea pipelines may have to be buried for reasons such as protection against trawlers, on-bottom stability, for better thermal insulation or for legislative requirements. Communication cables are also buried subsea for protection. The method of seabed trenching predominantly depends on the soil type, but ploughing is one of the most commonly used methods for pipeline and cable burial.
To plan for offshore operational time and to identify any areas of concern in ploughing operations, a ploughability assessment is carried out during the project design stage. The ploughability assessment for a given plough is based on geotechnical conditions at the site. The geotechnical condition of a site is characterized by geotechnical model with appropriate soil parameters. The assessment predicts the plough velocity and two force for a given trench and soil conditions. This paper presents an overview of the state-of-the-art ploughability assessment, provides in-depth view into soil mechanics of ploughing, and presents discussion on the limitations with regard to ploughability assessment results. The paper also presents parametric study results to better understand the sensitivity of soil properties in on ploughing performance.
Bitumen is field-proved to be a highly effective corrosion protection for steel armor wires in subsea power cables, umbilicals, and power umbilicals. Bitumen’s viscoelastic properties are known to influence the mechanical properties of cables, umbilicals, and power umbilicals. Still, common industry practice is to neglect bitumen and instead assume dry friction, also at bitumen-coated contact surfaces. This paper presents novel simulations of an umbilical with bitumen-coated armor wires using the cable and umbilical simulation software UFLEX2D. To the author’s knowledge such simulations have not been published in the scientific literature before. The UFLEX2D simulations show that the bending stiffness is highly sensitive to bitumen’s temperature and to some extent sensitive to the umbilical oscillation frequency. The paper also compares bitumen with dry friction. The simulations reveal that bitumen introduces a phase shift between the umbilical’s bending curvature and bending moment. This paper therefore introduces the complex bending stiffness which also includes this phase shift.
Umbilicals are used to connect offshore oil and gas production units, while subsea power cables connect power grids overseas. Power umbilicals are hybrids between umbilicals and power cables, as they include electric power phases in addition to traditional umbilical elements, such as steel tubes, electric signal cables, and fiber optic signal cables.
Subsea power cables, umbilicals, and power umbilicals often include steel armor wires for carrying axial load. Armor wires may also be used to tune the cable’s or umbilical’s submerged-weight-to-diameter ratio. Although the armor wires usually are galvanized, this may be insufficient corrosion protection as the cables and umbilicals are submerged in seawater for decades. A common industry practice is to coat the armor wires in bitumen. Bitumen is field-proven to be a highly effective corrosion protection for armor wires in subsea power cables and umbilicals.
Bitumen is a bottom fragment of crude oil distillation. It is a viscoelastic material, i.e. its mechanical properties are partly elastic, partly viscous. The viscoelastic properties are highly temperature-dependent and to some extent dependent on the frequency of which bitumen is being shear strained.
Belibassakis, K. A. (National Technical University of Athens) | Touboul, J. (Mediterranean Institute of Oceanography, Universite de Toulon) | Rey, V. (Mediterranean Institute of Oceanography, Universite de Toulon)
Propagation of water waves in coastal zones is strongly affected by the combined influence of currents and bathymetry variations. In the case of small amplitude waves, a coupled-mode model is developed in this work for treating the wave-current-seabed interaction problem, with application to wave scattering by non-homogeneous, vertically sheared current over general bottom topography. The scattered wave potential is represented by a series of local vertical modes containing the propagating mode and all evanescent modes, plus additional terms accounting for the satisfaction of the boundary conditions on a sloping seabed. Using the above representation, in conjunction with a variational principle, a coupled system of differential equations on the horizontal plane, with respect to the modal amplitudes, is derived. In the case of no shear the above coupled-mode system reduces to the one developed by Belibassakis et al (2011) for the propagation of smallamplitude water waves over variable bathymetry regions in the presence of ambient currents. Furthermore, if only the propagating mode is maintained in the local-mode series, the present coupled mode system reduces to a form compatible to the Extended Mild-Slope equation for surface waves interacting with a linear shear current recently derived and studied by Touboul et al (2015). Results are obtained by using second-order finite differences to discretize the equations which are numerically solved by iterations. Various representative test cases are considered demonstrating the effect of vorticity, which is assumed to be slowly varying in the horizontal directions, in conjunction with the additional effect of depth variations.
