bluuebunny/crossref_metadata_2025 huggingface dataset:
A Practical Spectral Fatigue Analysis Procedure for Topside of Floating Offshore Structures in West Africa
Authors: Chung Sungwook , Chun Minsung , Jang Kibok , Suh Youngsuk
Published: October 2015

In most offshore projects recently ordered, spectral fatigue analysis is required for design integrity. However, the spectral fatigue analysis is very complicated to implement since it has many variations for parameters and forms of input data, and the classification and commercial software packages are exposing limit to support all those variations. A topside fatigue analysis for a FPSO design in West Africa is one of such a challenging project due to the fact that the specification of the project requires spectral fatigue analysis considering 3-peak Ochi-Hubble wave spectrum, Wrapped normal wave spreading and sea state data with 3 wave components, main swell, secondary swell and wind sea. In this study, a practical spectral fatigue analysis procedure is introduced in order to implement the fatigue analysis using a commercial program SACS. Applying adaptive cosine spreading wave distribution which can approximate Wrapped normal wave spreading is devised for each sea state and the comparison between two wave spreading is carried out. Finally, the proposed methodology is justified by analyzing the characteristics of the sea state in West Africa.
A Random Solution to Ice-Induced Vibrations of Conical Structures
Authors: Liu Xiang , Chen Yingying , Gu Hai , Wu Jer-Fang
Published: October 2015

Offshore installations designed to withstand extreme ice actions, such as the multi-leg structures in Cook Inlet, the gravity based Molikpaq during its mobilization in the Beaufort Sea, lighthouses and channel markers in the Baltic Sea, jackets and mooring poles in Bohai Bay and multi-leg structures offshore Sakhalin, have experienced ice-induced vibrations (IIVs). Full-scale data from Bohai Bay also demonstrate that a conical waterline geometry of the structure does reduce the magnitude of the ice forces, but it still experiences IIVs that can be treated as a stochastic process. ISO 19906 recommends that the dynamic ice actions and the corresponding IIVs shall be considered in the design as the fatigue limit state (FLS). ISO 19906 provides the guidance for the time-domain random dynamic ice action on conical structures. The dynamic structural response to such ice action can take the form of a random vibration. As an alternative to the time-domain approach, random vibration analysis can also be done in the frequency domain by the spectral approach. In addition to the time-domain random dynamic ice action on conical structures provided in ISO 19906, a type of ice-force spectrum on conical structures has been developed. In this paper, a simplified single-degree-of-freedom system (SDOF system) and the ice-force spectrum are used to derive an analytical random solution to assess the IIVs of conical structures. As ISO 19906 points out that particular attention shall be given to dynamic actions on narrow structures and flexible structures, the developed random solution can be useful for designers to make a fast estimate of IIVs (i.e., displacement, velocity and acceleration) and to efficiently screen out the key design parameters of a conical ice-resistant structure.
Utilization of Wave Dissipating Caissons as an OWC Type Wave Energy Convertor
Authors: Ikoma Tomoki , Masuda Koichi , Eto Hiroaki , Kihara Kazuyoshi , Maeda Hisaaki , Takahatake Madoka
Published: October 2015

Wave energy convertors (WEC) of oscillating water column (OWC) types make compressed air due to vertical movement of a part of water column, which are surrounded by some walls in water. As a result, the compressed air works to rotate an air turbine which also works for an electrical generator. If a structural system can make to move water such like a water column, a shape of a structure would not matter.

Wave dissipating caissons installed in or on the existing production line can make to transfer a water wave to a flow and to reduce wave energy by making to overflow them. The caissons which can be overflowed by the water would actuate a water region vertically such like an oscillating water column system.

This study investigated if we can utilize wave dissipating caissons as a WEC. Wave dissipating caissons also decrease wave energy due to passing through themselves. Even so, when an air turbine can be installed onto the caisson in order to harvest wave energy, it would be possible to produce electricity at low cost. The study conducted model experiments using the model assumed that wave dissipating double-caissons were remodelled to OWC type WECs with side walls and evaluated performance of the primary conversion of them. In addition, effects of how to arrange the number of the WECs on the primary conversion efficiency were investigated with theoretical calculations based on the linear potential theory and the possibility of utilization of wave dissipating caissons as a WEC was examined in the study.
Efficient Fatigue Testing of Tubular Joints
Authors: Thibaux P. , Van Wittenberghe J. , Van Pottelberg E. , Van Poucke M. , De Baets P. , De Waele W.
Published: October 2015

Tubular joints are intensively used in off-shore structures for shallow waters. Depending on the sea conditions and the type of structure, the design can be fatigue driven. This is particularly the case for off-shore wind turbines, where turbulences are generating a fatigue loading. Any improvement of the fatigue performance of the tubular joint would be beneficial to reduce the weight and the cost of the structure. To assess efficiently the fatigue resistance of the tubular joint, a testing method has been developed based on the resonance principle. The complete circumference of the welded joint can be loaded, successively in the in-plane and out-of-plane modes at a frequency close to 20Hz.

