bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluating Methods for Control of an Offshore Floating Turbine
Authors: Fleming Paul A. , Pineda Isaac , Rossetti Michele , Wright Alan D. , Arora Dhiraj
Published: September 2014

Developing wind turbines on floating offshore platforms is a topic of growing interest. By using a floating platform, wind turbines can be located in areas with good resources and near to major load centers. However, there can be technical challenges with this approach, and one is the observed unstable interaction between the pitch speed controller and the platform motions. In this paper, we consider several controls-based methods for resolving this issue for a commercial-scale turbine on a tension-leg platform. The design of each method is reviewed and each is shown to resolve the unstable interaction. The methods are then compared through load suite analysis to assess the strengths and weaknesses of each approach. Finally, we present proposals for refining and combining these methods and further approaches for performance improvement that could be considered.
Reliability-Based Calibration of Partial Safety Factors for the Horizontal Axis Tidal Turbine Standard for Certification
Authors: Simeon Gaizka Zarraonandia , Ferreira Claudio Bittencourt
Published: September 2014

Tidal energy is nowadays one of the fastest growing types of marine renewable energy. In particular, horizontal axis tidal turbines (HATT) are the most advanced designs and the most appropriate for standardisation. It is however, in the interest of the industry to provide a set of standard practises in order to help in the process of designing this type of marine converters. DNV GL is producing this standard with the support of Alstom and as part of the ReDAPT project commissioned and co-funded by the ETI (Energy Technologies Institute).

The work undertaken by DNV GL to produce this standard involves the identification of the uncertainties that designers need to address during the design process. Unlike other marine structures, HATTs are usually located in very energetic areas where no other marine structure has been before. Site characterisation is one of the largest sources of uncertainty e.g. turbulence. Key inputs like turbine inflow conditions or predictions of extreme values are still grey areas due to the limited site measurements and the uncertainty of the metocean models. Numerical models of HATTs are still quite uncertain often dependent on experience of the people running them. As part of ReDAPT project there is an ongoing effort in validation and evaluation of the accuracy of these numerical models and some of the results are used in this calibration study.

The new standard for HATTs deals with the loading uncertainty in a whole new way by introducing a new parameter that is added to the traditional partial safety factors for loads. This new standard uses the traditional safety factors from the offshore structures standards and allows changing them based on the level of uncertainty that was introduced during the design process.

This paper describes the process of calibration of the partial safety factors in ULS for loads of HATTs that was part of the work in the creation of the new standard. The reliability based calibration involved the formulation of failure criteria, the identification of stochastic variables in the failure criteria, calculation of reliability against failure and ultimately the new set of partial safety factors for loads and a methodology for adjustment of the factors in a case by case basis.
Current Profile Data Sources for Engineering Design West of Shetlands
Authors: Jeans Gus , Fox Joe , Channelliere Claire
Published: September 2014

Current profile data sources considered for derivation of engineering design criteria West of Shetland are described. The region is impacted by a variety of oceanographic processes that combine to produce a complex current regime. Reliable quantification of the resulting current profiles is required for safe and cost effective offshore exploration and field development.

A key challenge to all offshore developments is acquisition of appropriate data. Site specific measurement remains the primary current profile data source for engineering applications, with full water column coverage at sufficient resolution required for riser design. Such in-situ data are generally expensive and time consuming to collect, so there is an increasing tendency for numerical model current data to be considered.

Model data are often relatively quick and inexpensive to obtain, with the added benefit of a much longer duration, potentially allowing inter-annual variability and extreme events to be captured. However, the accuracy and reliability of numerical model data remains questionable, or unproven, in many deepwater development regions.

This paper describes a recent study in which in-situ data remained the primary source for derivation of current profile criteria for engineering design. Short duration proprietary data were supplemented by additional public domain data from nearby sites in a regional synthesis, with critical results. The performance and benefits of readily available model data are also considered.
Numerical Evaluation of Energy Absorption in Ship-Offshore Fixed Platform Collisions
Authors: Travanca Joao , Hao Hong
Published: September 2014

Both the maritime traffic and the number of built offshore platforms have been continuously increasing over recent times. Among the structures built offshore, the fixed type constitutes the majority. The consequent diversity of plausible collision scenarios involving offshore platforms and passing ships must therefore consider aspects such as different ship size, different impact energy or different impact locations. For high energy collisions, large deformations are expected on both the platform and ship structures. It is expected that part of the energy absorption in the platform is confined to localized zones where plastic deformations take place, although the elastic strain energy may also be significant. For such impact problems, the amounts of strain energy in each structure are mainly dependent on the relative stiffness of the structures. By taking different ship and platform configurations as well as different contact points between the two bodies, different relative stiffness of the two structures can be tested in order to provide a clearer understanding of the dissipation of strain energy. The possible plastic deformation mechanisms are analyzed and simplified approaches are considered for prediction in comparison with the numerical results carried out by finite element analysis. Based on the results, some evaluations are made with respect to the code of practice in offshore platform design against ship impact.
FSI Analysis of Flow-Induced Vibration in Subsea Jumper Subject to Downstream Slug and Ocean Current
Authors: Lu Yaojun , Liang Chun , Manzano-Ruiz Juan J. , Janardhanan Kalyana , Perng Yeong-Yan
Published: September 2014

