bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow-Induced Vibration Marine Current Energy Harvesting Using a Centrally-Pivoted Cylinder
Authors: Stappenbelt Brad
Published: October 2022

The phenomenon of vortex-induced vibration (VIV) for elastically restrained cylindrical structures in cross-flows is relatively well investigated. The utility of this mechanism in harvesting energy from marine current and tidal flows is however arguably still in its infancy. With relatively few moving components, a flow-induced vibration based energy conversion device augers low complexity compared to the commonly employed turbine design. Despite the interest in this concept, a practical device has yet to emerge. It is desirable for optimal system performance to design for a very low mass or mass moment of inertia ratio. The device operating range in particular is maximised below the vortex-induced vibration critical point where an infinite resonant response region is realised. An unfortunate consequence of this requirement is large buoyancy forces that need to be mitigated by gravity-based, suction-caisson or anchor mooring systems. The focus of this paper is the testing of a novel VIV marine current energy-harvesting configuration that utilises a centrally-pivoted horizontal cylinder. The results of experimental investigations, utilising the University of Wollongong fluid mechanics laboratory towing tank, are analysed and presented. A reduced velocity test range of 0 to 27 was covered. Power take-off (PTO) damping ratios spanning from 0.041 to 0.392 were examined in order to determine the optimal conditions and hence the maximum device energy conversion efficiency.
Solitary Wave Propagation Using a Novel Single Fluid Finite Volume Method for Free Surface Gravity Waves
Authors: Roland Kjærgaard Qwist Jesper , Christensen Erik Damgaard
Published: October 2022

Accurate prediction of wave loads from extreme waves on individual beams in offshore jacket structures are important to ensure the structural integrity. As the load distribution along a beam is strongly tied to the wave kinematics it is crucial to have an accurate prediction of the surface elevation and the velocity field.

The open-source CFD code OpenFOAM contains two solution algorithms (interFoam and interIsoFoam), which are used for ocean wave simulations and wave load predictions. The algorithms are often applied using a too coarse spatial discretisation, which violates the assumption of a continuous solution at the free surface. The consequence is inaccurate and unphysical predictions even for propagation of a steady wave.

Our recently developed Direct Surface Description (DSD) solution algorithm in OpenFOAM is used to simulate a solitary wave with different cell aspect ratios. The solitary wave case shows that the DSD method provides accurate velocity fields and surface elevations both for square and horizontally elongated cells.
Comparison of Cutter Suction Dredger Slurry Concentration Soft Measurement Method Based on the Mechanism and Data Dual-Driven Model
Authors: Wang Bin , Fan Shidong , Xiong Ting , Zhu Hanhua
Published: October 2022

Aiming at the problem that the cutter suction dredger (CSD) slurry concentration measurement method (γ-ray concentration meter) is single, and the frequent failures affect the construction continuity. A soft measurement method of slurry concentration based on the mechanism and data dual-mode drive was proposed. First, analyze the process of cutter cutting and the force of the mud and sand particles inside of the cutter. Then, the concentration prediction model of the slurry transportation process was presented by digital simulation modeling technology. Second, a data-driven slurry concentration prediction model was established, using big data, machine learning, data mining, and other technologies, which learn the historical data of the dredger. To verify the accuracy of the two models, this paper designed three different parameters and five operating conditions on the experimental platform. Experiments show that the prediction performance of the two models is good. In most working conditions, the mechanism-driven slurry concentration model and the data-driven slurry concentration model maintain an error rate of less than 5%, but the prediction error of the mechanism-driven slurry concentration model is relatively large under the flow velocity at the suction port is 3.5 m/s.

Overall, the slurry concentration prediction model based on the mechanism & data has high prediction accuracy and can improve construction efficiency and reliability.
Subsea Buoyancy and Gravity Energy Storage System for Deep-Water Applications: A Preliminary Assessment
Authors: Novgorodcev Andre R. , Mols Frank , Jarquin Laguna Antonio
Published: October 2022

This article presents a preliminary assessment of a subsea buoyancy and gravity energy storage system (SBGESS). The storage device is designed to power an off-grid subsea water injection system to be installed at the Libra oil field in Brazil at 2000 m below sea level. Two 12MW floating wind turbines provide the energy supply. The system performance is evaluated according to historical wind data from reanalysis models, the water injection pumps’ power curves, the required daily water flow rate, and the maximum number of shutdowns allowed per year. A control strategy with three different operation modes and one energy-save sub-mode was implemented to optimise the size of the proposed energy storage system.
Effects of Rheological Properties on Primary Cementing of Irregular Horizontal Wells
Authors: Sarmadi Parisa , Frigaard Ian A.
Published: October 2022

Primary cementing is an industrial operation that is used to seal every oil and gas wells, of which there are many millions globally. The process aims to remove the drilling fluid and other residues from the annulus between the casing and the formation, replacing it with a cement slurry, which then hydrates and seals the well to prevent subsurface leakage. Leakage compromises well productivity and can have environmental and health consequences, e.g. polluted aquifers, methane emissions, and subsurface ecosystems. In this work, we focus on fluid displacement mechanics of primary cementing operations. Even in stable boreholes, several geometrical and operational irregularities can be seen while reading caliper data. Washouts, keyseats, and breakouts are examples of irregularities. The other irregularity arises from eccentricity which mainly originates from operational features. Eccentricity of the annulus significantly affects the cement placement flow and longevity of the wellbore.

