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### Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2016;139(2):021101-021101-15. doi:10.1115/1.4034615.

This work investigates the hydrodynamic effects of introducing interceptors on fast vessels. Interceptors are vertical flat blades installed at the bottom of the stern vessel. They cause changes in pressure magnitudes around the vessel bottom and especially at the end of the hull where they are located. The pressure variations have an effect on resistance, draft height, and lifting forces which may result in a better control of trim. This work uses a combination of computational fluid dynamics (CFD) and ultrareduced experimental tests. The investigation applies the Reynolds-averaged Navier–Stokes (RANS) equations to model the flow around the ultrareduced model with interceptors with different heights. Our model is analyzed based on a finite-volume method using dynamic mesh. The boat motion is only with two degrees-of-freedom. The results show that the interceptor causes an intense pressure gradient, decreasing the wet surface of the vessel and, quite surprisingly, the resistance. At last, this paper shows that, within a range, a better trim control is possible. The height of the interceptor has an important effect on interceptor efficiency, and it should be especially selected according to the length of the vessel and boundary layer thickness at the transom. The ultrareduced model tests were performed in the Current Channel of the Laboratory of Waves and Current of COPPE/UFRJ (LOC in Portuguese acronym).

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(2):021102-021102-8. doi:10.1115/1.4035139.

This paper describes an experimental study carried out in a laboratory flume with a smooth surface to investigate the effect of a surface wave on unidirectional current. The measured velocity data were analyzed within the framework of the phase averaging for combined wave–current flow and verified by velocity equations based on the phase-averaged Prandtl momentum-transfer theory. The results highlight the changes induced on the mean velocity profile, turbulence intensity, and Reynolds shear stress in a plane of symmetry due to the superposition of surface waves of different frequencies. Modifications in the mean velocities, the turbulence intensities, and the Reynolds shear stresses with respect to the current-only flow are explored. As the frequency of the surface waves in unidirectional current changes, the results show variations in the mean flow and in the turbulence statistics that may affect the local sediment mobility in the coastal region.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(2):021103-021103-11. doi:10.1115/1.4034921.

The motion of a ship/offshore platform at sea is governed by a coupled set of nonlinear differential equations. In general, analytical solutions for such systems do not exist and recourse is taken to time-domain simulations to obtain numerical solutions. Each simulation is not only time consuming but also captures only a single realization of the many possible responses. In a design spiral when the concept design of a ship/platform is being iteratively changed, simulating multiple realizations for each interim design is impractical. An analytical approach is preferable as it provides the answer almost instantaneously and does not suffer from the drawback of requiring multiple realizations for statistical confidence. Analytical solutions only exist for simple systems, and hence, there is a need to simplify the nonlinear coupled differential equations into a simplified one degree-of-freedom (DOF) system. While simplified methods make the problem tenable, it is important to check that the system still reflects the dynamics of the complicated system. This paper systematically describes two of the popular simplified parametric roll models in the literature: Volterra GM and improved Grim effective wave (IGEW) roll models. A correction to the existing Volterra GM model described in current literature is proposed to more accurately capture the restoring forces. The simulated roll motion from each model is compared against a corresponding simulation from a nonlinear coupled time-domain simulation tool to check its veracity. Finally, the extent to which each of the models captures the nonlinear phenomenon accurately is discussed in detail.

Topics: Simulation , Waves , Ships , Hull
Commentary by Dr. Valentin Fuster

### Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;139(2):021301-021301-8. doi:10.1115/1.4034923.

This study proposes marine engine centered data analytics as a part of the ship energy efficiency management plan (SEEMP). The SEEMP enforces various emission control measures to improve ship energy efficiency by considering vessel performance and navigation data. The proposed data analytics is developed in the engine-propeller combinator diagram (i.e., one propeller shaft with a direct drive main engine). Three operating regions from the initial data analysis are under the combinator diagram noted to capture the shape of these regions by the proposed data analytics. The data analytics consists of implementing Gaussian mixture models (GMMs) to classify the most frequent operating regions of the main engine. Furthermore, the expectation maximization (EM) algorithm calculates the parameters of GMMs. This approach, also named data clustering algorithm, facilitates an iterative process for capturing the operating regions of the main engine (i.e., in the combinatory diagram) with the respective mean and covariance matrices. Hence, these data analytics can monitor ship performance and navigation conditions with respect to engine operating regions as a part of the SEEMP. Furthermore, development of advanced mathematical models for ship performance monitoring within the operational regions (i.e., data clusters) of marine engines is expected.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(2):021302-021302-19. doi:10.1115/1.4035141.

