bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Wind Loads and Damping on Heading Stability of FPSOs
Authors: Zangeneh Razieh , Thiagarajan Krish P. , Cameron Matthew
Published: October 2017

Tanker vessels used for offshore oil production and storage are kept at station by turret mooring systems, enabling the vessel to weathervane in the direction of dominant environmental loads. The disruption of heading equilibrium for a turret-moored tanker was predicted by experiments and numerical studies. A vessel was observed to lose control in head sea condition, wherein for wavelength from 0.73 < λ/L < 2 (L-ship length) the model drifted to a large angle of 45–60 degrees (Thiagarajan et al. 2013). Previous numerical analyses conducted by the authors identified that this heading drift reduced remarkably in the presence of wind. This finding is confirmed by an experimental study and reported in this paper.

A geometrically scaled down version of a turret-moored FPSO at 1:120 scale of a prototype VLCC was tested at the Alfond W2 Wind & Wave Ocean Engineering facility of the University of Maine. This lab is a unique facility equipped with a high-performance wind machine over a multidirectional wave generator, and can create regular or random sea-states with wind speeds up to 7 m/s. The tests reported here were conducted with regular waves under two wind speeds (12 and 25 m/s full scale). It was observed that the presence of an initially bow wind can minimize the heading instability. The reason for this observation is described by analyzing the effect of the wind induced moments on the equilibrium condition.

Free-decay tests were also conducted to investigate the contribution of the wind damping to the total damping. Measured results show that in the presence of wind, the damping values are higher than those estimated due to hydrodynamics only. It also has been discussed that this wind induced damping on FPSOs, can result in smaller heading angles. From this study, it is concluded that the wind can play a large role in the station-keeping dynamics of the moored-tankers.
Theoretical Analysis of the Performance of a Self-Righting Boat
Authors: Bai Tiechao , Ding Ziyou , Wang Xianzhou , Zhang Zhiguo , Zhang Fanchen
Published: October 2017

A self-righting body is a highly specialized boat that immediately rolls back to an upright position during a catastrophic event in which the vessel is turned over. This capability can be a crucial “fail-safe” when operating in high-sea conditions and coastal areas. This paper used a theoretical method to study the behavior of the righting ability of a boat. The analysis of self-righting process of a boat include stability and rolling capability of the boat design. The stability and rolling capability are correlated with each other. The derivation of the motion equations for stability is performed and the rolling damping is neglected to simplify the problems. The recovery times and rolling cycle are calculated by solving the motion equations based on the integration method. The recovery time and rolling cycle were obtained for further application. The equations could be applied to aid the design of a floating boat with self-righting capability. The calculation results were compared with the model test results. The comparisons show the reliability of the method used in this paper.
Considerations on Numerical Procedure for Stick-Slip Analysis of Drill String
Authors: Katsui Tokihiro , Inoue Tomoya , Izutani Kenta , Nagaishi Yuhi , Rheem Chang-Kyu , Matsuo Miki Y.
Published: October 2017

Stick-slip is one of the typical phenomenon which is observed in offshore drilling and considered as a critical problem for the drilling operation. The stick-slip makes a large fluctuation of drill bit rotation, even though the top of the drill pipe is rotating at a steady velocity and sometimes causes the damage of the drill bit. Additionally, it leads a crushing of the sediment layer which is a big problem especially for the scientific drilling [1][2][3]. The main purpose of the scientific drilling is to correct high quality core samples of sediment layers under the seabed. However, once the stick-slip occurs, it makes difficult to recover a high-quality sediment layer core sample. Therefore, it is necessary to detect the occurrence of stick-slip and its fundamental characteristics such as oscillation periods and amplitudes by simulation with the aid of surface drilling data, which can be monitored during the drilling operation to mitigate or prevent stick-slip. It would be advantageous to identify the characteristics of the stick-slip from the surface drilling data.

The past study [4][5][6] investigated a numerical method to analyze the stick-slip by solving the NDDE (Neutral Delay Differential Equation) which is derived from torsional vibration equation. A small-scale model experiment was conducted in a water tank to observe the stick-slip phenomenon, and the result from the analytical model is evaluated with that obtained from the experiments.