In this work, we study the effect of the rheological properties of fluids involved in the primary cementing on displacement flows in an irregular horizontal section of well. The focus of the work is on irregularities in the shape of enlargements. We use three-dimensional (3D) numerical simulations, using the Volume of Fluid (VoF) method to compute displacement of a drilling mud by a spacer within an annulus with series of enlargements. We use Bingham fluid and Newtonian fluid models for the mud and spacer, respectively. The 3D model allows us to capture the interface between fluids and study the effects of more local irregularities in detail, e.g. secondary flows and residual mud layer on the wall of the well. The rheology of the fluids beside the geometrical properties, irregularity and eccentricity, plays important role in determining the presence of unyielded residual mud layer. In this paper we describe the competition between viscosity ratio and density differences with geometrical properties.
Peridynamic Analysis of Floating Ice Under Transverse Pressure
Authors: Bachimanchi Pranitha , Saha Nilanjan
Published: October 2022

Offshore structures frequently interact with floating ice during its operational conditions in the arctic region. The structures are mainly designed in a slopping manner such that vertical loads are exerted, leading to out-of-plane failure of floating ice. In [1], the fracture pattern and failure process of ice floe with different dimensional conditions are studied through the classical approach. In the present study, a peridynamic approach has been adopted to analyze the fracture pattern of floating ice. Peridynamics [2] is a modified continuum mechanics that reformulates classical governing equations by replacing spatial derivatives with integrals. The main advantage of the novel approach is the incorporation of damage within governing equation. Unlike the classical method, it treats crack as an internal part of the body, eliminating the need for external kinetic relations to analyze crack growth. In [3], the peridynamic model of floating ice has been presented. Also, the crack growth pattern under transverse load with a pre-existing crack is studied. The current study validates the peridynamic model of floating ice, modeled as a peridynamic Mindlin plate [4] resting on Winkler elastic foundation [5] (representing sea surface), under static transverse pressure with classical theory. Then, the fracture pattern of the floating ice with and without pre-existing crack is studied and validated with the available literature.
Prediction of Structural Damages and Armament Accelerations of a Surface Naval Ship due to Underwater Explosions
Authors: Kim Hyunwoo , Cerik Burak Can , Choung Joonmo
Published: October 2022

Underwater explosion (UNDEX) caused by torpedoes yields the largest subsurface load that can be applied to a naval surface ship. The purpose of this paper is to estimate the extent of structural damages and accelerations acting on major armament systems by UNDEX for a large naval surface ship. The hull of the naval surface ship was modeled using Abaqus, a commercial finite element analysis code. By applying the Geers and Hunter UNDEX model provided by Abaqus, the primary shock wave and the secondary bubble wave based on doubly asymptotic approximation were implemented. The DSSE-HC model (Hosford-Coulomb combined with a localized necking model) enhanced for strain-rate effect was applied to estimate the extent of structural damages caused by UNDEX. Since the DSSE-HC model is not supported by Abaqus, the material subroutine was independently developed. The range of structural damages predicted by the DSSE-HC model was compared with that predicted by the constant failure strain model, which is widely used for UNDEX problems, to identify a difference in the structural damage. As a result, it was confirmed that there is a large difference in resulting damage. That is, the constant failure strain model predicted overestimated the extent of damage depending on the assumed failure strains. In addition, the accelerations acting on the various armament systems also showed significant differences. To predict the structural damage and acceleration of the armament system due to a UNDEX in the future, it was found that it is necessary to apply a fracture model with an advanced mechanical background rather than a simple fracture model.
The Long-Term Stress Range Distribution of Marine Risers Considering the Effect of Wave Directionality
Authors: Haghayeghi Zohreh Sadat , Ketabdari Mohammad Javad
Published: October 2022

The long-term distribution of stress cycles is a crucial part of the fatigue life assessment of offshore structures. This research studies the wave and current induced motion of a top tensioned riser connected to a semi-submersible platform. To account for the directionality, the platform’s directional response amplitude operator (RAO) is applied.

