Jacket Substructures Research Articles

Conceptual Design of Offshore Jacket Substructures Using Machine Learning

The offshore wind sector is mature and has led to standardized design methods for offshore substructures. The conceptual design phase is critical for efficiency and cost-effectiveness and forms the basis for further design iterations. As turbine capacity increases, so does the complexity of offshore substructures, making design more challenging. However, the design process still relies on the expertise of the design engineers. These engineers rely heavily on their experience and intuition when designing, which can lead to biases due to limited information. To address this problem, Machine Learning (ML) techniques offer a promising way to improve the accuracy and efficiency of the conceptual design of offshore substructures. The current study is limited to the conceptual design of jacket substructures and was conducted on a self-developed global dataset of real jackets. The ML-based approach proposed in this study is capable of learning from existing data, recognizing intricate relationships between design variables, and potentially providing more accurate estimates for the initial conceptual design of offshore jacket substructures.

Fragility and vulnerability development of offshore wind turbines under aero-hydro loadings

Wind technology is an effective solution for the carbon neutral over the world. Offshore wind turbines (OWTs) have therefore seen unprecedented development over the decades. Fragility and vulnerability of OWTs are vital in structural performance evaluation and risk assessment of infrastructural systems under natural hazards. Previous research mainly focused on the fragility of OWTs with monopile substructure in the normal operating or parked conditions. In the present study, a realistic situation with respect to the emergency shutdown mode is considered for the OWTs with monopile, tripod, and jacket substructures. A procedure to establish fragility and vulnerability models is developed. Numerical models using a combination of FE software are implemented and validated for dynamic analyses and pushover analyses. Then, the fragility and vulnerability models of the main structural subsystems including rotor, nacelle, tower, and substructure are separately established according to the developed procedure, so as to the fragility and vulnerability models of the whole wind turbine. Besides, impact of operational condition and characteristics of substructure on fragility and vulnerability is comprehensively compared and discussed.

Effect of Shear Key Interarray on the Bearing Capacity of Grouted Connections of Prepiling Jacket Substructures

Jacket structures and piles penetrating the seabed are connected through grouted connections. The shear key shape is designed to improve the shear strength of grouted connections. On a construction site, it is difficult to control the height error of the inner and outer steel tubes because of the environmental load. In addition, an installation error leads to an error in the arrangement of the shear keys, degrading the structure's performance. Among the construction methods for installing jacket structures, prepiling is more economical than the post-file method, but construction errors are likely. In this study, the fracture shape and maximum axial load capacity were evaluated according to the arrangement error of the shear key. It was found that when an arrangement error of the shear key occurred, the maximum axial load decreased to within 10%.

Evaluating operational strategies for the installation of offshore wind turbine substructures

The construction of offshore wind farms requires solving more and more complex logistical problems due to the increasing sizes of turbines and changing environments. In particular, the installation of substructures requires attention due to their significant impact on capital expenditures and the limited literature and guidelines available. In this paper, we develop a decision support tool consisting of a discrete-event simulation that allows for comparing strategies for the installation of offshore wind turbine substructures in terms of time and costs. We identify several combinations of transportation and installation strategies for monopile and for jacket substructures. The differentiation is based on the deployed vessels and the installation sequence of the components. The strategies are applied to the case of a wind farm in the North Sea. For both substructure types, we find that strategies involving a second installation vessel result in the shortest installation times, and those in which the installation vessel(s) take(s) care of both transportation and installation result in the lowest costs. Additionally, we quantify the performance increases as a result of a reduction of the most prominent bottlenecks and the sensitivity of the project performance to the start date. Finally, the results are discussed in relation to future market and technological developments in the field.

