Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2019;141(4):041101-041101-8. doi:10.1115/1.4041816.

This paper presents an analysis of stratification-dependent mean velocity profiles measured in a Norwegian coastal wind climate, and its comparison with models available in the literature. For this purpose, we use 3 years of observations from a 100 m meteorological mast located at the Frøya island (150 km west of Trondheim, Norway), equipped with a set of two-dimensional (2D) ultrasonic anemometers. The presented analysis is preceded by a general description of the site wind climate, the atmospheric stratification, the roughness length, and the surface layer height. Finally, the measured wind velocity profile is compared with selected models: the basic power and logarithmic law and the stability-corrected models: stability-corrected logarithmic wind profile, the Panofsky and Dutton model, the Peña boundary layer height corrected model, and the correlation-based Smedman-Högström model.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2019;141(4):041301-041301-9. doi:10.1115/1.4042000.

The objective of this research is to investigate the velocity of oil drops within the impeller of an electrical submersible pump (ESP) working with oil-in-water dispersion flows at different operational conditions. An experimental study was conducted using an ESP prototype with a transparent shell designed to enable flow visualization within the impeller channels. The tests were performed at three rotational speeds, 600, 900, and 1200 rpm, for three water flow rates, 80%, 100%, and 120% of the best efficiency point (BEP). A high-speed camera (HSC) with a lighting set captured images of the oil-in-water dispersion at 1000 frames per second. The images observation suggests the presence of a turbulent flow in the impeller. The turbulence, associated with high rotation Reynolds numbers, causes the oil drops to become smaller as the impeller rotational speed and the water flow rate increase. Despite this rotating environment, the oil drops generally have a spherical shape. Regarding the kinematics, the images processing reveals that the velocity of oil drops has a magnitude around a unit of m/s. The velocity depends on the oil drop position in the channel: oil drops that stay close to a suction blade (SB) have significantly higher velocities than oil drops that stay close to a pressure blade (PB). Considering a complex flow with water velocity profiles and pressure gradients, the analysis of oil velocity curves indicates the occurrence of accelerations that may be caused by drag and pressure forces acting on the oil drops.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041302-041302-9. doi:10.1115/1.4042096.

Global warming is inducing sea ice retreat, which is opening new shipping routes and extending the accessible area for resource exploration. This encourages an increasing research interest in sea ice behavior. With the sea ice melting, level ice is broken up by waves propagated from the open ocean, resulting in an environment where both floating ice floes and waves exist. Such wave–ice interaction can bring significant influences on the potential human activities. This work presents a series of numerical simulations to predict the behavior of a circular ice floe forced by regular waves, with different wavelength and wave amplitude conditions being investigated. The numerical model was validated against experiments, and it revealed good accuracy in predicting the rigid body motion of an ice floe, including some extreme cases that are difficult to model by previous methods. Two specific behaviors were observed during the numerical simulations, namely overwash and scattering. Both behaviors are discussed in detail to analyze their linear/nonlinear effect on the ice floe motion. The applied model could be used to provide valuable estimations for arctic engineering purposes.

Commentary by Dr. Valentin Fuster

Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2019;141(4):041501-041501-12. doi:10.1115/1.4041761.

The research investigates the influence of human expertise on the effectiveness of ice management operations. The key contribution is an experimental method for investigating human factor issues in an operational setting. Ice management is defined as a systematic operation that enables a marine operation to proceed safely in the presence of sea ice. In this study, the effectiveness of ice management operations was assessed in terms of ability to modify the presence of pack ice around an offshore structure. This was accomplished in a full-mission marine simulator as the venue for a systematic investigation. In the simulator, volunteer participants from a range of seafaring experience levels were tasked with individually completing ice management tasks. Recorded from 36 individuals' simulations, we compared ice management effectiveness metrics against two independent variables: (i) experience level of the participant, categorized as either cadet or seafarer and (ii) ice severity, measured in ice concentration. The results showed a significant difference in ice management effectiveness between experience categories. We examined what the seafarers did that made them more effective and characterized their operational tactics. The research provides insight into the relative importance of vessel operator skills in contributing to effective ice management, as well as how this relative importance changes as ice conditions vary from mild to severe. This may have implications for training in the nautical sciences and could help to inform good practices in ice management.

Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2019;141(4):041601-041601-9. doi:10.1115/1.4041893.

Short-term extreme response estimates are required in many areas of ocean and offshore engineering, such as steel risers design. As in many cases, the response in non-Gaussian, a theoretical solution, is usually not readily available for this purpose. Hermite transformation and Weibull-based models, among others, are some alternatives that have been used in connection with sampled response time series. In this work, a new approach is investigated. Recently, a four-parameter distribution known as the shifted generalized lognormal distribution (SGLD) has been presented in the literature. One of its main advantages is that it covers regions of skewness–kurtosis not covered by other distributions of common use in engineering. In this paper, the performance of this distribution is evaluated in the extreme values' estimation of the utilization ratios of steel riser sections. Three alternatives for using SGLD are investigated in two case studies of different dynamic behavior. The first one is a steel-lazy wave riser (SLWR) connected to a turret-moored FPSO (floating, production, storage and offloading unit) in 914 m water depth, and the second is a SLWR connected to a spread-mooring FPSO in a water depth of 1400 m. The results obtained by the SGLD-based analysis, which considered several simulation lengths, are compared to those obtained by means of an extreme value distribution fitted to episodical extremes obtained from many distinct realizations. The results of a traditional Weibull-fitting approach to the response peaks and those obtained with a Hermite transformation-based model are also presented for comparison.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041602-041602-9. doi:10.1115/1.4041718.

A gas explosion in an offshore platform may result in loss of life, pollution, and critical damage to facilities. Safety critical structural elements of these facilities have to be designed to withstand high explosion loads. The present study discusses methodologies for explosion risk assessment (ERA) of safety critical structural elements and introduces a coupled Eulerian–Lagrangian (CEL) method to improve the accuracy of the dynamic structural response under explosion loading. The design accidental load is defined by explosion risk analyses in terms of drag pressure, differential pressure, and overpressure. In current practice, an explosion pressure-time history is simplified into a triangular shape and uniformly applied to the surface of the impacted structures. This approach cannot account for the interaction between elastic waves (normally solved by the Lagrangian method) in the structure and compression waves (normally solved by the Eulerian method) in air. The CEL method which accounts for fluid–structure interaction has been experimentally validated and leads to more realistic predictions of the dynamic response of structures when compared to other analysis methods. The plastic strains derived from the CEL analysis can be approximately 50% lower than those predicted by Lagrangian analysis. Therefore, significant potential weight reduction can be achieved using the CEL method for gas explosion analysis.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041603-041603-9. doi:10.1115/1.4041643.

In this paper, a solution for estimating the relative position and orientation between two ships in six degrees-of-freedom (6DOF) using sensor fusion and an extended Kalman filter (EKF) approach is presented. Two different sensor types, based on time-of-flight and inertial measurement principles, were combined to create a reliable and redundant estimate of the relative motion between the ships. An accurate and reliable relative motion estimate is expected to be a key enabler for future ship-to-ship operations, such as autonomous load transfer and handling. The proposed sensor fusion algorithm was tested with real sensors (two motion reference units (MRS) and a laser tracker) and an experimental setup consisting of two Stewart platforms in the Norwegian Motion Laboratory, which represents an approximate scale of 1:10 when compared to real-life ship-to-ship operations.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041604-041604-10. doi:10.1115/1.4041999.

Pistons are fundamental structural elements in any engineering practices such as mechanical, civil, aerospace, and offshore engineering. Their strength strongly depends on buckling load, and such information is a major requirement in the design process. Euler's linear buckling equation is the most common and most used model in design. It is well suited for linear elastic members without geometrical imperfections and nonlinear behavior. Several analytical and experimental investigations of typical hydraulic cylinders have been carried out through the years but most of the available standards still use a linear approach with many simplifications. Pistons are slender beams with not-uniform cross section, which need a stronger effort than the classical Euler's approach. The present paper aims to discuss limitations of current DNV standards for piston design in offshore technologies when compared to classical numerical approaches and reference results provided by the existing literature.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041605-041605-13. doi:10.1115/1.4042182.

