Subsea Structures Research Articles

Development of an offshore ground motion prediction equation for peak ground acceleration considering path effects based on S-net data

Ground motion prediction equations (GMPEs) in offshore regions are important not only for earthquake early warning systems and strong motion prediction but also for evaluating the durability of subsea structures and tsunami risks associated with seafloor slope failures. Since soil conditions and propagation paths differ between onshore and offshore areas, it is desirable to develop a GMPE specific to the seafloor. Previous GMPE models have some problems, such as being influenced by buried observation equipment and path effects. In this study, to predict the distribution of seafloor seismic acceleration, we developed a new GMPE regressed on the peak ground acceleration (PGA) data of S-net using minimum necessary seismic parameters as explanatory variables. Residual analysis using the conventional GMPE emphasized the path effects through the offshore area, which were corrected by the depth up to the plate boundary. The new model successfully predicted PGA with smaller errors compared to conventional onshore and offshore GMPEs. The residuals between the observed and predicted PGAs were used to examine the factors responsible for the effects of the S-net site conditions. The new GMPE can predict PGAs within 300 km of the epicenter from the moment magnitude (Mw 5.4–7.4), focal depth, earthquake type, and source distance. In this model, the distance attenuation coefficient is smaller than in conventional models, and consequently, the PGAs along the trench axis that are amplified due to path effects can be reproduced. This means that PGAs will be unexpectedly larger than those estimated by conventional GMPEs even far from the hypocenter. Our model improves the accuracy of PGA prediction and avoids underestimation in assessing seafloor slope failure and earthquake resistance near the trench.Graphical abstract

Numerical simulations of the effects of KC number and entrapped water on the subsea cover hydrodynamic characteristics

ABSTRACT The entrapped water in subsea structures with openings significantly influences hydrodynamic forces and structural stability. This study utilises Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k − ω Shear Stress Transport (SST) turbulence model to investigate oscillatory flows around a subsea cover under varying conditions, including different levels of entrapped water (obtained by different opening dimensions) and KC numbers. Numerical simulations are validated through experimental data from oscillatory flows over a subsea cover on a plate. The study examines lift and in-line force coefficients to assess the impact of opening dimensions and KC numbers on hydrodynamic forces. Findings show that opening size primarily affects the lift coefficient due to changes in the behaviour of entrapped water. The influence of entrapped water decreases as both the KC number and opening size increase. A 30% opening yields the smallest lift-to-in-line ratio ( C Lrms / C Drms ) and reduced C Lrms and C Lstd values. Additionally, the flow fields around the cover are also discussed. Smaller openings reduce the amplitude of velocity, vorticity, and pressure inside the cover. Openings mitigate vortex formation and shedding caused by jets between the structure and the wall. The results also reveal a phase shift in lift coefficients for different opening sizes. These findings provide critical insights for the hydrodynamic behaviour and design of similar subsea structures.

Quantifying marine growth on pipelines with archival remotely operated vehicle footage – a case study

Abstract. Marine growth can substantially affect the hydrodynamic properties of subsea infrastructure. The ability to accurately quantify this growth is essential for designing and maintaining subsea structures. Traditionally, quantifying marine growth has been done by applying simple ratio equations putting in relationship the known distance of an object and its length observed in the image to the length of an unknown object only observed in an image. This method is very sensitive if measurements are not taken in the same plane. Hence, this study aims to establish a method for creating a 3D model of a subsea pipeline segment based on Structure from Motion (SfM). The proof of concept is done using a case study. For this study, Remotely Operated Vehicle (ROV) footage from a 2019 inspection survey is utilised. The scenario in which the procedure is tested represents a difficult, but realistic test case. Despite the lack of metadata, video quality and obstructions (e.g. watermarks), meaningful results were produced. The SfM workflow included masking obstructions (e.g. watermarks), and an iterative camera alignment procedure to generate 3D models, enabling the measurement of marine growth using either the models or ortho images created. While the resulting models were sufficiently accurate to provide meaningful measurements for harder growth types, the method faced challenges in accurately modelling softer growth types, suggesting the need for further refinement. Nevertheless, SfM was proven to be a more reliable method of measuring biofouling than the ratio approach that been traditionally used.

