DESIGN AND ANALYSIS OF FIXED OFFSHORE STRUCTURE – AN OVERVIEW

Various types of offshore structures including drilling rigs, platforms, etc. have been built. These days, they are requested to have better working performance in severer state of sea and weather, and as a recent trend, they are becoming larger in size and more complex.However, they appeared just several decades ago, and the data to design them is not accumulated so much as general ships. These structures are to be designed under the rational and synthetic estimation of behaviors in sea, and the safety is also to be confirmed experimentally or analytically.For that purpose, the authors have developed the total system of computer program for design and analysis for floating offshore structures. The system made it possible to realize the rational and consistent evaluation on the behaviors and the safety of them.This system consists of an integrated group of individual programs which includes hydrostatic calculation, tank capacity calculation, trim and stability calculation for intact and damaged condition, analysis on the oscilating motion of the structures among regular waves and wave force calculation, the statistical analysis on each response of the structures among irregular waves, etc.Furthermore, this system can be used individually or successively according to the needs in every stage of design or analysis.In this paper, outline of this system is presented with examples of calculation using this system for the existing semi-submersible catamaran drilling rig, from which the reasonable results were gained.

A probability distribution model for stress concentration factors in multi-planar tubular DKT-joints of steel offshore structures

The occurrence of Texas Gulf Coast hurricanes is analyzed using various statistical methods. Simple Poisson, periodic Poisson, and Markov chain models are fitted to the occurrence data for a site offshore of Mustang Island, near Corpus Christi. Use of the periodic Poisson model permits a relatively simple description of cyclical occurrence phenomena. The goodness of fit of the three different models is compared, using interarrival time, hazard function, and comparative maximum likelihood tests. The auto-covariance function for the data is investigated, and the effects of varying-record-Iengths are considered. Results found for other Texas sites are also compared with those found for the Padre Island site. While the simple Poisson and Markov chain models seem to provide a reasonable fit to the occurrence history, the periodic Poisson model is seen to provide a much better fit to the data. Reasons for the acceptance of a periodic model for hurricane occurrences are discussed. The cyclical hurricane occurrence pattern implied by the acceptance of the periodic Poisson model is shown to have a very significant effect on the design of both offshore and onshore structures. An important indication of this study is that the Texas coast is currently nearing the peak of cycle of high hurricane occurrence likelihood. INTRODUCTION The Gulf Coast of the United States is frequently visited by hurricanes, resulting in very large property damages. Since hurricanes are the most severe type of storm generally affecting the region, the high winds and waves associated with such storms produce the critical conditions for which many offshore structures must be designed. Offshore structure designers require some method of predicting hurricane occurrences over time spans of several years. Such prediction models may then be used to develop the suitable design criteria for a particular structure. Study of the Gulf Coast storm records reveals the rather irregular nature of hurricane arrivals at coastal sites. This irregularity suggests the treatment of hurricane occurrences as a stochastic process; that is, a random process in time. Hurricane occurrence probabilities may be obtained from the models of such a random process. This investigation is concerned with the development of probabilistic models for the prediction of hurricanes offshore of Mustang Island,, near Corpus Christi, Texas. (Figure 1) ';. The forecasting methods used in this study are based solely upon the available historical storm occurrence data. The methods utilized are valid only for long term forecasts made in ignorance of future weather conditions. Thus, the probabilistic treatment of long-term hurricane prediction serves as a complement to synoptic weather forecasting procedures, which are suitable only for short-term predictions. Various stochastic models for hurricane predictions with parameters derived from the storm data at the site are investigated and compared. DATA The assembly, categorizing, and interpretation of the available hurricane occurrence data is a very important step in making predictions of future storm likelihoods. The past record of storm occurrences is the information upon which the stochastic models of this study are based.

