ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V003T00A001. doi:10.1115/OMAE2018-NS3.

This online compilation of papers from the ASME 2018 37th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2018) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

Structures, Safety, and Reliability: Abnormal or Rogue Waves

2018;():V003T02A001. doi:10.1115/OMAE2018-77270.

This work is focused on the analysis of the wave action equation with full 4-wave interactions (Snl4). For this purpose, we have applied a state-of-the-art spectral wave model (Wave Watch III), using an exact method for the calculation of the full nonlinear Boltzmann interactions in the evolution of the wave spectrum. We emphasize the use of the exact WRT method [Van Vledder, 2006] for the computation of the Snl4 interactions instead of the approximate DIA method. The WRT algorithm includes the full Boltzmann integrations. We discuss how the WRT method is important in any assessment of rogue waves in the ocean and discuss how the enhanced spectral peak assists the formation of rogue waves packets. We demonstrate how the most nonlinear part of the peak of the spectrum is reduced in amplitude when the nonlinear interactions are instead computed using the DIA interactions. These results suggest that a clear understanding of the physics of nonlinear interactions and of rogue wave dynamics requires the use of the full Boltzmann interactions. Future work would include faster WRT computations so that practical forecasting/hindcasting can become possible using the full four-wave interactions.

Topics: Waves
Commentary by Dr. Valentin Fuster
2018;():V003T02A002. doi:10.1115/OMAE2018-77484.

As a reference point for the extreme Metocean conditions, the hurricane-scale classification is often used: that is a tropical storm becomes a hurricane if the wind speed reaches U ∼ 33m/s. In this paper, it is argued that such classification is not arbitrary, and indeed signifies change of the physical regimes in all environments near the air-sea interface: in the atmospheric boundary layer, at the surface, and through the upper ocean. This threshold is approximately the wind speed at which the drag coefficient was found to saturate in the field observations (U10 ≈ 32–33m/s), which saturation has received a lot of attention. Less known are the observations that below the surface, change of the upper-ocean mixing mechanism and of bubble dynamics occur at U10 > 35m/s. Directly at the surface, wave dynamics also undergoes essential transformations, from wave breaking (dissipation) being driven by evolution of nonlinear waves, to the breaking being forced directly by the winds, at U10 ≈ 34 m/s. It is therefore argued that the simultaneous change of physical regime in all the three air-sea environments cannot be coincidental, and consequences of the regime change for the Metocean modelling are discussed. As an important byproduct, parameterisation of wave-breaking probability is obtained in terms of the mean symmetry of surface waves. Such parameterisation allows us to estimate frequency of breaking events, based on time series of surface elevations, without explicitly detecting the breaking waves.

Commentary by Dr. Valentin Fuster
2018;():V003T02A003. doi:10.1115/OMAE2018-77753.

Freak wave is an unexpectedly large wave in ocean with extreme height and abnormal shape. The viscous effect is important in prediction of the flow patterns of the freak wave due to its strong nonlinearity. Thus, compared with the potential theory, more accurate information of flow field of the freak wave can be obtained by using the computational fluid dynamics (CFD) method. In laboratory test and numerical study, the focused wave is often adopted to substitute the freak wave in real sea. In this paper, we present a high accurate numerical model for large eddy simulation of the focused wave. In this model, the space filtered Navier-Stokes equations are solved on non-staggered grids by the finite volume. The fourth order compact scheme is adopted for discretization of both convection and diffusion terms of the governing equations. The standard fourth-order Runge-Kutta method is used for time advancement. The velocity-pressure coupling is ensured at each stage and the discretized equations are solved by strongly implicit procedure (SIP) method. The turbulence is simulated by the Smagorinsky model while the free surface is captured by using of the volume of fluid (VOF) method. The model is firstly validated by simulation of the cavity flow and linear wave. The simulation results are compared with theoretical values and published results, respectively. Finally, large eddy simulation of focused wave is presented. The comparison of the numerical results and measured data reveals that the proposed model is capable of reproducing the propagation and evolution of the focused wave.

Commentary by Dr. Valentin Fuster
2018;():V003T02A004. doi:10.1115/OMAE2018-77771.

This study proposes ideas to reproduce freak waves from observational data. The reproduced data will apply to investigations on freak wave impact to offshore structures. Four-dimensional variational method (4DVAR) was used for the freak wave reproduction. Under a dynamical constraint, 4DVAR minimizes the squared error between observation and model prediction by adjusting the initial condition iteratively. This study utilizes the Higher Order Spectral Method (HOSM) to predict the nonlinear wave evolution, which is essential for freak wave generation. Information on wave spectrum estimated beforehand by a wave model is also employed to stabilize the reproduction. To increase convergence speed with fewer efforts of coding, a type of ensemble-based variational method (a4dVar) was adopted. The a4dVar performs perturbed ensemble simulations to evaluate the gradient of the squared error and is easy to parallelize and implement. This paper conducted twin experiments of HOSM+a4dVar data assimilation. HOSM model generated the true state of the uni-directional wave field, and the spatiotemporal wave field was reconstructed from time series of one virtual wave gauge located in the model. It is assumed that the virtual wave gauge detected a freak wave. The estimation accuracy of linear estimation and HOSM estimation were compared.

Topics: Waves
Commentary by Dr. Valentin Fuster
2018;():V003T02A005. doi:10.1115/OMAE2018-77858.

The statistical properties of individual wave heights and wave crests from time series of recorded surface elevation are analyzed with a particular focus on the occurrence of extreme and rogue waves in the datasets. The datasets include surface elevation measurements from three different sensors: a wave buoy, a wave laser and a Saab wave radar — all situated at the Ekofisk field in the North-Sea and providing sea surface elevation measurements at 2Hz temporal resolution.

The resulting statistical properties of wave heights and wave crests are compared with common reference statistical distributions such as Rayleigh, Tayfun (1980) and Forristall (1978, 2000) distributions for wave heights and crest heights. In particular, the occurrence of rogue waves (H > 2.2Hs or C > 1.25Hs) in the datasets is investigated. Possible relations between the occurrence of rogue waves and spectral characteristics of the corresponding sea states are briefly discussed.

Commentary by Dr. Valentin Fuster
2018;():V003T02A006. doi:10.1115/OMAE2018-77904.

Modulation instability (MI) is one possible mechanism to explain the formation of extreme waves in uni-directional and narrow-banded seas. It can be triggered, when side-bands around the main frequency are excited and subsequently follow an exponential growth. In physical domain this dynamics translates to periodic pulsations of wave groups that can reach heights up to three times the initial amplitude of the wave train. It is well-known that these periodic wave groups propagate with approximately half the waves phase speed in deep-water. We report an experimental study on modulationally unstable wave groups that propagate with a velocity that is higher than the group velocity since the modulation frequency is complex. It is shown that when this additional velocity to the wave groups is small a good agreement with exact nonlinear Schrödinger (NLS) models, that describe the nonlinear stage of MI, is reached. Otherwise a significant deviation is observed that could be compensated when increasing accuracy of the water wave modeling beyond NLS.

Commentary by Dr. Valentin Fuster
2018;():V003T02A007. doi:10.1115/OMAE2018-77971.

The long-term trends of the expected largest waves in the ice-free Arctic waters from Laptev to Beaufort Seas was studied analyzing the ERA-interim reanalysis from 1979 to 2016. The analysis showed that the positive trend is largest in October and increased almost 70 cm in 38 years. For ships navigating the Northern Ship Route, it is important to know what the possible largest waves to expect during its cruise. In view of conducting the extreme value analysis, the uncertainty of the largest wave needs to be validated. However, the observation in the Arctic Ocean is limited. We, therefore, rely on the reanalysis wave products in the Arctic Ocean, whose uncertainty is yet to be determined. ERA-Interim and ERA-5 are compared in the Laptev, the East Siberian, Chukchi and Beaufort Seas. The comparison is relevant as the two products differ in its horizontal grid resolution and availability of the satellite altimeter significant wave height data assimilation. During 2010–2016 when the ERA5 is available, only a small difference from ERA-Interim was detected in the mean. However, the expected largest waves in the domain tended to be large for the ERA-5, 8% normalized bias. The tendency was quite similar with a high correlation of 0.98.

Topics: Waves , Arctic region , Ice , Water
Commentary by Dr. Valentin Fuster
2018;():V003T02A008. doi:10.1115/OMAE2018-78292.

The Wigner transform can be used to derive equations directly for the evolution of the autocorrelation of the sea elevation. This has been known in the literature as the derivation of the Alber equation, and applies to envelope equations. Wigner-Alber equations have been used to characterise spectra as either stable or unstable, and to predict Fermi-Pasta-Ulam recurrent dynamics for the unstable ones. Here we show that a systematic study of Wigner equations can improve this analysis in several respects, including: (i) the incorporation of accurate dispersion and (simple) wave breaking effects; and (ii) the characterization of the space and time scales over which localized extreme events emerge. More broadly this approach can be seen as a full modulation instability analysis for any measured spectrum. This work builds upon recent joint work with G. Athanassoulis and T. Sapsis.

Commentary by Dr. Valentin Fuster
2018;():V003T02A009. doi:10.1115/OMAE2018-78317.

The restricted duration of wave records, usually 20 or 30 minutes, introduces sampling variability, the statistical uncertainty due to limited number of observations, in estimates of sea surface characteristics. This uncertainty may have significant impact on derived wave parameters commonly used in design and marine operations, and quantifying it is also of importance for wave forecasting purposes as well as for investigations of extreme wave events such as rogue waves. The study shows, using numerical simulations, effects of sampling variability on the measures of wave field nonlinearity, the skewness and kurtosis coefficients of sea surface elevation, and on the wave crest. Wave data are simulated by the nonlinear wave model HOSM (Higher Order Spectral Method). The Pierson-Moskowitz and the JONSWAP spectrum with different gamma parameters and different directional energy spreading functions are used in the analysis and their effect on sampling variability estimates is demonstrated. Sea states where rogue waves were recorded in nature are considered. The results are compared with the ones obtained from linear wave model simulations. Consequences of sampling variability on description of sea surface nonlinearity are discussed.

Topics: Seas , Nonlinear waves
Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Collision and Crashworthiness

2018;():V003T02A010. doi:10.1115/OMAE2018-77158.

Ships and offshore structures are often exposed to various types of repeated impact loads, such as wave slamming, floating ice impacts and ship collisions which will cause large deformation or even fracture. With imperfections due to the process of construction or damage caused by accidents, the load carrying capacity of structures will decrease. This paper investigates the load carrying capacity of aluminum alloy plate with an initial crack under repeated impact loads by means of experiments and numerical simulations. In the experiments, the prepared specimens with crack and without crack are impacted repeatedly up to plate perforation by releasing a hemispherical-headed cylindrical hammer. Numerical simulations are carried out with ABAQUS/Explicit software. The numerical models are built according to the actual experimental conditions. Comparison of the numerical predictions with the experimental results shows reasonable agreement. It is found that aluminum alloy plates under repeated impacts are sensitive to initial cracks. The fracture mode and plastic deformation of aluminum alloy plates can also be affected.

Commentary by Dr. Valentin Fuster
2018;():V003T02A011. doi:10.1115/OMAE2018-77239.

The objective of this paper is to develop a realistic numerical model for ice–pre swirl stator (PSS) structure interaction problem under collision circumstance. PSS structure has been installed in front of a propeller of oil carrier or container vessel operating non-arctic region in order to increase efficiency of propulsion system, which was designed by Daewoo Shipbuilding and Marine Engineering (DSME). Recently, according to increase of port traffic in arctic region, it is being anticipated that market of PSS installed vessel operating in arctic region will be extended to pursue raised ship’s efficiency but in strength point of view, quantified assessment for dynamic interaction between PSS and sea ice is not specified in any design codes or classification rules. This standpoint will be led to an increased demand for the safe structural design of arctic vessel’s PSS structure subject to collision with sea ice.

In this study, DSME’s PSS structure design will be introduced and five types of numerical model to represent ice crushing behavior which were well-known though previous study will be investigated. The parameters of each material model of ice will be defined and more realistic numerical model for ice will be proposed through comparison study with experimental results. After that, the results of numerical simulations for a collision between PSS and sea ice with five numerical models using LS-DYNA software will be represented. In here, dynamic responses of PSS and failure behaviors of each ice model at each load case will be thoroughly evaluated. Collision loading scenarios and ice dimension were properly assumed based on polar class rule for propeller-sea ice interaction. Consequently, simplified analytic method and proper numerical model for ice corresponding to PSS-sea ice collision simulation will be proposed and discussed, respectively.

Commentary by Dr. Valentin Fuster
2018;():V003T02A012. doi:10.1115/OMAE2018-77485.

Sandwich structures with corrugated cores have attracted a lot of interest from industrial and academic fields due to their superior crashworthiness. In this paper, the dynamic response of metallic trapezoidal corrugated core sandwich panels under low-velocity impact loading is studied by conducting drop hammer impact testing. The sandwich panels composed of two thin face skins and trapezoidal corrugated core, were designed and fabricated through folding and laser welding technology. Main attention of present study was placed at the influences of the impact energy, impactor diameter and impact location on the impact force, deformation mechanisms and the permanent deflections of the trapezoidal corrugated core sandwich panels. Results revealed that the impact energy has significant effects on the dynamic response of the sandwich panel, whereas the impact diameter has little effects on it. The deformation mode of the front face sheet differs sharply when the impact location is different. The middle unit cell of corrugated core is compressed to the “M” shape under different low-velocity impact loading.

Commentary by Dr. Valentin Fuster
2018;():V003T02A013. doi:10.1115/OMAE2018-77964.

