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Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2017;139(5):051101-051101-16. doi:10.1115/1.4036950.

Multibody operations are routinely performed in offshore activities, for example, the floating liquefied natural gas (FLNG) and liquefied natural gas carrier (LNGC) side-by-side offloading case. To understand the phenomenon occurring inside the gap is of growing interest to the offshore industry. One important issue is the existence of the irregular frequency effect. The effect can be confused with the physical resonance. Thus, it needs to be removed. An extensive survey of the previous approaches to the irregular frequency problem has been undertaken. The matrix formulated in the boundary integral equations will become nearly singular for some frequencies. The existence of numerical round-off errors will make the matrix still solvable by a direct solver, however, it will result in unreasonably large values in some aspects of the solution, namely, the irregular frequency effect. The removal of the irregular effect is important especially for multibody hydrodynamic analysis in identifying the physical resonances caused by the configuration of floaters. This paper will mainly discuss the lid method on the internal free surface. To reach a higher accuracy, the singularity resulting from the Green function needs special care. Each term in the wave Green function will be evaluated using the corresponding analysis methods. Specifically, an analytical integral method is proposed to treat the log singularity. Finally, results with and without irregular frequency removal will be shown to demonstrate the effectiveness of our proposed method.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;139(5):051301-051301-9. doi:10.1115/1.4036676.

Recent environmental considerations, as salmon lice, escape of farmed fish and release of nutrients, have prompted the aquaculture industry to consider the use of closed fish production systems (CFPS). The use of such systems is considered as one potential way of expanding the salmon production in Norway. To better understand the response in waves of such bags, experiments were conducted with a series of 1:30 scaled models of closed flexible bags. The bags and floater were moored in a wave tank and subjected to series of regular waves (wave period between 0.5 and 1.5 s and wave steepness 1/15, 1/30, and 1/60). Three different geometries were investigated; cylindrical, spherical, and elliptical, and the models were both tested deflated (70% filling level) and inflated (100% filling level). Incident waves were measured together with the horizontal and vertical motion of the floater in two points (front and aft). Visual observations of the response were also done using cameras. The main finding from the experiments were that a deflated bag was more wave compliant than an inflated bag, and that the integrity (whether water entered or left the bag over the floater) was challenged for the inflated bags even for smaller waves (identified as wave condition B (1.0 m < H < 1.9 m) in Norwegian Standard NS 9415). A deflated bag is significantly more seaworthy than an inflated bag when it comes to integrity and motion of the floater.

Topics: Waves , Water
Commentary by Dr. Valentin Fuster

Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2017;139(5):051401-051401-9. doi:10.1115/1.4036385.

For subsea mining, the prediction of pressure loss due to the hydraulic transport of solid particles in the flexible pipe to connect the mining tool and the lifting system is important for the design of mining system. The configuration of the flexible pipe is expected to have an inclined part. In the present paper, the authors developed a mathematical model to predict the pressure loss in inclined pipes. The total pressure loss is expressed by the summation of the loss due to a liquid single-phase flow and the additional loss due to the existence of solid particles. The additional pressure loss can be divided into the variation in static pressure due to the existence of solid particles, the loss due to the particle-to-pipe wall friction and collisions, and the loss due to the particle-to-particle collisions. The empirical formula in horizontal pipes proposed by the other researchers was applied to the model of the last two losses. Furthermore, we carried out the experiment on hydraulic transport of solid particles in a pipe. In the experiment, alumina beads, glass beads, and gravel were used as the solid particles, and the inclination angles of the pipe were varied to investigate the effect of the pipe inclination on the pressure loss. The calculated pressure loss using the model was compared with the experimental data. As the results of the comparison, it was confirmed that the developed model could be applied to the prediction of the pressure loss in inclined pipes.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051402-051402-11. doi:10.1115/1.4036832.

Modeling depth of long-term pitting corrosion is of interest for engineers in predicting the structural longevity of ocean infrastructures. Conventional models demonstrate poor quality in predicting the long-term pitting corrosion depth. Recently developed phenomenological models provide a strong understanding of the pitting process; however, they have limited engineering applications. In this study, a novel probabilistic model is developed for predicting the long-term pitting corrosion depth of steel structures in marine environment using Bayesian network (BN). The proposed BN model combines an understanding of corrosion phenomenological model and empirical model calibrated using real-world data. A case study, which exemplifies the application of methodology to predict the pit depth of structural steel in long-term marine environment, is presented. The result shows that the proposed methodology succeeds in predicting the time-dependent, long-term anaerobic pitting corrosion depth of structural steel in different environmental and operational conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2017;139(5):051701-051701-10. doi:10.1115/1.4036675.

