Nonlinear Random Wave Loading on Fixed Offshore Platforms

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional (Monte Carlo) time simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated extreme responses (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. A more efficient version of the time simulation technique (ETS) was recently introduced which takes advantage of the correlation between the extreme surface elevations and their corresponding extreme responses by dividing the simulated extreme responses into a number of groups based on the magnitude of their associated extreme surface elevations. The probability distribution of response extreme values for each group is then calculated individually based on a relatively small number of simulations, and then the total probability theorem is used to derive the probability distribution of response extreme values. The ETS procedure is found to be many times more efficient than the CTS method. However, its efficiency and accuracy reduces for sea states of lower intensity. In this paper, a more efficient version of the ETS technique is introduced which takes advantage of the correlation between extreme values of the nonlinear response and their corresponding linear response extreme values (both quasi-static and dynamic linear responses are considered)). The three different versions of the ETS technique are compared by exposing a test structure to sea states of different intensity. It is demonstrated that the extreme linear response versions of the ETS technique are more accurate and efficient than the version based on surface elevation extreme values.

Non-linear time domain irregular wave simulations have been performed for the Kvitebjørn jacket platform located in the North Sea with the aim to quantify the dynamic amplification. The jacket is a slender structure installed in about 190m water depth. For each of the selected extreme sea states both quasi-static and dynamic response simulations have been carried out for several wave realizations using different seeds. Based on the quasi-static response and dynamic response, equivalent dynamic amplification factors (EDAFs) were calculated for different response measures in the jacket. EDAF is the factor one has to multiply the q-probability quasi-static response with in order to obtain an adequate estimate of the q-probability dynamic response. The EDAFs are to be used in ultimate limit state (ULS) and accidental limit state (ALS) analyses of the platform. Simulations were performed applying a Gaussian wave with Wheeler stretching and a second-order wave model. This paper focuses on the selection of wave kinematics method and on the establishment of the EDAF analysis procedure.

This study addresses the dynamic response control of deep-water jacket offshore platforms under typhoon and misaligned wave loads by proposing a Tuned Mass Damper (TMD)-based vibration suppression strategy. Typhoon loading is predicted using the Weather Research and Forecasting (WRF) model to simulate maximum wind speed and direction, a customized exponential wind profile fitted to WRF results, and a spectral model calibrated with field-measured data. Correspondingly, typhoon wave loading is calculated using stochastic wave theory with the Joint North Sea Wave Project (JONSWAP) spectrum. A rigorous Finite Element Model (FEM) incorporating soil–structure interaction (SSI) and water-pile interaction is implemented in the Opensees platform. The SSI is modeled using nonlinear Beam on Nonlinear Winkler Foundation (BNWF) elements (PySimple1, TzSimple1, QzSimple1). Numerical simulations demonstrate that the TMD effectively mitigates dynamic platform responses under aligned typhoon and wave conditions. Specifically, the maximum deck acceleration in the X-direction is reduced by 26.19% and 31.58% under these aligned loads, with a 17.7% peak attenuation in base shear. For misaligned conditions, the TMD exhibits pronounced control over displacements in both X- and Y-directions, achieving reductions of up to 29.4%. Sensitivity studies indicated that the TMD’s effectiveness is more significantly impacted by stiffness detuning than mass detuning. It should be emphasized that the effectiveness verification of linear TMD is limited to the load levels within the design limits; for the load conditions that trigger extreme structural nonlinearity, its performance remains to be studied. This research provides theoretical and practical references for multi-directional coupled vibration control of deep-water jacket platforms in extreme marine environments.

Committee Mandate Concern for the environmental and operational loads from waves, wind, current, ice, slamming, sloshing, green water, weight distribution and any other operational factors. Consideration shall be given to deterministic and statistical load predictions based on model experiments, full-scale measurements and theoretical methods. Uncertainties in load estimations shall be highlighted. The committee is encouraged to cooperate with the corresponding ITTC committee. Introduction The content of this committee's report is composed in accordance with its mandate by the expertise of its members. Compared to previous reports of the Loads committee the structure is slightly altered, while the topics covered remain basically the same, except that the present report covers ice loads more extensively, while giving less attention to hydroelasticity in waves. There is one section for each of the main types of loads acting on a ship or offshore structure. Hence, Section 2 focuses on wave loads, Section 3 on current and wind loads, while Section 4 concerns ice loads. Next, Section 5 is of a more generic character, concerned with characteristic loads and uncertainty. Finally, Section 6 is devoted to special topics, of which there was only one contribution, namely loads on free-fall lifeboats. Within each of the sections 2 – 5 there is generally one part focusing on ships and another part focusing on stationary offshore structures. On the lowest level we have distinguished between the different ways of assessing the loads, namely theoretical/numerical methods, laboratory tests and full-scale measurements, although this subdivision is not followed consistently through all sections. In the section on wave loads, a distinction is made between potential theory methods and field methods. The latter group contains numerical methods for solving the Navier-Stokes equations in some form, assessing the flow in the entire fluid field. Potential formulations normally use boundary element methods, although field methods can also be used with potential theory (e.g, Amini-Afshar et al. 2019). The committee has performed a benchmark study on heave/pitch motions and vertical bending moments for a large containership at zero speed in steep regular waves. Existing experimental results have been compared with numerical simulations performed by some of the committee members to investigate the performance of various numerical linear and nonlinear methods, ranging from strip theories to SPH and RANSE solvers. This study is presented in an appendix to the report. To avoid unnecessary overlap with other ISSC committees the focus has been on loads and rigid body responses. Structural dynamic responses, such as springing and whipping of ships, are left to the Dynamic Response Committee, but vortex-induced vibrations of slender structures and ice-induced vibrations are considered in the present report. Loads on offshore wind turbines (OWT) are covered to some extent, although the Offshore Renewable Energy Committee deals with OWT in general.