A Method for Recognition and Coordinate Reference of Autonomous Underwater Vehicles to Inspected Objects of Industrial Subsea Structures Using Stereo Images

To date, the development of unmanned technologies using autonomous underwater vehicles (AUVs) has become an urgent demand for solving the problem of inspecting industrial subsea structures. A key issue here is the precise localization of AUVs relative to underwater objects. However, the impossibility of using GPS and the presence of various interferences associated with the dynamics of the underwater environment do not allow high-precision navigation based solely on a standard suite of AUV navigation tools (sonars, etc.). An alternative technology involves the processing of optical images that, at short distances, can provide higher accuracy of AUV navigation compared to the technology of acoustic measurement processing. Although there have been results in this direction, further development of methods for extracting spatial information about objects from images recorded by a camera is necessary in the task of calculating the exact mutual position of the AUV and the object. In this study, in the context of the problem of subsea production system inspection, we propose a technology to recognize underwater objects and provide coordinate references to the AUV based on stereo-image processing. Its distinctive features are the use of a non-standard technique to generate a geometric model of an object from its views (foreshortening) taken from positions of a pre-made overview trajectory, the use of various characteristic geometric elements when recognizing objects, and the original algorithms for comparing visual data of the inspection trajectory with an a priori model of the object. The results of experiments on virtual scenes and with real data showed the effectiveness of the proposed technology.

Physics-Informed Neural Networks for Prediction of a Flow-Induced Vibration Cylinder

Abstract Flow-induced vibration (FIV) is a common phenomenon in ocean engineering for subsea structures with circular-shaped cross sections. A large amount of computational resources or experimental efforts are required to predict or measure the complicated motions and flow field surrounding a circular cylinder undergoing vortex-induced vibration (VIV). Physics-informed neural networks (PINNs) are powerful deep learning techniques for solving governing partial differential equations (PDEs) of dynamic systems as an alternative to complex numerical methods. In the present study, a framework is built employing PINNs for solving the Navier–Stokes equations to predict flows past an FIV cylinder using sparsely distributed spatiotemporal data inside the domain. The training process involves minimizing the supervised loss of flow data at these sparse points and the residuals of the governing PDEs. For the training of the PINN model, a moving frame around an FIV cylinder is used to collect the training flow data from two-dimensional direct numerical simulation results at a low Reynolds number. The structural displacements of the cylinder are also implemented in the residuals of the equations of the developed PINN. The performance of the PINN is evaluated by comparing the predicted contours of the surrounding flow velocities with the training data. The hydrodynamic forces prediction is achieved using the PINN-obtained flow field predictions combined with the force partitioning method.

Sensor platform for periodic integrity inspection of subsea structures with a permanently installed measuring system

Offshore structures are inspected with high costs and risks. In order to reduce both, the Fraunhofer IKTS has developed a sensor platform for automated periodic inspection by underwater vehicles like Remotely Operated Vehicles (ROV) or Autonomous Underwater Vehicles (AUV). The monitoring tasks are numerous. Temperature or pressure measurements are widely needed to ensure safe operation. Welded seams are a critical element of offshore infrastructure. If they fail due to high wave loads, extraordinary ice collision or due to fatigue behaviour, the entire structure becomes critical regarding the stability and safety. The Fraunhofer IKTS developed a technique for the permanent installation on subsea structures in order to enable a periodic inspection. The communication and energy supply are realized wireless by WiFi and inductive energy supply. The sensors used differ from application to application. For the welded seam inspection, shear wave laminable transducers ensure a flat design to prevent damage due to current or waves. Shear waves give a relative independency from fouling to make the interpretation of the data more comprehensive. The device has been tested in maritime environment for 2,5 years to ensure reliability. Furthermore, a modular setup with integrated transducers into the circuit board provide market relevant potential low prices on the long term. The connection unit has been designed in a way that also small ROV/AUV can operate the light-weight unit to ensure a broad applicability by almost any vehicle on the market.

