Jacket Structures Research Articles

Testing of jacket pile sleeve grouted connections exposed to shear forces and bending moments

Modern offshore jacket structures are designed with piles that are carrying large shear forces and bending moments. Even if the geometry of the grouted connection is quite simple the load transfer mechanism is complex. It involves friction, contact stresses, brittle material, abrasion, cyclic degradation etc. In order to improve our understanding of the behaviour and load-carrying capacity of grouted connections a laboratory test program involving 12 test specimens was executed. The specimens were full scale representation of a segment of a part of a pile sleeve connection close to the lower end. The tests are carefully planned to represent the real load and deformation conditions of the connection under cyclic loads. Two design geometries “Retracted” and “Extended” were tested. Four of the tests included reinforcement bars. The effect of the grout seal keeper plate was also investigated.The load-carrying capacity under cyclic loading was found. There were significant differences in the behaviour of the two different geometries.

Seismic vibration control of offshore jacket platforms using decentralized sliding mode algorithm

A semi active control algorithm, using MR dampers, is developed for reduction of responses of offshore jacket platforms induced by earthquake ground motions and the responses are obtained for both nonlinear and linearized drag forces. The MR dampers are placed at different levels of the jacket structure and the coupled structure-damper system is modelled in Simulink. Decentralized sliding mode controllers are designed to drive the response trajectories into the sliding surfaces and the command voltage to the MR dampers are generated with the help of clipped-optimal control algorithm. The results of the numerical simulations show that the designed semi active sliding mode controller is effective in reducing the structural vibrations caused by earthquake ground motions. As a result of linearization of the drag force, the effectiveness of the controllers is reduced and responses obtained are conservative in nature, i.e., the increase in controlled responses are more when linearized drag force is considered. The control system is found to be stable and robust; however, the positions and the number of MR dampers have quite significant impact on the performance of the controller.

Current blockage and extreme forces on a jacket model in focussed wave groups with current

This paper documents large laboratory-scale measurements of hydrodynamic force time histories on a realistic 1:80 scale space-frame jacket structure exposed to combined waves and in-line current. The aim is to investigate the fluid flow (and the associated hydrodynamic force) reduction relative to ambient fluid flow due to the presence of the jacket structure as an obstacle array, interpreted as wave–current blockage. Transient focussed wave groups, and embedded wave groups in a smaller regular wave background are generated in a towing tank, and 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 modelled as a uniformly distributed Morison stress field. Good agreement is achieved, both in terms of incident surface elevation as well as total force time histories, all using a single set of Morison drag (Cd) and inertia (Cm) coefficients. Substantial force reduction is observed under transient large crest relative to prediction from the present industry design guideline with the same Morison coefficients. We demonstrate the generality of our findings: without influence of Keulegan–Carpenter (KC) number effect, a single invariant set of Cd and Cm is all that is required to numerically explain and reproduce the measured total force time histories on a realistic jacket model for a large range of wave heights and non-zero current speeds.

Testing of jacket pile sleeve grouted connections exposed to variable axial loads

Modern offshore jacket structures are designed with piles that are carrying variable axial loads, large shear forces and bending moments. Even if the geometry of the grouted connection is quite simple the load transfer mechanism is complex. It involves friction, contact stresses, brittle material, abrasion, cyclic degradation, etc.Current design recommendations were developed based on relatively small scale test and with only a few tests performed with cyclic loads that include change of load direction. In order to study the capacity of grouted connections with shear keys for alternating axial load, four (4) segment specimens were tested. The specimens were full scale representation of a segment of a part of the annulus of an axially loaded pile sleeve connection.The load-carrying axial capacity under cyclic loading was found.

Axial behaviour of jacket piles for offshore wind turbines

To improve our understanding and to quantify the behaviour of a jacket foundation for offshore wind turbines, several tests have been conducted using a geotechnical centrifuge at 100g. The monotonic tension and compression tests carried out on single small-scale model piles (prototype dimensions: 1·8 m in diameter and an embedment depth of 40 m in dense sand) jacked at 1g and 100g show the influence of the set-up method on the axial ultimate capacity and on the tip and shaft resistances. The pile axial capacity in compression is improved, whereas the normalised displacement of the pile is bigger in tension when piles are jacked at 1g. Vertical cyclic loads on single piles jacked at 1g (pure tension tests or two-way tests) and horizontal cyclic loads on a four-pile jacket structure are applied to examine the behaviour under more realistic loading paths. The piles of the jacket structure are considered and studied as singles piles. Finally, the stability diagrams for the single and the jacket piles are discussed to visualise the significant difference in performance of a single pile against the comparable pile in the jacket.

