Structural performance assessment of fixed offshore platform based on in-place analysis

In-place analysis for offshore platforms is essentially required to make proper design for new structures and true assessment for existing structures, in addition to the structural integrity of platforms components under the maximum and minimum operating loads when subjected to the environmental conditions. In-place analysis have been executed to check that the structural member with all appurtenance&#180s robustness have the capability to support the applied loads in either storm or operating conditions. A nonlinear finite element analysis is adopted for the platform structure above the seabed and pile-soil interaction to estimate the in-place behavior of a typical fixed offshore platform. The SACS software is utilized to calculate the dynamic characteristics of the platform model and the response of platform joints then the stresses at selected members, as well as their nodal displacements. The directions of environmental loads and water depth variations have significant effects in the results of the in-place analysis behavior. The most of bending moment responses of the piles are in the first fourth of pile penetration depth from pile head level. The axial deformations of piles in all load combinations cases of all piles are inversely proportional with penetration depth. The largest values of axial soil reaction are shown at the pile tips levels (the maximum penetration level). The most of lateral soil reactions resultant are in the first third of pile penetration depth from pile head level and approximately vanished after that penetration. The influence of the soil-structure interaction on the response of the jacket foundation predicts that the flexible foundation model is necessary to estimate the force responses demands of the offshore platform with a piled jacket-support structure well.

The influence of mooring system in rogue wave impact on an offshore platform

Offshore platform is an important infrastructure for exploitation of marine resources, and the operation safety is a key issue in the offshore platform industry. Dropped risers and other dropped objects would threat the platform structure and personal safety. In order to analyze the risk of damage caused by the dropped risers, a practical engineering analysis is presented using the engineering simulation platform ANSYS/LS-DYNA, a useful and powerful tool with an explicit dynamic solver. The impact angle of the dropped object may have different impact consequences on the offshore platform deck in respects of dimension and strength. By using ANSYS/LS-DYNA, the dropped riser's impacts on the platform deck under different angles circumstances are simulated and analyzed. Based on the simulation results, the nonlinear fitting approaches of polynomial fitting and BP neural networks were used to analyze the nonlinear mapping between impact angles and collision results. The results of the fitting were analyzed and verified by LS-DYNA and the fitting accuracy of the two methods was further discussed. This method can be used in conventional risk analysis studies for evaluating the risk for offshore platform and subsea pipelines from dropped objects.

Risk identification and assessment methods of offshore platform equipment and operations

This paper presents a procedure a coupling of the finite element method and normal mode technique for obtaining the elastic dynamic response of offshore platforms subjected to arbitrary time and space-dependent forces. The finite element method deals with the complexities of the actual problem by reducing it to an assembly of structural elements, with known elastic and inertial properties, connected together at a discrete number of points on their boundaries. The normal mode technique is then used to reduce the resulting matrix equation of order N to the solution of N single degree-of-freedom equations with certain constraints on the assumed form of damping. An example analysis, utilizing this procedure and an actual offshore platform under the action of two different types of dynamic forces, is presented. These forcing functions are: wave forces as determined through the use of a fifth-order gravity-wave theory and seismic forces as generated by the north-south component of the May 18, 1940, El Centro, California, accelerogram. The platform is assumed to respond according to the assumptions associated with the Bernoulli-Euler beam and bar theory. INTRODUCTION In the field of offshore platform analysis and design, as in many other fields, the tools of analysis are derived through an evolutionary process. As knowledge and ability increase for more accurate prediction of the environmental forces on platforms, the methods of analysis must improve to maintain consistent accuracy. The American Petroleum Institute(l) in its Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms recognizes this evolutionary process and encourages designers of offshore platforms to use "all research advances available to them" The advent of high-capacity, high-speed digital computers has revolutionized the field of structural dynamic analysis. In the past the approach was to reduce the structural model to the simplest form, usually an "equivalent"single-or dual-degree-of-freedom system or "equivalent" beam or bar model for which known closed-form solutions to the dynamic equations were available. While this approach gave valuable insights into the nature of the dynamic response problem, it did not give the detailed elastic response information that is possible today. This paper presents a method which the authors believe is the next step in the evolutionary process of offshore-platform dynamic analysis. It is a combination of the finite element method and the normal mode technique. The finite element method more accurately takes into account the geometry of the system than the so-called "equivalent" methods. Although the finite element method and the normal 'mode technique have been known for a number of years, it is only with advanced computer technology that these approaches have been economically feasible for the analysis of large complex structures. This economic feasibility, the increased accuracy of predictions of dynamic forces on platforms in the operating environment, and the desire of operators to construct offshore platforms in deeper water and use them under more hostile operating conditions, will force designers to use more advanced methods of dynamic analysis.