Stress influence matrix on hot spot stress analysis for welded tubular joint in offshore jacket structure

The detailed finite element (DFE) simulation of welded offshore tubular joint can give realistic and sound hot-spot stress (HSS) estimates regarding fatigue design of jacket substructures for offshore wind turbines. However, the computational efforts and time costs due to complicated joint configurations and a large quantity of design load cases have prompted the design practitioners likely resort to a simple alternative, the SCF method, which is also rooted in the DFE simulation. The present study attempted to optimize the conventional DFE simulation procedure by applying a stress influence matrix (SIM) method and its modified version gSIM, along with discrete joint formulation techniques, to simplify and make efficient of the DFE process, and then incorporating point positioning techniques to eventually ease the HSS evaluation. Through an implementation case involving a KK tubular joint, the gSIM method proved its robustness, reliability and computationally efficiency in joint HSS evaluation practice.

Collaborative Strategy Heightens Efficiency of North Sea Predrilling Campaign

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30850, “Perfecting the ‘Perfect Well’: Lessons Learned From the Predrilling Campaign for Johan Sverdrup,” by Janne Vølstad and Stian Bærheim, Equinor, and Knut Henriksen, Baker Hughes, et al. The paper has not been peer reviewed. Copyright 2021 Offshore Technology Conference. Reproduced by permission. The predrilling campaign for the Johan Sverdrup development was a key contributor to the successful execution of the first phase of the megaproject. After a summary of the results of the campaign, the focus of the complete paper is to identify some of the key factors behind the success of the campaign and outline lessons learned relevant to future drilling campaigns. Since the completion of the predrilling campaign, the project has successfully tied back, completed, and begun production of eight predrilled oil producers and injection in 12 subsea injectors. Johan Sverdrup Field The field is approximately 150 km west of Stavanger in a water depth of 110–120 m. The top reservoir is situated approximately 1800 m below mean sea level. The reservoir features hydrostatic pressure and undersaturated oil with a low gas/oil ratio. The field has been developed in two phases. Phase 1, approved by Norwegian authorities in 2015, includes four bridge-linked platforms (Fig. 1). These platforms include a living-quarters platform (LQ) with utility system functions, a process platform (P1), a drilling platform (DP), and a riser platform (RP) with tie-in of onshore power. Phase 1 includes three injection templates tied back to the RP. All platforms have jacket substructures. Main power to the field is supplied from shore. The full field development includes 64 wells, both platform and subsea. The field came onstream in October 2019. Ramp-up of the predrilled wells went smoothly to a field production level of approximately 350,000 B/D. Phase 2 of the development was approved in 2019 and is expected to begin production in Q4 2022. Phase 2 includes development of another processing platform (P2), modifications of the RP and the field center, and five subsea production and injection templates. At peak, this field will account for approximately one-third of all oil production in Norway. Expected recoverable reserves are 2.7 billion BOE. The full field (Phase 1 and 2) expected oil production capacity is 105,000 std m3/d (660,000 B/D) at peak. Breakeven price for the full-field development is less than $20/bbl.

Development of a 3-legged jacket substructure for installation in the southwest offshore wind farm in South Korea

A suitable substructure for Offshore Wind Turbines plays an important role since it can efficiently reduce the financial costs of wind power projects. According to the track record of the Korean offshore wind farms, the 4-Legged Jacket (4LJ) substructure is the most utilized system; however, this design is still limited, specifically in the economic aspects. Thus, this study aims to design a more cost-effective substructure to enable large-scale deployment for Korean offshore projects. To achieve this objective, the new 3-Legged Jacket (3LJ) substructures together with various bracing topological forms (i.e., Pratt, Warren, and X-bracing) are developed. Results show that under the environment loads, dynamic responses obtained from the developed 3LJs are almost independent to the loading directionality; while there is a strong polarization in the case of 4LJ. Among the three cases of the 3LJs, the X-type topological form has the highest flexural stiffness together with the largest manufacturing and fabrication costs, while the Warren bracing system reaches ultimate strength earlier than the Pratt system. Therefore, the 3LJ substructure with a Pratt bracing system is suggested as a good alternative to the existing 4LJ system for Korean offshore wind farms, with reductions of up to 21% and 25% in total weight and number of weld joints, respectively.

