Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2015;137(6):061301-061301-7. doi:10.1115/1.4031493.

Semisubmersible floating platforms used in offshore deep or ultradeep water environments have hull structures that are comprised of vertical cylinders (columns) connected by braces, pontoons, etc. Several of the connections between these various members are susceptible to fatigue damage. In fatigue damage assessment or fatigue reliability analysis, a global structural response analysis is typically carried out using a finite element (FE) model where internal forces or stresses in the various members are evaluated for specified sea states measured at the site. Of specific interest in the present study is the fatigue reliability analysis of brace-column connection details in a semisubmersible hull unit for selected Brazilian environmental conditions. Stress concentration factors (SCFs) for the selected critical hot spots are applied to the nominal component stresses due to axial forces and biaxial bending. The hot-spot stress response spectra are used with various spectral methods—referred to as Rayleigh, modified Rayleigh (with bandwidth correction), and Dirlik—to estimate fatigue damage using Miner's rule. Uncertainties in some parameters used in the fatigue life assessment are considered and the probability of fatigue failure in the last operational year of the structure is estimated.

Commentary by Dr. Valentin Fuster

Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2015;137(6):061401-061401-8. doi:10.1115/1.4031392.

To understand the evolution of dynamic stiffness of damaged synthetic fiber mooring ropes, experimental investigations of polyester and high modulus polyethylene (HMPE) ropes are systematically performed utilizing a specially designed experimental system. An experimental procedure is proposed and test results show that the dynamic stiffness increases with increasing mean load and loading cycles, while decreases with increasing strain amplitude and damage level. The similarity criterion of dynamic stiffness is derived from the dimensional analysis for damaged ropes and verified by experiments. An empirical expression that accounts for the damage, mean load, strain amplitude, and loading cycles is proposed to describe the damage effect upon dynamic stiffness of synthetic fiber mooring ropes.

Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2015;137(6):061601-061601-10. doi:10.1115/1.4031312.

The long-term probability distributions of a spar-type and a semisubmersible-type offshore floating wind turbine response are calculated for surge, heave, and pitch motions along with the side-to-side, fore–aft, and yaw tower base bending moments. The transfer functions for surge, heave, and pitch motions for both spar-type and semisubmersible-type floaters are obtained using the fast code and the results are also compared with the results obtained in an experimental study. The long-term predictions of the most probable maximum values of motion amplitudes are used for design purposes, so as to guarantee the safety of the floating wind turbines against overturning in high waves and wind speed. The long-term distribution is carried out using North Atlantic wave data and the short-term floating wind turbine responses are represented using Rayleigh distributions. The transfer functions are used in the procedure to calculate the variances of the short-term responses. The results obtained for both spar-type and semisubmersible-type offshore floating wind turbine are compared, and the study will be helpful in the assessments of the long-term availability and economic performance of the spar-type and semisubmersible-type offshore floating wind turbine.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2015;137(6):061701-061701-8. doi:10.1115/1.4031669.

A dynamic model for undamped, water hammer-induced, radial vibration of long, thin-walled, laminated, filament wound pipes is derived. The model is based on the interaction of the unsteady flow parameters with the anisotropic elastic properties of the pipe. With the aid of integral transforms and generalized functions, an approximate solution of the derived governing equation is achieved and its implementation on a representative example is discussed.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(6):061702-061702-21. doi:10.1115/1.4031670.

Hooking events, defined as trawling gear becoming firmly “stuck” under a pipeline, rarely occur during bottom-trawling operations. However, hooking events can have detrimental consequences. There is no existing method for quantifying the hooking probability of bottom-trawling operations. In this study, an approach is proposed to quantify the trawl board hooking probability using simulation tools and statistical data. Numerical simulation use the SIMLA code to establish simplified hooking criteria. The criteria link the pipeline data to the fishing activities data, enabling the quantification of hooking probability. First, the numerical simulations of both pull-over and hooking events were compared with small-scale model test results. Reasonable agreement was reached. Based on the simulation results, simplified criteria for trawl board hooking were proposed. Finally, data from the EUROPIPE II pipeline section in the Norwegian sector were used as a case study. Data regarding free span as well as fishing activities in that region were used to obtain the statistical input. The Monte Carlo simulation technique was then used to estimate the hooking probability. Parametric studies were first performed to investigate the effects of important parameters. Then, based on the findings from the parametric studies, the hooking probability with the most reasonable parameters was estimated.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2015;137(6):061801-061801-10. doi:10.1115/1.4031327.

Flow-induced motion (FIM) experiments of a single circular cylinder or multiple cylinders in an array involve several configuration and hydrodynamic parameters, such as diameter, mass, damping, stiffness, spacing, Reynolds number, and flow regime, and deviation from circular cross section. Due to the importance of the FIM both in suppression for structural robustness and in enhancement for hydrokinetic energy conversion, systematic experiments are being conducted since the early 1960s and several more decades of experimentation are required. Change of springs and dampers is time consuming and requires frequent recalibration. Emulating springs and dampers with a controller makes parameter change efficient and accurate. There are two approaches to this problem: The first involves the hydrodynamic force in the closed-loop and is easier to implement. The second called virtual damping and spring (Vck) does not involve the hydrodynamic force in the closed-loop but requires an elaborate system identification (SI) process. Vck was developed in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan for the first time in 2009 and resulted in extensive data generation. In this paper, the second generation of Vck is developed and validated by comparison of the FIM experiments between a Vck emulated oscillator and an oscillator with physical springs and dampers. The main findings are: (a) the Vck system developed keeps the hydrodynamic force out of the control-loop and, thus, does not bias the FIM, (b) The controller-induced lag is minimal and significantly reduced compared to the first generation of Vck built in the MRELab due to use of an Arduino embedded board to control a servomotor instead of Labview, (c) The SI process revealed a static, third-order, nonlinear viscous model but no need for dynamic terms with memory, and (d) The agreement between real and virtual springs and dampers is excellent in FIM including vortex-induced vibrations (VIVs) and galloping measurements over the entire range of spring constants and velocities tested (16,000 < Re < 140,000).

