Jack-up Platform Research Articles

Dynamic Response Prediction Model for Jack-Up Platform Pile Legs Based on Random Forest Algorithm

Jack-up offshore platforms are widely used in many fields, and it is of great importance to quickly and accurately predict the dynamic response of platform pile leg structures in real time. The current analytical techniques are founded upon numerical modelling of the platform structure. Although these methods can be used to accurately analyze the dynamic response of the platform, they require a large quantity of computational resources and cannot meet the requirements of real-time prediction. A predictive model for the dynamic response of the pile leg of a jack-up platform based on the random forest algorithm is proposed. Firstly, a pile leg dynamic response database is established based on high-fidelity numerical model simulation calculations. The data are subjected to cleaning and dimensional reduction in order to facilitate the training of the random forest model. Cross-checking and Bayesian optimization algorithms are used for the selection of random forest parameters. The results show that the prediction model is capable of outputting response results for new environmental load inputs within a few milliseconds, and the prediction results remain highly accurate and perform well at extreme values.

Data-driven fast reliability assessment of offshore structures based on real-time sensor data

With the advancement of monitoring technologies for offshore structures, sensor data are becoming increasingly available in real-time. The objective of this paper is to develop fast methods for reliability assessment of offshore structures for the extreme and fatigue failure modes, based on the latest real-time sensor data, assumed to be four months. The continuously updated assessment informs operators on the failure probability over the next few years. Reliability analysis using real-time data is challenging owing to the need to extrapolate the observed data to more severe sea states and across many orders of magnitudes of probabilities, and there is a lack of related studies. Here, the environment comprises the short-term and long-term wave characteristics. Three data-driven reliability approaches are developed for three types of wave sensor data. The first approach applies statistical models to the response when wave data are unavailable. The second relies on Gaussian Process Regression (GPR) when only the wave spectral parameters are known. The third applies the Gaussian-experienced NARX network to predict the response time series when the wave time series are recorded. The proposed approaches are applied to a jack-up platform, and implementation of benchmark reliability analysis is enabled by establishing an emulator trained from dynamic simulations for fast generation of synthetic sensor data. The GPR approach is found to show good extrapolation capabilities and able to provide relatively accurate and robust reliability predictions.

Shaking table tests for seismic response of jack-up platform with pile leg-mat foundation on sandy seabed

The pile leg-mat foundation is a novel composite foundation designed for jack-up offshore platforms. The seismic response of such foundations in sandy seabeds is an issue that requires significant attention. In this study, shaking table tests were conducted under various input seismic waves to investigate the dynamic response of a seabed platform system subjected to seismic loads. The effects of the peak ground acceleration (PGA), seismic wave frequency, wave type, pile leg insertion depth, and pile length on the response of the seabed platform system were investigated. The results indicated that as the PGA increased, the acceleration response of the soil and platform model increased, increasing the accumulation of excess pore water pressure and the horizontal displacement of the platform model. The acceleration and horizontal displacement responses of the seismic wave with lower frequencies were higher than those with higher frequencies. The peak acceleration amplification factors under the measured and regular seismic waves showed significant differences, with the dynamic response of regular waves to the platform model being pronounced. Under seismic waves with low PGAs, the horizontal displacement of the platform model decreased with increasing pile leg insertion depth or pile length.

Predicting the ultimate bearing capacity of spudcan in stiff-over-soft clay based on a failure model varying with soil parameter

A jack-up platform is prone to a ‘punch-through’ accident when working on the stiff-over-soft clay foundation. To assess the risk of punch-through, it is necessary to plot the bearing capacity-penetration curve for evaluating the penetration depth of the pile leg or to calculate the ultimate bearing capacity of the spudcan and compare it with the preload. However, accurate prediction of spudcan bearing capacity is not easy. In this paper, the failure mode about the stiff-over-soft clay under different geological conditions when punch-through occurs is discussed using coupled Eulerian lagrangian (CEL) method, varying the strength ratio between soft and stiff soil layers, the thickness of stiff soil relative to the spudcan diameter, and the non-homogeneity factor of soil strength. Based on the soil failure model with varying soil parameters, a new calculation method for ultimate bearing capacity of spudcan is proposed and then verified against existing centrifuge test results. The results show that the shape of soil shear plane in stiff clay is significantly affected by the strength ratio of soil layer and the non-homogeneity factor of soil strength. And the presented new method whose prediction errors compared with centrifuge test results are less than 5%, has better calculation accuracy than the ISO (International Organization for Standardization) method, which is valuable in guiding the calculation of the ultimate bearing capacity of the spudcan and ensuring the safe operation of the jack-up drilling units on stiff-over-soft clay.

