Compliant Towers Research Articles

Interactive task learning via embodied corrective feedback

This paper addresses a task in Interactive Task Learning (Laird et al. IEEE Intell Syst 32:6–21, 2017). The agent must learn to build towers which are constrained by rules, and whenever the agent performs an action which violates a rule the teacher provides verbal corrective feedback: e.g. “No, red blocks should be on blue blocks”. The agent must learn to build rule compliant towers from these corrections and the context in which they were given. The agent is not only ignorant of the rules at the start of the learning process, but it also has a deficient domain model, which lacks the concepts in which the rules are expressed. Therefore an agent that takes advantage of the linguistic evidence must learn the denotations of neologisms and adapt its conceptualisation of the planning domain to incorporate those denotations. We show that by incorporating constraints on interpretation that are imposed by discourse coherence into the models for learning (Hobbs in On the coherence and structure of discourse, Stanford University, Stanford, 1985; Asher et al. in Logics of conversation, Cambridge University Press, Cambridge, 2003), an agent which utilizes linguistic evidence outperforms a strong baseline which does not.

Development of an equivalent static method for the approximation of the dynamic response of offshore structures

PurposeThe design of compliant towers in deep waters is greatly affected by their dynamic response to wave loads as well as by the geometrical and material nonlinearities that appear. In general, a nonlinear time history dynamic analysis is the most appropriate one to be applied to capture the exact response of the structure under wave loading. However, this type of analysis is complex and time-consuming. This paper aims to develop a simplified methodology, which can adequately approximate the maximum response yielded by a dynamic analysis by means of a static analysis.Design/methodology/approachVarious types of time history dynamic analysis are first applied on a detailed structural model, ranging from linear to fully nonlinear, that are used as reference solutions. In the sequel, a simplified analysis model is formulated, capable of reproducing the response of the entire structure with significantly reduced computational cost. In the next stage, this model is used to obtain the linear and nonlinear response spectra of the structure. Finally, these spectra are used to formulate a simplified design approach, based on equivalent static loads.FindingsThis simplified design approach produces good results in cases that the response is mainly governed by the first eigenmode, which is the case when compliant towers are considered.Originality/valueThe present paper borrows ideas from the area of earthquake engineering, where simplified methodologies can be used for the design of a certain class of structures. However, the development of a simplified methodology for the approximation of the dynamic behavior of offshore structures under wave loading is a much more complex problem, which, to the authors’ knowledge, has not been addressed till now.

On the consideration of wave and structural nonlinearities in the design of compliant towers through dynamic analysis

The purpose of this work is to highlight the importance of considering the wave and structural nonlinearities in the design through dynamic analysis of offshore platforms located in deep water. The paper studies the necessity of applying dynamic analysis in combination with the accurate calculation of the water particle kinematics. The example treated in this context is a compliant tower, set up in deep water. Nonlinearities are considered both in the calculation of the wave loading and in the structural analysis. The wave loading is calculated with the water particle kinematics drawn from a fully nonlinear 3D model, able to incorporate a spread of energy in the frequency spectrum and in the spatial directions. Comparisons are provided with the calculations associated with linear random wave theory and the steady wave theories of Airy and Stokes 5th. The unsteady calculations are based on the most probable shape of a large wave on a given sea state: the auto-correlation function of the underlying spectrum. In addition, the effect of directionality in the wave loading calculations is examined. The outcomes are quantified for all scenarios that are investigated. As far as the structural analysis is concerned, the effect of geometric nonlinearity and the effect of the nonlinear behavior of the foundation soil are examined. To this end, three cases are considered and compared. A geometrically linear dynamic analysis with linear soil properties, a geometrically linear dynamic analysis with nonlinear soil properties and a fully nonlinear dynamic analysis that takes into account both the geometrical nonlinearity and the nonlinear properties of the soil. The structural calculations are performed using the well-known structural analysis software SAP2000, enhanced by a special software developed to generate and directly apply the wave loading on the structural members. The results indicate that the consideration of the water particle kinematics associated with a fully nonlinear unsteady wave model for the evaluation of the wave loading leads to notable differences with respect to the linear, unsteady model. Significant are also the differences with respect to steady wave theories in terms of the structural response parameters that are examined. These differences are additionally affected by the applied directional spreading. Irrespectively of the adopted wave theory, the nonlinear dynamic analyses lead to significant discrepancies with respect to the linear ones. Ultimately, these types of structures respond dynamically under the effect of the wave loading. Therefore, the wave field energy distribution within the spectrum is of significant importance, as well as the choice of the corresponding period of the steady wave theory (Ttr−tr, TC, TP).

A scenario-based deepwater decommissioning forecast in the U.S. Gulf of Mexico

In the U.S. Gulf of Mexico, 21 structures have been decommissioned in water depth greater than 400 ft from 1989 to 2016, and circa 2016 deepwater inventories include 48 fixed platforms, three compliant towers, and 47 floating structures. All of the floaters were held by production circa January 2017, but 12 fixed platforms no longer produce and at least five of these structures have been converted to serve auxiliary roles as pipeline junctions. Several fixed platforms and a few floaters are long on their decline curve and are expected to be decommissioned within the next few years unless tieback opportunities materialize or an alternative use for the structure is found. The purpose of this paper is to present an integrated analytic framework to model structure decommissioning using a scenario-based approach. Using production models and cash flow analysis combined with scheduled removals for nonproducing structures, model results predict between 27 and 51 deepwater structures will be decommissioned through 2031 and between 12 and 25 removals are expected from 2017 to 2022.