This paper presents a multiphysics approach for characterizing flow-induced vibrations in a subsea jumper subject to pressure fluctuation due to downstream slugging and external vortex shedding effects due to ocean current. In this study the associated fluid properties, phase behavior, and slugging dynamics were all characterized at subsea condition using PVTSIM and OLGA programs, respectively; the outcomes were then applied to a two-way fluid-structure interaction analysis (FSI) to quantify the vibration response. To mitigate the resonant phenomenon, detailed modal analysis was also conducted to check the modal shapes and natural frequencies. Therefore, this study integrated the best practices in flow assurance study (OLGA and PVTSim), computational fluid dynamics simulation (CFD), and computational structure analysis (FEA), and provided a complete solution to the fluid-structure interaction involved in a subsea jumper. It is revealed that both the slugging flow and the external ocean current induce vibration response in a subsea jumper. Compared to the vortex-induced vibration due to the external current and the flow-induced vibration due to the internal flow, the pressure fluctuation due to the slug plays a dominant role in generating excessive vibration and fatigue failure of a subsea jumper. Although this study focused on a subsea jumper only, the same approach can be applied to subsea flowline, subsea riser, and other subsea structures.
Improving the Early Steel Design Process by Integrating Strength Calculation and Rule Checking Capabilities in a Ship Design Tool
Authors: Rox Nicolas
Published: September 2014

A well-founded determination of steel structure scantlings is essential during the early design process of a ship or an ocean structure. In the first 4 up to 6 weeks of a new building project, the major part of the final building costs has to be fixed. Amongst others a proper steel weight estimation is crucial. The weight depends on the structural dimensions which are determined mostly by experience and rarely by direct calculations. Therefore, a simple direct strength calculation tool has been integrated in a ship design software. The tool uses structural and general ship design information. Besides the structural constraints, posed by the project design, the steel designer has to fulfill also the structural constraints posed by the classification societies. Normally they are checked with software solutions provided by the societies. However these software tools are not well adapted to the early design process as various design parameters change frequently. For this purpose a link has been created between a rule scantling tool on one side and a design software on the other. The link allows an automated exchange of steel scantlings and project information. By this the modeling and design work as well as the structure scantling and steel weight determination is performed in the design software tool, while the rule scantling tool is only used for a quick assessment of class conformity. With the help of the mentioned methods, the structure can be pre-dimensioned directly based on the early design model in accordance with the classification rules. Furthermore the steel dimensions can not only be optimized with regard to local and global loads, but also with regard to design boundary conditions. In consequence the early steel design process is improved by a more accurate steel scantlings determination and results in a better optimized steel structure as well as severely reduced time spent on the steel iterations.
Finite Element Analysis of Pipeline Global Walking With Spanning and Lateral Buckling
Authors: Duan Gang , Tang Andy , Qi Xinhai , Zhong Jianxia
Published: September 2014

This paper investigates High Pressure and/or High Temperature (HPHT) design of a pipeline across an escarpment with significant seabed undulations and elevation change from drill center (DC) to riser touch down point (TDP). The pipeline has a tendency to walk towards the riser during pipeline start-up / shut-down (SD) operations due to seabed slope and riser bottom tension in a case study. A hold-back pile at the uphill pipeline end near DC to arrest walking, along with the riser bottom tension applied at the other end of the pipeline, results in significant tensions in the pipeline and substantial spans over escarpment. The pipeline spanning lengths, lateral buckling amplitudes and walking distances and their variations during the start-up / shut-down cycles are presented and discussed. Both wet-insulated single pipe (WISP) and pipe-in-pipe (PIP) options are studied and compared. Interactions among pipeline global walking, spanning and lateral buckling are observed. In-depth understanding of pipeline systematic response will help perform a safe and cost-effective pipeline design.
Analysis of Propagation of Long Waves in Shallow Water Using the KdV-Based Nonlinear Fourier Transform (KdV-NLFT)
Authors: Brühl Markus , Oumeraci Hocine
Published: September 2014

