to establish military bases in international waten. (e.g. [5, 6]). ... airport, conventional response analysis techniques .... Kansai airport which is supported by semi-.
This
publication
was
funded
lndus t r y Founda uon (Ryoichi
by a grant Sa sagawa,
from Japan Chairman}.
Shipbuilding
114 INTERNATIONAL SYMPOSIUM
Keynote Lectures Invited Lectures
Kobe International Exhibition Hall, Japan
cumlENT AND FUTUD DDmCTlONS IN VERY LARGE nOA TING STllUCIUIlE IlESEAJlCH AND DEVELOPMENT
R. Cen_iz Ertekin Department of Ocean Engineering School of Ocean and Earth Science and Technology University of Hawaii at Manoa Honolulu, HI 96822, USA KEY WORDS: Hydroelasticity, Very large floating structures (VLFS), floating airports and
bases, dynamic response. ABSTRACf The Quest for large scale exploration and utilization of ocean resources, as well as the scarcity of adequate urban land areas near densely populated coastal regions, will eventually require the use of very large floating structures (VLFS) in the oceans. Due to the enormous sizes of these structures, their designs and analyses must include a number of engineering approaches different from the conventional ones. It is the intent of this paper to critically review the current research and development on the design and analysis of VLFS, and also to attempt to point out the necessary future directions that we must look into to eliminate the technical difficulties present in making VLFS • reality.
1.
INTRODUCTION
Currently, it appears that there are three major reasons for planning to have. very larae floating structure in the oceans: i) to exploit the resources of the oceans at a larle scale (e.l. (I, 2]), Ii) to move land-based complexes, such as airports, offshore due to the lack of adequate land areas near coastal cities (e.g. [3, 4]), and ill) to establish military bases in international waten (e.g. [5, 6]). These and other possible uses of VLFS can only be reality if technical, economical and political conditions together are feasible. The definition of a VLFS is not that certain. However. the most appropriate ODe seems to be •....• floatina structure whose characteristics, especially its size and flexibility, require for its design, construction, and operation special considerations not required by conventional-sized floating structures. • [7] To this, we may add the special considerations required for its assembly since such a VLFS will likely to be constructed as many modules and
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later assembled (linked) in the ocean. Because of the very larle size of VLFS, possibly as larle as • SOOO m lonl floating airport, conventional response analysis techniques applied to even the largest semi-submenible type oil rigs cannot, in general, be used in designinl and analyzinl VLFS. One must employ one of a number of hydroelastic analysis methods to determine the local and global structural responses. Therefore, the existing methods of hydroelasticity with regard to VLFS response will first be reviewed to put a lilht on the future needs and extensions to thetories and calculation techniques. Since every application of • VLFS will have its own unique requirements, the type of the structure, such as a semi-submenible or a mat-like structure, e.l. a barge, used to fulfill owner's requirements and beyond (for example, to make it sea kindly), is an important factor in the selection of the specific analysis methods. Special attention will be paid to this factor. This will be followed by the near- and
long-term pote!ltial applications for a single and multi-module VLFS system, and the 'construction/aSsembly teehniques with their -inherent ..technical, logistical and economical problems. 2.
HYDROELASTICITY
"Hydroelasti;ity is that branch of science which is concerned with the motion of deformable bodies through liquids." [8] The field of hydroeiasticity was introduced to naval architects in the late 50s to explain the flutter phenomenon related to appendages, rudders, etc. [9]. With the pioneering works of [8], a unified approach to predicting main hull vibrations became available. There are basically two major approaches used in determining the deformations of a flexible body under the action of waves. One is the direct approach in which the equations of motion are written for the unknown deformations, and solved directly. The system may be discrete as, for example, in the nodaldisplacement approach or continuous as, for example, in the Euler-beam idealization. The other major approach is the modal-superposition method in which the equations of motion are written for the unknown principal coordinates. Then the displacement at any point is obtained by the superposition of the products of the mode-shape and the principal coordinate vectors. These two main approaches used in determining the dynamic behavior of VLFS are not unique to hydroelasticity; they have been borrowed from the field of mechanical vibrations. We will next briefly review the available hydroelasticity t.heories pertaininl to deformations of a VLFS. Because it is possible to use different approximations for modelling the structure and fluid, it is convenient to categorize them with respect to the spatial dimensions used in these models. It must be pointed out that the main reason for having so many approaches to the hydroelutic response is directly related to our inability to efficiently solve a large system of equations that model the dynamics of VLFS presently. As the computer hardware and accurate and efficient numerical methods advance rapidly, it will no longer be necessary to resort to some of these theories, especially the 2D ones, in the future. All the following theories discussed are based on the assumptions of linear structural displacements and fluid motions. Very little, if any at all, is known on the effects of nonlinearities on hydroelastic response.