Decommissioning cost estimation in the deepwater U.S. Gulf of Mexico – Fixed platforms and compliant towers

Decommissioning is the final stage in the life cycle of an offshore structure, where all wells are plugged and abandoned, the platform and associated facilities are removed, and the seafloor cleared of all obstructions created by the operations. From 1989 to 2012, 15 structures in water depth greater than 400 ft were decommissioned in the U.S. Gulf of Mexico, but none of the project cost have been publicly released. The purpose of this paper is to apply work decomposition algorithms developed by ProServ Offshore to estimate cost for well plugging and abandonment, conductor severance and removal, pipeline abandonment, umbilical and flowline removal, and platform removal for the 53 deepwater fixed platforms and compliant towers in the Gulf of Mexico circa January 2013. Decommissioning cost estimates are presented by stage and operator. Bullwinkle and Pompano are expected to be the most expensive fixed platform decommissioning projects in the Gulf of Mexico estimated at $265 million and $203 million, respectively. Total undiscounted decommissioning liability for the asset class is estimated to be $2.4 billion.

The Conquest of the Inner Space - Challenges and Innovations in Offshore Technology

Spectacular missions to the moon, the Mars and to other planets of our solar system has boosted public attention to the outer space for the last five decades. During the same period mankind has also conquered the inner space - with fantastic innovations and fabulous inventions - however, hidden under the sea surface, and hence widely unnoticed. While searching for new promising oil and gas fields the offshore industry operates in water depths that have never been explored before. Production systems already reach down to 2000 m, and will probably conquer greater water depths during the next years. Extreme environmental conditions in addition to these enormous water depths call for special solutions. This paper deals with recent developments of offshore deepwater production technology, e.g. SPARs, TLPs, Compliant Towers and huge mono-buoys. After a short survey of the development in ocean engineering from the first shallow water platforms in the Gulf of Mexico in 1947 to today’s deepwater production systems off the Brasilian coast the paper summarizes the principles of platform dynamics in waves to illustrate the advantages of specific design characteristics.

Quickwave: An Impulse Algorithm to Calculate Preliminary Design Shear and Moment Envelopes for Compliant Towers

Abstract A novel, simple method to calculate compliant tower (CT) level shear and moment envelopes for preliminary design has been developed, and verified by comparing with rigorous 3-D tower analyses. The approach relies on a vast experience base to define important features influencing the dynamic response of CTs, and a new closed-form solution for the acceleration (needed to construct design envelopes) of the tower caused by an impulsive-type wave load. The main benefits of the approach are: 1) the designer can iterate and quickly converge on a working preliminary design without resorting to time-consuming computer analyses; 2) the designer can quickly evaluate configurations for different water depths, pile arrangements, payload, steel grade, reserve buoyancy, and well counts.

Evaluating Deepwater Development Concepts

Summary This paper provides an overview of the current state of deepwater technology. It discusses the major factors that must be considered when evaluating potential deepwater development concepts, presents proven deepwater development concepts, and provides insight into the applicability of each. Two unproven "dry tree" concepts are presented to illustrate proposed new concepts. Approximate costs are included, leading up to general conclusions with respect to the evaluation process. The focus is primarily on the Gulf of Mexico. The Deepwater Market The incentives for developing deepwater fields are clear. World oil demand is expected to rise annually by an average of 890,000 B/D between 1993 and 1995. The U.S. has a serious balance of payments problem that is aggravated by the fact that it is importing more than half of its oil. The east and west coasts of the U.S., plus the section of the Gulf of Mexico east of Alabama, are restricted from drilling. Total deepwater discovered and potential reserves in the Gulf of Mexico are estimated to be 2 to 4 billion bbl and 10 to 12 billion bbl, respectively. More than 20 undeveloped discoveries exist in the Gulf of Mexico in water depths ranging from 2,400 to 7,520 ft. Development Options and Critical Issues Development Options. Deepwater fields can sometimes be developed with subsea tieback techniques without requiring a deepwater platform at the field itself. If a deepwater platform at the field is required, a wide choice of type exists. The list of proven and unproven (but highly developed) deepwater platform concepts includes (1) steel and concrete tension leg platforms (TLP's); (2) semi- and monohull, steel and concrete floating production systems (FPS's); (3) steel and concrete spars; (4) concrete deep draft floaters (DDF's); and (5) compliant towers (primarily steel). Some critical issues to consider when selecting concepts to include in a detailed evaluation study will be presented before describing the various deepwater development concepts and discussing the pros and cons of each. The pros and cons of the use of DDF's will not be included because the 45-ft draft limitation along the U.S. gulf coast will probably prohibit their use unless and until suitable techniques for mating an integrated deck to an already-moored hull are developed.