Analysis of the Drag-Reduction Ability of the Layout and Cross-Sectional Shapes of Subsea Structures in the Critical Flow Mode

Introduction. Slender structures of subsea energy production systems are under constant influence of currents and waves. Hydrodynamic loads result from the interaction of subsea pipelines, umbilicals, equipment supports with fluid flows, and lead to the vortex formation in the area behind the structures. Vortex-induced forces are the sources of the cyclic loading. They accelerate gradually the fatigue damage, which may result in a failure. One of the ways to reduce the loads on subsea structures is to alter the shape of a cross-section, taking into account the flow regime. Dependence of the resulting hydrodynamic loads on the cross-sectional shape and relative position of structures has not been studied in details for the uniform flow in the critical mode. The current work is aimed at filling this gap. The research objective is to consider the impact of the distance between the structures, and also, the presence of a D-shaped structure, placed upstream relative to the group of three cylinders of different cross-sectional shapes.Materials and Methods. The computational fluid dynamics approach was used in this work for numerical simulations of vortex-induced forces in the ANSYS Fluent software for cylinder with D = 0.3 m. Modelling was conducted with the Detached Eddy Simulation (DES) method, which combined advantages of the Reynolds-averaged Navier-Stokes equation (RANS) method and the Large Eddy Simulation (LES) method. The object of the research was the system of four structures in the 2D computational domain, which included the upstream D-shaped cylinder and the main group of three cylinders with the circular, squared and diamond shapes of the cross-section. The transient process was considered, where structures were under the influence of the uniform flow in the critical regime at Re = 2.5×10⁵.Results. Five sets of data were obtained in simulations for the time-dependent coefficients of the lift and drag forces: for the main system — of the D-shaped, circular, square and diamond structures, and also for the four systems — of only D-shaped, only circular, only square and only diamond shaped structures. Additional analysis was conducted for the effect of the distance between the structures on the amplitude of fluctuating hydrodynamic force coefficients. The obtained results are presented as time histories of coefficients of the lift and drag forces, frequency analysis and contours of velocity, pressure and vorticity fields. The results indicate a positive effect of the upstream D-shaped structure on reducing the drag force, acting on the central structure in the group of three cylinders located downstream.Discussion and Conclusion. The results of the performed studies facilitate the informed decisions regarding the arrangement of subsea structures in a group of four objects, depending on the cross-sectional shape and the distance between the structures. The upstream D-shaped structure provides reducing the hydrodynamic drag force acting on the central structure in the downstream group of three structures, thereby slowing the fatigue accumulation and increasing the time of safe operation.

Yoke-Type Elasto-Magnetic Sensor-Based Tension Force Monitoring Method for Enhancement of Field Applicability.

Tension members are key members that maintain stability and improve the strength of structures such as cable-stayed bridges, PSC structures, and slopes. Their application has recently been expanded to new fields such as mooring lines in subsea structures and aerospace fields. However, the tensile strength of the tension members can be abnormal owing to various risk factors that may lead to the collapse of the entire structure. Therefore, continuous tension monitoring is necessary to ensure structural safety. In this study, an improved elasto-magnetic (E/M) sensor was used to monitor tension force using a nondestructive method. General E/M sensors have limitations that make it difficult to apply them to operating tension members owing to their solenoid structure, which requires field winding. To overcome this problem, the magnetization part of the E/M sensor was improved to a yoke-type sensor, which was used in this study. For the development of the sensors, the numerical design and magnetization performance verification of the sensor were performed through eddy current solution-type simulations using ANSYS Maxwell. Using the manufactured yoke-type E/M sensor, the induced voltage signals according to the tension force of the specimen increasing from 0 to 10 tons at 1-ton intervals were repeatedly measured using DAQ with wireless communication. The measured signals were indexed using peak-to-peak value of induced voltages and used to analyze the signal change patterns as the tension increased. Finally, the analyzed results were compared with those of a solenoid-type E/M sensor to confirm the same pattern. Therefore, it was confirmed that the tension force of a tension member can be estimated using the proposed yoke-type E/M sensor. This is expected to become an effective tension monitoring technology through performance optimization and usability verification studies for each target tension member in the future.

