Jacket Structures Research Articles

Ultimate bending capacity of aged fixed platform by considering the effect of marine fouling

Abstract For offshore structures, especially jacket supported structure such as fixed platform and offshore wind turbine, the underwater area provides an ideal site for corrosion formation and marine fouling. In this marine environment, the structures are exposed to particularly aggressive corrosion and colonized by marine organisms involving a diversity of species. In this paper, a number of cases are conducted where an existing fixed platform is subjected to uniform corrosion. Furthermore, the platform is also subjected to marine fouling. The platform is assessed to investigate the effects of corrosion and marine fouling on its structural strength by determining the Reserve Strength Ratio (RSR) value. It is found the platform is limited to at least 50 years of service life when subjected to average corrosion. However, the platform is no longer considered as safe after 37 years when subjected to severe corrosion. Moreover, there is an RSR loss of 38% when marine fouling is introduced at the first 5 years which is detrimental to the overall safety of the platform. On the other hand, there is minimum RSR loss of only 5% for most cases when marine fouling is introduced but still detrimental to the overall safety of the platform. The obtained outcomes were documented and will be useful information to conduct condition assessment (or structural health monitoring) of aged fixed jacket structures.

Effects of geometrical parameters on the degree of bending (DoB) in two-planar tubular DT-joints of offshore jacket structures subjected to axial and bending loads

The fatigue life of tubular joints commonly found in offshore structures is not only dependent on the value of hot-spot stress (HSS), but is also significantly influenced by the through-the-thickness stress distribution characterized by the degree of bending (DoB). In the present research, data extracted from the finite element (FE) analyses carried out on 81 two-planar tubular DT-joint models was used to study the effects of geometrical parameters on the DoB values in DT-joints subjected to six different types of loading including two types of axial loading, two types of in-plane bending (IPB) moment loading, and two types of out-of-plane bending (OPB) moment loading. Generated FE models were validated using available parametric equations. The determination of DoB values in a tubular joint is essential for improving the accuracy of fatigue life estimation using the stress-life (SN) method and particularly for predicting the fatigue crack growth based on the fracture mechanics (FM) approach. Geometrically parametric investigation was followed by a set of nonlinear regression analyses to develop three DoB parametric formulas for the fatigue analysis and design of two-planar DT-joints.

Time and cost analysis of jacket structure load out using skidding

This paper conducted an analysis to determine time and cost required in the jacket structure loadout process using skidding method. Determining normal time and cost in accordance to the company’s budget for jacket structure loadout process using skidding with CPM. The purpose of this paper is to obtain project schedule with low cost using skidding with CPM to determine the time and cost needed for the jacket load out. The analysis result of time and cost by CPM for jacket structure loadout process using skidding is 9 days with a total cost US$ 68,196.

Mixing Efficiency of Compost Tea Brewing Machines with Various Airlift Pump Air Jacket Structures and Air Injection Pressures

Compost tea is an organic agriculture product that is used as a bio-fertilizer and biopesticide in modern agriculture. It is produced from compost using various brewing machines. The compost must be well mixed with water to support optimal extraction of nutrients and microorganisms. Therefore, in this study, the mixing efficiencies of brewing machines equipped with six different structured airlift pumps operating at six different air injection pressures (8, 9, 10, 12, 14, and 16 kPa) were investigated via comparative trials of the compost liquid circulation and the electrical conductivity and data analysis. Designs with total hole areas of 0.09 cm2 achieved the highest compost liquid mass flow rates and good mixing efficiencies. In addition, air mass flow rates increased with the number of air injection holes at fixed pressures. Together, these data indicated that the best air jacket footpiece design in this study was A2, which contained eight holes with diameters of 1.2 mm. This design achieved the highest pump efficiency, mixing time, and liquid compost mass flow at most air pressures. This paper will help designers and manufacturers of this type of machinery.

고정식 자켓형 해양구조물의 지반 물성치에 따른 구조 응답에 관한 연구

For a fixed jacket type offshore structure directly supported by the seabed, the structural behavior of offshore structure depends on the soil properties. Soil properties affect on the stiffness of the piles and the boundary condition in the structural analysis. The structural analysis is performed using PSI (Pile-Soil Interaction) suggested in the code and design rule. PSI analysis of the jacket structure is carried out after various soil types are selected according to the soil properties like internal friction angle, undrained shear strength, unit weight and so on. Three types of soil are selected by varying strength for a clay and sand, respectively. The structural analysis of the jacket structure is performed using these soils. The results about axial and lateral reaction force and the stress and displacement on the structure are compared. As a results, the structural response is smaller as the soil becomes more stiff. In conclusion, it is confirmed that the structural response of fixed jacket type offshore platform supported by seabed is sensitive to the change of soil properties.