Hull girder ultimate strength governs sagging and hogging failures, which is one of the most critical failure modes for a ship hull. The structural reliability analysis methodology has been used to develop common structural rules for tankers and bulk carriers. A linear model for bending moment in extreme weather with a nonlinear correction factor has been adopted in the analysis. It is difficult to conclude on the model uncertainty associated with nonlinear effects of bending moment as, until now, there are few studies addressing this topic. In this paper, the nonlinear effect on ship responses is analyzed, and the potential effect of climate change on ship responses is investigated with the improved three-dimensional (3D) Rankine Panel method using nonlinear wave input. The nonlinear wave input is generated by the higher-order spectral method (HOSM) wave model incorporating higher-order nonlinear effects, including nonlinear free-wave modulation as well as higher-order bound harmonics. The past and projected future wave climates of selected locations in the North Atlantic and North Norwegian Sea are considered.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2019;141(4):041701-041701-10. doi:10.1115/1.4042097.

This study focuses on the buckling of pipelines in shallow waters subjected to surface gravity waves. The wave-induced uplift forces on pipelines buried in sandy seabeds are investigated using Biot's consolidation model. Empathetic imperfection model proposed by Taylor and Tran (1994, “Experimental and Theoretical Studies in Subsea Pipeline Buckling," Mar. Struct., 9(2), pp. 211–257.) is used for the study. Thereafter, buckling analyses are performed on the pipeline with the combined temperature and the wave-induced loads. The differences in the critical buckling temperatures for the pipe with consideration of wave loads are analyzed within a range of sea states. The influence of wave loads is found significant for low burial depth ratios.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2019;141(4):041801-041801-10. doi:10.1115/1.4042180.

An improved constant panel method for more accurate evaluation of wave drift forces and moment is proposed. The boundary element method (BEM) of solving boundary integral equations is used to calculate velocity potentials of floating bodies. The equations are discretized by either the higher-order boundary element method or the constant panel method. Though calculating the velocity potential via the constant panel method is simple, the results are unable to accurately evaluate wave drift forces and moment. An improved constant panel method is introduced to address these issues. The improved constant panel method can, without difficulty, employ the near-field method to evaluate wave drift forces and moment, especially for multiple floating bodies. Results of the new evaluation method will be compared with other evaluation method. Additionally, hydrodynamic interaction between multiple floating bodies will be assessed.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041802-041802-7. doi:10.1115/1.4042178.

On several occasions, freak waves have been observed in the past, some causing severe damage. In order to model such extreme wave conditions, one possibility is to use focused waves of first- or second-order based on irregular sea-state wave spectra. The wave phase is chosen such that the waves focus at a predetermined location and time, but the individual wave components become steep and start breaking before the focus location for large amplitudes. In this study, transient wave packets are used for extreme wave generation. Extreme waves are generated that are higher and only break at the concentration point using the transient wave packets method implemented in the open-source CFD model REEF3D. Model validation is performed by comparison to experimental results. The generation of wave packets with an 8.3 times shorter focus distance is investigated and the wave is replicated in a shorter domain with a 9% higher crest. The method is further used to generate a steepness induced-breaking wave and calculation of extreme wave forces on an offshore structure is demonstrated.

Topics: Waves , Wave packets
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041803-041803-8. doi:10.1115/1.4042072.

Deepwater risers are susceptible to vortex-induced vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage in the inline (IL) direction may become equally important as the crossflow (CF) components. Accurate calculation of both IL and CF responses is therefore needed. Empirical VIV prediction programs, such as VIVANA “Passano et al. (2016, “VIVANA—Theory Manual Version 4.8,” Trondheim, Norway),” SHEAR7 “(Vandiver, J. K., and Li, L., 2007, “Shear7 v4.5 Program Theoretical Manual,” Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA),” and VIVA “Triantafyllou et al. (1999, “Pragmatic Riser VIV Analysis,” Offshore Technology Conference, Houston, TX, May 3–6, Paper No. OTC-10931-MS.)” are the most common tools used by the offshore industry. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e., all current profiles have to be assumed to be unidirectional or acting in the same direction. The purpose of this paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate that VIV fatigue damage using two-dimensional directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041804-041804-8. doi:10.1115/1.4041732.