In areas frequented by fishing vessels, trawl equipment or anchors may interfere with pipelines and cause damage through impact, potential hooking, and ensuing release of the pipeline. This load sequence of denting followed by global bending and springback results in a complex stress and strain history. Experiments have shown that fracture in an impacted pipe typically arises along the bottom of the dent, where the material suffers high compressive strains in the impact and hooking phase, and a rapid change to tension during the rebound phase. High compressive strains may reduce the strain to failure significantly for a succeeding tensile phase. A common trait of ductile damage models is to account for damage through nucleation, growth and coalescence of voids, which traditionally is thought to occur during tension. In this study, an uncoupled phenomenological Cockcroft-Latham-type fracture model accounting for anisotropic damage is used. The fracture model is implemented in the explicit finite element programme IMPETUS Afea Solver, and calibrated using material tests. Simulations show that the proposed fracture model is able to account for the observed behaviour.

Commentary by Dr. Valentin Fuster
2018;():V003T02A014. doi:10.1115/OMAE2018-78199.

A review of accidents involving collisions between ships and offshore platforms was carried out. There are reports and publications that present numbers, statistics and even details of the most important collisions between ships and offshore platforms, especially considering the North Sea region, but publications about accidents in Brazilian waters are rare. Thus, this paper reports the few existing publications that consider this problem in Brazilian waters and shows the results of eleven years of collecting data of collisions on Petrobras’ platforms.

Commentary by Dr. Valentin Fuster
2018;():V003T02A015. doi:10.1115/OMAE2018-78364.

An engineering study was performed in 2017 to develop a multi-span suspension bridge on floating foundations across the Bjørnafjorden in Norway. The bridge was approximately five kilometers long and consisted of three main suspension spans supported by four pylons (towers). Two of the pylons were supported on tension-leg platforms (TLP) due to large water depths. The bridge has to be resistant towards collisions from passing ships. However, submarine impacts to the submerged parts of the bridge were also a challenge due to the bridge location being close to an active submarine training field.

This paper focus on the response of one such TLP towards collisions from submarines transiting below the bridge. Nonlinear explicit finite element analysis is used to study the possible collision scenarios, and the response of the TLP and the resulting bridge motion is evaluated. Further, transient failure of a tether was investigated to assess possible consequences of rupture of one of the tethers.

Commentary by Dr. Valentin Fuster
2018;():V003T02A016. doi:10.1115/OMAE2018-78634.

When a platform is operating in a mooring, various vessels that frequently pass by result in severe accidental collisions of the platform. Thus, the kinematic response of the mooring platform should be investigated. A new analytical method, including a load analysis and kinematics analysis, is proposed in this paper. In the load analysis, the impact force is calculated using finite element method (FEM). In the kinematic analysis, closed-form analytical expressions based on multi-body dynamics are derived with the impact force as an input. Furthermore, the expressions are improved considering the fluid effect. A series of collision cases are implemented to validate the proposed method by FEM. The kinematic results solved by the proposed method agree well with FEM, which illustrates that the method is feasible and accurate. However, the proposed method taking around 30s, which is much shorter than 7200s by FEM, is proved to be more efficient.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Extreme Loading and Responses

2018;():V003T02A017. doi:10.1115/OMAE2018-77150.

The slamming phenomenon often occurs when the bottom of a ship hits the seawater with a relatively high velocity in irregular waves. In extreme sea conditions, the slam induced dynamic loads can be quite large resulting in local structural damages or global deformation of the panels, in particular when ships face head sea waves with high forward speed. The paper presents finite element simulations of an aluminum stiffened plate subjected to extreme slamming loadings. The selected plate is commonly used for the design and construction of the hull of high-speed vessels. The simulations are performed with the explicit LS-DYNA finite element solver and analyze the large plastic deformation of the stiffened plate. The effect of the heat affected zone due to the welding is also assessed using the mechanical properties of an aluminum alloy grade 5083-H116. The outcomes of this study identify the parameters that should have practical importance when estimating the extent of structural deformation due to slamming loads.

Topics: Aluminum
Commentary by Dr. Valentin Fuster
2018;():V003T02A018. doi:10.1115/OMAE2018-77629.

In this paper, the numerical model was developed by using the commercial code LS/DYNA to investigate the dynamic response of sandwich panels with three PVC foam core layers subjected to non-contact underwater explosion. The simulation results showed that the structural response of the sandwich panel could be divided into four sequential regimes: (1) interaction between the shock wave and structure, (2) compression phase of sandwich core, (3) collapse of cavitation bubbles and (4) overall bending and stretching of sandwich panel under its own inertia. Main attention of present study was placed at the blast resistance improvement by tailoring the core layer gradation under the condition of same weight expense and same blast load. Using the minimization of back face deflection as the criteria for evaluating the blast resistant of panel, the panels with core gradation of high/middle/low or middle/low/high (relative densities) from the front face to back face demonstrated the optimal resistance. Moreover, the comparative studies on the blast resistance of the functionally graded sandwich panels and equivalent ungraded ones were carried out. The optimum functionally graded sandwich panel outperformed the equivalent ungraded one for relatively small charge masses. The energy absorption characteristics as well as the core compression were also discussed. It is found that the core gradation has a negligible effect on the whole energy dissipation of panel, but would significantly affect the energy distribution among sandwich panel components and the compression value of core.

Topics: Explosions
Commentary by Dr. Valentin Fuster
2018;():V003T02A019. doi:10.1115/OMAE2018-77739.

The ANSYS/AUTODYN software was employed to investigate the dynamic response of the metallic sandwich panels subjected to air blast loading. The sandwich panels were composed of two face sheets and a trapezoidal corrugated-core. To validate the numerical models, the simulation results were compared with experimental data reported previously. In the simulation works, the process of shock wave propagation and the structural dynamic response were analyzed. Meanwhile, the influences of the stand-off distance between the explosive charge and the front face sheet on the fluid-structure interaction effect, dynamic response and the energy absorption of sandwich panels were investigated. Numerical results demonstrated that the impulse intensity decreased dramatically with the increase of stand-off distance. The slapping between the front face sheet and the back face sheet could be observed at the stand-off distances of 50 mm and 100 mm, while the sandwich panel exhibited the “strong core” response mode under the stand-off distance of 150 mm. Investigations into energy absorption characteristic revealed that the total energy absorption reduced with the increase of stand-off distance. The front face and corrugated-core provided the most contribution on total energy absorption. Moreover, the energy absorption proportion of corrugated-core had a positive correlation with the stand-off distance.

Commentary by Dr. Valentin Fuster
2018;():V003T02A020. doi:10.1115/OMAE2018-77845.

In this study, for the sake of evaluating the structural response taking account of the fluid-structure interaction effect of a ship under severe wave condition, a method for coupling the CFD and 3D FEA in a sequentially staggered manner, is developed. The elastic deformation of the ship is taken over, not only to the following FEA stages but also to the following CFD stages. In order to validate the developed two-way coupling method, and to investigate into the fluid-structure interaction effect on the ship, the comparisons among the straightforward (one-way) coupling method, the experimental results and the developed two-way coupling method are carried out, in terms of the wave-induced loads exerted on the ship, and the hydroelastic response. Both the weakly and strongly coupled methods are investigated. The fluid-structure interaction effect is found as a decrease of the natural frequency of vertical vibration mode of the ship; the natural frequency predicted from the developed two-way coupling method is slightly lower than that from the one-way coupling method.

Commentary by Dr. Valentin Fuster
2018;():V003T02A021. doi:10.1115/OMAE2018-77908.

To investigate the slamming pressure on the bottom of a wet-deck structure of a multihull vessel, the water impact problem of a stiffened steel panel is simulated by using a fully coupled ALE/FEM algorithm which is implemented in the commercial software LS-DYNA. The Lagrangian formulation is used to describe plane-strain deformations of the hull panel while the Eulerian formulation is applied to describe the fluid flow. The governing equations of this coupling problem are solved by using finite element method. The explicit finite element method is firstly validated through the comparisons of the slamming pressure and structural deflection between the numerical predictions and the published experimental data, for an elastic horizontal plate. Secondly, the parametric study of the mesh size in the impact domain of the FE model is performed. The total slamming forces obtained from three models are compared. To study the effects of the flexibility of the structure on the slamming load, the predictions of slamming pressure on several locations of the elastic panel are compared with the values obtained by using the rigid body model.

The water entries of the stiffened panel with two different deadrise angles, entry velocities, and thickness of plating are simulated. The results of the total slamming force, slamming pressure are presented and discussed.

Topics: Water
Commentary by Dr. Valentin Fuster
2018;():V003T02A022. doi:10.1115/OMAE2018-78355.

Steep breaking waves can result in high impact loads on offshore structures, and several model test campaigns have been conducted to assess the effect of horizontal wave slamming. High loads have been measured, and they can be challenging to withstand without significant deformation.

For wave slamming problems it is common to estimate the characteristic slamming load and assume that this will give an equivalent characteristic response. One challenge related to the slamming load is that it has a large variability in load level, the duration of the load and the shape of the overall load pulse. This variability can have a large impact on the estimated response to the characteristic load, causing a similar or larger variability in response. Due to the sensitivity to the structural response, it may be difficult to interpret large amounts of such data to arrive at a relevant design load without making overly conservative assumptions.

This paper investigates the sensitivity of the structural response to assumptions made in the material modelling and how the short term variability is affected if we instead of load use response indicators such as plastic strain and max deformation to arrive at a characteristic load. For this purpose, a simplified dynamic response model is created, and the recorded wave impact events can then be evaluated based on the predicted structural response from the simplified model.

It was found that the structural response is sensitive to the structural configuration. The assumed material behavior and hydro-elastoplastic effects were identified to greatly affect the structural response. A reasonable approach to arrive at the q-annual response seems to be to first estimate the q-annual extreme slamming load, and then run the structural analysis on several of the measured slamming time series with the estimated q-annual extreme pressure.

Topics: Stress
Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Fatigue and Fracture Reliability

2018;():V003T02A023. doi:10.1115/OMAE2018-77038.

The Weibull stress (Beremin, 1983) relates the local first principal stress and plastically strained volume to the probability of fracture. It requires two constants (m, σu) as input, which are generally regarded as material parameters. As the Beremin approach is used in the background to structural analysis rules, the Beremin parameters are now being used in other situations, including engineering analysis. However, the currently accepted way to find the Beremin constants requires twenty tests, which is considered to be unacceptably expensive for industrial application. Less expensive ways of finding the Beremin parameters have been published in the literature, but they have never been compared to the de facto standard. In this paper, the Beremin parameters were found by the de facto standard method and two other ways, and a comparison is made. It was found that the Beremin parameters can be estimated with reasonable accuracy with a method that uses on just two sets (six specimens) of fracture specimens by careful application of the method of Andrieu (2012). Also, a proposal is made for another method which is based only on the Master Curve temperature T0.

Topics: Stress
Commentary by Dr. Valentin Fuster
2018;():V003T02A024. doi:10.1115/OMAE2018-77292.

Up to now, the uncertainties in the fatigue utilization during operation (long-term, typical 20 to 30 years) phase have been widely investigated for various oil and gas installations while limited attentions have been paid to the fatigue damage during the transport phase. In normal, the fatigue damage during the transport phase is assumed to contribute limited proportions (for example, less than 5%) of the total fatigue damage for the whole life since the transport duration is at most several months. However, as the size of oil and gas installations increases, the fatigue damage during the transport phase may increase noticeably considering the inertia forces in moderate or severe sea states. During the transport, the weather encountered may deviate significantly from the long-term statistical values. It becomes crucial to determine the uncertainties in the calculated fatigue damage during the transportation phase. Nowadays, the uncertainties are mainly accounted for using Design Fatigue Factor (DFF) while the value of DFF may be different in different standards. In this paper, the fatigue damage for a topside module during the transport phase is studied. Three different vessels are to be used for comparison purpose. The uncertainties due to the sea states encountered are focused. Simplified approach is adopted to investigate the factors influencing fatigue damage. In addition, the calculated fatigue damage is also compared with the fatigue damage based on simplified fatigue analysis.

Commentary by Dr. Valentin Fuster
2018;():V003T02A025. doi:10.1115/OMAE2018-77294.

Fatigue damage at side structures are widely reported for ships because the external wave pressure reaches maximum at still-water level. At the same time, the ship side structures also withstand noticeable hull girder loads (vertical and horizontal bending moments). Correct assessment of the fatigue damage due to global and local load effects is of vital importance for side structures of marine vessels. Little research, however, has been carried out for the side structures of semi-submersibles. Like ships, the semi-submersible side structures experience both global and local load effects. Especially for external pressure, the effect of intermittent wet and dry surfaces on the side shell near or above the waterline needs to be considered. In this paper, a global analysis model of the Gjøa semi-submersible, shown in Figure 1, is built and the local response for the side structure is investigated. The contribution of fatigue damage due to global and local load effects are compared and presented.

Commentary by Dr. Valentin Fuster
2018;():V003T02A026. doi:10.1115/OMAE2018-77552.

Current standards (as an example, DNV-OS-E301[1] and API-RP-2SK[2]) do not account for mean load in the fatigue assessment of mooring chains. Both standards, provide S-N curves derived from experimental work without specifying the mean load for which they have been obtained or proposing a mean load correction function. This paper reports a fatigue analysis study of mooring chains under Tension Loading using a multiaxial fatigue criterion for two different mean loads. Multiaxial fatigue criteria enable to account directly for complex phenomena, such as residual stresses, non-proportionality of the stress tensor, among others. This paper presents an example of the implementation of the Dang Van fatigue criterion for studying the fatigue behavior of mooring chains under tension. It quantifies the effect of the mean load on the fatigue lifetime and the failure location. Furthermore, it also proposes a simplified approach to reduce the complexity and the computational time of the fatigue analysis using Dang Van fatigue criterion. The paper is organized as follows: in the first part an example of the fatigue assessment is reported. Two different loading conditions with the same load amplitude but different mean loads are studied. The assessment method is based on two steps: a mechanical analysis and a fatigue analysis. In the second part, a simplified fatigue assessment method is proposed. As part of this method, a ratio between the fatigue lifetimes of two loading conditions, which have the same load amplitude but different mean load, is formulated. This ratio has been obtained analytically using the geometric representation of the Dang Van fatigue Criterion. Finally, the paper ends with a discussion, based on recent works, regarding the formulation of the locus of the Dang Van criterion and the fatigue properties used for the calibration of this criterion.