Probabilistic analysis of the local joint flexibility in two-planar tubular DK-joints of offshore jacket structures under in-plane bending loads

Tubular DK-joints are among the most common joint types in jacket substructure of offshore wind turbines (OWTs) and oil/gas production platforms. The local joint flexibility (LJF), as an intrinsic feature of a tubular joint, is one of the factors affecting the global static and dynamic responses of an offshore structure. In the present paper, a total of 324 finite element (FE) analyses were carried out on 81 FE models of two-planar tubular DK-joints under four types of in-plane bending (IPB) moment loads. Generated FE models were validated based on the available experimental data, FE results, and parametric equations. FE analysis results were used to develop a set of probability density functions (PDFs) for the LJF factor (fLJF) in IPB-loaded DK-joints. Based on a parametric FE investigation, a sample database was prepared for the fLJF values and density histograms were generated for respective samples. Nine theoretical PDFs were fitted to the developed histograms and the maximum likelihood (ML) method was applied to evaluate the parameters of fitted PDFs. In each case, the Kolmogorov-Smirnov and chi-squared tests were used to assess the goodness of fit. Finally, the Birnbaum-Saunders model was proposed as the governing probability distribution function for the fLJF. After substituting the values of estimated parameters, four fully defined PDFs were presented for the fLJF in two-planar tubular DK-joints under four types of IPB loading.

Damage identification in offshore jacket structures based on modal flexibility

A modal flexibility-based method using finite element model updating technique for damage detection purposes is presented. The motivation for choosing modal flexibility is explained and the objective function for an iterative optimization process is formulated. The simulated platform FE model is established with single and triple damage cases where noise contamination and model uncertainty are considered. Monte Carlo simulation is used for random parameter study. A substructure optimization algorithm is proposed together with a threshold to lower the number of design variables. When the flexibility-based method employs only the first order modal parameter and translational degree of freedoms on merely 4 nodes, satisfactory results can still be derived under high level of contaminated noise and random errors on elemental elasticity modulus and mass density. The influence of random parameters, spatial-incompleteness and tiny damage cases are discussed with several important conclusions summarized. The proposed substructure optimization algorithm and crucial problems during optimization process are described. The algorithm is applied to two numerical examples including a simply-supported beam and a jacket-platform, the study based on equivalent model reveals better accuracy and efficiency of the proposed algorithm, even with 3% measurement noise and model uncertainty.

Effect of foundation behaviour on steel jacket offshore platform failure modes under wave loading

ABSTRACTIn this paper, the performance and failure modes of a newly designed platform have been studied numerically as a case study. In order to investigate the effects of soil condition on the behaviour and failure modes of the platform, two different soil profiles named SC1 and SC2 classified as weak and strong soils, respectively, have been investigated. Static Pushover analysis has been performed by considering wave loading as the lateral load pattern exerted on the platform. Different models in terms of different soil types and different pile sizing have been developed. A comprehensive discussion has been presented about the effect of these parameters on the modes of failure of the platform. Analysis results revealed that the strength proportionality between the foundation and the jacket part of the platform leads to a better performance of the platform in accordance with high indeterminacy of the jacket structure and helps prevent immature failure.

A Comparison of Fatigue Life Improvement Methods for an Existing Offshore Jacket Platform Structure

Most offshore Jacket platforms are installed in shallow water, less than 300 meters for drilling and production of oil or gas. They are normally fixed to seabed and constructed as three dimensional frameworks with tubular members as structural elements. The surrounding environment around offshore platform is affected by various environmental loads that comprise of wind, waves, currents and earthquake. The major load in such structures is wave loading, repetitive in nature which causes time varying stresses that results global and or local fatigue damage on the offshore steel structure. This study includes fatigue assessment of a selected jacket structure using ANSYS software. This study concentrates mainly on the stress reduction at joints by various methods under the applied cyclic loading. The various methods considered in this study are providing: joint cans with branch stubs, grouted connections, FRP wrappings, stiffeners at the joint section. A comparison of various methods for reducing the hotspot stresses which increases fatigue life are studied and the most effective one is recommended.

Conceptual jacket design by structural optimization

AbstractWe present an approach for sizing optimization of jacket structures and apply it to investigate the conceptual design of jackets for offshore wind turbines. Conceptual design is an input to early structural and financial models, and we assume simplified analysis and load models. A four‐legged jacket for the DTU 10‐MW wind turbine in 50‐m water depth is modelled by Timoshenko beam finite elements, and the structural dimensions of the beam cross sections are considered as continuous design variables. A structural optimization problem is formulated to minimize the jacket mass, with constraints on fatigue and ultimate limit states. The optimal design problem is then used to investigate how the optimized mass depends on the number of bays and the jacket leg distance. The conceptual design investigation led to a new conceptual design with 14% lower mass compared with the original conceptual design. We conclude that structural optimization can provide useful insights in the conceptual design phase and lead to a better starting point for the further design and planning processes.