A free-hanging riser (FHR) is a typical riser configuration seen in the disconnected drilling riser, the water-intake riser, and the deep-sea mining riser. In offshore productions, these marine risers will move back and forth in water and further generate an equivalent oscillatory current around themselves, due to the vessel motions. Both in full-scale marine operations and model tests, it has been reported that such oscillatory current leads to riser vortex-induced vibration (VIV) and therefore causes structural fatigue damage. Recently, there have been some attempts to numerically predict vessel motion-induced VIV on the compliant production risers, with emphasize on relatively large Keulegan–Carpenter (KC) numbers. In the real marine operations, the risers experience small KC number scenarios during most of their service life. Therefore, the investigation of vessel motion-induced VIV under small KC number is of great significance, especially considering its contribution to the fatigue damage. In this paper, numerical investigation of VIV of a FHR attached to a floating vessel is carried out. A new response frequency model for vessel motion-induced VIV under small KC numbers is proposed and implemented in vivana. Validation of the proposed numerical methodology is performed against the published experimental results, where a good agreement is achieved.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2019;141(4):041901-041901-11. doi:10.1115/1.4041669.

This paper describes the use of a lifting line model in order to determine the optimum loading on a marine turbine's blades. The influence of the wake and its geometry is represented though the use of a full wake alignment model. The effects of viscous drag are included through a drag-to-lift ratio. Results for different number of blades and tip speed ratios are presented. Various types of constraints are imposed in the optimization method in order to avoid abrupt changes in the designed blade shape. The effect of the constraints on the power coefficients of the turbines is studied. Once the optimum loading has been determined, the blade geometry is generated for a given chord and camber distributions. Finally, a vortex-lattice method is used to verify the power coefficient of the designed turbines.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;141(4):041902-041902-14. doi:10.1115/1.4041998.

A coupled numerical model has been developed and validated to study the fluid–structural interaction responses of a three-bladed tidal turbine in aligned waves and current. The unsteady blade element momentum (BEM) theory was combined with modal analysis for hydro-elastic calculation. Both the loading and deflection of the blade were studied. The dynamic loading on the blade due to structural deformation was much smaller than the wave-induced loading under linear wave conditions for the given condition. The linear response amplitude operators (RAOs) of the loads and the blade tip deflections were obtained and used to predict the linear responses. Although both sum- and difference-frequency responses can be identified from time domain simulations, the wave-induced load and the deflection of the blade are dominated by the first-order contributions. The maximum deflection of the blade tip could reach 1.3 m (203% of the means) in the flapwise direction and 0.35 m (210% of the mean) in the edgewise direction with a wave peak period of 11.3 s and a significant wave height of 5.5 m.

Commentary by Dr. Valentin Fuster

Technology Review

J. Offshore Mech. Arct. Eng. 2019;141(4):044001-044001-4. doi:10.1115/1.4042073.

Fire and explosion are a major cause of concern for refinery, gas processing, petrochemical, and gas installations. The safest way to deal with a fire is to prevent it. However, fire prevention strategies are not always successful, and fires can happen. Therefore, valves should be designed and tested to be fire-safe. This paper reviews valve design features that can help prevent fires, including secondary sealing between the ball and body in case of losing a soft seat, a graphite fire-safe ring design for stem and seat sealing, antistatic devices, and antistatic tests to ensure that the valve is fire-safe. In addition to design considerations, a fire-safe design should be validated through tests defined in standards such as API 607, API RP 6FA, and ISO 10497. The API RP 6FA tests reviewed in this paper include seven tests that check the operability of the valve from closed to open position. A case study was done to prove the operability of a fail close 38″ pipeline ball valve on an oil export pipeline in case of fire during the first 20 s from the open to the closed position. Thermal analysis on the body of the valve proved that there was no thermal expansion inside the valve after 150 s of fire. Additionally, the maximum radial displacement on the valve body after 150 s of fire was 0.34 mm which is negligible. Thus, the valve thermal expansion did not disturb the operation of the valve after 20 s.

Commentary by Dr. Valentin Fuster

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