An offshore platform has several modules that contain much of the equipment needed for oil and gas production, and these are placed on the limited space of the topside. Furthermore, the equipment layout should leave sufficient space in between to ensure operability, maintainability, and safety. Thus, the design problem to arrange the topside of an offshore platform can be difficult to solve due to the number of modules and equipment placed on the topside. This study proposes a method to arrange the offshore topside based on an expert system and multistage optimization in order to obtain the optimal arrangement that addresses various considerations and satisfies the given requirements. The proposed method consists of four components. First, an expert system is proposed to systematically computerize experts' knowledge and experience and to evaluate the feasibility of alternatives for the arrangement of the offshore topside. Second, a multistage optimization method is proposed to yield a better arrangement design by formulating the arrangement design problem as an optimization problem with two stages. Third, an arrangement template model (ATM) was proposed to store the arrangement data of the offshore topside. Fourth, the user interface was developed to run the expert system and for optimization. A prototype program was then developed to solve an floating, production, storage, and offloading (FPSO) topside problem in order to evaluate the applicability of the proposed method. The results showed that the proposed method can be used to obtain the optimal arrangement of an offshore topside.

Commentary by Dr. Valentin Fuster

### Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2017;139(2):021401-021401-8. doi:10.1115/1.4035225.

Technology qualification (TQ) centers on establishing an acceptable level of confidence in innovative aspects of new technologies that are not addressed by the normative standards and/or common certification procedures. Risk-based technology qualification aims to minimize the uncertainty and risk of potential failures in novel designs, concepts, or applications that are not covered by existing standards, industry codes, and/or best practices. The degree of success in a technology qualification process (TQP) depends on its potential for minimizing the uncertainty of a novel technology under assessment and the level of uncertainty arising from the qualification methods and basis. Due to the lack of generic reliability data, focused research and development, and in-service experience, it is necessary to employ risk-based qualification of new technology. In a risk-based TQ, the technology under consideration is decomposed into manageable elements to assess those that involve aspects of new technology and to identify the key challenges and uncertainties. The aforementioned requires risk ranking with the support of experts, who represent relevant technical disciplines and field experience in design, fabrication, installation, inspection, maintenance, and operation. Hence, it is vital to have a comprehensive approach for ranking the risk of potential failures in a TQP, especially to reduce the variability present in the risk ranking and the overall uncertainty. This paper proposes a fuzzy logic based approach, which enables the variability present in the risk ranking of a TQP to be minimized. It also demonstrates how to make risk rankings by means of an illustrative case.

Commentary by Dr. Valentin Fuster

### Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2017;139(2):021501-021501-9. doi:10.1115/1.4035244.

In this study, freeze-thaw cycles were conducted on samples of a fine grained soil from the Qinghai–Tibetan plateau which had been prepared with different dry unit weights. During freeze-thaw cycles, electrical resistivity was measured. The soil samples were also scanned by X-ray computed tomography (CT) before and after freeze-thaw cycles. Unconsolidated and drained (UD) triaxial compression test was performed to obtain the apparent friction angle and cohesion. Changes in the arrangement and connections between soil particles were analyzed so as to investigate the mechanisms of changes in the strength parameters. The electrical resistivity increased in all samples, regardless of the different original dry unit weights, which implies that in all cases the arrangement of soil particles became more irregular and attached area between soil particles was increased. These changes contributed to the increase of apparent friction angle. On the other hand, the CT scans indicated that, depending upon the original dry unit weight, freeze-thaw cycles induced strengthening or deterioration in particle connections, and thus apparent cohesion was increased or decreased. With three freeze-thaw cycles, changes in microstructure of soil samples led to increases or decrease in both the apparent friction angle and cohesion.

Commentary by Dr. Valentin Fuster

### Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2016;139(2):021601-021601-9. doi:10.1115/1.4034762.

Recently, the authors have proposed a new methodology for the vertical position of the center of gravity (KG) estimation of semisubmersibles at its production location. The procedure, called the Zero GM method, is based on the identification of a characteristic behavior of the floater around an induced neutral equilibrium condition. The proposed methodology has been already numerically explored in terms of stability and practical feasibility based on real data of an offshore production semisubmersible. As the procedure implies in temporarily taking the vessel to neutral–unstable upright equilibria (both considered unsafe conditions), experimental tests appear as a physical mean of demonstrating the procedure's practical application and safety. The objective of this paper is to present the results of a model test campaign where the Zero GM method has been experimentally investigated on an offshore semisubmersible. The model and its ballast weight distribution were especially designed to simulate the path of the KG in the process of finding the neutral equilibrium, both in calm-water and in waves. The effect of mooring lines was also assessed. The experimental results showed that around the neutral equilibrium a jump in the loll angle and a peak in the roll oscillation period appear, so that KG can be accurately estimated, even in the presence of (mild) waves and mooring. Capsize or risk of capsize has not been observed during the tests, even when relatively large unstable conditions were tested in waves.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2016;139(2):021602-021602-10. doi:10.1115/1.4034922.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(2):021603-021603-12. doi:10.1115/1.4034957.