Natural frequency and vibration analysis of jacket type foundation for offshore wind power

There are various types of foundation structure for offshore wind power, engineers may assess the condition of ocean at wind farm, and arrange the transportation, installation of each structure members, furthermore, considering the ability of manufacture steel structure as well, then make an optimum design. To design jacket offshore structure, unlike onshore cases, offshore structure also need to estimate the wave excitation effect. The aim of this paper is to study the difference of natural frequency between different kinds of structural stiffness and discuss the effect of different setting of boundary condition during analysis, besides, compare this value with the natural frequency of sea wave, in order to avoid the resonance effect. In this paper, the finite element analysis software ABAQUS is used to model and analyze the natural vibration behavior of the jacket structure.

Global slamming forces on jacket structures for offshore wind applications

This study investigates the global slamming forces due to plunging breaking waves on a jacket structure, based on the statistical analysis of the experimental data from the WaveSlam project. Hammer tests and wave tests were conducted in the project, and the data are used to reconstruct the time series of the global slamming force by employing a method based on linear regression. The used wave test data were acquired under one wave condition. A total of 3910 force time series are reconstructed and analyzed statistically to reveal the characteristics of the slamming force. For each force time series, six parameters are introduced to describe it, including the peak force, duration, impulse and rising time, etc. The variability and correlation of these parameters are investigated. The distribution of these parameters is modeled with various probability distributions. The results show the high variability of the slamming force and the importance of statistical analyses. Based on these statistical analyses, the slamming coefficient is estimated from the peak force. For a curling factor of 0.4, the mean slamming coefficient is about 1.29. When considering one standard deviation around the mean, the slamming coefficient varies from 0.70 to 6.78 for a curling factor ranging from 0.1 to 0.5. A representative time series of wave slamming force is obtained by averaging the individual force time series. Accordingly, a 3-parameter triangular force model and a 5-parameter exponential force model are proposed to describe the development of the slamming force in time.

Detailed Study on Breaking Wave Interactions With a Jacket Structure Based on Experimental Investigations

Wave breaking is one of the major concerns for offshore structures installed in shallow waters. Impulsive breaking wave forces sometimes govern the design of such structures, particularly in areas with a sloping sea bottom. Most of the existing offshore wind turbines were installed in shallow water regions. Among fixed-type support structures for offshore wind turbines, jacket structures have become popular in recent times as the water depth for fixed offshore wind structures increases. However, there are many uncertainties in estimating breaking wave forces on a jacket structure, as only a limited number of past studies have estimated these forces. Present study is based on the WaveSlam experiment carried out in 2013, in which a jacket structure of 1:8 scale was tested for several breaking wave conditions. The total and local wave slamming forces are obtained from the experimental measured forces, using two different filtering methods. The total wave slamming forces are filtered from the measured forces using the empirical mode decomposition (EMD) method, and local slamming forces are obtained by the frequency response function (FRF) method. From these results, the peak slamming forces and slamming coefficients on the jacket members are estimated. The breaking wave forces are found to be dependent on various breaking wave parameters such as breaking wave height, wave period, wave front asymmetry, and wave-breaking positions. These wave parameters are estimated from the wave gauge measurements taken during the experiment. The dependency of the wave slamming forces on these estimated wave parameters is also investigated.

Mesh Parametric Study for Fatigue Assessment of Tubular K-joints using Numerical Methods

Wind turbine jacket structures are complex structures, whose joints design is generally driven by fatigue. These joints, along with their complex welds, are of special interest in terms of cost reduction. Therefore, a thorough analysis and understanding of the background behind the assessment proposed in guidelines is motivated. The paper presents a study of the influence of meshing for the assessment of tubular K-joints following the hot-spot approach using numerical methods. The accuracy of the results is discussed for several mesh layouts. The influence of the mesh density, element shape and element type are investigated. Furthermore, a parametric study is performed in order to see the variation of the results for several diameters, thicknesses and brace inclination combinations. The hot-spot method is proved to be robust regarding mesh regularity. However, the efficiency of irregular mesh models is very low and a high number of elements is needed in order to find an asymptotic behaviour that tends to a constant solution for increasing mesh refinement. Conclusions can be drawn for which cases it is worth to invest time in semiautomatic meshing. A discussion is done regarding which element size and type is better regarding accuracy and computational time.