The recent activity in the design and construction of off-shore structures has imparted considerable impetus to research on the behaviour of soils in cyclic loading. Many new constitutive models for soil behaviour have been presented in recent years (Mroz, Norris and Zienkiewicz, 1978; Bazant and Krizak, 1976). They attempt at simulating the experimentally observed failure of soils due to cycling of the load. Soils are complex two-phase materials but even for simpler ideal elasto∕ plastic materials, the solution of shakedown under cyclic loading for the situations of interest to geotechnical engineers are not available. Though structural engineers have been aware of this problem for considerable time and have evolved their design criteria for safety against fatigue failure of structural elements, the only exact solution available for a continuum is that of a thick cylinder subjected to fluctuating internal pressure (Symond and Neal, 1951; Symond, 1952) Zienkiewicz et al (1976) were the first to point out the possibility of progressive settlement of a footing under a particular system of cyclic loads. Their conclusions were based on repeated number of finite element analyses. The authors have attempted here to obtain approximate solutions of shakedown loads on strip footings subjected to repeated vertical and horizontal loads and moments for certain specified load paths, using very few finite element analyses in conjunction with shakedown theorems (Melan, 1938). The results are presented in the form of design graphs.

The analysis and design of offshore structures is a complex complicated process. It is an iterative process involving several underlying assumptions, approximations and uncertainties which remain unresolved. The structures have to withstand an environment that can be predicted only with great uncertainties and must rest on soils whose properties and behaviour vary randomly and are known only to an approximate degree of accuracy. Therefore, the design of an offshore structure not only requires a thorough understanding of the principles of mechanics but also the expert human knowledge acquired from many years of experience.

A method for adapting existing earthquake response spectrum techniques to the analysis and design of offshore structures is presented. The offshore structure is modeled as a lumped parameter system and a nonlinear fluid resistance law is assumed. The nonlinear hydrodynamic drag effects are approximated satisfactorily by a linearizing technique except for the high frequency portion of the response spectrum where a special relation is shown. Rules are given for the construction of approximate earthquake response spectra using design estimates for the maximum values of ground displacement, velocity, and acceleration and certain characteristics of the structure. With the procedures set forth in this paper, earthquake response spectra that include fluid inertia and drag effects can be developed and used for offshore structures, and much of the information already available on response spectrum techniques for land-based structures can easily be adapted for use by designers of ocean structures. Examples are given of the application of the method to offshore structures. INTRODUCTION Earthquake response spectra are used extensively in the design of land-based structures to resist lateral forces associated with earthquake motions. However, the use of response spectra directly in the design of offshore structures is hindered by the effects of fluid-structure interaction. If the effects of fluid-structure interaction can be satisfactorily accounted for in such a manner that response pectra techniques can be extended to the design of offshore structures, then the wealth ofresponse spectrum information that is presently available for land-based structures can be used by designers of ocean structures. In this paper, an offshore structure is modeled as a lumped-parameter multi degree of freedom system as shown in Fig. 1. The hydrodynamic forces acting at each node as the system moves through the surrounding fluid are considered to be those computed from Morison's equation and include nonlinear hydrodynamic drag forces. It is noted that the energy dissipated by waves created at the surface as the structure moves through the water is not included in this formulation. These nonlinear forces will be linearized in such a manner as to ensure the existence of classical normal modes 2 in the linearized equations of motion and each resulting modal equation will contain an equivalent linear hydraulic damping coefficient. Procedures will then be given for the construction of an approximate earthquake elastic displacement response spectrum (which includes the effects of fluid-structure interaction) for each mode of vibration in the system. The procedures presented in this paper were developed by comparing computer solutions of the nonlinear equations of motion with computer solutions of the linearized equations of motion using actual earthquake records. 3

