Jacket Platform Research Articles

Experimental investigations on wave impact pressures under the deck and global wave forces and moments on offshore jacket platform for partial and full green water conditions

The wave induced dynamic pressures under the deck and the global wave forces and moments on an offshore platform model were measured for three different model orientations and for partial and full green water conditions in random wave fields of different energy levels. It is found that for partial and full green water conditions, the global vertical wave force dominates when compared to horizontal wave forces. The orientation of the structure to wave direction has some effect, only for 22.5° orientation of the deck to wave direction. Shoreward maximum wave force can reach 3 times the maximum seaward wave force. For zero air gap condition, the highest value of normalized peak vertical force can reach a value of 42, which is 5 times higher than the normalized significant uplift force value. The normalized downward peak force value can reach as high as 8 for full green water condition due to the weight of overrunning water staying on the top of deck slab. For zero air gap condition, the normalized peak impact pressure under the deck can reach as high as 13.2 and 21.5 near the leading and trailing bottom side of the deck.

Optimal design of brace-viscous damper and pendulum tuned mass damper using Particle Swarm Optimization

We implemented Particle Swarm Optimization (PSO) to determine the most optimal properties of the brace-viscous damper system (BVDS), pendulum tuned mass damper (PTMD), and combined BVDS-PTMD system in mitigation of dynamic response of a jacket platform. To do this, a scaled prototype of an offshore jacket is numerically simulated in ANSYS subjected to ocean waves. Concerning BVDS, at each story, the variables being optimized are (1) BVDS configuration such as chevron, toggle, diagonal; (2) damping coefficient, and (3) brace area. The properties of PTMD being optimized are the damping ratio (ξd), frequency ratio (β), structural excitation frequency ratio (α), and mass ratio (m¯). To do the optimization, the results of ANSYS are exploited to calculate the PSO cost function (standard deviation of deck displacements (σsd)). Regarding the deck displacement and base shear force, the optimization results proved that the optimal BVDS-PTMD combined system outperforms the optimal BVDS and the optimal BVDS performance is better than that of the PTMD system. Both in the optimal BVDS and integrated BVDS-PTMD, the chevron configuration for the top floor and toggle configuration for the first to third floors are evaluated as the optimum arrangement by the optimization algorithm.

Spudcan-pile interaction in thick soft clay and soft clay overlying sand: A simplified numerical solution

A drilling jack-up is often deployed to perform drilling activities or well intervention at an existing fixed jacket platform. Depending on the clearance distance between the aft spudcan and the nearest pile, the installation of the spudcan may have an adverse effect on the platform pile which in turn could compromise the integrity of the jacket structure. This paper presents a simplified method for assessing the effect of spudcan installation on the pile foundation of a jacket platform in thick soft clay and soft clay overlying sand layers. Adopting a two-stage uncoupled approach, in which predetermined free-field lateral soil displacements are applied to a beam-column model with soil modelled using p-y curves, the soil displacements around a penetrating spudcan is estimated by an analytical solution derived from spherical cavity expansion theory. With appropriate consideration for the volume of soil displaced by the penetrating spudcan for the cases of thick soft clay and consideration for soil squeezing for soft clay overlying sand, the computed free-field lateral soil displacements are in fair agreement with the measured profiles in centrifuge tests and can be used in a beam-column model to estimate the induced bending moment in the pile in practice.The pile response to the free-field lateral soil displacements is assessed using a beam-column model incorporating p–y curves, considering the effect of soil remoulding caused by the installation of the spudcan where appropriate. A numerical model which considers the evolving lateral soil displacements due to the progressive advancement of the penetrating spudcan is also studied. The computed pile bending moment profiles are consistent with the measured profiles in the centrifuge tests and those computed by 3D large deformation finite element (LDFE) model. The proposed practical method is simple but yet effective to assess the influence of the installation of a spudcan foundation on an adjacent pile of a jacket platform in thick soft clay and soft clay overlying sand.