The characteristics of surface gravity waves present significant variations as they propagate through non-homogeneous currents, and these variations are further modified by the effects of depth inhomogeneities that occur in variable bathymetry regions; Jonsson (1990), Thomas & Klopman (1997). For example, the wave amplitudes could present a significant enhancement within the streaks associated with Langmuir circulation; see, e.g., Smith (2001). Furthermore, large amplitude waves can be produced in cases when obliquely propagating waves interact with opposing currents, see, e.g., Mei (1983, Ch. 3.7). This situation could be further enhanced by inshore effects due to sloping seabeds, and has been reported to be connected with the appearance of “giant waves”; see, e.g., Lavrenov & Porubov (2006). Such kind of models have been developed for surface waves crossing weak current jets or steps with horizontal shear, see, e.g., Smith (1987), McKee (1994, 2006), Belibassakis (2007). The study of spatial variation of waves and the investigation of scattering of realistic wave spectra over irregular sub-wavelength scale currents, with the effects of bottom irregularities, can be supported by theoretical models treating the simpler problem of monochromatic waves interacting with steady currents. Another important aspect is the incorporation of the effects of shear flow due to the vertical structure of the boundary layer of environmental currents, as e.g., tidal currents; see Soulsby (1990). Several authors developed models to treat the effects of shear current on water wave propagation; see, e.g., Kirby & Chen (1989), Swan & James (2001), Nwogu (2009), and the references cited there.
This paper presents an extended method to simulate the coupled dynamics of top tensioned risers (TTRs) connected to Tension Leg Platforms. Coupling between TTR and TLP occurs at the tensioner system and the riser guides.
In this paper, a new TTR model that considered effects from tensioner system and riser guide is integrated into existing CABLE3D and COUPLE codes. Tensioner system is consisted of 4 tensioners each of which is treated as a linear spring and acting as concentrated force on TTRs. The motion of TLP is transmitted to TTRs through upper deck and riser guides. The vertical friction between TTRs and upper deck or riser guides is neglected. Results for several applications of up-dated CABLE3D and COUPLE are presented. First, a single TTR under harmonic excitation at the fairlead is analyzed. Corresponding TTR tensions and motions are obtained in time domain. Then a TLP with 9 TTRs and 12 Tendons is treated as a global system exposed to wind and waves.
Risers, such as top tension riser (TTR) and steel catenary riser (SCR), are usually used associated with TLP to secure the oil/gas transportation work so the coupling effects between TLP and risers are needed to be taken into consideration. Chen (2006) used COUPLE to calculate coupled dynamic analysis of a mini TLP with tendon and the riser system. The computation of the force interactions between each individual riser and the corresponding attaching point on the TLP is obtained at each time step. But if TTR is considered, generally, there are two kinds of support to provide top tension for TTR: buoyancy can and tensioner system. Huang (2013) has developed a new module in CABLE3D and COUPLE to consider such dynamic effects on both of them on a Truss spar. In his work, individual tensioner cylinder is treated as a linear spring while buoyancy can is modeled as constant buoyancy force. In TLP, the tensioner system is more popular and more widely utilized.
In this paper, the static and dynamic simulation of a TTR connected to TLP through a tensioner system and with a keel riser guide at near surface have been successfully achieved by extending CABLE3D's capabilities based on Huang (2013) . Firstly, the excitation on the TLP in the dynamic simulation is assumed to be a harmonic motion. Then the dynamic analysis of the TLP coupled with the TTRs and tendon systems is performed. The updated CABLE3D has been integrated into existing COUPLE. The main purpose of this study is to simulate TTR line tensions and TLP’s motions by using this extended model, and to show its capability in current and future research work.
Traditionally to model a thermal effluent out of the power plants, either the formula method or the two-dimensional numerical model could be used. However too many constraints restrict the formula method and real circumstances cannot be reflected. In this study, thermal diffusion simulations of the power plant are carried out through the use of two-dimensional shallow water equations. The flow field of this study with the tidal influence is also simulated simultaneously by the use of MIKE 21 hydrodynamic module. Simulation results from the two-dimensional hydrodynamic model, however, could be somehow under-estimated if the buoyancy is ignored. In order to know the small difference between the surface temperature and the depth-averaged temperature, we use FLOW-3D software to figure out whether and how much the heat transfer of buoyant plumes is ignored in two-dimensional simulation. It can be seen that the further the distance from the outfall is, the less the vertical temperature profile difference will be. That is to say the buoyant effect can be ignored when the distance increases from outfall in shallow water area, as well as validated in the integral model theory for turbulent buoyant jets.