Commentary by Dr. Valentin Fuster
2018;():V003T02A027. doi:10.1115/OMAE2018-77888.

The container carriers represent a challenge in the design process, where an important role play the estimation of design loads and their response. The wave-induced vibrations are often referred to as whipping as a transient phenomenon due to wave impacts and springing as a resonance phenomenon due to oscillating loads. These hydro-elastic vibratory phenomena’s are the most important consequences of the continuous increase in the ship length and low stiffness of the hull girder. While this is a concern in the marine industry, this phenomenon is not clearly considered in the design. Therefore, there are uncertainties associated with the wave-induced vibration intensity and the probability of occurrence of whipping to the sea state that needs to be clarified.

This paper address the effect of the relationship of the characteristics of whipping vibration to the sea state, evaluating the results of a series of time-domain computational simulations. A numerical method that combines a three-dimensional panel method and FEA proposed by one of the co-authors is employed for evaluating the load effects. The calculations are performed for the respective short-term sea states. This study presents the probability of occurrence and vibration intensity of the whipping vibration experienced by a 6500 TEU container carrier in a North Atlantic Ocean route.

Topics: Containers , Vibration , Ships
Commentary by Dr. Valentin Fuster
2018;():V003T02A028. doi:10.1115/OMAE2018-77982.

The issue of hydroelasticity caused by hull vibration has become an unavoidable problem in the design and verification of large ships. Driven by environmental protection and economical efficiency, the size of ships are increasingly larger, and the resulting springing and whipping response and their effects on fatigue damage has been paid more and more attention especially for ultra large container ships (ULCS). Many classification societies typically check fatigue damage caused by vertical bending when considering springing, while it needs to be emphasized that large container ships can suffer severe torsional loads compared to other large ships due to wide breath and big hatch openings. In the existing stress calculation method, the finite element analysis method obviously has a high calculation accuracy. However, there are so much work to do with FEM model established, and partially refined, operated at all sea states etc., which not only requires much time, but also higher computing equipment. Therefore, in this paper, a simplified calculation method of fatigue damage considering the effect of bending and torsion is proposed, and a 21000TEU will be calculated by this method. The wave loads on the hull structure will be estimated based on the 3D linear hydroelastic theory coupling horizontal and torsional vibration, and the stress caused by bending and torsion will be obtained respectively. Finally, the fatigue damage is calculated by spectral analysis method considering high frequency springing loads. Then the effect on large container ships’ fatigue due to bending and torsional vibration is discussed.

Commentary by Dr. Valentin Fuster
2018;():V003T02A029. doi:10.1115/OMAE2018-78032.

The objective of this work is to analyze the fatigue reliability of fillet welded cruciform joints considering the uncertainty of the load and capacity. The weld shape is defined by multivariate normally distributed variables, which represent the position variations of the shape control points on the fillet welds. Finite element analyses are performed to calculate the fatigue notch factors of the weld root and toe, where the fatigue crack is usually initiated. Various weld shapes associated with various correlation conditions and weld quality levels are generated and the corresponding probability distributions of the fatigue notch factors are obtained by using the Monte Carlo simulation method. Sensitivity analyses are carried out to identify which location is more important for the fatigue notch factors. Within the context of the local strain approach, a critical fatigue notch factor that can exactly trigger fatigue failure is proposed. Its statistical descriptors are determined by using the Monte Carlo simulation method, in which the nominal stress range, material properties and fatigue damage at failure are treated as random variables. The limit state functions of the weld root and toe are formulated based on the actual and critical fatigue notch factors. The first order reliability method is applied to evaluate the fatigue reliability. The cruciform joint, composed by two fatigue-prone locations, is evaluated as a series system of components. Two different loading conditions, which make the cruciform joints load-carrying and non-load-carrying respectively, are considered.

Topics: Fatigue , Reliability
Commentary by Dr. Valentin Fuster
2018;():V003T02A030. doi:10.1115/OMAE2018-78147.

Most materials for offshore applications are tested for brittle fracture resistance at a single temperature related to the minimum design temperature and by a single fracture test method. It is much rarer to perform tests at multiple temperatures to compare the fracture performance across a range of temperatures and testing methods. EWI recently compared the fracture toughness transition behaviors for an X70 steel pipe across Charpy V-notch (CVN), single-edge notched bending (SENB) crack-tip open displacement (CTOD), and single-edge notched tension (SENT) CTOD test geometries. This showed variability of the material behavior better described by the inhomogeneous behavior models considered for welded joints. It also suggested the possibility that near the ductile-to-brittle transition temperature, the toughness under SENT CTOD may be higher than for SENB CTOD testing where the failure mode is brittle fracture.

The testing methods used full-size CVN and nearly full-thickness CTOD specimens in bending, as limited by the pipe curvature of the 219-mm diameter pipe with 35.4-mm wall thickness. The SENT CTOD specimens were pre-cracked in bending with the same dimensions as the SENB specimens, but are then cut down to place the pre-cracked crack tip at approximately one quarter of the thickness through the resulting specimen. This modification places the tip in the higher constraint region for the tension test.

Girth welds in the same X70 pipe were prepared using a pulsed GMAW process with ER80S-D2 welding wire. Similar testing was performed with weld centerline notches for the CVN and CTOD specimens. The transition behavior was related between the three testing methods for the weld centerline at the mid-wall of the pipe thickness.

Using representative values equivalent to the minimum of three tests, the SENT values were 4.8 to 4.9 times the values for the SENB tests.

Commentary by Dr. Valentin Fuster
2018;():V003T02A031. doi:10.1115/OMAE2018-78188.

It is known that the fatigue strength decreases in corrosive environment and many experiments were carried out to comprehend the decrease in fatigue strength in corrosive environment. In order to comprehend the actual state, a cycle speed of fatigue test loads should correspond to a wave frequency. Therefore, an experiment in the long life region is practically difficult, then the corrosion fatigue data available for the life assessment of the structure is quite limited.

In this study, the fatigue strength of the welded joints in long life service was evaluated according to the calculations of corrosion fatigue crack propagation subjected to the random loadings which followed an exponential distribution. In the crack propagation calculations, the progress of corrosion wastage from the plate surface and the resultant stress increase were considered simultaneously. In the high stress and the short life region, the decrease in fatigue strength due to the accelerated crack propagation in corrosive environment was dominant because the progress of corrosion wastage was little. On the other hand, in the low stress and the long life region, the decrease in fatigue strength became dull as longer the fatigue life because the corrosion fatigue crack propagation was suppressed by the corrosion wastage, but after that the fatigue strength showed the precipitous decrease due to the increase in stress resulted by the progress of corrosion wastage.

Commentary by Dr. Valentin Fuster
2018;():V003T02A032. doi:10.1115/OMAE2018-78596.

Stiffened plates with cracked damage are often subjected to constant amplitude and/or variable amplitude cyclic loads in sea environment. Under the stress-controlled asymmetric low-cycle fatigue loads, the coupling effect of low-cycle fatigue crack propagation and accumulative plasticity contributes to the increase of accumulative mean strain of cracked structures. Low-cycle fatigue crack growth and the increase of whole strain of cracked structures will change the bearing capacity of cracked structures. In this paper, experimental study on crack propagation and strain accumulation of cracked stiffened plate under low cycle fatigue load has been conducted. AH32 steel is used to make stiffened plate specimen with crack symmetrically located about stiffener. The accumulative strain of the cracked stiffened plate specimens during low-cycle fatigue crack propagation was obtained. From the experiments for cracked stiffened plates under the low-cycle fatigue loading, it is found out that the crack propagates firstly in the weld and then also gradually takes place in the stiffener. The stress ratio of low-cycle fatigue load and stiffener stiffness have been investigated in the experimental study and it is found out that these parameters significantly affect the low-cycle fatigue crack growth life and accumulation strain of the cracked stiffened plate specimens.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Probabilistic and Spectral Wave Models

2018;():V003T02A032a. doi:10.1115/OMAE2018-77124.

The paper addresses the problem of deriving the nonlinear, up to the second order, crest wave height probability distribution in front of a vertical wall under the assumption of finite spectral bandwidth, finite water depth and long-crested waves. The distribution is derived by relying on the Quasi-Deterministic representation of the free surface elevation in front of the vertical wall. The theoretical results are compared against experimental data obtained by utilizing a compressive sensing algorithm for reconstructing the free surface elevation in front of the wall. The reconstruction is pursued by starting from recorded wave pressure time histories obtained by utilizing a row of pressure transducers located at various levels. The comparison shows that there is an excellent agreement between the proposed distribution and the experimental data and confirm the deviation of the crest height distribution from the Rayleigh one.

Commentary by Dr. Valentin Fuster
2018;():V003T02A033. doi:10.1115/OMAE2018-78216.

This paper discusses the uncertainty in extreme wave analysis from different sources. Poor data quality and small sample size will lead to uncertainty in the extreme wave analysis. Extreme value estimation methods are developed based on various assumptions, and each would lead to unique estimation results. In addition, the cause of extreme waves varies among regions, directly affecting the extreme behavior. The aim of this study is to provide insight into how the uncertainty of extreme wave estimation is influenced by the different source of uncertainty, namely data uncertainty, method selection and extreme behavior at each location.

Key parameters to describe the extreme wave events are the frequency of occurrence and its tail-behavior. We use these two parameters as a benchmark to assess the extreme wave characteristics. We focus on four regions, namely Gulf of Mexico, North Sea, Adriatic Sea, and North West Pacific. Meteorological cause of extreme events and known extreme wave behavior are reviewed based on previous studies. Model inter-comparison revealed the shortcomings of wave models to reproduce extreme wave events, and the magnitude of data error was unique to each location.

Numerical experiments were conducted to evaluate the possible impact from poor data quality and small sample size on epistemic uncertainty. Case study based on representative parameters of Gulf of Mexico and North Sea revealed the difference between two locations. These results provide a benchmark for the source of uncertainty and its impact on extreme wave analysis. Among them, extreme waves dominated by tropical cyclones were most vulnerable to have large epistemic uncertainty. The importance to adequately quantify epistemic and aleatory uncertainty is reconfirmed.

Topics: Waves , Uncertainty
Commentary by Dr. Valentin Fuster
2018;():V003T02A034. doi:10.1115/OMAE2018-78386.

A spectral description of the wave spectrum is usually required in the design of offshore structures, and a generalised form of the JONSWAP spectrum is often used. The JONSWAP spectrum involves parameters that allow flexibility in the specification of the spectral peak, which is important for the response of both fixed and floating structures, but particularly for the floating structures. The peak of the wave spectrum is also important in nonlinear effects that for example contribute to the probability of a large crest occurring in a sea state time series realisation in a model basin, used to test a design platform.

There has been a number of studies focused on the uncertainties of JONSWAP parameter estimates. We review these to establish an overview of the present understanding of the uncertainties, and we undertake further analyses to investigate the sensitivity of the uncertainties to the method of analysis and the types of data typically available for analysis, including both time series and spectral data.

Topics: Uncertainty
Commentary by Dr. Valentin Fuster
2018;():V003T02A035. doi:10.1115/OMAE2018-78417.

Absence of mathematically justified criteria during development of the wind energy input and wave breaking energy dissipation source terms in Hasselmann equation (HE), used as the framework of modern operational wave forecasting models, lead to creation of plethora of parameterizations, having enormous scatter, disconnected from the physical background and obeying dozens of tuning parameters to adjust the HE model to the specific situation. We show that it’s possible, based on analytical analysis and experimental observation data, to create the new set of source terms, reproducing experimental observations with minimal number of tuning parameters. We also numerically analyze six historically developed and new wind input source terms for their ability to hold specific invariants, related to HE self-similar nature. The degree of preservation of those invariants could be used as their selection tool. We hope that this research is the step toward the creation of physically justified tuning-free operational models.

Commentary by Dr. Valentin Fuster
2018;():V003T02A036. doi:10.1115/OMAE2018-78754.

This study focuses on the computation and analysis of the energy content of a wave train and the influence of nonlinear components, such as nonlinear wave profile as in Stokes wave and phased locked breathers, on the content. To this end, an overview of a state-of-the-art nonlinear Fourier analysis tools for the nonlinear Schrödinger equation is presented. Experimental measurements from a set of performance tests of the directional wave basin at Oregon State University were analyzed using this tool and the energy contents, both from the linear spectrum and nonlinear spectrum, were calculated. The deviation of the energy content from linear analysis and its relationship to the level of nonlinearity of the wave train is investigated. The Benjamin-Feir parameter presents the degree of nonlinearity of the wave train. An increasing energy deviation was observed for increasing nonlinearity of the wave field. Spatial evolution of such behavior is also investigated. It was confirmed that the significant difference from the linear energy is due to increase in the nonlinear components and the more distance the wave train could travel (without substantial dissipation) the more erratic and more significant energy deviations were observed.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Probabilistic Response Models

2018;():V003T02A037. doi:10.1115/OMAE2018-77263.