Spatio-Temporal Variation Characteristics of Spudcan-Footprint and Surrounding Soil of Jack-up Platform—Taking a Well Site in Chengdao Area in Abandoned Yellow River Subaqueous Delta as an Example

Spatio-Temporal Variation Characteristics of Spudcan-Footprint and Surrounding Soil of Jack-up Platform—Taking a Well Site in Chengdao Area in Abandoned Yellow River Subaqueous Delta as an Example

Environmental parameter design for jack-up platforms based on FEM-BPNN-JPA coupled approach

This study proposes a novel coupled approach, integrating the non-linear finite element model (FEM), BP neural network (BPNN), and joint probability analysis (JPA), to conduct the ocean environmental parameter design for jack-up platforms. Two bottom constraint methods are involved in the FEM, namely the contact surface model and the hinge constraint model, to conduct the structural response analysis. Subsequently, based on the calculated structural responses from FEM, the BPNN is employed to predict the jack-up response under varying combinations of wind speed, wave height and current velocity. This FEM-BPNN coupled method proves to be an effective and efficient tool for determining the jack-up responses under different environmental load combinations for JPA. Based on the optimal univariate probability distributions of each environmental parameter, multi-dimensional copulas are adopted to construct the conditional and joint probability distributions. These distributions, in turn, yield environmental design parameters under different design criteria and corresponding horizontal displacements of the jack-up platform. Results show that environmental design parameters and corresponding horizontal displacements are smaller compared to those obtained from the univariate probability distribution for the same return period. The FEM-BPNN-JPA coupled approach proves to be effective in estimating environmental design parameters by considering parameter correlations.

Combined VHM bearing capacity of pile leg-mat foundation on marine clay seabed

The pile leg-mat foundation is a new type of jack-up wind power installation platform foundation. Since the foundation needs to bear the horizontal and bending moment loads caused by wind and waves for long period of time, the combined vertical, horizontal and moment (VHM) bearing capacity of the pile leg-mat foundation in the soft clay seabed is crucial in evaluating the safety of the jack-up platform. In this study, the effects of pile leg insertion depth L and the gradient of soil shear strength κ on the VHM bearing capacity of pile leg-mat foundation were analyzed. The numerical results indicate that the pile leg insertion depth significantly enhances the horizontal bearing capacity of the foundation. As the pile leg insertion depth increases, the horizontal load distribution of the foundation gradually shifts from the mat to the pile leg. However, the vertical and moment bearing capacity of the foundation is mainly provided by the mat and not significantly improved by the pile leg insertion depth. Based on the numerical results, the VHM failure envelope of the pile leg-mat foundation was established, and the method for assessing the bearing capacity of the pile-leg mat foundation was proposed.

Effects of spudcan installation processes on the bearing capacity of foundations under combined loading in soft-over-stiff clay

The bearing capacity of spudcan foundations under vertical, horizontal, and moment loading must be adequately assessed prior to the operation of jack-up platforms. However, studies the effects of combined loading on spudcan foundations in soft-over-stiff clays, considering the effect of installation processes, have been limited. In this study, the two-dimensional remeshing and interpolation with small strain method was used to simulate the large deformations experienced by the soil during the installation of a spudcan, and a three-dimensional model was employed for the small strain analysis regarding the bearing capacity. The undrained shear strength of stiff clay influenced the failure mechanisms of the soil during vertical and moment loading. However, it had minimal impact during horizontal loading. Therefore, in this study, an equation for the bearing capacity factors for vertical and moment loading conditions is proposed. The properties and parameters of the combined bearing capacity failure envelope with respect to different dimensions are analysed and corresponding equations for the failure envelope are proposed, which aid the on-site assessment of jack-up platforms prior to them becoming operational.

Carbon net zero transition: A case study of hydrogen storage in offshore salt cavern

In a world of energy transition and net zero pledges, low-carbon energy solutions are central in the fight against climate change. Hydrogen plays a key role in a clean energy future, but there are still economic and infrastructural challenges to be overcome. Hydrogen storage in large volumes is one of the value chains current limitations. This paper presents a feasibility study of an offshore blue or green hydrogen storage in salt caverns created by leaching, within potentially identifiable salt deposits extent of the Gulf of Mexico coastline (US). In this case study, the hydrogen station will have caverns to store hydrogen and some caverns to dispose of CO2 from the blue hydrogen (Carbon Capture and Storage – CCS technology). The construction design includes a fixed platform for the H2 HUB and a jack-up platform for drilling. It presents the conceptual design of wells for leaching the caverns and hydrogen injection and withdrawal cycles.

