Jacket Structures Research Articles

Current blockage in a numerical wave tank: 3D simulations of regular waves and current through a porous tower

This paper introduces a new numerical approach for the estimation of the global hydrodynamic loads on space-frame offshore structures exposed to combined waves and current. We provide numerical evidence for reduced fluid loading on offshore structures – current blockage, which serves as an extension to the analytical, computational and experimental work of Taylor et al. (2013) and Santo et al. (2014a, 2014b). A full 3D free-surface turbulent flow is simulated for a porous tower in a numerical wave tank. This is intended to model waves and current through a jacket or compliant tower, both space-frame structures. Comparisons are made between the numerical simulations and experiments conducted by Allender and Petrauskas (1987) on a scale-model jacket structure from the Gulf of Mexico, and the current blockage model presented previously in Taylor et al. (2013). Three different flows are simulated: steady current, regular waves with no current and regular waves with an in-line current. Overall, good agreement in terms of peak total forces is achieved, showing that the force reduction on such structures due to current blockage effects is real and significant. Additional information on force time history and flow visualisation are presented from the numerical simulations. Flow visualisation for waves and current reveals that the form of the global mean wake is simple at the structure but becomes complex well downstream. The simple form of the flow at the tower is responsible for the global force reduction being predictable using a modified version of the Morison equation (Morison et al., 1950). This paper also demonstrates the novel use of a porous tower as a simple representation for the complex geometry of real space-frame structures when exposed to combined large waves and significant in-line current, an approach which could be considered for possible incorporation into offshore design practice.

Soil-structure interaction on the response of jacket-type offshore wind turbine

Jacket structures are still at the early stage of their development for use in the offshore wind industry. The aim of this paper is to investigate the effect of the soil-structure interaction on the response of an offshore wind turbine with a jacket-type foundation. For this purpose, two different models of flexible foundation-the p-y model and the p-y model considering pile groups effect-are employed to compare the dynamic responses with the fixed-base model. The modal analysis and the coupled dynamic analysis are carried out under deterministic and stochastic conditions. The influence of the soil-structure interaction on the response of the jacket foundation predicts that the flexible foundation model is necessary to estimate the loads of the offshore wind turbine structure well. It is suggested that during fatigue analysis the pile group effect should be considered for the jacket foundation.

Failure Analysis and Redesign on Jacketed Polypropylene Reaction Kettle of Propylene

Jacketed polypropylene reaction kettle is the key equipment of polypropylene reaction setting. Cracks and leakage occurred in the kettle wall near the lower thermometer pipe branch, which is a widespread phenomenon in this refinery. The results showed that the cracks were caused by low cycle fatigue. High edge stress concentration resulted from incorrect design of the jacket structure is responsible for the fatigue cracks. An improved measurement which is available and simple was introduced.

Reassessment of the jacket structure due to uniform corrosion damage

A number of offshore platforms operating in the years were built with the outdated criteria. A reassessment analysis of the structures should be conducted for safety and maintenance. Reassessment of a jacket structure subjected to corrosion damages was analysed in this study. Programme SACS was used for modelling and conducting pushover analysis for the jacket structure. There are two types of corrosion damage, ‘general corrosion’ and ‘localised corrosion’, and the general corrosion was applied on the structure. Corrosion rates of the jacket structure could be divided into three parts, atmospheric zone, splash zone and full immersion zone. Through the pushover analyses using SACS, the relation between time and RSR/RSR0 was established and it was verified which zone had more corrosion influence to the jacket structure.

Random seismic response analysis of jacket structure with Timoshenko's beam theory

Today many complex three-dimensional (3D) models, typically finite element method (FEM) models, have been used in the analysis of jacket platforms. However, these comprehensive models are not readily adopted in engineering practice, especially during the preliminary design stage. The main reasons are the large computing resources required and the lack of detailed information for setting up the FEM model. In this paper, a simplified response analysis method is proposed to make preliminary estimate of the random seismic responses on jacket platforms. Based on Timoshenko's beam theory, the equation of bending motion was proposed and solved by using the classical Ritz method combined with the pseudo-excitation method. A computer code for computing random seismic responses was also developed and verified. A six-legged jacket platform was used as an example, whereby three types of Ritz functions were used to illustrate the validity of the model. The results of this study were compared with those obtained from a well-established complex 3D FEM model. The findings of the comparison showed that the accuracy of the simplified model was better than 95%. It was concluded that the proposed simplified method was effective, useful and practical in seismic design of platforms.