Hydraulic model tests and numerical simulations show that long sinusoidal waves that are generated in very shallow waters are not stable but show modifications of the free surface as function of propagation in time and space. First, with increasing distance from the wave maker the wave becomes asymmetric and develops into a bore-shaped wave. Second, with further increasing distance more and more additional wave crests appear from the front of the bore (undular bore). The shallower the water depth, the more additional wave components can be observed. In extremely shallow water, the periodic sine waves completely disintegrate into periodic trains of solitons. At Leichtweiss-Institute for Hydraulic Engineering and Water Resources (LWI), TU Braunschweig, a nonlinear Fourier transform based on the Korteweg-deVries equation (KdV-NLFT) is implemented and successfully applied in Brühl [1] that provides an explanation for this nonlinear phenomenon and allows the prediction of the dispersion and propagation of long sinusoidal waves in shallow water.
Assessment of the Feasibility of the Arctic Sea Transportation by Using Ship Ice Transit Simulation
Authors: Valkonen Janne , Riska Kaj
Published: September 2014

Arctic sea transportation has drawn a lot of attention in the recent years. The possibility of using a shorter route between Europe and Asia interests many actors in the shipping industry. Benefits from the shorter route may, at first, seem attractive. However, there might be factors affecting the feasibility of the route that are not obvious at first. The estimated transit speed along the Arctic route is not necessarily reached due to ice and other prevailing conditions. Simplified methods can underestimate the actual transit time, so the use of advanced methods is advisable. A thorough assessment of ship performance along the selected route can reveal factors that affect the feasibility of trans-Arctic shipping.

This paper presents how the ship transit along an Arctic route can be simulated and how the ship performance can be assessed based on the simulation results. In this paper comparative results of ship performance in different ice conditions are shown and the benefits and challenges of the ship ice transit simulations are discussed. The effect of the input ice conditions to the ship performance estimates and how the ice information from various sources can be used for the simulations are discussed.

Ship performance in the Arctic transit is tested using a probabilistic model called COSSARC for ship performance simulation in ice and open water. The tool can be used for assessing the economic feasibility of ship designs and transport concepts. One of the main benefits of the ship ice transit simulations compared to the simpler methods is that ice ridges are described in the realization of ice conditions statistically and the ship performance in ridges is modeled. A ship might get stuck in ridges and might be forced to wait for assistance from an icebreaker, or a double acting ship might travel slower than anticipated through ridge fields. This increases the transit time significantly, which is not necessarily revealed by simpler methods. The main outcome from the ship ice transit simulations described in this paper, is a more realistic estimate of transit time for a given route. This can be used as input for economic or other assessments. It is possible to estimate the probability of getting stuck in ice ridges from the ship ice transit simulation results, and thus the need for icebreaker assistance can be assessed. The probabilistic simulation of ship performance can be done in the design phase of the ship to assess various design concepts or while selecting what kind of ship or fleet of ships is the most suitable for the given transport task.
Mode Shape Variation for a Low-Mode Number Flexible Cylinder Subject to Vortex-Induced Vibrations
Authors: Gedikli Ersegun D. , Dahl Jason M.
Published: September 2014

The excitation of two low-mode number, flexible cylinders in uniform-flow is investigated to determine effects of structural mode shape on vortex-induced vibrations. Experiments are performed in a re-circulating flow channel and in a small flow visualization tank using object tracking and digital particle image velocimetry (DPIV) to measure the excitation of the cylinder, to estimate forces acting on the structure, and to observe the wake of the structure under the observed body motions. Previous research has focused on understanding the effect of in-line to cross-flow natural frequency ratio on the excitation of the structure in an attempt to model the excitation of multiple structural modes on long, flexible bodies. The current research investigates the impact of structural mode shape on this relationship by holding the in-line to cross-flow natural frequency constant and attempting to excite a specific structural mode shape. It is found that the combination of an odd mode shape excited in the cross-flow direction with an even mode shape in the in-line direction results in an incompatible synchronization condition, where the dominant forcing frequency in-line may experience a frequency equal to the cross-flow forcing frequency, a condition only observed in rigid cylinder experiments when the natural frequency ratio is less than one. This is consistent with the first mode being excited in both in-line and cross-flow directions, however this leads to an asymmetric wake. The wake is observed using DPIV on a rigid cylinder with forced motions equivalent to the flexible body. A case of mode switching is also observed where the even in-line mode exhibits an excitation at twice the cross-flow frequency; however the spatial mode shape in-line appears similar to the first structural mode shape. It is hypothesized that this situation is possible due to variation in the effective added mass along the length of the cylinder.
Trajectory Tracking of Underactuated Surface Vessels Based on Hierarchical Sliding Mode
Authors: Liu Cheng , Zou Zaojian , Yin Jianchuan
Published: September 2014

