Jacket Structures Research Articles

Warp-knitted fabric structures for a novel biomimetic artificial intervertebral disc for the cervical spine

As an attempt to better replicate the complex kinematics of a natural disc, a novel biomimetic artificial intervertebral disc replacement (bioAID) has been developed containing a swelling hydrogel core as nucleus pulposus, a fiber jacket as annulus fibrosus and metal endplates to connect the device to the adjacent vertebrae. The first prototype consisted of a weft-knitted fiber jacket, in which only a single fiber was used to create the jacket structure. This can endanger the structural integrity of the complete device upon yarn damage. Therefore, in this study, several warp-knitted textile structures were assessed to (1) ensure structural integrity, (2) while allowing for swelling constraint of the hydrogel and (3) behaving as one integrated unit similar to the natural IVD. Moreover, the fiber jacket should (4) act as a scaffold that allows bone ingrowth to ensure long-term stability and (5) have a good durability, (6) be wear resistant and (7) have good manufacturing feasibility with good quality control. In this study, 4 different stitch patterns, including 2 × 1 and 1 × 1 lapping with and without a pillar stitch, were produced. The effect of the stitch pattern and stitch density on the fabric mechanical properties and device swelling and compressive strength was assessed. As a next step, the effect of using multiple layers of fabrics, mimicking the layered structure of annulus fibrosus, on the functional capacity of the bioAID was characterized. All textile structures were capable of limiting the swelling of the hydrogel while withstanding its internal pressure and showing sufficient wear resistance. However, only the 2 × 1 and 2 × 1 with pillar stitch had a pore size range that was suitable for cell infiltration to facilitate osseointegration as well as having the highest strength of the complete device to ensure safety under compression loading. Incorporating different number of jacket layers of these two stitch patterns did not show any significant effect. When also taking the structural parameters into consideration, the 2 × 1 lapping design with 4 layers was able to constrain hydrogel swelling, provide a high compressive strength, could facilitate cell infiltration and had dimensions within the range of a natural intervertebral disc.

Reliability assessment of an offshore wind turbine jacket under one ultimate limit state considering stress concentration with active learning approaches

At present, offshore wind turbine jacket structures are generally modeled with beam elements. The local joint flexibility and stress concentration of the joints are hardly considered. In this work, one jacket model is developed to consider joint stress concentration by using super-elements. Another jacket model is also developed with pure beam elements. The stress concentration is investigated between the two jacket models. Moreover, a general framework is proposed to conduct the reliability analysis with active learning Kriging approaches and one typical ultimate limit state is studied. At first, the developed jacket models are validated with the numerical simulation results of National Renewable Energy Laboratory (NREL) technical reports. Secondly, global sensitivity analysis is carried out to reduce the random parameters for the reliability analysis. In the end, the reliability analysis is conducted with active learning Kriging approaches. The results show the probabilities of failure of the beam model are very sensitive to the assumed stress concentration factors. More importantly, it is noticed that active learning Kriging approaches can efficiently and accurately assess the probabilities of failure. Compared with the traditional Monte Carlo simulation approach, active learning Kriging approaches can reduce computational loads by hundreds of times and have the same accuracy.

Estimating initiation conditions for extrusion buckling of driven open-ended piles

The potential for pile tip damage and extrusion buckling has become an increasing concern for the offshore wind industry following the trend toward the use of large-diameter thin-walled pile foundations, either as monopiles or as part of a jacket structure. Extrusion buckling may be triggered by an initial pile tip fabrication imperfection or pile damage during transport and handling. It may also be triggered during driving through strong heterogeneous sediments. Numerical analysis of the problem requires advanced and computationally expensive techniques because of the three-dimensional, dynamic and large-deformation nature of the pile and soil deformations. The problem is addressed here by combining coupled Eulerian–Lagrangian simulations with geotechnical centrifuge model test data in dry dense sand. Both approaches consider pre-dented piles with the objective of estimating the soil strength, characterised by the cone resistance, at which minor initial damage will start to propagate. Simplified analysis of structural dent propagation and radial stresses generated in the soil during advance of a deformed pile led to a simple calculation procedure to assess the potential for extrusion buckling. Reasonably good agreement was achieved between the proposed calculation method, numerical simulations and centrifuge tests data, rendering the approach a valuable screening tool for practical application.