Probabilistic prediction of the spudcan extraction from sand-over-clay based on monitored data

Upon completion of the operation on a site, spudcan foundations are pulled out of the seabed before relocating the jack-up platform to another candidate site. It is critical to predict the peak extraction resistance before commencing the jack-up extraction. Though sand-over-clay seabed is commonly found in active marine oil and gas exploration zones, the models for estimating the extraction resistance are very rare. This paper proposes a Bayesian prediction model based on 51 groups of centrifuge tests on foundations extracting from sand overlying clay soils. Model factors are introduced to define the uncertainties in the prediction. Combined with the monitored data during foundation extraction, possible breakout range of resistance and depth is predicted and updated in real time. The most likely breakout resistance and depth in the area are also estimated. The results show that (1) the predicted range of breakout resistance and depth may occur are concentrated around the measured values with the update of the monitored data; (2) the relative error between the most likely breakout depth and the measured value is kept within 10%; and (3) regardless the shape of the foundation, the Bayesian prediction model can effectively predict the extraction behaviour of the foundation.

Centrifuge tests and numerical simulations on vertical bearing capacity of mat foundation on marine clay seabed

Mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed, which avoids the problems of excessive insertion and extraction difficulties of spudcan foundation on soft clay seabed. Evaluating the vertical bearing capacity of the mat foundation on the clay seabed is of great significance for the safe operation of jack-up platform in marine environment. In this research, the vertical bearing characteristics of mat foundation on horizontal and sloping clay seabed were studied through centrifuge tests. Then, numerical simulations were conducted to study the vertical bearing capacity of the full-scale mat foundation on clay seabed, in which the effects of the large mat foundation size, pile leg insertion depth, preloading, consolidation degree of soil, and soil plasticity were analyzed. The results show that the vertical bearing capacity of the mat foundations on the sloping seabed is reduced, and the soil under the mat foundation shows obvious sliding failure. The contribution of the insertion depth of pile legs on the horizontal bearing capacity is more obvious than that of the vertical bearing capacity. The enhancement of the vertical bearing capacity is mainly due to the dissipation of the excess pore pressure and the increase of the mean effective stress of the soil. The effects of preloading, consolidation degree, and soil property on the vertical bearing capacity of the mat foundation are significant.

Time evolution of elastic stiffness coefficients for a jack-up spudcan in clay during operation

Current design practice of spudcans in clay generally ignores the influence of installation and subsequent operation on the spudcan elastic stiffness. In reality, soil consolidation around the post-installation spudcan can cause the elastic stiffness to evolve with time, thus affecting the safe and stable operation of jack-up platforms. This paper reports a numerical study dedicated to this specific subject. The spudcan installation and the time evolution of elastic stiffness coefficients during operation are continuously simulated using an effective stress large deformation finite element technique. The effects of soil, loading, embedment and geometric conditions were evaluated and interpreted. It was found that the non-uniform growth of the shear modulus caused by soil consolidation during operation results in the time evolution of the elastic stiffness and makes the deformation mechanism asymmetric. The overlap degree between the involved zone of the deformation mechanism and the growth zone of the shear modulus (or the so-called averaging effect) affects the gradient of time evolution. Based on the parametric study, a generalized framework is present for predicting the elastic stiffness coefficients of spudcans for different scenarios and consolidation times, thus providing a complete view of the time evolution of elastic stiffness for the offshore in-site assessment.

Capacity of spudcan foundation on dense sand overlying clay under combined loading

The evaluation of spudcan foundation capacity under combined vertical, horizontal and moment loading is critical for the site-specific assessment of jack-up rigs. However, the investigation on the behavior of a spudcan on the surface of a sand layer overlying clay under combined loading is limited. This paper reports the results from a series of finite element analyses conducted to explore the failure envelope of spudcan on dense sand overlying clay before punch-through, considering the effects of the preload level on spudcan, the thickness ratio and the relative density of sand, and the underlying clay strength through parametric study. The numerical results provide insights that when the sand layer is sufficiently thick, the failure envelope expands with the increase of vertical load level, leading to higher horizontal and moment capacities; however, for the spudcan on a relatively thin sand layer, the horizontal and moment capacity may decline with rising vertical load level, resulting in eccentricity and shrinkage of the failure envelope. New design formulas for the failure envelope are proposed covering a comprehensive range of soil parameters and layer geometries of practical interest, which facilitates their general application in the site-specific assessment of the jack-up platforms.