Trajectory tracking is an importance practice in ship motion control field. It attracts more attention recently due to its difficulties. Trajectory tracking requires the ship to arrive pinpoint location at exact time. It is a underactuated system because the degrees of freedom of control inputs are fewer than the degrees of freedom that needed to be controlled. In this paper, a hierarchical sliding mode controller and a common sliding mode controller are proposed to deal with the trajectory tracking problem of underactuated surface vessels. Simulation results validate the tracking performance of the proposed controllers. The closed-loop stability is testified by the Lyapunov stability theorem.
The Effect of Wind Data Resolution on the Predicted Energy Generation of an Offshore Wind Farm
Authors: Drenick Chelsea , Prowell Ian , Dolan Dan
Published: September 2014

To estimate the potential energy of an offshore wind farm, wind data and an assessment framework are necessary. Depending on the type of wind data and the software used for the assessment, many techniques are available to calculate the expected energy generation. This paper investigates how the resolution of the wind data affects the estimated final energy output and revenue predicted for an offshore wind farm. Four wind resolution sets are utilized in the analysis: a 5-year 10-minute time history, a 5-year 6-hour time history, a wind rose, and a wind speed and direction average. The first two data sets are analyzed using a time history analysis procedure that determines the energy generated at each time step including wake loss effects. The third and fourth data sets are analyzed using a probabilistic analysis method. The four analysis procedures are utilized at a variety of locations off the East Coast. At each site, the expected energy as well as revenue is presented for each of the data types so that the trends in varying the wind data resolution can be determined. Conclusions are made based on the accuracy and possible bias associated with low resolutions for estimating potential energy for a specific location. Findings illustrate the value of a time history, as compared to a more simplistic probabilistic analysis, to support conclusions about expected energy production and revenue generated.
Scour Zone Characterization by Deep Impinging Jet
Authors: Kebede Yulian A. , Gabr Mohammed A. , Kayser Mohammad F.
Published: September 2014

Scour downstream of many hydraulic structures such as culverts and spillways may be treated as analogues to jet scour. This study presents the characterization of a fluidized zone by an impinging jet using a recently developed In Situ Erosion Evaluation Probe (ISEEP). Jet embedment was varied from 0.61 m (2 ft) to 2.43 m (8 ft) in the laboratory with a jet velocity ranging from 3 m/s to 6 m/s using an external pump. A number of piezometers were installed radially and with depth around the probe to characterize the extent of the fluidized zone (zone where effective stress reaches zero). Results indicated that the shape of the embedded fluidized zone changes from spherical to elliptical with increasing impinging distance. The nature of the zone expansion was categorized into three phases: initial, transition, and steady state. A jet velocity of 3.8 m/s resulted in a lateral distance of the fluidization zone that extended 0.3 m from the probe. At jet velocities of 4.2 m/s and 4.5 m/s, the lateral distance of the fluidization zone reached about 0.40 m and 0.45 m, respectively. Vertically, a jet velocity of 4.2 m/s fluidized the soil up to 0.3 m above the jet (probe tip) at 2.4 m embedment depth. At an embedment depth of 2.4 m, this maximum fluidization zone occurs as a closed fluidization. The dimensions of this zone are a function of the applied jet velocity (considering the values used in this study).
Review of Low-Constraint Fracture Resistance Testing With SENT Specimens
Authors: Zhu Xian-Kui , McGaughy Tom , Orth Fabian , Jennings Jon
Published: September 2014

Fracture resistance is an important material property and characterized by a J-integral resistance curve (J-R curve) or a crack-tip opening displacement (CTOD) resistance curve. ASTM standard specimens with deep cracks are subject to bending dominant forces, leading to high crack-tip constraint conditions and conservative fracture resistance curves.

Actual cracks occurring in line pipes and welds are often shallow ones dominated by tensile forces, resulting in low constraint conditions. Shallow cracks have been shown to generate elevated fracture resistance curves in comparison to standard deep-crack results. To reduce the over-conservatism of standard resistance curves and to produce more realistic toughness properties to meet the needs of strain-based design approaches for pipelines, different procedures and technologies have been developed over the years to determine the low-constraint fracture resistance curves by use of the single edge-notched tension (SENT) specimens. This includes the multiple specimen method developed and standardized by DNV for J-R curve testing, the single specimen method developed by CanMet for J-R and CTOD-R curve testing, and the single-specimen method developed by ExxonMobil for CTOD-R curve testing. This paper delivers a technical review of existing fracture test methods using SENT specimens, and discusses the advantages and limitations of each method.
Nonlinear Drop Simulations of a Christmas Tree in Deep Waters
Authors: Majed Arya , Cooper Phil
Published: September 2014