Fatigue Life and Crack Initiation in Monopile Foundation by Fatigue FE Analysis

The construction of new renewable energy infrastructures and the development of new ocean resources continues to proceed apace. In this regard, the increasing size and capacity of offshore wind turbines demands that the size of their accompanying supporting marine structures likewise increase. The types of marine structures utilized for these offshore applications include gravity base, monopile, jacket, and tripod structures. Of these four types, monopile structures are widely used, given that they are comparatively easy to construct and more economical than other structures. However, constant exposure to harsh cyclic environmental loads can cause material deterioration or the initiation of fatigue cracks, which can then lead to catastrophic failures. In this paper, a 3D fatigue finite element analysis was performed to predict both the fatigue life and the crack initiation of a welded monopile substructure. The whole analysis was undertaken in three steps. First, a 3D non-steady heat conduction analysis was used to calculate the thermal history. Second, a thermal load was induced, as an input in 3D elastoplastic analysis, in order to determine welding residual stresses and welding deformation. Finally, the plastic strain and residual stress were used as inputs in a 3D fatigue FE analysis in order to calculate fatigue crack initiation and fatigue life. The 3D fatigue finite element analysis was based on continuum damage mechanics (CDM) and elastoplastic constitutive equations. The results obtained from the 3D fatigue finite element analysis were compared with hot spot stresses and Det Norske Veritas (DNV-GL) standards.

Experimental investigation of breaking regular waves slamming on offshore wind jacket structure

A series of experiments were carried out to investigate the breaking regular wave slamming on an offshore wind jacket structure in a wave flume. The breaking wave characteristics, global slamming forces and local pressure distributions are analyzed. The effects of relative location between jacket structure and breaker location, bottom slope, water depth, incident wave period and wave height on the slamming force and pressure distribution of the jacket structure are investigated. The results show that the maximum value of normalized peak pressure and location will be significantly affected by the bottom slope and water depth. Modified empirical formulas are proposed to calculate the slamming force and slamming duration for the offshore wind jacket structure.

Identification of Damage in Offshore Jacket Structure by Utilizing Artificial Neural Networks

Identification of Damage in Offshore Jacket Structure by Utilizing Artificial Neural Networks

Effects of drainage conditions and suction pressure on tensile response of bucket foundations: An experimental study

Suction buckets acting as supports for jacket structures may constitute a viable foundation solution for offshore wind turbines. While monopiles dominate the industry, suction buckets remain uncommon despite their advantages in transitional water depths. One of the reasons stems from challenges of achieving optimal design due to scarce experience and knowledge, as the mechanisms governing foundation response to different drainage conditions are poorly understood. This paper contributes with new insights into tensile behavior of suction buckets based on experimental evidence. A defining property of bucket foundations is the ability to withstand extreme uplift forces on account of suction, which represents the core subject of this study. All tests involved a medium-scale model installed in sand in a pressurized environment. A novel feature of this research consists of examining the relative importance of soil density. By varying the relative density (40–90%) and the uplift rate (0.05–500 mm/s), a full range of drainage and soil conditions was explored. Suction pressure generated during partially drained or fully undrained uplift tends to reach similar values regardless of initial relative density. This key finding demonstrates how the soil state is altered due to jacking installation and cyclic pre-shearing. Regression models are established to capture the dependency of tensile capacity and initial stiffness on uplift rate.

Preliminary assessment of potential pile tip damage and extrusion buckling

Pile tip damage has become of particular concern for the offshore wind industry due to the extensive use of large-diameter, thin-walled piles for monopiles and jacket structures. The potential for damage has increased in recent developments due to the occurrence of embedded boulders within the seabed sediments (e.g. glacial tills) or partially weathered soft rocks (e.g. limestones, chalk). To assess the risk of pile tip damage, this paper presents a simple schematic approach to estimate: i) whether pile tip damage might occur following impact with embedded boulders; and ii) what soil strength (e.g. as reflected in the cone tip resistance qc) might initiate progressive distortion of an initial minor dent (extrusion buckling). The methodology was developed based on results of centrifuge model tests and large displacement finite element analysis, encompassing: i) undamaged thin-walled piles installed through embedded layers of different sized boulders; and ii) initially ‘pre-dented’ piles driven through homogeneous sand triggering progressive distortion. Numerical analysis was used to develop predictive guidelines for conditions that might cause significant pile tip damage. The guidelines were validated by comparison with results from model tests and also through well documented field cases where extrusion buckling occurred. The paper provides guidance for choosing the critical parameters involved in the calculation processes to enable direct application in design, allowing assessment of whether more detailed investigation and analysis should be undertaken.