Hydrodynamic shape optimization is one of the key tools for the development of new system concepts in offshore engineering. This paper presents a fully automated numerical procedure for optimum adjustment of shapes to enviromnental conditions. Nonliear programming algorithms vary the design parameters to find a minimum of the objective function within a few iterations. The paper presents the optimization of a mono-tower, a buoy, a semisubmersible and a TLP. The resulting hull shapes are characterized by minimum wave loads and motions. Introduction To ensure the future supply of hydrocarbons offshore technnology stretches its frontier into deep water regions and areas subject to harsh environmental conditions. A large variety of different system concepts have to be developed for specific offshore operations at these frontiers. For reducing economic risks offshore operators improve cash flow by achieving an early start of production. In Norway, for example, the lead time to bring fields on strmun has been cut down by 60–70% to a period of 15–20 months within a few years1. To support this process the development of system concepts and design have to be performed in decreasing space of time. It should be emphasized that these preliminary design decisions govern major parts of later expenses2. The design of complex systems like offshore structures is an iterative process involving different fields of engineering. The main ohjective in the design of a new structure is the development of a solution, which serves best the operational requirements and meets all economic and engineering constraints. Primary factors influencing the design of offshore structures are environmental conditions and their impact on system behaviour. Wave induced motions and loads influence operability as well as structural dimensioning and the associated construction cost. The quality of the final design is based on the knowledge and the experience of the design team. Expensive and time consuming trial and error parameter studies are necessary to achieve a good design. Although more systematic approaches, like Nonlinear Programming (NLP), are not new in designing offshore structures they are rarely used. Chou3 presented a remarkable paper proposing an analytical procedure for the optimum design of offshore structures. The minimization problem is solved by means of calculus of variations. An optimtun displacement distribution along the vertical axis is developed. Due to the analytical formula used for the evaluation of forces and motions the geortmtry is restricted to hydrodynamically transparent bodies of revolution with vertical axes4. Chow derived optimal shapes for a buoy and a platform supported by four columns applying ISSC spectra. The same restriction with respect to geometry applies to the work of Saitos5. The heave exciting force of a buoy is minimized for selected wave frequencies. A semisubmersible is optimized with respect to heave motion in a design sea state applying the objective function proposed by Chou. Forces are calculated on the basis of a slender body assumption. Viscous effects are neglected. Kagemoto6 optimized the arrangement of vertical floating cylinders in waves using the penalty function method SUMT (Sequential Unconstrained Minimization Technique) of Fiacco and McCormick7. Alternativeiy three objective functions were investigateed: the total horizontal force, the total vertical force and the mean drift force for a specific wave length. Force evaluation is based on the hydrodynamic interaction theory of Kagemoto and Yue8 which restricts the procedure to vertical cylinders. Arrangmnents of va

Chapter 7 - Loads and Dynamic Response for Offshore Structures

Design and construction of offshore structures.

To investigate the residual strength of damaged offshore structures, tests were conducted on two tubular steel columns which were fabricated by cold-rolling and welding. The test specimens had diameters of 1.02 and 1.53 m with a wail thickness of 6.73 mm. The corresponding diameter to- thickness ratios were 151 and 227. A local dent was introduced into each specimen to simulate damage due to impact. The depth of the dents was 2.7% and 5.5'% of the specimen diameter for the smaller and the larger-diameter specimens, respectively. During the process of indentation, data was collected on the denting behavior and energy dissipation characteristics of the specimens. The dented specimens were then tested axially with fixed-fixed end condition. The results of the tests showed a minimal effect of the dent on the axial strength in comparison with the teats of undamaged specimens. The fixed-fixed end conditions appear to have been the major reason for the relative insensitivity of the specimens to the effects of the dent. 1. Introduction Offshore platforms may incur structural damage from a number of causes. In addition to operational and environmental loads which in severe cases can overload and consequently damage the structure, an offshore platform may be subjected to ship collisions or accidental impact from falling objects. Of these, ship/platform collisions are considered to be the most consequential and appear to occur fairly frequently.1 The damage to a member of an offshore platform may take one of the following forms, depending on the type of structure and the event producing the damage:Local denting of the tube wallOverall deformation of the memberCombination of local denting and overall deformation of the member The damage may vary from a slight dent to a total collapse of the structure although less than severe damage is more common.1 Yet, significant damage to offshore platforms occurs in somewhat more isolated incidences. It has been reported that dents with a depth of tile diameter or permanent deflection of 0.4 the length of the member occur almost every two years in the North Sea2 and there have been a total of 560 various accidents around the world between 1970 and 1981.1 Such damage can significantly affect the structural integrity of an offshore platform resulting in the possibility of serious environmental consequences as well as jeopardizing the safety of the crew. Therefore, assessment of the effect of possible damage should be of concern to engineers involved in the design of offshore structures. If it can be rationally determined, the residual strength of damaged members should be included in the analysis to determine the overall integrity of a structure. A significant amount of research addressing this problem has been undertaken in the last several years.2,3,4,5,6,7,8,9 A review of the state of the art has been presented by Ellinas.1 Probably, the most significant omission in the reported research has been the lack of experimental work on tubular members fabricated by cold-rolling and welding, which is the usual method of fabrication for offshore platforms, and of the consideration of members with large diameter-to-thickness ratios.