Since this research aims to calibrate Weibull distribution parameters, a long-term analysis of stress cycle distribution for all sea states is run. A finite element model is developed to solve riser dynamics under the effect of waves, currents, and top vessel motions. The code is solved for 87 sea states in the time domain, and then the stress cycles are derived by the Rainflow counting method. Finally, the long-term stress ranges distribution is fitted to a Weibull distribution. Comparison of the results in cases with and without directionality, shows that directional assumption leads to a broader distribution for stress ranges.
Knowledge Elicitation and Digitization Using FRAM to Inform Automation of Marine Operations in Ice
Authors: Musharraf Mashrura , Smith Doug , Smith Jennifer , Veitch Brian
Published: October 2022

Autonomous systems are becoming more common in the maritime industries. Some marine operations are more straightforward to automate than others. For example, the navigation of local vessels can be more easily automated on account of well-established standard operating procedures and relatively predictable operating conditions. Complex operations, such as ice management for ocean going ships, are less amenable to automation. Ice management is complex in nature and often takes place under challenging circumstances that include harsh weather, time constraints, and multiple information sources. The conditions are dynamic, and it is difficult to predict precisely how the circumstances will evolve. Successful ice management depends both on prior managerial planning and on-the-job adjustments made by the seafarers in dynamic operational conditions. To reach the level of autonomy equivalent to expert seafarers, the machine needs to assess, reason, decide, and act like them. One way to achieve this is to train the machine by the experts. This paper presents how the Functional Resonance Analysis Method (FRAM) can facilitate such training by connecting the human (expert seafarers) and the machine. A FRAM model of the ice-management operation will be used to 1) understand the ice management operation from experts and visualize it, 2) identify the salient features used and the on-job-adjustments made by experts during the operation, and 3) guide the digitization of the collected knowledge for ice-management automation.
Energy-Maximising Control Philosophy for a Cyclorotor Wave Energy Device
Authors: Ringwood John V. , Ermakov Andrei
Published: October 2022

Recently, cyclorotors, utilizing lift rather than buoyancy forces for energy extraction, have been proposed and inherit many of the appealing features characteristic of modern wind turbines. In particular, the ability to spill energy though foil pitching allows the device to remain at rated power despite significant variations in input power level, while additional cyclorotor features, including variable submergence depth and rotor radius, also permit a significant degree of modulation of the device structure, and energy absorption characteristics, offering considerable flexibility. These configuration flexibilities, in addition to torque control of the rotor/generator shaft (also characteristic of wind turbines) offers the control engineer considerable freedom in adjusting the device characteristics to maximise the effectiveness of the device in capturing wave power, while maintaining structural integrity and minimizing harmful stresses on system components. However, such flexibility also provides a significant challenge in the form of a multivariable control problem for a system described by significantly nonlinear hydrodynamics. This paper describes a proposed hierarchical control system for a cyclorotor wave energy device, utilizing submergence depth, rotor radius, foil pitch angles, and shaft torque as control inputs. The hierarchy involves the separation of the control actuators into two classes: structural (or slow) control effectors, and wave-by-wave (or fast) control effectors. In particular, the paper will examine the interaction between the two levels of the control hierarchy and the need, if any, for simultaneous optimisation of the control parameters at both levels.
Geohazard Study of Mudslide Event at a Subsea Escarpment and Structural Response of Crossing Flowlines: Part I – FEA Analysis
Authors: Hayes Kevin , Jang Hyunchul , Lai Lawrence , Musto Anthony
Published: October 2022

Subsea earthquakes have the potential to cause mudslides on the seafloor. These extreme events can have a significant impact on the structural integrity of subsea flowlines and umbilicals. A numerical geohazard study is performed on a subsea mudslide event occurring at an escarpment. The study objective is to determine the structural response of flexible lines crossing the escarpment area when subjected to an extreme mudslide event and to determine both the survivability of the lines and the need for proposed anchoring devices.

The study utilizes several numerical methodologies of increased complexity, precision, and cost, such as Finite-Element-Analysis (FEA), Computational-Fluid-Dynamics (CFD), and Fluid-Structure-Interaction (FSI).

This paper focuses on the FEA analysis, with the CFD and FSI analyses presented in a separate paper [1]. The FEA methodology reduces conservatism over conventional approaches by applying the mud debris flow effects on the flexible lines with a one-way CFD to FEA coupling and a morphing escarpment bathymetry. A CFD simulation is performed on a rigid seabed bathymetry to extract the mudslide velocity field at every second. This velocity field is applied to the Abaqus FEA simulation using Abaqus/Aqua with a one-way coupling approach, thereby allowing the FEA model loading on the flexible flowlines to be updated from the load created by the mudslide motion.