Engineering analysis of the dynamic characteristics of an electrical jacket platform of an offshore wind farm under seismic loads

From the perspective of the power transmission economy, offshore electrical platforms will be an inevitable choice for developing large-scale offshore wind farms in the open sea. In this study, a finite element numerical model of the electrical platform was built, and the dynamic responses under seismic loads were analyzed. The validity of the numerical model was verified by the corresponding physical model test. On this basis, a time-history analysis of the strain, acceleration, and displacement response of the electrical platform was carried out. The results and trends of dynamic responses under different wave actions were obtained. The influence of water and air environments and unidirectional and bidirectional seismic excitations on the dynamic response of the structure was analyzed. Consequently, the strain peaks of the structures under air conditions are all greater than those under water conditions, with an increment mean of 12.88%, and an increment mean of the peak acceleration of 10.76%. The peak value of the acceleration response is 30.49% higher than that of the unidirectional earthquake, and the peak value of the strain response is 25.78% higher than that of the unidirectional earthquake. It is indicated that bidirectional seismic excitation is more detrimental to the safety of structures.

Fatigue Crack Monitoring of T-Type Joints in Steel Offshore Oil and Gas Jacket Platform.

Several approaches have been used in the past to predict fatigue crack growth rates in T-joints of the offshore structures, but there are relatively few cases of applying structural health monitoring during the non-destructive testing of jacket platforms. This paper presents an experimental method based on the sensing of the piezoelectric sensors and finite element analysis method for studying the fatigue cracks in the offshore steel jacket structure. Three types of joints are selected in the current research work: T-type plate, T-type tube-plate, and T-type tube joints. The finite element analysis model established in the current study computes and analyzes the high stress and high strain regions in the T-type joints. The fatigue damage in the T-type joints was successfully detected by utilizing both the finite element analysis and experimental methods. The results showed that fatigue cracks of the three types of joints are prone to appear at the weld toe and spread in the welding direction. The fatigue damage location of T-type plate and T-type tube-plate joints is more concentrated in the upper weld toe area, and the fatigue damage location of the T-type tube joint is closer to the lower weld toe area.

An Efficient Monte-Carlo Simulation for the Dynamic Reliability Analysis of Jacket Platforms Subjected to Random Wave Loads

The growing demand for the application of jacket platforms in deep water requires more attention on the assessment of structural reliability. This paper is devoted to the dynamic reliability analysis of jacket platforms subjected to random wave loads with the Monte-Carlo simulation (MCS), in which a sample size of the order of magnitude of 104 to 105 for repeated time–history analyses is required for small failure probability problems, and a duration time up to three hours needs to be considered in the time–history analyses for a specific sea condition. To tackle the difficulty involved in the MCS, the explicit time-domain method (ETDM) is used for the required time–history analyses of jacket platforms, in which truncated explicit expressions of critical responses with regards to the contributing loading terms are first established and then used for numerous repeated sample analyses. The use of ETDM greatly enhances the computational efficiency of MCS, making it feasible for the dynamic reliability analysis of jacket platforms under random wave loads. A jacket platform with 11,688 degrees of freedom was analyzed for the evaluation of dynamic reliability under a given sea condition, indicating the accuracy and efficiency of the present approach and its feasibility to practical structures.

Elastic-Plastic Seismic Performance for Jacket Platform Based on Improved Modal Pushover Method

Elastic-Plastic Seismic Performance for Jacket Platform Based on Improved Modal Pushover Method

Design optimization of jacket offshore platform considering fatigue damage using Genetic Algorithm

Due to the substantial volume of materials used in the structural members of jacket platforms, weight optimization is of critical importance. A Genetic Algorithm is a powerful tool that allows for consideration of many parameters in the optimum design of platforms simultaneously, a feature hardly found in ordinary circumstances. Regarding the cyclic loads derived from waves, fatigue is one of the most fundamental issues in the design and optimization of offshore platforms. In this study, using the Genetic Algorithm, a real platform is optimized with two optimization scenarios named (CF) and (WCF). The CF-Optimization considers constraints, including fatigue life in connections, stress and buckling design requirements in structural members, horizontal displacement at the working point level, and structural adequacy control of connections, while the WCF-Optimization ignores fatigue constraint in the mentioned constraints. The results show that (CF) and (WCF) optimizations reduced the structural weight of the platform by the ratios of 13% and 15%, respectively. Moreover, an investigation of the results obtained from WCF-Optimization reveals that ignoring fatigue could significantly lead to unreliable results in the design optimization of the jacket platform.