Low-Frequency Forces on Tubular Spaceframe Towers: Analysis of Cognac Data

Abstract The Cognac field data indicate that the low-frequency force response components are not negligible and are significantly correlated to the wave envelope process. Although this is a well-known phenomenon in floating structures, it had previously not been validated from data recorded on fixed platforms. Simulation studies based on a vertical rigid cylinder subjected to Hurricane Frederic sea states show significant differences in applied low-frequency drag forces obtained using the Wheeler and Delta stretching schemes used to approximate near-surface fluid kinematics. Previous studies focusing on the peak hydrodynamic forces, as used for the design of fixed platforms, have shown a significant difference between predictions based on Wheeler and Delta stretching. The present study reveals that the Cognac field data could be used to discriminate between the Wheeler and Delta stretching schemes in terms of the low-frequency forces, which are potentially important in the design of compliant towers. However, such a discriminatory study would require detailed structural and hydrodynamic modeling of the Cognac tower.

Comparative Risk Assessment of Alternative TLP Systems: Structure and Foundation Aspects

Abstract Offshore operators can choose from a wide variety of alternative production systems including those based on fixed platforms, tension leg platforms (TLPs), compliant towers, floating production systems, and remote subsea systems. These systems involve varying degrees of technical innovation and technological risk. Life cycle costs, including initial costs and operating costs over the life of the project, need to be carefully considered to define systems that will minimize cost and maximize return on investment. The life cycle includes all stages of the project, including design, construction, operation, and decommissioning. To address decision-making issues in the selection of alternative systems, eighteen sponsoring organizations under the leadership of Amoco helped define the Methodologies for Comparison of Alternative Production Systems (MCAPS) project. Conducted during a two-year period (1988-1990), this project developed and illustrated an engineering procedure to assist the process of making rational comparisons among design alternatives for offshore production systems. The example developed to illustrate application of MCAPS was a TLP concept designed for Gulf of Mexico conditions. Four TLP alternatives were analyzed and compared; these involved three production riser alternatives and two tendon system alternatives. This paper addresses the structural and foundation aspects of the two tendon system alternatives. A companion paper (Stahl et al., 1991) addresses the production riser alternative, the alternative system economic evaluations, and the comparison processes. The structure aspects addressed in this paper include both first-phase coarse qualitative and quantitative evaluations, and second-phase detailed quantitative evaluations. The detailed quantitative evaluation is illustrated with structural damage in extreme conditions, including potential overloading of the hull, tendons, and the foundation piles. The paper concludes that MCAPS is not a simple process. It is an intensive interdisciplinary process requiring good teamwork and extensive familiarity with the system being analyzed. The judgmental elements of the process are considerable. The potential benefits of the process include: an improved understanding, qualitatively and quantitatively of risk mechanisms; identification and mitigation of hazards; improved cost-effectiveness of designs; and improved safety. The specific risk numbers generated are less important than the design improvement insights that are developed; these numbers can only serve as a guide to focus attention on those aspects of the project which generate the most risk and to indicate where cost-effective risk reduction measures can be implemented.

Random Extreme Wave Analysis of Deepwater Structures

Abstract This paper reviews various wave analysis procedures for designing deepwater structures under an extreme seastate. The random wave analysis procedures suitable for fixed stiff platforms and compliant towers are discussed. The random wave analysis procedures are then applied to a 1350-ft water depth fixed platform. The reduction in design force levels due to random waves is indicated by comparing with the conventional regular wave analysis approach. The second harmonic effects due to waves can be easily identified through the dynamic response spectrum which has two peaks occurring at the peak frequency of the input wave spectrum and the natural frequency of the structure. The study also shows that the expected extreme value estimated based on the upcrossing approach agrees well with the snapshot peak response derived from a wave record containing an extreme wave height.

Strategies for Fatigue Design of Deepwater Platforms

After reviewing strategies used for designing the 1000-ft Cognac platform, this paper will describe recent developments, which have been applied to preliminary designs of fixed platforms in up to 1600 ft of water, and to compliant towers in 2000 ft. The first is a strategy in which the waterline geometry and tower dimensions are coordinated so as to minimize wave forces at the natural frequency. This was first suggested by John Nath in 1967, and has come to be known as the “Nath hole” strategy [1]. The second is rationalization of allowable hot spot stress in terms of the Weibull shape parameter, “squiggly.” For structures which have been optimized as described in the foregoing, the trend of fatigue versus natural period can be much less severe than described by the author at the BOSS-82 conference [2].

Optimal Cable Configurations for Passive Dynamic Control of Compliant Towers

Whether in storm or normal seas, the ideal offshore cable-stayed compliant tower moves in harmony with the wind and waves. A properly tuned cable configuration is the key to controlled dynamic response. Formulated here are a nonlinear dynamic model involving wind, wave, current, tower, and cable interactions and a cable optimization algorithm. The objective function, the rms tower rotation, is minimized subject to appropriate constraints involving compatible system geometries and loads, as well as bounds on the platform level accelerations needed for human comfort. Tower motion is limited to a plane.