In this study, the numerical model is applied for the stick-slip analysis not only of the drill pipe model but also the actual drill pipe in operation. The solutions of the NDDE is depend on not the initial value but the initial history of the solution, because NDDE contains a delayed function term. Especially, the initial history settings have much effect on the numerical solution of NDDE in case of the actual drill pipe. Additionally, to solve the NDDE for stick-slip analysis, we must set some model parameters concerned with the frictional torque on drill bit. The present study investigated the effects of the initial history and the model parameters settings on numerical solutions in detail and presented an procedure to determine the appropriate settings of the initial history and the model parameters by reference to the measured top drive torque.
A Study on an Iceberg Drift Trajectory
Authors: Andersson Leif Erik , Scibilia Francesco , Imsland Lars
Published: October 2017

Iceberg drift forecast is a challenging process. Large uncertainties in iceberg geometry and in the driving forces — current, wind and waves — make accurate forecasts difficult. This article illustrates from a data set that even if the uncertainties in current, wind and waves are reduced the forecast using a dynamic iceberg models stays difficult, because of the sensitivity of the model to different parameters and inputs. Nevertheless, if the uncertainty of the current driving force on the iceberg is reduced by measuring the current at the iceberg location, it is possible under specific conditions to estimate the approximate iceberg shape. This iceberg shape geometry can be used directly in the dynamic iceberg model.
Explosion Risk Analysis on the Liquefaction Process of LNG-FPSO at the PFD Level
Authors: You Wonwo , Park Jaeuk , Lim Youngsub
Published: October 2017

The potential risk of an offshore processing facility is the major important part in the oil and gas industry due to its limited space causing difficulties in evacuation. An offshore processing facility is normally exposed to flammable oil and gas in the operating phase. Especially, uncontrolled hydrocarbon leaks or ruptures of the equipment present main threats. These failures can lead to fire and explosion disaster. Some studies have proposed fire and explosion assessment methodologies and made fire and explosion assessment tools. These tools can provide risk assessments result using physical effect modelling software and following the related standards or engineering practices according to accident scenarios. Nevertheless, existing fire and explosion assessment procedures are still not comprehensive enough to applicate a specific process due to its complexity and are not clear which stage in a project is appropriate for applying it.

This paper focuses only on explosion accidents and discusses the development of an explosion risk analysis procedure possible to apply at process flow diagram (PFD) level. The explosion risk analysis procedure using PFD has 6 steps; modelling of a process, scenario selection, inventory calculation, frequency calculation, consequence modelling and risk estimation. It starts at modelling of a specific process using process simulation software, HYSYS. The process modelling can be optimized by the existing methods and finally provide the PFD for the specific process. In the scenario selection step, the information required to perform a risk analysis is identified. The inventory calculation conducts to calculate the inventory of a defined segment after sizing of the equipment in the PFD. The frequency calculation consists of leak frequency and ignition probability. The leak frequency can be calculated with historical database and the ignition probability can be calculated with a specific ignition probability model. The consequence modelling is conducted by using physical effect modelling software, PHAST. It can provide the distance to specified overpressure. Finally, at the risk estimation step, the risk results are evaluated. This procedure can help to applicate a specific process easily and provide explosion risk assessment tool at PFD level.

This paper conducts the case study for a liquefied natural gas floating production storage offloading (LNG-FPSO) which is one of the representative offshore processing facilities. Especially, a natural liquefaction process in a LNG-FPSO, which liquefies the processed natural gas to store in a storage tank of a LNG-FPSO, is the most important process in terms of cost and risk. In the situation the most of ongoing or prospective projects for LNG-FPSO adopt dual mixed refrigerants (DMR) liquefaction process, the representative configurations of the DMR liquefaction processes are evaluated and compared. It can help decision making through providing which configuration has an advantage in terms of explosion accidents.
Parametric Analysis of Crushing and Squeezing Loads Over a Flexible Pipe During Installation Procedure
Authors: Guedes Mendonça Heloisa , de Arruda Martins Clóvis
Published: October 2017

Launching of flexible pipes in deep water is one of the most critical operations since it can damage the metallic layers. The effects of installation loads on the pipe, such as the crushing applied by tracks of the caterpillars on the launching vessel and the squeezing resulting from the tensile armors traction are investigated in this paper. The aim of this work is to present a parametric analysis of variables responsible for the flexible pipe failure during laying operation. In order to perform this analysis, a tridimensional model using Finite Element Method is evaluated considering the actual profile of the metallic layers and the real geometry of the caterpillar tracks. Important parameters, as the stiffness of the caterpillar tracks and the installation loads, are analyzed to identify how these variables affect the yielding of the metallic layers. The investigations of the pipe failure based on the crushing and squeezing loads are stated considering different launching depths. The results of this parametric investigation are presented, and a safe configuration for the launching is proposed, based on the tridimensional model investigated and on in each of the analysis performed.
Ultimate Bearing Capacity Assessment of Hull Girder With Asymmetric Cross-Section
Authors: Li ChenFeng , Fu Peng , Ren HuiLong , Xu WeiJun , Guedes Soares C.
Published: October 2017