A key finding from the FEA study is that although the mudslide event creates large displacements in the flexible lines, it does not significantly pull the flexible flowlines over the escarpment, thereby eliminating the need for planned anchors and saving significant capital expenditure and installation costs.
Artificial Neural Network Surrogate Modeling for Offshore Wind Turbines Under Multi-Hazards
Authors: Wang Xiaowei , Heo YeongAe
Published: October 2022

Surrogate modeling of offshore wind turbines (OWTs) under multiple hazards such as wind, wave, and earthquakes provides an expedient path toward rapid risk assessment and management of these structures. This paper explores the applicability of artificial neural network (NN) for the surrogate modeling using a synthetic database based on finite element analyses of a typical OWT under combined loading conditions due to winds, waves, and earthquakes. A multi-layer NN with multiple output variables that can effectively capture the OWT’s multi-hazard behavior is developed. Considered input variables include peak ground accelerations, reference wind speeds, wave heights and periods, and damping ratios. A 5-fold-cross-validation-based optimization is conducted to identify the optimal number of hidden layers (N) and the associated optimal number of neurons in each layer (Q). It is found that N = 2 and Q = 12 are optimal for the OWT. The proposed NN model can evaluate the monopile maximum bending moment, and displacement/rotation at mudline with errors less than 30% in general.
Three-Dimensional Simulations for Geometric Optimization of a Shoreline Hybrid Wave Energy Converter
Authors: Koutrouveli Theofano , Das Neves Luciana
Published: October 2022

Hybridisation of different wave energy conversion (WEC) technologies appears to be a promising approach to harness the available wave energy resource most efficiently over a wide range of metocean conditions and overcome some of the challenges facing the roll out of a competitive wave energy industry. This underlying idea, hybridisation, is in the genesis of a novel device being further developed in the OCEANERA-NET project WEC4PORTS, which had been proved under the SE@PORTS project. Said novel hybrid wave energy converter (HWEC) combines two well-known wave energy conversion principles, an oscillating water column (OWC) and a multi-reservoir (4 reservoirs in the present configuration) overtopping device (OTD) and has been developed for installation in multipurpose breakwaters — i.e., a structure that retains its primary function of providing sheltered conditions for port operations to develop and includes electricity production as an added co-benefit. In this paper focus is given to the three-dimensional numerical model developed with OpenFoam for comparing alternative configurations of this breakwater-integrated HWEC, by studying the hydrodynamic efficiency. In total, five alternative geometries were compared. A frequent sea-state off the northern Atlantic coast of Portugal near the northern outer rubble-mound breakwater of the port of Leixões — the research prototype — is considered.
Computational Methods for Wave Structure Interaction Modelling in Coastal Environments Under Consideration of Bathymetric Attributes
Authors: Dempwolff León-Carlos , Martin Tobias , Windt Christian , Melling Gregor , Bihs Hans , Goseberg Nils
Published: October 2022

In recent years, increasing ship-sizes and associated increasing wave loads have led to a demand for prediction tools quantifying the ship-induced loads on waterways. Depth-averaged numerical models, using a free surface pressure term, are a prominent method to obtain the relevant design parameters. These models incorporate the wave deformation processes due to attributes of complex bathymetries, while allowing for an efficient simulation of large computational domains. The non-hydrostatic shallow water equations model REEF3D::SFLOW uses a quadratic pressure approximation and high-order discretisation schemes. This paper presents the implementation of a pressure term to account for the displacement of the free surface by solid moving objects. Two test cases verifying the implementation are shown based upon the analytical 1-dimensional solution of the wave propagation due to surface pressure and the estimation of Havelock angles. These verification tests are the first step towards a holistic model, combining a large scale model with CFD simulations near waterway banks.
On Simulating Variability of Sloshing Loads in LNG Tanks
Authors: Remmerswaal Ronald A. , Veldman Arthur E. P.
Published: October 2022

Local pressure measurements during liquid impacts in LNG tanks show a large variability, due to the free-surface roughness and the liquid fragmentation resulting from free-surface instabilities initiated by the gas escaping redfrom the closing gas pocket. It also makes the experimental discrimination of the effect of any multiphase parameter (density ratio, compressibility, phase transition) on impact loads more complex. The main challenges are (i) to capture both experimentally and numerically the development of the free surface instabilities; and (ii) to quantify their effect on the variability of local pressures.

Numerical modeling of the initiation of the free surface instabilities requires an accurate VOF-variant. Surface tension forces are modelled via a Young-Laplace jump condition, where the interface curvature is computed using a local height function. A parabolic interface reconstruction is required to ensure convergence of the curvature in space and time. Advection is performed with a modified EMPFA method. Spurious velocities are avoided by careful treatment of the (almost) discontinuities of the flow field. The model will be demonstrated on various simulations of breaking waves.
System Simulation-Based Feasibility and Performance Study of Alternative Fuel Concepts for Aquaculture Wellboats
Authors: Æsøy Lene , Piehl Henry , Nerheim Ann Rigmor
Published: October 2022

Climate change and population growth are increasing the need for sustainable food production. The ocean covers around 70% of the earth’s surface area and there is a great potential in utilizing this for ocean farming. However, the maritime traffic needs to decrease the emissions per transported mass with 50% by 2030 to meet the Paris Agreement. With an increased ocean space utilization, the vessel fleet will increase due to operational demands. Hence, with increased energy consumption, the need for decarbonization will increase further.