Horizontal and vertical axis wind turbines on existing jacket platforms: Part 1 – A comparative study

The wind resource offshore is often excellent due to a higher average wind speed and lower turbulence intensity. Jacket structures are the most common offshore platforms for extraction of oil and natural gas in relative low water depths. When the offshore resources run out, these structures must be displaced to another area containing underground resources or removed in the case of reaching their design life. Therefore, one possible procedure to reduce the carbon footprint on the planet, allowing society to rely on promising sources of ’clean’ energy while salvaging these oil and gas platforms, is to consider the transformation of these oil and gas platforms into offshore wind turbine support structures. The present research focus on the possibility of converting such structures for gas extraction into offshore platforms for wind turbines. In this study, a comparison between the behavior of horizontal and vertical axis wind turbines on the same offshore platform is presented. In this comparison, two different software programs are used: MATLAB and SAP2000. The model proposed is a new simplified tool used to study the structural analysis of the jacket structures, developed and summarized in 10 steps, adopted to evaluate the behavior of the platform with the wind tower configurations.

Assessment of strengthening, modification, and repair techniques for aging fixed offshore steel platforms

Jacket platforms are widely used for hydrocarbon production in the oil and gas offshore field so far. After years of operation in jacket platforms, a structural integrity check is required to investigate whether the structure of the jackets is fit for purpose or not. So, maintenance, repair, and strengthening become an ongoing and continual process during the jacket platform's life. Corrosion, fatigue, vessel collisions, heavy dropped objects, and severe storms are the main reasons for damages. These events lead to perforation, dent, failure, and strength reduction of jacket elements. In this paper, a set of finite element models is used to model each damage case. Moreover, Strengthening, Modification, and Repair (SMR) techniques observe the members' behavior and the efficiency of the repair action. Also, multi-criteria analysis is presented for prioritizing the SMR techniques using the technique for preference by similarity to ideal solution (TOPSIS) method. The results and insights, the ultimate strength for both damage and repaired cases, provide a supportive view to the engineers to design a proper repair method. What is more, the results provide an ideal solution in selecting alternatives for repair methods.

Seismic response analysis of fixed jacket-type offshore structures based on power spectrum density Driven input

ABSTRACT Offshore platforms in seismically active areas should be properly designed to survive within the face of intense earthquakes without a global structural collapse. This paper scrutinizes the seismic performance of a newly designed and established jacket type offshore platform based on the API-RP2A normalized response spectra during seismic events. A finite element model is developed from a typical jacket type offshore platform taking into the effect of the interaction among structure, pile, and soil components. The seismic responses of jacket type offshore platforms subjected to random earthquake excitations are evaluated by means of the power spectral density (PSD) analysis. Dynamic characteristics, the response function, output PSD and transfer functions for various elements of the platform are discussed. The spectrum compatible PSD is directly used to estimate the peak structural responses and determine the dynamic response of offshore jacket platforms that meet the required level of engineering practice for preliminary design.

Memory-event-triggering H ∞ reliable control for networked jacket platforms against earthquakes and stochastic actuator faults

This paper deals with the problems of networked modelling and memory-event-triggering reliable control for steel jacket-type platforms in network environments. First, a networked model of the jacket platform against earthquakes and probabilistic actuator faults is established. Based on this model, a memory-based event-triggering communication scheme is introduced to account for the constrained communication resources. A distinct feature of the proposed triggering scheme is that it has great potential to identify and trigger the significantly changed data than the existing ones without ‘memory’. Then, by formulating the network-based closed-loop platform system as a stochastic delay system, some sufficient conditions are derived for co-designing the desired controller and the triggering scheme. Finally, comparative simulation results are provided to demonstrate the effectiveness and merits of the proposed triggering and control co-design method. It is shown that compared with some existing event-triggering control methods, the proposed memory-event-triggering reliable controller is effective to mitigate vibrations of the jacket platform and is potentially advantageous to save more network bandwidth and control cost.

