Experimental Study on the Vortex-Induced Motions (VIM) of a Semi-Submersible Floater in Waves

2017 ◽  
Author(s):  
António Maximiano ◽  
Arjen Koop ◽  
Jaap de Wilde ◽  
Rodolfo T. Gonçalves
Keyword(s):  
Wave Height ◽  
Model Tests ◽  
Air Bearing ◽  
Wave Direction ◽  
Sea State ◽  
Natural Period ◽  
Calm Water ◽  
Two Factors ◽  
Induced Motion ◽  
Nominal Response

Recent studies suggest that the vortex-induced motions (VIM) of a semi-submersible found in model tests over-predicts the response in the field, which may lead to an over conservative design of the mooring and riser system. Within the Vortex Induced Motion Joint Industry Project (VIM JIP), run by MARIN and University of São Paulo (USP), possible reasons for this over-prediction are investigated using model tests and CFD [1–6]. A model test campaign was carried out at MARIN to test different candidates that might explain the observed differences. The results obtained with an air bearing setup regarding damping, mass ratio, draft variations and sinusoidal tow velocities, have been published elsewhere [6]. The present publication focuses on the influence of waves on the VIM response. The model was a generic bare hull semi submersible with four rounded square columns at scale 56.5. A simplified mooring system consisting of four springs was designed to match the desired natural period in the sway and yaw direction. The model was towed at different velocities, corresponding to the range of reduced velocities where the highest response is expected. A VIM tow test campaign was carried out in calm water as a benchmark. The model was then tested at 7 different wave-current conditions, and the results are compared with the benchmark case. The results suggest that two factors are important for VIM response in waves: the wave height, and the relation between wave and current direction. Comparing to calm water conditions, a reduction of 15% on the peak nominal response was found for a smaller sea state (HS = 2m, TP = 10s), however with a higher significant wave height (HS = 4m, TP = 10s) the peak nominal response was reduced by 30%. Depending on the combination of current-wave direction, the influence of the same sea state (HS = 4m, TP = 10s) on VIM response can be negligible (transverse seas) or result in a 30% reduction of the peak nominal response for collinear sea and current. This is a relevant finding since most research on the topic has focused on collinear conditions, and VIM tests in waves with transverse or oblique conditions are rare [7–9]. Comparing the calm water VIM response obtained with the air bearing setup, published in [6], with the soft mooring configuration reported here, the latter shows a generally smaller response, with a narrower lock in region. Nevertheless the peak response is found to be similar for both experimental setups.

Model Tests With the HVS Semisubmersible for Dry Tree Application

2014 ◽  
Author(s):  
Johyun Kyoung ◽  
Chan-Kyu Yang ◽  
Kostas Lambrakos ◽  
Jim O’Sullivan
Keyword(s):  
Model Tests ◽  
Base Case ◽  
Natural Period ◽  
Depth Limit ◽  
Wave Basin ◽  
Induced Motion ◽  
Motion Performance ◽  
Heave Motion ◽  
Keel Plate ◽  
Measured Response

The global hull motion performance of the HVS semisubmersible for dry tree application is investigated with model tests. The HVS semisubmersible, which has been validated for low heave motion and VIM (Vortex Induced Motion) response, was modified for dry tree application. As a base case, the modification includes a keel plate with riser keel guides at the level of the pontoons. The keel plate is optimally designed to increase the hull heave period to compensate for the heave period reduction in the HVS semisubmersible due to the riser tensioners for the dry tree application. The plate also provides additional viscous damping that decreases the heave response at the heave natural period. The model tests were performed to investigate the in-place hull motion performance for the Gulf of Mexico environmental conditions. The pneumatic riser tensioners were modeled using a spring with dual stiffness. Because of the water depth limit in the wave basin, a truncated mooring was used to simulate the full scale prototype mooring system. An alternate modification to the HVS semisubmersible that includes pontoon plates was also tested and the measured response was compared to the response of the base case. The measured hull responses were correlated with MLTSIM, a Technip in-house nonlinear time-domain 6-DOF motion analysis program.

scholarly journals Capability of Jason-2 Subwaveform Retrackers for Significant Wave Height in the Calm Semi-Enclosed Celebes Sea

2020 ◽  
Vol 12 (20) ◽  
pp. 3367
Author(s):  
Kaoru Ichikawa ◽  
Xi-Feng Wang ◽  
Hitoshi Tamura
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Leading Edge ◽  
Sea State ◽  
Significant Wave ◽  
Contamination Sources ◽  
The Pacific ◽  
Unique System ◽  
Calm Water ◽  
Good Agreement

