bluuebunny/crossref_metadata_2025 huggingface dataset:
The Development of MOPSO-Based Dynamic Routing Algorithm for the Inspection of Autonomous Underwater Vehicle
Authors: Lin Yu-Hsien , Huang Lin-Chin , Chen Shao-Yu
Published: October 2017

In this study, the authors developed the dynamic routing algorithm combining an image detection technique to support the optimal route plan of Autonomous Underwater Vehicle (AUV) inspecting an offshore wind farm affected by ocean currents. A modular structure is applied to program design by the graphical language, LabVIEW (Laboratory Virtual Instrument Engineering Workbench). The modular structure is composed of 6-DOF (Six Degrees-of-Freedom) motion module, a self-tuning fuzzy control module, a stereo-vision detection module, and a dynamic routing module. In terms of path planning for inspection, several Pareto frontiers are solved iteratively according to two objectives, namely, cruise time and energy consumption. Performances obtained from MOPSO (Multi-Objective Particle Swarm Optimization) -based dynamic routing algorithm would be in comparison with those from SOPSO (Single-Objective Particle Swarm Optimization) -based dynamic routing algorithm. In addition, selections of fixed weight and dynamic weight of MOPSO-based dynamic routing algorithms would be discussed in the environment with or without ocean currents. Eventually, the image inspection mode is not only beneficial for optimizing feasible routes but it can also identify features of obstacles for positioning.
Optimizing the Design of Unbonded Flexible Pipelines With More Realistic Predictions of pH and H2S Content in the Annulus
Authors: Ke Li , Taravel-Condat Carol , Kittel Jean , Mingant Rémy , Duret-Thual Claude , Querez Virginie
Published: October 2017

Due to its high metallic confinement, the annulus of unbonded flexible pipelines is a specific and mild corrosive medium for carbon steel armour wires. This environment presents high supersaturation levels of dissolved iron, leading to pH values far above thermodynamic equilibrium. Furthermore, the permeation of acidic gases (such as CO2 and H2S) through the polymer pressure sheath occurs at very low flow rates. Since the annulus is supersaturated with dissolved iron, part of the H2S is consumed as it slowly arrives into the annulus. Therefore, the annular medium contains low levels of gas far below those predicted by standard thermodynamic models, and less H2S is available to trigger sour cracking.

The recent development of harsher oil field conditions (higher water depths, increased CO2 content, presence of H2S...) induced the need to refine the design of flexible pipes to propose more cost effective solutions. As pH and H2S content are key parameters for the selection of steel grades, taking into account the supersaturation and the H2S consumption in the annulus allows major optimization of flexible pipes by using for instance steel grades with higher strength. Therefore, extensive experimental work was conducted over the past years to better characterize the annulus and predict more realistic pH and H2S levels. In this paper, the following developments are presented:

– A kinetic corrosion model named FlexCor was derived from numerous corrosion tests done at various CO2 pressures in confined configuration, with in-situ pH monitoring. These tests were performed over long durations (3 months) in order to capture the effective long term supersaturated pH. The kinetic model is able to simulate the transient and stationary phases of the supersaturated pH evolution up to 45 bara of CO2, providing a good fit with the experimental data. The tests also demonstrated that the annulus environment remains supersaturated even at high CO2 partial pressures.

– A methodology taking into account the H2S consumption was developed based on extensive long-term small scale and full scale testing (> 2 years), where low flow rates of H2S were imposed. The experimental results show that H2S consumption is far from negligible, even when the annulus is not fully flooded. This H2S consumption methodology was certified by an Independent Verification Agency and is now being applied on commercial projects.
3D Path Following and Tracking for an Inspection Class ROV
Authors: Arnesen Bent O. , Lekkas Anastasios M. , Schjølberg Ingrid
Published: October 2017

This paper presents 3D path tracking simulations and path-following by experimental testing with a small inspection class ROV. The ROV is underactuated, but moves freely in surge, heave and yaw (4 DOF). Paths used are a spiral for path-tracking and a lawnmower pattern for path-following, where both paths are related to a proposed structural setting and a mission. The structure is a cylindrical fishing cage used for fish farming in the aquaculture, and the task is to perform inspection with equipped cameras and sensors while tracking or following the path.
The Effect of Turbulence Model on the Response of a Large Floating Wind Turbine
Authors: Eliassen Lene , Bachynski Erin E.
Published: October 2017