This study presents an analytical model of flexible riser and implements it into finite-element software abaqus to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into abaqus to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser–seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051702-051702-9. doi:10.1115/1.4036377.

For a trenched and buried pipeline, the propensity to upheaval buckling (UHB) is a major design concern. Predictive UHB design is typically required at the outset to determine both trenching and backfilling requirements. Additional rockdump schedule can be established by analyzing post pipelay out of straightness (OOS) survey data incorporating appropriate safety factors based on a structural reliability analysis (SRA). The normal approach is to examine the as-laid pipeline imperfection survey statistics and data accuracy. The structural reliability analysis and load factor calculation are typically performed a priori based on the assumed initial imperfections using the universal design curve methodology. A new pseudo-energy method for UHB and OOS is proposed and discussed in this paper based on the variational principle and modal analysis. The approach takes into account the effects of varying effective axial force, trench imperfections, and vertical uplift resistance, by combining both axial friction and lateral resistance methods into a unified model. A new concept, effective uplift resistance and associated load, is also introduced to deal with nonuniform backfill cover. Adjacent imperfections and backfill profiles are considered in detail. A finite element (FE) model is developed to consist of three-noded quadratic pipe elements using abaqus Ver 6.12, and iterations of FE analyses are performed to demonstrate the tangible benefits of the approach specifically for UHB OOS design in relation to target trenching and backfilling, leading to improved reliability and potential cost saving in UHB OOS design and rockdump installation.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051703-051703-11. doi:10.1115/1.4036372.

Recently, the flexible cryogenic hose has been preferred as an alternative to exploit offshore liquefied natural gas (LNG), in which helical corrugated steel pipe is the crucial component with C-shaped corrugation. Parametric finite element models of the LNG cryogenic helical corrugated pipe are established using a three-dimensional shell element in this paper. Considering the nonlinearity of cryogenic material and large geometric structural deformation, mechanical behaviors are simulated under axial tension, bending, and internal pressure loads. In addition, design parameters are determined to optimize the shape of flexible cryogenic hose structures through sectional dimension analysis, and sensitivity analysis is performed with changing geometric parameters. A multi-objective optimization to minimize stiffness and stress is formulated under operation conditions. Full factorial experiment and radial basis function (RBF) neural network are applied to establish the approximated model for structure analysis. The set of Pareto optimal solutions and value range of parameters are obtained through nondominated sorting genetic algorithm II (NSGA-II) under manufacturing and stiffness constraints, thereby providing a feasible optimal approach for the structural design of LNG cryogenic corrugated hose.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051704-051704-9. doi:10.1115/1.4036375.

In the literature, the continuous line bucket (CLB) lifting system and the pipe lifting system (PLS), as two typical mineral lifting methods in deep sea mining (DSM) systems, have been discussed since the 1960s. The purpose of this paper is to determine an appropriate lifting method for deep sea mining systems at different working conditions. The determination is based on the comparison of the analysis results of the two typical lifting methods considering the technology performance and the profitability. The analysis is based on a numerical calculation performed in a matlab environment. This paper shows the comparison of the results of the CLB system and PLS in terms of the lifting efficiency, the energy consumption, and the profitability. The research reported in this paper can be utilized to select a proper lifting method for a DSM project depending on its specific criteria analysis.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051705-051705-10. doi:10.1115/1.4037084.

The motion of a semisubmersible drilling rig must be checked in advance to satisfy the safety criteria of the rig. However, the complexity of the rig's connections makes it difficult to analyze the rig motion during the drilling operation because it is connected to the seabed by the blow-out preventer (BOP). The rig's connections consist of several pieces of risers, a telescopic joint, and a riser tensioner system. Also, from a macroscopic perspective, the risers should be regarded as flexible threads. Therefore, this study developed a rig motion analysis program considering the deformable effects of flexible risers and the full connectivity of the drilling rig. Flexible multibody dynamics (FMBD) based on the absolute nodal coordinate formulation (ANCF) is adapted for the mathematical modeling of the risers and joints. Acting as an external disturbance, a hydrodynamic force and current force are exerted on the drilling rig and the risers, respectively. The drilling rig is fully modeled including the riser tensioner system, telescopic joint, flexible risers, and upper and lower flex joints. The motion analysis with and without connections was fulfilled to verify the effect of connectivity. Moreover, we observed that the movement of the drilling rig increases as the current speed increases. Finally, the simulation is successfully applied to check the motions and tensions of the drilling rig in moderate and storm conditions.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051706-051706-11. doi:10.1115/1.4037063.