Finite element computations were used to investigate the feasibility of the concept. Then, an X-node was made and successfully tested to investigate the stress distribution along the weld. The experimental results were compared with finite element computations, giving a good agreement.
Influence of Structural Compliance and Slope Angle on Ice Loads and Dynamic Response of Conical Structures
Authors: Ziemer Gesa , Evers Karl-Ulrich , Voosen Christian
Published: October 2015

Model tests in ice have been conducted at the Large Ice Basin of HSVA with cylindrical and conical, compliant structures exposed to drifting level ice to investigate the influence of slope and compliance on the ice load and its breaking frequency. Main goal of the test campaign was to study the importance of structural feedback during ice-cone interaction. This is a major issue e.g. for numerical simulation of offshore structures during design phase. Four shapes were tested: 50°, 60°, 80° and 90° slope angle. The cylinder was tested in order to define the worst case scenario regarding magnitude of ice load and severity of ice-induced vibrations. Stiffness and natural frequency of the structure were chosen similar to typical values for offshore wind turbine support structures. All shapes were tested both in a compliant and fixed configuration.

The breaking frequency was found to be more pronounced for the lower slope angles where the ice failed in flexural failure only, while a transition to crushing failure as observed on a cylindrical structure takes place at 80° cone angle already. This results in significantly higher ice loads on the 80° cone than on those with lower angles, but a reduced risk of severe ice-structure interaction due to the unsteady nature of the mixed mode breaking process. Although the breaking frequency is rather constant e.g. during ice impact on the 60° cone, it was not possible during the model tests to match the ice drift speed and the dynamics of the structure in a way that causes resonance. However, model test results prove that there is a risk of conical structures with low natural frequencies and low stiffness in ice plane being excited by periodic ice failure in their natural frequency, thus response amplification may take place and pose a risk to the structural integrity of conical offshore structures exposed to sea ice.

This paper presents the model test setup, analysis of the results, and general findings.
Global Buckling Mitigation of Buried Pipelines: Lateral Resistance Assessment
Authors: Gallegillo Martin , Hardouin Guillaume
Published: October 2015

This paper presents an approach to rock cover design for un-trenched pipelines installed on the seabed and rock-dumped for protection against dropped objects, anchor chain impact and fishing/trawling activities. This is found in some North Sea locations which present challenging conditions for trenching while protection is necessary due to intensive fishing activities.

Under these circumstances the pipeline must remain within the rock berm and, hence, it must be designed against global buckling. Whereas there are clear design guidelines addressing upheaval buckling behaviour, the resistance to lateral buckling to maintain a pipeline within the rock berm has received less attention in the literature.

The aim of this paper is to present a method to design a rock berm to mitigate against lateral buckling of rock-dumped pipelines based on the horizontal out-of-straightness survey data provided to the designer. The main challenges associated with this activity at different design phases are also introduced, including the use of analytical tools as well as detailed finite element analysis.
Identification of Nonlinear Manoeuvring Models for Marine Vessels Using Planar Motion Mechanism Tests
Authors: Ross Andrew , Hassani Vahid , Selvik Ørjan , Ringen Edvard , Fathi Dariusz
Published: October 2015

A nonlinear manoeuvring model in three degrees of freedom is presented. MARINTEK’s approach to the numerical determination of this manoeuvering model’s parameters is then shown. Finally the process of taking advanced experimental methods, and utilising MARINTEK’s numerical tools to generate an advanced model available for use in VeSim, MARINTEK’s in-house simulator tool, is shown as a demonstration that the model combined with numerical tools is of great use in making manoeuvring predictions.
Small-Scale FE Modelling for the Analysis of Flexible Risers
Authors: Rahmati M. T. , Alfano G. , Bahai H.
Published: October 2015

Flexible risers which are used for transporting oil and gas between the seabed and surface in ultra-deep waters have a very complex internal structure. Therefore, accurate modeling of their behaviour is a great challenge for the oil and gas industry. Constitutive laws based on beam models which allow the large-scale dynamics of pipes to be related to the behaviour of its internal components can be used for multi-scale analysis of flexible risers. An integral part of these models is the small-scale FE model chosen and the detailed implementation of the boundary conditions. The small scale FE analyses are typically carried out on models of up to a few meters length. The computational requirements of these methods limit their applications for only multi-scale structural analysis based on a sequential approach. For nested multi-scale approaches (i.e. the so called FE2 method) and for multi-scale multi-physic analyses, e.g. fluid structure interaction modeling of flexible risers, more efficient methods are required. The emphasis of the present work is on a highly efficient small-scale modelling method for flexible risers. By applying periodic boundary conditions, only a small fraction of a flexible pipe is used for detailed analysis. The computational model is firstly described. Then, the capability of the method in capturing the detailed nonlinear effects and the great advantage in terms of significant CPU time saving entailed by this method are demonstrated. For proof of concept the approach is applied on a simplified 3-layer pipe made of inner and outer polymer layers and an intermediate armour layer made of 40 steel tendons.
An AUV Yumeiruka for Seabed Topography Survey
Authors: Ishibashi Shojiro , Yoshida Hiroshi , Sugesawa Makoto , Ohta Yutaka , Fan Frank Hsiao
Published: October 2015