Thermal diffusion simulations of the power plant are usually carried out through the use of two-dimensional shallow water equations. The Navier-Stokes equations are simplified by a depth averaging procedure (Benqué et al., 1982). Abbaspour et al. (2005) simulated thermal pollution of the power plant in the coastal area of the Persian Gulf by using MIKE 21 hydrodynamic model developed by DHI Water and Environment (DHI, 2003).
The flow field of this study is also simulated by the use of MIKE 21 hydrodynamic module. The model now allows the use of triangular meshes for increasing the computational grids in the concerned region (DHI, 2011).
Today, driven piles are commonly used to fix offshore foundations for wind energy converters in the sea bed. However, the installation noise level due to pile driving is far beyond the limit which should be kept for the protection of marine life. Not only the critical noise emission, but the demand of an installation at greater water depths and the increase of converter performances results in larger loads which cannot be founded economically with monopiles. By using drilled injection piles for the foundation of offshore wind energy converters the installation noise can be reduced to an acceptable level and the increasing demand of foundations in deeper water as well as for converters with higher performance can be met.
To reduce the installation noise to acceptable level the nowadays commonly used driven piles can be replaced by drilled injection piles with a tubular steel core to install the foundations of offshore wind energy converters to the sea bed (Fig. 1). Bigger pile dimensions needed for foundations of offshore wind energy converters in deeper water as well as for converters with higher performance will even increase the noise level due to pile driving. Therefore an alternative installation method is needed. Since it is expected that drilling of piles by hammer drill produce essentially lower acoustic emissions and since the load capacity could easily be adapted by the number of piles used for the foundation, this installation method could be an alternative to pile driving.
Wind energy converters and their foundations are loaded vertically by dead loads and horizontally by wind, current and wave loadings. The vertical loadings are carried by the skin friction and by tip pressure of the pile. However, the load carrying behavior of horizontally loaded piles is influenced by the stiffness of the piles, which is influencing the design of the steel structure. Thus, the flexibility of piles due to the soil stiffness and the scour should be considered in static and fatigue design to achieve economic foundation systems, e.g. by non-linear spring stiffness applicable in framework calculations. For the determination of the spring stiffness numerical parameter studies are carried out. These parameter studies consider injection piles made of circular hollow sections 200 ˟ 30 mm (inner support link) reinforced by casings (CHS 355.6 ˟ 20 mm) in their top region. For the filling of the support link, the gap between the support link and the casing concrete is used. In a first step the non-cohesive soil conditions at the FINO1 research platform located north of Borkum Island in the North Sea are considered. Due to the time consuming numerical calculations the results for a generic soil condition (Herion et al., 2015) considering cohesive soil layers will be presented in a second step, planned for ISOPE 2017.
In this paper, the effect of an Offshore Wind Farm (OWF) on the surrounding wave field is numerically investigated in frequency domain, using a BEM numerical model based on the potential flow theory. The analysis is performed for regular waves of various periods and incident wave directions and for irregular waves with variable peak period and significant wave height. Specific cases of regular and irregular waves are compared, revealing the differences between the regular wave model and the real sea state. By numerically simulating the incident wave and the scattering effects caused by the OWF, indications are provided regarding the impact of the OWF on the local wave climate.
In recent years, the increasing energy demand has led to a growing interest towards the efficient exploitation of renewable energy sources. Under this framework, offshore wind energy has become an increasingly attractive option, offering multiple benefits and addressing effectively the well-known obstacles and problems associated with the exploitation of wind energy onshore (Henderson et al., 2003; Breton and Moe, 2009). Consequently, the offshore wind energy sector is continuously growing and this has resulted into large-scale commercial deployment of Offshore Wind Farms (OWFs), especially in the coastal and offshore areas of northern Europe (EWEA, 2015). So far, most OWFs operating in Europe have been installed at shallow waters of average depth equal to 22.4 m and at an average distance from the shore equal to 32.9 km (EWEA, 2015). Moreover, the deployed support structures correspond mainly to fixed bottom configurations, i.e. monopile, tripod and jacket (EWEA, 2015).