The paper presents results from analyses of full scale measurements of the dynamic response of two steel jacket platforms in the North Sea. The measurements discussed in the paper were, for both platforms, recorded in extreme environmental conditions. The main emphasis is on describing the structural system behavior, on the composition of the dynamic response and on the evaluation and characterization of the environmental loading mechanisms. It is found that the stress response is mainly wave induced and quasi-static. The quasi-static response is non-Gaussian and consistent with theoretical predictions based on Morison-type wave loading. Several other non-linear wave load effects are also apparent. The structural system is close to linear in stiffness and mass. The resonant response is, however, non-Gaussian due to nonlinear damping mechanisms. The paper discusses also the implications which the observed non-linear wave loading and structural damping may have for potential deep water jackets. INTRODUCTION In compliance with the requirements of the Norwegian Petroleum Directorate seven fixed platforms in the Norwegian sector of the North Sea have been instrumented for collection of environmental data and platform performance data (E- and P- data). The main purpose of the collection and analyses of this data is "through a check of the design assumptions and platform behavior, to be able to assess the safety of the load carrying structure and foundation in the production phase". An additional purpose is to gain experience with operative platforms that may be useful when considering new platform concepts The data collection programmes which lasted for periods of two to three years have created a valuable data base for analysis and research on platform behavior in different environmental conditions. This paper describes the dynamic behavior of two steel jacket platforms in both moderate and extreme environmental conditions, as evaluated from the full scale measurements. Similar results for gravity structures and an articulated tower have been presented in ref. /1/ and /2/. The main emphasis of this paper is on the description of the behavior of the structural system including the soil-structure interaction, on the composition and nature of the dynamic response and on characterization of the waves and the hydrodynamic loading. Furthermore, the nature of the observed behavior is compared with theoretical models of the platform behavior arising from the commonly used design assumptions. DESCRIPTION OF THE PLATFORMS The two platforms dealt with are Ekofisk 2/4H and Valhall QP. Sketches of the platforms are given in Figure 1. Both platforms are four legged quarters platforms situated in approximately 70 metres water depth. Ekofisk 2/4H is K- braced with single corner piles while Valhall QP is X-braced with double corner piles. Both platforms are connected to other platforms with bridges.

A well-validated Finite Element (FE) model of a jacket structure (OC4 jacket) with pile foundation together with a Computational Fluid Dynamic (CFD) model (OpenFOAM-based) set-up for extreme waves on the structure are applied to perform systematic comparative analyses of the dynamic response of the structure to extreme wave loads with and without scour effects. The pile foundation model is considered by implementing a non-linear soil model to the foundation using load-deflection (p-y) approach to capture the non-linear behaviour of the soil for large pile foundation displacements. The results show that, the scour protection improves the foundation bearing capacity and consequently, decreases the failure probability in terms of both serviceability and ultimate limit states. Moreover, it enhances the dynamic response of the structure under extreme load events caused by breaking waves. Accordingly, an innovative solution for scour protection of offshore-fixed bottom structures has been introduced by “Biodegradable Soft Rock” (sand filled containers produced from bio-degradable geotextile product) which is the newest development in this section.