Determination of elastic stiffness coefficients for spudcan foundations in a spatially varying clayey seabed

Determining the elastic stiffness coefficient of spudcan foundations is a challenging task due to the natural variability of soil properties coupled with the complexity of loading conditions. Uncertainty in the elastic stiffness coefficient is a potential risk factor for the safe and stable operation of offshore jack-up platforms, and there are no guidelines for operators to assess its confidence level. This paper conducted a study on this specific subject using the three-dimensional random finite element method. The influence of soil strength heterogeneity, the coefficient of variation and the scale of fluctuation of the shear modulus, embedment depth and soil backflow on the deformation mechanism and elastic stiffness coefficient of spudcans were systemically discussed. The results suggest that the above parameters, combined with the soil's spatial variability, can alter the deformation mechanism, thereby the elastic stiffness coefficient. If spatial variability is ignored in platform design, the safety probability of spudcans is only 50%. So, a matrix algebraic expression is proposed to accurately predict the elastic stiffness coefficient in spatially variable clay at any given confidence level. The study findings may facilitate incorporating spatial variability into elastic stiffness research and supplement the calculation method in the existing jack-up assessment guidelines.

Modeling of an underwater drilling platform for very near shore applications

The development of the blue economy involves the construction of marine infrastructures (harbors, marine wind farms, underwater data centers, etc.). All these activities require a geological investigation of the sea bottom. For nearshore and very near-shore applications the most common solution is represented by the adoption of fixed jack-up platforms; however, transportation and deployment of this kind of infrastructure are quite expensive and time-consuming. Also, the adoption of floating drilling technologies that are currently used for offshore applications is often difficult or impossible in shallow waters. In this work, an underwater drilling platform designed for near-shore applications is investigated. The proposed system is designed and optimized for rapid deployment in different operational conditions. This work is focused on the description of this innovative solution including simulation models that have been developed for this activity. Also, simulation models represent an original contribution because they are able to simulate complex interactions of the drilling pod with the surrounding water, with the support ship and with the excavated sea bottom.

Wave–current-structure blockage: Development of one-way hydro-structural coupling for a jack-up platform in waves and current

This paper introduces a novel application of porous block approach to derive hydrodynamic forces on a generic jack-up platform in waves and current using OpenFOAM. The objective is to account for fluid force reduction due to wave–current blockage effect. In parallel, following Santo et al. (2018), a one-way coupling is developed and presented to map the resolved hydrodynamic forces from OpenFOAM to structural beam elements forming the legs of the jack-up platform in ABAQUS (so called OF-ABAQUS coupling solver) to derive dynamic response of the platform when it is at elevated condition (or in-place). Sensitivity checks are performed to ensure the developed OF-ABAQUS coupling solver is robust. The development is compared with ABAQUS simulations, which make use of the standard Morison equation with relative velocity formulation to derive the applied hydrodynamic forces. Waves with in-line current are considered. At the structural resonance, we observe reduction in the amplitude of the dynamic response, which is attributed to the wave–current-structure blockage effect manifested as additional fluid damping not present in the standard Morison and present industry recipes.

Jack-up platform leg optimization by topology optimization algorithm-BESO

Jack-up platforms are widely used for the offshore oil and gas exploration, constructions and offshore wind farms. The jack-up platform legs are import component, which not only support the whole weight but also withstands various external loads, such as wind, current and wave load. A topology optimization algorithm-BESO (bi-directional evolutionary structural optimization) is proposed in this paper for the jack-up leg optimization. Based on the sensitivity analysis, the optimization strategy with sensitivity filtering and updated have been applied. And a simple numerical example is used to verify the proposed optimization method. A case jack-up platform is introduced to implement the leg structure optimization. After a series of iterations, a new leg structure is obtained. And the ultimate analysis, Eigen analysis and buckling of optimized leg structure optimized structure are all discussed compared with the traditional structures (K-type, the reverse(rev) K-type and X-type). To further evaluate the performance of the optimized leg structures, an experiment test is designed in the wave tank considering the current and wave load. Also, the optimized leg structure and the traditional ones are all manufactured to perform the experiment. The results show that the optimized leg structure has better ultimate bearing capacity, Eigen analysis and dynamic response with lighter weight under the same environmental loads. It is indicated that the BESO topology method is superior in finding best structure of the jack-up platform leg.