Japan Agency for Marine – Earth Science and Technology (JAMSTEC) has been recently advanced the development of several underwater vehicles for the survey of mineral resources buried under Japan’s territorial sea. One of them is Yumeiruka. It has characteristic canard wings and rudders which are attached on its fore and rear parts. Since they cause good maneuverability, good survey results using acoustic observation devices can be obtained. And it has a high performance compact inertial navigation system which was developed as Japan’s first INS for an underwater vehicle by the cooperation between JAMSTEC and a Japanese manufacture. Yumeiruka has now conducted sea trials in order to improve its performance and functionality.
Distributed Propulsion: A Novel Concept for Inland Vessels
Authors: Hagesteijn Gerco , van der Meij Karola , Thill Cornel
Published: October 2015

In the drive to reduce the fuel consumption of inland vessels, one of the main limitations, the risk of propeller ventilation was investigated. The aim was to reduce the resistance of the vessel, while at the same time an acceptable margin against propeller ventilation had to be assured. A typical inland vessel has up to two high loaded relative large propellers, with tip regions in the lighter loading condition above the water line. To eliminate the risk of propeller ventilation, these ships often have inbuilt shape features such as tunnels and skirts, only having the intention to avoid air being drawn by the rotating propeller.

These devices however have a noticeable own resistance, imposing a mortgage to the ship’s total resistance. Current design practice for these devices is that as long as you cannot quantify a risk, take care of a sufficient safety margin. With the research done within the EU-funded STREAMLINE project new insights were gained into the prediction of air suction. As a result, the margin against ventilation could be reduced which resulted into a large reduction of the resistance of the vessel.

Reducing the high loading of propellers means to gain efficiency with at the same time reducing the risk of propeller ventilation as the suction by the propeller(s) can be decreased as well. This leads to the idea of a “distributed thrust” concept (DTC). Within STREAMLINE DST developed a sample case for such a novel design with six thrusters, with the aim to reduce ventilation against zero and to achieve maximal performance improvements. The measurements were carried out in MARIN’s Depressurized Wave Basin (DWB). These tests were carried out with a 1:10 scale ship model, in sailing condition, and depressurized conditions. In this way, the correct representation of cavitation and possible ventilation bubbles and vortices is ensured, resulting in a correct physical behavior. At the same time synchronized high speed video recordings were made to acquire insight in the occurring phenomena.

Within the project CFD calculations were carried out, aimed at characterizing the performance, loads, cavitation nuisance and ventilation risk in full-scale operating conditions. Validation of these calculations was done using the towing tank results.
Development and Experimental Calibration of Numerical Model Based on Beam Theory to Estimate the Collapse Pressure of Flexible Pipes
Authors: Lacerda Jefferson , Lourenço Marcelo I. , Netto Theodoro A.
Published: October 2015

The constant advance of offshore oil and gas production in deeper waters worldwide led to increasing operational loads on flexible pipes, making mechanical failures more susceptible. Therefore, it is important to develop more reliable numerical tools used in the design phase or during the lifetime to ensure the structural integrity of flexible pipes under specific operating conditions. This paper presents a methodology to develop simple finite element models capable of reproducing the behavior of structural layers of flexible pipes under external hydrostatic pressure up to collapse. These models use beam elements and, in multi-layer analyses, include nonlinear contact between layers. Because of the material anisotropy induced by the manufacturing process, an alternative method was carried out to estimate the average stress-strain curves of the metallic layers used in the numerical simulations. The simulations are performed for two different configurations: one where the flexible pipe is composed only of the interlocked armor, and another considering interlocked armor and pressure armor. The adequacy of the numerical models is finally evaluated in light of experimental tests on flexible pipes with nominal internal diameters of 4 and 6 in.
Uncertainty of Sea Trials Results Used for Validation of Ship Manoeuvring Simulation Models
Authors: Gavrilin Sergey , Steen Sverre
Published: October 2015

Increasingly complex marine operations dictate higher need for precise and reliable modelling. For the last decades several different approaches to modelling of ship manoeuvring were developed, including scaled model testing, numerical and empirical methods. Increasingly, time-domain simulation models for ship manoeuvring are developed and used for training and planning of marine operations. Especially when using the simulation models to plan operations, it is essential that the simulation model is properly validated. There is a need for better and more standardized validation methods for such simulation models. A key issue is the uncertainty of the data used in the validation. Typically, the validation will be against full scale trials results.