The objective of this study is to investigate the variation of neutral axis of ship hull girder due to asymmetric geometry or asymmetric load, and its influence on the ultimate strength of hull girder. In order to account for asymmetric geometries and loads of hull girders, the force equilibrium and force-vector equilibrium criteria together with a minimum convergence factors (error) method, are employed to determine the translation and rotation of neutral axis plane of symmetric or asymmetric hull cross-section in the application of Smith’s method at each step of curvature of the hull girder. The ultimate strengths of Dow’s 1/3 frigate model with three predefined structural integrity states, one intact and two damaged respectively, are investigated by the improved Smith’s method for a range of variation of heeling angles. The influence of asymmetric geometry and load on the motion of neutral axis plane and on the ultimate strength are analyzed and discussed. The results show that the improved iteration strategy together with the MCFM is self-adapting and more accurate in searching the translation and rotation of neutral axis plane. Finally, the envelope curves of the bending moments in the three predefined integrity states are obtained, which can be used for assessing ultimate strength of hull girders under combined vertical and horizontal wave bending moments.
Conceptual Design and Analysis of a Submerged Wave Energy Device in Shallow Water
Authors: Hayatdavoodi Masoud , Ertekin R. Cengiz , Thies Jason T.
Published: October 2017

Preliminary conceptual design of a submerged wave energy converter (WEC) device in shallow water is presented. The WEC consists of a fully submerged, horizontal, flat plate that is restricted to vertical oscillations due to surface waves. Thin rails are used to guide the vertical oscillations and restrict the motion in all other directions. The vertical oscillation of the plate is converted to electricity by use of a direct drive power take-off (PTO) device located on the seafloor. The PTO is a linear generator consisting of a translator, directly linked to the plate by a solid shaft, and a stator. The plate oscillation is controlled by use of a spring, and by the damping effect of the PTO. The oscillations of the horizontal plate is determined by coupling the fluid governing equations with the equation of vertical motion of the horizontal plate that consists of sum of all vertically acting forces on the plate, including the vertical wave-induced force, the frictional force due to the guide rails, the spring force, and the damping force due to the PTO. The fluid flow is governed by use of the Level I Green-Naghdi equations. We also used the Navier-Stokes equations coupled with the volume of fluid method, solved through OpenFOAM. Comparison and discussion of the results from the two theoretical approaches are provided. The wave energy device has a very simple configuration, and the energy output is independent of the direction of incoming waves. Moreover, the entire device is fully submerged at all times and hence it is protected from the impact of breaking waves on the surface. It is concluded that the proposed wave energy device can be a reliable solution for wave energy harvesting in shallow to intermediate water, while minimizing the common challenges seen in many WEC devices.
Case Studies of Petroleum Production Systems With the Flow Performance Index (FPI)
Authors: Mendes José Ricardo P. , Bordalo Sergio N. , Celis Ariza Sergio Fernando , Miura Kazuo
Published: October 2017

In this work, the Flow Performance Index (FPI) is introduced to guide the analysis of the performance of well systems for petroleum production. For some time now, the oil industry has been investing in the technological advancement of the instrumentation of its wells and flow lines; therefore, the volume of acquired data is quite substantial. Nevertheless, these data are still scantly used and stored in isolated databases where sharing the data is difficult, forcing the professionals to waste time, searching and organizing information, rather than spending time on decision-making processes. Consequently, there is a need to organize and integrate the available data from the different sources and areas of petroleum engineering. The FPI may be employed to handle large amounts of field data (measured periodically) in a rational approach to integrate the data. The FPI allows the assessment of the technologies used in wells for completion and artificial lift, and the performance of wells and flow lines; it may be used for monitoring production and to aid in the diagnosis of flow assurance problems; it could also be employed for benchmark studies and comparison of field production systems. A few examples of applications of the FPI are presented here, comparing the performance of vertical, directional and horizontal wells, sand control technologies, and monitoring of production. Further, the concept of the FPI is extended for gas-lift wells, and a more general formulation is proposed to include mechanical-lift systems. The examples given herein have proven the usefulness of the FPI, in different areas of an upstream business unity in Brazil.
A Tribute to the Extraordinary Research Work Within the Domain of Fatigue and Fracture of Offshore and Ship Structures Carried Out by Prof. Stig Berge and Per Jahn Haagensen at the Norwegian University of Science and Technology
Authors: Horn Agnes Marie , Macdonald Kenneth A.
Published: October 2017

The motivation of this paper is to highlight the importance of the work carried out during the last decade by Prof. Haagensen and Prof. Berge at the Norwegian University of Science and Technology NTNU, with the aim to inspire and motivate young engineers to continue their important and valuable research within fatigue and fracture. This paper will focus on their historical contribution to the research within fatigue and fracture of offshore and ship structures.

Stig Berge is a professor of Marine Technology at the Norwegian University of Science and Technology NTNU. He has spent his academic carrier focusing on fatigue of offshore and ship structures; he has published more than 70 papers and articles in well-known journals and conferences since 80’s.

Per Jahn Haagensen is currently an Emeritus professor at Department of Mechanical Engineering and Logistic Faculty of Technology. He has spent his whole research carrier within fatigue and fracture related topics mainly for the offshore industry. He is especially known for the different fatigue improvement methods which have been developed since the 90’s. He has until recently been an active member of the International Institute of Welding (IIW).