Wellboats are one of the main contributors to the CO2-emission related to traditional fish farming production of Atlantic Salmon, not counting the actual feed production. Within the well-boat the fuel system has the highest potential to reduce the carbon footprint. However, one of the major challenges with the renewable fuels are their low energy density compared to fossil fuels. New, promising energy concepts have emerged, but they lack a quantifiable performance evaluation within the complete aquaculture supply chain. Further, the choice of the vessel’s engine, generator and consumer modules affect each other. For example, changing to a fuel with lower energy density may lead to a larger fuel storage tank which then limits the available space for the fish tank.

The main objective of this study is to use graphical system simulation to model and evaluate the performance of new concepts for low- and zero-carbon fuels. Based on these simulations, new fuel concepts can be compared with conventional marine diesel systems. The simulation model can be used to detect operational limitations with respect to range, transported mass (fish), vessel size, weight, and refueling possibilities. Further, it can also be used to identify bottlenecks in terms of size, weight, and voyage limitations.

Based on the outcome of the wellboat system simulation model, the feasibility of fuel systems can be evaluated and used as recommendation for the design of new wellboats. The industry needs feasible solutions which are sustainable in terms of reliability and cost. The insights gained from using the simulation model aim to assist the industry to reach their emissions reduction goals by 2030, and to provide decision support for retrofitting the fuel and energy system of wellboats.
Conductor Instability – Experience From 10 Years of Monitoring Drilling Operations on Subsea Wells in Harsh Environments
Authors: Molven Håkon , Pöcher Per Kristian , Martinsen Vegard , Holden Harald
Published: October 2022

Subsea production and exploration wells are subjected to loads from the drilling riser and subsea BOP. Over time, BOPs have grown in size and weight resulting in larger wellhead loads. The main task of a subsea conductor is to transfer these loads into the seabed. The bending loads applied on the wellhead will be transferred into the conductor, which again will transfer the loads through the cement and into the soil.

In cases where the load exerted on the soil and cement exceeds the capacity, the soil around the conductor fails. Further can the cement filling the cavity between the conductor and the soil crack and consequently lose its intended function. Support degradation reduces the conductor’s ability to support the well. More load will have to be taken by the conductor and casing structure.

4Subsea has delivered the Subsea Wellhead Integrity Monitoring (SWIM) service for operations on subsea wells for over 10 years. Sensors monitoring the motion and loads near the subsea wellhead have been mounted on more than 300 drilling campaigns. For the last couple of years, more than 70% of the drilling operations with semi-submersible drilling rigs on the Norwegian Continental Shelf (NCS) have been monitored using this SWIM service. As a part of the SWIM service, integrity parameters showing whether the conductor gives the intended amount of support are calculated. Loss of conductor support during drilling operations has been observed in several of these campaigns. An introduction to these integrity parameters will be given together with a discussion on the integrity issues that can be detected using these parameters.

The main purpose of this paper is to summarize the observations of conductor instability over the last 10 years. Examples of monitoring results showing how the instability issues were detected will be presented. Examples of actions taken to react to conductor instability will also be discussed. Finally, statistics on some core metadata such as water depth, region, and well type will be summarized and discussed.
Propeller Wake Measurement and Uncertainty Assessment by a Towing-Tank LDV
Authors: Long Xin , Liu Liwei , Zhang Zhiguo , Ye Bin , Wang Xianzhou
Published: October 2022

A towing-tank underwater Laser-Doppler Velocimeter (LDV) system equipped with an optical fp50 shift probe and a high-precision 3D moving coordinate frame was developed to measure propeller wake. The Nd: YAG-laser head was available in a power range from 50mW up to 500mW, different wavelengths and mounted directly in the probe. The towing carriage speed of initial point and predetermined moved points were repeatedly measured by the 2D probe. The test duration was changed to obtained appropriate sample sizes, which was applied into the later test. Uncertainty analysis of LDV calibration follows ITTC-recommended procedures and guidelines. The uncertainty assessment of test data follows ASME standard procedures to quantify the comparisons and reach conclusions regarding the quality. The uncertainty analysis results of the repeated uniform flow measurements on initial point presented high measurement accuracy of the probe. The uncertainty analysis results and the comparison between the initial point and predetermined moved points verified the high moving accuracy of the coordinate frame in uniform flow measurements. All completed repeated uniform flow measurements were considered as an important contribution to estimate systematic uncertainty of the whole LDV system. The propeller wake field of a modified E1619 was measured in open water test. According to the test condition, an in-house viscous CFD code was used to carried out simulation. The test results agreed well with the CFD results. The uncertainty assessment was carried out following the same standard procedures. This LDV measurement system is reliable enough, and its advantage of non-intrusive measurement can be perfectly presented in propeller wake measurement.
On the Validity of Miner’s Rule and Its Application in Offshore Pile Design Practice
Authors: Stark Anne , Breidenstein Sebastian , Grabe Jürgen
Published: October 2022