Dynamic analysis of an integrated offshore structure comprising a jacket-supported offshore wind turbine and aquaculture steel cage

A novel integrated offshore design, JOWT-SC, which combines a jacket-supported offshore wind turbine structure (JOWT) with an aquaculture steel cage (SC), is proposed in this paper. The enclosed steel cage in the integrated design can adjust to changes in water depth, preventing the fish from escaping. The jacket structure is equipped with two decks and a 10 MW reference wind turbine that provide steady electricity for the offshore fish farm. The numerical modeling and time-domain simulations were conducted in software SACS, and the dynamic responses of the JOWT-SC were also compared with the baseline JOWT, that is without a steel cage. The considered environmental conditions include waves measured from the southeast open sea area of Fujian in China, several wind speeds that represent normal operating conditions for the reference wind turbine, and a constant current flow. The results of the study demonstrate that the aquaculture steel cage significantly influences the dynamic responses of the integrated JOWT-SC substructure, and the pile-foundation shear responses are more sensitive to the wave frequency. In particular, the cage increases the hydrodynamic responses of the JOWT-SC at the mudline under the combined action of winds, waves, and currents, compared with the case where the JOWT-SC is only subjected to wind loads. Overall, the results of this study can be used as a reference for the realization of novel designs in practical engineering.

A Data-Driven Intelligent Prediction Approach for Collision Responses of Honeycomb Reinforced Pipe Pile of the Offshore Platform

The potential collision between the ship and the pipe piles of the jacket structure brings huge risks to the safety of an offshore platform. Due to their high energy-absorbing capacity, honeycomb structures have been widely used as impact protectors in various engineering applications. This paper proposes a data-driven intelligent approach for the prediction of the collision response of honeycomb-reinforced structures under ship collision. In the proposed model, the artificial neural network (ANN) is combined with the dynamic particle swarm optimization (DPSO) algorithm to predict the collision responses of honeycomb reinforced pipe piles, including the maximum collision depth (δmax) and maximum absorption energy (Emax). Furthermore, a data-driven evaluation method, known as grey relational analysis (GRA), is proposed to evaluate the collision responses of the honeycomb-reinforced pipe piles of offshore platforms. Results of the case study demonstrate the accuracy of the DPSO-BP-ANN model, with measured mean-square-error (MSE) of 5.06 × 10−4 and 4.35 × 10−3 and R2 of 0.9906 and 0.9963 for δmax and Emax, respectively. It is shown that the GRA method can provide a comprehensive evaluation of the performance of a honeycomb structure under impact loads. The proposed model provides a robust and efficient assessment tool for the safe design of offshore platforms under ship collisions.

Assessment of Dynamic Effects of Wave Loads in Fatigue Analysis for Fixed Steel Offshore Structures

This paper presents an algorithm and develops a formula to evaluate the dynamic effect of wave loading on fixed steel offshore structures (jacket structures) through the fatigue damage ratio. Applying the algorithm and formula proposed in this paper to evaluate the dynamic effect of wave loads in fatigue analysis for 03 Jacket structures built at increasing water depth under one specific marine condition and provide specific recommendations on the limits of application of quasi-static and dynamic methods in the fatigue analysis of the jacket structures. This research is really necessary because currently, the current standards (API, DnV) only stop at evaluating the dynamic effects of wave loads acting on the Jacket structure in the strength analysis. These standards propose a limit for quasi-static or dynamic analysis based on the "3.0 s or 2.5 s rule" (use the quasi-static method when Tmax≤ 3.0 s or ≤ 2.5 s), and it is advised that they only apply to waters within the North Sea and the Gulf of Mexico. This paper has demonstrated that it is not appropriate to use the specified standards for the North Sea and the Gulf of Mexico to select the method of fatigue analysis of the jacket structure in marine conditions outside the study area of the standard. Hoped that this paper will be a reference for engineers when choosing a fatigue analysis method for jacket structures in specific marine conditions at the location where the jacket structure has been installed. Doi: 10.28991/CEJ-2023-09-02-016 Full Text: PDF

Efficient optimization design method of jacket structures for offshore wind turbines

