8th World Seminar on Seismic Isolation, Energy Dissipation and Active Vibration Control of Structure

The Project INDEPTH (Development of INnovative DEvices for Seismic Protection of PeTrocHemical Facilities), supported by the Environment and Sustainable Development Programme of the European Commission Research Directorate General (Contract EVG1-CT2002-00065), has the objective to develop and apply innovative seismic isolation and/or dissipation devices for critical structures at petrochemical facilities, such as cylindrical/spherical tanks, thereby reducing the seismic risk at such facilities in highly-seismic areas, where a limited number of such applications exists, apart very few in LNG tanks. It is known, however, that a seismically isolated structure subjected to an earthquake can experience large displacements, potentially overstressing the attached piping. Moreover, isolation systems are generally designed for given value of supported mass and their performance can be non-optimal when the mass is varying, as in case of tanks, where the level of liquid frequently changes during day-to-day operations. In the framework of the Project, new devices are developing to solve specific issues such as the lack of isolation systems, where performance is independent of the mass associated with the liquid level inside tanks, the lack of low-cost, light-weight new isolators and the “compensation” for high displacements associated with an isolated structure in order to avoid piping overstress. The paper describes the general problem of seismic risk at petrochemical facilities, the approach adopted for the development of INDEPTH Project, the first results (selected structures, seismic hazard, numerical models developed) and the expected achievements. The Project started in Sept., 1st, 2002 , has an overall duration of three years and, at the moment of the Seminar, is at the end of its 1st Year. 1. PROBLEM AND ITS SIGNIFICANCE It is well known that some parts of Europe are in areas of high seismic risk, and within this area there are numerous refineries and other petrochemical/industrial facilities. The motivation for the project is to increase the seismic performance reliability of such facilities, both existing and future, in the face of high earthquake risk, through the development of new and innovative devices and techniques. Concern for society, quality of life of surrounding communities and impact on the environment, forces policy makers to assess the risk of hazardous material releases from refineries and petrochemical facilities during earthquakes. In fact, a major seismic event in an industrialised area could damage (and has damaged in the past) process equipment, storage facilities and transfer (or lifeline) systems. The start of fires and the release of airborne toxic gases and/or volatile liquids may obstruct post-earthquake rescue operations and have major social and economic consequences. An example in case is the initiation of multiple fires at the Tupras oil refinery and loss of production after the Izmit earthquake in Turkey [1, 2, 3, 4]. The concerns relevant to the abovementioned highlighted problems are relevant to the increment of seismic reliability of existing and future facilities, the assessment of risk (i.e. casualties, releases of hazardous materials, damage to structures, equipment and facilities) and the support to post-event rescue operations (risk of fires, release of toxic fluids, etc.). INDEPTH Project is mainly focussed on the first item, i.e. to increment the seismic reliability of existing and future facilities, through the development of innovatives anti-seismic devices. 2. PROPOSED APPROACH The approach of the Project to the problem is articulated on the following main topics: • selection of critical structures from walkthroughs performed at Aspropyrgos refinery, Greece and Huelva LNG facility, Spain; selection of main design parameters of the devices • definition of site-specific and generic seismic hazard • design and manufacturing of the devices • numerical analyses to confirm the design parameters vs. the expected dynamic behaviour of the devices, identification of specific fluid-soil-structure-interaction (FSSI) problems and experimental validation through shaking table tests • quantification of technical/economical/safety benefits with respect to the conventional stateof-the-art measures presently adopted and potential application to retrofitting. A selection of the most vulnerable structures found in petrochemical facilities has been made on the basis of the experience of the team. These structures are LNG and critical product storage tanks (such as firewater tanks), spherical storage vessels (often containing ammonia or LPG). For each of these components, an optimal solution (in terms of technical effectiveness and economic feasibility) capable of reducing the seismic vulnerability and enhancing the performance reliability is to be developed in this project. In particular, for each structure, a seismic isolation and/or energy dissipation system will be selected as an alternative (and an improvement) over the more traditional seismic design or retrofit concepts. The project objectives will be pursued by: • Development of new concept seismic isolators and flexible joints for accompanying interconnected piping, conceived by taking into account the specific needs of variable-mass cylindrical tanks/spheres, recognized as some of the most critical components (for their potentially dangerous contents and by their inherent seismic vulnerability) in a petrochemical facility. • Use of updated state-of-the-art analytical techniques to compute and measure the effects of fluid-soil-structure interaction (FSSI) on the seismic behaviour of structures in both their existing configuration and with the proposed new isolation/energy dissipating devices, subject to the appropriate performance requirements. • Quantification of the technical and economic benefits of such devices, including consideration of the need for protection from fire and chemical attack due to the highly aggressive and corrosive operating environments. • Preparation of guidelines for selection of proper isolation and/or energy dissipating devices. The active participation of end-users in the formulation of the main objectives assures that the scientific/technological goals will be relevant to industry requirements and focus on problemsolving research. Furthermore, to ensure that real operational issues associated with actual industrial plants are addressed, candidate structures will be determined following a limited vulnerability screening of selected portions of the Aspropyrgos Refinery and the Huelva LNG facility, located in highly seismic areas of Greece and Spain, respectively, following input from facility personnel. In the framework of the Project, new devices are developing to solve specific issues such as the lack of isolation systems, where performance is independent of the mass associated with the liquid level inside tanks, the lack of low-cost, light-weight new isolators and the “compensation” for high displacements associated with an isolated structure in order to avoid piping overstress. 3. THE SELECTED STRUCTURES Three types of structures found in petrochemical facilities have been selected as examples for the application of isolation devices in this project: • Above grade LNG storage tanks. • Above grade vertical storage tanks containing liquid product or firewater. • Spheres containing pressurised (and possibly refrigerated) liquid product. Each of these structures represents a different set of economic and technical challenges when applying base isolation devices. Large LNG storage tanks of the order of 160,000 m3 volume are found in LNG export liquefaction and import regasification facilities (the latter being more relevant to risk of seismic loads in Europe due to demand for gas). The LNG tank stores natural gas in a liquefied state at a temperature of -168°C, which represents a considerable quantity of stored chemical energy that would be released should a failure due to seiami event result in the liquefied gas coming in contact with atmospheric oxygen, with a significant risks to the surrounding population, should a containment failure occur because of an earthquake. LNG tank designs are, for the most part, designed as above ground structures with concentric steel inner (containing the liquefied gas) and outer reinforced/prestressed concrete shells. Design of the tank to withstand seismic loads presents specific issues, particularly anchorage of the inner tank to the outer tank structure. At present, there are four sites with base-isolated LNG tanks. In all cases, the use of isolation units was expensive and had significant impact on construction schedules. The challenges therefore for applying base isolation to LNG tanks within the INDEPTH project is to develop an economic isolator unit that can be used for the expected range of tank sizes in Europe, that can be installed with minimum impact on construction schedule, and that can accommodate the unique variability of the mass of stored product (from full to near empty). Conventional product storage (containing crude oil, ethylene, benzene and other aromatics) and firewater tanks containing liquid, up to over 100,000 m3 in volume, are the most common type of storage unit in petrochemical facilities. Containment of these products (harmfully to the environment and extremely flammable)during a seismic event is therefore desirable. Their typical construction costs are relatively low (unless anchorage is required) and consequently, base isolators would need to be economic to be viable. For anchored tanks, the cost of isolators can be offset against the cost to include anchorage in the design, and the cost of modifications to the tank foundation system. It is in this application that the INDEPTH project will focus on (economic) fibre reinforced rubber bearing isolators.