The wind turbine design standards advise choosing one of two recommended turbulence models for load simulations of offshore wind turbines. The difference in fatigue loads for the two turbulence models is relatively small for bottom-fixed wind turbines, but some floating wind turbines show a higher sensitivity to the chosen turbulence model. In this study, the motions and mooring line fatigue damage of two semi-submersible floating wind turbines are investigated for three different wind speeds: 8 m/s, 14 m/s and 20 m/s, and three different wave states for each wind speed. For both concepts, the CSC 5 MW and the CSC 10 MW, the low-frequency surge response is important for the mooring line tension, and the simulations using the Kaimal turbulence model give the largest variation in tension at the surge eigenfrequency. However, using the Mann turbulence model in the load simulations give a higher response in the range of the blade passing frequency (3P). The CSC 10 MW has a higher aerodynamic thrust relative to the CSC 5 MW, and will therefore have a larger surge response at the lower frequencies than the CSC 5 MW. At the lowest wind speed, where the variation in mooring line tension at surge eigenfrequency is high, the fatigue damage is larger if the Kaimal turbulence model is applied to the load simulations. However, at the highest wind speed, using the Mann turbulence model in the simulations, give a higher mooring line fatigue damage.
Subsea Pipeline UHB OOS Design: Structural Reliability Analysis
Authors: Liu M. , Cross C.
Published: October 2017

Upheaval buckling (UHB) is a major design concern for a trenched and buried subsea pipeline operating at high temperature and pressure. A predictive assessment is necessary during the detailed engineering design and optimisation to evaluate and define any measure that may be utilised for UHB mitigation such as deep trenching, backfilling, blanket or spot rockdumping. A pre-emptive UHB structural reliability analysis (SRA) has to be performed prior to pipeline installation based on the typical trench imperfection out of straightness (OOS) statistics. The SRA results are updated once survey data is made available. A rockdump schedule can be established by incorporating appropriate safety or load factors to address uncertainties in the design parameters and as-built OOS survey measurement accuracy.

This paper examines the basis for processing the OOS features from survey data and stochastic distributions assumed for SRA with a view to improving the SRA OOS analysis. A number of OOS issues are considered. To cut conservatism an alternative distribution and interpretation is proposed for the key SRA input parameters with regards to imperfections and survey resolution. The random imperfection height assumption used in the current SRA practice for UHB is thus challenged — the rationale and argument for an alternative approach are constructed through a review of stochastic process theory, additional integrity criteria, a parametric analysis and evaluation of multiple OOS survey data sets. To add to the strength of the argument, a range of engineering issues are discussed in the context of stochastic distributions of imperfections. A worked example and case study is presented leading to a rationally reduced load factor and rockdump volume requirement for OOS UHB mitigation and protection.
Simplified Hydrodynamic Design Procedure of a Submerged Floating Tube Bridge Across the Digernessund of Norway
Authors: Xiang Xu , Minoretti Arianna , Eidem Mathias Egeland , Belsvik Kjell Håvard , Aasland Tale Egeberg , Vodolazkin Mikhail
Published: October 2017

The paper will look into the hydrodynamic loads and responses on the proposed Submerged Floating Tube Bridge (SFTB) through the Digernessund by the Norwegian Public Roads Administration (Statens vegvesen, NPRA). The aim is to show how different hydrodynamics aspects during the prelimiary design can be simply addressed under the given environmental conditions. Different SFTB systems are introduced as the first step. A simplified method based on modal analysis is introduced and implemented for evaluation of the motions and stress, bending moments along the bridge. Firstly, a 2D Boundary Element Method (BEM) solver is developed and verified, which is further used for solving the hydrodynamics coefficients of different bridge cross sections. The 3D hydrodynamic coefficients of pontoons are solved by the commercial software AQWA. The analysis procedure of the simplified method for the global SFTB responses is presented. The Eigen periods of the Bjørnefjord SFTB is re-calculated by the present model as a first validation of the implementation. The loads and responses of the bridge under given wave conditions are then estimated. The evaluation of the possibility of vortex induced vibrations of the current SFTB design is given.
A Numerical Method for Representing Retardation Functions With Complex Exponentials
Authors: Liu Fushun , Jin Lei , Chen Jiefeng , Li Wei
Published: October 2017

