Construction Of Offshore Structures Research Articles

Novel multivariate design concept for floating wind turbines by Gaidai multivariate reliability method and deconvolution scheme

Development of novel risk and reliability assessment methods is intended to support safer construction of offshore structures, subjected to environmental wave loads. Current study investigated 10-MW FWT (i.e., Floating Wind Turbine), operating under realistic environmental conditions. While increasing operating safety, enhanced risk and reliability assessment methods may eventually help reduce manufacturing and maintenance costs. Excessive structural dynamics being usually caused by environmental stressors, acting on structural system. Environmental loads resulting from ambient wind and wave motions are typical for offshore structures. Current work advocates a novel risk and reliability assessment methodology that allows for reliable forecasting of failure/damage risks, arising from excessive FWT structural dynamics. Recently developed Gaidai multivariate reliability methodology along with state-of-the-art deconvolution method had been employed. Unlike existing reliability approaches such as Weibull-type, GP (i.e., Generalized Pareto), POT (i.e., Peaks Over the Threshold), etc., the recommended methodology does not rely on any pre-assumed functional class, when extrapolating failure probability functional tail. Practical advantages of the suggested multivariate reliability methodology combined with deconvolution scheme over, that is, 4-parameter Weibull’s extrapolation method had been demonstrated. Suggested methodology makes effective use of even limited underlying datasets, enabling robust and accurate projections of multidimensional structural system failure/damage risks. Overall methodological performance suggests that numerically stable and accurate extreme dynamics forecasts for FWT structural bending moments might be obtained, utilizing suggested multivariate reliability methodology. Deconvolution extrapolation approach being more numerically stable than parametric extrapolation techniques, due to its non-parametric nature.

The bivariate region frequency extreme value analysis of significant wave height and mean wave period in the South China Sea

A comprehensive understanding of extreme significant wave height (SWH) and mean wave period (MWP) benefits greatly the design and construction of offshore structures. In this paper, we present the bivariate region frequency analysis method to spatially investigated the extreme sea state. Based on univariate region frequency analysis approach, this bivariate method provides a chance to combine the similar joint characteristics of extreme sea state conditions at various locations with a specific area. The regional assessment of extreme SWH and MWP is conducted in the South China Sea employing ERA5 wave datasets from 1978 to 2021. Based on multivariate L-moments, the SCS is divided into 35 homogeneity regions which experience discordance test and homogeneity test. The Huangyan Island and Nansha Island near area are chosen to verify the model effect. By comparing the theoretical and empirical density, a series of diagnostic figures indicate the goodness of fit between empirical and theoretical distributions.

Saddle Strength Analysis on Jacket Structure During Roll Up Procedures in Fabrication Phase

Offshore platform is a structure that serves as a well drilling facility to explore, extract, store and process petroleum and gas located under the seabed. There are several phases in the construction of offshore structures (fixed jacket platform) from preparation to construction. Fabrication phase is the earliest stage in the manufacture of a fixed offshore platform (jacket structure) which is carried out by the fabricator. One of the procedures at the fabrication phase is roll up procedure. In this final project research, an analysis will be carried out regarding the configuration of the rigging, crane, and sadle support used in the jacket structure during roll up procedure. After the analysis of jacket structure modeling using the SACS software, the results of rigging configuration can be obtained, which were the slings spesifications used to connect the structure with cranes, with sling diameter of 2.75 inch and SWL value of 64 MT. As for the sling used to connect the jacket with winch has a diameter of 1.5 inch snd SWL value of 18 MT. Furthermore, the cranes used have a capacity of 75 and 78 MT, respectively. In the load analysis of the jacket structure, it was found that saddle support received the largest reaction when the jacket was at an 80-degree slope with the largest reaction value of 102.74 MT. In the saddle support local analysis, the result of maximum equivalent von-mises stress obtained was 43,486 MPa, with the saddle allowable stress of 345 Mpa, therefore the UC value of 0.24 can be obtained.

Dynamic analysis of shallow embedded jacket offshore structures under wave forces

In this paper, the stress of temporary structures during the construction of offshore structures is analysed through the study of the temporary buried drilling platform. Firstly, The fluid flow is controlled by the modified RANS equation (the Reynolds-average equation) and Forchheimer saturation resistance model, the free surface is tracked by volume of fluid (VOF) method, and the κ–ε closed equation is solved. The finite element numerical simulation software is used to establish a multi-factor coupling calculation model of the stress of temporary buried offshore structures. The maximum wave force of temporary structures under different buried depths, different structural sizes, different load periods and amplitudes is simulated, and the stability of Long Yuan wind turbine pile foundation drilling platform under the maximum wave force is analyzed. The research results of this paper can be used for dynamic design and analysis of wave load conditions, structural design dimensions and load periods in the engineering design of embedded offshore structures in shallow water under wave loads, so as to calculate the wave load values of structures reasonably and provide concrete theoretical basis for the establishment of offshore systems.

