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A review of Very Large Floating Structures (VLFS) for coastal and offshore uses
Ocean Engineering 109 (2015) 677–690
Contents lists available at ScienceDirect
Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Review
A review of Very Large Floating Structures (VLFS) for coastal and offshore uses Miguel Lamas-Pardo a,1, Gregorio Iglesias b,2, Luis Carral a,3 a b
University of Coruna, 15403 Ferrol, Spain University of Plymouth, PL4 8 AA Plymouth, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2014 Accepted 4 September 2015 Available online 22 October 2015
Very Large Floating Structures (VLFS) have sparked tremendous interest and been the focal point of several articles. The Megafloat is particularly well known for coastal use. The aim of this article is to review the concept of VLFS, showing how they are deployed for both coastal and offshore areas. For these offshore areas, the MOB project (Mobile Offshore Base) is the design that has been most fully developed. Although the Megafloat has been widely studied, attention should also be given to other VLFS for offshore purposes. Among these is the MOB mentioned earlier, as well as other VLFS, including the Pneumatic stabilized platform (PSP) or Versabuoy. These floating structures have been designed in response to logistic developments, mainly to create floating harbours and airports, both on the coast and offshore. They have a wide variety of functions. After providing an overview of each VLFS, the different models will be compared. Their advantages and disadvantages will be assessed according to the depth in which they work and their proximity to the coast. Another comparison is then made between the VLFS and other floating structures that have already been in use on the coast and offshore: pontoons, barges, ships and semisubmersible platforms. It must be added that all of the VLFS are only at the design stage, with the exception of the MegaFloat in Tokyo Bay, the only manufactured VLFS in existence.. These projects have not been carried out. Nevertheless, they have inspired research on behaviour-related problems in VLFS design. One area in particular is hydroelasticity. For coastal waters, the increase in costs of real estate and the sensitivity towards the protection of coastal areas will have an impact on the development of these structures in the 21st century. Their use in open ocean water- offshore- requires further studies in order to lower the costs and to offer more reliable solutions. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Very Large Floating Structures Mobile Offshore Base Floating harbour Floating airport Maritime urbanism
Contents 1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 1.1. The VLFS: responding to the sustainable development of the sea in the 21st Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 1.2. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 1.3. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.4. Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.4.1. Coastal VLFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.4.2. Offshore VLFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.5. Historic evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 Mega-Float . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.1. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680
E-mail addresses: [email protected] (M. Lamas-Pardo), [email protected] (G. Iglesias), [email protected] (L. Carral). Tel.: þ34 649033643. Tel.: þ44 1752 586 131. 3 Tel.: þ34 609 224 026. 1 2
http://dx.doi.org/10.1016/j.oceaneng.2015.09.012 0029-8018/& 2015 Elsevier Ltd. All rights reserved.
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2.2. Constructive system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.3. Stages of the research program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.4. Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.5. Technical challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 3. Mobile Offshore Base, MOB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 3.2. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 3.3. Operational requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.4. Propulsion system and DP alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.5. Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.6. Load Transfer System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 3.7. Proposals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 3.8. Hybrid Mobile Offshore Base, by Aker Maritime ASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 3.8.1. Semisubmersible concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 685 3.9. Joint Mobile Offshore Base, by McDermott . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 3.9.1. Nonlinear Compliant Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 4. Pneumatically Stabilized Platform, PSP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 4.1. Operational concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 4.2. Modularity and construction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 4.3. Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 5. Versabuoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 6.1. Advantages and disadvantages of each VLFS design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 6.2. Comparison of each VLFS design with structures currently used for coastal and offshore development.. . . . . . . . . . . . . . . . . . . . . . . . . 688 6.3. Final conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689