Background. The development and production of hydrocarbons on the sea shelf in the World Ocean have a long history. Currently, about 35 % of the world's hydrocarbon production is produced in the offshore area. The dynamics of the growth of oil production in the sea is now more than 5 times higher than the dynamics of the growth of production on land. The construction and operation of offshore platforms are associated with high risks of both technogenic and economic characters. According to the World Offshore Accident Dataset (WOAD), more than 200 accidents on offshore platforms occur each year, or about 4 % of accidents in the total number of accidents at sea facilities in the world. Since the average cost of drilling rigs in the sea is 3-5 times greater than on land, ensuring their safety and analyzing the hazard risk for this type of facility is a priority. Currently, the use of existing methods of hazard analysis and risk assessment for offshore drilling platforms is hindered by the fact that there are no methods for determining the extent of accidents on the water surface. The problem of estimating the parameters of the strait of hazardous substances during their passage to the water surface remains unexplored. Aims and Objectives. Investigation of the influence of the hydrocarbon composition of petroleum products (on the example of kerosene TS-1) on the parameters of the spill of fire-hazardous liquids during spills on the water surface. Methods. In the work, experimental research methods were used, which consisted in observing the behavior of petroleum products (on the kerosene TS-1 as an example) during their passage to the water surface. Results. The key effect of naphthenic hydrocarbons on the change in the area of the oil spill across the water surface is proved.

Tubular joints are integral components of offshore structures and as such, many codes and standards address their design. The fatigue damage process of tubular joints in offshore structures is inherently variable and uncertain. In addition, potentially, there are important bias and uncertainties introduced by the analytical models used in design codes and standards. Therefore, in the development of reliability based fatigue design and requalification criteria, it is important to recognize these differences and to develop the best estimates of joint fatigue durability. This paper summarizes a study to identify the primary sources of bias and uncertainty associated with assessment of fatigue lives of tubular joints in offshore platforms. Based on the extensive screened test data, the important uncertainty sources of tubular joint fatigue durability has been examined, and the uncertainty models are developed to facilitate development of reliability based fatigue design and requalification criteria and fatigue inspection guidelines for tubular joints in offshore platforms. Introduction The development of the reliability based offshore platform design and requalification guidelines involves defining the scope (class of structure, load cases, etc.) and code format - including definition of characteristics values; selecting reliability measures; assessing uncertainties in loads and resistance; establishing target reliability levels; and determination of factors of safety, and load and resistance factors. Tubular joints are one of the main technical problems and high cost areas in the design, construction and maintenance of steel, template-type offshore structures. The uncertainty assessment of the tubular joints is therefore one of the most important components in the development of the reliability based design and requalification criteria for offshore platforms. The nature of uncertainties of the joint capacity and durability and methods for their modeling and analysis is a key in reliability based structural design of offshore platforms. Similarly, it is a key in development of realistic inspection and maintenance programs for these structures. In the context of development of criteria for design and requalification of platforms1 and floating facilities2 in the Bay of Campeche and development of inspection and maintenance programs for these facilities3,4, this study was intended to address a paradox that had developed from inspections of platforms in the Bay of Campeche.5 This paradox was that when inspections were made of joints in older platforms that were evaluated using detailed fatigue analysis methods6 to have fatigue lives that were in the range of months to years, no cracking could be detected. However, damage was found at other joints where it was not predicted or expected.5 This raised the question of: what was the confidence with the fatigue analyses? The objective of this paper is to summarize development of uncertainty models of the fatigue strength of tubular joints. Following a brief review of uncertainty of tubular joint fatigue durability, the sources of uncertainties in tubular joint fatigue durability are logically organized and quantitatively and qualitatively evaluated. Based on this evaluation, the uncertainty models are developed to facilitate development of reliability based fatigue design and requalification criteria, and guidelines for the inspection and maintenance of template-type offshore platforms.