The design of a naval vessel requires accurate estimation of the extreme loads and motions that it will experience during its lifetime. Operation in large seaways in which the ship-wave interaction is highly nonlinear and transient leads to design events such as maximum internal loads due to global wave bending, local slamming loads, extreme roll, combinations of the global wave bending and local slamming, and many others. In this article, a method is presented that allows for nonlinear analysis to be used to predict events with user-specified rareness. The core of the method combines probability, frequency, and time-domain analyses to generate short time-window sea environments that lead to extreme dynamical events. The Office of Naval Research Tumblehome geometry is analyzed for the extreme roll angle when advancing in stern quartering irregular seas. 1. Introduction The design of a naval vessel requires accurate estimation of extreme loads and motions that it will experience during its lifetime. Specific quantities of interest are the maximum slamming load during wet-deck impact, maximum acceleration at different locations on the vessel, maximum green-water load on the bow structure or helicopter deck, maximum roll angle, or frequency of occurrence of capsize, to name a few. It is important to recognize that a ship lifetime is decades long, and the exposure time in different severe storms over the lifetime is of the order of weeks, if not months. Furthermore, because of the random nature of the sea and, hence, the dynamical response of the ship, the extreme response is also random and should be characterized statistically. This means that a single lifetime realization in a given seaway by either model tests or numerical simulation only gives one sample of the extreme response, and multiple lifetime realizations are required to characterize the extreme response.

Evaluation of dynamic responses under extreme environmental conditions is important for the structural design of offshore wind turbines. Previously, a modified environmental contour method has been proposed to estimate extreme responses. In the method, the joint distribution of environmental variables near the cut-out wind speed is used to derive the critical environmental conditions for a specified return period, and the turbulence intensity (TI) of wind is assumed to be a deterministic value. To address more realistic wind conditions, this paper considers the turbulence intensity as a stochastic variable and investigates the impact on the modified environmental contour. Aerodynamic simulations are run over a range of mean wind speeds at the hub height from 9–25 m/s and turbulence levels between 9%–15%. Dynamic responses of a monopile offshore wind turbine under extreme conditions were studied, and the importance of considering the uncertainties associated with wind turbulence is highlighted. A case of evaluating the extreme response for 50-year environmental contour is given as an example of including TI as an extra variant in environmental contour method. The result is compared with traditional method in which TI is set as a constant of 15%. It shows that taking TI into consideration based on probabilistic method produces a lower extreme response prediction.

Among different types of loads on jacket structures, wave loads, especially breaking wave loads, are the most likely to threat the stability of the structure; therefore, the correct estimation of the wave loading of an offshore jacket structure is crucial for the design of these structures. Despite the importance of the correct estimation of breaking wave induced forces on jacket structures, so far, no slamming formulae to predict these forces are available in the design standards and guidelines or in other publications. Moreover, the implications of such extreme wave load events and the associated uncertainties for the dynamic response of the entire jacket structure, including the response of the foundation piles, are still not fully clarified. This PhD study attempts to improve the understanding of processes associated with the interaction of waves and jacket structures and to develop reliable wave slamming formulae for the prediction of breaking wave-induced loads on jacket structures. First, the present knowledge is analysed to identify the processes involved in the interaction as well as the related knowledge gaps. Second, the data available from previous tests performed on a truss structure under breaking and non-breaking waves in the Large Wave Flume (GWK tests) in Hannover are analysed to identify the most relevant influencing parameters and to provide reliable slamming formulae for breaking waves on legs and braces of jacket structures. Third, using a CFD model set-up for the waves generated in the large wave flume GWK and a CSD model for the truss structure tested in GWK, the laboratory tests are reproduced and a methodology is proposed to predict total forces induced by near-breaking and breaking waves on jacket structures. Finally, the proposed methodological approach including the slamming force formulae developed in this study is implemented to calculate total forces by breaking waves on a full-scale jacket structure (OC4 jacket). The Finite Element FE model of the OC4 jacket is extended by pile foundation model and the structural performance of the entire structure was examined by comparing the results by those of the numerical models developed for the same jacket structure. Finally, the dynamic response of the jacket structure with pile foundation to breaking waves is systematically analysed to achieve a substantially improved understanding of the processes involved in the wave-jacket-pile foundation interaction.

Semi-active control of jacket platforms under wave loads considering fluid-structure interaction