Development of jacket substructure systems supporting 3MW offshore wind turbine for deep water sites in South Korea

This study aims to develop jacket substructures supporting a 3MW Offshore Wind Turbine (OWT) in water depth ranges of 25–40 m. A simplified structural design process is introduced and influences of jacket topological forms and brace systems on their dynamic performances are assessed. The results conclude that the topological configuration of the jacket substructure has a significant influence on the dynamic characteristics of the whole system, indicating a high priority when designing a jacket substructure for OWTs. Furthermore, two jacket substructures having the lowest material costs and the best performances under the Korean ocean environmental conditions are suggested. Additionally investigated in this study is a sensitivity of environmental loading directions on the structural performances of the jacket substructures. All the findings in this study might be very useful to assist the structural engineers when they design the support structures for large-scaled OWTs in the near future in South Korea.

Feasibility of modal expansion for virtual sensing in offshore wind jacket substructures

The present paper investigates the feasibility of modal expansion-based virtual sensing in the context of offshore wind jacket substructures. For this specific application, issues have been reported when expanding wind-driven brace vibrations and wave-driven vibrations in the splash-zone based on a sensor network placed solely above the sea level. These limitations are addressed in this paper by extending the sensor network with sub-sea vibration sensors and a wave radar sensor, which allow for capturing local brace vibration modes and the wave-driven vibration response. The brace expansion is thus improved by including the local brace vibration modes in the expansion basis, while the representation of wave-driven vibrations is improved by including load-dependent Ritz vectors computed based on input from the wave radar sensor. The merit of the proposed extension is explored using a numerical model of an offshore wind turbine supported by a jacket substructure in a simulation setting with different operational and environmental conditions. It is documented that the extended setup provides an improvement in the expansion-based estimation of both wind- and wave-driven vibrations. The former improvement is particularly relevant for operational cases, while the latter is relevant for idling cases. Despite the documented improvements, a systematic reduction in the expansion quality is observed for higher wind speeds in operational cases for both the basic and the extended setup. It is contended that this phenomenon is due to the operational variability of the controller, which violates the fundamental assumption of the structural system being linear and time-invariant.

STRUCTURAL ANALYSIS OF JACKET FOUNDATIONS FOR OFFSHORE WIND TURBINES IN TRANSITIONAL WATER

Most of the offshore wind turbines (OWT) recently installed in Europe, China and North America are in shallow water. However, unlocking the full potential of OWT lies in deeper waters. Jacket substructures have presented themselves as a reliable foundation concept for transitional water depth. This study focuses on the structural static and dynamic analysis of the traditional jacket substructures (with X and K bracing) and the recently patented three-legged twisted jackets (with a twisted angle of 30 and 60 degrees) for deployment in transitional water (beyond 60 m). To facilitate comparison, the dimensions of all the jackets remain the same, while, the geometric configurations are distinct. Static analysis was implemented to better understand the global load bearing behaviour of the jackets. First, the global displacement patterns at the tower top are compared. The individual reactions at mud-line were investigated, followed by the evaluation of the maximum von Mises stress. Subsequently, this research went on to investigate the effect of dynamic loading. In this dynamic analysis, three main critical points were considered, including the wave point (67 m), the platform and the tower top. A modal analysis was performed to compute the mode shapes and natural frequencies for all the jackets. The first five modes of all the jackets were also checked against the results available for the OC4 project. A similar analytical approach was adopted for the structural design of monopile or tripod foundations for offshore wind turbines. The results showed that in the static analysis both the traditional jackets and the twisted jackets were safe under the provided load combination. The twisted jacket proved to possess excellent structural behaviour compared to the traditional four-legged jackets, while maintaining the merits of lower material usage with fewer nodes. Analysing the von Mises stress revealed that the maximum stress occurred at the transition piece and close to the working platform. The modal analysis results of the jackets demonstrated that the twisted jackets (30 and 60 degrees) with the first natural frequency of 0.29 and 0.31 Hz fell under the soft-stiff design category whereas the traditional four-legged jackets were classified as stiff-stiff designs. The discovered structural performance of OWTs equipped with various jacket foundations contributes to the preliminary structural selection and optimal design of foundations of OWTs to be installed in transitional water.