This paper presents the design and analysis of an improved lightweight, bottom-founded, offshore platform for multi-purpose use in support of field development operations in water depths to 500 feet and possibly greater, and in severe as well as in mild marine environments. The concept, an innovation based on current tripod-type structures, utilizes a novel framing support system which results in an extremely efficient design with a broad range of applicability. The development of diverse applications for the concept and an assessment of its limits of applicability are discussed. This paper is of interest to the designer, the engineer, or others wh O are seeking an alternative, straightforward and cost effective approach to the design and installation of offshore platforms. The data presented is useful to those assessing the economic feasibility of an offshore field development. The paper discusses platform performance data, including member stresses, deflections, and natural periods of vibration, and offers criteria related to the environment, payload, and water depth for which the concept is best suited. Estimated platform weight and cost data versus water depth for various environments and development scenarios are presented and compared with data from more conventional platform concepts. It is demonstrated that the improved concept offers significant advantages over current designs. OBJECTIVE OF THE DESIGN This design concept was developed in response to the ever growing need for more cost effective solutions to the development of our offshore hydrocarbons reserves. The objective of the design was to develop an offshore structure that 1) is very lightweight, 2) has a relatively high deck load capacity, 3) is suitable for use in a large range of water depths, 4) is adaptable for the pre-drilling and completion of wells, 5) can be fabricated in a relatively short period of time, 6) can be installed with conventional lifting and/or drilling equipment, and 7) can be easily relocated for reuse at another offshore site where the water depth differs greatly from the first site. GENERAL DESCRIPTION The basic concept is a tripod structure as illustrated in Figures 1 to 3. The configuration shown in these figures is called a "Right Angle" configuration because it appears, when viewed from the side, as a 90° right angle. The right angle configuration offers greater access for the drilling of wells. In a "symmetrical" configuration the apex of the tripod is placed at the centroid of the triangular base. The symmetrical configuration is suitable for those applications that do not have wells. The principal component of the structure is the column. The column is designed as a separately installed element which is lifted or floated into place offshore in a single piece. It may contain risers, umbilicals, sumps, caissons, and other facilities as needed for offshore operations. The column is designed to support a cantilevered equipment deck.

The measured static and dynamic response characteristics of four jacket-type platforms in water depths up to 373 feet are described. Fundamental periods and damping were determined both by free vibration tests and spectral analysis of random response data. For each platform, measured end-on and broadside natural periods were nearly equal with values ranging from 1.5 to 1.9 seconds; the theoretically predicted natural period for one of these platforms was close to the measured value. Damping estimates range from approximately 1.0 to 5.0 percent of critical depending on the methods of testing and data analysis. For one platform spectra of both deck motions and wave heights are presented; measured amplitudes of response are compared to theoretical predictions. Motions predicted using regular Airy wave theory overestimate the measured motions for the tested platforms and the low sea states considered. INTRODUCTION Importance of Dynamics. In the design of offshore platforms, the possibility of dynamic response must be considered in the member force analysis. For platforms installed in active earthquake regions dynamic response analysis is mandatory. For regions where extreme waves and high cycle fatigue control the design, the need to consider dynamic response is not as clear. Estimates of dynamic response should first be made with simple models to determine if costly detailed analyses are required. Simple models are also quite powerful for examining a range of dynamic response conditions thus fostering a better understanding of dynamic platform behavior and allowing a more appropriate and efficient use of detailed analysis. In addition, with reference to the subject of this paper, the simple models indicate which parameters are important enough to be considered for experimental investigation. As a first approximation wave forces are often assumed to vary sinusoidally and the familiar steady-state response spectrum for single degree freedom (SDF) systems is used to estimate the importance of considering dynamic response (Figure 1). For this model and idealized force, dynamic amplification factor, AF, is dependent only on the perio9 ratio, R, and the system damping, S. Clearly, AF is large and very sensitive to damping for values of R near one (resonance). Using these curves to estimate the dynamic effect of a 12 second force on a four second structure (R =3.0), AF is seen to be about 1.12, i.e., maximum dynamic response is 1.12 times the maximum static response. For this period ratio AF is nearly independent of damping. Thus dynamic response to extreme waves is predicted to be small and insensitive to damping on the basis of this model. On the other hand, for the fatigue life calculation the waves of interest are much smaller with periods ranging from three to six seconds. For the same example four second platform, corresponding R values range from 0.75 to 1.5. In this region of the response spectrum AF is large and strongly dependent on damping. In fact, when damping is small, the AF at resonance can be expressed approximately as(Equation available in full paper)