Drop weight impact tests and numerical simulations have been performed to examine the plastic behavior and failure of clamped rectangular cross section tubes subjected to transverse loads. The selected indenter is a hemisphere with diameter of 20 mm. The tube lengths are 125 and 250 mm, and they are struck at the midspan and the quarter-span. The impact point along the width direction is located at the central position and displaced 10 mm from the center, respectively. The results show that the impact location affects strongly the plastic behavior and failure of the tubes. The impact location displaced along the width increases the energy absorbing capability of the tubes accompanied with an asymmetrical deformation mode. The experimentally recorded force–displacement responses and failure modes show good agreement with the numerical simulations, performed by the LS-DYNA finite-element code. The numerical results show the process of crack initiation and propagation and provide the details to analyze the structural plastic deformation and failure of the tubular specimens under transverse loads. The impact characteristics of the rectangular tubes are well presented based on the relevant failure modes observed in beams, plates, and circular tubes. Moreover, the influence of the impact location on the strength of tube specimens is characterized, and the collapse mechanism of rectangular tubes is described.

Commentary by Dr. Valentin Fuster

### Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2016;139(2):021701-021701-7. doi:10.1115/1.4034695.

Soil resistance to pipeline axial displacement plays a key role in the ratcheting process known as “pipeline walking.” Still, it is not yet fully understood. New frameworks to address the different geotechnical aspects involved have recently been published. However, the current practice has been to lump all the time-dependent effects back into a single “equivalent” friction factor, based on a representative pipeline velocity. This paper argues that defining a single velocity as representative of the pipeline expansion (or contraction) is not trivial. While the pipeline ends might move a couple of meters in the few hours it takes to heat up, somewhere close to the middle it will move a few millimeters only. As a result, different levels of soil drainage, for example, are observed along the same pipeline, during the same loading. This paper presents the results of “true” velocity-dependent pipeline walking analyses and compares them to those obtained using constant equivalent friction factors. For the particular cases analyzed, the difference between the results obtained with the two approaches ranged from negligible up to about 30%. Examples show that the results of velocity-dependent pipeline walking analyses are significantly influenced by how the temperature changes over time along the pipeline length. The velocity-dependent model employed describes the axial soil resistance as a hyperbolic function of the pipe velocity. Additional aspects which are expected to influence the soil response (e.g., consolidation time between movements, progressive compression, and consolidation hardening) have been neglected.

Commentary by Dr. Valentin Fuster

### Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2016;139(2):021801-021801-8. doi:10.1115/1.4034760.

The effect of tandem spacing on the flow-induced motions (FIM) of two circular cylinders with passive turbulence control is investigated using two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes equations with the Spalart–Allmaras turbulence model. Results are compared to experiments in the range of Reynolds number of 30,000 < Re < 100,000. The center-to-center spacing between the two cylinders is varied from 2 to 6 diameters. Simulation results predict well all the ranges of FIM including vortex-induced vibrations (VIV) and galloping and match well with experimental measurements. For the upstream cylinder, the amplitude and frequency responses are not considerably influenced by the downstream cylinder when the spacing is greater than 2D. For the downstream cylinder, a rising amplitude trend in the VIV upper-branch can be observed in all the cases as is typical of flows in the TrSL3 flow regime (transition in shear layer 3; 2 × 104 < Re < 3 × 105). The galloping branch merges with the VIV upper-branch for spacing greater than three-dimensional (3D). Vortex structures show significant variation in different flow regimes in accordance with experimental observations. High-resolution postprocessing shows that the interaction between the wakes of cylinders results in various types of FIM.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(2):021802-021802-6. doi:10.1115/1.4035140.

Passive turbulence control (PTC) in the form of two straight roughness strips with variable width, and thickness about equal to the boundary layer thickness, is used to modify the flow-induced motions (FIM) of a rigid circular cylinder. The cylinder is supported by two end springs and the flow is in the TrSL3, high-lift, regime. The PTC-to-FIM Map, developed in the previous work, revealed zones of weak suppression (WS), strong suppression (SS), hard galloping (HG), and soft galloping (SG). In this paper, the sensitivity of the PTC-to-FIM map to: (a) the width of PTC covering, (b) PTC covering a single or multiple zones, and (c) PTC being straight or staggered is studied experimentally. Experiments are conducted in the low turbulence free surface water channel of the University of Michigan, Ann Arbor, MI. Fixed parameters are: cylinder diameter D = 8.89 cm, m* = 1.725, spring stiffness K = 763 N/m, aspect ratio l/D = 10.29, and damping ratio ζ = 0.019. Variable parameters are circumferential PTC location αPTC$∈$ (0–180 deg), Reynolds number Re $∈$ (30,000–120,000), flow velocity U$∈$ (0.36–1.45 m/s). Measured quantities are amplitude ratio A/D, frequency ratio fosc/fn,w, and synchronization range. As long as the roughness distribution is limited to remain within a zone, the width of the strips does not affect the FIM response. When multiple zones are covered, the strong suppression zone dominates the FIM.

Commentary by Dr. Valentin Fuster