The design and fabrication of offshore structure depend on material science and solid mechanics, such as steel processing technology and welding and materials testing and evaluation and so on. In this paper, CTOD (Crack Tip Opening Displacement) toughness test for weld joint of heavy thickness S355G10+N steel has been carried out to evaluate the fracture resistance of this weld joint under Submerged Arc Welding(SAW). The results show that (1) the weld joint of heavy thickness S355G10+N steel tested possesses excellent toughness. That is to say the materials in the weld joint possess enough capacity resisting the crack and propagation. Therefore the welding procedure specification evaluated can be applied to fabricate offshore structure. What is more, being at the case of as-weld condition for the weld joint the CTOD test was carried out herein, hence the offshore structure fabricated with the welding procedure specification evaluated can avoid the post welding heat treatment. (2) Different positions in weld joint have their special toughness. The position which is 2mm away from the fusion line (located in base metal) (FL+2) possesses best toughness (average value is 0.825mm). The fusion line position (FL) possesses slightly worse toughness (average value is 0.672mm). The weld metal position possesses worst toughness (average value is 0.553mm). This distribution character is different from other references reported and comparision has been made in this study. (3) Being a lower fracture toughness in weld metal, as a design suggestion high stresses should be avoided in this position.

Results from recent climate investigations indicate that significant climatic variations occur over periods of time which may be small compared to the design life of a structure. A procedure is developed for quantifying the effects such variations produce in the expected distribution of extreme wave heights. Wave height distribution from hindcasts indicate that this modification to the conventional treatment of extremes can lead to significantly different estimates for wave heights associated with a given recurrence interval. INTRODUCTION Prior to 1970, there was only minimal interest in the United States focused on climatic variability. Normal values of precipitation, temperature, wind, and other weather elements were based on a 20-yr record that was assumed, to represent the long-term mean climate. The time-scale of climate change was popularly believed to be on the order of 10,000's of years and related to advances and retreats of major ice sheets in the Pleistocene. However, during the last 20 years or so researchers investigating different aspects of climate change have begun to amass a large set of evidence which indicates that climatic variability can occur on time scales which significantly affect the activities of man. Since 1970, frequently occurrences of what might once .have been termed "abnormal" weather has led to a much more widespread acceptance of the importance of short-term climatic variability. Droughts in Africa, North America, and Europe, widespread flooding in the Mississippi Valley, the dearth of landfall Atlantic hurricanes, and the extreme cold of the winter of 1976-1977 in the eastern United States are but a few facets of the shifting patterns of world-wide weather in the 1970's. In spite of the general acceptance of the existence of short-term climate fluctuations, practically all planning and design for offshore and onshore structures is done with no direct allowance for this variability, Indirectly, of course, it is expected that, by accumulating a sufficient record length, one can approximate the statistical population of wave heights and water levels, since various past climate states will be included in the sample. However, extrapolations based on data from mixed populations can lead to significant problems in the estimation of extremes (Borgman and Resio, 1977; Resio, 1975). Consequently, it is desirable to examine interrelationships between climate and wave extremes and to base long-term wave height estimates on some combined physical statistical model, if possible. This is complicated by uncertainty of climate forecasts and the increased complexity of the statistical analyses in such an approach. An alternative suggested by Goldman (1977) is to rely more heavily on deterministic estimates of wave heights from dynamical models; but this approach cannot be incorporated into an analysis of risk in a straight forward manner. It is therefore advantageous to follow through an analysis of tile relationship between extreme wave heights and climate fluctuations in spite of increased complexity and uncertainty.

The design and assessment of offshore structures, fixed or floating, against extreme and abnormal accidental events require special advanced analysis capabilities and can be dimensioning in certain situations. Such accidental events entail vessel collision/allision, dropped objects, fire, and explosion conditions. Only vessel impact and dropped objects are addressed herein. Allisions involving high impact speeds are also discussed. The structures considered include fixed platforms, floaters (Spar, FPSO, and Semi-submersible), and mobile offshore units. The paper shows how nonlinear FEA and simplified methods are applied both at the design stage and as assessment tools following an incident of accidental loading. It demonstrates that by making reasonable assumptions concerning the energy dissipation parameters during a collision/allision scenario, significant savings in computational and analysis efforts are realized while the reliability of the results is maintained. Both traditional low kinetic energy and abnormal high energy scenarios are evaluated. The paper also presents proposed repair solutions to some typical damage conditions and compares the different codes and standards (ISO, NORSOK, and API) with regards to the acceptability criteria. Introduction The design of offshore structures must account for accidental loadings with low probability of occurrence. Such accidental loadings include vessel collision/allision, dropped objects, fire, and explosion conditions. This paper is dedicated to vessel impact and dropped objects. Fire and blast resistance will be covered in future publications focusing on offshore applications. The FABIG (2009) and ASCE (2010) are currently the two main bodies dealing with fire and blast resistant structures with emphasis on buildings and petrochemical facilities. In addition to accidental supply ship impacts, some recent offshore facilities had to consider high speed ship bow impacts or allusions in order to ensure adequate safety especially when storage of petroleum or liquefied gas is involved. The objective of this paper is to present recent analysis capabilities that make it possible to realistically predict the energy absorption capabilities of offshore structures due to ship collision. This is important in order to ensure that the extent and consequence of the damage is acceptable. The acceptability criteria are set in the regulations and codes of practice as dependent on the associated collision risk level and consequence assessment. In the ISO standards; for example, hazards are categorized into three main groups, according to annual exceedance probability or return period:–Group 1 - hazards with 10–2 annual probability or return period of the order of 100 years–Group 2 - hazards with 10–3 annual probability or return periods of the order of 1000 to 10000 years–Group 3 - hazards with 10–4 annual probability or return periods well in excess of 10000 years Extreme conditions correspond to the 100 year conditions and are treated as ultimate limit state (ULS) events while accidental or abnormal events are associated with the 1000 or 10,000 year abnormal limit state (ALS) scenarios. A review of requirements in offshore regulations, standards and codes is given in Ghoneim (2008).

An experimental study on wave forces on a vertical cylinder due to spilling breaking and near-breaking wave groups

This paper gives a general overview of the analysis of offshore structures under fire loads. Attention is drawn to the different requirements of component fire testing, which may give misleading emphasis to certain types of analysis. It is shown that the basis of structural behaviour under fire loading is the change in material properties. However, it is the effects of thermal strains which are most likely to promote structural instability, leading to collapse. Non-linear structural analysis has ban USCXI to illustrate fire loading effects, but it is likely that the offshore design industry will employ linear elastic structural analysis in many designs. The design of seeondary structures to reskt fire loadings is discussed. INTRODUCTION The design of steel offshore structures to resist fire loadings is a new technology. Analytical techniques are not particularly well developed [1,2] or understood by either practicing engineers who develop the designs or safety specialists, who overview the design process. What is well understood is the technology of fire component testing [3,4], and it is this knowledge which is perhaps misleading the development of the analysis of steel structures under fire loadings. Recent research into structural fire performance has concentrated on the numerical simulation of furnace fire tests. Whereas such work [5] can lead to economies in component testing programs it does little to increase the fundamental understanding of full-scale structures in real fires. Offshore fires involve a hydrocarbon inventory, and embrace jet, pool and running liquid fire types. The description and calculation of fire loadings is not addressed in this paper. Material Behaviour The behaviour of steel structures under fire loading conditions is determined largely by temperature related material properties. Foundation data is available from a number of sources [6,7,8,9]. It is conventional to split the material properties into two groups. These are then termed the thermal and the mechanical material properties. Thermal properties are:specific heat,thermal conductivitycoefficient of thermal expansion.The mechanical properties are:yield stress (Fy),Youngs modulus (E),Shear modulus (G). Almost without exception, the material properties of steel vary to a greater or lesser extent with temperature. The stress-strain behaviour of a typical structural steel is shown in Figure 1. Note the loss of a sharp yield point and the drop in performance between 400 deg C and 750 deg C. When a steel type has been given enhanced properties by heat treatment, then the properties are likely to fall-back to those associated with the chemical composition when a threshold temperature of about 600 deg C is attained in a fire. By contrast the material properties associated with microstructure and hence chemical composition are both enduring and pertinent to fire conditions. In Figure 2 the behaviour of Austenitic stainless steel, low carbon structural steel and 'fire-resisting' steel are compared. In the range 600 deg C to 800 deg C the stainless steel retains more of its basic strength.