In the study reported in this paper it was found by inspection of repeated tests results that even under relatively calm environmental conditions outcomes of sea trials can be highly uncertain. However, it is very expensive to investigate uncertainty of each type of trial experimentally. Therefore it can be very beneficial to estimate it by means of a simulation model.

This paper presents results of analysis of full scale turning circle trials of research vessel “Gunnerus”. Turning circle trials with 20° and 35° rudder angle executed both to starboard and port sides are analyzed. Experimental uncertainty analysis is performed. Effectiveness of IMO correction procedure is discussed. Also paper describes a method for determining uncertainty of trial results due to environmental effects by means of simulation model and compares them with experimental uncertainty. The method is based on Monte-Carlo techniques.
Nonlinear Effects on Local Mechanics of Freak Waves
Authors: Fujimoto Wataru , Waseda Takuji
Published: October 2015

The local properties of freak waves, such as geometry and particle velocities, are still to be investigated and are essential in the limit state design of ships or offshore structures. We have focused on two factors for this research. The first is nonlinearity higher than 3rd order since local steepness around freak waves will be large. The 4th order nonlinearity deforms a perturbed regular wave like a crescent and also causes a longitudinal asymmetry that means that the shape of the wave is asymmetrical in the propagation direction. The second is higher wavenumber components that will increase particle velocities. We tried to research the effects of these two factors on freak waves with Higher Order Spectral Method. Consequently, a crescent shape and longitudinal asymmetry in freak wave shape were found. In addition, higher wavenumber components increased the maximum horizontal velocity of freak waves significantly. These results show that the 4th order nonlinearity and higher wavenumber components are important for local freak wave kinematics, as well as for determining structural impacts, the motion of floating objects, and wave breaking.
Piezoelectric Vibration-Based Energy Harvesters for Oil and Gas Applications
Authors: Silva L. Loureiro , Monteiro P. C. C. , Savi Marcelo A. , Netto Theodoro A.
Published: October 2015

Monitoring and control of subsea systems in remote ultra deep water scenarios is challenging as well as an opportunity for development and application of new technologies. One of the major problems is providing continuous power to sensors and actuators, independent of electrical umbilical cables. A conventional solution is the use of electrochemical batteries. However, problems can occur using batteries due to their finite lifespan. The need for constant replacement in remote locations can become a very expensive task or even impossible. Piezoelectric energy harvesters have received great attention for vibration-to-electric energy conversion over the last years. The evaluation of the power output of devices for different excitation frequency and amplitude of vibration has an important role in the design of such devices. This work describes the methodology to design a prototype that can be used in a pipe subjected to flow induced vibrations. Numerical model has been developed to reproduce the electromechanical coupling mechanism aiming at estimating the output voltage of the piezoelectric harvester. The results show the potential of piezoelectric materials for this application.
Discrete Element Modelling of Pack Ice Interaction With Floating Structures
Authors: Dai Jie , Peng Heather
Published: October 2015

This paper presents a two-dimensional numerical model for ship-ice interaction simulatiion using the discrete element method (DEM). The simulation was conducted for a broken ice field with hundreds of circular ice floes and various combinations of ice conditions. A viscous-elastic ice rheology was adopted to model the dynamic behavior of each individual ice floe. Both ship-ice and ice-ice contacts were considered in the interaction force. Environment forces, including wind force and wave force, were calculated by empirical formulas. An algorithm was developed to log each contact and solve motions of individual ice floe and the ship. The resistance of ship advancing in ice was predicted and compared with model test results.
Reduction of Tophole Risks Through Improved Application of Geophysical and Geotechnical Techniques
Authors: Barwise Andy , Salisbury Richard , Wood Gareth , Van Der Kraan Micha , Carrington Tim , Thompson Giles
Published: October 2015

The offshore oil and gas industry spends over $60bn per year on oil and gas wells and of this some $6bn, or around 10% is eaten up by geological and geotechnical problems such as stuck pipe, lost circulation, well bore instability, shallow water flows and other problems. On top of this are the environmental costs of the oil spills that can result from lost well control, and perhaps most importantly the human costs in terms of injuries and loss of life resulting from some of the worst incidents.

This paper lists the geohazards within and around a well, the drilling risks implied by these geohazards, and the impact they can have on the planning and drilling of offshore wells.

Current practice in geophysical and geotechnical site investigation techniques which, when correctly applied and interpreted, can help to reduce the risks and costs associated with the ‘Top-hole’ section is summarised and discussed (the Top Hole section is defined as the depth to the base of the first pressure containment string).

Finally, a systematic approach to assessing and mitigating top-hole geo-risks through a multi-disciplinary geoscience and engineering approach is described.