Risk assessment of aging offshore jacket platform group: a case study on “B” Field platforms

Offshore jacket platforms are designed to fulfil their intended purpose in oil and gas exploitation at designated service life periods. During the operational development, a number of field operators extend their platform service life if the reserves are still available. This technical paper proposes the method to assess the structural integrity of several aging platforms in a field with a simplified approach. Taking the case of “B” Field platforms installed in the mid-1990s, the detailed procedure and example are discussed thoroughly. Five indicators furnished with quantification procedures are also presented to propose the ranking methodology. This paper also proposes the method of jacket platforms grouping based on several similarity criteria. The results of this approach can be used as a baseline for more detailed structural integrity assessment for each group representation.

Structural Model Updating of Jacket Platform by Control Theory Using Vibration Measurement Approach

The identification of variations in the dynamic behavior of structures is an important subject in structural integrity assessment. Improvement and servicing of offshore platforms in the marine environment with constant changing, requires understanding the real behavior of these structures to prevent possible failure. In this work, empirical and numerical models of jacket structure are investigated. A test on experimental modal analysis is accomplished to acquire the response of structure and a mathematical model of the jacket structure is also performed. Then, based on the control theory using developed reduction system, the matrices of the platform model is calibrated and updated. The current methodology can be applied to prepare the finite element model to be more adaptable to the empirical model. Calibrated results with the proposed approach in this paper are very close to those of the actual model and also this technique leads to a reduction in the amount of calculations and expenses. The research clearly confirms that the dynamic behavior of fixed marine structures should be designed and assessed considering the calibrated analytical models for the safety of these structures.

Offshore petroleum rigs/platforms: An overview of analysis, design, construction and installation

Jacket platforms are one of the most important and regularly used types of offshore structures for oil and gas extraction that have a big impact on the economy of the countries. In this paper, all aspects including design, analysis, construction and installing of the jacket type offshore structure, are summarized and classified. This type of structure is one of the specified platforms for shallow water, and for long term service, it also has the ability to carry large deck loads. This paper aims to present general guidance about the planning, design and construction of offshore jacket (template) platforms. Jacket platforms are fixed type platforms which are attached to the seabed using piles which provide stability against the wind, wave and current loads. Also, this type of offshore platform has a high initial and maintenance cost because of its exposure to corrosion, and cannot be reused after the end of its service period. Jacket platforms are most suitable for shallow water having no better alternative while it has the cost disadvantage for deep water.

Computational Fluid Dynamics Analysis of Rigs-to-Reefs (R2R) Jacket Structures

Platform decommissioning activities have been increasing due to unproductive fields and unstable oil prices. One of the decommissioning methods used by the oil and gas companies is by converting the platform into an artificial reef through the rigs-to-reef (R2R) programme. The programme benefits the marine life and increases the marine productions. In this study, the dynamic responses and flow characteristics of jacket platforms were investigated using computational fluid dynamics (CFD) analysis. Three jacket structures with different sizes were used to investigate the suitability of the structure as a potential artificial reef. The pressure exerted on the structure as well as the back eddies and upwelling phenomenon were also investigated. This is to ensure the settlement of the coral larvae and attract marine life to inhabit around the artificial reef. The results show that the platform size and configurations are the significant criteria to design any artificial reef. The pressure on the jacket member is in the acceptable range. Higher efficiency index of back eddy and upwelling could also be obtained by smaller jacket structures.