Guidelines on Asset Management of Offshore Facilities for Monitoring, Sustainable Maintenance, and Safety Practices.

Recent activities in the oil and gas industry have shown an increasing need for monitoring engagements, such as in shipping, logistics, exploration, drilling, or production. Hence, there is a need to have asset management of these offshore assets (or facilities). Much of the offshore infrastructure is currently approaching or past its operational life expectancy. The study presents an overview on asset management of offshore facilities towards monitoring, safe practices, maintenance, and sustainability. This study outlines the major considerations and the steps to take when evaluating asset life extensions for an aging offshore structure (or asset). The design and construction of offshore structures require some materials that are used to make the structural units, such as offshore platform rigs, ships, and boats. Maintaining existing assets in the field and developing new platforms that are capable of extracting future oil and gas resources are the two key issues facing the offshore sector. This paper also discusses fault diagnosis using sensors in the offshore facilities. The ocean environment is constantly corrosive, and the production activities demand extremely high levels of safety and reliability. Due to the limited space and remote location of most offshore operations, producing cost-effective, efficient, and long-lasting equipment necessitates a high level of competence. This paper presents the guidelines on asset monitoring, sustainable maintenance, and safety practices for offshore structures. In this study, the management of offshore structures were also presented with some discussions on fault monitoring using sensors. It also proposes sustainable asset management approaches as guidelines that are advised, with policy implications.

Experimental Case Studies about Uniplanar SHS Joints with Full-Overlapped Top Connection

Squared hollow steel profiles are commonly used in the construction of offshore structures or building facades. By welding two or more pipes, typical joints are created that are specific for different areas of applications. These joints are less resistant than straight pipes due to the geometrical heterogeneity and the complex stress behavior of the welding. Standards define these joints, but there are restrictions imposed regarding the material or geometry. This paper focused on full-overlapped joints with squared hollow section profiles and on-top connection, which are disregarded in current standards. The aim was to figure out the influence of the inclination angle on the resistance of the joint. In the analysis, experimental and numerical studies were performed. Four different inclination angles commonly used on construction sites were the focus. It was discovered that there is a total diminishment of 46% in the load bearing capacity between the steepest and the most obtuse angles. The structural behavior is non-linear and is influenced by the value of the angles. The second aspect is related to the influence of the steel profile, which is evaluated by a comparison between a squared profile and two circular profiles. It was discovered that the joint made with squared profiles has a higher bearing capacity than the one made with circular profiles, a statement valid for similar thicknesses of the elements.

Offshore structures, sea mud accumulation and mangrove regeneration: Insights from the Vietnamese Mekong Delta, Vietnam

Offshore structures have been prioritized for controlling eroded muddy coasts, particularly in Southeast Asia. However, these structures have had limited accumulation of sea mud and a low level of mangrove regeneration. The reality shows that sea mud accumulation and mangrove regeneration greatly contribute to controlling erosion and establishing muddy coasts. Little is known as to how sea mud accumulation and mangrove regeneration have been integrated or referred to during designing and construction of offshore structures. I used construction of offshore structures along the Vietnamese Mekong Delta coast for answering the above question. This study was undertaken using critical review of the technical designs, semi-structured interviews with relevant stakeholders, analysis of satellite images and field measurements in pursuit of the objectives. The results show that sea mud accumulation and mangrove regeneration were not adequately considered by the design and construction of offshore structures. Attenuation of the wave energy and the transplantation were prioritized instead. Physical visibility and survival of interventions were two criteria used for designing the structure. Until now, most sites have been still in liquid sediment. Nether sea mud accumulation nor mangrove regeneration were recorded. The findings provide a technical reference to redesigning current offshore structures to improve their efficacy in accumulating sea mud and promoting mangrove regeneration in the region and in Southeast Asia in the future.

Performance of Coral Aggregate on Mechanical and Durability Properties of Concrete - A Review

Coral concrete is a type of aggregate and is generally light and porous, with rough surface, little adherence to the attachments and high concentration of sea salts. Coral concretes are mainly required in the Construction of offshore structures. This types of aggregate complies low grade of strengths, little corrosive resistance and is highly brittle. These features influences the workability, mechanical properties, and durability properties of developed concrete. The objective is to analyse the performance of coral aggregate on workability, mechanical and durability properties such as (shrinkage, chloride) of concrete. In this backdrop, this paper gives an account of properties of low stability hydrated products of the Portland cement under sea condition, low strength and stiffness of coral aggregates having large connected porosity with weak interface microstructure between the cement matrix and coral aggregates. Keywords: Coral aggregate, Physical & Chemical Properties, Strength, Durability properties.