Numerical time- or frequency-domain techniques can be used to analyze motion responses of a floating structure in waves. Time-domain simulations of a linear transient or nonlinear system usually involve a convolution terms and are computationally demanding, and frequency-domain models are usually limited to steady-state responses. Recent research efforts have focused on improving model efficiency by approximating and replacing the convolution term in the time domain simulation. Contrary to existed techniques, this paper will utilize and extend a more novel method to the frequency response estimation of floating structures. This approach represents the convolution terms, which are associated with fluid memory effects, with a series of poles and corresponding residues in Laplace domain, based on the estimated frequency-dependent added mass and damping of the structure. The advantage of this approach is that the frequency-dependent motion equations in the time domain can then be transformed into Laplace domain without requiring Laplace-domain expressions of the added mass and damping. Two examples are employed to investigate the approach: The first is an analytical added mass and damping, which satisfies all the properties of convolution terms in time and frequency domains simultaneously. This demonstrates the accuracy of the new form of the retardation functions; secondly, a numerical six degrees of freedom model is employed to study its application to estimate the response of a floating structure. The key conclusions are: (1) the proposed pole-residue form can be used to consider the fluid memory effects; and (2) responses are in good agreement with traditional frequency-domain techniques.
Prediction of Calcium Carbonate Scaling in Pipes Using Artificial Neural Networks
Authors: Paz Paulo A. , Caprace Jean-David , Cajaiba João F. , Netto Theodoro A.
Published: October 2017

Scaling problems in oil production happen frequently. There are chemical and mechanical treatments to fight with this problem. Knowing if they will happen under particular circumstances of temperature and pressure is the best way to avoid or to control their apparition. This paper shows the most important characteristics that improve the calcium carbonate scaling accumulation in pipelines in order to understand how kinetic and thermodynamic characteristics affects the scaling process. Monitoring the scaling process using the Saturation Index was considered a method for scaling solutions, this method uses characteristics as solution pH, temperature, ion concentrations, among others. The influence that temperature and initial ion concentration have over the solution pH of the scaling system was studied using an experimental test in a batch reactor, this allows to follow the scaling process in real time through pH changes. The prediction of calcium carbonate scaling process represents another objective in this work. Numerical analysis based on artificial intelligence as Multi Layer Perceptron and Probabilistic Neural Network were used. A good learning process and a good prediction model for the ANN methods was shown when the experimental SI and predicted SI were compared. It was possible to confirm the accuracy of the ANN methods using external experimental tests.
Pipeline Regulations in the Norwegian Petroleum Industry: Experiences, Follow-Up and Statistical Summaries of Incidents
Authors: Sundby Trond , Anfinsen Kjell Arild
Published: October 2017

The Petroleum Safety Authority Norway (PSA) is an independent government regulator responsible for safety, emergency preparedness and the working environment in the Norwegian petroleum industry.

Norway’s regulations for petroleum operations offshore and on land are risk-based, and give great emphasis to principles for reducing health, safety and environmental (HSE) risk. They have been developed over more than 40 years, changing from detailed prescriptive regulations in the early days to the present requirements, which are largely formulated in performance-based (functional) terms. They specify requirements for the various aspects, characteristics or qualities which a product, process or service must possess.

In our regulations there are requirements for reporting incidents and specifically to report on damages to load-bearing structures and pipeline systems. About 2900 incidents with risers and pipelines were reported on the Norwegian Continental Shelf (NCS) from 1975 to January 2017. Of these, 2/3 related to pipelines and 1/3 to risers, and they were of varying degrees of severity. The incidents with highest risk for personnel concerned risers and more than 80 reported major incidents involving flexible risers occurred in the period from 2000–2016.

This paper will briefly summarise the development of the regulations and give examples from the pipeline regulations and the way the PSA follows up the industry today. We will also present statistical data of incidents related to risers. Since the start of the trends in risk level project (RNNP) on the Norwegian Continental Shelf (NCS) in 1999–2000 there has been 84 reported riser incidents (classified as major in the PSA RNNP-project) where most of those incidents are related to flexibles. There has been no major accident related to risers on the NCS in the same period and very few leaks. 8 of 11 reported leaks for pipelines and risers within the safety zone has been from flexible risers, with relatively low leak rates.
Three Dimensional Velocity Field Underneath a Breaking Rogue Wave
Authors: Alberello Alberto , Pakodzi Csaba , Nelli Filippo , Bitner-Gregersen Elzbieta M. , Toffoli Alessandro
Published: October 2017

Wave breaking has large impact on stresses and loading on marine structures, but it is not yet accounted for in the design process. A numerical investigation is here presented to fully assess the three-dimensional velocity field underneath a breaking wave. The breaking onset is achieved by modulating an initial monochromatic wave with infinitesimal sideband perturbations. The latter triggers a nonlinear energy transfer, which allows one individual waves to grow and break once the steepness has overcome a specific threshold. Numerical simulations of the Navier-Stokes equations are carried out by means of the open source CFD code OpenFOAM. To speed up the simulation process, the nonlinear evolution of the perturbed Stokes wave is first obtained with a High-Order Spectral Method (HOSM) until the onset of breaking; surface elevation and velocity field are then transferred to the CFD for the final stage of the breaking process. The fully three-dimensional turbulent kinematic field is presented and discussed with reference to the velocity field predicted by the theory.
A UAV SAR Prototype for Marine and Arctic Application
Authors: Li Wei , Zhang Houxiang , Osen Ottar L.
Published: October 2017