Current design practice simplifies complex loading scenarios, i.e. highly cyclic loads from varying direction offshore, using classification methods by combining different cycle packages with constant frequency, mean load level and amplitude. This procedure assumes the resulting accumulated deformation in the soil to be independent of the ordering of the cycle packages, i.e. the validity of miner’s rule. This paper presents an experimental study on the validity of Miner’s rule in non-cohesive soils based on high cyclic direct simple shear tests. The test program comprises monotonic and high cyclic direct simple shear tests in fine silica sand, which is routinely used at Centre of Offshore Foundation Systems. The paper investigates the effect of ordering of the cycle packages on the resulting cyclic deformation accumulation for different loading scenarios with varying mean and cyclic load level. The results are evaluated in terms of the accumulated volumetric and the resulting shear strain. Based on the literature, Miner’s rule is assumed to be valid for a deviation of the relevant variables of < 20 %. Conclusions with respect to the influence of cyclic preloading are drawn and related to current design practices.
Decline in Slope Stability as a Consequence of Gassy Soil in Submarine Slopes on the Balearic Promontory
Authors: Kaminski Pauline , Grabe Jürgen , Sager Thore F. , Urlaub Morelia
Published: October 2022

Submarine landslides can entail a substantial hazard for offshore infrastructure as they are capable of triggering tsunamis and may develop into highly mobile turbidity currents capable of breaking seabed cables. Despite considerable research activity, the trigger mechanisms for such landslide events cannot be clearly defined. Recently, marine gas occurrence has been investigated as a possible trigger mechanism. The behaviour of a fine-grained gassy soil is influenced by a variety of micromechanical processes; amongst destructuring due to fracture formation or gas bubble expansion, and bubble flooding with subsequent cavity collapse. Capturing and modeling these processes in order to assess the destructive potential of enclosed gas bubbles in submarine slopes is to date a considerable scientific challenge. With the help of a large number of Finite Element Limit Analyses (FELA), which are based on laboratory tests on a gravity core from the western Mediterranean Sea, submarine slope stability in the respective region was evaluated. Based on these analyses, gassy soil can be defined as a preconditioning factor but not as a capable trigger mechanism for submarine landsliding.
Integrated Optimization of Hybrid Steam-Solvent Processes in a Post-CHOPS Reservoir With Consideration of Wormhole Networks
Authors: Zhao Min , Yang Daoyong
Published: October 2022

In this paper, an integrated technique has been developed to evaluate and optimize performance of hybrid steam-solvent processes in a post-CHOPS reservoir with consideration of wormhole networks. A reservoir geological model is developed and calibrated by history matching reservoir pressure with the oil, gas, and water production rates as the input constraints, while its wormhole network is characterized with a newly developed pressure-gradient-based (PGB) sand failure criterion. Once calibrated, the reservoir geological model incorporated with the wormhole network is then employed to evaluate and optimize performance of hybrid steam-solvent processes under various conditions, during which the net present value (NPV) is maximized with an integrated optimization algorithm by taking injection time, soaking time, production time, and injected fluid composition as controlling variables. It is found that a huff-n-puff process imposes a positive impact on enhancing oil recovery when wormhole network is fully generated and propagated. Among all solvent-based methods, a pure CO2 huff-n-puff process shows a better performance than flue gas, while the addition of alkane solvents leads to a higher oil recovery compared with that of the CO2 only method. Since the addition of C3H8 and n-C4H10 will significantly decrease heavy oil viscosity and enhance oil swelling, all hybrid steam-solvent injection achieves high oil recovery by taking advantage of both thermal energy and solvent dissolution. It is found that the NPV reaches its maximum of C$5.36 × 107 when the steam temperature is 200°C for the optimized hybrid steam-solvent scenario.
Semisubmersible in Service Experiences on the Norwegian Continental Shelf
Authors: Ersdal Gerhard , Halsne Marita , Oma Narve , Andersen Terje , Langøy Morten , Bjørheim Lars G.
Published: October 2022

Semisubmersible Mobile Offshore Units (MOUs) have been used on the Norwegian Continental Shelf since the mid 1960’s and since the late 1980’s also as production units. During these years a significant amount of experience data has been gathered and systemized.

This paper gives an overview of reported incidents in the period between 2000 and 2020, related to the structure and the maritime systems on semisubmersibles operating on the Norwegian Continental Shelf. Incidents, accidents and damages are reported by operators and rig owners to the PSA. These are summarised in this paper and includes cracks, dents from wave actions, anchor impact on structures, horizontal and vertical wave impacts and ship collisions. In addition, failure of the dynamic position keeping system, mooring system breakage and incidents related to stability and ballasting have been reported.

The paper provides numerical values of the occurrence of many of these incidents, where possible, in addition to an overview of the number of semisubmersibles active on the Norwegian Continental Shelf. In addition, a listing of accidents and incidents are provided and to some extent a summary of the incidents are provided for a possible further evaluation of common causes, importance and the possible consequences of these accidents, incidents and damages.