Hybrid joining of jacket structures for offshore wind turbines – Determination of requirements and adhesive characterisation

Due to the advantageous wind conditions, many new structures are planned offshore. The construction of offshore structures with ever increasing sizes requires new concepts to cope with the particular requirements thereof. One often overlooked possibility is adhesive bonding as a structural joining method, which upholds significant optimisation potential. This paper, the first in a series of two, illustrates the potential of adhesive bonding on the widely used jacket structure. For that, the structural concept is adapted towards ease of manufacturing, hybrid joints are defined, adhesives are selected according to requirements resulting from structural and climatic constraints, structural verifications performed, and the validation therefor presented in preliminary small-scale tests—which will be upscaled in the second part of this series. This paper thus serves as a roadmap for practitioners, and outlines the various steps to evaluate the potential of adhesive bonding in complex structures.

Fiber element modeling of circular double-skin concrete-filled stainless-carbon steel tubular columns under axial load and bending

Circular concrete-filled double-skin steel tubular (CFDST) columns with external stainless-steel are high-performance composite columns that have potential applications in civil construction including the construction of offshore structures, bridge piers, and transmission towers. Reflecting the limited research performed on investigating their mechanical performance, this study develops a computationally efficient fiber model to simulate the responses of short and slender beam-columns accounting for the influences of material and geometric nonlinearities. Accurate material laws of stainless steel, carbon steel, and confined concrete are implemented in the mathematical modeling scheme developed. A new solution algorithm based on the Regula-Falsi method is developed to maintain the equilibrium condition. The independent test results of short and slender CFDST beam-column are utilized to validate the accuracy of the theoretical solutions. The influences of various column parameters are studied on the load-axial strain [Formula: see text] curves, load-lateral deflection [Formula: see text] curves, column strength curves, and interaction curves of CFDST columns. Design formulas are suggested for designing short and beam-columns and validated against the numerical results. The computational model is found to be capable of simulating the responses of CFDST short and slender columns reasonably well. Parametric studies show that the consideration of the concrete confinement is important for the accuracy of the prediction of their mechanical responses. Furthermore, high-strength concrete can be utilized to enhance their load-carrying capacity particularly for short and intermediate slender beam-columns. The strengths of CFDST columns computed by the suggested design model are in good agreement with the test and numerical results.

CTOD Evaluation of API X80MO PSL2 TMCP Steel Welded Joints Used in Offshore Structures Based on API RP 2Z

Offshore structures and equipment, used both for oil extraction and wind power generation, are constantly exposed to extreme application conditions, such as low temperatures, corrosion, and cyclic loading. Thus, to ensure the integrity of such components, the use of materials with excellent mechanical properties combined with good weldability is required. In this sense, the main committees, institutes, and normative societies have created pre-qualifications, codes, and programs to evaluate and certify the weldability of steels applied in the offshore industry. API RP 2Z prequalification is one such standard that is designed to ensure that steels developed for these applications have excellent fracture toughness in the heat affected zone (HAZ). In this context, the effect of heat input on the HAZ toughness of welded joints of API X80MO steel plates was evaluated, using the CTOD (Crack Tip Opening Displacement) parameter. It was found that this steel, of 37 mm thickness, welded with heat inputs of 0.8 and 4.5 kJ/mm, presented weldability characteristics and fracture toughness behavior suitable for API RP 2Z standard specification. Therefore, API X80MO steel plates, produced by controlled rolling followed by accelerated cooling, is an option for application in the construction of offshore structures and components.

Simulation of underwater concrete movement in flowing water using DEM-CFD coupling method

With the construction of offshore structures and underwater projects, increasing requirements are put forward for underwater concrete (UWC). Based on the DEM-CFD coupling method, this study simulates the diffusion and movement of UWC in flowing water, and discusses the effects of concrete properties, pouring velocity and water flow velocity on UWC pouring. According to the simulation results, the mortar at the top and edge of UWC is easily washed away by flowing water in the process of pouring, resulting in the loss of UWC. Additionally, improving concrete cohesiveness, increasing concrete pouring velocity and decreasing water flow velocity could significantly reduce the loss rate of UWC.