As offshore reservoirs are depleted, the seabed may subside. Furthermore, the extreme crests estimates are now commonly higher than obtained previously due to improved understanding of statistics of non-linear irregular waves. Consequently, bottom fixed installations which have previously had sufficient clearance between the deck and the sea surface may be in a situation where wave impact with the deck must be considered at relevant probability levels.

In the present paper, we investigate the long-term area statistics for maximum crest height under a fixed platform deck for 2nd order short crested and long crested sea based on numerical simulations as a function of platform deck dimension for jackets. The results are for one location in the northern North Sea, but some key results are also reported and verified for a more benign southern North Sea location. Time domain simulations for long crested and short crested waves over a spatial domain with dimension of a platform deck are performed, and relevant statistics for airgap assessment determined.

Second order waves are simulated for the different cells in the (Hs, Tp) scatter diagram for Torsethaugen two-peak wave spectrum for long-crested and short-crested sea. A total of 1000 3-hour sea states are generated per cell, and time series generated for 160 spatial points under a platform deck.

Short-term and long-term statistics are established for the maximum crest height as function of platform dimension; inline and transverse to the wave direction, and over the area. Results are given for the linear sea and for the second order time series.

The annual q-probability estimates for the maximum crest height over area as a function of platform dimension is determined for a location at the Norwegian Continental Shelf by weighting the short-term statistics for the individual cells in the scatter diagram with the long-term probability of occurrence of the sea state. To reduce the number of numerical second order simulations, the effect of excluding cells that have a negligible effect on the long term extreme crest estimate is discussed. The percentiles in the distribution of maximum crest (over area) in design sea states that corresponds to the extreme values obtained from the long-term analysis are determined for long crested and short crested sea. The increase in the extreme crest over an area compared to the point in space estimate is estimated for both linear and second order surface elevation.

Commentary by Dr. Valentin Fuster
2018;():V003T02A038. doi:10.1115/OMAE2018-77579.

A turret structure can be a major design driver for FPSO systems. Therefore, careful attention needs to be given to the estimation of extreme loads on this structural element.

This paper presents an all seas long-term analysis of the extreme global restoring forces acting on a FPSO turret structure, and a comparison with the results obtained through the contour line approach.

The analysis is performed in the time domain using a coupled model, where the floater is modelled in the software SIMO, and the risers and mooring lines are represented by a Finite Element Model in RIFLEX.

The characteristic responses of the turret structure with q-annual probability of exceedance are estimated from a full long-term analysis where both the short and long-term variability are considered. These results are compared to those obtained through the long-term estimate from the contour line approach when assuming 90th percentile for the worst sea state with q-annual probability of exceedance.

The results from the full long-term analysis will allow us to verify the adequate percentile level to be used with a contour line approach when estimating extreme turret structure loads.

Topics: Stress , FPSO
Commentary by Dr. Valentin Fuster
2018;():V003T02A039. doi:10.1115/OMAE2018-77785.

This paper aims to demonstrate how to estimate strains of fixed structures considering cases with nonlinearities based on parameters determined from one linear case. Both simulated and experimental data have been evaluated. A finite element model was used to obtain the simulated responses. Accelerations and strains were measured along the application of random loading to a fixed structural model for the experimental data. Operational Modal Analysis has been considered in the time domain in order to identify the modal properties. Nonlinearities are included as friction is imposed on the models.

Commentary by Dr. Valentin Fuster
2018;():V003T02A040. doi:10.1115/OMAE2018-78283.

Offshore structures are typically required to withstand extreme and abnormal load effects with annual probabilities of occurrence of 10−2 and 10−4 respectively. For linear or weakly nonlinear problems, the load effects with the prescribed annual probabilities of occurrence are typically estimated as a relatively rare occurrence in the short term distribution of 100 year and 10 000 year seastates. For strongly nonlinear load effects, it is not given that an extreme seastate can be used reliably to estimate the characteristic load effect. The governing load may occur as an extremely rare event in a much lower seastate. In attempting to model the load effect in an extreme seastate, the relevant short term probability level is not known nor is it known whether the physics of the wave loading is captured correctly in an extreme seastate. Examples of such strongly nonlinear load effects are slamming loads on large volume offshore structures or wave in deck loads on jacket structures subject to seabed subsidence.

The present paper is concerned with the long term distribution of strongly nonlinear load effects and a methodology is proposed which incorporates CFD analysis in a long term Monte Carlo analysis of crest elevations and wave kinematics. Based on a long term time domain simulation of a linear surface elevation, a selection of events is run in CFD in order to obtain a database of linear and corresponding fully nonlinear wave fields with the possibility of wave breaking included. In the subsequent long term analysis, a large linear event is then replaced by the closest matching event in the database. A technique is developed to Froude scale the database results and shift the origin in time and plane so that the database of typically only 100 events give a close match to all the events in the simulation.

The method is applied to the simple case of drag loading on a cylinder which is truncated above the still water level such that only the largest waves impact with the structure. It is observed that whereas the Event Matching method agree well with a second order model for return periods lower than 100 years, the loading on the cylinder is significantly larger for longer return periods. The deviation is caused by the increasing dominance of wave braking in the largest crest and illustrates the importance of incorporating wave breaking in the analysis of wave in deck loading problems.

Topics: Waves
Commentary by Dr. Valentin Fuster
2018;():V003T02A041. doi:10.1115/OMAE2018-78411.

Due to subsidence, unrealistic environmental assumptions in design or other factors, many fixed offshore installations around the world are prone to large impact forces on the super-structure. Too often, unexpected damages are found, and more accurate analyses reveal the need for reinforcement to comply with the relevant rules and regulations. The common analytical practice is to compute impact forces using 5th order Stokes waves in a CFD solver, with the crest height set to the relevant return period. Using this approach implicitly introduces a series of unrealistic assumptions, and cannot necessarily be expected to produce the force of the desired return period. In the present work, we propose a framework to better estimate the true waves producing the 100-year wave-in-deck forces in a typical North Sea environment. This is done by a long-term analysis of wave-in-deck force, hierarchically using several screening filters and solvers of different complexity. The main facilitator is an efficient High-Order Spectral Method, which at the final level is used as initial and boundary conditions in CFD. The resulting waves are short crested and may break — producing very high crest velocities that are normally not accounted for in standard engineering practice.

Topics: Waves , Design
Commentary by Dr. Valentin Fuster
2018;():V003T02A042. doi:10.1115/OMAE2018-78745.

The present paper deals with the estimation of the short-term extreme motions of a spar floating wind turbine in parked rotor conditions, through a 1:30 at-sea experiment, carried out at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy). Thanks to some favorable local environmental conditions of the site, several wind-generated sea states with relatively low significant wave height (Hs < 0.50 m) have been collected during the experiment. These sea states are scale models of ocean storms, which are relevant hydrodynamic design conditions for the spar platform. The 30-minutes extreme values of the model structure motions have been estimated for all the six degrees of freedom, using the Weibull Tail Method (WTM), and the results obtained are presented in the paper. Such estimations are 1:30 scale models of the 3-hours extreme values of the spar motions in parked rotor conditions and may be directly used for design purposes.

Commentary by Dr. Valentin Fuster
2018;():V003T02A043. doi:10.1115/OMAE2018-78763.

This study investigates the use of efficient surrogate model development with the help of polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a simple moored offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response results from the contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for any sampled Hs and Tp; in such simulations, typically, a set of random phases (and deterministic amplitudes) define a wave train consistent with the defined sea state. These random phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty — from Hs and Tp on the one hand, and the phase vector on the other — in a nested manner that is shown to efficiently yield long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations, which serve as the truth system. Success with the method suggests that similar inexpensive surrogate models may be developed for assessing the long-term response of various offshore structures.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Reliability of Mooring and Riser Systems

2018;():V003T02A044. doi:10.1115/OMAE2018-77064.

Rigorous methods of probabilistic evaluations on long-term extremes are integral components in reliability research of offshore structures against overload events. Assessment across all conceivable sea states requires accounting for variabilities of long-term environmental loads and short-term stochastics, traditionally captured through extensive sampling or numerical expectation integration. The amount of environmental load variables render numerical integrations across high dimensions computationally prohibitive, while industry requirements of high return periods demand large Monte Carlo samples of timedomain dynamic analyses. Subset simulation offers a promising alternative to classic methods of statistical analysis, dividing ultra-low probability problems into subsets of intermediate probabilities. The methodology is uniquely advantageous for the assessment of heavy-tail overload events, which are unpredictably severe and occur at exceedingly rare frequencies. Subset simulation is experimented on a mooring case study situated in the hurricane-prone Gulf of Mexico, with the structure exposed to a joint-probabilistic description of wave, wind and current loads. The devised methodology is found to successfully evaluate hurricane-stimulated extreme events at ultra-low probabilities, beyond the feasible reach of Monte Carlo simulation at reasonable lead times.

Topics: Simulation , Mooring
Commentary by Dr. Valentin Fuster
2018;():V003T02A045. doi:10.1115/OMAE2018-77460.

The last years Statoil has replaced some of our seabed mooring chain segments. Some of these chains have corrosion pits caused by Microbiologically Influenced Corrosion (MIC).

In 2016 and 2017 one full length of a seabed chain segment, including anchor, was retrieved from a SEMI at approximately 300m water depth in the North Sea. The chain has been 20 years on the seabed. The corrosion on the chain was carefully documented, and showed significant levels of MIC. The extent of the MIC showed a strong dependency on seabed contact and how well the chain was buried in the sediments. The observed MIC is caused by Sulphate Reducing Bacteria (SRB). After corrosion identification, the chain has also been subject to full scale fatigue testing.

This paper presents the technical condition of the seabed mooring chain, describing the different levels of MIC, typical SRB corrosion attacks, and the results from the fatigue testing.

Commentary by Dr. Valentin Fuster
2018;():V003T02A046. doi:10.1115/OMAE2018-78508.

A model of a compressed air energy storage unit is tested for tow-out and installation in the Ocean, Coastal & River Engineering (OCRE) Portfolio of the National Research Council of Canada (NRC). The proposed prototype accumulator is a cylinder of 36 m in diameter and 12 m in height, which will be installed at the bottom of Lake Ontario at about 60 m water depth. The model of the accumulator with scale 1:21.5 was fabricated at the Design and Fabrication Unit of NRC. Appropriate ballast systems were designed and applied for the tow out, installations and release mechanism tests. The model scale test was conducted to examine the hydrodynamic behavior of the accumulator during tow-out and set down operations. NRC’s Towing Tank and Offshore Engineering Basin test facilities were used for the tasks. In this paper only the installation case of the accumulator is reported and discussed. Relevant numerical simulations are also carried out. Comparisons of the numerical results with the experimental results show good agreement for the compared cases.

Commentary by Dr. Valentin Fuster
2018;():V003T02A047. doi:10.1115/OMAE2018-78559.

This paper presents an efficient methodology for multi-objective design optimization of drilling riser in ultra-deep water considering maximum operability window and minimum weight of drilling riser system. As exploration activity moves to ultra-deep waters, the associated drilling cost increases, putting pressure on the operators to expand the drilling operability and reduce costs. Drilling systems are an integral part of oil and gas exploration particularly in deep waters. The drilling riser design requires a time-consuming design loops and scenarios analyzed with different FEM models, such as connected mode, drift-off, hang-off, recoil analysis, emergency disconnection, etc. The main purposes of this work is to improve the safety and cost-effective for drilling riser design by employing multi-objective optimization based on metamodel. The Radial Basis Function (RBF) metamodel is constructed by the design of experiment sampling and is utilized to solve the problem of time-consuming analyses. In the optimization module, multi-objective optimization by a non-dominated sorting genetic algorithm II is performed. Thereby, RBF optimum solutions forming a Pareto set are obtained and compared with accuracy analysis to determine their validity. The optimization results indicate that the proposed optimization strategy is valid and provide an efficient optimization design method for drilling riser in ultra-deep water.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Reliability of Renewable Energy Systems

2018;():V003T02A048. doi:10.1115/OMAE2018-77248.

In present work, two combined wind turbine (WT) and wave energy converter (WEC) systems have been concerned. One is a fixed-bottom system, referred as “MWWC” (monopile-WT-WEC combination); the other is a floating system, referred as “TWWC” (TLP-WT-WEC-combination). Comparative study of the hydrodynamic responses of the two combined systems has been done by numerical simulations in the time domain. Hydrodynamic loads of the supporting structures 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 supporting tower and the WEC floater. Firstly, the effect of different power-take-off (PTO) parameters, wave periods and the displacements of the WEC on the performance of the WEC’s wave energy production of the two combined systems under typical wave cases has been investigate, and preliminary optimal values for the PTO damping stiffness of the two combined systems have been obtained and compared; secondly, the effect of the horizontal contact force between the supporting tower and the additional WEC floater of the two combined systems have been further investigated, which is important for both the fatigue and extreme loads design of the supporting tower. Finally, a new strategy for MWWC system by adding horizontal PTO dampers between the supporting tower and the WEC floater has been proposed and investigated, which is helpful for both reducing the horizontal contact force and using the relative horizontal motion to produce power.

Commentary by Dr. Valentin Fuster
2018;():V003T02A049. doi:10.1115/OMAE2018-77376.

With continuous innovation and development of the wind power technology, the offshore wind turbine is rapidly developing. However, it also has difficulties in construction at sea and other shortcomings. One-step installation technique of the composite bucket foundation (CBF) provides a new way to solve the difficulties in the construction of offshore wind turbine at sea. And the integrated floating transport technique of the offshore CBF is the key link in the one-step installation technique. In this paper, by controlling three factors, such as draft, wave height and speed, the contact force between the vessel and CBF in the integrated floating transportation process is studied with experimental method. The relationship and the effect between the three factors and the contact force is analyzed. The experimental results are collected and analyzed to verify the safety of the integrated floating transportation of offshore CBF.