Study of local scour around rectangular and square subsea caissons under steady current condition

Numerical simulations were conducted to investigate steady current scour around rectangular and square subsea caissons. The caissons, which are representative of subsea structures, have a submerged height ranging from 0.5 to several times their longest base dimension. The flow model used is based on the Reynolds Averaged Navier-Stokes equations, and the scour model simulates bed load and suspended load transport to predict the evolution of the seabed profile. The aim of this study is to investigate the mechanisms of local scour, specifically the contribution of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour. The model is validated through comparisons with experimental measurements, indicating reasonable agreement. Based on the numerical model results, the mechanisms of local scour around square and rectangular caissons are found to be qualitatively similar. If the flow is directed perpendicular to one of the caisson faces, both the horseshoe vortex and the local streamline contraction contribute to scour. The maximum scour depth is located at the two upstream corners of the caisson due to the combined effects of the horseshoe vortex and streamline contraction. When the flow approaches the caisson with a diagonal attack angle of 45°, the two upstream faces act like a streamlined wedge that splits the incoming flow. The study found that the horseshoe vortex almost disappears and the scour is primarily caused by the local velocity amplification at the two side corners. If the flow attack angle is 45°, the scour does not initiate near the upstream corner without the contribution from the horseshoe vortex. These findings are expected to serve as a valuable theoretical foundation for the design of scour protection.

Feasibility study of a method for tuning wave energy converters

A Wave Energy Converter (WEC) cannot operate efficiently if left alone floating to the mercy of the random sea surface. For a WEC to be efficient, its natural frequency should be close to the peak frequency of the local dominant waves. This paper introduces a method for tuning any natural frequency of a WEC to the peak frequency of a wave spectrum and converting dynamic responses into mechanical power. An innovative lightweight and dual-drag force subsea structure with a huge amount of entrapped seawater can be attached to any vessel using cables. This structure increases the roll mass moment of inertia of a medium size barge, and the mass of a small heaving floater, reducing their natural frequencies. The tension in the cables is converted into mechanical power. The study focuses on the Galápagos Islands, where swells with a typical 13 s peak period and significant wave height between 1 to 3 m dominate the wave climate. For the dynamic systems based on roll and heave responses, annual averages of mechanical power are 178 and 73 kW with efficiencies of 49 and 67% respectively. The method developed in this paper can improve the efficiency of existing and novel WECs.

Advanced Underwater Measurement System for ROVs: Integrating Sonar and Stereo Vision for Enhanced Subsea Infrastructure Maintenance

In the realm of ocean engineering and maintenance of subsea structures, accurate underwater distance quantification plays a crucial role. However, the precision of such measurements is often compromised in underwater environments due to backward scattering and feature degradation, adversely affecting the accuracy of visual techniques. Addressing this challenge, our study introduces a groundbreaking method for underwater object measurement, innovatively combining image sonar with stereo vision. This approach aims to supplement the gaps in underwater visual feature detection with sonar data while leveraging the distance information from sonar for enhanced visual matching. Our methodology seamlessly integrates sonar data into the Semi-Global Block Matching (SGBM) algorithm used in stereo vision. This integration involves introducing a novel sonar-based cost term and refining the cost aggregation process, thereby both elevating the precision in depth estimations and enriching the texture details within the depth maps. This represents a substantial enhancement over existing methodologies, particularly in the texture augmentation of depth maps tailored for subaquatic environments. Through extensive comparative analyses, our approach demonstrates a substantial reduction in measurement errors by 1.6%, showing significant promise in challenging underwater scenarios. The adaptability and accuracy of our algorithm in generating detailed depth maps make it particularly relevant for underwater infrastructure maintenance, exploration, and inspection.