The authors are members of a working group of the Offshore Site Investigation and Geotechnics (OSIG) committee of the Society of Underwater Technology (SUT) who are drafting guidelines on the subject.
State-of-the-Art Within Ship Weather Routing
Authors: Simonsen Martin Hjorth , Larsson Erik , Mao Wengang , Ringsberg Jonas W.
Published: October 2015

Increased fuel prices and public awareness of environment impacts from shipping have attracted large efforts in maritime sector to increase its energy efficiency as a factor of competitiveness. Weather routing has become a recognized measure, which can partly help to achieve the targets as well as enhancing safety. A routing system requires a reliable optimization algorithm to consider a ship’s operational costs, expected time of arrival, and cargo safety etc. simultaneously. Hence, the service provided by a weather routing system is highly dependent on a properly selected optimization algorithm and associated input parameters.

In this paper the concept of weather routing is broken down into many elements for further analysis. Focus is given to algorithms, constraints and weather forecasts used in the optimized routing plan. Two different aspects of state-of-the-art have been considered. The first is a study of software already in use and the second is a study of methods investigated in the research community. Furthermore, this paper also provides examples of development trends, for example the fatigue based routing, and the risk based routing, as well as its integration with onboard monitoring systems for more reliable weather and ship specific response information.
Surface Temperature Fluctuations in Ice Indentation Tests
Authors: Turner Joshua D. , O’Rourke Brian J. , Jordaan Ian J. , Taylor Rocky S.
Published: October 2015

Ice-structure interactions produce numerous, short-lived high pressure zones which transfer the majority of the load to the structure. Ice indentation tests have been used to observe the fluctuations in surface temperature at these zones. Previous investigations have shown that temperature fluctuates inversely to the applied load during cyclic loading. These fluctuations are believed to be due to rapid melting and refreezing cycles in the high pressure zones.

A recent series of small-scale indentation tests investigated surface temperature fluctuations for indentor speeds at three orders of magnitude (0.21 mm/s to 21 mm/s). Freshwater granular ice specimens were grown in cylindrical steel moulds using seed ice and distilled water. A 70 mm diameter indentor with a radius of curvature of 89.6 mm was indented into the ice to depths between 10 and 15 mm. Seven thermocouples were installed in the indentor and made flush with its surface. These were used to measure the surface temperature of the ice during indentation. All tests took place at −10 degrees.

Notable changes in temperature response were observed with changing speed. A steady increase in temperature was observed during low speed tests. Medium speeds lead to fluctuations in temperature due to the presence of spalling and cyclic loading that is consistent with the results of others. Some evidence of the inverse relationship between load and temperature can be observed in the highest speed tests, but the thermocouples sampling frequency was too low to accurately respond to high frequency loading and could not provide clear results.
Tensioned Step Riser Configuration for Ultra-Deep Application
Authors: Tian Yuan , Hou Yucheng , Pires Fabio , Tan Zhimin
Published: October 2015

This paper presents and compares the results of a research study for an ultra-deep water field development. A total of three riser configurations were modeled with the commonly used commercial software, and both extreme and fatigue analysis were performed. The advantage of tensioned step riser configuration, a patented and innovative riser configuration specially designed for ultra-deep water application, is demonstrated over other traditional configurations such as free-hanging configuration and double-hump lazy wave configuration.

The analysis results indicated that the proposed tensioned step riser configuration has largely favorable features in extreme and fatigue performance for deep water application. The tensioned step riser configuration should be treated as a strong candidate for ultra-deep water flexible developments under substantially harsh environmental loading with worse vessel responses.
Performance Assessment of a Floating Coaxial Ducted OWC Wave Energy Converter for Oceanographic Purposes
Authors: Portillo Juan C. C. , Henriques Joao C. C. , Gato Luis M. C. , Gomes Rui P. F. , Falcão Antonio F. O.
Published: October 2015

This paper presents a numerical study on a floating coaxial ducted OWC wave energy converter equipped with a biradial air turbine to meet the requirements of an oceanographic sensor-buoy. The study used representative sea states of the Monterey Bay, California, USA. The geometry of the coaxial ducted OWC was hydrodynamically optimized using a frequency domain approach considering a linear air turbine. Afterwards, a time domain analysis was carried out for the system equipped with a biradial turbine. The turbine rotor diameter and the optimum generator’s control curves were determined, based on results for representative sea states. Results show that mean power output fulfills the requirement for oceanographic applications (300–500W) using a turbine rotor diameter of 0.25 m. Furthermore, the system’s performance is strongly influenced by the inertia of the turbine and the generator rated power. These results confirmed the suitability of using the coaxial ducted OWC as a self-sustainable oceanographic sensor-buoy.
Vibration and Shock Loads Assessment of the Reciprocating Compressor on an Offshore Platform
Authors: Nezamian Abe
Published: October 2015