With the gradual implementation of offshore wind energy production, the future tendency is to expand into the deeper water. The jacket foundations will take the place of the present monopile foundations when the water depth increases. The foundations account for the majority of the construction cost for offshore wind farms, and the structural optimization of jackets will bring lucrative economic benefits. Structural optimization is a complex iterative process that requires huge computing costs. Therefore, this paper proposes a structural optimization method based on surrogate models to solve this problem effectively and swiftly obtain optimized design schemes of lightweight jackets for offshore wind turbines. The structural responses of jacket wind turbine systems under the equivalent static extreme loads with a recurrence period of 50 years are mainly considered in structural optimization design, and the key optimization variables of jackets are determined by parameter sensitivity analysis. The finite element models of jackets are transformed into surrogate models, and the genetic algorithm is employed to optimize the surrogate models directly. The optimized jackets are additionally verified through coupled dynamic analysis, besides, buckling strength and fatigue life are also checked. And local refined optimizations are carried out for the failure members. According to the optimized design schemes of lightweight jackets for 30 m, 50 m and 70 m water depths, it is demonstrated that the structural optimization design method is adequate and efficient for jackets of wind turbines. Parameter sensitivity analysis can cut the number of optimization variables in half to improve the optimization efficiency. Furthermore, the application of surrogate models can significantly speed up the optimization process by saving about 98.61% of the original time consumed. The optimization design method of the jackets for offshore wind turbines proposed in this paper is suitable for practical engineering, with high precision and efficiency.

Detection of damage in offshore jacket structure using artificial neural network

With industrial growth over the years, geologists have begun to search for offshore sources of oil and natural gas. Most offshore oil-production platforms are jacket type. Offshore jacket structures consist of welded steel space frame. Tubular pipes are opted as they reduce hydrodynamic loads making them highly durable structures. However due to natural phenomenon like cyclones, earthquakes, etc. these structures can get adversely damaged. Apart from that the structure also degrades due to corrosion and fatigue due to the severe conditions it is always exposed to. In many cases physical inspection of the structure for checking damages is not possible due to unfavourable onsite conditions or lack of trained professionals. Hence there is a need to develop alternatives solutions. In recent years much research has been done in the application of Artificial Neural Networks (ANN) in Civil Engineering. ANN are computation systems that are inspired by biological neural systems. This paper aims at creating an ANN to identify damage in an offshore jacket structure using the modal parameters. The training set for the ANN is obtained by using a finite element software. The ANN is then tested using a test set and then it will be used to predict the structural damage in an offshore jacket structure.

Numerical Analysis of Supply Vessel Collision with Tripod Jacket Structures

Numerical Analysis of Supply Vessel Collision with Tripod Jacket Structures

Effect of Shear Key Interarray on the Bearing Capacity of Grouted Connections of Prepiling Jacket Substructures

Jacket structures and piles penetrating the seabed are connected through grouted connections. The shear key shape is designed to improve the shear strength of grouted connections. On a construction site, it is difficult to control the height error of the inner and outer steel tubes because of the environmental load. In addition, an installation error leads to an error in the arrangement of the shear keys, degrading the structure's performance. Among the construction methods for installing jacket structures, prepiling is more economical than the post-file method, but construction errors are likely. In this study, the fracture shape and maximum axial load capacity were evaluated according to the arrangement error of the shear key. It was found that when an arrangement error of the shear key occurred, the maximum axial load decreased to within 10%.

Effect of Soil-Pile-Structure Interaction on Behaviour of Offshore Jacket Structure

Offshore jacket structures are generally supported on pile foundations. Studies have evaluated the effect of soil-pile interaction on design of foundation; however, effect of soil-pile-structure interaction on behaviour of jacket structure has received little attention. Hence, present study focuses on understanding the influence of soil conditions, soil-pile interaction and foundation modeling approach on deformation characteristics and member forces of jacket structure. Linear static analysis on a typical 4-legged battered jacket structure is performed using STAAD Pro., subjected to dead/live loads and environmental loads. The water depth of 120 m and wave heights of 5 m, 10 m and 15 m are considered for this study. Morison’s equation considering Stokes 5th order wave theory is used for calculation of wave forces on jacket structure. The supports are modelled as skirted pile groups. The piles are represented by springs, with stiffness calculated using soil-pile interaction analysis. The soil conditions used in the study are varied for different compaction states of sand and different consistency states of clay and various parameters such as lateral/vertical deflection, support reactions, forces in leg, beams and bracings have been compared. It is concluded that ignoring soil-pile-structure interaction during analysis of jacket structure would underestimate the lateral/vertical deflections, support reactions, shear force and bending moment for leg, and axial forces in beam and plan bracing of the structure. No substantial impact of the interaction on other member forces is observed. Jacket structure founded on clayey soil exhibits higher deformation and member forces as compared to sand in general. The study is expected to help structural engineers in adopting an appropriate modelling approach, considering soil-pile-structure interaction, to obtain more realistic response for jacket structures.