A test guyed tower structure has been installed and maintained in 293 ft of water in the Gulf of Mexico. The experimental objectives, structural design, and tower construction are described in this paper. The test data presented illustrate that the wave induced dynamic response of the tower can be accurately simulated and that the spud can foundation performs satisfactorily. The field test results provide further confirmation that the guyed tower is technically feasible and a practical structure to serve as an offshore drilling and production platform. INTRODUCTION The guyed tower is a bottom founded offshore platform concept which differs in two major ways from a steel jacket platform. First, in contrast to a conventional jacket platform which uses pilings to transmit both the wave and gravity loads to the soil, the guyed tower uses a guyline and clump weight system to dissipate the wave energy and a spud can foundation to transfer the gravity loads to the soil. Second, since the sway period of the guyed tower is greater than the design wave period, the principal structure inertial forces always oppose the principal wave forces instead of adding to the total load as occurs on a conventional platform. Because of these two advantages the concept offers an economically attractive alternative to conventional platforms in water depths from 600 to at least 2000 ft (1). In order to advance the development of the guyed tower, a one-fifth scale test structure was installed in the Gulf of Mexico during October of 1975 (2). Measurements of the structure's response to environmental forces corresponding to design level conditions were recorded and subsequently analyzed. This paper describes the main features of the design, fabrication, and installation of the test structure and presents highlights from the data obtained. TEST OBJECTIVES The main objectives of the test were to obtain data and information about the installation, dynamic response, guying system behavior, and spud can (foundation) behavior of a guyed tower structure in an actual offshore environment. The installation procedures that were tested included towing of the structure to location, upending, and setting on bottom by adding preload ballast to the spud can as well as deploying the guy lines, clump weights, and anchors, and tensioning the guylines to insure proper setting of the anchors. The dynamic response of a guyed tower is more difficult to predict compared to fixed-bottom platforms because the tower motions are of sufficient magnitude to require consideration of the interaction between the structure velocity and the wave particle velocity. Further, preliminary analytical and model studies indicated that the response of the tower to waves is phased such that the principal inertial forces always oppose the principal wave forces. Thus the field test data served to substantiate the predicted dynamic response of the tower when subjected to actual ocean waves, wind, and current.

This paper is devoted to the random vibration analysis of jacket platforms under wave loads using the explicit time-domain approach. The Morison equation is first used to obtain the nonlinear random wave loads, which are discretized into random loading vectors at a series of time instants. The Newmark-β integration scheme is then employed to construct the explicit expressions for dynamic responses of jacket platforms in terms of the random vectors at different time instants. On this basis, Monte Carlo simulation can further be conducted at high efficiency, which not only provides the statistical moments of the random responses, but also gives the mean peak values of responses. Compared with the traditional power spectrum method, nonlinear wave loads can be readily taken into consideration in the present approach rather than using the equivalent linearized Morison equation. Compared with the traditional Monte Carlo simulation, the response statistics can be obtained through the direct use of the explicit expressions of dynamic responses rather than repeatedly solving the equation of motion. An engineering example is analyzed to illustrate the accuracy and efficiency of the present approach.

Prolonged exposure to hand-arm vibration (HAV) due to use of hand-held power tools leads to an increased occurrence of symptoms of disorders in the vascular, neurological, and osteo-articular systems of the upper limbs called hand-arm vibration syndrome (HAVS). Biodynamic responses of the hand-arm system to vibration can be suggestive parameters that give us better assessment of exposure to HAV and fundamental data for design of low-vibration-exposure power tools. Recently, a single axis hand-arm vibration system has been installed in the Japan National Institute of Occupational Safety and Health (NIOSH). The aims of this study were to obtain the fundamental dynamic characteristics of an instrumented handle and to validate the performance and measurement accuracy of the system applied to dynamic response measurement. A pseudo-random vibration signal with a frequency range of 5-1,250 Hz and a power spectrum density of 1.0 (m/s2)2/Hz was used in this study. First the dynamic response of the instrumented handle without any weight was measured. After this measurement, the dynamic response measurement of the handle with weights mounted on the handle was performed. The apparent mass of a weight itself was obtained by using the mass cancellation method. The mass of the measuring cap on the instrumented handle was well compensated by using the mass cancellation method. Based on the 10% error tolerance, this handle can reliably measure the dynamic response represented by an apparent mass with a minimum weight of 2.0 g in a frequency range of 10.0 to 1,000 Hz. A marked increase in the AM magnitude of the weights of 15 g and 20 g in frequency ranges greater than 800 Hz is attributed not to the fundamental resonance frequency of the handle with weights, but to the fixation of the weight to the measuring cap. In this aspect, the peak of the AM magnitude can be reduced and hence should not be an obstacle to the biodynamic response measurement of the human hand-arm system. On the basis of the results obtained in this study, we conclude that this hand-arm vibration test system can be used to measure biodynamic response parameters of the human hand-arm system.

Steel tubular bridge pier with inner cruciform plates has high seismic performance. In this paper, the applicability of static-dynamic verification method for seismic design of steel tubular bridge piers with inner cruciform plates is studied. The ultimate strength and state of steel tubular stub columns with inner cruciform plate subjected to compression and bending were investigated through finite element analysis. The validity of the proposed design formula of the failure strain which was obtained from the stub column analysis was examined by analysis on tubular steel bridge piers with inner cruciform plates under cyclic loading. The dynamic response analysis of the same steel bridge piers was carried out on the basis of the spring-mass model and the fiber element model. These results indicated the static-dynamic verification method of the seismic response analysis through the spring-mass model are good agreement with dynamic verification method of seismic response analysis by the fiber element model.

Mechanical properties of soft human tissues under dynamic loading