Stress concentration due to in-plane bending (IPB) loads in ring-stiffened tubular KT-joints of offshore structures: Parametric study and design formulation

Although tubular KT-joints are commonly found in offshore jacket structures and despite the crucial role of stress concentration factors (SCFs) in the evaluation of the tubular joint's fatigue performance, the SCFs in internally ring-stiffened KT-joints subjected to in-plane bending (IPB) loads have not been investigated so far and no design equation is available to determine the SCFs for IPB-loaded joints of this type. In the present research, data extracted from the finite element analysis of 118 models, which were verified using test results obtained from an experimental investigation, were used to study the effect of geometrical parameters on the weld-toe SCFs of central and outer braces in internally ring-stiffened tubular KT-joints subjected to four different types of IPB loads. Parametric study was followed by a set of the nonlinear regression analyses to develop SCF parametric equations for the fatigue analysis of ring-stiffened KT-joints under IPB loadings.

Stress concentration factors induced by out-of-plane bending loads in ring-stiffened tubular KT-joints of jacket structures

In the present paper, data extracted from the finite element analysis of 118 models, which were validated against the test results obtained from an experimental study, were used to investigate the geometrical effect on the weld-toe stress concentration factors (SCFs) in internally ring-stiffened tubular KT-joints subjected to four different types of out-of-plane bending (OPB) loads. Although the tubular KT-joints are commonly found in offshore jacket structures and despite the crucial role of SCFs in the evaluation of the tubular joint׳s fatigue performance, the SCFs in internally ring-stiffened KT-joints subjected to OPB loads have not been investigated so far and no design equation is available to determine the SCFs for OPB-loaded joints of this type. In the present research, geometrically parametric study was followed by the nonlinear regression analysis to develop a new set of SCF parametric formulas for the fatigue design of ring-stiffened KT-joints under the OPB loadings.

Sea Ice Problems in Bohai Bay Oil and Gas Exploitation

ABSTRACT Zhang, D.; Xu, N.; Yue, Q., and Liu, D., 2015. Sea ice problems in Bohai Bay oil and gas exploitation. Sea ice infests the Bohai Bay every winter. Normally, ice condition is slight but sometimes heavy ice conditions may occur. It has been difficult for offshore designers to determine the conditions and the ice loads. Two jackets were pushed over by ice and others were designed too conservative as ice load were overestimated. In recent years some of the platforms have met severe ice induced vibrations, which has caused harmful accidents. In this paper, sea ice problems in oil and gas exploitation of Bohai Bay are briefly introduced. The dynamic behavior of ice resistant jacket structures and the potential failure modes that arise from cyclic ice actions are described.

Optimization of the shear tool deployment system for removal of steel piled jackets

It is estimated that there are more than 200 steel piled platform installations in the North Sea which have reached the end of their life and have to be cut and removed. In comparison with other cutting methods, the shear tool cutting method offers the greatest potential regarding optimizing the time of the operation. However, the existing shear tool is susceptible to blade bending–related failures caused by heave motions of the heavy lift vessel to which the tool is attached. While there are many studies related to the operability and workability of sea surface structures, this study focuses on the operability and workability of an underwater structure, the shear tool system. In order to improve the performance of the existing shear tool, the new shear tool system has been designed as an equalizer with additional counter weights; thus, this system acts as a passive heave compensator. In total, six different design concepts have been assessed using a multicriteria sensitivity analysis, and the most promising concept has been selected. Afterwards, the performance of the new design was compared with the performance of the existing tool by calculating their operability and workability for the removal of the benchmark (Northern North Sea jacket) structure. The workability calculation was based on 15 years of environmental conditions and results indicate that the new design leads to 50% workability increase.

Effect of seabed slope on pile behaviour of fixed offshore platform under lateral forces

Fixed offshore platforms supported by pile foundations are required to resist lateral load due to wave and current forces. The response to environmental loads is strongly affected by the soil structure (pile) interaction. The forces exerted by waves are most dominant among the lateral environmental forces which governing the jacket structures design especially the foundation piles. The present investigation is to perform a static wave analysis on a typical fixed offshore platform for extreme environmental conditions, and to study the effects of the combined lateral and vertical loads on pile group foundation. The three dimensional modeling and analysis of the offshore platform are done using finite difference method. The present analysis was done under static condition considering the structural and the environmental loads at extreme environmental conditions, by reaching the state of static equilibrium. A parametric study has been done by varying the seabed slope to examine the variation in soil-structure interaction behaviour of piles. It has been found that the lateral displacement at the pile top and at the seabed level increases as the seabed slope increases. It is also noticed that the depth at which the maximum shear force and bending moment occurs from the pile top increases as the slope of the seabed increases.