Experimental investigations on the fatigue resistance of automatically welded tubular X-joints for jacket support structures

The development within the offshore wind sector towards more powerful turbines combined with increasing water depth for new wind parks is challenging both the designer as well as the manufacturer of bottom fixed support structures. Besides XL-monopiles, the market developed an innovative and economic jacket support structure which is based on automatically manufactured tubular joints combined with standardized pipes. Besides the improvements for a serial manufacturing process the automatically welded tubular joints show a great potential in terms of fatigue resistance e.g. due to a smooth weld geometry without sharp notches. However, these benefits are not considered yet within the fatigue design process of automatically manufactured jacket substructures according to current standards due to the lack of suitable S-N curves. Therefore, 32 axial fatigue tests on single and double-sided automatically welded tubular X-joints have been performed to determine a new hot spot stress related S-N curve. Based on these constant amplitude fatigue tests a new S-N curve equal to a FAT 126 curve was computed which implicitly includes the benefits of the automatically welding procedure.

Recovering More Than 70% From the Johan Sverdrup Field

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 30525, “Toward a Recovery Ambition of More Than 70% for the Johan Sverdrup Field,” by Eli Eikje, SPE, Tone Nedrelid, and Elisabeth Bratli, Equinor, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4-7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. Before the giant Johan Sverdrup barrel of oil, operator Equinor and its license partners set a recovery ambition of greater than 70% for the field. The complete paper discusses key elements driving this ambition. These include the field size and reservoir characteristics, early assessments and investments for improved oil recovery (IOR), data acquisition, reservoir monitoring, and digitalization. Introduction With a recoverable volume range of 2.2-3.2 billion BOE, Johan Sverdrup is a giant oil field approximately 150 km west of Stavanger, the third-largest oil field on the Norwegian Continental Shelf (NCS). The reservoir features hydrostatic pressure and undersaturated oil with a low gas/oil ratio (GOR). The first phase of the field came on stream in October 2019. A predrilling campaign included eight oil producers and 12 injectors. The field is being developed in two phases. Phase 1 includes four bridge-linked platforms: Living quarters with utility-system functions Process platform Drilling platform Riser platform (RP) with tie-in of onshore power Three injection templates are tied back to the RP. All platforms have jacket substructures. Main power to the field is from shore. The full field development includes 64 platform and subsea wells. Phase 2 of the development was approved in 2019 and is expected to start production in Q4 2022. It includes development of another processing platform, modifications of the RP and the field center, five subsea production and injection templates, and power-from-shore supply. The central area with highest hydrocarbon thickness and the highest relief to the free water level was chosen for the field-center location.

Johan Sverdrup’s Digital Operations Drive Efficiency, Safety

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30477, “Johan Sverdrup: The Digital Flagship,” by Paal Frode Larsen, Tor Tønnessen, and Florian Schuchert, Equinor, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4-7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The sheer size of the Johan Sverdrup field development (expected recoverable reserves of 2.7 billion bbl, with projected operations of more than 50 years) has earned it designation as the operator’s digital flagship. The field has driven digital solutions and work flows that have the potential to be scaled up for the benefit of the operator’s entire portfolio. The complete paper describes the development of the “digital field worker” at Johan Sverdrup, an initiative that has changed the approach toward not only construction and completion but also operations. Introduction The field is approximately 150 km west of Stavanger in the North Sea in a water depth of 110-120 m (Fig. 1). The top reservoir is approximately 1800 m below mean sea level. The reservoir is characterized by hydrostatic pressure and undersaturated oil with a low gas/oil ratio. A predrilling campaign included eight oil producers and 12 injectors. Additional wells will be drilled to achieve the expected Phase 1 oil production capacity of 70,000 m3/d. The field has been developed in two phases. Phase 1 was approved by Norwegian authorities in 2015 and includes four bridge-linked platforms, all with jacket substructures. Primary power to the field is supplied from shore. The full field development includes 64 wells, both platform and subsea wells. The field came onstream in October 2019. Ramp-up of the predrilled wells proceeded smoothly to a field-production level of approximately 350,000 B/D. Phase 2 of the development was approved in 2019 and is expected to start production in Q4 2022. At peak, this field will account for approximately one-third of all oil production in Norway with record low emissions. The expected full-field (Phase 1 and 2) production capacity is 105,000 m3/d at peak. Breakeven price for the full-field development is less than $20/bbl.