This article aims to present their main findings and conclusions from their long academic carrier. While the authors have strived to convey in a single paper an overview of the careers and important contributions, the Professors themselves may well have chosen to place a different emphasis on their work.
Numerical Solution of Rapid Freezing of Sea Water on Cold Substrates
Authors: Dehghani Saeed R. , Muzychka Yuri S. , Naterer Greg F.
Published: October 2017

Rapid freezing of sea water on a cold substrate of spongy ice is investigated. The mechanism of transient ice accretion on cold substrates is different than slow freezing of salt water. An investigation of rapid freezing in this paper fills a gap of knowledge related to periodic icing of salt water on marine and offshore structures. The equation of transient heat conduction through brine-spongy ice is analyzed. Rapid freezing causes complete salt trapping, which makes the salinity constant and stable at the phase interface during the solidification. A thin layer of salt water is considered in contact with a spongy substrate. A finite difference method is employed to calculate the rate of solidification of the brine layer and consequently the thickness of ice accumulated. The discretization is based on the Method of Lines (MOL) which is a useful numerical-iterative method for boundary moving problems. Numerical results show that colder substrates and brine layers have the potential to create a thicker layer of new ice.
Wave-Induced Current in a Seakeeping Basin
Authors: van Essen Sanne , Lafeber Wim
Published: October 2017

During tests in MARIN’s wave basins, it was observed that large-scale current patterns may develop under the influence of wave generation and absorption. The velocity of these currents is very low, so they generally do not influence the behaviour of models. However, for specific experiments at low speeds — wave added resistance tests with small models or current drag tests — a residual current may influence the results significantly.

A good understanding of the residual circulation in a wave basin is essential to improve the quality of the tests performed. The wave-induced current patterns were observed during tests in MARIN’s Seakeeping and Manoeuvring Basin (SMB). The patterns may develop in several ways under the influence of waves in a basin. End effects of Stokes drift (mass transport due to second-order wave effects) can play a role, as the water has to return at the end of the basin. The SMB has the capability to generate oblique waves. It therefore has a wave-damping beach along two sides of the basin. Similar to ‘real’ beaches, they may cause alongshore currents and rip currents under the influence of oblique and perpendicular waves respectively. During the tests, floaters in the form of oranges were distributed in the basin after wave generation. They were tracked using a camera system. The images were processed such that the surface current patterns in the basin were visualized, and an estimate of the velocities was obtained. Additional local acoustic current meter measurements were used to check the order of magnitude of these velocities.

Based on these tests, it was concluded that different patterns may occur in the basin, with the largest velocities after oblique wave generation. Typical surface velocities are in the order of 1 to 2 cm/s, non-uniformly distributed over the basin. Due to this non-uniformity and because decay is slow (memory effects), very sensitive added resistance and current drag tests may have to be corrected for a measured current velocity in the future.
Estimation of Undisturbed Geothermal Gradient in Wells From Measured Drilling Data: A Numerical Approach
Authors: Cantinelli Sevillano Lucas , De Andrade Jesus , Sangesland Sigbjørn
Published: October 2017

The undisturbed geothermal gradient is a key thermal boundary that drives heat transfer processes occurring in oil and gas wells throughout their lifetime. However, the temperature distribution with depth is somewhat uncertain, and this is often assumed to be a linear approximation from the mudline to the bottom of the well. During drilling, the circulating temperature may significantly affect the rheology of the drilling fluids and the cement setting processes. Therefore, erroneous estimates of the wellbore temperature may affect the overall performance of the drilling phase and subsequent well operations. Further, it is important to know the accurate temperature distribution within the formation for assessment of the petroleum prospectivity through source rock maturation and reservoir quality.

This paper presents a numerical methodology to estimate the undisturbed geothermal gradient while drilling in offshore wells. This methodology may also be applied to onshore wells by simplification.

The new approach is based on an in-house axisymmetric wellbore transient thermal model, in which the equations are solved using the finite difference method. The model computes the heat transfer between the well and riser system with the surroundings. However, other computational codes may also be used following the framework presented in this study. The computer code should provide a detailed representation of the geometry of the wellbore, the physical properties of the drilling fluid and formation, the suitable thermal boundary conditions and temporal discretization. The temperatures of the fluid at the inlet of the drillstring and at the bottom hole assembly (BHA), in the annulus A, are used as input to the numerical model that iteratively adjusts the undisturbed geothermal gradient, which generated the temperature recordings while drilling.

The paper comprises cases studies of hypothetical wells drilled in relevant offshore areas in the world, each with their distinctive and variable geothermal gradient, defined by the different rock formations encountered. Uncertainties regarding the thermal properties of the rock were also considered to ascertain the robustness of the code. The water depth of the drilling site was also observed to impact the convergence of the algorithm. The results obtained by the numerical approach are in good agreement with the expected values of the undisturbed formation temperatures.