This paper presents an investigation of the still water sink trajectories of an accidentally dropped Christmas Tree (XT). A 3D coupled nonlinear dynamics/hydrodynamics model of the XT is constructed and simulations executed from surface to seabed. XT initial water entry orientations are randomly varied in a Monte-Carlo simulation to predict the seabed impact zone. Still water simulation results are compared to simplified cone-angle type method predictions. The effect of XT flat panels on excursion mitigation is investigated. Finally, the influence of non-uniform current effects is studied.
A Probabilistic Approach to Fatigue Safety and Integrity of TTRs
Authors: Mousavi Mir Emad , Upadhye Sanjeev , Vijayaraghavan Vishnu , Haverty Kevin
Published: September 2014

Probabilistic methods can improve the reliability of fatigue damage evaluation in top tensioned (production) risers because they tend to provide less biased estimators on their safety, which can be used for more reliable decision making concerning their design. Such methods consider the collective impact of uncertainties in the riser system, which is not accurately assessed in conventional fatigue analysis. The large factors of safety that are commonly used in deterministic-based fatigue damage assessment tend to assure the high safety of the design, still they are generic factors that do not take advantage of available data for accurate quantification of system safety. This paper presents a probabilistic method toward fatigue reliability and integrity analysis of TTR systems. By using rules of probability, a simplified method is developed to estimate the probability of failure of the TTR system in its lifetime, considering the uncertainties with the Palmgren-Miner rule, the cyclic loads, and the fatigue strength of the components, and other analysis approximations. The method is then used for a comparative assessment on the fatigue reliability of the TTR components and calculating its fatigue Integrity Index. The method is illustrated in a case study and is used to provide recommendations that could possibly improve the TTR fatigue design by reducing its cost, increasing its safety, and maximizing its integrity.
Wave Drift Forces and Resonance Analysis on Two Ships Arranged Side by Side
Authors: Zhang Yu , Yu Dapeng , Fu Shixiao , Guo Fei , Wei Wei
Published: September 2014

In recent years, with the development of new ships and further utilization of marine resources, multi-body floating systems are widely used in practice. Compared with the single floating body, the movement in multi-body floating system is not only affected by the external environment, but the interaction between the bodies cannot be neglected. So analysis of hydrodynamic performance of a multi-body floating system is of great importance.

In this paper, a multi-body system consisting of two side-by-side ships is studied. The code AQWA® is used for its hydrodynamic performance analysis in frequency domain. Its hydrodynamic parameters are compared with those of the related single-ship system and the difference is obvious. The two-ship system shows a peak in motion different from single-ship system at some frequencies and its wave exciting forces have period effects. Also, negative values appear in added masses, which never occur for a single-body floating system. When the gap between the two ships is changed, there is a significant trend that the wave frequency of the peak value decreases with the gap size between the two ships.

In addition, this paper also discussed the length of wave and distance of ships ratio that the motion resonance usually happens. Through the analysis of this dimensionless parameter, a conclusion about resonance between two parallel ships is deducted.
Evacuation, Escape and Rescue EER Analysis of Offshore Plant Based on Human Behavior Model
Authors: Ha Sol , Ku Namkug , Roh Myung-Il , Li Xing
Published: September 2014

Escape, evacuation, and rescue (EER) analysis is one of vital roles to protect and save personnel in the event of a major accident on offshore oil and gas production platforms. However, most of EER analysis method for evaluating EER strategies of offshore plants relied on deterministic or probabilistic methods including several risk assessment tools. This study focused that all partial processes of EER analysis are similar to the things of passenger ship escape and evacuation: “Means of Escape”, “Simplified Evacuation Analysis”, and “Advanced Evacuation Analysis”. Since various methodologies and some authorized rules for EER plans and resources were already studied in the domain of passenger ship, this study tried to adopt them to offshore plant fields with their minor revisions. In the evacuation process of EER analysis, especially, a human behavior model was adopted to simulate the evacuation sequence in more detail. It was supposed that the human behavior model is based on the velocity-based model, and consists of individual, crowd, and emergency behaviors. As an example, the method is applied to some offshore plant projects and compared with the result of conventional EER analysis.
Study on Development of the Floating Pier for Damage Protection Control of Ship in Tsunami
Authors: Masuda Koichi , Masuda Mitsuhiro , Minami Kiyokazu , Ikoma Tomoki , Ohno Masato , Nakamura Fuminori
Published: September 2014

When the tsunami attacks in harbors, the vessels moored at the wharf may be unable to be evacuated. In such a case, damages such as, breaking the mooring tethers, grounding on a wharf, drift to land areas and the collision with buildings are assumed. And, after suffering a disaster, the base for supporting medical treatment and a life is necessary to disaster victims. In this research, the floating tsunami protection wharf (FTPW) is proposed. This present paper describes the tsunami simulation and the motion analysis of vessel moored at the FTPW using the MPS Method. In this research, the 3D-MPS method is applied. The applicability of the FTPW was examined. Moreover, the feasibility of the disaster victim support base on the FTPW deck was examined.
Some Considerations on TTR Wake Interference Analysis
Authors: Dai Wentao , Moorthy Dakshina , Xing Shutao
Published: September 2014