1. Introduction
1.2. Definition
1.1. The VLFS: responding to the sustainable development of the sea in the 21st Century
A Very Large Floating Structure (VLFS) or Very Large Floating Platform (VLFP) is a unique concept of oceanic structure that embraces a range of unprecedented parameters, as shown in the table below (Suzuki et al., 2006): Their size and flexibility require special consideration in terms of design, analysis, construction, assembly and operation. Apart from the parameters shown in Table 1, the VLFS also have the following characteristics:
In recent years, the demand for developable land around the coastal cities has increased significantly, for residential purposes as well as industrial and logistic uses (Suzuki et al., 2006; Wang and Tay, 2011). Highly congested areas are in need of further expansion. Nevertheless, the land available on the coast is often fully developed; the sea is the only option for expansion. Dubai and Singapore currently stand out in terms of coastal development. Similarly, floating houses already exist in Holland. With more than half of Holland's land surface below sea level, people from this country have proposed the concept of floating towns. These include green houses, shopping centres and floating residential areas (Fig. 1). There are also plans to develop floating airports and ports both near the coast and in the open sea. The latter is mainly for strategic reasons. In cities, floating farms may make it possible to provide arable land and food products for a growing human population. At the same time, the integrity of the ecosystem is maintained (Wang and Tay, 2011). The New York Sun Works Center has built a sustainable engineering science barge on the Hudson; this shows that city gardens can be developed on a floating structure in a sustainable way. Along similar lines, salmon-producing countries, such as Norway, the US, Canada (Fig. 2) and Chile, have offshore farms to ensure a constant supply of fresh fish (Per Heggelund, 1989) With this technology, humans can populate the ocean surface. Proposed by the architect Vicente Callebaut Bélgica (Fig. 3), the Lilypad Floating Ecopolis (Wang and Tay, 2011) is a visionary project to accommodate urban populations on an island in the shape of a water lily. Pernice (2009) proposes other ideas for floating cities. However, the size of an airport and/or port is huge compared to that of the existing floating structures, such as pontoons, barges, ships and offshore platforms. Hence, the concept of Very Large Floating Structure (VLFS) comes into its own (Zhang et al, 2015).
Long design life: 50 years for the MOB and 100 years for a MegaFloat Float (Suzuki et al., 2006)
Low maintenance costs. Durability and resistance to fatigue, key concepts in VLFS for material selection, design and manufacturing.
Fig. 1. Visionary semi-aquatic town in the Netherlands, Source: Wang and Tay (2011).
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However, as there are many types of VLFS, it makes more sense to classify them by location: I. Coastal VLFS. II. Offshore VLFS. 1.4.1. Coastal VLFS These can only be used in calm waters- often within a bay, cove or lake-close to the coastline. Coastal VLFS proposals take on the pontoon design, as its simplicity is suitable for calm water areas. These large floating pontoons have been called MegaFloats. However, due to their small draft in relation to their length, they are also referred to in literature as “mat-like” VLFS. Their structure is simple, with a trunk that offers high stability, low manufacturing/maintenance costs and easy repairs. Compared to other types of marine structures, they are highly flexible, so elastic deformations are more important than their movement as a rigid body. Thus, hydro-elastic analysis is key to the design of pontoon-type VLFS; most VLFS studies focus on this analysis: (Mamidipudi and Webster, 1994; Yago and Endo, 1996; Utsunomiya et al, 1998; Kashiwagi, 1998; Ohmatsu, 1999; Wang and Tay, 2011; Kim et al., 2014).
Fig. 2. Salmon farms at Vancouver, Canada. Source: Wang and Tay (2011).
1.4.2. Offshore VLFS In the open sea, the wave height precludes pontoon type VLFS. Alternative geometries for offshore VLFS are therefore proposed. At first glance, the semisubmersible type seems the most appropriate. This paper deals with the most representative examples: – Mobile offshore Base (MOB). – Pneumatically Stabilized Platform (PSP). – Versabuoy.
Fig. 3. Lilypad floating ecopolis. Source: www.vincent.callebaut.org.
Table 1 VLFS parameter ranges. Parameter
Minimum
Maximum
Length Displacement Cost
1000 m 106 tons $5000 million
10,000 m 107 tons $15,000 million
1.3. Applications VLFS are designed primarily for floating airports and ports, for calm waters on the coast or on open sea. However, there could be other uses, including: – