Measured Dynamic Behavior of North Sea Jacket Platforms

This paper presents how non-linear waves and wave loading induce non-Gaussian dynamic response of fixed offshore structures, and the effects this may have to fatigue life and extreme dynamic response of such platforms. Furthermore, the validity of different theoretical wave and wave load models is discussed. The basis for the paper is full-scale measurements of waves and dynamic response recorded on a jacket platform in the North Sea and time domain computer simulations of the response of this platform using a finite element structural model. INTRODUCTION Fixed deep water platforms will because of the large water depths and high deck masses behave highly dynamically. Such platforms must therefore be designed against irregular environmental load processes with the energy distributed over a wide frequency range. This demands the use of stochastic methods for dynamic response analysis both for prediction of fatigue life and extreme load effects. The practical procedures for such analysis which are available today are based on Gaussian environmental excitation processes, linearized load models, linear structural models, and, consequently, on Gaussian dynamic response. Full-scale measurements from existing platforms have, however, indicated that the dynamic response of fixed structures generally is non-Gaussian [1, 2, 3]. In this paper it is, based on the results of systematic analysis of full-scale measurements of waves and dynamic response, discussed which are the sources of non-Gaussian dynamic response and what are the consequences of this to fatigue life and extreme response. Furthermore, the capabilities of existing methods for non-linear dynamic response analysis is outlined through comparison of measured and simulated dynamic response. The effects of the observed type of non-linear load effects to fatigue life and extreme response of deepwater platforms are discussed in [4, 7]. FULL-SCALE MEASUREMENTS The Instrumented Platform The applied data are recorded on the Val hall quarters platform. This platform is four legged and x-braced double corner piles. The water depth is 70 m. Typical dimensions for wave loaded structural elements are 1.5 m for the legs and 0.7 m for the bracing. The platform is connected with a bridge to two neighboring platforms. The platform is instrumented for measuring of wind speed and direction, wave heights, current speed and direction, strain in one of the braces of an x-joint and platform accelerations at cellar deck and mudline levels. The platform is shown in Figure 1 together with the two connected platforms. In Figure 2 a more schematic picture of the platform is given which also indicates the location of the different sensors. The Applied Data The data involved in the study are the wave data, current data and strain data in the location indicated in Figure 3. Key parameters for the different data types are given in Table 1. The standard length of each recording period is 20 minutes. The study included 28 recording periods, all representing sea states where the mean wave direction was within the 30 degrees sector indicated in Figure 3. The distribution of the recording periods with respect to sea state severity is given in Table 2

Renewable energy provides a solution for complex current and future social and environmental problems whereas offshore industry has a large potential for providing renewable energy for future. Currently, offshore technology making use of wind for energy generation purpose becomes a hot spot with highly advanced research and development going on on one side and complex and critical problems present and difficult to solve on the other. This paper is trying to study problems related to the quantification of the hydrodynamic and aerodynamic loads for the design of offshore wind turbine support structures in the offshore wind farm. Both the hydrodynamic and aerodynamic conditions in the offshore site are extremely complex resulting in the difficulty of reasonable determination for the external loads on the wind turbine support structures. However, due to the increasing global demands for future energy solution, the design, analysis and optimization of offshore wind turbine is nevertheless an important issue. The paper first gives an introduction of the offshore wind farm and the complexity of the offshore environment. Wave load is explored with introduction of existing wave load models, comparison of their characteristics while the focus is placed on the nonlinear wave load by means of the Stokes higher order wave theory. Properties of a single regular wave based on methods of linear wave theory and Stokes higher order wave theory are compared which lead to differences in the results of wave load models when these two different methods are used. Wind load model is introduced briefly, followed by the introduction of current methods for determination or approximation of combined wave and wind load and also recommendations for practice. Park effect of the wind load and wave load is also introduced at limited depth in the latter stage as a direction for future research. Conclusion and recommendations based on all the above are therefore given at the last section of the paper.

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.

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.

Jacket platforms are often fitted with sacrificial anodes to prevent marine corrosion. These anodes are attached to the surface of the tubular members of the jacket, increasing the hydrodynamic loads. Industry typically accounts for this using empirical global factors. The study aims to assess the increase in wave loads caused by the presence of anodes using a 1:20 scale model of a representative jacket. Experiments were conducted for two cases: a bare jacket and a jacket fitted with anodes. Regular and random waves were generated and hydrodynamic loads on these jackets were measured for three different angles of wave attack – 0°, 45° and 90°. Comparisons of wave loads from the different sets of experiments were made and salient conclusions are drawn. For the limited cases undertaken, it was found that the jacket with anode contributed significant increase in the wave loads (maximum case) to an extent around 2.5% for regular waves (wave period = 1–3 s) and 17% for random waves (peak wave period = 3 s).

Model tests were undertaken to aid the design of deck strengthening and protection for the Ekofisk 2/4 Tank. The detailed design work considered the possibility that wave impact on the deck and modules contained thereon might occur under extreme conditions of subsidence, tide and waves. The model tests investigated an unusually large number of parameters, and the resulting data were used to provide design values of shear force and overturning moment on the platform as a whole, forces on the deck, forces on modules, pipework and other structural components, and wave crest elevations both around and within the deck structure. Various unconventional data analysis procedures were devised in order to take account of non-linearities in extreme wave conditions. The results of using different analysis procedures and the final choice of design values are presented. INTRODUCTION The Ekofisk field started to produce oil around 1971. More than a decade later the sea-bed around Ekofisk was found to be subsiding at a rate of about 0.5m per year. By the end of 1986 the subsidence had reached 3. 8m, and was predicted to reach at least 6m by 2011, the expiry date of Phillips's operating licence. Subsidence of this magnitude was not accommodated within the original design, and Phillips therefore undertook a major and urgent programme of re-design and modifications on all the affected platforms. Some of the earlier work and model testing associated with the steel jacket platforms has been reported previously [1]. The jacket platforms making up most of the Ekofisk complex were raised in July and August 1987. A field layout plan is shown in Figure 1. Early feasibility studies showed that jacking was not practical for the central Tank structure. One of the options for the Tank was to leave the decks in place, and to strengthen locally so as to withstand the impact of the '100-year' wave with only minor damage. A plan and elevation of the Ekofisk tank, showing the +20m and +30m decks, is shown in Figure 2. The consequence of leaving the +20m and +30m decks in place would be that the +20m deck and modules contained thereon would be wetted and subjected to wave impact. The idea of allowing the '100-year' wave to impact a deck structure and modules is a major departure from conventional design practice, where a residual air gap ensures that no part of the deck enters the water. The situation was complicated by the unusual design conditions - shallow-water non-linear waves, with wave scattering from both the outer perforated wall and from the inner tank structure. Phillips engaged the original design company, Doris Engineering, and performed a complex programme of model tests to provide the necessary design data. The problem was considered well beyond the capability of any theoretical model. The analysis of these model tests posed a number of difficult problems, and considerable effort went into developing special analysis techniques, and then cross-checking the results - to ensure the best possible understanding of the data and the most reliable input to the design.

API RP2A WSD is a design code in practice for design of jacket platforms in the Persian Gulf but is based on the Gulf of Mexico environmental condition. So for the sake of using this code for the Persian Gulf, it is better to perform a calibration based on this specific region. Analysis and design of jacket platforms based on API code are performed in a static manner and dynamic analysis is not recommended for such structures. Regarding the fact that the real behavior of the offshore jacket platforms is a dynamic behavior, so in this research, dynamic analysis for an offshore jacket platform in the Persian Gulf under extreme environmental condition is performed using random time domain method. Therefore, a new constructed offshore jacket platform in the Persian Gulf is selected and analyzed. Fifteen, 1-h storm, simulations for the water surface elevation is produced to capture the statistical properties of extreme sea condition. Time series of base shear and overturning moment are derived from both dynamic and static responses. By calculating the maximum dynamic amplification factor (DAF) from each simulation and fitting the collected data to Weibull distribution, the most probable maximum extreme (MPME) value for the DAF is achieved. Results show that a realistic value for DAF for this specific platform is 1.06, which is a notable value and is recommended to take into practice in design of fixed jacket platform in the Persian Gulf.

An analytical approach and numerical solution to determine coupled aeroelastic and hydroelastic response of floating offshore wind turbines of arbitrary shape to combined wind and wave loads is presented. The model considers simultaneously the aerodynamic and hydrodynamic loads on an FOWT and integrates these with finite element method for structural analysis due to the combined loads. The hydrodynamic and aerodynamic loads are determined based on the linear wave diffraction theory and steady blade element momentum method, respectively, and the solution is obtained in frequency domain. The structure may be fixed or floating, located in arbitrary water depth, and may host single or multiple wind towers. The model captures the complete translational and rotational motions of the body in three dimensions, and the elasticity of the blades, tower and the floating platform. To assess the performance of the model, rigid and elastic responses of a FOWT to combined wave and wind loads are computed and compared with available laboratory measurements and other theoretical approaches where possible, and overall very good agreement is observed. The model developed in this study addresses directly three shortcomings of existing approaches used for the analysis of FOWTs, namely (i) determination of the elastic responses of the entire structure including the floating platform, (ii) analysis of the motion and elastic response of FOWTs in frequency domain, and (iii) assessment of responses of FOWTs with single or multiple wind towers.

This paper describes results from analyses of fullscale measurements of dynamic response, recorded under extreme environmental conditions, of three gravity platforms in the North Sea. The paper concentrates on evaluation of the composition of the dynamic response, on characterization of the loading mechanisms for the environmental loading and on description of the behaviour of the structural systems. Furthermore, the consequences of some of the observed effects for deep water platforms are discussed. INTRODUCTION During the 24 November 1981 one of the most severe hurricanes ever registered in the North Sea passed the STATFJORD oil field and the FRIGG gas field, see Fig.(l). This paper describes the dynamic behaviour of three gravity platforms, evaluated from fullscale measurements recorded during the maximum sea state of this hurricane. The measurements from this storm do not contain anything on the structural behaviour which has not been found from other data logged under storm conditions. However, since the environmental conditions in this storm were so extreme, the dynamic behaviour of the platforms in this case may be taken as representative for the behaviour under design conditions. Therefore, this recording period was selected for presentation in this paper. DESCRIPTION OF THE PLATFORMS The platforms are FRIGG TCP2, STATFJORD 'A' and STATFJORD 'B' and they are all of the CONDEEP design. The platforms are shown in Fig.(2). FRIGG TCP2 is a three legged platform at 100 m water depth and is connected to another gravity platform with a bridge and a pipeline. The soil conditions under the platform are characterized by dry sand in the upper layers and with clay in the deeper layers. The caisson of the platform penetrates the sea bottom by steal skirts. The platform was installed in 1977 and is found to be settled at the time for the measurements to be discussed in this paper. STATFJORD 'A' is a freestanding three legged platform at 150 m water depth. The soil conditions on the STATFJORD field are characterized as soft clay. As for FRIGG TCP2, the cassion penetrates the sea bottom with steel skirts. The platform was installed in 1977, and as FRIGG TCP2, it is found to be settled. STATFJORD 'B' is a four legged platform on the same water depth as STATFJORD 'A'. The STATFJORD 'B' caisson is differently designed compared to STATFJORD 'A' and FRIGG TCP2 as it has a flat bottom and no steel skirts. The platform was installed in August 1981, and the hurricane at 24 November is the first strong storm it experienced. EXPECTED DYNAMIC BEHAVIOUR In order to introduce the terminology and concepts used in the description of the measured behaviour, and to introduce a reference model for the discussion of the measurements, a short outline of the commonly accepted theoretical model for simulation of random vibrations induced by environmental loads on gravity platforms are given in the following.

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

Nonlinear wave and currents loads analysis of offshore wind turbines

This paper deals with prediction of cavitation noise of full scale ship's propeller from model experiment and its comparison with full scale measurements performed on two ships, a cargo liner and a training ship. It is proposed to estimate separately the tonal noise and the broadband noise because of their different mechanisms of noise generation. The acoustic-mechanical reciprocity principle is applied for estimating the tonal noise, and for the broadband noise a semi-empirical formula is applied for estimation in low frequencies. Full scale measurements on actual ship were arranged to acquire far-field noise and to eliminate extraneous noise reflected from sea surface. The comparison between predictions and full scale measurements has shown agreement amply proving that the proposed method of cavitation noise prediction is valid for practical purposes.

A common method of estimating the sheltering effects of different breakwater locations and layouts is to carry out physical model wave disturbance tests. Such tests have been carried out in different laboratories throughout the world for many years. But to our knowledge no reports are available in the literature showing comparison between model measurements and field measurements. The trend is that we know more and more on the wave cl imate along our coasts. Hence we have a better basis to make our economical calculations on breakwaters. We therefore also want to operate our models on a more absolute basis rather than on a comparative basis. The trend in recent years has also been to study breakwater locations and layouts in order to minimize mooring forces and ship movements. On this background VHL found a comparison between model test results and field measurements necessary. Full scale measurements of waves were carried out in two harbours by VHL during the winter 1976/77. This paper will present the results of the comparison of the model and the full scale measurements in Berlevag and Vard0 fishing harbours on the open coast of Finnmark in the northern part of Norway (Fig. I) . The model tests, as well as the full scale measurements, have been sponsored by the Norwegian State Harbour Authorities.

Wind and wave loadings have a predominant role in the design of offshore structures in general, and articulated tower in particular for a successful service and survival during normal and extreme environmental conditions. Such towers are very sensitive to the dynamic effects of wind and wind generated waves. The exposed superstructure is subjected to aerodynamic loads while the submerged substructure is subjected to hydrodynamic loads. Articulated towers are designed such that their fundamental frequency is well below the wave frequency to avoid dynamic amplification. Dynamic interaction of these towers with environmental loads (wind, waves and currents) acts to impart a lesser overall shear and overturning moment due to compliance to such forces. This compliancy introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of articulated towers. Prediction of the nonlinear behaviour of these towers in the harsh ocean environment is difficult. However, simplified realistic mathematical models are employed to gain an important insight into the problem and to explore the dynamic behaviour. In this paper, various modeling approaches and solution methods for articulated towers adopted by past researchers are reviewed. Besides, reliability of articulation system, the paper also discussed the design, installation and performance of articulated towers around the world oceans.

An analytical method of computing the frequency response of single degree of freedom (DOF) oscillators with nonlinear damping is described. The author proposes an energy-type nonlinear damping model and the corresponding stationary probability density with white noise input can be obtained explicitly. A theorem is presented which gives an interesting result, in terms of the Krylov-Bogoliubov approximation, concerning the modeling and identification of nonlinear internal damping in flexible structures. This analysis also serves as a contribution to random vibration theory by providing a method of computing the first- and second-order statistics (steady-state probability density, correlation function, and spectral density) of nonlinearity damped oscillators with white noise input.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>