Two instrumental series of events in the past several years have provided the impetus for API Subcommittee 2 (SC 2) to action a new strategy for the API Series 2 standards. Beginning in the early 1990's, with the formation of ISO TC 67 / SC 7 and subsequent development of the ISO 19900 series of standards for offshore structures, API started to map a long term strategy for the API documents, while actively participating in the preparation of the ISO suite. As a part of the Gulf of Mexico infrastructure response to the hurricanes Ivan, Katrina and Rita, API chartered the Hurricane Evaluation and Assessment Team (HEAT) to assess adequacy of existing standards and provide recommendations for modifying provisions in the standards. While aggressively pursuing in 2006 and 2007 the publication of six documents related to hurricane issues, and in consideration of the progressive publication of the ISO 19900 series documents. API SC 2 leadership developed a strategy to restructure the content of its standards to align more closely with the ISO document portfolio subdivided in general standards and the structural form standards, and providing thus a more straight forward approach in making updates to the technical provisions impacting multiple standards. The ISO Offshore Structures suite of standards consists of more than a dozen documents organized in two categories, first, guidance and requirements for technical disciplines common to more than one functional concept and secondly, guidance and requirements for the various concepts. With more than half the standards now published and nearly 80% to be published by 2009, baseline practices for many offshore structural facilities will utilize these standards in the near future. In fact, following the publication of the ISO standards, many regions of the world are expected to adopt them as a basis for offshore platform design. This paper outlines the plan to be implemented over the next few years concerning the restructuring sequence, alignment and merging of the ISO standards and API standards, and highlights benefits and challenges of this strategy. Introduction The development of offshore structures standards is an interesting and complex blend of pushing new technical boundaries, reacting to the consequences of some significant natural events and aligning with economic and safety initiatives. In the case of the offshore structure standards, some of the work has piggy-backed upon the more general standards initiatives while other activities are more specifically related to offshore conditions, e.g. damage due to hurricanes passing over a number of offshore facilities. Standards have often become a focal point of interest following a defining event, such as Hurricane Hilda in the 1960's, or in conjunction with the time when a product or industry has headed toward commodity status, as illustrated by the initiation of the API standards activities in the 1920's. In the past half-century, standards have often been driven, either directly or indirectly, by legislative or regulatory directives. The offshore structures standards were event driven in the beginning and both event and legislative/economic driven today. The path to having a single set of globally applicable standards is an outgrowth of the political and legislative process establishing the European Union. This " event?? drove what were initially regional standards with some global applicability to collaboratively bringing regional efforts into a single coherent direction. Though regional activity still exists today in the European Union, much of it is coordinated to produce a single set of standards.
A method of analysis is presented to predict the nonlinear dynamic behavior of compliant offshore structures, in particular guyed tower platforms. The structural model is analyzed in time domain using the normal mode superposition approach. Nonlinearities due to fluid-structure interaction are considered. The nonlinear stiffness of the mooring system is included by suitable modification of the forcing function. Secondary overturning effects due to large deflection are introduced by equivalent lateral forces at each mass point. In analogy to the treatment of the nonlinear mooring stiffness, the nonlinearity of the soil-pile foundation can be accounted for by applying external forces and moments. By way of example, the results of dynamic analyses performed on a guyed tower designed for the North Sea are included.
As the offshore industry moves into deeper water, the dynamic behavior of structures becomes a very important parameter in the overall design procedures. In particular, the cyclic nature of wave loads has a significant effect on the fatigue life of the structure. This paper presents a procedure for fatigue analysis where the dynamic response of the structure is analyzed through a spectral approach. The sea waves which constitute the forcing function acting on the structure are represented as energy spectra; the response is obtained in spectral terms and is subsequently interpreted according to probabilistic concepts.As part of.this study we have considered the characteristics of the distribution of the response. In the usual approach to fatigue analysis one assumes that the peaks of the response follow a Rayleigh distribution. This assumption is valid if the Cartwright-Longuet-Higgins' measure of bandwidth (E) is equal to zero. However, in practical fatigue applications, this idealization is rarely, if ever, encountered. Thus, in order to better characterize narrow banded phenomena, we have investigated descriptions which do not assume E = o. An investigation of caisson designs indicates that fatigue results using the more accurate response distributions can differ appreciably from those obtained with the Rayleigh distribution.
A strategic approach towards the desigrn of a newly built semi submersible, or the redesign and selection of an existing vessel is presented. The paper also addresses the practical implementation of this design strategy into a computational tool based on large scale parametric and optimization analyses in a logical sequence, in which a number of knowledge-based rules derived from operational constraints, design experience, codes of practice and other design considerations are utilized and enforced. Application of this strategy to a specific case shows how, by choosing suitable controlling parameters matching environmental constraints and field requirements, an appropriate design may be obtained. INTRODUCTION Market Overview At present there are well over 170 semi submersible vessels in the world market [1]. Widely used as drilling, construction and/or support vessels, they have also been successfully used as floating production system (FPS) since 1975, when the Transworld 58 was deployed in the Argyll field. As deeper water and marginal fields are being developed, their employment as production vessels IS becoming more attractive both technically and economically. In the North sea, the Balmoral [2] and the Ivanhoe/Rob Roy [3] fields have both been recently developed successfully by semi submersible FPS, while the Emerald field FPS is at the time of writing being converted from a drilling vessel [4]. Elsewhere, taking advantage of the short development time and market availability, around ten drilling units have already been converted and employed as production vessels since 1977 in the Campos Basin [5]. As far as drilling and support vessels are concerned, due to the increasing level of offshore activities and to the operational superiority of semi submersibles over other vessel types [6], an increase 10 the number of new vessels being built may also be expected in the future. The Design Spiral In the initial design of a new semi submersible or the redesign of an existing vessel for new operational circumstances, a vast variety of choices are open to the designer. Given the constraints imposed by codes of practice, operational requirements and economics, a wide range of design parameters such as hydrostatic stability, hydrodynamic response, allowable payload and cost must be considered, and rational decisions must be made in order to reach the final design. Within the designs or redesign exercise, the existence of inter-relationships between both constraints and characteristics of different natures makes it inevitable that a final design can only be approached iteratively. A typical top level design spiral with particular attention paid to technical issues is shown in figure 1. While different routes exist to reach the :preliminary design, and while it is possible to perform sensitivity analyses on different design parameters by making use of conventional design tools, difficulties frequently arise when large amounts of costly information are available but are found difficult to digest and interpret. The lack of succinct information, suitable tools and experience can all slow down the convergence of the design significantly and give rise to unnecessary development expense and time wastage.
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