Satellite altimetry is a unique system that provides repeated observations of significant wave height (SWH) globally, but its measurements could be contaminated by lands, slicks, or calm water with smooth surface. In this study, capability of subwaveform retrackers against 20 Hz Jason-2 measurements is examined in the calm Celebes Sea. Distances between contamination sources and Jason-2 observation points can be determined using sequentially assembled adjacent waveforms (radargram). When no contamination sources are present within a Jason-2 footprint, subwaveform retrackers are in excellent agreement with the Sensor Geophysical Data Records (SGDR) MLE4 retracker that uses full-length waveforms, except that Adaptive Leading Edge Subwaveform (ALES) retracker has a positive bias in a calm sea state (SWH < 1 m), which is not unusual in the Celebes Sea. Meanwhile, when contamination sources exist within 4.5 km from Jason-2 observation points, SGDR occasionally estimates unrealistically large SWH values, although they could be partly eliminated by sigma0 filters. These datasets are then compared with WAVEWATCH III model, resulting in good agreement. The agreement becomes worse if swells from the Pacific is excluded in the model, suggesting constant presence of swells despite the semi-enclosed nature. In addition, outliers are found related with locally-confined SWH events, which could be inadequately represented in the model.

scholarly journals Sea State Determination from Ship-Based Geodetic GPS

2014 ◽  
Vol 31 (11) ◽  
pp. 2556-2564 ◽  
Author(s):  
James Foster ◽  
Ning Li ◽  
Kwok Fai Cheung
Keyword(s):  
Wave Height ◽  
Wave Model ◽  
Ocean Waves ◽  
Sea Surface ◽  
Wave Direction ◽  
Sea State ◽  
Dominant Wave ◽  
Model Predictions ◽  
Economic Vitality ◽  
Buoy Data

AbstractOcean waves have a profound impact on navigation, offshore operations, recreation, safety, and the economic vitality of a nation’s maritime and coastal communities. This study demonstrates that ships equipped with geodetic GPS and a radar gauge can provide accurate estimates of sea state. The Research Vessel (R/V) Kilo Moana recorded 1-Hz data for the entire period of a 10-day cruise around the Hawaiian Islands. Solving for precise kinematic positions for the ship and combining these solutions with the ranges from the ship to the sea surface provided by the radar gauge, it was possible to retrieve 1-Hz estimates of the sea surface elevation along the cruise track. Converting these into estimates of significant wave height, strong agreement was found with wave buoy measurements and hindcast wave data. Comparison with buoy data indicates the estimates have errors on the order of 0.22 m, or less than 11% of the wave height. Using wave model predictions of the dominant directions, the data were processed further to correct for the Doppler shift and to estimate the dominant wave period. Although relatively noisy in locations where the predicted wave directions are expected to be poor, in general these estimates also show a good agreement with the wave buoy observations and hindcast wave estimates. A segment of the cruise that formed a circuit allowed for testing the consistency of the ship-based estimates and for determining a dominant wave direction, which was found to agree closely with model predictions.

Model Tests on the Parametric Resonance of the Deep Draft Semi-Submersible Under Regular and Irregular Waves

2020 ◽  
Author(s):  
Chenglin Li ◽  
Liwei Yu ◽  
Shuqing Wang ◽  
Zhe Tian ◽  
Xujie Wang
Keyword(s):  
Wave Height ◽  
Parametric Resonance ◽  
Model Tests ◽  
Irregular Waves ◽  
Mooring System ◽  
Mooring Lines ◽  
Natural Period ◽  
Beam Wave ◽  
Wave Flume ◽  
Pitch Direction

Abstract Model tests on the parametric resonance of the deep draft semi-submersible (DDS) under regular and irregular waves are conducted in the wave flume of the Ocean University of China (OUC). Model tests with and without mooring lines are conducted under various regular and irregular waves with different wave angles. From the results of the model without mooring lines, it is found that parametric resonance generally occurs when wave period is about the natural period of heave and about half of the natural period of pitch or roll. The steady amplitude of the parameter resonance is larger under larger wave height. In heading waves, parametric resonance occurs in the roll direction, while parametric resonance occurs in both roll and pitch direction for the beam wave. In irregular waves, parametric resonance is practical non-ergodic. For the model without mooring lines, parametric resonance does not occur under the constraints of the mooring system.

