Challenges for a Total System Analysis on Deepwater Floating Systems

In the design of floating systems, three major system components need special attention: the floater, the mooring system, and the riser system. This paper will focus on one the most unique areas in the structural design of these components, namely, the fluid-induced effects on floating structure systems and the associated structural response. Due to the rapid growth in the offshore field, particularly in deep waters, this analysis is seeing a phenomenological growth, and considerable research is ongoing in this area, with steady advancement in the design procedure. The state-of-the-art in the treatment of the individual components of the floating structure, namely, the floater, the mooring system, and the riser system will be briefly described. The importance of their interactive coupling effects with fluid is the special subject that will be emphasized. In particular, the ‘Total System Analysis’ of the floating system consisting of all three of its dynamic components will be laid out. A possible systematic approach for the complete system and various simplifications available for an efficient practical solution will be elaborated. The paper will conclude with a discussion of the present-day deep water design challenges that remain and the research that is needed to meet these challenges. INTRODUCTION OF OFFSHORE STRUCTURES The design life span of offshore structures ranges from a few years to as many as 25 years based on their applications. Often their useful life extends beyond their design life, and they remain operational longer. In most instances, the floating offshore structures are required to stay in position in all weather conditions. Offshore structures [1] are defined by either their function or their configuration. The functions of an offshore structure may be one of the following (even though multiple functions may be possible for a structure): Exploratory Drilling Structures: A Mobile Offshore Drilling Unit [MODU] configuration is largely determined by the variable deck payload and transit speed requirements. Production Structures: A production unit can have several functions, e.g. processing, drilling, workover, accommodation, oil storage and riser support. Storage Structures: Used in storing the crude oil temporarily at the offshore site before its transportation to the shore for processing. The configuration of offshore structures may be classified by whether the structure is a fixed structure, either piled or gravity, a compliant or articulated structure, or a floating structure. The requirements of a floating structure are that it be moored in place and that, subject to the environment, the floater remains within a specified circle of operation from a desired mean location, which is generally achieved by mooring lines (or a dynamic positioning system). FLOATING OFFSHORE SYSTEM Floating offshore system [1] illustrated in Fig. (1) consists of three principal structural components: *Address correspondence to this author at the Offshore Structure Analysis, Inc.13613 Capista Drive, Plainfield, IL 60544, USA; [email protected] Floating hull: providing the space for the operation of the production work, and the space for storage of supplies, Mooring system: providing a connection between the structure and the seafloor for the purposes of securing the structure (generally called station-keeping) against environmental loads, and Risers – achieving drilling operation or product transport. The station-keeping may also be achieved by a dynamic positioning system solely using thrusters, or in combination with mooring lines. The dynamic positioning, however, will not be part of this paper. The mooring lines provide the restoring force to the floater. Fig. (1). Floating offshore systems. TOTAL SYSTEM ANALYSIS It appears from the research done to date on the subject of coupled system and the results presented here that, for some systems, the complete coupled system analysis is important, whereas it does not provide appreciably different responses for other floating systems. However, whether such analysis is warranted for a particular system cannot be easily determined a priori at the present time with the present state of knowledge about the systems. More comprehensive reChallenges for a Total System Analysis The Open Mechanics Journal, 2008, Volume 2 29 search into such analysis is required to generate such guidelines. Therefore, further systematic research should be made to answer the lingering question regarding the importance of the coupled system analysis [2-20]. The goal should be to generate a region of applications chart for various systems that can be used by the designer as a guide to determine the level of sophistication required in the analysis of a given floating system. Fig. (2). Sequential approximation from a Total System Analysis (TSA) to a more practical method. The ‘total system analysis’ (TSA) is defined here as the analysis of the complete floating system, in which no empirical simplifications are made regarding the loading or the responses of the system components. Thus, the system considers the fluid structure interaction of the entire system, including the coupling among them concurrently. Ideally, it means that both the fluid loading and structure response will include the entire fluid-floater-mooring-riser interaction, (with the incorporation of any soil interaction, as needed) without an ad hoc empiricism. In other words, the fluid flow past the system is represented in a computational fluid dynamics (CFD), while the complete structure system is described in a Finite Element (FE) representation. However, while it is generally possible to incorporate such a complete model in software with the current state of knowledge, in practice, it becomes an insurmountable problem and a prohibitively time-consuming task to accomplish under the present hardware environment. Therefore, the complete analysis must be simplified to achieve a practical limit, but in a logical step with fewer empiricism. These simplifications are proposed here, which may be achieved in a hierarchical sequence, as illustrated in Fig. (2). The sequence is shown starting with the most complete analysis and ending with the simplest and most used analysis adopted today in the design of an offshore system. Thus, any intermediate step in the figure may be considered a refinement in the analysis. Of course, one can come up with a variation of the proposed intermediate steps. But the proposed ones appear logical. In the complete analysis, the system includes the floater, and all the mooring lines and risers. The fluid field past this complete system is described in a CFD code and the mutual interaction of all the components with the fluid is solved in terms of Navier Stokes equation, in which the floater is represented as a rigid body, but the mooring lines and risers are input in a structural FE code. In the next simplification (step 2), the fluid loading on the floater is determined by the more complete numerical wave tank analysis (NWT, to be elaborated later). In this case, the mooring lines and risers are individually analyzed by a 3-D FE analysis, and iteration is performed to achieve convergence. In step 3 of simplification, the floater NWT is approximated before coupling with the mooring/riser finite element (FE) analysis. If further simplification is needed, then the simple 3-D diffraction theory may be used for the floater (step 4). For simplification step 5, instead of full 3-D approach, the appendages (mooring lines and risers) may be handled by a segmented ‘2-D strip CFD’ to represent the 3-D fluid flow field. Finally (in step 6), the risers and mooring lines are solved by a structural finite element analysis or like, in which the fluid loading on them is determined via empirical formulas. This is the most common method of design technique for the offshore system today. For some systems, this simplified technique may become too simplified an approach. In these cases, one practical method of simplification is to consider that the presence of the floater and other appendages will not influence the fluid loading on the mooring/riser component and vice versa. Thus, for a practical analysis under the present-day hardware platform, the following practical coupled TSA method illustrated in a flow chart in Fig. (3) is recommended. In this method the floater is considered a rigid body for which a nonlinear time domain analysis is invoked. The mooring lines and risers are individually analyzed using CFD method. Even though it is still a timeconsuming process, the larger and faster hardware can handle it today. For risers, such CFD analysis is being successfully carried out and good progress has been achieved with execution time that is not prohibitively high on available hardware. The method may be extended to the mooring lines with a modest amount of effort. A time domain method will be needed for both the floater and the appendages, so that at each time step, iterations may be performed among their boundaries for convergence, before the next time step is reached. Initially some further simplification may be warranted in the CFD analysis, e.g., strip 2-D for a 3-D CFD. Additionally, as illustrated in the chart (Fig. 3), these types of mooring line and riser analysis will be able to provide sufficiently accurate hydrodynamic coefficients for risers and mooring geometry for application to the complete coupled system after appropriate validation. Once the coefficients are generated, the simpler empirical methods that are used today can also be applied in a design achieving higher accuracy in the results. The technique, while still computationally time consuming, is possible with today’s fast com30 The Open Mechanics Journal, 2008, Volume 2 Subrata Chakrabarti puters and will provide better and more accurate response of the floating system without arbitrarily choosing these coefficients or depending on limited small scale test data. Once such analysis is extensively made, a data bank of coefficients may be generated for future use in the analysis of the coupled system. In the subsequent sections, the state of the art for the analysis of these systems will be discussed starting with the uncoupled system followed by the individual components.