Probabilistic site characterisation using monitored spudcan installation data

Predicting spudcan penetration is essential for reducing risk during the installation of an offshore mobile jack-up platform. However, predictions made prior to installation often deviate from those recorded in practice, mainly because inherent uncertainties in seabed soils are not adequately considered. In this paper, the characteristics of the seabed are quantified using monitored spudcan installation data by developing a Bayesian framework coupled with Metropolis algorithm-based Markov chain Monte Carlo (MCMC) simulation. Applied to sand overlying clay conditions multiple readings from the load–penetration curve are used to derive statistical interpretations of seabed parameters such as sand thickness, sand friction angle and the undrained shear strength profile of clay. Illustrative examples demonstrate the proposed method using data from geotechnical centrifuge tests and a monitored offshore jack-up. The accuracy of the method is also retrospectively examined against a database of 66 centrifuge tests. The results show that the proposed method can provide consistent soil parameters with reduced uncertainty regardless of the influence of the updating sequence, number of MCMC chains, scale factors and the prior distributions applied. This method offers the offshore industry a new methodology to statistically interpret seabed conditions by incorporating monitored data. Potentially it could be combined with spatial interpolation methods to infer ground conditions and more accurately predict future spudcan installations.

Numerical Analysis of Dynamics of Jack-Up Offshore Platform and Its Seabed Foundation under Ocean Wave

Jack-up offshore platform is a type of important marine structure, which is mainly used for satellite launch, oil exploitation, and other engineering tasks in the offshore area. The offshore platform is bound to be subjected to wave loading in the course of use. Whether it can withstand the wave impact is an important engineering problem. To solve this engineering problem, the self-developed fluid–structure–foundation interaction coupling model OlaFlow-ABAQUS is used to explore the dynamic response characteristics of a jack-up offshore platform and its seabed foundation under three conventional wave conditions (wave height is 3, 5, and 7 m, respectively) in a coupled way. The numerical results show that only a small amplitude of periodic sloshing occurs for the jack-up offshore platform under the three conventional wave conditions. The maximum sloshing amplitude is up to 8 cm, and there is no visible residual displacement. It is indicated that there is no plastic deformation zone in the seabed foundation near the pile legs of the jack-up platform. It can thus be concluded that the jack-up platform has excellent stability under conventional wave conditions. Under conventional wave loading, momentary liquefaction occurs in the seabed foundation around the pile legs of the platform, and the maximum liquefaction depth is about 1 m. This study indicates that the coupling model OlaFlow-ABAQUS for the fluid–structure–foundation interaction is feasible, and has some advantages to study the dynamic response and to evaluate the stability of large-scale marine structures and their seabed foundations under ocean waves.

Estimation of Overall Fatigue Life of Jack-up Leg Structure

Jack-up platforms are designed to work in three conditions: Transit, Preloading and Operating. The fatigue life of the jack-up platforms in operating condition will be determined to be similar to that of offshore fixed steel structures. In preloading conditions, fatigue damage is usually ignored. Up to now, the fatigue damage of the jack-up leg structure in transit conditions has been calculated at approximately 20% of the total fatigue damage of the jack-up leg structure in two conditions (transit and operating). The approximate calculation method is usually accepted by consultants and register agencies. If the approximation is used, the fatigue life of the jack-up leg structure will be calculated only as for the jacket structure of the fixed offshore structure, with 80% of the allowable fatigue life based on standards. The approximation proved to have many disadvantages: the different travel times of each jack-up rig cannot be mentioned; hot spots that need to be maintained during the transit condition have not been pointed out; it is difficult to guarantee the safety of the jack-up leg structures in the transit condition. In order to overcome the limitations of the approximation method, this paper will propose a method to predict the overall fatigue life of the jack-up leg structure in three main problems. Firstly, we use the analysis method of fatigue of fixed steel offshore structures for jack-up leg structures in operating conditions. Secondly, we suggest a method to analyze the fatigue of the structures in transit conditions. Herein, motion analysis and determination of inertia forces on the leg structure are performed by the Boundary Element Model (BEM) in SACS software. Then the inertia forces are assigned to a Finite Element Model (FEM) in SACS to decide the internal forces of the structures. Hotspot stresses are determined by combining nominal stress from FE analysis results with a concentrated stress factor from the analysis of joint local models in the ANSYS program. Then, fatigue damage and fatigue life of hotspots of the structure are determined in the transit condition. Finally, a formula is suggested to determine total fatigue damage in operating conditions and transit conditions with different cases in relation to different fraction factors. These results are used to predict fatigue life corresponding to the most dangerous cases of structural joints. These new suggestions are applied to fatigue analysis for jack-up Tam Dao 05. Currently, the Tam Dao 05 platform has been operating in the Vietnam East Sea. Doi: 10.28991/CEJ-2022-08-03-06 Full Text: PDF