A state-of-the-art review of the needs and problems involved in developing improved methods of inelastic dynamic analysis of offshore soi1-pi1estructure systems subjected to extreme earthquake shaking has been completed. It is concluded that dev.e10pment of a design-oriented computer code for three-dimensional, nonlinear dynamic analysis of fully coupled offshore soil-pi1e-structure systems is warranted. Complementary studies of several components of the system model are also required in order to develop user guidelines for parameter selection. The results of this review and a proposed course of action in terms of superstructure and substructure modeling strategy, computer code structure, and parameter selection are described. INTRODUCTION Recent years have seen an increase in the opportunities to pursue development of oil and gas resources in frontier areas which lie within seismically active regions of the world. Renewal of development activity in the Santa Barbara Channel and the recently held lease sales in the Gulf of Alaska and offshore Southern California and the 'East Coast of the United States have quickened the pace. These opportunities bring with them the need to consider earthquake ground motions in the process of criteria selection and design of offshore platforms. The overall philosophy which has developed in recent years to guide the selection of earthquake design criteria for many onshore structures involves a bi1eve1 approach. In principle, this philosophy prescribes a lower level of ground shaking which the structure should be designed to resist without significant damage. Although not always defined quantitatively, this lower level design requirement is intended to represent a condition of strong ground shaking that has a reasonable likelihood of occurring once during the life of the structure. A second higher level of ground shaking is prescribed, which the structure is to be designed to resist without catastrophic failure or loss of life. This higher level of ground motion is intended to represent a condition which has a low probability off being exceeded the life of the structure. As reflected by the current edition of the API-RP2A Guidelines, this bi1eve1 philosophy for defining earthquake design criteria has been adopted by the oil industry for use in offshore platform design. In the terminology used in RPZA, these two design requirements are referred to as "strength" and "ductility" levels, respectively. The implication of the "strength" level requirement is that this base or lower level of ground motion must be resisted in the elastic range. The use of the term "ductility" level underscores the desire to allow, and specifically provide for, controlled inelastic behavior in platform and foundation elements under the unlikely occurrence of severe shaking which is associated with the upper level design event. Although this desire to provide for inelastic behavior under severe levels of earthquake shaking is strongly motivated by the desire to combine cost efficiency with safety in design, there is also good engineering justification.

Offshore platform has long been in a complex and time-varying marine environment, would be inevitably influenced by environmental load and accidental loading, and result in the occurrence of various kinds of accidents or even disasters. The risk assessment for ocean platform has become an important issue in marine engineering field. However, the quantitative calculation of risks in the marine engineering industry is extremely difficult. The reason may attribute to the fact that there are few relevant statistical databases; and the mechanism of collision damage of marine structures is complex. The numerical simulation of impact caused by falling object is carried out by nonlinear finite element method (FEM). The PLS-RBF network is used for nonlinear interpolation purpose by combining the partial least square regression with the radial basis function (RBF) neural network. Therefore, the trained network can be implemented to obtain accurate collision results under various conditions by nonlinear interpolation. The feasibility and effectiveness of the proposed mapping approach for nonlinear finite analysis are testified by the simulation validation.