Forecasting Current Velocity and Profile in a Strait Water Using Warped Gaussian Process

Abstract Ocean current forecast is vital for developing tidal energy and construction of offshore structures in the strait waters. This paper developed a short-term ocean current forecasting approach, which consists of the measured data preprocessing, kernel function selection, and data forecasting using the warped Gaussian process (WGP). A preprocessing using the wavelet thresholding method was proposed to enhance the quality of the measured raw data. The theory of WGP and the commonly used kernel functions were briefly introduced. The sliding time window and one-step ahead strategies were employed to increase the accuracy of predictions. Observations collected during an ocean current measurement campaign executed in a strait water on the coast of the East China Sea were used as an example data set. The current velocity and profile were forecasted and validated using the example data set as an illustration of the framework of the developed approach. The effects of window length, kernel function, and time interval on the WGP forecasting efficiency and precision were investigated. The forecasting performance of the developed WGP model was discussed by comparing it with the standard Gaussian process prediction (GP) model. The current profile with a 95% confidence interval was also predicted by the developed WGP model at a certain point. The validation shows that the developed model is efficient in the short-term ocean current forecast.

Development of Entering Order and Work-Volume Assignment Algorithms for the Management of Piping Components in Offshore Structure Construction

In the early 2010s, with rising oil prices and increasing purchase orders for offshore structures for deep-sea resource development, the shipyards that took these orders suffered unexpected losses. Unlike the construction of commercial carrier vessels, the construction of offshore structures necessary to develop deep-sea resources is difficult to manage due to the complexity of the outfitting process of the topside structure, which is a plant for gas and oil production and treatment. Piping components in particular, which comprise most of the design items, are difficult to manage because they involve 2 to 3 times the man-hours and up to 10 times the quantity of items compared to commercial carrier vessels. Due to not only high man-hours and quantity but also large fluctuations caused by design changes and long procurement lead times, process delays that result in delayed compensation frequently occurred. In response, Samsung Heavy Industries developed an integrated management system for piping components. This study describes the entering order optimization algorithm and work-volume assignment optimization algorithm, which are the core algorithms of this system. The entering order optimization algorithm determines the optimal installation order considering the procurement status of the piping components and the installation readiness status of the installation work site, through which it determines the entering order of the piping components. The algorithm seeks to accelerate the completion rate of installation of the piping components. Next, to minimize delivery delays of sub-contractors to the shipyard, this study developed a work-volume assignment optimization algorithm that can equalize the load on multiple sub-contractors considering the raw material readiness status and the production capacity of the sub-contractors, in terms of materials that must be ordered from external sub-contractors among the piping components whose entering order was determined. Finally, applying the algorithm developed using actual shipyard data resulted in an accelerated completion rate of installation and improved balance of load in terms of volume assigned to the sub-contractors.

Algorithm development for the spool installation sequence of outfitting works in offshore structure

In the early 2000s, shipyards in South Korea were the most competitive in the world for commercial vessels because of their accumulated production and quality management capabilities. In the mid-2000s, the rising trend of international oil prices and predicted market growth of offshore structures caused large shipyards in South Korea to start focusing on the construction of offshore structures. However, these shipyards suffered massive losses because they did not properly reflect the characteristics of offshore structures, which differ from those of commercial vessels. Such losses can largely be classified into engineering losses and production losses. This study aimed to address problems related to outfitting installation work from a production perspective. An algorithm was developed to define an efficient spool installation sequence and was verified through actual data of offshore structure. The developed algorithm should make it possible to plan spool installation work by determining an efficient spool installation sequence. In addition, the algorithm can assist in prompt decision making and eliminate wasted cost and time by providing with an efficient spool installation sequence.

Study on Supporting Structure Performance of Deep Soft Soil Foundation Pit near Sea under Waves, Tides, Vibration, and Unbalanced Loads

In addition to the vibration loads caused by offshore structure construction, the offshore foundation pit can also be affected by additional hydraulic loads such as waves, tides, and typhoons, hindering our understanding of the mechanical performance of the foundation pit supporting structure. In order to illuminate this mechanical performance under waves, tides, hammering, and unbalanced loads, this paper analyzed the supporting structure with theoretical and numerical simulations. The results were verified with field monitoring data. Moreover, the influences of loads, including construction vibration loads and unbalanced loads, on structure and soil were discussed. By applying these results to the engineering design, this research could provide a reference for studying mechanical properties of relevant supporting structures and the construction of offshore foundation pits.