Investigation on the dynamic behavior of the ring-stiffened double-shell structure due to subsea collisions

The ring-stiffened double-shell structure is currently widely utilized in the equipment for ocean engineering, of which the response analysis under subsea impact is important to accomplish the safety and highly difficult Marine missions. However, due to the complex characteristics of the research object including fluid-solid coupling contact, large deformation, short response time, and multiplet structure, the dynamic behavior of the impacted ring-stiffened double-shell structure is hard to analyze accurately by traditional numerical methods. To overcome challenges, a fresh modeling and solving approach for the impacting mechanism of the structure body is put forward by this paper. The method is based on the Arbitrary Lagrange-Euler and Fluid-Structure-Interaction (ALE-FSI) theory. A stiffness matrix of a penalty function and steady outer pressure setting is presented to build the underwater interaction model. Then, the ALE-based description method is proposed to simulate the collision process. The results indicate that the adopted modeling and solving methods for the subsea structural collision have better revealed the dynamic response process of subsea structure to impact. There isn’t any evident damage spreading to the surrounding region, and the structural damage is primarily contained in the collision area. When an underwater impact is applied to the double shell structure, the non-pressure shell provides better protection. The core academic achievement of this work is providing an effective approach for subsea impacting mechanism and nonlinear dynamic response of ring-stiffened double-shell structure which can be used in different scenarios. It can offer universal references to optimize the construction of underwater equipment and determine the level of impact damage to submarines.

Physical modelling of subsea structure removal via thermally induced pore pressure

Given the requirements of environmental agencies, the decommissioning of subsea structures, including removal, recycling, restructuring or re-adaptation, is inherent to deepwater oil exploration activities. During removal by lifting, the possible development of negative excess pore pressure (i.e., suction) in the soil beneath the structure significantly hampers the recovery of subsea infrastructure. This study presents a method for potentially reducing this suction by heating the fine soil under the foundation during uplift to produce excess thermally-induced positive pore pressure, which may reduce or eliminate suction. Reduced-scale physical modelling studies of mudmat/skirted like foundation structures showed that the negative pore pressure generated during pullout was in certain amount counterbalanced by the thermally-induced positive pore pressure, facilitating their removal. However, this process depends on the magnitude of the sustained loading with respect to the pullout resistance. Upward displacements take place just after heating, without further load increments and the displacement rate is dependent on the level of sustained load.

Tuning Wave Energy Converters to local wave conditions

A Wave Energy Converter (WEC) cannot operate efficiently if left alone floating to the mercy of the random sea surface. Moreover, high waves may impose destructive forces while contributing nil energy production in return. For a WEC to be efficient, its natural frequency should be close to the peak frequency of a wave spectrum. At the resonance or another lower natural frequency, dynamic responses are more regular, and thus, the power take-off system (PTO) can operate in a more efficient manner. This paper introduces a general systematic methodology for tuning the natural frequency of a resonant WEC for the desired degree of freedom and converting dynamic responses into power. A barge is used as a case study to tune its roll and heave natural period, which in general are much lower than the peak period of swells. The roll natural period is increased by hanging subsea structures to the sides of the vessel by means of mooring lines, see Fig. 1. The size of the structures and their entrapped water increase the roll mass moment of inertia to achieve the desired period. For the heave response, the natural period raises by adding skirts to the sides of the vessel, increasing the entrapped water. The resonant motions are significantly suppressed by means of the drag force acting on the subsea structures. Thus, the extreme drag forces acting on the structures act as a damper to the vessel response. These forces are transferred to the mooring lines, which can be converted into a rotary motion of a shaft of the PTO. Our application focuses on the Galapagos Islands, where narrow, low-frequency southerly swells with a typical 13 s peak period dominate the wave climate. This study allows for a systematic design of resonant-type WECs, which is necessary for the development of technical and economic feasibility studies of various novel and existing concepts.
 