Reliability Evaluation of Jacket-type Offshore Platforms Subjected to Wind, Wave, and Current Loads

The aim of this study is to evaluate the reliability and sensitivity of jacket-type offshore platforms under wind, wave, and current loads. The reliability of the platforms are comprehensively determined based on the deformation and energy of the jacket platform. The Young’s Modulus; the yield strength; the strain hardening rate; the diameter and thickness of members; the water depth; the wave height and period; the wave velocities on the surface, middle, and base of the platform; the drag coefficient; the inertial coefficient; and the horizontal concentrated scaled loads are defined in the study as random variables. The sensitivity of the performance functions to the above-mentioned variables is determined, and the variables that most influence the performance of the platform are identified. Main conclusions are drawn as follows: the increasing mean and decreasing standard deviation of wall thickness in Type-1, Type-4, and Type-2 jacket members can effectively reduce the deformation and total strain energy of platform structures. For jacket structures in the plastic state, increasing the mean and decreasing the standard deviation of yield strength is a reasonable method for reducing deformation. Maximum deformation and strain energy are highly correlated whether the load level is high or low, and the correlation of these indicators with the variables considered gradually increases with the load level.

Developing a SIM strategy for offshore jacket platforms based on the FE model updating and a novel simplified method

Lack of data of the jacket structures may result in a high uncertainty about the structures ability to endure harsh marine environmental conditions. An experimental measurement from structural behavior under harsh marine environmental conditions is necessary to reduce this uncertainty in practical structural monitoring process of jacket structures. As an alternative, a novel and robust simplified methodology based on the empirical investigation could be a practical approach of better understanding the structures integrity. Hence, these methodologies are often termed as model reduction schemes. In this article, model reduction schemes include simplified platform model and static reduction. This work introduces a structural integrity assessment strategy for jacket structures based on the finite element (FE) model updating and a novel simplified method. Hereof, model reducing and model updating procedure is established based on a novel optimization technique. Since the number of measured degrees of freedom is most of the time restricted in practice, this paper represents a methodology using the cross model cross mode (CMCM) method in combination with an iterative procedure which uses limited, spatially incomplete modal information. One of the important advantages associated with the proposed method is that measuring the rotational degrees of freedoms, which is most of the time very expensive and inaccurate, is not needed.This research is an empirical study on a laboratory model of a jacket structure with the aim of establishing Refined Simplified FE Model (i.e. RSM) to conduct damage detection. The studied jacket structure is a reduced scale 2D frame fabricated and tested at the Laboratory of Modal Analysis of the Tabriz University, Iran. Hereof, experimental vibration tests are accomplished on a physical 2D platform model to record dynamic responses and acquire dynamic features of damaged structure. In addition to eliminate the uncertainty effects in the damage detection results, RSM technique is employed because of practical considerations and also this technique provides a fast damage zone diagnosis procedure. Also, improved reduction scheme is utilized based on static reduction scheme to carry out damage detection in jacket structure. Improved reduction technique eliminates the bad influence of model reduction procedure on the proposed new methodology. The results demonstrate that the presented new technique is completely successful in conducing damage identification in jacket structures.

A framework to assess structural integrity of ageing offshore jacket structures for life extension

The structural integrity assessment of ageing structures for possible life extension has been identified as a growing challenge in the oil and gas industry worldwide. Although the available guidelines provide a general assessment process, it is necessary to have more detailed guidelines. This can be achieved by adding relevant theories and models which can capture time-dependent structural degradation more precisely. To address this, a new framework for possible life extension is proposed in this paper. The proposed framework provides more precise corrosion models, new damage theories and assessment guidelines to predict the remaining fatigue life and check the structural adequacy in all the limit states during the whole extended service life. Initially, the paper presents the proposed framework in detail. The framework approach is then applied to an ageing jacket as a case study and results are compared with conventional approach. The proposed approach results in a remaining life of ten years as compared to one year using the conventional approach. Thus, the jacket structure can be safely operated for an additional nine years using the proposed approach. Recommendations are also made on increasing the remaining fatigue life using life improvement techniques. Finally, the applicability, significance and validity of the proposed framework are discussed.