This study is an attempt to clarify some key issues in the top tension riser (TTR) wake interference analysis for offshore oil and gas risers. A wake model for strakes was developed from a series of test results conducted by a major operator, and analyses were done using this wake model to compare with analysis results using Blevins’ model, which is originally generated for bare pipes and has been commonly used in the industry for strakes. The effect of lift coefficient reduction factor was also studied and results are illustrated. Last, the effectiveness of removing strakes to improve interference performance was investigated, and results show that in some situation removing strakes could actually increase the total drag coefficients and drag forces.
The Identification of Power-In Region in Vortex-Induced Vibration of Flexible Cylinders
Authors: Rao Zhibiao , Resvanis Themistocles L. , Vandiver J. Kim
Published: September 2014

This paper proposes a novel method to identify the power-in regions of long flexible cylinders subjected to vortex-induced vibration (VIV). It also attempts to address a practical problem: “Will a secondary power-in region appear after the primary power-in region is covered with suppression devices?” The source of data is a recent model test on a 38 meter long flexible cylinder, densely instrumented with fiber optic strain gauges and accelerometers.

For pipes with partial coverage of suppression devices in uniform flow, the bare region would be expected to be a power-in region and the section with suppression devices is expected to be a power-out region. Experimental data from these types of tests are used to benchmark the proposed power-in zone identification method. The method is then used to identify the power-in zones on a bare cylinder in a sheared flow.

This paper also explores the occurrence of secondary power-in regions that may exist, when suppression devices are placed in the primary power-in zone. Secondary power-in regions were observed. Lessons learned from the power-in region identification in sheared flows will be a useful tool for designer/engineers choosing where to place suppression devices.
Effect of Geometric and Material Discontinuities on the Reeling of Pipelines
Authors: Liu Yafei , Kyriakides Stelios
Published: September 2014

Reeling remains one of the most efficient methods for installing pipelines offshore. The process results in plastic bending, straightening, and reverse bending to strain levels that can be as large as 2–3%. Thus, despite many years of practice, occasional failures during the reeling and unreeling process continue to take place resulting in costly disruptions and repairs. A common cause of such failures is local buckling that can precipitate fracture. This paper presents the results of a study of how discontinuities in geometry and mechanical properties can lead to buckling and failure. Large-scale nonlinear finite element models are used to simulate the reeling/unreeling of pipelines. The pipeline is modeled using shell elements and contact with the hub of the reel is treated appropriately. The elasto-plastic behavior of the steel is modeled using nonlinear kinematic hardening. Typically, a section of pipeline is taken through a winding and unwinding cycle on a reel of a given radius at a constant value of tension. Discontinuities in wall thickness and yield stress such as those that can occur at girth welds are shown to result in sharp local changes in curvature that extend over 3–4 pipe diameters accompanied by severe local straining and ovalization. The combination of these can lead to local buckling. Increase in the applied tension can reduce these at the expense of additional ovalization of the pipeline.
Pipeline Roll During Reel-Lay Installation due to Piggyback DEH Cable and Local Residual Curvature
Authors: Endal Geir , Rao Venu , Ragupathy P. , Sriskandarajah T.
Published: September 2014

Offshore pipelines may require a Direct Electric Heating (DEH) cable for auxiliary heating purposes which will be strapped as close to the pipe wall as possible at pre-determined intervals for efficient thermal performance particularly during start-up and shut-down conditions. The straps are provided with tension to maintain the orientation of the DEH cable intact. The positioning of DEH cable causes differential buoyancy and gravity loads and the associated positioning vectors giving a tendency to rotate the pipeline in the catenary.

If the pipeline inplace buckling design by residual curvature method for controlled thermal expansion during operation is adopted for the pipeline with a DEH cable, the additional imperfection in the pipeline will contribute to the pipeline’s tendency of rotation together with the imbalance in buoyancy and weight. This paper discusses the analytical approach used based on the Stiffened Catenary Calculations Method and Simplified Energy Balance Approach Method to determine the roll, the pipeline may experience to reach stable equilibrium in combination with the residual imperfection. The total work required to bend and twist the pipe from surface to seabed is estimated for twist angles at touch down between 0 and 180 degrees and in water depth of 360m.