 Figure 1: Schematic representation of resonant WEC with tuned roll natural period

Optimization design of remotely operated vehicle (ROV) for Madura Strait Area

Nowadays, the development of unmanned vehicles is significantly increased. Since the remotely operated vehicle (ROV) has more advantages than Diver. Then, ROV has an important role in underwater inspection and research in various sectors, such as offshore infrastructure development. ROV will give some critical information to the operator about an underwater inspection like bathymetry, pipeline inspection, and the other crucial subsea structure that the divers can’t reach. The optimization design has been carried out to determine ROV operational requirements in Madura Strait by Total-Based Method. ROV will be divided into 3 leading function groups, they are Features, Survive and Operate. Every group will be divided into Function Breakdown Structure (FBS). FBS will be divided again into Work Breakdown Structure (WBS). The optimum design will be determined by linking every breakdown function. The results are the ROV has 2MP 1080P resolution for camera, 2000 lumens for lighting, 50 m detection area for sonar and echosounder, 100 N grip force for Gripper, equipped with stainless steel frame, 2 hours long for operational duration, 5kg.f for thruster, operated with cable 300 m long and equipped with 6-unit thruster.

Prediction Method for RUL of Underwater Self-Enhancement Structure: Subsea Christmas Tree High-Pressure Valve Actuator as a Case Study

Underwater pressure-bearing structures are produced in practice by means of pressure self-enhancement methods in order to improve the stress distribution and enhance the pressure-bearing performance. On the other hand, the pairs equation shows that stress is an important factor influencing the degradation of the structure. In fact, improving the stress distribution will not only improve the pressure-bearing performance, but will have an impact on the life degradation trend. Thus, pressure self-enhancement affects the structural life by changing the stress distribution. With this in mind, this paper considers the effect of pressure self-enhancement on the service time of subsea structures, and a Bayesian network (BN)-based method that can be used to predict the remaining useful life (RUL) of underwater self-enhanced structures is proposed. The method also takes into account the influence of multiple sources of structural factors in order to predict the RUL of the structure more accurately. The life degradation process of an all-electric Christmas tree valve actuator is used as a case study. The prediction results are compared with data in the literature to verify the validity of the method. The results have implications for guidance on the O&M assurance of underwater production systems.

Optimal design of a single-point electromechanical cable subsurface mooring

ABSTRACT Single-point electromechanical (EM) cable subsurface mooring (EMC-SSM) is an innovative subsea structure used to measure several ocean parameters throughout a water column in real time. However, designing an EMC-SSM that can adapt to rough sea conditions is challenging owing to the relative fragility of EM cables. In this study, an optimisation design method based on parameter sensitivity analysis was proposed to improve the responses of EMC-SSM. The unreasonable design and optimisation value were determined by evaluating the slopes of the sensitivity curves of four response parameters: the minimum and maximum tensions, maximum declination angle, and maximum curvature to the net buoyancy of the main float and big sensor package (BSP); drag; added mass coefficient of the BSP; and net weight and hydrodynamic coefficient of small sensor packages. We found that the proposed optimisation design method is effective in identifying and optimising the unreasonable configurations of mooring similar to the EMC-SSM.

A framework for a net environmental benefit analysis based comparative assessment of decommissioning options for anthropogenic subsea structures: A North Sea case study