Effect of spudcan penetration on laterally loaded pile groups

Engineering cases, where jack-up rigs with large-diameter spudcan foundations operate near a jacket platform supported by pile groups, are increasing. Therefore, the group interaction of lateral-loaded pile groups should be re-evaluated. A pile group analysis method that is able to evaluate the effect of spudcan penetration on the pile group interaction is proposed in this study. This method is based on the modified Poulos method and is universally used in current ocean engineering practices. The pile group interaction-induced additional pile head deflection of a pile group, when subjected to both spudcan penetration and the influence of another pile group, is initially determined by elastic analysis. Next, the nonlinear foundation beam (NFB) method is adopted to calculate the head deflection of an isolated pile, which we utilise to calculate the head deflection of the pile group after considering the effects of spudcan penetration. Finally, the Y multiplier is obtained by several pilot modifications of p-y curves of the isolated pile in the NFB model, whose variation during spudcan penetration indicates the effect that such penetration has on the pile group interaction. Predicted results were compared with those of the centrifuge model test, and further with numerical simulation results, in order to prove the validity of the method in analysing a free-head and a fixed-head pile group.

Laboratory tests on a hybrid SDR approach for jacket platforms via improved dynamic-reduction system

The improvement of robust skills for early defect recognition for fixed marine structures is crucial to avoid the possible catastrophe caused by structural degradation. This paper presents an approach for defect recognition based on vibration measurements that is capable of detecting the defect to individual members of offshore jacket platforms, when limited, spatially incomplete modal datum is available. Generally, model reduction is better than modal expansion. Hereof, this paper investigates the developed new defect recognition method using improved dynamic-reduction system. The approach needs only one mode shape. The approach first updates the modal datum. Then, an efficient technique is applied to create a transformation matrix. This matrix is utilized to correct / or refine the nominal eigenvectors so that the updated model is well-matched with the experimental modal data. In this regard and also to conquer the uncertainties between numerically and experimentally identified features, laboratory vibration tests are accompanied on a physical platform model. The efficiency of the approach is validated by different destruction scenarios foreseen on the physical model. The proposed defect recognition approach in conjunction with iterative dynamic reduction approach yields the best defect location and severity estimate. Proposed technique is considered because of practical considerations and expansion of such methodologies would be extremely useful for offshore jacket structure.

Renewable Wave Energy Potential for the Sustainable Offshore Oil Platforms in South China Sea

The Jacket platform needs gas and diesel to run its turbines, and in the end, they produce catastrophic emissions annually. The environmental concerns regarding these platforms have forced us to utilize an alternative source of energy that is sustainable and clean. In this study 51 locations, are of interest where oil and gas activities are in progress at present in the shape of a jacket platform or pipelines. The significant wave height and wave period scatter diagram data are collected from the platforms in the South China Sea. The linear wave theory is used to find the wave power. The given time period is converted into an equivalent time period first before wave energy is determined. The study shows that location no. 20 is the ideal location to deploy the wave energy converter Pelamis P2 with a potential mean wave power of 6.61 kW/m A single unit of Pelamis P2 can produce on an average electricity output of 91.37 kW/m including, the losses and machine efficiencies, whereas a wave farm can generate an average output of 62 GWh/yr. The electricity supply of 70.3 % of the minimum and 14.1 % of the maximum energy demand, while using only wave energy converter. If hybrid wind and wave energy system is used, then energy production will increase. The results show that the wave farm could also reduce the use of natural gas up to 17.6E06 m3/year, avoiding the emission of 12000 tonnes of CO and 54000 tonnes of NOx annually, and can save up to RM 20 billion annually with the reduction of natural gas emissions.

Assessment of wind-induced responses for offshore jacket platforms based on high frequency force balance tests

This paper focuses on the wind sensitivity of large offshore jacket platforms subjected to hurricanes, and a 1:100 scaled model is tested in a boundary wind tunnel under 0~360° wind directions using the high frequency force balance (HFFB) technique. Based on the experimental results, a corrected frequency-domain calculation method for evaluating wind-induced responses is derived, which can eliminate the resonance effects of the test model. The results show that: the first seven or higher-order mode combinations of platforms should be adopted to evaluate the dynamic wind-induced responses; the different mode coupling effects show a complicated pattern, especially in the across wind direction, and the complete quadratic combination method is suggested; the root mean square (RMS) displacement and acceleration of the derrick present approximate O shapes at different heights, and the maximum values reach 9.35 cm and 49.98 cm/s2. Therefore, the wind-induced vibration of large offshore jacket platforms should be under stronger control to ensure their safety. [Received: April 15, 2018; Accepted: December 13, 2019]