Strain estimation for offshore wind turbines with jacket substructures using dual-band modal expansion

Structural fatigue is a design driver for offshore wind turbines (OWT). In particular, the substructures, like jackets, are strongly affected by fatigue. Monitoring the fatigue progression in the welds is vital for the maintenance and a potential lifetime extension. However, inspections of critical locations are costly due to the limited accessibility of the mostly submerged jacket. Considering the high number of potentially critical welds, it is regarded as economically unfeasible to equip all fatigue hot spots with sensors. Thus, an indirect method to monitor the fatigue progress of the structure and point out critical locations is desirable. For a consistent support of ongoing maintenance, it has to yield reliable results for varying operational and environmental conditions. This paper applies a virtual sensing approach to jacket substructures. From a small set of sensors on the tower, fatigue at every desired location of the jacket is estimated using dual-band modal expansion. Simulations using the OC4 jacket design are performed to show potentials and limitations of the method. Namely fatigue progress on leg welds of K-joints is predicted with high accuracy over a wide range of load cases. However, some difficulties in fatigue prediction of X-joints due to the occurrence of local modes and limitations in the extrapolation of wave loading have to be resolved in future work.

Automatically Welded Tubular X‐Joints for Jacket Substructures: Prediction of the Technical Fatigue Crack Location

ABSTRACTTo increase the competitiveness of jacket substructures compared to monopiles a changeover from an individual towards a serial jacket production based on automated manufactured tubular joints combined with standardized pipes has to be achieved. Therefore, this paper addresses fatigue tests of automatically welded tubular X‐joints focusing on the location of the technical fatigue crack. The detected location of the technical crack is compared to numerical investigations predicting the most fatigue prone notch considering the structural stress approach as well as the notch stress approach. Besides, the welding process of the automated manufactured tubular X‐joints is presented.

Integrated Construction of Jacket Substructures to Reduce Construction Cost for Offshore Wind Turbines

Integrated Construction of Jacket Substructures to Reduce Construction Cost for Offshore Wind Turbines

Ultimate Strength of Completely Overlapped Joint for Fixed Offshore Wind Turbine Jacket Substructures

This paper presents an innovative eccentric jacket substructure for offshore wind turbines to better withstand intense environmental forces and to replace conventional X-braced jackets in seismically active areas. The proposed eccentric jacket comprises of completely overlapped joint at every joint connection. The joint consists of a chord and two braces in a single plane. The two braces are fully overlapped with a short segment of the diagonal brace welded directly onto the chord. The characteristic feature of this joint configuration is that the short segment member can be designed to absorb and dissipate energy under cyclic load excitation. The experimental and numerical study revealed that the completely overlapped joint performed better in terms of strength resistance, stiffness, ductility, and energy absorption capacity than the conventional gap joints commonly found in typical X-braced jacket framings. The eccentric jacket could also be designed to becoming less stiff, with an inelastic yielding and local buckling of short segment member, so as to better resist the cyclic load generated from intense environmental forces and earthquake. From the design economics, the eccentric jacket provided a more straightforward fabrication with reduced number of welded joints and shorter thicker wall cans than the conventional X-braced jacket. It can therefore be concluded based on the results presented in the study that by designing the short segment member in accordance with strength and ductility requirement, the eccentric jacket substructure supporting the wind turbine could be made to remain stable under gravity loads and to sustain a significantly large amount of motion in the event of rare and intense earthquake or environmental forces, without collapsing.