One fatality due to horizontal wave impact have been experienced in the period evaluated and a continued focus on air gap and wave in deck loading is important. However, these issues have later been better standardized by the class societies and the focus from PSA now is to ensure that the operators and rig owners are operating according to these standards. The main challenge for semisubmersibles continues to be operational issues related to stability and ballasting incidents, both because of the observed frequency and the potential consequence of such incidents.
Storm Models for the Calculation of Extreme Wind Speed
Authors: Laface Valentina , Arena Felice , Romolo Alessandra
Published: October 2022

This paper proposes a new approach for the calculation of extreme wind speed. Specifically, the storm approach widely employed for long-term statistics of ocean storms is reviewed and adapted for the analysis of windstorms. This approach is based on the substitution of the sequence of real event at a given site with a sequence of simplified events characterized by a kind of equivalence with the actual ones. Basing on that, analytical solution for the calculation of the return period of a storm event whose peak exceeds a given threshold is developed. The windstorm model characterizes the simplified windstorm event by means of two parameters which are the maximum average wind speed over the windstorm and a duration parameter determined by imposing that the maximum expected gust in the simplified event is the same of that in the actual one. First, an attempt is made to assimilate the peak gust to a “maximum expected gust” and to relate this quantity to the windstorm duration. Then, in order to validate the new model, a data analysis is carried out to estimate average wind speed return values with the proposed approach and results are compared with those provided by well-established peak over threshold method.
Optimization of Oil Recovery and CO2 Sequestration in Tight Reservoirs During CO2 WAG Flooding With Hydraulic Fracture
Authors: Li Bao Zhen , Zhang Jian , Kang Xiaodong , Wang Liqi , Tang Engao , Zhou Wensheng
Published: October 2022

Tight oil reserves are of great significance in oil exploration and development in China. CO2 flooding is promising in tight reservoirs due to its high mobility and better injectivity, and water alternative gas (WAG) flooding could improve sweep efficiency and oil recovery, which is extensively used. However, its injectivity in tight reservoirs is limited. The feasibility of improving CO2-water alternations performance in tight reservoirs by hydraulic fracturing was investigated in this study.

In this paper, numerical reservoir simulation was conducted to simulate CO2 WAG flood process with typical reservoir characteristics and hydraulic fracture properties of the YC formation. The optimization of hydraulic fracturing during CO2 alternative water injection process were performed with numerical simulation, response surface and multiple response optimization method, which could be done with acceptable accuracy by performing a limited number of runs, hence, reducing the engineering time, effort and money. With the simplified equation between the NPV or CO2 storage and different hydraulic fracture parameters, operators can determine the best combination of all the factors for different objectives. The results show that moderate fracture length, large fracture height and small water-gas ratio could achieve the optimal economic benefits and CO2 storage in low permeability reservoirs.

This work can provide a better understanding of the physical mechanisms and key parameters affecting the effectiveness of CO2 WAG flooding for enhanced oil recovery in the tight formation.
Hydrodynamic Interactions of Multiple Surface-Piercing Struts by Smoothed Particles Hydrodynamics
Authors: Vernengo Giuliano , Demirel Yigit Kemal , De Marco Muscat-Fenech Claire , Gaggero Stefano , Villa Diego
Published: October 2022

The hydrodynamics of surface piercing struts and pile-like structures in general presents some very complex fluid dynamics phenomena that are worth investigating. Among them there are the forward wave pile-up at stagnation, the wave breaking with flow reversal like that observed in Bidone-type hydraulic jumps, flow separation caused by interactions of steep free-surface waves and the turbulent wall boundary layer. Such a type of flow structures plays a key role in several engineering applications, ranging from naval architecture to civil and ocean engineering.

The hydrodynamic analysis of different surface-piercing struts in tandem configuration has been done in the framework of a Smoothed Particle Hydrodynamic approach available through the open-source software DualSPHysics, developed to exploit the GP-GPU architecture to speed up the computation. A numerical wave tank has been set-up to carry out calm water tests. Beyond the influence of the forward speed, the analysis has focused on the effect of three main geometric parameters establishing the configuration: the longitudinal and the lateral distance among the vertical, surface-piercing, struts and the relative size among them.