Pushover Analysis Pile of ULA Jacket Platform Towards Earthquake Loads by SNI-1726:2012

This study aims at analyzing pushover on pile jacket structure based on SNI-1726:2012 standards for earthquake loads. Push­over analysis or ultimate strength was performed on ULA jacket plat­form operated by PHE ONWJ. The jacket platform is operated in the northwestern part of the Java Sea. Design of earthquake loads on ULA jacket platform structure with large seismic accele­ration was obtained. From seismic analysis, dynamic response occurred on ULA jacket platform is evaluated, with natural struc­ture period of 1.256 seconds. The largest shear base values were in X and Y directions resulted from seismic acceleration of PGA 0.2g. On member check, the critical part happens to be one of the members of WD2, and on joint check, the largest UC occurred on 401L joint, both were due to the aforementioned earth quake ace­leration. Pushover analysis was then performed to obtain the value of RSR (Reserve Strength Ratio). The analysis yields an RSR value which far exceed the limiting criteria of API RP 2A WSD of RSR ≥ 0.8. Therefore it could be concluded that ULA jacket platform structure is within the range of low consequence category.

Damage analysis of ship collisions with offshore wind turbine foundations

Nowadays, a large number of wind turbines are being installed offshore due to more stable and steady flow of wind at sea and also less noise and visual impact compared to onshore wind farms. With the growing number of offshore wind installations, particular attention should be paid to the safe operation of assets. Offshore wind assets are subject to extreme environmental conditions and high dynamic stresses caused by wind, waves and currents. More importantly, they are largely exposed to hazards associated with collision with either commercial ships or infield support vessels passing closely at high speeds. To date, the damage analysis of collisions between infield support vessels and offshore wind turbine foundations has received very limited attention. In this study, a numerical nonlinear finite element analysis (NLFEA) approach is developed to evaluate the damage to wind turbine foundations when stricken by an offshore support vessel. The model is applied to a case study where 4000 tons class vessels collide with two common types of fixed-bottom foundations, namely monopile and jacket structure in shallow and deep waters respectively. Various accident scenarios are identified and the resulting damage to wind turbine foundations are analyzed. The number, location and the extent of damage to the members in each scenario are determined and the effects of reinforcement on the structure response are evaluated. The results of this research provide a good understanding of the factors that affect magnitude of damage caused by ship-wind turbine collision accidents and give an insight on how the next generation of wind turbine foundations can be designed in a more “collision-friendly” way.

Inverse Estimation of Local Slamming Loads on a Jacket Structure

Slamming loads from plunging breaking waves feature a high impulsive force and a very short duration. It is difficult to measure these loads directly in experiments due to the dynamics of the structures. In this study, inverse approaches are investigated to estimate the local slamming loads on a jacket structure using hammer test and wave test data from a model scale experiment. First, a state-of-the-art approach is considered. It uses two deconvolution techniques to first determine the impulse response functions and then to reconstruct the wave impact forces. Second, an easier applicable approach is proposed. It uses linear regression with the ordinary least square technique for the force estimation. The results calculated with these two approaches are highly identical. The linear regression approach can be extended to account for the loads transferred among different locations. This leads to lower and theoretically more accurate estimation of the loads compared to the previous two approaches. For the investigated case, the total impulse due to the wave is 22% lower. The estimated forces by the extended approach have a resolution at the millisecond level, which provides detailed information on the shape of the forces. The approach is an important tool for statistical investigations into the local slamming forces, and further on for the development of a reliable engineering model of the forces.

Risk Analysis of Return Support Material on Gas Compressor Platform Project

On a fixed platforms project are not only carried out by a contractor, but two or more contractors. Cooperation in the construction of fixed platforms is often not according to plan, it is caused by several factors. It takes a good synergy between the contractor to avoid miss communication may cause problems on the project. For the example is about support material (sea fastening, skid shoe and shipping support) used in the process of sending a jacket structure to operation place often does not return to the contractor. It needs a systematic method to overcome the problem of support material. This paper analyses the causes and effects of GAS Compressor Platform that support material is not return, using Fault Tree Analysis (FTA) and Event Tree Analysis (ETA). From fault tree analysis, the probability of top event is 0.7783. From event tree analysis diagram, the contractors lose Rp.350.000.000, - to Rp.10.000.000.000, -.