SAR (Synthetic Aperture Radar ) systems are special types of radar that produce high resolution images (comparable to optical sensors) in all weather conditions, night and day. SAR sensors have many applications in marine and arctic applications. In this paper a compact SAR prototype system is developed for UAV (Unmanned Aerial Vehicle) platform. The radar is based on FMCW (Frequency-Modulated Continuous-Wave) radar mode. The system integrates a high performance RTK (Real Time Kinematic) GPS and IMU (inertial measurement unit) based motion compensation module, FPGA (Field Programmable Gate Array) based controller and signal processing module. It has a resolution of 0.3 meter with the weight below 2 kg. It has been test and verified on the guide rail, car and integrated on a rotary UAV. The system will extend the capability of UAV in the marine and arctic remote sensing area.
Influence of Second-Order Difference-Frequency Wave Loads on the Floating Wind-Wave Hybrid Platform
Authors: Lee Hyebin , Bae Yoon Hyeok , Kim Kyong-Hwan , Park Sewan , Hong Keyyong
Published: October 2017

A wind-wave hybrid power generation system is a floating offshore energy platform which is equipped with a number of wind turbines and wave energy converters (WECs) to harvest energy from various resources. This wind-wave hybrid platform is moored by eight catenary lines to keep its position against wind-wave-current environment. In most cases, the resonant frequency of horizontal motion of moored platform is very low, so a resonance is hardly seen by numerical simulation with linear wave assumptions. However, the incident waves with different frequency components are accompanied by sum and difference frequency loads due to the nonlinearity of the waves. Typically, the magnitude of the second-order wave loads are small and negligible, but once the second-order wave loads excite the platform at its natural frequency, the resonance can take place, which results in adverse effects on the platform. In this paper, the second-order difference frequency wave load on the wind-wave hybrid platform is numerically assessed and time domain simulation by coupled platform-mooring dynamic analysis is carried out. As a result, the horizontal motions of the platform was highly excited and the increased motions led higher top tension of the mooring lines compared with the case of linear wave environment. Especially, the combination of the wind and wave loads excited the horizontal motions more and made the mooring top tension far higher than wave load was only applied. With regards to the second-order difference frequency wave load, the result with the Quadratic Transfer Function (QTF) is compared to the one with Newman’s approximation. As the simulation results between them was insignificant, the Newman’s approximation can be used instead of the complete QTF to reduce the computational effort.
Numerical Investigations of the Extraction of Submerged Foundations by Coupled CFD-DEM
Authors: Kanitz Manuela , Grabe Juergen , Hager Alice , Goniva Christoph , Kloss Christoph
Published: October 2017

Offshore structures are founded on submerged foundations. The excavation of submerged foundations in the sea bed is a difficult task to accomplish when it comes to the decommissioning of these offshore structures. The extraction resistance is a lot higher than the pressure acting on the structure due to hydrostatic pressure, earth pressure and its self-weight. Once the extraction begins, a negative pore water pressure is created until inflowing pore water compensates this negative pore water pressure. This depression is hindering the extraction of the submerged foundation. Additionally, the resistance is dependent on the embedment depth of the structure, the soil properties as well as the extraction velocity, which influences the dimension of the negative pore water pressure. The numerical investigation of this dynamic problem is a limitation for continuum based approaches like the Finite Element Method (FEM) due to the occurring large deformations. These results from the soil bed failing under the movement of the structure and hence starting to flow. Additionally, in order to estimate the created depression, the investigation of the water-soil-interaction is crucial, as the change of the pore water pressure plays a significant role. Therefore, it is necessary to analyze the behavior of the soil particles and the pore water pressure. In order to do this, a coupled Euler-Lagrange approach, namely the combination of Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM), is used. In these simulations on one hand, the liquid phase, e.g. the water, is considered as a continuum, while on the other hand, for the solid phase, e.g. the soil, a particle representation is chosen. Hence, it is possible to compute the particle-particle — as well as the fluid-particle-interactions. The calculations are carried out with the open source software package CFDEMcoupling®, which combines the discrete element code LIGGGHTS® with CFD solvers based on OpenFOAM®. This paper introduces the coupled CFD-DEM approach to simulate the extraction of a submerged plate in the soil bed. In this work, the soil grains are idealized by spherical particles of different diameters. In order to consider effects of dilatancy and contractancy in the soil bed, different relative densities are investigated. Additionally, a variation of the extraction velocity of the plate is carried out to examine the dependence on the creation of negative pore water pressure. For each case, the extraction resistance is calculated. The flow velocity and the pressure distribution in the vicinity of the structure are analyzed.
Comparison of Field Measurement and Numerical Simulation of the T-Shaped Barge Motions During the Topside Floatover Installation
Authors: Yu Wentai , Li Huailiang , Zhu Shaohua , Wang Alan M. , Xie Peng , Luo Hanbing
Published: October 2017

This paper presents the comparison of field measurement and numerical simulations of the barge motions in the successful floatover installation of one mega topside in Bohai Bay, China.