A comparison study on jacket substructures for offshore wind turbines based on optimization

Abstract. The structural optimization problem of jacket substructures for offshore wind turbines is commonly regarded as a pure tube dimensioning problem, minimizing the entire mass of the structure. However, this approach goes along with the assumption that the given topology is fixed in any case. The present work contributes to the improvement of the state of the art by utilizing more detailed models for geometry, costs, and structural design code checks. They are assembled in an optimization scheme, in order to consider the jacket optimization problem from a different point of view that is closer to practical applications. The conventional mass objective function is replaced by a sum of various terms related to the cost of the structure. To address the issue of high demand of numerical capacity, a machine learning approach based on Gaussian process regression is applied to reduce numerical expenses and enhance the number of considered design load cases. The proposed approach is meant to provide decision guidance in the first phase of wind farm planning. A numerical example for a National Renewable Energy Laboratory (NREL) 5 MW turbine under FINO3 environmental conditions is computed by two effective optimization methods (sequential quadratic programming and an interior-point method), allowing for the estimation of characteristic design variables of a jacket substructure. In order to resolve the mixed-integer problem formulation, multiple subproblems with fixed-integer design variables are solved. The results show that three-legged jackets may be preferable to four-legged ones under the boundaries of this study. In addition, it is shown that mass-dependent cost functions can be easily improved by just considering the number of jacket legs to yield more reliable results.

New proposal to express notch stress approach results by equivalent SCFs

For the fatigue design of welded structures, such as jacket substructures for offshore wind energy turbines, the structural stress approach is state of the art. Standards and guidelines also allow the application of the notch stress approach for complex welded components, weld roots, or to consider the advantages of the real weld geometry if measured or after grinding within the fatigue design. Since the theoretical background as well as the application of both approaches differs widely, it is not possible to directly integrate the fatigue notch factor Kf according to the notch stress approach into the design formalism of the welded structures based on the structural stress approach. Therefore, this paper derives a methodology to transform the elastic fatigue notch factor Kf into an equivalent stress concentration factor. By applying the introduced concept design engineers are able to selectively combine the advantages of both approaches without adapting the approved software tools to perform the fatigue design of complete welded structures.Furthermore, the presented concept of the equivalent stress concentration factor is an effective tool to perform a direct comparison of the relative fatigue resistance according to both approaches – structural stress approach and notch stress approach – on structural stress concentration level rather than on the level of damages or lifetime. This opportunity of comparison is applied on a tubular X-joint of a jacket substructure. The computed stress concentration factors are in good accordance between both approaches which is similar to the lifetime based results given in literature for application of both approaches on tubular K- and Y-joints.

A comprehensive fatigue load set reduction study for offshore wind turbines with jacket substructures

Designing jacket substructures for offshore wind turbines demands numerous time domain simulations to face different combinations of wind, wave, and current states. Regarding sophisticated design methods incorporating structural optimization algorithms, a load set reduction is highly desirable. To obtain knowledge about the required size of the design load set, a study on fatigue limit state load sets is conducted, which addresses mainly two aspects. The first one is a statistical evaluation of random subsets derived from probabilistic load sets with realistic environmental data obtained from the research platform FINO3. A full set comprising 2048 load simulations is gradually reduced to subsets and the results are compared to each other. The second aspect is a systematic load set reduction with the assumption of unidirectional wind, waves, and current. Firstly, critical directions are determined. Then, unidirectional load sets are systematically reduced. The corresponding damages are compared to those obtained from probabilistic load sets for eight test structures. It is shown that the omission of wind-, wave-, and current-misalignment does not necessarily imply an excessive simplification, if considered wisely. The outcome of this study can be used to decrease the numerical effort of the jacket design process and the levelized costs of energy.