The winding and unwinding of a pipeline in the reeling installation process involve repeated excursions into the plastic range of the material, which induce ovality, elongation, and changes to the mechanical properties. The reeling/unreeling process involves some back tension required to safeguard the pipe from local buckling. This study examines the effects of winding/unwinding a pipe on a reel at different values of tension on the induced ovality and elongation and the resulting degradation in collapse pressure. In Part I, a model testing facility is used to simulate the reeling/unreeling process in the presence of tension. The combination of reel and tube diameters used induces a bending strain of 1.89%. A set of experiments involving three reeling/unreeling cycles at different levels of tension is performed on tubes with diameter-to-thickness ratios (D/t) of 20 and 15.5 in which the progressive changes in cross-sectional geometry and elongation are recorded. Both ovalization and elongation are shown to increase significantly as the back tension increases. A second set of experiments on the same two tube D/ts is performed in which following a reeling/unreeling cycle at a chosen level of tension, the tubes are collapsed under external pressure. The collapse pressure is shown to decrease significantly with tension, which is primarily caused by the reeling/unreeling-induced ovality. Part II presents models for simulating reeling and the induced structural degradation. The experimental results in Part I are used to evaluate the performance of the models.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(5):051707-051707-12. doi:10.1115/1.4037064.

Part II presents two modeling schemes for simulating the reeling/unreeling of a pipeline, with the aim of establishing the degrading effect of the process on the structural performance of the pipeline. A three-dimensional (3D) finite element model of the winding/unwinding of a long section of pipeline onto a rigid reel is presented first. The second model applies the curvature/tension loading history experienced at a point to a section of pipe in contact with a rigid surface of variable curvature. Both models use nonlinear kinematic hardening plasticity to model the loading/reverse loading of the material. The 3D model first demonstrates how the interaction of the problem nonlinearities influences the evolution of deformation and load parameters during reeling/unreeling. The two models are subsequently used to simulate the three-reeling/unreeling cycle experiments under different levels of back tension in Part I. The ovality-tension and axial elongation-tension results are reproduced by both models with accuracy for the first cycle, adequately for the second cycle, and are overpredicted for the third cycle. The two models are also used to simulate the reeling/unreeling followed by collapse of the tubes under external pressure experiments. Both models reproduce the measured ovality-tension results and the corresponding collapse pressures accurately. Since the two-dimensional (2D) model is computationally much more efficient, it is an attractive tool for estimating the effect of reeling on collapse pressure. Questions that require exact tracking of the winding/unwinding history and the interaction of the pipe with the reel are best answered using the 3D model.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Geotechnics

J. Offshore Mech. Arct. Eng. 2017;139(5):052001-052001-12. doi:10.1115/1.4036371.

Torpedo piles installed by dynamic penetration have been used as anchors in the Brazilian offshore oil production infrastructure practice for two decades. Dynamic penetration aided by fluidization of the soil during pile penetration is now being contemplated as a method of installation that would allow deeper penetration. The two key design questions in connection with torpedo piles are how far they penetrate and what their pullout capacity is. In a companion paper, the authors addressed the first question, whereas in the present one the second question is attended through laboratory tests using model piles, essentially pipes simulating torpedo piles without wings. The model piles were installed in two different ways: by fluidization, which enabled the piles to sink by their own weight, and by monotonic jacking. Pullout tests were then performed on the model piles in both fluidized and nonfluidized sandy soils prepared at two initial relative densities. Results from the laboratory tests indicate that shaft uplift capacity of fluidized piles is essentially independent of the sand initial relative density. The measured values of the coefficient of lateral earth pressure (Ks) derived from the fluidized model tests are lower than those reported for other methods of pile installation, in some cases being lower than K0. Finally, the shaft resistance of fluidized piles increases after installation as the soil reconsolidates and particles rearrange.

Commentary by Dr. Valentin Fuster

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