The Reciprocating Compressor is becoming one of the common classes of mechanical equipment and plays an important role in offshore oil field re-vitalisation. A wide variety of vibration and failure problems may occur in reciprocating compressors, associated supporting frames and offshore support structure. Excessive vibration problems usually occur when a mechanical natural frequency of the support structural system or compressor manifold system is excited by a pulsation or mechanical excitation source. Since reciprocating compressors generate high pulsation levels at numerous harmonics, which in turn produce shaking forces, vibration and failure problems in these systems are common. These can lead to failures of the compressor, bearings system and or offshore supporting structure/platform. In addition, special problems can occur due to the torsional natural frequencies and the high harmonic torques, due to the compressor loading. Shock absorbers can be designed to control and mitigate the vibration from the compressor applied to the supporting structure. In order to design an appropriate number and location for shock absorbers, it is important to determine the vibration characteristics of the reciprocating compressor on the offshore platform and assess the interactions between it and the supporting structure. This paper gives an overview of the challenges and also discusses various aspects of vibration and shock load assessment of the reciprocating compressor on an offshore platform. Application of the proposed methodology were presented for an offshore structural unit.
Residual Stress Measurement, Finite Element Mapping and Flaw Simulation for a Girth Welded Pipe
Authors: Ficquet Xavier , Serasli Karim , Kingston Ed J.
Published: October 2015

Optimising the structural integrity of an oil and gas pipeline is hugely important for its productivity and hence profitability. The structural integrity of a pipeline is influenced by factors such as: stress (i.e. applied and residual), material properties, environment, and the size and orientation of contained flaws. For example, whilst in operation, the integrity of a pipeline can be extended by reducing its applied stresses e.g. the flow and pressure of the oil and gas running within. Prior to operation however the integrity of the pipeline can easily be extended by reducing the residual stresses generated during installation or even “negatively pre-loading” the pipeline using residual stresses to help cancel out some of the applied stresses. Therefore understanding the distribution of residual stresses within a pipeline can be of great benefit to Oil and Gas engineers.

In this paper, complementary residual stress measurement techniques are used to obtain near surface and through-thickness residual stress distributions in a fully circumferential butt welded pipe. The deep hole drilling (DHD) method was used to obtain the axial and hoop residual stresses along radial lines through the pipe wall. Near surface measurements on the outer surface of the pipe were obtained using the incremental centre-hole drilling (ICHD) method.

The measurements were made only at limited points in and adjacent to the circumferential weld. In order to estimate the complete residual stress field present in the pipe, a mapping procedure utilising a finite element (FE) method was implemented. This entailed introducing the measured residual stresses into a FE model of the pipe as an initial condition and allowing redistribution. Naturally, the stresses at the measurement locations should remain at their initial values. Consequently, the method was developed to allow redistribution while retaining the measured values. The paper provides these estimates of the full residual stress state present in the pipe based on this mapping procedure. The FE model was then used to simulate the influence of various sizes of flaw on the mapped residual stresses field. An assessment of the acceptability of areas of loss of the wall thickness in internally pressurised pressure vessels was then performed.
Hydrodynamics of Podded Propulsors With Highly Loaded Propellers and in Extreme Azimuthing Conditions
Authors: Islam Mohammed , Jahra Fatima , Ryan Ron , Hedd Lee
Published: October 2015

State of the art CFD capabilities has enabled the accurate prediction of forces and moments on the propeller as well as on the pod-strut body due to small to moderate azimuthing angles. The capability of CFD to predict the hydrodynamics at extreme azimuthing angles is yet to be demonstrated. The aim of this research is to develop a simulation capability to capture most of the dynamics of podded propulsion systems in regular to extreme operating conditions. The numerical methodologies to evaluate the hydrodynamic characteristics of podded propulsors in puller configurations in extremely oblique inflow and highly loaded condition in open water and the associated results are presented in this paper.

A numerical study is carried out to predict the hydrodynamic forces of a podded propulsor unit in various extreme static azimuthing conditions. An unsteady Reynolds-Averaged Navier Stokes (RANS) solver is used to predict the propulsive performance of the podded propulsor system in puller configuration using both steady and unsteady state solutions. To obtain insight into the reliability and accuracy of the results, grid dependency studies are conducted for a podded propulsor in straight-ahead condition. RANS solver simulation technique is first validated against measurements of a puller podded propulsor in straight ahead condition for multiple loading scenarios. The propeller thrust and torque as well as the forces and moments of the pod unit in the three coordinate directions in straight-ahead condition and at static azimuthing angles in the range of −180° to 180° at advance coefficient of 0.20 are then compared with that of the measurements. Additionally, the velocity and pressure distribution on and around the pod-strut-propeller bodies are presented as derived from the RANS predictions. Analysis demonstrates that the RANS solver can predict the performance coefficients of the podded propulsor in extreme azimuthing and in the highly loaded conditions within the same level of accuracy of the same order of magnitude of the experimental results.
Validation of Bulbous Bow Optimisation by Viscous CFD
Authors: Vroegrijk Erik , Whitworth Stewart , Caldas Alejandro
Published: October 2015