The motion measurements of the T-shaped floatover barge HYSY228 and the environmental field measurements were conducted to guide the entire floatover operation. The motion monitoring system can provide up to centimeter-level accuracy to monitor the six-degree-of-freedom barge motions. The environmental monitoring systems include one buoy system measuring the wave height, period and direction, an Acoustic Doppler Current Profilers (ADCP) for the profile current speed and direction, an anemometer, and a tidal gauge.

Numerical simulation is carried out for the barge motions in the standby stage of floatover installation in mooring condition. The motions of the barge and mooring force responses are obtained by the commercial code SESAM using the measured environmental data. Two different methods, Newman’s approximation and Second-order Transfer Function (QTF), are used to predict the second order wave force. The wave component (high frequency) and the second order component (low frequency) are discussed. Statistical results are compared with those from the field measurement. Fair well agreement is achieved.
A Scaling Model for Droplet Characteristics in a Spray Cloud Arising From Wave Interactions With Marine Objects
Authors: Bodaghkhani Armin , Muzychka Yuri S. , Colbourne Bruce
Published: October 2017

The objective of this study was to develop a model for predicting spray droplet size and velocity distributions during spray cloud formation arising from a wave impact with an object. The study looked at scaling issues and developed a scaling model to relate the spray characteristics measured in a tow tank to large-scale spray formations arising from wave impact with vessels. Several phenomena related to spray scaling have been studied to develop the scaling rules in large enclosures. These are wave theories for deep water, air entrainment process during the wave impact, water sheet disintegration and droplet size distribution as well as the scaling of two-phase flow interfaces (water/air). The focus of this study was on atomization and particle motions, and the thermodynamic part of scaling was ignored. The formation of upstream droplets caused by a wave impact on the bow of a vessel is the result of sheet and droplet breakup. Scaling models related to the process of air entrainment, which is caused by the wave impact, water sheet breakup, and spray cloud formation, were investigated to implement a comprehensive scaling model. A mathematical formulation, considering the aforementioned phenomena, was developed to calculate the final average droplet diameter and maximum run-up velocity. The effects of initial wave characteristics, the geometrical characteristics of the water sheet at the moment of water impact, and a spray parameter, on the final average droplet diameter were investigated. Predictions of wave characteristics and final droplet diameter are compared with previously published field observation data.
Application of a Hybrid Boussinesq-Panel Model for Motion Predictions of a Moored Sevan-Floater in Finite Water Depth
Authors: You Jikun , Glomnes Einar Bernt
Published: October 2017

This paper presents the applications of an efficient hybrid time-domain simulation model for predicting moored Sevan-floater motions in irregular waves and finite water depth. The irregular incident waves are modeled by the extended Boussinesq equations, which can capture wave-wave interactions and the low-frequency long waves accurately in finite and shallow water depth. By imposing the incident wave kinematics on the surface of the floater, a panel model based on Rankine source method is applied for the calculation of wave forces and corresponding floater motions. The contributions from low-frequency components in incident waves as well as their diffraction effects are included in the wave force calculations. Validation of the irregular waves simulated by the present numerical model are performed against experimental data. Then, the simulated moored floater motions are compared with model test results and results based on Newman’s approximation. The general good agreements with experimental results demonstrate the present model can be used as an alternative for this problem while Newman’s approximation shows non-conservative results.
Numerical Modeling of the Hydrodynamic Performance of Hydrofoils for Auxiliary Propulsion of Ships in Regular Head-Waves
Authors: Ahmed Tamer M. , Welaya Yousri , Abdulmotaleb Serag M.
Published: October 2017

In this paper, a detailed hydrodynamic analysis of the thrust generated by an oscillating hydrofoil is presented. The hydrofoil is mounted at the bow of a platform supply vessel as a means of auxiliary propulsion and, is vertically oscillating due to the ship’s heave and pitch motions in head-waves. Firstly, responses of the ship without hydrofoil are obtained using a 3-dimensional frequency-domain panel method. The responses of the ship with hydrofoil are then obtained by taking into account the extra forces and moments due to the presence of the foil. Secondly, three methods are used to calculate the average generated thrust, namely: a quasi-static method, a simplified frequency-domain method and, a dynamic stall method. All three methods are compared and appraised against numerical/experimental data available in the literature, for a similar hull form.
Interference of Top Tensioned Risers for Tension Leg Platforms
Authors: Zhang Hanqing , Smith Derek
Published: October 2017