Although OWFs may contribute significantly to the coverage of the increasing energy demands, their installation and operation should be implemented by considering not only economic and engineering factors, but, additionally, by assessing and predicting reliably possible negative environmental impacts in the corresponding marine environment (e.g. undesirable effects on local wave climate, changes in sediment transport patterns, loss of biodiversity, etc). Considering that today most OWFs operate at shallow water depths and at relatively small distances from the shore, the aforementioned environmental impact assessment becomes crucial also for local communities, since the existence of any possible negative environmental impacts may affect directly the human activities (e.g. fishing, leisure activities, such as surfing, etc) and operations in the corresponding coastal environment.
In the present paper, ten-year long three-hourly time series of wind and wave data, based on hindcasts of WAVEWATCH III model, are analyzed and modelled as nonstationary stochastic processes. The study area covers the region [100 E, 70 W] × [60 S,66 N]. The initial time series is decomposed by means of the nonstationary modelling, and the residual stationary part is used as input to a Fuzzy Inference System (FIS) in combination with an Adaptive Neuro-Fuzzy Inference System (ANFIS) for the prediction of future values of wind and wave parameters. For comparison purposes, the FIS/ANFIS models are also applied to the initial nonstationary series without making any decomposition. The performance of both forecasting procedures is assessed by means of well-known error measures. It should be noted that FIS/ANFIS models are coupled for the first time with a nonstationary time series modelling for forecasting purposes.
Wind and wave data are very important for a number of applications connected with the open ocean and coastal activities. Thus, shortand/ or long-term forecasting of them is of great practical utility in areas ranging from navigation safety and oil response contingency planning to coastal erosion and ocean climate change. Third generation spectral wave models (the WAMDI group 1988; Tolman 1991; Booij et al 1999) run on a regular basis providing us with long-term data of good quality with high spatial and time resolution without gaps, and, thus, can be used for forecasting purposes (either off-line or in near realtime); see, e.g., Roulston et al (2005), Reikard and Rogers (2011). However, and because their numerical implementation is quite complicated, they require great computational power and high CPU time.
On the other hand, various researchers treat the forecasting problem by means of various soft computing techniques. Some of them utilize Artificial Neural Networks (ANN); see, e.g., Deo et al (2001), Rao and Mandal (2005), Jain and Deo (2007). Some others use Fuzzy Inference Systems (FIS) in combination with Adaptive Neuro-Fuzzy Inference Systems (ANFIS); see, e.g., Kazeminezhad et al (2005), Özger and Sen (2007), Mahjoobi et al (2008), Zamani et al (2008), Sylaios et al (2009), Akpinar et al (2014). These techniques require less computational effort and they are easy to be applied.
For a special arrangement of multiple floating bodies, it is known that body-generated waves will be confined inside of the bodies due to hydrodynamic interactions; which is referred to as ‘cloaking’ phenomenon. In order to study this phenomenon in more detail and its application to engineering problems, we have developed an accurate computer code and an optimization scheme using GA (genetic algorithm) and presented some results for the diffraction problem. In the present paper, the floating body to be cloaked is assumed to be freely oscillating in regular waves and multiple bodies surrounding the central body are assumed to be controlled with an external dynamic system with damping and restoring forces. Study is made on the energy conservation relation and the scattered and radiation wave patterns when the cloaking is realized and also on the absorption of wave energy through the work done by the external force of damping installed in the surrounding cylinders.
In recent years, structures with complex shape or multiple floating bodies are increasing for the purpose of marine energy utilization. The wave drift force on these structures may increase by hydrodynamic interaction effects, and especially in the case of multiple floating bodies they may collide each other. Therefore the wave drift force should be calculated accurately and studied how to reduce it.
In general, the incident wave is disturbed by a floating body and thus the disturbance wave (consisting of scattered and radiation waves) is generated. In the cloaking phenomenon studied by Newman (2013), the disturbance wave could be zero and the incident wave was not disturbed by surrounding a central body with a finite number of smaller bodies and optimizing the location and size of those surrounding bodies. This phenomenon can be applied to reduce the wave drift force.