Experimental observations on the ice pile-up in the conductor array of a jacket platform in Bohai Sea

Massive ice pile-ups have been frequently observed in the conductor array of offshore jacket platforms in Bohai Sea. Once formed, such rubble piles were very difficult to clear manually and therefore studies on the ice rubble piles and their forming process were needed. This study investigates the ice pile-up behavior in the conductor array of a jacket platform in Bohai Sea by conducting a series of model tests in the ice tank at Tianjin University. The laboratory-scale model jacket was mounted on a rigid carriage and towed through the ice sheet at different speeds, directions and water levels. Observations on the test phenomena indicated that the water level and ice attack direction had a significant influence on the ice failures in front of the ice-breaking cones and the conductor array, thereby resulting in different ice pile-up patterns in the conductor array. The ice drift velocity only affects the pile-up dimensions, normally higher ice velocity resulting in higher pile-up heights. Also, based on the measured ice pile-up dimensions, the distribution of the ice rubble pile in the conductor array was found to be non-homogeneous, both laterally and longitudinally, and the rubble pile was found “grounded” on the bottom of the model conductor array in some cases, which might bring about additional load to the jacket structure. Furthermore, the horizontal ice forces acting on the conductor array due to ice rubble building were also measured, and comparisons of the ice forces based on the present tests with the calculated results from ISO algorithm were made. The R value of 5–6 might be more applicable for the present structure under the present ice conditions.

Wave impact load on jacket structure in intermediate water depth

The aim of this study is to estimate impact forces on a jacket using the improvised methodology and to propose a slamming coefficient for the jacket structures. Four types of structures have been discussed in this study, single cylinder (SC), legs with connecting braces in wave direction, legs with connecting braces transverse to wave direction and jacket structure. The SC case was analyzed using the improvised methodology and the obtained slamming coefficient was validated with the existing literature. Based on this initial validation, the methodology was extended for other models. Four loading cases were identified based on the location of the wave breaking. The impact load estimated was compared with theoretical formulas given by Goda et al. (1966), Wienke and Oumeraci (2005) for all models. The slamming coefficient (Cs) was found for wave characteristics having spectral bandwidth ratios of 1 and 0.75. For both the wave characteristics, the Cs was found to be higher for the loading case, when the wave breaks in front of the structure. The Cs varies between 2.2 and 4.6 depending upon the wave characteristics, with maximum obtained for 0.75. The method used in this paper can be used for estimating impact loads on complex structures.

Dynamic lateral response of suction caissons

Deeper water installations of offshore wind turbines may be supported by jacket structures. This study investigates the dynamic response of suction caissons for jackets by analysing 3D finite element models in the frequency domain. The numerical modelling was firstly validated by analytical solutions for pile foundations. Groups of crucial dimensionless parameters related to the soil profile and the foundation geometry are identified and their effects on the response of suction caissons are studied. Static stiffness coefficients are presented in a form of mathematical formulas obtained by fitting the numerical results, pertaining foundations with different slenderness ratios and embedded in different soil profiles.Sensitivity of the dynamic impedances of suction caissons on the skirt length was showed in this study. Moreover, the results for the suction caissons indicated that the overall dynamic response is profoundly affected by the relative thickness of the soil layer and by the variation of soil stiffness with depth.

Bucket foundation model testing under tensile axial loading

The present study focusses on bucket foundation behaviour under long-term cyclic loading. The paper analyses testing results of a bucket foundation model exposed to cyclic tensile loading. The model, with dimensions of 1 m in diameter and 0.5 m in skirt length, was, installed in dense water-saturated sand. Slow monotonic loading tests and cyclic tensile loading tests were performed (up to 40 000 load cycles) including tests with mean cyclic loading in tension, which is a unique testing condition. High-quality data were documented for load, displacement, and pore pressure response. Conclusions have been drawn regarding static and cyclic loading stiffness and displacement development during long-term cyclic loading. Four cyclic loading tests induced partially drained soil conditions and showed that pore pressure can accumulate during the long-term loading. Post-cyclic monotonic tensile loading tests showed up to 25% reduction in load capacity of the foundation. The research results provide valuable information for the design of an upwind bucket foundation under a jacket structure.