The novelty of the numerical framework is the ability to provide reliable and satisfactory estimates of the undisturbed geothermal gradient for wellbores with any configuration, lithology and rock properties. These estimates are based on temperature measurements of the circulating drilling fluid at the BHA and account for uncertainty in rock thermal properties; in reasonable time using standard engineering computers.
Statistical Analysis of Fatigue Test Data
Authors: Johnston Carol
Published: October 2017

The offshore environment contains many sources of cyclic loading. Standard design S-N curves, such as those in DNVGL-RP-C203, are usually assigned to ensure a particular design life can be achieved for a particular set of anticipated loading conditions. Girth welds are often the ‘weak link’ in terms of fatigue strength and so it is important to show that girth welds made using new procedures for new projects that are intended to be used in fatigue sensitive risers or flowlines do indeed have the required fatigue performance. Alternatively, designers of new subsea connectors, used for example in tendons for tension leg platforms, mooring applications or well-heads which will experience cyclic loading in service, also wish to verify the fatigue performance of their new designs. Often operators require contractors to carry out resonance fatigue tests on representative girth welds in order to show that girth welds made using new procedures qualify to the required design S-N curve. Operators and contractors must then interpret the results, which is not necessarily straightforward if the fatigue lives are lower than expected.

Many factors influence a component’s fatigue strength so there is usually scatter in results obtained when a number of fatigue tests are carried out on real, production standard components. This scatter means that it is important first to carry out the right number of tests in order to obtain a reasonable understanding of the component’s fatigue strength, and then to interpret the fatigue test results properly. A working knowledge of statistics is necessary for both specifying the test programme and interpreting the test results and there is often confusion over various aspects of test specification and interpretation.

This paper describes relevant statistical concepts in a way that is accessible to non-experts and that can be used, practically, by designers. The paper illustrates the statistical analysis of test data with examples of the ‘target life’ approach (that is now included in BS7608:2014 + A1) and the equivalent approach in DNVGL-RP-C203, which uses the stress modification factor. It gives practical examples to designers of a pragmatic method that can be used when specifying test programmes and interpreting the results obtained from tests carried out during qualification programmes, which for example, aim to determine whether girth welds made using a new procedure qualify to a particular design curve. It will help designers who are tasked with specifying test programmes to choose a reasonable number of test specimens and stress ranges, and to understand the outcome when results have been obtained.
Ultimate Compressive Strength Assessment of Damaged Plates
Authors: Amante Diogo do Amaral M. , Pasqualino Ilson P. , Estefen Segen F.
Published: October 2017

This paper presents the assessment of the ultimate compressive strength of damaged steel plates with a local dent. The explicit nonlinear finite element code from ABAQUS program was employed to simulate dynamically the dent damage. Therefore, the residual stresses due to the damage were considered in subsequent numerical compression analyses. A parametric study of plates considering residual stresses, geometric imperfections and dents was accomplished. Results showed that local dent is the dominant parameter for the ultimate strength of damaged plates. For a particular dent depth, the effect of welding induced geometric imperfections and residual stresses can be disregarded from the analysis. Afterwards, analytical expressions were developed to estimate the ultimate strength of damaged plates.
Experimental Investigation of Residual Ultimate Strength of Damaged Metallic Pipelines
Authors: Cai Jie , Jiang Xiaoli , Lodewijks Gabriel , Pei Zhiyong , Zhu Ling
Published: October 2017

The ultimate strength of metallic pipelines will be inevitably affected when they have suffered from structural damage. The present experiments aim to investigate the residual ultimate bending strength of metallic pipes with structural damage based on large-scale pipe specimens. Artificial damage such as dent, metal loss, crack and combinations thereof is introduced on the pipe surface in advance. The entire test project consists of 34 seamless pipes with a relative low Diameter-to-thickness (D/t) ratio around 21.3, among which four intact specimens and thirty damaged specimens have been carried out for mutual comparison. Extensive measurements on structural damage and pipe geometries including wall thickness and outer diameter are performed. The material properties are measured by tensile tests with specimens from both pipe longitudinal and hoop direction. The four-point bending tests are performed to investigate the structural behaviors of metallic pipes. The bending strength associating with failure mode of each specimen is documented extensively, and the bending moment-curvature curves are presented and discussed. The fundamental research of experiments on damaged pipes in the present paper will be deployed for the following numerical and analytical research in the near future.
Hydroelastic Analysis and Experimental Validation of a 350,000 DWT Very Large Crude Carrier
Authors: Lin Yuan , Ma Ning , Wang Deyu , Gu Xiechong
Published: October 2017