The paper further discusses the numerical finite element analyses performed to estimate the amount of twist in the pipeline as the pipe is paid from the vessel. The analytical and numerical approaches support the view that the pipeline will have restoring moment to lay the pipeline with DEH cable in the sideways for pipe laid on seabed. Comparisons are made with the post lay test data from other projects to confirm the findings from the assessment are in agreement. The unlikely scenario of pipeline being laid with residual curvature section in a position other than in the horizontal plane and its effect on the lateral buckling behaviour on application of steady state temperature and pressure was also investigated and the results are presented in the paper.
Stochastic Analysis of a Nonlinear Energy Harvester Model
Authors: Spanos P. D. , Richichi A. , Arena F.
Published: September 2014

Floating oscillating-bodies are a kind of wave energy converter developed for harvesting the great amount of energy related to water waves (see Falcão [1] for a review).

Although the assumptions of small-wave and linear behavior of oscillating system are reasonable for most of the time during which a floating point harvester is in operation, nonlinear effects may be significant in extreme sea states situations.

In this paper a nonlinear dynamic analysis of a point harvester wave energy converter is conducted. The model involves a tightly moored single-body floating device; it captures motion in the horizontal and vertical directions. The stiffness and damping forces, being functions of the displacement and velocity components, make the system nonlinear and coupled.

For the input forces, the erratic nature of the waves is modeled by a stochastic process. Specifically, wind-generated waves are modeled by means of the JONSWAP spectrum.

The method of statistical linearization [2] is used to determine iteratively the effective linear stiffness and damping matrices and response statistics of the system and to proceed to conducting a dynamic analysis of the harvester model.

The reliability of the linearization based approach is demonstrated by comparison with time domain integration, Monte Carlo simulation, data. This approach offers the appealing feature of conducting efficiently a variety of parameter studies which can expedite preliminary evaluations, inter alia, of competing design scenarios for the energy converter in a stochastic environmental setting.
On the Aleatory and Epistemic Uncertainty of the Wave Resource Assessment in the North West Pacific
Authors: Kidoura Yusuke , Wada Ryota , Waseda Takuji
Published: September 2014

This paper presents the results of quantifying uncertainty associated with the wave resource assessment in the North West Pacific. This study classifies uncertainty into 2 types: aleatory uncertainty and epistemic uncertainty. We defined the source of aleatory uncertainty as inter-annual variability of wave power and the source of epistemic uncertainty as difference between a model and an observation. Spectral analysis of inter-annual variability and correlation coefficient analysis between inter-annual variability and climate index such as Arctic Oscillation are conducted to quantify aleatory uncertainty. The results revealed that aleatory uncertainty in North West Pacific differs from that in North Atlantic. Regarding the cause of inter-annual variability, seas surrounding Japan can be classified into 4 regions: the sea of Okhotsk and Eastern Japan with high correlation with the North Pacific Index, the Japan Sea with Arctic Oscillation and Southern Japan with the typhoon frequency. These results provide the characteristic of uncertainty in wave power assessment in the North West Pacific, and they are helpful to establish standards of wave resource assessment in Japan.
Design Aspects of Large Diameter Turret Systems: Part 1 — Moonpool Piston Mode
Authors: de Vries Igor , Molin Bernard , Barbagallo Alexandre , Cinello Alexandre , Mamoun Naciri
Published: September 2014

The maximum displacement of turret moored floating production and storage units (FPSO, FLNG) has been steadily increasing over the past decades and is likely to increase even further in the future. Furthermore, some of these turret moored units are designed to keep position in even the harshest of environments (e.g. 10,000-year storms) and very deep water (e.g. more than 2000m depth). As a consequence, the demands on the capacity of mooring systems are also increasing and larger turret sizes with relatively smaller chaintable openings result. The turret cylinder is neither completely open (due to strength considerations) nor completely closed (due to installation and inspection requirements). Therefore, the mass of water inside the turret will to a degree be forced to move with the turret, but also to a degree be able to enter and exit the turret. As a result, a combination of inertial, hydrostatic, and piston mode effects similar to those found in open moonpools can occur. An additional complicating factor is that the turret is surrounded by an open annular space which influences the flow of water around the turret bottom. Both piston mode and inertia of water inside the turret will impose additional loads on the turret and its support system. It is therefore imperative that these loads be taken into account in the design phase. A number of tools, both experimental and computational may be used to obtain estimates of loads. However, whichever method is chosen, a necessary step is to identify the natural periods of the piston mode taking place in the turret. Investigations, whether numerical or experimental, must obviously focus on these periods. In this paper, analytical, computational (CFD) and experimental methods to obtain natural piston mode periods of turret moonpools are described. The methods are applied to simplified, yet realistic, turret models. Results from these different methods are compared. Respective figures of merit are discussed.
Nonlinear Wave Amplification Around Column-Based Platforms in Steep Waves
Authors: Stansberg Carl Trygve
Published: September 2014

The wave diffraction and upwelling around vertical columns such as platform legs and offshore wind turbine foundations is studied. Nonlinear contributions in steep wave conditions are particularly addressed. Data and analyses from a previous experiment with fixed cylinders in deep water regular and random waves are reviewed, and essential findings are highlighted. Measured extreme crests are clearly higher than linear predictions, connected with an increase in basic harmonics as well as with second-order components. Improved results are obtained with fully second-order modeling, while this is still incomplete, and such analyses can be computationally demanding as well as non-robust.