Taxpayers and operators worldwide have significant current liabilities associated with decommissioning of offshore Oil & Gas (O&G) assets. Consequently, decommissioning is at the forefront of industrial, governmental, and non-governmental agendas. Decommissioning is a highly complex activity with health, safety, environmental, social, economic, and technical implications. Increasing scientific evidence supports that manmade subsea structures create hard, artificial reef habitats that provide ecological and social benefits to society. Given the significant uncertainty regarding how subsea structures should be retired at the end of their operational lifetimes, it is necessary for governments, taxpayers, and operators to understand the risks and benefits associated with potential decommissioning options. Currently, the North Sea decommissioning process is based on the policies and direction of the Oslo and Paris Convention’s (OSPAR) Decision 98/3 and follow comparative assessment (CA) multiple-criteria decision analysis (MCDA) guidelines to determine the best overall strategy for decommissioning subsea structures; however, CA MCDA processes can be biased, ambiguous, difficult to use, interpret, and replicate, and limited in their consideration of multigenerational benefits. Consequently, to assist decision-makers in understanding and evaluating options and associated benefits for decommissioning subsea structures, this study adapted the net environmental benefit analysis (NEBA) framework to supplement and strengthen the CA process for evaluating decommissioning options for offshore O&G facilities. The net environmental benefit analysis based comparative assessment (NEBA-CA) framework is presented that addresses the growing need for a practical, quantitative, scientifically robust, defendable, and transparent MCDA approach to determine optimized decommissioning strategies for subsea assets. Increased transparency in CAs will provide an additional layer of credibility with regulators and society. The approach is data driven and a desktop analysis mainly relying on existing data. Using a North Sea case study, this work demonstrates the ability of NEBA-CA to resolve inherent complexity in comparing decommissioning options, thereby supporting operators in working with regulators to decommission assets in a way that maximizes ecosystem service benefits to society while managing site-related risks and costs. The NEBA-CA framework supplements and strengthens the standard CA process by 1) incorporating quantified metrics including multigenerational ecosystem service benefits and risks, 2) excluding front ranking (scoring) or weighting of metrics, and 3) providing consistent graphical displays to support visual differentiation of options and metrics.

Validation of earthquake analysis methodology of a suction-caisson foundation-structure through model testing

Suction caissons are one of the most widely used foundation solutions for subsea structures and wind farms. Seismic response of subsea structures is however seldom documented properly, often just treated as a foundation capacity issue applying a quasi-static acceleration and not considering the inertial interaction between the structure and the soil. The more relevant tasks to document are the motions of the unit and the response of the externally connected flowlines and equipment/systems on the unit.Based on a case study located in the Shah Deniz field in the Caspian Sea, model centrifuge tests and numerical modelling were carried out to validate the global response of a 4-caisson supported manifold structure subject to seismic motions in soft clay. The centrifuge tests were carried out at 58 g at the centre for geotechnical modelling at UC Davis. To simulate the soil-structure interaction, a series of non-linear springs defined by kinematic hardening models were used in analyses with the ABAQUS software. This development includes the algorithms for determining the required model parameters. A very good agreement between recorded response from the centrifuge test and calculated response from the FE-analyses was achieved.The development and validation of the soil model presented in this paper is an improvement in design methodology for caisson foundations subjected to earthquake loading. The non-linear soil springs are well suited to incorporate in more detailed structural analyses where an accurate representation of the foundation response is required. The paper also briefly describes how the subsequent earthquake design analyses were performed for the Shah Deniz manifold structures making use of the validated soil spring model and the added value it gave to the project.

DYNAMIC PROPERTIES OF SAND FOR EARTHQUAKE RESPONSE ANALYSIS IN THE BAY OF CAMPECHE

The Bay of Campeche is located in a region of moderate to high seismic activity related to the active triple junction between the North American, Caribbean, and Cocos plate boundaries. Therefore, the fixed offshore platforms and subsea structures in the Bay of Campeche must be designed against earthquake loading. A database was developed of classification and index properties tests, in situ measurements of shear wave velocity (Vs) using downhole P-S suspension seismic velocity logging, in situ piezocone penetration tests, resonant column tests to characterize the shear modulus and material damping ratio at small shear strains (10−5 % to about 0.1 %), and strain-controlled cyclic direct simple shear tests to evaluate the decrease of shear modulus and the increase of material damping ratio at large shear strains (0.1 % to about 10 %) performed on sand from the Bay of Campeche, including sands with no carbonate content to 100 % carbonate content, retrieved from the seafloor to a penetration depth of 120 m below seafloor. The database was tailored specifically to develop empirical correlations for the Bay of Campeche sand to determine Vs when in situ measurements of Vs are not available and to develop numerical modeling to predict the variation of the normalized shear modulus (G/Gmax) and material damping ratio (D) as a function of shear strains (g) when dynamic test results are unavailable for all the sand layers. The equations developed to calculate Vs and the curves of G/Gmax-g and D-g of Bay of Campeche sands are recommended for preliminary or perhaps even final seismic site response evaluations.