Hydroelastic response of Very Large Crude Carrier (VLCC), such as springing and whipping, is very important for the fatigue strength analysis. In the present paper, numerical simulation and model test are conducted simultaneously on the springing responses of a 350,000DWT VLCC. The numerical simulation results are obtained by combining 2-D strip method and 3-D Finite Element Method (FEM) in the frequency domain. Meanwhile, the segmented model tests are conducted in three conditions. Additionally, the experimental time histories of wave bending moments in regular and irregular waves are presented. In conclusion, the methods and results presented in this paper have shown that the springing would be much easier when the VLCC is in ballast condition or the full loading conditions with reduced stiffness. The springing response will happen, when the wave encounter frequency is near to the half and one third of the ship’s flexural natural frequency. The numerical method presented is available for springing response analysis quickly with acceptable accuracy.
Two Tandem Cylinders With Turbulence Stimulation in FIV Power Conversion: CFD With Experimental Verification of Interaction
Authors: Ding Wenjun , Sun Hai , Xu Wanhai , Bernitsas Michael M.
Published: October 2017

Flow induced vibrations of two rough, rigid, tandem-cylinders on springs are investigated for power conversion for Reynolds number 30,000 ≤ Re ≤ 120,000. Passive turbulence control (PTC) in the form of roughness strips is employed to enhance FIV and increase the power harness efficiency of the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) converter. Numerical simulations are performed using two-dimensional, Unsteady Reynolds-Averaged Navier-Stokes equations with the Spalart-Allmaras turbulence model. The center-to-center spacing ratio d / D of the two cylinders is set as 2.0 or 2.57 with mass ratio m* = 1.343 , damping ratio ζ = 0.26, and stiffness K = 1,200 N/m. Amplitude response, frequency response, interaction, energy harvesting, and conversion efficiency are presented and discussed. The main conclusions are: (1) In the VIV region at Re = 60,000, the amplitude response, frequency response, harnessed power, and power conversion efficiency of the upstream cylinder is the same for the two spacing ratios. Due to the shedding effect, the motion of the downstream cylinder for spacing ratio d/D = 2.0 is more severely suppressed than spacing ratio d/D = 2.57, which reduces the harnessed power and conversion efficiency for the downstream cylinder. (2) In the galloping region at Re = 110,000, due to the different impingement of the shed vortices on the downstream cylinder, the upstream cylinder harnesses more power and has higher energy conversion efficiency for spacing ratio d/D = 2.0 than d/D = 2.57.
On a Systematic Identification of Key Factors on Safety of Marine Structures and Their Potential Treatment Methods
Authors: Cui Weicheng
Published: October 2017

Safety of marine structures is one of the most important problems to be concerned by all stakeholders. Prof. Torgeir Moan of Marine Technology at the Norwegian University of Science and Technology has dedicated almost all his academic life in investigating various issues in this field and he and his colleagues and students have made significant contributions to the analysis methods. He is certainly deserved to be dedicated a special symposium in honor of his contribution. The present author was greatly influenced by him and worked for the similar issues. In this paper, the author will re-discuss the issue of the safety of marine structures from a philosophical point of view based on his own experience. The purpose of this presentation is to identify the key factors on safety of marine structures and to suggest the potential treatment methods for these factors. It is expected that this discussion is of some value for the future researches in this field of safety of marine structures which was greatly focused by Prof. Torgeir Moan.
Effects of Nonlinear Riser-Soil Interaction Model on Fatigue Design of Steel Catenary Riser Under Random Waves
Authors: Kimiaei Mehrdad
Published: October 2017

Steel Catenary Risers (SCRs) are one of the main components in development of oil and gas fields in deep waters. Fatigue design of SCRs in touch down zone (TDZ) is one of the main engineering challenges in design of riser systems. Nonlinear riser-soil interaction models have recently been introduced and used widely in advanced structural analysis of SCRs. Due to hysteretic nonlinear behavior of the soil, SCR system will show different structural response under different loading time histories.

This paper investigates the effects of nonlinear riser-soil interaction in the TDZ on fatigue performance of an example SCR subjected to randomly generated waves. Sensitivity of fatigue life of the system, location of the critical node and the maximum stress range to different wave realizations and different soil types are discussed in detail.
Investigation of Ultimate Limit State Safety Margins in the Structural Design Rules
Authors: Lofthaug Kristoffer , Brubak Lars , Bøe Åge , Steen Eivind
Published: October 2017

A study to document the Ultimate Limit State (ULS) safety margins built into the DNV GL rules for Bulk and Tanker is presented. Critical structural members were identified together with the load level at which these members start to develop permanent buckling sets exceeding normal fabrication tolerances. These critical load levels are then compared with the local ULS rule strength limits in order to have a measure for the structural safety margins and hull redundancy.

Non-linear finite element (NFLE) analyses were performed to estimate the structural response for different focus areas (critical structural members). Typically, critical members in bottom, deck, transverse bulkhead and hopper were chosen. Cargo hold models were developed both with linear finite elements, [1,2] and non-linear finite elements, [3]. In the non-linear FE analysis, the structural safety factor for ULS was defined as the load level giving permanent plastic deformation equal to the permissible distortion (production tolerances) for structural members. The non-linear FE results were compared with the maximum permissible load level with respect to buckling and yielding according to DNV GL Ship rules [1] inclusive Common Structural Rules for BC&OT (CSR) [4].