I-D Su.ctare, 2-D Flald Model The VLFS is modeled as an Euler or Timoshenko beam and the fluid is modeled by strip theory [8]. The 2-D velocity potential can be determined by Frank's close-fit method [10] for mono-hulls [8,14] or multi-hulls [11, IS] or by the boundary-element method [12]. Because strip theory is not capable of predicting longitudinal wave loads, the theory used in [11] can be augmented by a suitable 3-D approximation as in [13]. Note that the dimensions refer to the longitudinal direction for the structure and transverse plane for the fluid. 2.1
1.2
2-D Structure, 2-D Fluid Model Consider a mat-like floating structure with a draft much smaller than its beam and length. In such a case, it is possible to approximate the structure as 2-D in the horizontal plane as in [16, 17] and use the orthotropic plate theory for the structural model. Because the draft is small, it should also be possible to use the zero-draft Green-function method to solve the hydrodynamic problem [18]. We are not aware of the use of this 2-D structure, 2-D Fluid model proposed here. The computational advantage of this method should be very significan~ allowing one to predict the. . dynamic response of a VLFS with deck space as large as SOOOm by l000m without much difficulty. However, the structure should be mat-like, unlike, for example, the proposed Kansai airport which is supported by semisubmerged columns with footings [49]. 2.3
2- D Structure, 3- D fluid Model The structural model is the same as in Section 2.2, i.e. orthotropic plate theory is used. However, the fluid model is based on the 3-D potential theory. This model was used by [17] to determine the motions of a mat-like VLFS which is proposed as a floating airport. It is important to note a novel solution procedure applied to the hydrodynamic problems in [16, 17t that is once the diffraction problem is solved in 3-D, the solution of the radiation problem can be obtained by a simple matrix multiplication without the need to invert the influence coefficient matrix of the radiation potentials. This remarkable result should receive great attention in future studies.
2."
3-D Structure, 2-D fluid Model The VLFS is modeled by 3-D frame finite elements and the fluid is modeled by using the 2-D potential theory [19, 20, 21, 22]. The cross-section in plane deformations were also
iiItftided m tflete wOrb. The 3-~ SU'iiCnanJ model by FEM allows one to use the modesuperposition method. In [22], comparison of predictions based on a rigid and flexible ship with experiments cOllfirmed the expected result that the assumption of rigid body fails in accUrately predicting the bending moments and shear forces.
eqaatioll" ...,... .lJ'-eIl ~ dW~ of struc:tunr members beu. stendet' iD comparison with the wavelenath, may provide acceptable predictions for semi-submenible type VLFS in preliminary design studies. This approach was used by [29] in studyinl the hydroelastic behavior of a 10-module VLFS. 2.6.3
Iateraetloe Theory In calculating the hydrodynamic coefficients and loads. the multiple-scattering and direct matrix methods can be combined to solve the 3-D potential problem [30]. In (31). 8 semi-submersible modules were linked together around a ring and their dynamic behaviors were analyzed using the interaction theory. Their predictions -showed good agreement with the experimental data.
2.5
3-D Structure, 3-D Fluid Model In this-i!pproach. the structure is modeled by frame or plate elements and the fluid is modeled by the 3-D linear potential theory [23. 24]. The mode-superposition method is used to solve for the coupled problem of fluid-structure interaction. The single-plane symmetry composite source distribution method used in [23] was later extended to two-planes of symmetry [2S] to achieve further reduction in the size of the problem. The use of the 3-D fluid model which leads to the solution of diffraction and radiation potentials for all modes of motion. including the flexible ones. is the best way to solve the hydroelasticity problem within the limitations of linearity assumption. However. the size of the problem may become so large that it may not be possible to employ this approach in analyzing some VLFS because of present limitations of computer hardware.
2.7
RMFC Structure If a VLFS is made up of many modules which are linked together by connectors. it is expected that the modules themselves will be more rigid than the connectors. One can then use the rigid-module, flexible connector (RMFC) approach to reduce the size of the problem significantly. since each module will only have six-degrees-of-freedom. There are a number of ways to solve this problem very efficiently.
EMFC Structure, MoriSOD'sFluid Model In cases where connector stiffness is not significantly less than the module stiffness, it may become necessary to assume that the entire VLFS is flexible. This may be the case when the connectors are very stiff, allowinl only small rotations. Such a structure may be modeled as an elastic module, flexible connector (EMFC) structure. The fluid loadinl can be provided through the use of Morison's equation and the coupled equations of motion can be' sglved directly for the nodal displacements in II finiteelement frame model [28, 48). The frequencyindependent hydrodynamic coefficients used in this method may lead to significant inaccuracies especially at high wave frequencies. To overcome this limitation. it has recently been proposed by (32) that the hydrodynamic and exciting force/moment coefficients are obtained from 3-D potential theory to obtain a better approximation.