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(6):061802-061802-9. doi:10.1115/1.4031578.

The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horizontal plane, is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed-dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time-domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface, and vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results is presented. Using linear dynamic stability analysis, the influence of hydrodynamic coefficients on the plate's stability is investigated as a simplified alternative to a semidisplacement vessel.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2015;137(6):061901-061901-9. doi:10.1115/1.4031277.

The performance of an asymmetrical rolling cam as an ocean-wave energy extractor was studied experimentally and theoretically in the 70s. Previous inviscid-fluid theory indicated that energy-absorbing efficiency could approach 100% in the absence of real-fluid effects. The way viscosity alters the performance is examined in this paper for two distinctive rolling-cam shapes: a smooth “Eyeball Cam (EC)” with a simple mathematical form and a “Keeled Cam (KC)” with a single sharp-edged keel. Frequency-domain solutions in an inviscid fluid were first sought for as baseline performance metrics. As expected, without viscosity, both shapes, despite their differences, perform exceedingly well in terms of extraction efficiency. The hydrodynamic properties of the two shapes were then examined in a real fluid, using the solution methodology called the free-surface random-vortex method (FSRVM). The added inertia and radiation damping were changed, especially for the KC. With the power-take-off (PTO) damping present, nonlinear time-domain solutions were developed to predict the rolling motion, the effects of PTO damping, and the effects of the cam shapes. For the EC, the coupled motion of sway, heave and roll in waves was investigated to understand how energy extraction was affected.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Geotechnics

J. Offshore Mech. Arct. Eng. 2015;137(6):062001-062001-11. doi:10.1115/1.4031328.

The effect of pile–soil interaction on structural dynamics is investigated for a large offshore wind turbine (OWT) in the hurricane-prone Western Gulf of Mexico (GOM) shallow water. The OWT has a rotor with three 100-meter blades and a monotower structure. Loads on the turbine rotor and the support structure subject to a 100-year return hurricane are determined. Several types of soil are considered and modeled with a distributed spring system. The results reveal that pile–soil interaction affects dynamics of the turbine support structure significantly, but not the rotor dynamics. Designed with proper pile lengths, natural frequencies of the turbine structure in different soils stay outside dominant frequencies of wave energy spectra in both normal operating and hurricane sea states, but stay between blade passing frequency intervals. Hence, potential resonance of the turbine support structure is not of concern. A comprehensive Campbell diagram is constructed for safe operation of the offshore turbine in different soils.

Commentary by Dr. Valentin Fuster

Technology Review

J. Offshore Mech. Arct. Eng. 2015;137(6):064001-064001-6. doi:10.1115/1.4031668.

Offshore structures are exposed to severe operating conditions because energy resource development has recently extended toward deeper seabed and lower temperature regions. Hence, fracture toughness evaluation for very thick and high strength steels is one of the most important parameters required for the structural integrity assessment of offshore structures. Fracture toughness is known as a property which describes the ability of a material containing a crack to resist unstable brittle fracture. Crack tip opening displacement (CTOD) and J integral are the most commonly employed parameters as fracture criteria in elastic plastic fracture mechanics (EPFM). There have been extensive research efforts to clarify the relationship between CTOD and J integral in elastic plastic regime. Plastic constraint factor (PCF) in the relationship between CTOD and J integral can serve as a parameter to characterize constraint effects in fracture involving plastic deformation. In this regard, the characteristics of the PCF are of significant importance in EPFM analysis. In this study, we evaluated fracture toughness of American Petroleum Institute (API) 2 W Gr. 50 steel in terms of CTOD in various temperatures using single edge notched bend (SENB) specimens. Test specimens are fabricated by submerged arc welding (SAW) and flux cored arc welding (FCAW). In addition, CTOD values are compared to absorbed impact energy with respect to the weld metal (WM) and heat affected zone (HAZ). Then, we investigated PCFs with respect to several regions of the weldment at various temperatures. Experimental values of PCFs were calculated and then compared against the predicted values according to the American Society for Testing and Materials (ASTM) standard. CTOD values of WM by SAW is found to be about three times higher than that of FCAW at −10 °C, and CTOD values calculated by the ASTM standard are approximately 30% lower than the CTOD according to British Standard (BS). In addition, the maximum of 40% discrepancy is observed in PCFs obtained between the experiment and the predicted values according to the ASTM standard. This may lead to too conservative fracture toughness estimation for the welded joints of API 2 W Gr. 50 steel when using PCF by ASTM. Based on the accurate estimated PCF values obtained from this study, it is believed that rational fracture design of offshore structures is possible.

Commentary by Dr. Valentin Fuster

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