Commentary by Dr. Valentin Fuster
2018;():V003T02A050. doi:10.1115/OMAE2018-77382.

An offshore integrated anemometer mast (OIAM) is proposed to integrate the offshore installation of the foundation and steel mast into one operation. It can be prefabricated onshore and transported to the installation site which is simple in construction, rapid in speed and saving in cost. The most critical technique of the OIAM is the self-floating towing technique with a reasonable subdivision inside the floating tank. These subdivisions of OIAM facilitate the floating stability in case the occurrence of a damaged compartment. The tank will be ballasted to lower OIAM down and to penetrate these shallow skirt plates underneath into the sea bed after a free-floating towing process. To study and predict the dynamic behaviors of the OIAM in different sea conditions considering different towing critical factors, the hydrodynamic software MOSES is used to simulate the three-dimensional motion of the OIAM in the towing operation. Before that the integrated installation technique is introduced in terms of structure design, towing operation, and sinking on-site. The numerical results show that multiple subdivisions guarantee an adequate floating stability, and the hydrodynamic features indicate that a relative small dynamic response in some extreme sea state can be obtained by this OIAM structure. The self-floating technique of OIAM in sea is highly competitive for saving cost by using less expensive equipment in towing transportation.

Commentary by Dr. Valentin Fuster
2018;():V003T02A051. doi:10.1115/OMAE2018-78519.

A key challenge in the Offshore Wind industry is assuring the life-cycle structural integrity of wind turbine foundation monopiles. This is due to harsh environmental aspects as well as the loading regime (i.e. constant exposure to wave and wind forces introducing both fatigue and corrosion damage). Welding is a widely used joining technique for the manufacturing of offshore monopile structures. However, this is an aggressive process that introduces high levels of residual stress, which in turn may lead to reduced fatigue life, corrosion cracking resistance and accelerated degradation mechanisms. This study presents evidence that a measurement-informed strategy could be used towards developing a more reliable structural integrity assessment procedure for offshore monopile structures by taking into account the effect of residual stresses. A welded mock-up, 90 mm thick, 2600 mm wide and 800 mm long plate, was fabricated using a typical double-V welding procedure following current industrial practice. The contour method of residual stress measurement was employed to map residual stresses in the welded mock-up as well as in the CT specimens extracted from the weld region of the plate for future fatigue tests. Residual stress measurement results show that the mock-up plate contained tensile residual stresses above yield in the core of the weld, while the extracted CT specimens had lower though still significant residual stress levels. These results indicate that if the initial residual stresses are not carefully considered during fatigue or corrosion cracking tests, the results from the CT specimens alone will likely result in misleading structural life estimations.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Risk Analysis and Management

2018;():V003T02A052. doi:10.1115/OMAE2018-77009.

Condition monitoring technique has been widely applied in Maritime to ensure safe operation and minimise unscheduled downtime. However, in practice, ship operators need to assure that a failure mode is indeed monitored by the sensor intended for it, and the sensor has sufficient accuracy and precision for its purpose. Additionally, for a reliable condition monitoring technique, issues such as sensors degradation or drift that will reduce the data quality over time must be addressed. All these require that ship owners to select a monitoring system with the best suitable sensors technology while is economically viable. In this paper, tunnel thruster was used as a case study to demonstrate the basic approach to develop a reliable condition monitoring technique through Failure Mode, Effects and Criticality Analysis (FMECA). Based on failure modes, four types of condition monitoring techniques were identified including Vibration Monitoring, Acoustic Emission Monitoring, Wear Debris /Water in Oil Monitoring, and Thermal Monitoring, where vibration monitoring is discussed in detail as an example for defining the sensor specification. For a reliable condition monitoring technique, prediction of sensor reliability will be especially useful in the situation where sensors systems can degrade over time in service. Using temperature sensors as an example, a Bayesian network (BN) modeling approach has been carried out for assessing sensor reliability affected by aging.

Commentary by Dr. Valentin Fuster
2018;():V003T02A053. doi:10.1115/OMAE2018-77010.

Offshore structures are large-scale and multifunctional products with high requirements for reliability. In this paper, the reliability simulation method based on fault tree analysis for offshore structures is studied. For non-repairable systems, the Weibull distribution is used to fit the failure distribution function of the bottom events of fault trees. The Monte Carlo method is used to sample the lifetime of bottom events, and the unreliability of the top event is calculated through the structure function of the fault tree. For repairable systems, the time arrays of state changes are generated through sampling failure and maintenance time of the components. The system failure time of each simulation is calculated based on minimal cut sets of the fault tree. The method is applied in the reliability analysis of the electrical power system of offshore platforms to prove its feasibility and effectiveness.

Commentary by Dr. Valentin Fuster
2018;():V003T02A054. doi:10.1115/OMAE2018-77618.

Production efficiency is a key factor for oil & gas companies to have a good performance in a low oil prices environment, and planned shutdowns are events that impact crucially in this factor.

This technical paper objective is to explain how, starting from a point of high planned losses due to shutdowns bad performance (13%), the application during only one year of best practices compiled from oil & gas and other business experience is able to change things up to achieve an impressive outcome (3%).

Long term planning, meticulous preparation and efficient execution are achieved when best practices are applied with accountability, rigor and common sense.

Maintenance and Integrity shutdown plans compliance is also improved as they are considered inside a big picture that allows us to schedule them in the more efficient way to assure our HSE standards, and helped by some new technologies as the so called NII (Non-Intrusive Inspections).

Standards, procedures, deliverables, communication channels, materials control, organigrams… are the specific tools to be used and refed to implement the lessons we learn at the end of each shutdown.

Sharing regularly our experience with other operators is also a source of best practices and cross-pollination.

Maintaining this quality standard along the time and improving to the extent possible will drive us to excellence in the middle term. As our teams become more and more involved on this new method, they start to feel more comfortable and propose improvements, being more and more committed as they see good results arriving.

Commentary by Dr. Valentin Fuster
2018;():V003T02A055. doi:10.1115/OMAE2018-77700.

As offshore oil & gas activities are currently evolving towards more productive yet complex situations, the availability of efficient safety metrics has become essential in the early stages of offshore oil & gas projects to underline potential major accidents hazards and clearly communicate safety criticalities. Inherent safety has demonstrated to be a widespread concept in offshore risk management strategies, but there are few preliminary studies in the existing literature about systemic indexing to orient the conceptual and basic design stages of the project lifecycle. In the present work, a methodology for the selection of inherently safer solutions was developed as a support tool for decision-making in early design activities of offshore oil & gas installations. The expected inherent safety performance of alternative design options is assessed by means of a comprehensive set of key performance indicators (KPIs) based on the simulation of consequences of offshore accident scenarios and credit factors of the possible loss of containment events from offshore equipment. The proposed KPIs aim to capture the hazard level of single units and to address selectively multiple targets of the potential threats than personnel and process equipment on the installation, such as marine organisms on the sea environment. Moreover, overall aggregated KPIs were introduced as a sound synthetic measure of the inherent safety performance of the offshore system. The method was applied to the assessment of alternative designs of an offshore production facility, particularly characterized by environmental and safety concerns. The results from the case study evidenced the capability of the proposed method in ranking the potential and credible critical units of each alternative configuration and identifying the relative magnitude of targets contributions to the global safety profile of the installation.

Commentary by Dr. Valentin Fuster
2018;():V003T02A056. doi:10.1115/OMAE2018-78264.

Risk-based in-service inspection enhances structural integrity and minimizes the number of potential unwanted incidents in relation to offshore concrete wind turbine structures. Risk-based in-service inspection enables in-situ inspections, evaluation of inspection results, condition assessment of the offshore wind turbine structures and recommends optimal future inspections. The vagueness, uncertainties and variability present in the risk-based in-service planning process jeopardizes the final inspection recommendations and structural integrity of the structure under consideration. Hence, it is vital to develop methodologies for the systematic utilization of expert knowledge to minimize variability in risk-based in-service assessments. This manuscript suggests a fuzzy set theory-based approach to minimize the risk assessments’ variability and unwanted activities. The suggested fuzzy set theory-based approach enables the identification of the potential hot points that encompass high-risk regions needing priority in future inspections with optimal effort and higher productivity. This manuscript also discusses the use of available inspection methods to enhance the structural integrity of offshore concrete wind turbines subject to deterioration, as well as the use of fuzzy theory-based risk-based in-service inspection assessments for making optimal in-service inspection recommendations.

Commentary by Dr. Valentin Fuster
2018;():V003T02A057. doi:10.1115/OMAE2018-78396.

The risks associated with mooring of ships are a major concern for port and maritime authorities.

Sea waves and extreme weather conditions can lead to excessive movements of vessels and mooring loads affecting the safety of ships, cargo, passengers, crew or port infrastructures.

Normally, port activities such as ships’ approach manoeuvres and loading/unloading operations, are conditioned or suspended based solely on weather or wave forecasts, causing large economic losses. Nevertheless, it has been shown that some of the most hazardous events with moored ships happen on days with mild sea and wind conditions, being the culprit long waves and resonance phenomena. Bad weather conditions can be managed with an appropriate or reinforced mooring arrangement.

A correct risk assessment must be based on the movements of the ship and on the mooring loads, taking into account all the moored ship’s system.

In this paper, the development of a forecast and warning system based on the assessment of risks associated with moored ships in port areas, SWAMS ALERT, is detailed.

This modular system can be scaled and adapted to any port, providing decision-makers with accurate and complete information on the behaviour of moored ships, movements and mooring loads, allowing a better planning and integrated management of port areas.

Commentary by Dr. Valentin Fuster
2018;():V003T02A058. doi:10.1115/OMAE2018-78616.

Floating Production Storage and Offloading (FPSO), a significant offshore oil-gas production system, faces a variety of risks in the process of operation. Vapor cloud explosion (VCE) caused by combustible gas leakage is likely to occur on the topside of FPSO. As an initial accident, VCE has an effect on surrounding devices, leading to subsequent consequences and ampliative scale of the accident. The process, known as the domino effect, can result in severe consequences, indicating that it is necessary to analyze characteristics and impacts of the domino effect on FPSO. In this study, the most risky equipment is determined. VCE overpressure on device surfaces caused by gas leakage of this most risky equipment is calculated, and the results are used for analyzing the domino effect based on Bayesian network.

Commentary by Dr. Valentin Fuster
2018;():V003T02A059. doi:10.1115/OMAE2018-78635.

The need to develop an integrated dynamic safety and risk analysis model for decommissioning offshore jacket structures is driven by the risky, expensive and complex nature of the operation. Many of the existing risk analysis techniques applicable to offshore assets failed to recognise and capture evolving risks during different stages of the decommissioning operation. This paper describes risk-based safety model to conduct quantitative risk analysis for offshore jacket decommissioning failure. First, a bow-tie technique is developed to model the accident cause-consequence relationship. Subsequently, a Bayesian belief network is used to update the failure probabilities of the contributing elements and thus, provides a more case-specific and realistic safety analysis when compared to the static nature of a bow-tie. This paper also presents the application of experiential learning in the dynamic safety analysis. The proposed technique is tested using a real-life case study from the Shell Brent Alpha platform. An algorithm to limit the effect of generic failure data was also developed. It is observed that the proposed technique helps to identify hazards shortly before they occur and sensitivity analysis revealed the most critical elements of the operation that must be managed to prevent catastrophe and consequently, reduce associated costs of remediation.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Structural Analysis and Optimization

2018;():V003T02A060. doi:10.1115/OMAE2018-77012.

The paper deals with water cut-off walls for offshore waste landfill facilities that are used to prevent leakage of toxic substances to the surrounding water. A new type of a widely used water cut-off wall called Steel Pipe Sheet Piles (SPSP) is investigated with experimental measurements as well as numerical simulations. The main objective is to found out the influence of tidal fluctuations in the sea on the outflow of the contaminant and the results show that the outflow of contaminants is lower when tidal fluctuations are present compared to the case with only a mean water level. The influence of different filler materials on this behavior is also investigated.

Commentary by Dr. Valentin Fuster
2018;():V003T02A061. doi:10.1115/OMAE2018-77018.

Modal parameters identification of offshore structures is important for many engineering applications, such as damage detection, structural health monitoring, etc. Operational modal analysis has been widely used for large structures. However, measured signals are inevitably contaminated with noise and may not be clean enough for identifying the modal parameters with proper accuracy. The traditional methods to estimate modal parameters in noisy situation are based on over-determined system to absorb the “noise modes” firstly, and then using the stability diagrams to distinguish the true modes from the “noise modes”. However, it is difficult to sort out true modes when the signal noise ratio is low, especially, the “noise modes” will also tend to be stable as the model order increases. This study develops a noise reduction procedure for polyreference complex exponential (PRCE) modal analysis based on ambient vibration responses. In the procedure, natural excitation technique (NExT) is firstly applied to get free decay responses (auto- and cross-correlation functions) from measured (noisy) ambient vibration data, and then the noise reduction method based on solving the partially described inverse singular value problem (PDISVP) is implemented to reconstruct a filtered data matrix from the measured data matrix. In our case, the measured data matrix is block Hankel structured, which is constructed based on the free decay responses. The filtered data matrix should maintain the block Hankel structure and be lowered in rank. When the filtered data matrix is obtained, the PRCE method is applied to estimate the modal parameters. The proposed NExT-PDISVP-PRCE scheme is applied to field test of a jacket type offshore platform. Results indicate that the proposed method can improve the accuracy of operational modal analysis.

Commentary by Dr. Valentin Fuster
2018;():V003T02A062. doi:10.1115/OMAE2018-77028.