Dry tree top-tensioned risers (TTRs) are widely used on floating production systems such as TLPs and Spars for drilling, completion, workover and production. The interference between neighboring TTRs is an important consideration which has a direct impact on the total TTR payload budget and the wellbay size for floater sizing and cost. Since the realistic sizing of a floater is essential towards the concept selection process for a field development, TTR interference should be addressed at the early stages of an offshore oilfield development. If the floater is a tension leg platform (TLP) and the field has strong current with associated extreme waves, riser interference may be very challenging and can have direct impact on riser design and the sizing and layout of the TLP. The waves and the oscillating motions of the TLP will have effects on riser interference. The oscillating motion of the TLP can excite the vibrational motion of the risers, and the wave-induced velocity of water particles and the motions of the risers with the movement of the TLP increases the relative flow acting on each riser. The combined effects will increase the deflection of the risers and thus the likelihood of riser interference. The industry has not seen an acceptable interference analysis approach yet which can account for the combined effects of current, waves, and TLP motions.

This paper proposes two engineering approaches for the interference analysis of top tensioned risers for tension leg platforms with the combined effects of current, surface waves, and associated floater motions being addressed.
Benefits and Deep Water Install Ability Challenges of Residual Curvature Method for Lateral Buckling Mitigation
Authors: Karjadi Erwan , Cooper Phil , Smienk Henk , Kortekaas Ferry
Published: October 2017

One way to control lateral buckling in the operation phase for High Pressure High Temperature (HPHT) pipelines is by deliberately introducing residual curvature sections at intervals along the pipeline by adjusting the straightener settings of the pipelay tower, as described in a patent held by Statoil [1]. This method has been applied with reel-lay installation for a number of shallow water pipelines in Europe (Statoil’s Skuld project and Total’s Edradour project).

The paper presents the benefits as well as the feasibility of the use of Residual Curvature Method (RCM) to control lateral buckling for deep water applications which involves high top tension in the overbend and high pressure and twist of the RC section in the sagbend. The study cases consider the application of the method for pipelines in 1850m water depth which are pushing the pipe top tension close to the limit of the capacity of the tensioners of Heerema Marine Contractor’s (HMC) Reel-lay vessel the Aegir. There are some challenges of the application of the residual curve method for deep water pipelines. Due to high top tension, some potential issues are investigated during lowering of the curved section from the straightener, passing the tensioners and through the J-lay tower into the water to the seabed. Detailed analyses have been performed to check the interaction of the residual curved pipe section against the tensioners (the effect of the squeeze load on the RC section) and to assess the maximum bending moment generated when the residual curved section is under high top tension below the tensioners against the Load Controlled Condition (LCC) for local buckling bending moment limit. Another consideration is the increase of hydrostatic pressure in deep water which could limit the allowable bending moment in the sagbend when lowering the curved sections to the seabed. Discussions are presented to the feasibility of the concept including the proposed ways of mitigation for the aforementioned potential issues.

The paper will also show an improved prediction of pipe twist/roll by comparing a published analytical 2D plane solution against the 3D FEA model prediction. The improved prediction, which considers the out of plane bending component of the pipe catenary, results in an increase of pipe twist in the sagbend section. This reduces the bending moment in the residual curved section when entering the sagbend and increases the probability to roll the curved section over to the horizontal plane on the seabed.
Wave and Current Generation in Wave Flumes Using Axial-Flow Pumps
Authors: Babarit Aurélien , Delvoye Simon , Arnal Vincent , Davoust Laurent , Wackers Jeroen , Bonnefoy Félicien
Published: October 2017

We investigate a new concept for wave and current generation. It consists of axial-flow pumps driven such as to generate an oscillatory flow through an orifice located at one end of the flume. Oscillations of the flow lead to the generation of water waves at the free surface. If the average of the flow is different from zero, a current is generated that superposes on the waves. In this study, we explored the technical capabilities of this concept and the influence of geometric parameters on wave and current generation. We used numerical and experimental modelling. Most noticeably, the numerical results indicate that this concept is well suited for the generation of long and high waves. An experimental setup has been designed and built. We used it to make an experimental proof of concept for the wave and current generation, including waves propagating against the current.
Optimising Foundation Skirt Geometries for Reliable Foundation Capacity and Installation
Authors: Bransby M. F. , O’Driscoll D. , Zhu H. , Randolph M. F. , Drummen T.
Published: October 2017

Increasing numbers of subsea structures related to wells and pipelines are being placed on the seabed as part of typical subsea or tie-back developments. Given the proliferation of these structures and the marginal cost of offshore developments, controlling installation and fabrication costs for subsea structures can be key to project viability. Skirted mudmats are often the most cost-effective foundation type, and particular additional design focuses on optimising their cost by minimising foundation weight and installation time.