scholarly journals Global ship accidents and ocean swell-related sea states

2017 ◽  
Vol 17 (11) ◽  
pp. 2041-2051 ◽  
Author(s):  
Zhiwei Zhang ◽  
Xiao-Ming Li
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Environmental Issues ◽  
Wave Model ◽  
Economic Losses ◽  
Wave Direction ◽  
Sea State ◽  
Mean Wave Period ◽  
Numerical Wave ◽  
Wind Sea

Abstract. With the increased frequency of shipping activities, navigation safety has become a major concern, especially when economic losses, human casualties and environmental issues are considered. As a contributing factor, the sea state plays a significant role in shipping safety. However, the types of dangerous sea states that trigger serious shipping accidents are not well understood. To address this issue, we analyzed the sea state characteristics during ship accidents that occurred in poor weather or heavy seas based on a 10-year ship accident dataset. Sea state parameters of a numerical wave model, i.e., significant wave height, mean wave period and mean wave direction, were analyzed for the selected ship accident cases. The results indicated that complex sea states with the co-occurrence of wind sea and swell conditions represent threats to sailing vessels, especially when these conditions include similar wave periods and oblique wave directions.

scholarly journals RESEARCH Of) THE MODEL SHELTERING INVESTIGATION OF A HARBOR

1974 ◽  
Vol 1 (14) ◽  
pp. 124
Author(s):  
Ho-Shuong Hou
Keyword(s):  
Wave Height ◽  
Model Test ◽  
Wave Diffraction ◽  
Wave Pattern ◽  
Model Tests ◽  
Test Results ◽  
Wave Direction ◽  
Wave Condition ◽  
Wave Refraction ◽  
Sheltering Effect

The Model Sheltering Experiment deals with the planning arrangement of the proposed harbor and is done by the worst wave condition (with respect to wave direction, wave height, and wave period) which probably occurred on the proposed harbor. The objectives of this experiment are to get the wave pattern of the harbor basin and to understand the various phenomena of the wave refraction, diffraction, and reflection caused by model test due to different harbor arrangement, and to comprehend the sheltering effect of the outer breakwaters. From the analysis of these test results, harbor planning of the most effective arrangement - the most economic length of breakwaters and the most ideal width of harbor entrance could be selected. For the purpose of analyzing results of model tests; comparison of theoretical wave diffraction calculation is proposed.

Documentation of Operational Limits of Free-Fall Lifeboats by Combining Model Tests, Full-Scale Tests, and Computer Simulation

2009 ◽  
Author(s):  
Wojciech E. Kauczynski ◽  
Per Werenskiold ◽  
Frode Narten
Keyword(s):  
Wave Height ◽  
Model Test ◽  
Free Fall ◽  
Model Tests ◽  
Full Scale ◽  
Test Results ◽  
Extrapolation Methods ◽  
Basic Performance ◽  
Calm Water ◽  
Full Scale Tests

Historically, approval of lifeboats is based on a “calm water” philosophy through the SOLAS regulatory regime. In spring 2005 during offshore installation tests in calm water, unacceptable structural deflection of the roof for one type of free-fall lifeboat was revealed. Immediate actions were initiated by the Norwegian Oil Industry Association (OLF) and Statoil, including the goal of studying and documenting the main performance factors for free-fall lifeboats at up to a 100-year weather condition. In addition, OLF has request to develop relevant criteria for in depth classification of performance, and finally to upgrade urgently, when relevant, all free-fall lifeboats operating on the Norwegian continental shelf to the agreed standards. The basic performance criteria of free-fall lifeboat systems in emergency conditions are: structural strength, acceleration loads on passengers during water impact, boat forward speed immediately after water entry, and the manoeuvring away to a safe distance from the installation. Within the OLF-project, MARINTEK has performed an extensive model test program (over 25000 tests) with the 14 different types of free-fall lifeboats (launched by vertical drop or from a skid). Boat performances have been examined in different weather conditions, ranging from still water up to 11m wave height (regular waves and wind) or 7m (irregular significant wave height with corresponding wind). Calm water model test results have been compared to full-scale test results. In order to extend prognosis of the lifeboat performances up to 100-year storm condition (Hs = 15.7m), special extrapolation methods have been developed for studying the three basic performance areas, augmented by computer simulations applied for higher sea states. This paper presents example results and experiences gained from the model tests, full-scale tests and combined use of simulations and model test results. Experimental model test set-up and applied analysis and extrapolation methods are reviewed. Finally, the application of newly proposed performance and technical criteria is discussed.