The structural method for shipbuilding remains invariant for a century, the introduction of light materials in other fields, aeronautics and automotive, involves saving weight without risking structure failures. The most difficult part is to incorporate these materials in the shipbuilding industry, because the impact in cost and qualified personnel remains as a handicap. The purpose of this study is to review the shipbuilding process specifically the joints between hybrid panels and primary structural elements without a high cost in resources or material. A honeycomb core sandwich is used as a hybrid material, and the optimized mixed joint is raw steel, the clamp is directly welded to the primary members, glued to the panel with two adhesive strengths, and the mathematical formula used to simulate the debonding at the interface is based on the Cohesive Zone Model, proposed by Al-fano and Crisfield. The numerical simulation analyses consist in topological and parametric optimization, and debonding effects are simulated using contact non-linearity and fracture mechanics approaches.

Commentary by Dr. Valentin Fuster
2018;():V003T02A063. doi:10.1115/OMAE2018-77032.

The purpose of this paper is to present theoretical solutions based on an improved energy method for predicting the helical buckling (HB) behavior of pipes in vertical, inclined, and horizontal wells.

The energy method has been applied to solve the pipe-in-pipe’s (PIP) helical buckling behavior since Lubinski, et al [2] in the 1950’s. However, in the preceding studies, the energy methods are not yet completely correct because the pipe’s potential energy of the distributed contact normal force induced by the helical buckling was considered to be negligible. This deficiency caused improper deductive procedures.

In this paper, the energy method is improved by adding the term of the potential energy of the distributed contact normal force. With this improvement, not only can the PIP’s critical helical buckling forces be successfully derived, but it also provides a deeper insight on the PIP’s helical buckling onset, as well as the post helical buckling behavior.

For inclined and horizontal wells, equations are provided to determine the critical forces required to initiate the helical buckling mode for both “long” and “short” pipes. In addition, the post buckling behavior is also described, and a new concept of helical buckling zone (HBZ) for “short” pipes is introduced based on the force-pitch plots as an area in-between the helical buckling’s onset curve and the classical Lubinski curve.

Finite element ABAQUS models have also been utilized to confirm the analysis using the improved energy method. And the ABAQUS results show remarkable agreement with the theoretical solutions.

Topics: Pipes , Buckling
Commentary by Dr. Valentin Fuster
2018;():V003T02A064. doi:10.1115/OMAE2018-77110.

During the process of workpiece productions in metal forming industries, it is necessary to control the results of the reshaped piece to ensure its quality. A common procedure of metal plate forming processes is given by the application of an upper and lower die. Therefore, ribbed die configurations can be used. To simulate the forming process of metal workpieces, the Finite Element Method (FEM) is a feasible tool. In this paper, a parametric model of a ribbed die structure is developed with the specification that only small imperfections on the workpiece surfaces will appear after the forming process. The workpieces in this paper are plates with thickness values equal and greater than 20mm. Furthermore, the springback behaviour of the different workpieces will be in the main focus of the proposed analyses. The results of the simulations are used to developed different types of holder configurations instead of the lower die. This concept might further reduce the costs of forming processes of large metal plates.

Commentary by Dr. Valentin Fuster
2018;():V003T02A065. doi:10.1115/OMAE2018-77334.

The acceleration responses at certain points of the longitudinal-transverse stiffened conical shells in special frequency region are major matters of concern. Because the finite element models of the longitudinal-transverse stiffened conical shells have to be employed to calculate the vibration response of the structure at all frequencies under consideration, it requires a large amount of computational cost when the optimization is conducted. In order to optimize the vibration response of the longitudinal-transverse stiffened conical shell, the surrogate modeling method is used in this study to approximate the frequency-acceleration response function which makes the vibration response optimization affordable. Since different surrogate models often perform differently in different regions of the design space, an ensemble of surrogate models is utilized to maximize the overall accuracy over the whole design space. The ensemble of surrogates is a weighted combination of Kriging model, radial basis function (RBF) and support vector regression (SVR). The weights of the ensemble of surrogates vary in different regions and are determined by the estimated errors of the surrogate models at the study point. The smaller the estimated error is, the higher the weight is. Then the prediction of ensemble of surrogates is compared to the individual surrogate’s, and the results show that the accuracies of the ensemble of surrogates in peak regions are significant higher than its components. Based on the ensemble of surrogates, a vibration optimization of a longitudinal-transverse stiffened conical shell is conducted using genetic algorithm (GA). The design variables of the optimization are the thickness of the longitudinal-transverse stiffened conical shell and the height of stiffened structure. The objective is to minimize the highest acceleration of the shell and the calculations of the peak accelerations are approximated by the built ensemble of the surrogates. The constraints include the weight of the stiffened conical shell and structure size combination. The optimization results show that the proposed approach is efficient in optimization of the vibration response of longitudinal-transverse stiffened conical shells.

Commentary by Dr. Valentin Fuster
2018;():V003T02A066. doi:10.1115/OMAE2018-77340.

In recent years, water jet propulsion unit has been widely used in the field of high speed ship. Compared with traditional propeller, water jet propulsion unit has excellent maneuverability with high speed, and lateral force generated by water jet propulsion unit can reduce the radius of turning. High speed ship with water jet propulsion has higher efficiency, lower noise. However, water jet propulsion unit needs to be opened in stern transom plate, and it causes the water jet force when ship is operating, all of these will affect the local strength of stern. It remains to be researched whether the vibration generated by water jet excitation force has a significant influence. These problems are designers worried about. To solve these problems, this paper builds the finite element model of stern contains water jet propulsion unit, considering hull deck load, broadside load, bottom load, bulkhead load and water jet load, checking the local strength of stern. Analysis of vibration problem, considering the influence of added mass of entrained water, dividing stern into deck, bottom, water jet propulsion unit, stern transom plate and other local structure, calculating natural frequencies of plate, panel and grillage of each local structures. Comparing the results with shaft frequency and blade frequency, checking the reserve frequency, judging whether water jet propulsion unit on vibration problem meets standards, providing reference for the following hull design.

Commentary by Dr. Valentin Fuster
2018;():V003T02A067. doi:10.1115/OMAE2018-77474.

The minimal ballast water crude oil tanker has the design of a trapezoidal-shape inclined bilge entirely instead of the regular U-shaped tanker, also known as the trapezoidal tanker, which reduces the amount of the ballast water. This type of ship does not only reduce the cost on ballast water management, but also is beneficial to the environment for reducing the risk of water pollution. Since it is a new design, there are no applicable specifications for the assessment of structural strength at present. In order to find out characteristics of the yielding and buckling strength of this type of ship, the strength of a conventional tanker and a trapezoidal tanker are calculated by a finite element method and then compared with a variety of cases. It can be seen that the trapezoidal tanker has lots of advantages in strength and lighting weight.

Commentary by Dr. Valentin Fuster
2018;():V003T02A068. doi:10.1115/OMAE2018-77546.

Generally, equipment or mechanical parts are worn out by abrasion, impact, erosion, and environmental corrosion. To reduce these losses, the welding that wears the surface of the material with abrasion or corrosion resistance is called overlay welding. Corrosion and high temperature oxidation are very important factors in overlay welding material selection because they have a great effect on wear rate. Typical equipment is made of carbon steel, stainless steel or a material that is not wear resistant. Therefore, overlay welding is applied to parts where the parts themselves are subject to severe wear or corrosion. The diffusion of chlorine and alkaline ash during the combustion process causes foiling, corrosion, and erosion in the water wall panel & tube which is a heat transfer device. In order to protect the water wall panel & tube from various chemical and physical phenomena occurring during the combustion process, heat-resistant alloy steel such as Inconel is overlay welded on the outer surface of carbon steel pipe. However, since the length of the water wall panel & tube is more than 7 meters, the deformation after overlay welding is very large and the straightening work requires a lot of time and money. Also, since cooling water flows through the pipe during welding, the temperature of the cooling water also affects the welding deformation. In this study, welding process sequence and coolant temperature are optimized to minimize welding deformation during overlay welding.

Commentary by Dr. Valentin Fuster
2018;():V003T02A069. doi:10.1115/OMAE2018-77627.

Concrete platforms have proven to be a viable offshore solution for deep-water projects according to the documented performance of over 40 years [1]. Since concrete has resistance to seawater corrosion, reduced maintenance costs and motions, concrete structures have become attractive to the offshore oil and gas industry [2, 3]. There are many floating concrete structures such as barge, ship, platform and LNG/LPG terminals. While most concrete offshore platforms are the fixed structures, floating concrete platforms have been used for drilling, extraction /storage and production units for oil and gas including heavy lifter. There are two representative floating concrete facilities on the Norwegian shelf, Troll B and Heidrun A [4, 5]. Troll B is the first concrete semi-submersible product unit and Heidrun A is the floating concrete tension leg platform. The concrete FPSO (Floating Production Storage and Offloading), however, was assessed to be inferior to the steel FPSO in respect of feasibility. In this study, an optimization design of the cross-section for cylinder shaped concrete platform is presented in order to promote the feasibility of a concrete hull for an FPSO. Optimal aspect ratio (depth to diameter) with economic analysis is examined and tangerine-shaped section which can utilize the excellent material properties of the concrete in compressive strength is developed. Exterior wall having a small curvature are applied to transmit the load to interior bulkhead. A parametric sensitivity study was conducted to find the optimal curvature of the exterior wall. Subsequently, an arch-shaped wall is applied to center wall in order to improve the upper and lower tension force caused by bending moment. The tendon was placed in the center of any cross-section, taking into account the tensile force that can be caused by additional asymmetric loads. The effect of tangerine shaped cross-section can be clearly identified by comparing the results with the circular shaped cross-section.

Topics: Concretes , Cylinders , FPSO
Commentary by Dr. Valentin Fuster
2018;():V003T02A070. doi:10.1115/OMAE2018-77655.

The present work aims at better understanding and predicting the thermal and structural responses of aluminium components subjected to welding, contributing to the design and fabrication of aluminium ships such as catamarans, lifesaving boats, tourist ships and fast ships used in transportation or in military applications. Taken into consideration the moving heat source in MIG welding, finite element models of plates made of aluminium alloy are established and validated against published experimental results. Considering the temperature-dependent thermal and mechanical properties of the aluminium alloy, thermo-elasto-plastic finite element analyses are performed to determine the size of the heat affected zone (HAZ), the temperature histories, the distortions and the distributions of residual stresses induced by the welding process. The effects of the material properties on the finite element analyses are discussed, and a simplified model is proposed to represent the material properties based on their values at room temperature.

Commentary by Dr. Valentin Fuster
2018;():V003T02A071. doi:10.1115/OMAE2018-77746.

To meet the needs of weight-light of ships, topology optimization for oil tankers has been proven to be more effective and promising. Nevertheless, due to its characteristics of topology optimization, massive restriction conditions and load-cases, long-time calculation and differences of the strain energy between different load-cases, there are still problems during this process. The objective of this article is to discuss topology optimization design of primary support members in cargo tank region for oil tankers. Here we select SIMP algorithm as the topology algorithm to achieve optimization design of the cargo tank structures. Main control parameters of topology optimization such as optimization subjects, optimization variables, design regions, restrictions, weight of load-cases and volume fractions are all in consideration. The modeling methods and analytical methods of topology optimization are expounded based on HCSR. According to the study, we conclude a detail topology optimization process of primary support members for oil tankers under complex constraints and complex forces. A case is given of an oil tanker with a single longitudinal bulkhead to explain the algorithm and achieve clear topology configuration.

Commentary by Dr. Valentin Fuster
2018;():V003T02A072. doi:10.1115/OMAE2018-77756.

There is an increasing interest in the lightweight design of ship and offshore structures, more specifically, choosing aluminum alloys or other lightweight high-performance materials to build structure components and ship equipments. Due to its better mechanical properties and easy assembly nature, extruded aluminum alloy stiffened plates are widely used in hull structures. When the load on the hull reaches a certain level during sailing, partial or overall instability of stiffened plate makes significant contribution in an event of collapse of the hull structure. It is very necessary to investigate the ultimate strength of aluminum alloy stiffened plate to ensure the ultimate bearing capacity of large aluminum alloy hull structure. Most of studies of the ultimate strength of stiffened plates deal with stiffened plates with T–shaped stiffeners. Stiffeners of other shapes have seldom been explored. In this research, the ultimate strength of six different cross–section aluminum alloy stiffened plates and one steel stiffened plate was studied based on the non–linear finite element analysis (FEA). Taking into account stiffness, weight and other issues, the new cross–section aluminum stiffener has finally been concluded for replacing the original steel stiffener in upper deck of a warship.

Commentary by Dr. Valentin Fuster
2018;():V003T02A073. doi:10.1115/OMAE2018-77825.

Decommissioning is an emerging sector in the UK and Norway, accounting for 2% of total industry expenditure in 2010 increasing to 8% in 2017. In accordance with existing regulations in the North Sea (OSPAR), dumping, and leaving wholly or partly in place disused offshore installations within the maritime area is prohibited. Over the next eight years, 200 platforms are expected to be removed in the North Sea.

There are a number of methods to remove offshore installations: Piece small, Reverse installation and Single lift. In the Single lift approach the jacket or the topside is removed in one piece, minimizing significantly the time offshore and therefore the safety and health incidents. But the Piece Small and Reverse Installation are the most common methods and are established and secure although are time consuming and labor intensive [1].

Several potential candidates for single lift technology at varying levels of technical readiness were considered in the past. The majority of the concepts remained on the drawing board, while some were awaiting project commitment. The only that was matured further than this is the Pioneering Spirit. Yme, its first commercial lift, gave this concept the “proven” status.