Subsea foundations must be designed to withstand all applied loads during their design life (e.g. during set-down, tie-in, hydrotest, operation etc.) with suitable reliability. Using skirts, peripheral or internal, to improve the sliding resistance is an efficient solution provided the self-weight of the subsea structure on set-down is sufficiently large to ensure installation of the skirts (even for the strongest likely seabed conditions), but can lead to significant cost increases if additional ballast is required to ensure this.

The paper examines how foundation skirt geometries can be optimised in order to provide sufficient foundation in-place capacity whilst minimising the amount of self-weight required for their installation. Parametric studies are presented that show how the sliding capacity of individual skirts is affected by the weight of the structure, and also the spacing and position within the foundation plan.
Prototype Reynolds Number VIV Tests on a Full-Scale Rigid Riser
Authors: Yin Decao , Lie Halvor , Baarholm Rolf J.
Published: October 2017

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 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 prototype Re regime.

In 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 cross-flow (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.
Study on the Remaining Fatigue Life of FPSO Based on Spectral Analysis
Authors: Yu Lei , Ren Huilong , Liu Xudong , Sun Xiaoxiong , Peng Yakang
Published: October 2017

The fatigue failure, as one of the traditional failure modes of ship hull structures [1], has been widely concerned in recent years. For FPSO converted from large oil tankers, it is critical to predict and extend their service life. The analysis has been finished in compliance with the fatigue damage calculation procedure on the basis of spectral method. A three dimensional model has been performed to represent the entire hull structure. The Hot-Spot Stress Approach is employed to determine the stress transfer function for a location where the fatigue strength is to be evaluated. The fatigue damage resulting from combining the damage from each of the short-term conditions can be accomplished by the use of a weighted liner summation technique. The remaining fatigue life of the FPSO is calculated by the method of spectral analysis to determine the fatigue damage of the oil tanker during the operation period and the FPSO working period respectively. According to the results, the inspection and maintenance of hull structures can be effectively carried out.
On the Comparison of the Dynamic Response of an Offshore Floating VAWT System When Adopting Two Different Mooring System Model of Dynamics: Quasi-Static vs Lumped Mass Approach
Authors: Cevasco Debora , Collu Maurizio , Hall Matthew , Rizzo Cesare M.
Published: October 2017

The interest in floating offshore wind turbines (FOWT) has been growing substantially over the last decade and, after a number of prototypes deployed [1], the first offshore floating wind farms have been approved and are being developed. While a number of international research activities have been conducted on the dynamics of offshore floating HAWT systems (e.g. OC3-Phase IV2, OC4-Phase II3), relatively few studies have been conducted on floating VAWT systems, despite their potential advantages [2]. Due to the substantial differences between HAWT and VAWT aerodynamics, the analyses on floating HAWT cannot be extended to floating VAWT systems.

The main aim of the present work is to compare the dynamic response of the FOWT system adopting two different mooring dynamics approaches. Two version of the in-house aero-hydro-mooring coupled model of dynamics for VAWT “FloVAWT” [3] are used: one which adopts a mooring quasi-static model, and solves the equations using an energetic approach [4], and a modified version of FloVAWT, which uses instead the lumped-mass mooring line model “MoorDyn” [5]. The floating VAWT system considered is based on a 5MW Darrieus type rotor supported by the OC4-Phase II3 semi-submersible.

The results for the considered metocean conditions show that MoorDyn approach estimate larger translational displacements of the platform, compared to the quasi-static rigid approach previously implemented in FloVAWT. As expected, the magnitudes of the forces along the lines are lower, being part of the energy employed for the elastic deformation of the cables. A systematic comparison of the differences between the two approaches is presented.
Joint Probability Distribution of Environmental Conditions for Design of Offshore Wind Turbines
Authors: Horn Jan-Tore H. , Krokstad Jørgen R. , Amdahl Jørgen
Published: October 2017