In this paper the authors attempt to validate the use of CFD in bulbous bow optimisation studies using two separate data sets. Based on the first data set a comparison is made between results derived using both RANSE CFD and Potential Flow. It is shown that the relative ranking of hull designs will be substantially different, as a result of the influence of shear resistance not being taken into account correctly by Potential Flow codes. The second data set is used to compare RANSE CFD results with comparative towing tank tests and full scale measured data. The results clearly show that full scale savings predictions derived with viscous CFD compare well with both towing tank predictions and full scale measurements. The remaining uncertainties are considered sufficiently small to allow for the large investment decisions to be made associated with bulbous bow modifications. The paper concludes with a view towards future possibilities for this kind of optimisation analysis.
Validation of Crack Growth for an Finite Element Based ECA Procedure for Reel-Laid Pipelines With Under-Matching Girth Welds
Authors: Sriskandarajah T. , Zhou Daowu , Cao Lingjun
Published: October 2015

There is a concern on the fracture integrity of the partially over-matching or under-matching weld during reel-lay installation where there is large plastic strain in the pipe. Conventional ECA procedures such as BS7910 and DNV-OS-F101 are applicable for fully over-matching welds only, due to limitations in the reference stress solution (or limit load solutions).

The ECA procedure based on 3D finite element (FE) analysis was developed for partially over-matching welds or under-matching. The methodology has been successfully applied to several projects of industry-wide significance, with partially over-matching welds in offshore pipelines. This paper provides a case study validating the crack growth from FE based ECA methodology against the large scale bending trial test where the pipe containing the notched defect was pre-strained under a series of straining cycles. A comparison of the crack growth between 3D FEA and the large scale bending test was presented.
Mechanics of Ice Rubble Over Multiple Scales
Authors: Bailey E. , Taylor R. , Croasdale K. R.
Published: October 2015

The mechanics of ice rubble plays an important role in many different engineering applications, including ice-structure interactions with oil and gas infrastructure, river and lake engineering, and ship-ice interactions in northern shipping lanes. Of particular interest are the massive accumulations of rubble formed by shear or compression in the ice cover, which consolidate to form sea ice ridges that can be hazards to such structures. These are common ice features in Arctic and sub-Arctic environments and as a result often govern the design loads for ships, coastal and offshore structures operating in these environments. In addition, ridge keels can scour the seafloor in relatively shallow waters posing a threat to pipelines and other subsea facilities. It is not clear what load an ice rubble feature can exert on a given structure and how it will deform. It will depend on a number of parameters including the age of the feature, its composition and structure, and its strength and failure behaviour. This paper will examine the mechanical properties of ice rubble over multiple scales. The paper will begin at the one block level, describing how ice block properties vary over time, before advancing to look at the bonding/sintering processes that occur between two ice blocks and eventually the processes that take place between multiple ice blocks (i.e., ice rubble) and large scale sea ice ridges. Particular attention will be paid to the effects temperature and pressure have on ice rubble, as these parameters are believed to be important to our understanding of its behavior.
Prediction of Extreme Tensions in Mooring Lines of a Floating Offshore Wind Turbine in a 100-Year Storm
Authors: Hsu Wei-ting , Thiagarajan Krish P. , MacNicoll Michael , Akers Richard
Published: October 2015

Floating offshore wind turbines (FOWTs) contribute to an emerging green energy technology, by exploiting higher and consistent wind speeds above the ocean. There are several challenges facing the design of mooring system of FOWTs, including installation costs, stability of light-weight minimalistic platforms, and shallow depths (50–300m). The extreme tension in mooring lines of a light displacement platform in shallow-water is dominated by snap loads. This is because light pre-tension requirements in the line may be insufficient to prevent the mooring lines from being exposed to wave motion induced slack and shock events.

