This is the first of four companion papers that present a comprehensive assessment of the effect of sea floor subsidence on the Valhall complex of platforms. The study has included an estimate of the increase in platform failure probability as a function of increased subsidence. Subsidence raises the effective mean still water level and increases the potential for inundation of the deck for extreme storm conditions. Deck wave slam forces generate significantly greater platform loading and lead to: (a) higher levels of structural inelastic response and increased risk of structural failure as well as (b) water reaching the cellar deck, and hence affecting operators as well as equipment. The paper focuses on addressing the first of these two issues. A structural assessment study was performed to address the significance of present and future levels of subsidence on the safety of three North Sea platforms. The study included a systematic assessment procedure that addressed each of the factors that impacted structural integrity issues and reliability concerns. Such factors included: ultimate strength analysis methodologies, tubular joint formulations, group pile effects, and soil-structure interaction, which are described in this paper, as well as deck impact force formulations (Pawsey et al., 1998, “Characterization of Environmental Loads on Subsiding Offshore Platforms,” 17th International Conference on Offshore Mechanics and Arctic Engineering, Lisbon, Portugal, July) component and system reliability modeling (Jha et al., 2000, “Assessment of Offshore Platforms Under Subsidence—Part II: Analysis and Results,” ASME J. Offshore Mech. Arct. Eng., 122, pp. 267–273), and acceptance criteria (Stahl et al., 1998, “Acceptance Criteria for Offshore Platforms,” 17th International Conference on Offshore Mechanics and Arctic Engineering, Lisbon, Portugal, July). This paper presents the assessment procedure, as well as the modeling approach. The paper also discusses the consequence classification of the three platforms and state-of-the-art soil mechanics techniques that lead to a significant increase in the tensile capacity of the foundation. [S0892-7219(00)00204-1]

A platform complex in the North Sea is used to illustrate development of reliability acceptance criteria. For platforms in the Norwegian sector of the North Sea, reliability acceptance criteria have their basis in progressive collapse limit state requirements and impairment/FAR (fatal accident rate) limits. A number of parallel supporting approaches to developing reliability acceptance are presented. These supporting approaches include economic risk assessment, in particular, the ALARP principle (as low as reasonably practicable), and historically accepted reliability levels associated with prior practices and experiences in various offshore operating arenas. The reliability acceptance criteria are found to be consistent with recommended or implied acceptance criteria in offshore design and assessment practices in other parts of the world. [S0892-7219(00)01003-7]

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Design of steel offshore structures has traditionally been based on elastic analysis to determine the distribution of forces through the structure, for an envelope of design cases. Checks are then performed on a component basis to ensure that no element of the structure fails to meet the governing criteria. Uncertainties in estimating the design loads and component strengths are allowed for by the application of safety factors. The analyses undertaken are linear and take no account of the reserve strength, and the robustness of the system. This paper discusses how the use of X-bracing leads to more robust designs that are: lighter, less expensive to fabricate and easierto install. In this paper we will use the results of previous studies, [1,2] and a number of pushover analysis on two, third generation (1990's), central North Sea platforms, to show that:The .....use of X-bracing..." [3] increases structural robustness in the jacket, without sacrificing cost, or constructability.Omitting the horizontal members on the vertical framing will not compromise the robustness of an X-braced jacket, subjected to hydrodynamic loading, if adequately designed X-braced horizontals are used. Introduction The main aim of structural design and fitness for purpose assessment is to ensure that a structure adequately fulfills its requirements with respect to serviceability and safety. Most offshore jacket structures possess an inherent reserve strength that is greater than the strength of the critical components. This is derived from a variety of sources, the principal one being the nonlinear structural interaction between components through plastic deformation and load redistribution, which in redundant systems allows mobilization of alternative load paths. Some structures display considerable levels of reserve strength whereas others suffer from a sudden drop in capacity as soon as one critical member fails. The reduction in ultimate capacity of a jacket structure due to damage to individual members can be measured by the so-called Damage Strength Ratio (DSR). This is effectively identical to the Reserve Strength Ratio (RSR), but is calculated with specific member(s) removed. DSR analyses have been used to prioritize inspection and also to check the strength or reliability implications of damage events. As the industry moves towards system based design it is important during the design cycle that the reliability of different framing configurations be assessed. In this paper we examine the results of several pushover analyses performed for two platform designs which were selected to have similar water depth, deck payload, hydrodynamic loading and vertical framing but different horizontal framing patterns. The discussion will focus on the advantages of considering system behavior, and robustness at the conceptual design level without compromising steel weight, fabrication and inspection costs. Consideration will also be given to the effects of structural robustness on inspection/repair philosophy, and changes in operational requirements during the design life. Definitions Redundancy Fixed offshore structures are typically statically indeterminate, When the number of unknown forces and/or moments in a structure exceeds that which can be determined by statics, the structure is said to be indeterminate. The forces and/or moments that cannot be determined using statics are said to be superfluous or redundant.

An assessment of three offshore platforms in the Valhall field of the North Sea is described in this paper. Ultimate strength and reliability analyses were performed for the jacket-type platforms for four levels of sea floor subsidence. The nonlinear structural models of the platform elements, joints, and associated soil-structure interaction are summarized in a companion paper by Gebara et al., 2000, “Assessment of Offshore Platforms Under Subsidence—Part I: Approach,” ASME J. Offshore Mech. Arct. Eng., 122, pp. 260–266. Similarly, environmental loads and reliability acceptance criteria are presented by Pawsey et al., 1998, “Characterization of Environmental Loads on Subsiding Offshore Platforms,” OMAE98, Lisbon, Portugal, and Stahl et al., 1998, “Acceptance Criteria for Offshore Platforms,” OMAE98, Lisbon, Portugal, respectively. This paper describes the procedures employed and results obtained for ultimate strength and reliability analysis. Ultimate platform strength was calculated using a pushover analysis in which environmental loads for increasingly rare events were applied incrementally until the platform failed. Failure was defined as formation of a limiting mechanism in the platform structure or foundation. Structural reliability analysis was performed to estimate the annual probability of platform failure as a function of subsidence level. The platform structural system was modeled in terms of two major subsystems that were assessed as likely to fail: (i) the platform deck, and (ii) the platform jacket and piles. Uncertainties were included in the loads, structural components, and soil strengths based on industry experience and data. First and second-order reliability methods (FORM/SORM) were used to estimate the annual failure probabilities. Consideration was given to the correlated nature of the failure modes due to the common environmental loads. The reliability results were compared with predefined acceptance criteria to judge the suitability of continued operations. [S0892-7219(00)00304-6]