The structural safety factor shows a typical value of 1.2–1.4, and for most cases the plate is governing dimensioning structural member. This study has identified significant structural safety margins, typically 20–40% above rule acceptance level for typical highly utilized local areas in Bulk and Tanker hulls.

It is to be noted that global Hull Girder Capacity is not addressed in present paper.
Lazy-Wave Buoyancy Length Reduction Based on Fatigue Reliability Analysis
Authors: Ribeiro Machado da Silva Vinícius , Sagrilo Luis V. S. , Vignoles Mario Alfredo
Published: October 2017

When the profit scenario of an industry changes, the continuity of some projects can be at risk. The current downturn of the oil and gas industry force managers to take hard decisions about the continuity of projects, resulting in delays, postponements or even the cancellation of forecasted projects. In order to keep with these projects, the rush for cost reduction is a reality and the industry is pushing the involved parties to be aligned with this objective. The Brazilian Pre-Salt region, characterized by ultra-deep waters, is an example of this scenario. Subsea structures represented by flexible risers, which are responsible for the flow assurance of oil, gas and water, are forecasted to have a demand about 4.000 km in the next years. Usually, in these type of applications, lazy-wave configurations are adopted, increasing the costs of the solution with the necessity of the buoyancy modules acquisition. The smaller the buoyancy length is the cheaper the project become, reducing the necessary amount of buoys and the time spent for its installation. These type of solutions can probably carry with it a high level of conservatism, imposed by the use of standardized safety factors, and can potentially be optimized with the adoption of probabilistic approaches within the chain of analysis. The objective of this paper is to assess the possibility of buoyancy length reduction of lazy-wave configurations by using structural reliability methods of analysis. The focus stays on the evaluation of the fatigue of the armour wires located at the bend stiffener region, one of the most critical failure mode for the design of flexible pipes in offshore Brazilian installations. As already discussed in Ref. [1], many variables can influence on such kind of analysis. Based on this previous study, the first six random variables, identified to be the most important ones, are taken to carry out the analysis. The fatigue reliability approach considers four 6” flexible riser configurations: an original lazy-wave, a lazy-wave with less 30% of buoyance length, another one with less 50% of buoyance length and a free-hanging configuration. Failure probabilities and safety factor calibration curves are shown for each presented configuration and compared among themselves. The results indicate the possibility of defining a lazy-wave configuration with smaller buoyancy lengths, reaching 75% of reduction without changing the preconized high safety class at last year of its operational time. Safety factor curves shows to have similar behavior no matter the configuration considered. Structural reliability analysis comes as a potential method to help engineers to have a better understanding on the driving random variables of the problem, giving a support for the actual cost reduction scenario and for better decision-makings based on quantified risk.
Viscous Drift Force and Motion Analysis of Semi-Submersible in Storm Sea States Compared With Model Tests
Authors: Yang Limin , Falkenberg Erik , Nestegård Arne , Birknes-Berg Jørn
Published: October 2017

Standard analysis models applied for motions of moored floaters are based on potential flow perturbation methods with wave frequency response governed by first order wave forces and low-frequency response governed by second-order difference frequency wave forces. These models have been shown to have limitations in extreme sea states where nonlinear wave excitation and viscous drag forces above still water level may dominate. This effect is particularly visible for the low frequency excitation since the potential flow contribution goes to zero for long waves. In the present study non-linear wave excitation and viscous drag contributions on a semi-submersible is modelled by Morison’s load formula since the columns and pontoons are slender elements. A numerical simulation model is developed using SIMO [6], in which viscous forces and damping are included by the drag term of Morison equation and with drag coefficients recommended from DNV-RP-C205 [1]. Low frequency surge responses calculated by the combined potential flow drift forces and viscous drag from Morison load model are compared with model tests for waves only and for combined wave and current conditions.

A simplified formula for current and viscous effects on wave drift force, generalized to non-collinear conditions is presented and compared with model test results.
Improvement on the Accuracy of Mean Drift Force Calculation
Authors: Liu Yujie , Falzarano Jeffrey M.
Published: October 2017

Accurate computation of the drift forces and moments is required to study the higher order steady excitations on the floater in waves. One of the components of the drift forces and moments is regarding the relative wave elevation. To obtain this term, we need an accurate estimation of the wave elevation. If the field point is exactly on the waterline, we will encounter some difficulty to get the potential value.