In this paper, we present a comparative analysis of a semi-submersible based FOWT exposed to a 100-year storm condition, based on model test data and numerical simulations of well-known industry standard software. The data was obtained from a 1/50th-scale FOWT with the wind turbine modeled after the NREL 5MW wind turbine. The software, OrcaFlex, was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic load for FOWT analyses. The numerical simulation of the moored FOWT in a 3-hour storm was executed in both the frequency-domain and the time-domain to determine the dynamic behavior of the platform and mooring system, respectively. Snap–type impact events were observed in both test data and numerical simulation. Tension maxima were fitted into extreme value distributions and comparisons are made between simulated and measured data. It is seen that snap events follow a different exceedance probability distribution compared to the cycle-to-cycle tension maxima.
Hydrodynamic Modeling of Large-Diameter Bottom-Fixed Offshore Wind Turbines
Authors: Bachynski Erin E. , Ormberg Harald
Published: October 2015

For shallow and intermediate water depths, large monopile foundations are considered to be promising with respect to the levelized cost of energy (LCOE) of offshore wind turbines. In order to reduce the LCOE by structural optimization and de-risk the resulting designs, the hydrodynamic loads must be computed efficiently and accurately. Three efficient methods for computing hydrodynamic loads are considered here: Morison’s equation with 1) undisturbed linear wave kinematics or 2) undisturbed second order Stokes wave kinematics, or 3) the MacCamy-Fuchs model, which is able to account for diffraction in short waves. Two reference turbines are considered in a simplified range of environmental conditions.

For fatigue limit state calculations, accounting for diffraction effects was found to generally increase the estimated lifetime of the structure, particularly the tower. The importance of diffraction depends on the environmental conditions and the structure. For the case study of the NREL 5 MW design, the effect could be up to 10 % for the tower base and 2 % for the monopile under the mudline.

The inclusion of second order wave kinematics did not have a large effect on the fatigue calculations, but had a significant impact on the structural loads in ultimate limit state conditions. For the NREL 5 MW design, a 30 % increase in the maximum bending moment under the mudline could be attributed to the second order wave kinematics; a 7 % increase was seen for the DTU 10 MW design.
Influence of Porosity on the Integrity Properties of Butt Welded Super Duplex Stainless Steel Umbilical Tube
Authors: An Xiaoxue , Dobson Alan , Chan Chun Yip
Published: October 2015

Super Duplex Stainless Steel (SDSS) tube is the main component in steel tube umbilicals for the transportation of hydraulic fluids and chemicals, or gas injection into the subsea well. The umbilical tube is made from seamless tube joined together by girth weld. Therefore, porosity is one of the main weld defects that could be discovered within the welds, and these defects can have significant impact on the execution of the whole project, in term of both schedule and cost.

This paper reviews the design codes and standards relating to the porosity acceptance criteria, and discusses the recent investigation on the influences of porosity (number, size and location) to the stress distribution within the weld. The study was performed using both FEA and practical fatigue test. The results demonstrated that the distance of the pores to the weld surface is a critical parameter to the increase and localisation of stress as the pore starts to interact with the weld root or cap. In addition, significant pore stress interaction has been observed when a bending moment is introduced to the weld. The study has demonstrated that the current porosity acceptance criteria applied to umbilical manufacture in the subsea industry are typically conservative and can be modified.
The Analysis of Optimized Ship Routings in the North Pacific by Considering Different Routing Strategies
Authors: Lin Yu Hsien , Fang Ming Chung
Published: October 2015

This study principally adopts the weather forecast data of WAVEWATCH III (WW3) model and Ocean Surface Currents Analysis – Real time (OSCAR) system as inputs of our weather routing system. For the sake of different purposes of routing problems, the proposed 3DMI (3-Dimensional Modified Isochrone) method considers two types of routing strategies, i.e. ETA (Estimated Time of Arrival) routing and FUEL (FUEL-saving) routing, with different constraints, such as land boundaries, significant wave heights, engine revolution speeds and roll responses. Eventually, it is demonstrated that the robustness of the proposed method seems to be a solution to multi-objective problems by adjusting the safety threshold for the trade-off of ship efficiency and economics.
Numerical Modelling of Fixed Oscillating Water Column Wave Energy Conversion Devices: Toward Geometry Hydraulic Optimization
Authors: Simonetti Irene , Cappietti Lorenzo , El Safti Hisham , Oumeraci Hocine
Published: October 2015

The Oscillating Water Column (OWC) is one of the simplest and most studied concepts for wave energy conversion. The commercial scale diffusion of the OWC technology is, however, strongly dependent upon the device optimization. Research at a fundamental level is therefore still required. Analytical, numerical and experimental models are necessary tools for advancing in the knowledge of the system and thus promoting its passage at the commercial level. In this work, a simplified frequency domain rigid piston model has been applied to preliminary select expected ranges of air pressures and air velocities for the instrumental set up of laboratory experiments. The set up of a Computational Fluid Dynamic (CFD) model implemented in the open source OpenFOAM®1 environment is then presented. The multiphase model solves incompressible 3D Navier-Stokes equations, using Large Eddy Simulation (LES) for turbulence modelling, and adopts a Volume of Fluid method (VOF) to track the air-water interface. A preliminary validation of the model with physical tests data is conducted. The numerical approach seems to be promising for an accurate simulation of the OWC device energy conversion process. Hence, the validated model can be a useful research tool for different problems, particularly for systematic parameter studies to extend the range of conditions tested in the laboratory.