Performance of CPT-based methods to assess monopile driveability in North Sea sands

Offshore pile driving is a high-risk activity as delays can be financially punitive. Experience of pile driving for offshore jacket structures where pile diameters are typically < 2 m has led to the development of empirical pile driveability models with proven predictive capability. The application of these methods to larger diameter piles is uncertain. A major component of driveability models involves estimating the static resistance to driving, SRD, a parameter analogous to pile axial capacity. Recent research on axial capacity design has led to improved models that use Cone Penetration Test, CPT data to estimate pile capacity and include for the effects of friction fatigue and soil plugging. The applicability of these methods to estimating pile driveability for larger diameter piles is of interest. In this paper, recent CPT based axial capacity approaches, modified for mobilised base resistance and ageing, are applied to estimating driveability of 4.2 m diameter piles. A database of pile installation records from North sea installations are used to benchmark the methods. Accounting for factors such as pile ageing and the relatively low displacement mobilised during individual hammer blows improves the quality of prediction of pile driveability for the conditions evaluated in this study.

Fatigue Design Recommendations for Conical Connections in Tubular Structures

Conical connections are important structural members for the integrity of most types of welded tubular structures. They are for example used in traditional jacket structures for oil and gas production and in monopiles for support of wind turbines where an optimal design is aimed for. From contact with the industry, it is noted that there is uncertainty about the basis for the stress concentration factors (SCF) for conical connections in design standards for fatigue assessment. This is related to how fabrication tolerances are accounted for and how a transition in thickness from the cone to the tubular or the cylinder should be made to minimize stresses due to thickness transitions and fabrication tolerances. Analytical expressions for stress concentrations at conical transitions are outlined in this paper to get a better understanding of the effect of thickness of the cone and the cylinder. By a proper basis for fatigue design, it is possible to control additional stresses from thickness transitions and fabrication tolerances at these connections.

Evaluation of ice-induced fatigue life for a vertical offshore structure in the Bohai Sea

In China, the oil and natural gas resources of the Bohai Sea are mainly marginal oil fields. Therefore, it is necessary to build both ice-resistant and economical offshore platforms. These platforms underwent severe ice-induced vibrations during winter season, which not only can disturb the crews and endanger facilities available on the platforms, but also may evoke fatigue damage in tubular joints. Hence, it is vital to carry out a fatigue analysis for ice-resistant structures, which are built in the Bohai Sea. At the present, a holistic approach with the aim of analysis of ice-induced fatigue life is still not available for vertical offshore structures. Based on the field observations over several years in Bohai Bay, in this study, an interaction between ice and a vertical-leg jacket structure is analyzed. Then, a fatigue life assessment method is presented for sea ice conditions and structural characteristics, which would be applicable to the Bohai Sea as well. Eventually, a jacket structure in the Bohai Sea is numerically simulated using ANSYS software. Besides, a comparison regarding the fatigue damage of the jacket structure under steady-state and random vibrations is made, and the fatigue life is accordingly calculated using the total damage.

고정식 자켓구조물의 기하형상 조건에 따른 구조거동특성

고정식 자켓구조물의 기하형상 조건에 따른 구조거동특성

On gradient‐based optimization of jacket structures for offshore wind turbines

AbstractDuring the bidding or very early design phases of jacket structures for offshore wind turbines, there may be very limited information available on meteorological conditions, soil conditions, turbine specifications, etc. However, it is still important to quickly produce near‐optimal designs with production costs similar to that of the final support structure. Numerical optimization methods can be used to this purpose. This paper investigates three gradient‐based optimization methods, where preliminary designs are produced by mass optimization. The mass is reduced by changing tube diameter and thickness of the structural members, and the optimization considers both frequency and fatigue constraints. The three methods are based on (1) damage equivalent loads, (2) quasi‐static analysis, and (3) dynamic analysis. The optimizations are conducted using in‐house software JADOP (jacket design optimization), and the optimized designs are evaluated using state‐of‐the‐art integrated time‐domain simulation software FEDEM Windpower. The findings show that each analysis can be applied with success. However, if excitations of structural frequencies contribute significantly to the overall damage, special care must be taken with quasi‐static and static modeling. It is observed that wave loading does not contribute considerably to the fatigue damage. Additionally, the aerodynamic loading does not change significantly with changes of tube geometry within the optimization ranges. The optimized designs are partly driven by reducing stress concentration factors, which can be achieved by reducing the chord diameter to thickness ratio. Thus, the optimized designs resemble each other to a certain extent.