Description of the proposed paper This study falls within the context of a research project which aimed to draw up ergonomic guidelines for future offshore platform designs. The participation of ergonomists during the design process together with the other actors involved contributed to a greater understanding of the activities to be developed in the new areas. It is always possible to identify existing situations which when analyzed offer partial clarification of future activities connected to the space to be designed. Using this information in the planning of new designs enables the best adaptation of the design to its users. During the two year duration of the project, trips were made to operating platforms with the aim of observing and following activities, and interviewing workers in different sectors, which supported the guidelines drawn up for these areas. Application Ergonomics has been challenged to contribute from the beginning of the design process for the enrichment of the technical specifications. The objective is not only to enlarge information quantity, but mainly to ensure that relevant and substantial information will be available so as to designers be thoroughly knowledgeable about the real work conditions on the offshore platform and therefore the real demands for the design project. And, since time is paramount in this sort of design project, the earlier this information is available to the designers the higher will be the chance of programming the substantial characteristics to each environment, hence avoiding critical situations as the ones presently observed. Results and conclusions Ergonomic analyses were carried out on the accommodations module areas and positive and negative aspects were identified on the different platforms. On the basis of this analysis, guidelines were drawn up for the basic design of new platforms. This involved guidelines for the layout concept design, inter-connectivity amongst sectors and the flow of people and activities. Significance of subject matter Making ergonomics part of offshore platform designs enables the design process to be guided by the logic of the work activity. Although a standard accommodations module cannot be created, we saw the possibility of creating tools that aid the use of guidelines in a participatory process in new projects.

The structural analysis and design of the Heidrun tension leg platform presented many technical Challenge a that had to be overcome in a fast and efficient manner to meet the demands of a very tight schedule. Some are inherent in the nature of a compliant structure such as a TLP with its complicated interaction between the structure, the mooring system and the environment and the importance of proper weight and at ability control during the different phase a of construction and operation. However, many other a were unique to the Heidrun platform. The Heidrun hull is the first large offshore structure built entirely of high strength lightweight aggregate (LWA) concrete. The topside modules form an integral component of the structural system of the platform. The interface between the steel topside modules and the concrete hull proved to be a very complex engineering task requiring very detailed analyses, tight fabrication and installation tolerances as well as testing of a number of key components. Ringing loads were included in the structural design of the platform for the first time in an offshore structure. The transportation of the large topside modules, mounted transversely on barges, and their mating to the hull was unique. The reserve capacity built into the design to provide additional safety in the critical components of the structure. The paper presents an overview of the multitude of challenges encountered during the detail design of the platform. Introduction The Heidrun TLP introduced several structural innovations not seen previously on any offshore platform. Examples of these include the concrete Module Support Beams (MSBS) as a main structural member of the platform and for supporting the topside modules, unique solutions for connecting the topside modules to the substructure, exclusive use of high strength LWA concrete in the construction of the hull, design and fabrication of very large diameter columns, and design of the platform for the structural effects of ringing. This paper gives an overview of the structural analysis and design activities required in connection with the development of the Heidrun TLP and concentrates on design aspects which are novel or required more extensive consideration due to the nature of the TLP configuration or construction method. A short description of the platform is provided followed by the most significant aspects of thehydrodynamic and structural analyses. Special analyses and some of the unique design considerations associated with the hull and topsides design are presented. For completeness brief descriptions are given of the construction sequence of the hull, the foundations and the independent design verification performed for the structural design.

The VSC-HVDC technology has emerged as the preferred solution for large-scale transmission of offshore wind power generation due to its flexibility, controllability, adaptability, and environmental friendliness. As a core component of VSC-HVDC, the lightweight design of the offshore converter platform plays a crucial role in determining the economic viability of VSC-HVDC. However, high-voltage and large-capacity offshore converter platforms involve numerous internal components and complex spatial distributions of multiphysical fields, posing significant challenges to achieving compact and lightweight designs. This paper aims to deeply explore the distribution laws of multi-physical fields and their influencing factors of the offshore converter platform through multi-physical field simulation technology, provide theoretical support for research in related fields, and offer references for the future design of offshore converter platforms.

As the result of safety factors considered in load and resistant values for the design of new offshore platforms, a higher resistance compared to the 100 year return period environmental loading, is achieved in the structure. This type of design is usually performed using linear methods of structural analysis. In this case the structure practically has a high reserve capacity which is disregarded in the design phase but may be considered in special cases. One of these important cases is when an existing structure is assessed and evaluated after elapse of its structural life. In this case using the platforms reserve capacity, through a non-linear analysis such as push-over, would be logical and economic. In this paper initially the factors affecting a platform non-linear analysis and methods for considering non-linear characteristics of structures will be discussed. Then the results of an investigation on the behavior of three Persian Gulf platforms due to wave loading using the non-linear push over analysis method of USFOS software are presented. Finally, the results of these analyses are compared with the API RP2A’s assessment criteria [1].