This paper will provide a more accurate method to calculate the wave elevation around the body and discuss the mathematical justifications. The accuracy will be validated by the comparison against the commercial software. When using more panels in the commercial software, the results will converge to those from our in-house program. We observe a large discrepancy in the relatively higher frequency range. The sharp spikes in the original method are not observed in the improved method. Therefore it is necessary to adopt the proposed method in order to provide more reasonable predictions of the mean drift forces.
Study on the Extreme High Water Levels and Wave Heights of Different Return Periods in Laizhou Bay, China
Authors: Zhou Chunyan , Zheng Jinhai , Zhang Jisheng , Fu Xiaoying
Published: October 2017

Based on good simulation results during storm events in 2009, MIKE21 was used to study the extreme water level, current velocity and wave height in Laizhou Bay, China. 95 extreme weather processes during 1988–2012 were simulated. For each event, coupled hydrodynamic and wave modules of MIKE21 was chosen to calculate the maximum water level and current velocity. The Gumbel distribution method, commonly used for estimating return-period values of marine hydrodynamic variables, is adopted in this study. The extreme high water level of 50-year return period in Laizhou Bay can reach 2.6–3.8 m; and that of 100-year return period can be as high as 2.8–4.6 m. The 50-year and 100-year return-period values of current velocity can reach up to about 2.8 m/s and 3.2 m/s respectively, both around the Yellow River mouth. Wave height strongly depends on water depth, water level rise, wind speed and direction. The results provide parameter reference for structure design in the Laizhou Bay.
Comparing Different Metamodelling Approaches to Predict Stress Intensity Factor of a Semi-Elliptic Crack
Authors: Keprate Arvind , Ratnayake R. M. Chandima , Sankararaman Shankar
Published: October 2017

This paper examines the applicability of the different meta-models (MMs) to predict the Stress Intensity Factor (SIF) of a semi-elliptic crack propagating in topside piping, as an inexpensive alternative to the Finite Element Methods (FEM). Five different MMs, namely, multi-linear regression (MLR), second order polynomial regression (PR-2) (with interaction), Gaussian process regression (GPR), neural networks (NN) and support vector regression (SVR) have been tested. Seventy data points (SIF values obtained by FEM) are used to train the aforementioned MMs, while thirty data points are used as the testing points. In order to compare the accuracy of the MMs, four metrics, namely, Root Mean Square Error (RMSE), Average Absolute Error (AAE), Maximum Absolute Error (AAE), and Coefficient of Determination (R2) are used. Although PR-2 emerged as the best fit, GPR was selected as the best MM for SIF determination due to its capability of calculating the uncertainty related to the prediction values. The aforementioned uncertainty representation is quite valuable, as it is used to adaptively train the GPR model, which further improves its prediction accuracy.
Mathematical Analysis of Possible Range of Stable Drilling in Offshore Riserless Drilling System
Authors: Matsuo Miki Y. , Inoue Tomoya , Sakaguchi Hide
Published: October 2017

In both of scientific and industrial drilling, it is not desirable to any have accompanying vibrations, large fluctuations, or other unexpected dynamics disturbing a steady and stable drilling process. One of the origins of such disturbances can be the flexibility of the drill pile; drill pipe is easily deformed by external perturbations because it is very long and slender. To specify the stable range of parameters of drill pipe, we theoretically and numerically analyzed a Cosserat-type mathematical model of drill pile. The values of the parameters of this model were set to appropriate values representing actual riserless drilling situations adopted in the operations of the scientific deep-sea drilling vessel Chikyu owned by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). We concluded that the buckling of pipe should be treated as a possible accident, but buckling does not happen in the normal operations of the Chikyu.
Support Optimization for Piping System With Machine Learning
Authors: Ham Jongho , An Jungeun , Kim Bongjae , Choi Jaewoong , Kim Booki
Published: October 2017

Piping stress analysis is performed by the manipulations of support type, location and pipe arrangement based on many specific design criteria. A classical way to find good engineering solutions satisfying design criteria among lots of combinations is obviously time-consuming work. In field practice, it also highly depends on engineer’s experiences and abilities.

This paper proposes a hybrid method by combining several global search optimization algorithms and predication model generation in order to automatically control the combinations of support types as the engineering solutions. Here, we use some efficient and popular algorithms such as genetic algorithm, swarm intelligence and Gaussian pattern search to develop initial design of experiments. From the set of the initials, we build and update a prediction model by applying machine learning algorithm such as artificial neural network. As a result of using the hybrid method, the engineering solution is sufficiently optimized for the classical solution.

Design variables for this problem are the types of restraints (or the pipe support type). The nonlinearity conditions such as gaps and frictions are also treated as key design variables. Each restraint is initially identified as a binary set of design variables, and transformed to integer numbers to run on the n-dimensional design space. The number of dimension corresponds to the number of pipe supports. Currently, pipe stress analysis problems are divided into a certain size that is enough to run on one computer for project management purpose. If we have bigger system with more design variables to consider, the hybrid machine learning method plays a key role in saving computation time with the help of additional parallel computation technique.