Effects of Current Velocity and Profile on Loading of Off Shore Jacket Structure

Effects of Current Velocity and Profile on Loading of Off Shore Jacket Structure

Blockage and relative velocity Morison forces on a dynamically-responding jacket in large waves and current

This paper documents large laboratory-scale measurements of hydrodynamic force time histories on a realistic 1:80 scale space-frame jacket structure, which is allowed to respond dynamically when exposed to combined waves and in-line current. This is a follow-on paper to Santo, Taylor, Day, Nixon and Choo (2018a) which used the same jacket structure but very stiffly supported. The aim is to investigate the validity of the Morison equation with a relative velocity formulation when applied to a complete space-frame structure, and to examine the fluid flow (and the associated hydrodynamic force) reduction relative to ambient flow due to the presence of the jacket structure as an obstacle array as well as the dynamic structural motion, interpreted as wave–current-structure blockage. Springs with different stiffness are used to allow the jacket to respond freely in the incident wavefield, with the emphasis on high frequency modes of structural vibration relative to the dominant wave frequency. Transient focussed wave groups, and embedded wave groups in a smaller regular wave background are generated in a towing tank. The jacket is towed under different speeds opposite to the wave direction to simulate wave loading with different in-line uniform currents. The measurements are compared with numerical predictions using Computational Fluid Dynamics (CFD), with the actual jacket represented in a three-dimensional numerical wave tank as a porous tower and modelled as a uniformly distributed Morison stress field derived from the relative velocity form. A time-domain ordinary differential equation solver is coupled internally with the CFD solver to account for feedback from the structural motion into the Morison distributed stress field. An approximate expanded form of the Morison relative-velocity is also tested and is recommended for practical industrial applications. Reasonably good agreement is achieved in terms of incident surface elevation, dynamic model displacement as well as total hydrodynamic force time histories, all using a single set of Morison drag (Cd) and inertia (Cm) coefficients, although the numerical results tend to slightly overpredict the total forces. The good agreement between measurements and numerical predictions and the generality of the results shows that the Morison relative-velocity formulation is appropriate for a wide range of space-frame structures. In these tests, this gives rise to additional damping of the dynamic system which is equivalent to 8% of critical damping. This is significantly larger than both the structural and hydrodynamic damping combined (which is about 1%) as quantified through free vibration (push test) in otherwise stationary water.

A global slamming force model for offshore wind jacket structures

Under certain harsh environmental conditions, jacket structures supporting offshore wind turbines might be exposed to plunging breaking waves, causing slamming forces that affect the structural integrity and fatigue life. The slamming forces should thus be properly considered during the design, but a suitable force model specifically for jacket structures is currently in absence. In this study, a five-parameter force model is developed for estimating global slamming forces due to plunging breaking waves on jacket structures, based on statistical analyses of experimental data from the WaveSlam project. The force model is developed by considering a total of 176 individual breaking waves, under six wave conditions. For each individual breaking wave, the time history of the slamming force is calculated based on hammer test data in addition to wave test data, and the wave parameters are acquired from a wave elevation measurement. The acquired time histories and wave parameters are then used to determine the parameters involved in the force model, including two exponential parameters (i.e. α1 and α2) and three dimensionless coefficients for the expressions of wave-dependent parameters (i.e. duration coefficient ζ1, rising time coefficient ζ2, and peak force coefficient ζ3). It is found that α1, α2, ζ1 and ζ2 are approximately constant, and ζ3 follows a lognormal distribution. The quantile that determines ζ3 should be carefully selected so as to provide a conservative prediction. A quantile of 95% is suggested in this paper, and it is found to be conservative based on the verification of the developed force model. Therefore, for a given sea state, this force model can give a deterministic and conservative prediction of the slamming force time history, regardless of the randomness of slamming forces. Challenges for the application of the force model are also addressed.

Structural optimization with several discrete design variables per part by outer approximation

The article proposes an optimal design approach to minimize the mass of load carrying structures with discrete design variables. The design variables are chosen from catalogues, and several variables are assigned to each part of the structure. This allows for more design freedom than only choosing parts from a catalogue. The problems are modelled as mixed 0–1 nonlinear problems with nonconvex continuous relaxations. An algorithm based on outer approximation is proposed to find optimized designs. The capabilities of the approach are demonstrated by optimal design of a space frame (jacket) structure for offshore wind turbines, with requirements on natural frequencies, strength, and fatigue lifetime.

Structural reliability assessment of an in-service offshore steel jacket structure using as built data*

This article considers the structural reliability of in-service offshore steel jacket structure with respect to Push Over Plastic Collapse mechanisms. From a structural integrity (SI) perspective, differing methodologies are required when undertaking assessments of these structures in comparison to those applied during the design phase. This difference is explicitly outlined and the associated impacts (and implications) when justifying the continue service under expected loading conditions are demonstrated. An in-service steel jacket is modelled with structural members (comprising cans) initially considered with design parameters (independent and correlated) followed by the as built case. The differing collapse mechanisms and associated probability of structural collapse is highlighted.