A Parametric Study on Effect of Wave Height, Water Depth and Support Conditions on Behaviour of Offshore Jacket Structure

Fixed offshore jacket structures are constructed for facilitating oil/gas exploration and production. These structures and their foundations are designed to resist large vertical and lateral loads. Various factors including water depth, wave height and support conditions would affect the response of jacket structures. However, few studies have focused on understanding the response of offshore jacket structure due to variation in these factors. In this context, the present work evaluates response of typical X-braced, square base, 4-legged battered jacket structure using STAAD Pro. under combined vertical and lateral environmental loading. The variables included are water depth (60 m, 90 m and 120 m), wave height (5 m, 10 m and 15 m) and two foundation modelling approaches (viz., fixed at pile location and with defined pile stiffness). The connection between structure leg and pile location is modelled to simulate realistic connection. Deck loads, wind forces and current velocities are considered constant for this study. The wind and wave loads have been applied in parallel, perpendicular and diagonal directions with respect to jacket structure. The wave forces are calculated by Morrison’s equation. For obtaining pile stiffness, medium dense sand layer is considered as foundation soil. The increase in water depth and wave height results in corresponding linear increase in lateral deflection and support reactions, the effect of water depth being more prominent. Moreover, lateral and vertical deflection, shear force and bending moment in the legs, the axial forces in the lower tie beams and plan bracings, and support moments are observed to increase when pile locations are modelled with appropriate vertical, lateral and rotational stiffness instead of fixed support. The effect of water depth on member forces is higher as compared to wave height. The present work deliberates on the mechanisms/reasons to explain the observed results and contributes in direction of framing decision matrix for design optimization of jacket structures.

Dynamic Analysis of Offshore Wind Turbine Supported by Jacket Substructure under Wind and Wave Loading

Burning of fossil fuel for the production of energy causes severe global warming effects. Renewable energy sources like solar, wind and tidal etc. are the alternative renewable energy sources which contribute in the reduction of adverse global warming effects. Wind turbines are being used for extracting wind energy from several years. Wind blow is continuous with limited disturbance in the offshore region when compared with main land. Offshore wind energy extraction is in research stage at many locations and implemented in European countries. Prediction of response of wind turbine supporting systems is essential in the design to withstand the environmental loads such as wind, wave, current and seismic etc. In the present study, a horizontal axis offshore wind turbine (HAWT) supported on an offshore jacket structure is considered and the response studies are performed. The jacket is considered at a water depth of 51m, thus total height of the jacket is 61m with a free board of 10m. A wind turbine of 5MW capacity is considered to be on top of jacket structure. The height of the wind tower is assumed as 70m, and a transition structure of 4m height is positioned in between jacket and tower. A free vibration analysis is performed to estimate the natural frequencies and mode shapes of the jacket supported wind turbine. The modal analysis is carried out using ANSYS static structural module. The response analysis under wind, wave, current and aerodynamic drag loads is performed using SACS 13.2 software. Wind force is estimated based on API 2005 provisions. The aerodynamic forces on the wind turbine blades are evaluated using Betz Theory. Wave loading is calculated using Morison equation and linear Airy’s wave theory. A parametric study is carried out by varying wave period from 6s to 20s. As the structure is symmetric about longitudinal and lateral directions, a wave directional analysis is also carried by considering 0o and 45o wave directions. The structural responses are studied for the combined wind, wave and current loads. Cut-in, rated, cut-out and storm conditions are simulated by modelling wind and aerodynamic loads on the tower, wind interacting area of the jacket and blades. Wave period and direction are varied to simulate different wave conditions. It is observed that the structural response is increasing as the wind velocity is increasing and wave period is decreasing.

Applicability of machine learning approaches for structural damage detection of offshore wind jacket structures based on low resolution data

Applicability of machine learning approaches for structural damage detection of offshore wind jacket structures based on low resolution data

A topology optimization methodology for the offshore wind turbine jacket structure in the concept phase

This paper proposes a topology optimization methodology for offshore wind turbine (OWT) jacket structure design in the concept phase. First of all, modal analysis and load calculation of a 5 MW reference OWT are performed. On the basis of finite element analysis, the displacement and von Mises stress of multi-jacket support structures in various ultimate loading conditions are explored. Furthermore, structural topology optimization formulation aims to minimize a weighted normalized objective for the OWT jacket structure. Therefore, several optimum structures are obtained by adjusting weighting coefficients. For the coupling effect between the structure and applied load, load recalculation, displacement, and stress reanalysis are performed on the jacket structure based on the topological configuration under a specific weighting factor. Comparisons are made between the reference structure and optimized structure. Without the addition mass, the fundamental natural frequency of the optimized structure is marginally increased, but the maximum displacement and von Mises stress are drastically lowered, indicating that the optimized structure outperforms its reference counterpart. These findings conclusively demonstrate that the proposed TO technique for the design of OWT jacket structures is feasible and preferable.