The Yme MOPU, owned by Repsol, was a jack-up type platform standing on three steel legs of 3.5 m diameter. The dry weight of the MOPU was approximately 13,500 t. The Yme MOPU was a challenging unit to remove mainly for three reasons: The platform motions due to the lack of stiffness in the leg support, its position in contact with the caisson wellhead platform, and the fact that the legs could not be pre-cut before the operation. The method selected to remove the platform was Single lift, using the dynamically positioned platform installation and removal vessel Pioneering Spirit.

The lifting arrangement consisted of 12 lift beams combined and connected in pairs to yokes. Five specifically designed yokes were installed. The yokes connect the TLS with the MOPU. The structural integrity of each interface was assessed with FE analysis. The Ballast system was used to provide additional clearance. Pioneering Spirit has a total of eighty-seven ballast water tanks, including four so called ‘Quick Drop Ballast Water Tanks’. The removal of the MOPU was performed successfully the 22nd August 2016, after two days work offshore.

Commentary by Dr. Valentin Fuster
2018;():V003T02A074. doi:10.1115/OMAE2018-78045.

A geometric model is established based on pre-existing model tests, and the rigid module flexible connector (RMFC) concept is adopted to perform hydrodynamic analysis of the modular floating structure. To obtain knowledge of the impact of tri-axial stiffness combinations, a large number of cases are designed and the dynamic performances are plotted. In order to reduce the extreme response induced by the structural resonance, damping components are designed and integrated with elastic connectors. Cases of tri-axial combinations with different damping ratios are also put into dynamic analysis. Finally, qualitative knowledge of the impact of connection properties on the dynamic response of modular floating structures is obtained and advices on the design and optimization of connecting structures are proposed.

Commentary by Dr. Valentin Fuster
2018;():V003T02A075. doi:10.1115/OMAE2018-78048.

It is critical to incorporate an appropriate constitutive model into a finite element analysis model to evaluate the nonlinear and dynamic effects on offshore structures subjected to dropped object impact loads. This paper demonstrates the high sensitivity of a dynamic constitutive model to mechanical properties of a specific offshore structural steel and its effect on nonlinear transient finite element analysis results for a steel plate system subjected to dropped object impact loads. Available stress-strain data obtained from dynamic tensile tests are used to validate the numerical results. The dynamic constitutive model recommended by Det Norske Veritas (DNV) was examined at both constitutive level and structural level. This paper proposes constitutive model parameters to improve the DNV’s recommendation.

Commentary by Dr. Valentin Fuster
2018;():V003T02A076. doi:10.1115/OMAE2018-78136.

At the Madeira River, north of Brazil, a natural phenomenon threatens the integrity and normal operation of an on-site hydroelectric power plant; thus, assemblies of containment structures, called logbooms, are installed across the river in order to protect the power plant installations. A truss-based nonlinear finite element method numerical tool is developed with the objective of designing and analyzing these assemblies. Initially, only the influence of the upstream velocity field is considered, and future modifications to account for the debris are expected. Code and solution verifications show that the tool converges reasonably well; the numerical error is about 0.2% of the theoretical value, and the uncertainty is about the same order: the results agree with analytical solutions from the simple catenary model. Finally, the method is validated by comparing numerical and experimental data; a satisfactory agreement is obtained, ascertaining the accuracy of the method: differences between experimental and numerical results are no higher than 6% and the trend of the tension force as a function of the free stream is followed by the numerical method.

Commentary by Dr. Valentin Fuster
2018;():V003T02A077. doi:10.1115/OMAE2018-78479.

The Central Processing Unit (CPU) parallel algorithm based on Computing Unified Device Architecture (CUDA) has shown great power of computing speedup ability. What performance will the new technique show in the field of structural computation? We choose the Gauss elimination method as the research object. In this study, the parallel Gauss elimination is realized in CUDA on GPU. Furthermore, we carry out two groups of numerical experiments. The first group investigates the effect of Matrix Bandwidths (MBs) and Node Numbers (NNs) on speedup ratio. The second one compares our method with the commercial software by analyzing two actual structural problems in ocean engineering.

Topics: Computation
Commentary by Dr. Valentin Fuster
2018;():V003T02A078. doi:10.1115/OMAE2018-78665.

When fragments entering water at high speed, velocity attenuation coefficients of the fragments calculated by different scholars were quite different. Through theoretical analysis and numerical simulation, the velocity attenuation characteristics of the fragments with different head shapes, water entry angles and aspect ratios are studied in this paper. Numerical results show that for the fragments with different head shapes, the main factors affecting the velocity attenuation of the fragments are different. In particular, the influences of the pier’s rough effect on the velocity attenuation of the cylindrical fragments (aspect ratio ≥ 0.5) and pie fragments (aspect ratio < 0.5) are different. As the water entry angle is decreased, the fragment velocity attenuation is slowed down by the “deflection effect” of the fragment. Considering the reduction of drag coefficient, a modified velocity attenuation formula is presented for the pie fragments during high-speed water entry.

Topics: Water
Commentary by Dr. Valentin Fuster
2018;():V003T02A079. doi:10.1115/OMAE2018-78675.

The noise of ship structure is mainly transmitted by two types which are air sound and structural sound. As a kind of mechanical energy, sound is produced by the sound source and goes through various transmission paths to the recipient. This process is a process of constant loss of energy. Therefore, according to aspects of noise generation, output, transmission and reception, the principle of cabin noise control can be divided into four aspects which are cabin structure acoustics design, noise source control, noise transfer path and individual protection at the end of cabin. In order to determine the best noise reduction measures, noise control measures should be considered on the basis of three principles of science, advanced nature and economy. Statistical energy analysis (SEA) graph method is compared a series of adjacent loss factor matrices in the SEA model with the data structure of graphs in graph theory, a plurality of transmission path of SEA model can be obtained by giving different weights to adjacent matrix loss factor matrices in SEA model. The problem of finding maximum energy transfer path in the SEA model is actually equivalent to the issue of seeking shortest path in the graph theory. In order to reduce the cabin noise of the ship structure, it is necessary to know the main source and the main energy conduction path of the noise cabin. The problem is translated into K shortest path problem in graph theory. In this paper, acoustical sensitivity analysis of noise reduction design parameter is developed according to sound energy transmission of two layer cavities structure, which can guide the noise reduction design of the ship cabin. The proposed cabin noise control method is applied to the problem of overproof cabin noise, and the optimal noise control scheme is obtained.

Topics: Noise control , Ships , Seas
Commentary by Dr. Valentin Fuster
2018;():V003T02A080. doi:10.1115/OMAE2018-78757.

Due to increase of global demand for renewable energies, assessment of environmental forces become an important aspect of offshore structures which is the key to ensure optimized and reliable development of this industry. The present study aims to bring out an integrated approach to drag coefficient estimation for circular cylinders covered by biocolonization. Hard and long flapping (Kelp) species were chosen as the study area to demonstrate the effect of biofouling. An integrated approach including the percentage of cover, surface roughness ratio, biofouling species, and aggregating pattern was employed to investigate the steady state drag force coefficient. Several experimental data were collected from previous researchers considering afore-mentioned parameters.

This paper proposes two independent equations for hard and long flapping biofouling using regression analysis method. Both equations were compared with experimental data; a multivariate statistical analysis by Taylor diagram was also performed resulting in reasonable values.

It is worth mentioning that the present work is part of a wider study concerning the effects of biofouling components on the hydrodynamic forces of structures such as jackets and floating offshore structures. The first part of the study considered the investigation of the existing data.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Ultimate Strength

2018;():V003T02A081. doi:10.1115/OMAE2018-77246.

Collision and grounding accidents account for more than half of all accident cases in most cases. Such marine accidents cause severe structural damage to the ship and lead to marine pollution as well as life and financial loss. For preventing the loss of property and pollution, and preparing a countermeasure, it is needed to predict a residual hull girder strength after accident regardless of someone’s special skill. The aims of this study are to i) investigate the residual hull girder strength by quantitative approach with collision location (height and penetration), ii) develop an empirical formula for calculating a residual hull girder strength which whoever can calculate in association with collision locations. In this study, three kinds of ships such as very large crude oil carrier, Suezmax, and Aframax class double hull oil tankers are selected as target struck vessels. And, the Intelligent Supersize Finite Element Method (ISFEM) is applied to assess the residual hull girder strength of damaged structures after collisions. Based on the ISFEM results, an empirical formula for calculation of residual hull girder strength is developed as a function of the collision depth and penetration. The developed formula in this study can be applied by anyone, and rapidly calculate its strength for preventing sequential events (collapse, fuel spill, etc.) after collision.

Commentary by Dr. Valentin Fuster
2018;():V003T02A082. doi:10.1115/OMAE2018-77357.

For evaluating reliable load carrying capacity of actual hull girder, experiments for similar scale model are necessary. The object of the present paper is to figure out a reliable FE analysis method in the similar scale model experiment regarding hull girder ultimate strength. The compared results between the true hull girder of a typical ultra large container ship (ULCS) and the scale experimental model created by the similarity criterion proposed in this paper assessed by finite element method (FEM) under longitudinal bending moment considering the effects of initial deflections are obtained guaranteeing the similarity in both elastic and inelastic range during the progressive collapse of plates, stiffened panels and hull girder. Finally, a series of elastic-plastic large deflection analyses is conducted to ensure the failure mode of hull girder is consistent with the actual ship.

Topics: Girders , Collapse , Hull
Commentary by Dr. Valentin Fuster
2018;():V003T02A083. doi:10.1115/OMAE2018-77402.

The purpose of the present study is to investigate dynamic ultimate strength of global hull girder of container ships using large scale non-linear finite element analysis. A series of time domain non-linear FE-simulation is carried out using large scale FE models of a 8000 TEU container ship where a hogging moment is applied to the midship section. 5 types of finite element models (three full models, a half hold model, a 1 transverse model) are used. These models adopt elasto-plastic material model which includes strain rate effect.

The hogging moment which is modeled by sinusoidal impulse is applied to these models, and collapse mechanism as well as dynamic hull girder ultimate strength is investigated by varying the load time duration. Moreover effects of load time duration, mass inertia, strain rate and analysis models are investigated in detail. It is found from the present study that ultimate strength as well as collapse mode are significantly dependent on load time duration of hogging moment.

Commentary by Dr. Valentin Fuster
2018;():V003T02A084. doi:10.1115/OMAE2018-77602.

Offshore pipelines operating in a harsh environment are usually subjected to combinations of bending moment and axial loadings in addition to internal pressure. Due to the corrosive media transported in the pipelines and corrosive substances within seawater and soil outside the pipelines, local corrosion defects will generate on the pipeline’s inner and outer walls, reducing its ultimate bearing capacity.

This paper presents a series of full-scale failure tests and nonlinear finite element analysis (FEA) to study the bending capacity and failure mode of corroded pipelines with outside locally-thinned-areas (LTAs) subjected to combinations of internal pressure, axial compressive force and bending moment. The LTAs are loaded in compression to simulate corrosion. Material tests of API 5L X56 seamless pipe steel were conducted and the stress-strain relationship was obtained. FEA results of the moment versus curvature relation, bending capacity and local buckling behavior of each specimen model matched the experimental results very well, validating the accuracy of this simulation. Additional FEA is then performed to investigate the effect of corrosion geometric parameters, such as corrosion depth, corrosion width, and corrosion length, on the ultimate moment. Among them, the width is of the greatest impact, followed by is the depth, the length impact can be ignored.

Topics: Pressure , Pipelines
Commentary by Dr. Valentin Fuster
2018;():V003T02A085. doi:10.1115/OMAE2018-77747.

Most of stiffened panels subjected to bi-axial compression mainly in transverse direction collapse with the buckling deformation of one times one half-wave in a local panel between longitudinal stiffeners and transverse frames. The authors defined this collapse mode as “local panel buckling mode”. In this study, the collapse behavior of the stiffened panel with local panel buckling mode is investigated in detail. Then, a simple design formula to estimate ultimate strength of a stiffened panel with local panel buckling mode is derived based on the collapse behavior. This formula is composed of a formula to predict the ultimate strength of a rectangular unstiffened panel subjected to uniaxial transverse compression, and the effects of stiffeners, bi-axial compression and von Mises yield condition are added to the formula. The ultimate strength calculated by the proposed formula is in good agreement with FEA results. Finally, the proposed formula is compared with an existing method and formulae used in the CSR-OT, CSR-BC and H-CSR. As a result, it is confirmed that the proposed formula has sufficient accuracy and high availability.

Commentary by Dr. Valentin Fuster
2018;():V003T02A086. doi:10.1115/OMAE2018-77759.

The Smith’s method is stipulated by the International Association of Classification Societies in the Common Structure Rules as a standard method for estimating ultimate/residual strength of hull girder in both intact and damaged conditions. However, for the latter case where the effective hull cross-section is asymmetric and the neutral axis of damaged cross-section not only translates but also rotates, the additional force vector equilibrium also needs to be applied so as to determine the neutral axis plane. The commonly adopted iterative methods for the two-force-equilibrium problem do not always converge for the desired accuracy. This paper proposes a Particle Swarm Optimization based iteration method to trace the motion of the neutral axis plane of asymmetric cross sections. The translation and rotation of the neutral axis are taken as the two dimensions of particles in the model, and the force equilibrium error and the force vector equilibrium error are the objective functions. The neutral axis is determined by performing a random search within the entire range of possible position of neutral axis. The proposed method has been implemented and validated for the case of the DOW’s 1/3 frigate model, the analysis of efficiency and accuracy shows that the method performs in general better than traditional ones.

Commentary by Dr. Valentin Fuster
2018;():V003T02A087. doi:10.1115/OMAE2018-77786.