The design process for offshore wind turbines includes a fatigue life evaluation of the structure with the relevant environmental conditions at the specified wind farm location. Such analyses require long-term distributions of the environmental parameters including their correlation. In general, the significant wave height, wave peak period and mean wind speed are the most important parameters for describing offshore environmental conditions. However, due to the low side-to-side damping level of offshore bottom-fixed wind turbines, wave directions misaligned with the wind direction may excite low-damped vibrational modes. As a consequence, the accumulated fatigue damage in the wind turbine foundation may change, compared to collinear wind and waves. In the current work, an extension to the three-parameter environmental joint probability distribution is presented, with the resulting distribution being a function of the significant wave height, peak period of the total sea, mean wind speed and the wave directional offset compared to the mean wind heading i.e. the wind-wave misalignment. The sea states within a 1-year return period for Dogger Bank are presented, as well as the 10- and 50-year environmental contour lines and extreme wind-wave misalignment angles.
Long Term Analysis of Semi Submersible Offset
Authors: Sandbakken Sindre S. , Haver Sverre , Larsen Kjell
Published: October 2017

During the design of risers, it is necessary to calculate the 100-year (ULS) top-end motion with reasonable accuracy. The ULS offset is usually taken as the most probable 3-hour maximum response in a storm modeled with 100-year wind and 100-year waves when current forces are neglected. This design approach is assumed to be conservative. In this paper, the level of conservatism is considered by performing a Peak Over Threshold (POT) and All Sea States (ASS) stochastic long term analysis.

The 3-hour maximum offset is assumed to be Gumbel distributed for the POT and ASS analysis. Gumbel parameters are estimated using method of moments for all metocean combinations. Response surfaces are established as a function of wind speed, W, significant wave height, Hs, and spectral peak period, Tp, to represent the short term variability of the response problem. The long term distribution of the most probable maximum response is established for the POT analysis. Similarly, a long term probabilistic model for W, Hs and Tp is established for the ASS analysis. Finally, the long term analyses are performed by combining the short term and long term variabilities using IFORM.
Extreme Response Prediction of Steel Risers Using a Four Parameter Distribution
Authors: Calderon Ibarra Miguel Alfonso , Sousa Fernando Jorge Mendes de , Sagrilo Luís Volnei Sudati , Low Ying Min
Published: October 2017

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 SLWR (steel-lazy wave riser) connected to a turret-moored FPSO in 914m water depth, and the second is a SLWR connected to a spread-mooring FPSO in a water depth of 1400m. 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 and Hermite transformation-based model are also presented for comparison.
Real-Time Data for Risk Assessment in the Offshore Oil and Gas Industry
Authors: Paltrinieri Nicola , Landucci Gabriele , Salvo Rossi Pierluigi
Published: October 2017

Recent major accidents in the offshore oil and gas (O&G) industry have showed inadequate assessment of system risk and demonstrated the need to improve risk analysis. While direct causes often differ, the failure to update risk evaluation on the basis of system changes/modifications has been a recurring problem. Risk is traditionally defined as a measure of the accident likelihood and the magnitude of loss, usually assessed as damage to people, to the environment, and/or economic loss. Recent revisions of such definition include also aspects of uncertainty. However, Quantitative Risk Assessment (QRA) in the offshore O&G industry is based on consolidated procedures and methods, where periodic evaluation and update of risk is not commonly carried out. Several methodologies were recently developed for dynamic risk analysis of the offshore O&G industry. Dynamic fault trees, Markov chain models for the life-cycle analysis, and Weibull failure analysis may be used for dynamic frequency evaluation and risk assessment update. Moreover, dynamic risk assessment methods were developed in order to evaluate the risk by updating initial failure probabilities of events (causes) and safety barriers as new information are made available. However, the mentioned techniques are not widely applied in the common O&G offshore practice due to several reasons, among which their complexity has a primary role. More intuitive approaches focusing on a selected number of critical factors have also been suggested, such as the Risk Barometer or the TEC2O. Such techniques are based on the evaluation of technical, operational and organizational factors. The methodology allows supporting periodic update of QRA by collecting and aggregating a set of indicators. However, their effectiveness relies on continuous monitoring activity and realtime data capturing. For this reason, this contribution focuses on the coupling of such methods with sensors of different nature located in or around and offshore O&G system. The inheritance from the Centre for Integrated Operations in the Petroleum Industries represents the basis of such study. Such approach may be beneficial for several cases in which (quasi) real-time risk evaluation may support critical operations. Two representative cases have been described: i) erosion and corrosion issues due to sand production; and ii) oil production in environmental sensitive areas. In both the cases, dynamic risk analysis may employ real-time data provided by sand, corrosion and leak detectors. A simulation of dynamic risk analysis has demonstrated how the variation of such data can affect the overall risk picture. In fact, this risk assessment approach has not only the capability to continuously iterate and outline improved system risk pictures, but it can also compare its results with sensor-measured data and allow for calibration. This can potentially guarantee progressive improvement of the method reliability for appropriate support to safety-critical decisions.