An alternative, simplified empirical nonlinear correction approach based on these findings, previously suggested for engineering design prediction of extreme crests in random waves, is validated against a re-analysis of the fixed column data. In addition, validation is also made against model test data for two floating production semis. The method agrees well with the data, taking into account the simplicity of the method and the complexity of the problem, and is a tool for early design phases. Detailed studies will still require model tests and CFD development.
A Comparative Study of Nonlinear Shallow-Water Wave Loads on a Submerged Horizontal Box
Authors: Hayatdavoodi Masoud , Ertekin R. Cengiz
Published: September 2014

This paper is concerned with calculations of the two-dimensional nonlinear vertical and horizontal forces and overturning moment due to the unsteady flow of an inviscid, incompressible fluid over a fully-submerged horizontal, fixed box. The problem is approached on the basis of the Level I Green-Naghdi (GN) theory of shallow-water waves. The main objective of this paper is to present a comparison of the solitary and cnoidal wave loads calculated by use of the GN equations, with those computed by Euler’s equations and the recent laboratory measurements, and also with a linear solution of the problem for small-amplitude waves. The results show a remarkable similarity between the GN and Euler’s models and the laboratory measurements. In particular, the calculations predict that the thickness of the box has no effect on the vertical forces and only a slight influence on the two-dimensional horizontal positive force. The calculations also predict that viscosity of the fluid has a small effect on these loads. The results have applications to various physical problems such as wave forces on submerged coastal bridges and submerged breakwaters.
Experimental and Numerical Investigation of Ship Roll Damping With and Without Bilge Keels
Authors: Kristiansen Trygve , Ruth Eivind , Firoozkoohi Reza , Borgen Henning , Berge Bjørn Ola
Published: September 2014

This paper describes an experimental and numerical study on the effect of bilge keels on a specific offshore vessel. Forced roll and free decay experiments were carried out at MARINTEK in a joint project between VARD, DNV GL and MARINTEK during Nov. 2012. The 1:25 ship model was tested both with and without bilge keels. 19 forced roll periods were tested at 4 roll angles (2, 4, 6 and 10deg). Free decay tests were performed for initial roll angles 2, 4, 6, 10 and 15deg. Damping was extracted and compared with numerical simulations from a potential flow strip theory program (VERES) and a commercial CFD tool (StarCCM+).

Both qualitative and quantitative agreement between the experimentally and numerically obtained damping parameters was documented. Discrepancies were identified and discussed. The three different methods of investigation gave together a basis for discussing the effect of bilge keels. From the present study, it is recommended that both experimental and numerical work in combination, for both forced roll and free decay tests, should be performed to assess the damping efficiency of bilge keels.
Numerical Analysis of Second-Order Wave Loads on Large-Volume Marine Structures in a Current
Authors: Shao Yan-Lin , Helmers Jens Bloch
Published: September 2014

A time-domain Higher-Order Boundary Element Method (HOBEM) based on cubic shape functions for second-order wave-current-body interaction developed by Shao & Faltinsen [1] is further refined by investigating the feasibility of adopting the unstructured meshes on the free surface and body surfaces from an open source mesh generator [2]. When the steady local flow effect is considered in the time-domain boundary-value-problem formulation, the advection terms in the free surface are part of the sources of numerical instability. In this paper, the advection terms are taken care of in an implicit way in a 4th order Runge-Kutta scheme with much better stability. Some numerical examples extensively studied in the literature are studied in order to validate the present numerical model.
On the Torsional Friction of Freely Rotating Short Fairing
Authors: Lie Halvor , Yin Decao , Liapis Stergios
Published: September 2014

Vortex-induced vibrations (VIV) model tests were carried out by Shell on a 38 m flexible pipe at the MARINTEK Ocean Basin in March 2011, Trondheim. As part of the model test campaign, the pipe with short fairings under two different torsional friction conditions between riser and fairings was tested in uniform currents. Test with ‘high’ friction show small responses, tests with ‘low’ friction show distinct large responses, even though, the spectra analysis shows that the responses seem to be dominated by vortex induced forces.

Decay tests were carried out on a riser section with fairings to further investigate the torsional friction effect. Friction coefficients were calculated by ‘Coulomb damping model’, which is proved to be a suitable model for fairing-riser friction. The friction coefficients could be directly used to determine the damping ratio and implemented in numerical models. Such decay tests are especially useful to design, manufacture and install fairings in order to achieve a proper friction level and avoid unstable responses such as galloping.