The plastic forming processes involved in the production of UOE pipes alter significantly the yield strength of the original steel plate. Numerous studies indicated that the work hardening and Bauschinger effect are the main factors influencing the alteration of the yield strength. Moreover, apart from the forming process itself, the flattening executed on strips sampled from the formed pipe appears to have also nonnegligible effect on the final yield strength that is used as quality index of the formed pipe. Therefore, this study tracks the yield strength of UOE pipe made of API-X70 steel with various thickness-to-diameter ratios by FE-simulation of the forming and flattening processes so as to identify the factors influencing the yield strength of the UOE pipe. The results show that the flattening process constitutes a critical phase in which steel experiences large loss of its tensile yield strength.

Commentary by Dr. Valentin Fuster
2018;():V003T02A088. doi:10.1115/OMAE2018-77820.

Container ships are widely used and their strength has always been the concern of designers and engineers. Additionally, container ships for a given size are of similar arrangements and structures because the containers are of same standards worldwide. So investigation into the strength of typical structures is meaningful. This paper focuses on the ultimate strength of typical longitudinal girders with openings locating between double bottoms which are subjected to longitudinal compression and have not been extensively studied yet. On one hand, the ultimate strengths of longitudinal girders with openings are calculated by Nonlinear Finite Element Analysis (NFEA), and effects of opening sizes, opening locations, boundary conditions and initial deflections are studied, and prediction expressions of ultimate strengths are proposed by varying opening sizes based on benchmarks of intact structures. On the other hand, uniform corrosion is introduced into these structures, and its influences on ultimate strength are also investigated. The results are representative and can be of reference values for similar structures.

Topics: Containers , Girders , Ships
Commentary by Dr. Valentin Fuster
2018;():V003T02A089. doi:10.1115/OMAE2018-77855.

In this study, for the stiffened panels subjected to the in-plane cyclic compression loads, the two followings are clarified. One is a generating process of the cumulative buckling deformation at panel parts of stiffened panels. The other is the effect of the cumulative buckling deformation on the ultimate strength of stiffened panels. To clarify them, the cyclic compression loading experiments were carried out with two stiffened panel specimens by using Multi Axis Loading System in National Maritime Research Institute (NMRI) in Japan. For one stiffened panel specimen, the thirty-one sets of compression test cases were conducted with different strokes and for each case. The number of cycles in each set was 100. While, for the other, it was subjected to the cyclic compression loads until it collapsed. In addition, Finite Element Method (FEM) analyses for stiffened panels subjected cyclic compression loads are carried out with the same condition as the experiments by using commercial FEM software, LS-DYNA.

Commentary by Dr. Valentin Fuster
2018;():V003T02A090. doi:10.1115/OMAE2018-78399.

This paper deals with the buckling assessment of stiffened panels, typical of ship and offshore structures. In particular, rather slender structures, involving very thin plates and stiffeners with limited cross section, are considered.

In order to limit the computational burden of a fully nonlinear progressive collapse analysis but, at the same time, to retain some benefits of a numerical finite element model assessment, an improved linearized buckling procedure is proposed in this work. The idea is to linearize the computation in a suitable point based on the design load of the structure, so that the results of a linearized buckling analysis are more informative in practice. Results are assessed with nonlinear collapse analyses, comprehensively considering various loading cases. Eventually, a hands-on buckling assessment procedure is suggested, which can be implemented in rules and design process if adequately validated by a suitable number of test cases and, possibly, experimental data.

Topics: Buckling , Collapse
Commentary by Dr. Valentin Fuster
2018;():V003T02A091. doi:10.1115/OMAE2018-78510.

The paper presents a method to assess stresses in sandwich plates with periodic microstructure in the post-buckling region. The fundamental assumption is that the microstructure is linear elastic and von Karman non-linearity is present only at macro-, i.e. panel-scale. This enables response evaluation by equivalent homogenous continuum with constant stiffness properties. It is shown that the homogenization cancels shear-induced, odd-terms in microstructural field variables such as displacements and stresses. These are of fundamental importance when strength of the panel must be assessed. The localization approach utilised here neglects the actual position of the microstructure and thus is able to capture only the maximum values of these stress fluctuations. The validation of the present approach is carried out with 3D FE analyses and a good agreement is shown. The shear-induced periodic stresses are of fundamental importance when the strength in the post-buckling region is assessed based on linear elastic basis.

Commentary by Dr. Valentin Fuster

Structures, Safety, and Reliability: Well Integrity and Reliability Assessment

2018;():V003T02A092. doi:10.1115/OMAE2018-77214.

A structured technology development process targeting to combine industry and Statoil’s experience has produced an engineering approach for wellhead fatigue analysis that is verified against measurements of load and load effects in actual subsea wells.

This paper outlines Statoil’s wellhead fatigue analysis approach, which is based on the new industry standard for wellhead fatigue analyses, DNVGL-RP-E104, ref. [1].

Parts of the methodology has been presented in previous papers. The present paper provides a birds eye view, putting all the pieces together into one coherent methodology.

The development and validation of an engineering approach for estimating the bending moment in the surface casing, between the wellhead housing and top of cement, will be presented in detail; this has previously been referred to as load sharing between wellhead and conductor.

The wellhead fatigue analysis approach is based on a “coupled model”, which in this case means that the conductor with PY-soil springs are included in the model, compatible with industry recommendations [1], with the following main characteristics:

• The lower boundary condition is modelled as a conductor in soil with a bending stiffness equivalent of the well system.

• Soil and template interaction is modelled by discrete springs.

• The global riser load analysis is run with long crested waves and head sea. Directionality of the waves are handled by reduction factors applied to the damage rate. Alternatively, directionality effects may be included by running multiple wave directions with short crested waves.

• Fatigue capacity of the hotspots in the well system is represented by ΔM-N curves generated from detailed FE models. Typically, ΔM-N curves are established for connectors, welds between housings and casings, and for the wellhead housings.

The paper includes validation against full scale measurements for a wellhead of preloaded type. In addition, it is demonstrated how the approach can be used for wellheads where the high-pressure housing may rotate inside the low-pressure housing. For this case, the validation is performed against a full 3D solid element model.

The analysis approach presented is computationally effective and it will hence enable increased focus on sensitivity analyses. Analysis work is moved from time consuming local- and global analysis, to effective post-processing of data.

Uncertainty in the input parameters has been found to significantly influence the fatigue estimate. Understanding these effects is considered vital for making conscious decisions on the fatigue life of a well. See e.g. [8], [10] and [20].

As pointed out already in 1985 by Valka et.al., ref. [5], and also by Milberger et. al., ref. [6], the cement level, and the relative motion of the two housings, represent large uncertainties. Macke et. al, ref. [10], showed that the additional uncertainty due to cement level and friction between housings exceeds the levels covered by the traditional fatigue safety factor of DFF = 10. A method is proposed to handle this in a consistent manner.

Topics: Fatigue
Commentary by Dr. Valentin Fuster
2018;():V003T02A093. doi:10.1115/OMAE2018-78102.

In recent years, lower oil prices have forced many oil companies to reduce capex costs by revitalizing brown fields, rather than developing new green fields. At the same time, the offshore drilling rig market has seen many old rigs, typically used for shallow water operations, being scrapped, leaving new generation, deep and ultra-deep water MODUs as the only viable option for new drilling campaigns. Based on the above, wellhead fatigue on older assets, especially in harsh, shallow water environments, has started to gain a central role during the planning phases of workover and intervention operations. In recent years, Suncor Energy began investigating an extension to its Terra Nova field, which began production in 2002. The field uses subsea wells tied back to an FPSO which is moored in 95m of water off Canada’s eastern Grand Banks, an area frequented by icebergs. Drilling operations for the field extension were planned to commence in summer 2017, and continue with a year-round drilling campaign using a Cat 6 MODU. Since the extension would involve sidetracks and interventions from existing wellheads, a series of wellhead fatigue studies were undertaken using a variety of industry recognized methodologies [1] to understand the levels of fatigue accumulation. Although there has been no evidence of wellhead fatigue damage, Suncor chose to take a very prudent and proactive approach, aimed at minimizing fatigue, and maintaining fatigue life for potential future drilling operations. An Instrumented Wellhead Load Relief (iWLR) system was installed, which is designed to restrain BOP motions, thereby reducing the wellhead loads considerably. The load reduction system virtually eliminates additional fatigue accumulation for the planned operations. Additionally, the instrumentation system enables the precise monitoring and tracking of loads applied at the wellhead for future analysis. This paper describes the engineering challenges needed to develop and install the iWLR system in a harsh, shallow water, arctic environment. This area is characterized by very stiff soils pitted with iceberg scours, where subsea equipment must be protected within 10m deep excavated drill centers to prevent iceberg collisions in the relatively shallow water. Additionally, the paper describes how the instrumentation system was integrated with the BOP MUX cable communication system, for the first time, to enable real time monitoring of BOP motions using high accuracy gyroscopes and load cells which monitor dynamic iWLR tether forces. A topside data gathering and processing system was developed to present wellhead loads based on the indirect method, with new algorithms established to account for the tether forces. Finally, the paper presents some preliminary high-level results, showing the efficiency of the system based on measured data.

Commentary by Dr. Valentin Fuster
2018;():V003T02A094. doi:10.1115/OMAE2018-78110.

The operating boundaries and associated watch circles of an offshore drilling campaign are traditionally assessed by technical analysis including simulations of rig, riser and conductor/casing response to determine the mechanical limits and confirm the design limits. V a r i o u s inputs such as weather conditions, mud weights, tension settings and other critical operational aspects have to be defined in the office. The obtained results are then incorporated into a paper based Well Specific Operating Guidelines (WSOG). This traditional WSOG approach may prove particularly limiting in harsh environment and ultra-deepwater operations [1]. Traditional WSOG limits can be overly conservative, predicting excessive non-productive time and, in some cases, indicating too high risks. In some occasions these aspects may prevent planned drilling operations from being sanctioned.

In 2016, Total Exploration & Production (TEP) drilled a complex well at a record water depth of 3,404 meters in challenging environmental conditions offshore Uruguay, with up to Beaufort 10 gales, 16m maxima waves and significant ocean current often above 2 knots. These extreme conditions for the drilling campaign highlighted the necessity of an enhanced Riser Management System (RMS). This paper presents the enhanced RMS, which includes among other features, the upgrade of the BOP inclinometers with high quality sensors, soil stiffness tracking, casing monitoring, wellhead integrity and fatigue monitoring, VIV detection functionality and enhanced operability monitoring. The paper further introduces the use of Dynamic Watch Circles (DWC) and Dynamic Operability Envelopes (DOE) based on real time monitored data and actual weather conditions. Finally, the paper discusses the potential benefits of implementing an electronic WSOG (eWSOG™) replacing the traditional WSOG for extreme drilling operations.

Commentary by Dr. Valentin Fuster
2018;():V003T02A095. doi:10.1115/OMAE2018-78301.

In order to facilitate real-time monitoring of accumulated wellhead fatigue damage, it is necessary to measure the wellhead bending moment in real-time. This paper presents a novel method to estimate the wellhead bending moment in realtime using acceleration and inclination data from the motion reference unit (MRU) sensors installed on BOP and LRJ, riser tension data and a trained neural network model. The method proposed in this paper is designed with a Recursive Neural Network (RNN) model to be trained to estimate the wellhead bending moment in real-time with high accuracy based on motion MRU sensor data and riser tension time series of a few previous cycles. In addition to the power of modeling complex nonlinearities, RNNs provide the advantage of better capturing the dynamic effects by learning to recognize the patterns in the sensor data and riser tension time series.

The RNN model is trained using virtual sensor data and wellhead bending moment from a finite element (FE) model of the drilling riser subjected to irregular wave time domain analyses based on a training matrix with limited number of significant height (Hs) and peak period (Tp) combinations. Once trained, tested and deployed, the RNN model can make real-time estimation of the wellhead bending moment based on MRU sensor data and riser tension time series. The RNN model can be an efficient and accurate alternative to a physical model based on the indirect method for real-time calculation of wellhead bending moment using real-time sensor data. A case study is presented to explain the training procedures for the RNN model. A set of test cases that are not included in the training dataset are used to demonstrate the accuracy of the RNN model using Root Mean Squared Error (RMSE), Normalized Root Mean Squared Error (NRMSE) and coefficient of determination (R2) as a metrics.

Topics: Machinery , Sensors
Commentary by Dr. Valentin Fuster
2018;():V003T02A096. doi:10.1115/OMAE2018-78521.

In recent years due to use of drilling risers with larger and heavier BOP/LMRP stacks, fatigue loading on subsea wellheads has increased, which poses potential restrictions on the duration of drilling operations. In order to track wellhead and conductor fatigue capacity consumption to support safe drilling operations a range of methods have been applied:

• Analytical riser model and measured environmental data;

• BOP motion measurement and transfer functions;

• Strain gauge data.

Strain gauge monitoring is considered the most accurate method for measuring fatigue capacity consumption. To compare the three approaches and establish recommendations for an optimal approach and method to establish fatigue accumulation of the wellhead, a monitoring data set is obtained on a well offshore West of Shetland. This paper presents an analysis of measured strain, motions and analytical predictions with the objective of better understanding the accuracy, limitations, or conservatism in each of the three methods defined above.

Of the various parameters that affect the accuracy of the fatigue damage estimates, the paper identifies that the selection of analytical conductor-soil model is critical to narrowing the gap between fatigue life predictions from the different approaches. The work presented here presents the influence of alternative approaches to model conductor-soil interaction than the traditionally used API soil model.

Overall, the paper presents the monitoring equipment and analytical methodology to advance the accuracy of wellhead fatigue damage measurements.

Commentary by Dr. Valentin Fuster

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