The mean and the rms values of the unsteady near-field free surface elevation have been analyzed and compared among the selected cases and interaction effects are studied in comparison to the free surface obtained for the equivalent single strut configuration.
Increase of Wave Drift Forces in Severe Seastates Due to Wave Frequency Viscous Damping
Authors: Fonseca Nuno , Ommani Babak
Published: October 2022

Calculation of wave drift forces in the context of mooring analysis is usually performed by potential flow boundary element methods. For monohulls with large waterplane area, such as floating, production, storage and offloading vessels (FPSOs), the assumption is that heave and pitch damping is dominated by inviscid radiation of waves. The present study investigates the effects of heave and pitch wave frequency viscous damping on the horizontal wave drift forces of a FPSO. Experimental evidence indicates the presence of non-negligible viscous damping in case of moderate and severe seastates. Such observation is confirmed by computational fluid dynamic (CFD) simulations of forced harmonic motions with different periods and amplitudes. On a second step, a numerical analysis shows that the surge wave drift forces increase significantly at the low frequency range when realistic heave and pitch viscous damping is considered. The frequency range where standard potential flow calculations appear to underestimate wave drift forces coincides with the range where design seastates have most of the energy. Finally, the wave drift coefficients calculated with realistic additional viscous damping are compared with empirical coefficients identified from model test data. The identification procedure follows a second order signal analysis technique known as cross bi spectral analysis.
Wave Energy Converter Optimal Design Under Parameter Uncertainty
Authors: Giorcelli Filippo , Sirigu Sergej Antonello , Pasta Edoardo , Gioia Daniele Giovanni , Bonfanti Mauro , Mattiazzo Giuliana
Published: October 2022

In the field of renewable technologies, the possibility to obtain energy exploiting seas and oceans’ wave motion has been known for a long time. Devices that transform wave energy into electric energy exploiting wave motion are called Wave Energy Converters (WEC). Following the design studies carried out in recent years, the research now proceeds towards the development of useful processes for the optimization of these devices. In this work we develop a preliminary robust optimal design process for the WEC system devices, in order to increase their reliability and robustness. Robust optimal design is a probabilistic optimization method for realistic optimization problems, in which, the uncertainty that occurs between real-world implementations and their ideal project value is taken into account. This method studies these parameters and finds suitable solutions to avoid unsatisfactory system performances and designs which can compromise their performances. Therefore, the process final purpose is to obtain a robust optimum instead of a global optimum. In this work, we developed the robust design optimization strategy for the design of a pitching wave energy converter, able to minimize its Levelized Cost of Energy (LCoE). This is done exploiting information given by two selected robustness indexes.
Design Methodology Evolution: Transition From O&G FPU to FOWT
Authors: Zhang Shaosong , Son Daewoong , Peiffer Antoine
Published: October 2022

Floating offshore wind is an emerging industry. The first floating offshore wind turbine designed by Principle Power was deployed offshore Portugal to support a 2-MW wind turbine generator in 2011. The turbine is supported by a proprietary semi-submersible concept named WindFloat. Since then, more than ten floating offshore wind turbines including eight additional WindFloat platforms have been installed and commissioned to produce power offshore. There are significant experiences built from the engineering delivery of the FOWT units.

Compared with offshore oil and gas industry, the offshore wind energy industry is still young. A great part of the design methodology for FOWT is inherited from the offshore oil and gas industry. But there are many design aspects which are unique to the FOWT because of the operational differences between FOWT and FPU for oil and gas industry. This paper will give a brief description of the design methodology for FOWT based on the experience built from the delivery of multiple WindFloat floating foundations. The methodology, which is inherited from oil and gas, but being improved or customized for FOWT will be discussed in detail. The subjects to be addressed in the paper include the engineering interface between WTG and the floating substructure, the floating stability assessment, wind load simulation, the determination of the duration of the numerical simulations and the number of random seeds, and the validation with model test results.

As the wind turbine power capacity becomes larger, the floating substructure and its associated mooring system follows a trend of growing dimensions, which is more comparable with the FPU. The growth of the floating offshore wind industry will leverage the lessons learned from offshore oil and gas industry.
A New Approach for Rock Strength Estimation Through a Semi-Point Load Strength Index and Correlation With Destructive and Nondestructive Tests
Authors: Abugharara Abdelsalam , Mafazy Salum , Butt Stephen
Published: October 2022

Multi scale projects of civil, mining, and oil and gas drilling require to determine rock strength as main property of surface and subsurface rocks for safe structures, economic mining operations, as well as drilling performance optimizations. The Unconfined Compressive Strength (UCS) as a main rock strength test is considered a time consuming for sample preparation, costly, and unaffordable. However, it is required for correlations for rock strength obtained from other tests. A semi-Point Load Strength Index (Semi-PLSI) is proposed in this research as a new approach to estimate rock strength. The objectives of this research include reducing testing costs, simplifying sample preparation and testing procedure, precisely localizing, stabilizing, and balancing specimens while testing, and improving accuracy and repeatability. For this test, various sample dimensions and length to diameter ratios are prepared from granite rock type. A 400 (kn) Geomechanics Frame is used for testing and an advanced data acquisition system is utilized for data recording. Data obtained from Semi-PLSI are correlated with data obtained from two main strength types including UCS that represent the shear fracture strength and indirect tensile strength (IT) that represent the tensile fracture strength. Data correlation also includes measurements of ultrasound primary and secondary wave velocities that represents nondestructive data measurements. The good correlation agreements and the verity ways of estimating the strength enrich this new approach of the Semi-PLSI proposed for UCS estimation and pave the road of more research.