The Concept of the Optimum Alignment of Discharge Pipelines Due to the Minimization of the Wave Forces

1992 ◽  
Vol 25 (9) ◽  
pp. 211-216
Author(s):  
A. Akyarli ◽  
Y. Arisoy
Keyword(s):  
Wave Height ◽  
Wave Force ◽  
Wave Forces ◽  
Wave Direction ◽  
Incoming Wave ◽  
Pipeline Route ◽  
Optimum Alignment ◽  
The Cost ◽  
Resultant Wave

As the wave forces are the function of the wave height, period and the angle between the incoming wave direction and the axis of the discharge pipeline, the resultant wave force is directly related to the alignment of the pipeline. In this paper, a method is explained to determine an optimum pipeline route for which the resultant wave force becomes minimum and hence, the cost of the constructive measures may decrease. Also, the application of this method is submitted through a case study.

scholarly journals Estimation of Significant Wave Heights from ASCAT Scatterometer Data via Deep Learning Network

2021 ◽  
Vol 13 (2) ◽  
pp. 195
Author(s):  
He Wang ◽  
Jingsong Yang ◽  
Jianhua Zhu ◽  
Lin Ren ◽  
Yahao Liu ◽  
...  
Keyword(s):  
Deep Learning ◽  
Wave Height ◽  
Significant Wave Height ◽  
Incidence Angle ◽  
Ocean Waves ◽  
Global Scale ◽  
Learning Technology ◽  
Sea State ◽  
Significant Wave ◽  
Wave Heights

Sea state estimation from wide-swath and frequent-revisit scatterometers, which are providing ocean winds in the routine, is an attractive challenge. In this study, state-of-the-art deep learning technology is successfully adopted to develop an algorithm for deriving significant wave height from Advanced Scatterometer (ASCAT) aboard MetOp-A. By collocating three years (2016–2018) of ASCAT measurements and WaveWatch III sea state hindcasts at a global scale, huge amount data points (>8 million) were employed to train the multi-hidden-layer deep learning model, which has been established to map the inputs of thirteen sea state related ASCAT observables into the wave heights. The ASCAT significant wave height estimates were validated against hindcast dataset independent on training, showing good consistency in terms of root mean square error of 0.5 m under moderate sea condition (1.0–5.0 m). Additionally, reasonable agreement is also found between ASCAT derived wave heights and buoy observations from National Data Buoy Center for the proposed algorithm. Results are further discussed with respect to sea state maturity, radar incidence angle along with the limitations of the model. Our work demonstrates the capability of scatterometers for monitoring sea state, thus would advance the use of scatterometers, which were originally designed for winds, in studies of ocean waves.

scholarly journals EXTREME WAVE PRESSURES AND LOADS ON A PILE-SUPPORTED WHARF DECK - INFLUENCES OF AIR GAP AND WAVE DIRECTION

2018 ◽  
pp. 5
Author(s):  
Andrew Cornett
Keyword(s):  
Offshore Structures ◽  
Model Tests ◽  
Scale Model ◽  
Extreme Waves ◽  
Wave Direction ◽  
Coastal Structures ◽  
Extreme Wave ◽  
Wave Impact ◽  
The Past ◽  
Water Depths

Many deck-on-pile structures are located in shallow water depths at elevations low enough to be inundated by large waves during intense storms or tsunami. Many researchers have studied wave-in-deck loads over the past decade using a variety of theoretical, experimental, and numerical methods. Wave-in-deck loads on various pile supported coastal structures such as jetties, piers, wharves and bridges have been studied by Tirindelli et al. (2003), Cuomo et al. (2007, 2009), Murali et al. (2009), and Meng et al. (2010). All these authors analyzed data from scale model tests to investigate the pressures and loads on beam and deck elements subject to wave impact under various conditions. Wavein- deck loads on fixed offshore structures have been studied by Murray et al. (1997), Finnigan et al. (1997), Bea et al. (1999, 2001), Baarholm et al. (2004, 2009), and Raaij et al. (2007). These authors have studied both simplified and realistic deck structures using a mixture of theoretical analysis and model tests. Other researchers, including Kendon et al. (2010), Schellin et al. (2009), Lande et al. (2011) and Wemmenhove et al. (2011) have demonstrated that various CFD methods can be used to simulate the interaction of extreme waves with both simple and more realistic deck structures, and predict wave-in-deck pressures and loads.

Sign in / Sign up

Export Citation Format

Share Document