Failure mode of the ice sheet loading at the junction of the upward and downward breaking cones

Ice-breaking cones were installed on several vertical jacket structures to reduce severe ice-induced vibrations in the Bohai Sea, China. The failure mode of the ice sheet loading at the junction of the upward and downward breaking cones is our most concern due to the worry that the crushing failure mode may arise at the junction of the upward and downward breaking cones which has much larger diameter than the original vertical leg. Based on field observation and field data the failure mode of the ice sheet loading at the junction of the upward and downward breaking cones is described, and the fracture process is explained by subsequent analysis.

20 Years Decommissioning Experience - Evolution of Innovative Solutions

"Decommissioning is an increasing market sector which has been gradually gathering momentum particularly in the North Sea. The forecast expenditure for removing existing platforms increases with time. This comes together with the increasing complexity of the decommissioning operation. Experience and expertise in this field are key for success. Saipem have been contracted to perform a number of ‘removals’ ranging from subsea templates, subsea pipelines, seabed debris clearance, jacket structures and topside modules. This paper provides an account of our experience gained over the last 20 years performing decommissioning activities. The paper presents the evolution of the techniques developed and focuses in particular on the Lift and Tow method developed after 2004 for a number of subsea applications. Problems always materialise post contract award due to inadequate data. The paper gives a detailed description of the Lift and Tow method along with various innovative techniques developed for this method, ranging from lifting operations supported by motion forecasting through to personnel access onto the structures."

Probabilistic Assessment of Degree of Bending in Tubular X-Joints of Offshore Structures Subjected to Bending Loads

Fatigue life of tubular joints in offshore structures is significantly influenced by the degree of bending (DoB). The DoB exhibits considerable scatter calling for greater emphasis in accurate determination of its governing probability distribution which is a key input for the fatigue reliability analysis of a tubular joint. Although the tubular X-joints are commonly found in offshore jacket structures, as far as the authors are aware, no comprehensive research has been carried out on the probability distribution of the DoB in tubular X-joints. In the present paper, results of parametric equations available for the calculation of the DoB have been used to develop probability distribution models for the DoB in the chord member of tubular X-joints subjected to four types of bending loads. Based on a parametric study, a set of samples was prepared and density histograms were generated for these samples using Freedman-Diaconis method. Twelve different probability density functions (PDFs) were fitted to these histograms. In each case, Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, after substituting the values of estimated parameters for each distribution, a set of fully defined PDFs have been proposed for the DoB in tubular X-joints subjected to bending loads.

Application of model reduction techniques to jacket structures

Offshore structures like jackets are exposed to various kinds of external loading conditions and examination of broader range of modes and natural frequencies are required for their dynamic analysis. In this study, four model reduction techniques — two modal truncation methods and two balanced truncation methods — are introduced and applied to a jacket structure. The reduction results are compared with those of the conventional Paz condensation methods. Four new methods show better fidelity, especially in high frequency ranges than the conventional methods.

Short-term probability distribution of the extreme values of offshore structural response by an efficient time simulation technique

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. Wave loading on slender members of bottom-supported jacket structures is frequently calculated by Morison's equation. Due to nonlinearity of the drag component of Morison wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of their extreme response probability distributions are not available. To this end, the conventional time simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated response extreme values (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, a more efficient version of the time simulation technique (ETS) is introduced to derive the probability distribution of response extreme values from a much smaller sample of simulated extreme values. The ETS procedure is found to be many times more efficient than the CTS method.

Energy dissipation in high-energy ship-offshore jacket platform collisions

Ship collisions with offshore structures may be characterized by large amounts of kinetic energy that can be dissipated as strain energy in either the ship, or the installation, or shared by both. In this paper a series of FE numerical simulations are performed with the aim of providing a clearer understanding on the strain energy dissipation phenomenon, particularly upon the ship-structure interaction. Ships of different dimensions and layouts are modelled for impact simulations. Likewise, three platform jacket models of different sizes and configurations are considered. The collision cases involve joints, legs, and braces and are simulated for several kinetic energy amounts of the vessels and different impact orientations. An overview of the plastic deformation mechanisms that can occur in both ship and jacket structure is also given. The results from the various models with different collision scenarios are compared in terms of the strain energy dissipation with respect to the different ship/installation strength ratios. From the FEA simplified approaches are also derived in terms of the relative stiffness of the two structures for assessing the responses and energy absorptions of the two structures. The conclusions drawn from this study can be applied to a broader range of collision assessment of offshore steel jacket platforms subjected to high-energy ship impacts.