Wave-Induced Seabed Response around a Dumbbell Cofferdam in Non-Homogeneous Anisotropic Seabed

Cofferdams are frequently used to assist in the construction of offshore structures that are built on a natural non-homogeneous anisotropic seabed. In this study, a three-dimensional (3D) integrated numerical model consisting of a wave submodel and seabed submodel was adopted to investigate the wave–structure–seabed interaction. Reynolds-Averaged Navier–Stokes (RANS) equations were employed to simulate the wave-induced fluid motion and Biot’s poroelastic theory was adopted to control the wave-induced seabed response. The present model was validated with available laboratory experimental data and previous analytical results. The hydrodynamic process and seabed response around the dumbbell cofferdam are discussed in detail, with particular attention paid to the influence of the depth functions of the permeability K i and shear modulus G j . Numerical results indicate that to avoid the misestimation of the liquefaction depth, a steady-state analysis should be carried out prior to the transient seabed response analysis to first determine the equilibrium state caused by seabed consolidation. The depth function G j markedly affects the vertical distribution of the pore pressure and the seabed liquefaction around the dumbbell cofferdam. The depth function K i has a mild effect on the vertical distribution of the pore pressure within a coarse sand seabed, with the influence concentrated in the range defined by 0.1 times the seabed thickness above and below the embedded depth. The depth function K i has little effect on seabed liquefaction. In addition, the traditional assumption that treats the seabed parameters as constants may result in the overestimation of the seabed liquefaction depth and the liquefaction area around the cofferdam will be miscalculated if consolidation is not considered. Moreover, parametric studies reveal that the shear modulus at the seabed surface G z 0 has a significant influence on the vertical distribution of the pore pressure. However, the effect of the permeability at the seabed surface K z 0 on the vertical distribution of the pore pressure is mainly concentrated on the seabed above the embedded depth in front and to the side of the cofferdam. Furthermore, the amplitude of pore pressure decreases as Poisson’s ratio μ s increases.

경량 모델을 활용한 해양구조물 시공 용접장 산출

경량 모델은 플랜트 건조 과정에서 설계 검토 및 공정 관리를 위해 널리 사용되고 있다. 경량 모델은 주로 삼각망으로 구성되어 있으며, 파일 사이즈가 작고, 형상 가시화에 최적화 되어 있다. 용접은 플랜트 건조 과정에서 가장 많은 시간과 비용이 소요되는 공정으로 효율적인 관리를 위해 정확한 물량 산출이 필요하다. 본 연구에서는 빠르고 쉽게 접근할 수 있는 경량 모델 기반으로 용접선을 추출할 수 있는 방법을 제안한다. 삼각망을 기반으로 에지 및 면을 분류하여 경계 표현법으로 부재에 대한 모델을 구성한다. 용접선은 두 부재가 맞닿는 부분이며, 교차 여부를 모델, 면 및 에지 하위 순서로 진행하여 효율적으로 검색하여 빠른 시간 안에 용접선을 추출할 수 있도록 하였다. 또한 부가적으로 용접타입 및 자세 정보를 생성하였다.

DESIGN AND ANALYSIS OF FIXED OFFSHORE STRUCTURE – AN OVERVIEW

Construction of offshore structures is more complex than onshore structures in terms of structural response and its loading system. The high dependency on the environment with high level of uncertainties is the main factor that contributes to the complexity of design and construction process. Better understanding on the type of structures and load exposed to it are required to maintain the high integrity of the operation life. An overview of the design and analysis of offshore structure in the case of fixed platform will be discussed in this paper. It comprises of the fundamental principle of wave dynamic, the dominant load acting on the structure, method used to quantify the responses and probability of failure that should be considered at the design stage. It is mainly to provide understanding and guideline for the purpose of the design phase. Description in details can be obtained through further reading based on references that have been cited in the content.

Numerical Model of Plastic Destruction of Thick Steel Structural Elements

Abstract In the shipbuilding industry, the risk of brittle fracture of the structure is limited by using certified materials with specified impact strength, determined by the Charpy method (for a given design temperature) and by supervising the welding processes (technology qualification, production supervision, non-destructive testing). For off-shore constructions, classical shipbuilding requirements may not be sufficient. Therefore, the regulations used in the construction of offshore structures require CTOD tests for steel and welded joints with a thickness greater than 40 mm in the case of high strength steel and more than 50 mm in the case of other steels. Classification societies do not accept CTOD test results of samples with a thickness less than the material tested. For this reason, the problem of theoretical modeling of steel structure destruction process is a key issue, because laboratory tests for elements with high thickness (in the order of 100 mm and more) with a notch are expensive (large samples, difficulties in notching), and often create implementation difficulties due to required high load and range of recorded parameters. The publication will show results and conclusions from numerical modeling of elastic properties for steel typical for offshore applications. Calculations were carried out at the Academic Computer Centre in Gdańsk.