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Research Papers: Ocean Space Utilization

J. Offshore Mech. Arct. Eng. 2017;140(1):011201-011201-9. doi:10.1115/1.4037488.

Numerical simulations and experiments of an elastic circular collar of a floating fish farm are reported. The floater model without netting structure is moored with nearly horizontal moorings and tested in regular deep-water waves of different steepnesses and periods without current. Local overtopping of waves was observed in steep waves. The focus here is on the vertical accelerations along the floater in the different conditions. The experiments show that higher-order harmonics of the accelerations matter. A three-dimensional (3D) weak-scatter model with partly nonlinear effects as well as a 3D linear frequency-domain method based on potential flow are used. From their comparison against the measurements, strong 3D and frequency dependency effects as well as flexible floater motions matter. The weak-scatter model can only partly explain the nonlinearities present in the measured accelerations.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;140(1):011301-011301-6. doi:10.1115/1.4037486.

Collisions and grounding accidents of ships, but also the failure of the hull-integrity, can lead to oil leakage. Examples are the Rena in 2011, the Hebei Spirit in 2007, and the much known accident of the Prestige in 2002. Consequently, research regarding the enhancement of the structural design to increase the safety-level of ships in case of accidents is important. In this paper, the use of a rubber bag as a second barrier is presented as an alternative concept to prevent oil leakage in case of accidents. The influence of the rubber bag is investigated using the exemplary simulation of a ship collision. A simplified tanker side structure as well as a box-shaped rubber bag is analyzed with the finite element (FE) method. The material model for the rubber bag is calibrated with tensile tests to obtain the required material parameters. The reaction forces and the associated penetration depth are analyzed. The comparison is done between the structure with and without the rubber bag. For the latter, the general behavior of an empty tank in a ship side structure is compared with the large-scale experimental results. Furthermore, an additional increase of the collision resistance of the ship due to the rubber bag without changing the common structural design is discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2017;140(1):011501-011501-10. doi:10.1115/1.4037472.

Polycrystalline isotropic ice was selected as the material of choice for this fundamental study on the mechanical behavior of ice. Two essential properties of the ice structure are the porosity and degree of anisotropy (DA). On the one hand, it is clear that these two factors have a great influence on the mechanical properties of the material. On the other hand, however, they are strongly dependent on the laboratory procedure used to fabricate the ice samples. Thus, in this work, three procedures to produce ice samples are analyzed. For this purpose, the structural and mechanical properties observed in uniaxial compression tests are discussed for each sample fabrication procedure. Then, after the most suitable fabrication procedure has been determined, the viscous behavior of isotropic ice is analyzed and discussed using the results of simple compression test at different temperatures and axial strain rates.

Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2017;140(1):011601-011601-9. doi:10.1115/1.4037789.

In this paper, the long-term extreme response of a vessel rolling in random beam seas and the associated reliability evaluation are addressed. The long-term response analysis is based on the upcrossing rates of the roll motion under different sea states. Generally, for nonlinear roll motion in random seas, the high-level roll response is sensitive and closely related to the nonlinear effects associated with the restoring and damping terms. Therefore, assessing the corresponding statistics of the random roll motion with low probability levels is difficult and time-consuming. In this work, the Markov theory is introduced in order to tackle this problem. Specifically, for the dead ship condition, the random roll excitation moment is approximated as a filtered white noise process by applying a second-order linear filter and an efficient four-dimensional (4D) path integration (PI) technique is applied in order to calculate the response statistics. Furthermore, the reliability evaluation is based on the well-known Poisson estimate as well as on the upcrossing rate calculated by the 4D PI method. The long-term analysis and reliability evaluation of the nonlinear roll motion in random seas, which consider the variation of the sea states could be a valuable reference for ship stability research.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2017;140(1):011701-011701-15. doi:10.1115/1.4037727.

Marine drilling riser is subject to complicated environmental loads which include top motions due to mobile offshore drilling unit (MODU), wave loads, and current loads. Cyclic dynamic loads will cause severe fatigue accumulation along the drilling riser system, especially at the subsea wellhead (WH). Statoil and BP have carried out a comprehensive model test program on drilling riser in MARINTEK's Towing Tank in February 2015. The objective is to validate and verify software predictions of drilling riser behavior under various environmental conditions by the use of model test data. Six drilling riser configurations were tested, including different components such as upper flex joint (UFJ), tensioner, marine riser, lower marine riser package (LMRP), blow-out preventer (BOP), lower flex joint (LFJ), buoyancy elements, and seabed boundary model. The drilling riser models were tested in different load conditions. Measurements were made of microbending strains and accelerations along the riser in both in-line (IL) and crossflow (CF) directions. Video recordings were made both above and under water. In this paper, the test setup and test program are presented. Comparisons of results between model test and RIFLEX simulation are presented on selected cases. Preliminary results show that the drilling riser model tests are able to capture the typical dynamic responses observed from field measurement, and the comparison between model test and RIFLEX simulation is promising.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(1):011702-011702-11. doi:10.1115/1.4037538.

Slender offshore structures in deep water subjected to currents may experience vortex-induced vibrations (VIV), which can cause significant fatigue damage. Extensive experimental researches have been conducted to study the VIV in the past several decades. However, most of the experimental works have small-scale models and relatively low Reynolds number (Re)—“subcritical” or even lower Reynolds number regime. There is a lack of full understanding of the VIV in prototype Re flow regime. Applying the results with low Re to a full-scale riser with prototype Re might have uncertainties due to the scaling effects. In addition, the surface roughness of the riser is also an important parameter, especially in critical Re regime, which is the case for prototype risers. In the present study, two full-scale rigid riser models with different surface roughness ratios were tested in the towing tank of MARINTEK in 2014. Stationary tests, pure crossflow (CF) free oscillation tests, and forced/controlled motion tests were carried out. Several conclusions could be made: The drag coefficient is dependent on the Re number and surface roughness ratio. At critical and supercritical flow regimes, the displacement amplitude ratio is less sensitive to Re than that at lower Re. The displacement amplitude ratio in subcritical flow regime is significantly larger than that in critical and supercritical flow regimes. Two excitation regions for the ‘smooth riser’ and one excitation region for the “rough riser” are identified.

Commentary by Dr. Valentin Fuster

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