2.6.1
2.8
2.6
Hasklad-Hanaoka Relatloashlp It is well known that the diffraction loads can be obtained from the knowledge of radiation potentials alone for a rigid body [e.l. 33). This remarkable relationship has recently been extended to elastic bodies at zero-forward speed by (34). With this new result, it is possible {o determine the generalized diffraction loads without the need to calculate the diffraction potential. It is also shown in [34] that the proof that the H-H relationship can be used in hydroelasticity is valid in both the direct and mode-superposition methods of structural analysis.
3-D Fluid Model In this model. 3-D potential theory is used to determine the hydrodynamic coefficients and existing forces/moments [26. 27, 23]. If the fluid coupling, i.e. the effect of the presence of one module on the hydrodynamics of another modute; -is weak, then the problem may further be simplified by calculating the hydrodynamic coefficients and loads for a single module and then solving the equations of motion in which there is only mechanical coupling due to the presence of connectors [12. 27]. 2.6.2
MorisoD's EquatioD Some studies have shown that Morison's
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1.9
Irrqular-Sea Respoue The standard Irregular-sea response techniques can, in general, be applied to hydroelasticity in short or lo~-crested confused seas [26, 3Sk 'However, as it is pointed out by [36], the spatial homogeneity of the environmental loads due to waves, winds and currents is in doubt when one studies the irregular-wave response.of a VLFS which may be as long as SOOOm.Since the standard spectral methods assume that the environment is spatially homogeneous, the accuracy of such calculations is suspect. In this regard, there is a research need to extend the approach of [37] to include a spatially inhomogeneous environment which is affected by the presence of a VLFS. 3.
STRUCTURE TYPE
There are basically two types of structure that can be used for VLFS. One is the mat-like structure which is either mono or multi-hull of rectangular cross sections, and the other is semisubmersible type structure with columns and pontoons. Although it is expected that because of the small water-plane area of columns, the dynamic response of a semi-submersible will always be less than a mat-like structure, there are unanswered questions with regard to the technical feasibility of connector design in a VLFS which consists of many semi-submersible modules [16]. There is a need for research to determine if, indeed, it is possible to design connectors that can transfer deck loads to the pontoons without causing any structural failure. Such problems must even be more pronounced in the case of semi-submerged columns which are not supported by pontoons [4]. The structure type obviously depends on the major function of the VLFS itself. In addition to the VLFS applications listed in Section I, we can list the following applications, some of which are suggested in [38]: Oil Storage, Aquaculture, Convention center, Toxic Waste Facilities, Rocket Launch Pad, At-Sea Industrial Complex. The material type to be used in the construction of a VLFS is also very important. The prestressed and reinforced concrete appears to be the most promising material that can be used in VLFS construction [42]. 4.
CONSTRUCTION/ ASSEMBLY /POSITIONING A VLFS can be monolithic or multi-
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module, however, the construction of a monohull VLFS which may be several kilometers lona is obviously impossible, i.e. such a construction yard does not exist. Unlike the gravity type fixed oil-production structures, the assembly of the various pieces of a floating mono-hull at sea is impossible. Therefore, if a mono-hull is chosen as a VLFS, then its size will be determined by the availability of a construction yard with a suitable dry dock or slipway. See [41] for an alternative and innovative continuous construction method. On the other hand, a VLFS which is formed by many modules, separately constructed on land, appears to be the most feasible solution. Building a module with its deck dimensions in the order of 100m is currently possible in a number of fabrication sites around the world. Although, the construction of a multimodule VLFS, be it a mat-like or semisubmersible type, is possible with the current technology, the assembly of modules is a serious problem that must be solved. It is recently reported in (39] that, in linking two large platforms, even small motions of either one prevented the linkina of the connectors in random seas. Without a clear and safe solution to this problem, it does not appear to be possible to assemble modules. We must seek a solution to this, perhaps the most important, problem that can only be solved by taking quantum leaps in technology. In addition to the problems associated with linkage, the deslgn of connectors is a very difficult problem that must be solved. If flexible or hinged-type connectors are not technically feasible, then the only alternative remaining is the riaid connection; however, it is not at all clear how modules can be riaidly connected at sea under the actions of the environmental loads. There are three major positioning methods: (i) permanent spread mooring, (ii) dynamic positioning by thrusters, and (Hi) single point mooring, Although the choice is to a great extent a matter of economics, it is clear that the positioning of a VLFS compared with conventional floating structures is a difficult task, and therefore, more study is needed on this, especially on' the economical feasibility of positioning. See [40] for some preliminary study results. Since positioning can only be satisfactorily determined with the' knowledge of drift loads, there is a areat need to develop theories on drift load predictions for elastic floating bodies under the action of waves. Spch theories do not exist today.
5.
IXPEJUMENTS
One must validate the predictions for the local and global hy
