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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

Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2017;139(5):051501-051501-9. doi:10.1115/1.4036478.

Sloping structures are widely used in ice-infested waters because of their ability to reduce ice loading by inducing a bending failure in ice sheets. From model test data, a significant velocity effect on the breaking load of ice sheets has been reported. In this paper, the ice–fluid interaction process is investigated by adopting the Euler–Bernoulli beam theory for the ice sheet and the potential theory for the underlying fluid domain. Accounting for the inertia effect of the ice sheet and the hydrodynamics of sea water beneath the ice sheet, the results demonstrate a velocity effect on the ice breaking loads in-plane deformation, which compare well with the available model test data. Moreover, our model formulation and implementation is such that the solutions for different ice velocities can be obtained rapidly from the reference solution, which facilitates the development of a real-time simulator. It is also shown that the velocity effect depends on the ice compressive strength and the angle of sloping structure.

Topics: Stress , Ice , Failure , Fluids
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

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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