Dynamic characteristics of an offshore wind turbine with breaking wave and wind load

In this paper, the response characteristics of an offshore wind turbine (OWT) structure under breaking wave forces and wind forces are studied. A 3D numerical model, based on solving the viscous and incompressible Navier-Stokes equations and the volume of flmethod, is employed to estimate the breaking wave forces on an OWT structure (6.0-m diameter monopile). The calculated wave forces are then applied with the wind forces on the OWT structure modeled in the computer program HAWC2 to understand the nature of its response. The effects from the aerodynamic damping and the foundation fl exibility on the structure's response are also discussed. In recent years, a large number of fioffshore wind turbines (OWT), which are supported by monopiles, tripod structures, and jacket structures, have been planned or constructed in shallow waters to capture the abundant wind energy source. Among them, a 'monopile-transi- tion piece-tower' type of structure seems to be preferred to support an OWT in shallow waters. For the design of a monopile structure installed on a fl at bottom, the Morison equation has generally been used to estimate wave forces on the structure. Moreover, as the frequencies of the environmental loads on a structure generally stay away from the structure's natural fre- quency, the vibration of the structure is not a 'major' problem in the designing stage of the structure. However, in the case where an OWT structure is installed in a submerged shoal, the water waves may experience severely nonlinear wave deformations on their propagation, and the nonlinear waves generated toward the structure can give rise to higher local pressures and impulsive forces on the structure. Because the breaking wave impact forces normally act in a very short time, this can also cause large horizontal accelerations at the nacelle. Furthermore, the repeated occurrence of breaking waves in every season can potentially affect the fatigue life of the structure. Therefore, the accurate estimation of breaking wave impact forces and dynamic responses induced by the forces is of great importance. Several researchers have investigated breaking wave impact forces and the dynamic responses induced by the forces throughout the numerical and experimental approaches. Hu and Kashiwagi (1) applied the Constrained Interpolation Profi le (CIP) method for studying the wave impact phenomena and violent wave-structure interactions. Christiansen et al. (2) studied the wave run-up and the extreme wave forces on an OWT foundation under the plunging breaker by using Navier-Stokes solver. Marino et al. (3) presented a numerical procedure by using Boundary Element Method (BEM)-MEL (Mixed Eulerian-Lagrangian) to simulate the extreme response of an OWT structure. Bredmose et al. (4) used a focused wave technique in Open-Foam to estimate the breaking wave impact forces on a cylindrical pile. Mokrani et al. (5) used a NS-VOF approach to study the slamming forces on a vertical wall. The wave impact on a rectangular column using smoothed particle hydrodynamics (SPH) was studied by Cummins et al. (6). However, although various researchers have stud- ied the breaking wave impact forces on the structure, little information is currently available

Discussion and Dynamic Characteristics of the Offshore Jacket Structure

In this paper, based on the finite element method, the K type common jacket structure connection in the form of positive connection and reverse connection conditions is being do comparative analysis. Research focus on dynamic characteristics, including of power spectrum analysis under the action of random vibration and fatigue properties of two kinds of connection type structure. The calculation results show: the inverted K type connection of jacket structure is about 10% increase in the stiffness and the dynamic performance, the inverted K type connection form is more worthy of popularization and application in engineering.

Design of a novel drilled-and-grouted pile in sand for offshore oil&gas structures

New offshore oil and gas exploration has placed renewed emphasis on developing structures in relatively complex geological conditions. Due to the damaging nature of impact driving, traditional steel piles used to support jacket structures, are not ideally suited to specific soil types, such as carbonate sands. Drilled and grouted piles are commonly used to support structures in these soil conditions. This paper describes a novel drilled pile, which has been developed specifically to provide a cost effective installation process while maintaining the benefits of grouted piles. The installation process negates the need for temporary casing in weak soils and minimizes the number of offshore operations. In this paper, the installation methodology and post-installation performance of a large scale onshore field trial is described. The installation process was successfully demonstrated with a 1.9 m diameter test pile installed in fine sand to 17.7 m depth in under 3 h. The performance of the pile, as measured in a tension static load test, was shown to compare favorably with existing pile design methods.

해상풍력발전기 자켓 지지구조물의 최적설계 및 신뢰성해석

Since the support structure of an offshore wind turbine has to withstand severe environmental loads such as wind, wave, and seismic loads during its entire service life, the need for a robust and reliable design increases, along with the need for a cost effective design. In addition, a robust and reliable support structure contributes to the high availability of a wind turbine and low maintenance costs. From this point of view, this paper presents a design process that includes design optimization and reliability analysis. First, the jacket structure of the NREL 5-MW offshore wind turbine is optimized to minimize the weight and stresses, while satisfying the design requirements. Second, the reliability of the optimum design is evaluated and compared with that of the initial design. Although the present study results in a new optimum shape for a jacket support structure with reduced weight and increased reliability, the authors suggest that the optimum design has to be accompanied by a reliability analysis during the design process, as well as reliability based design optimization if needed.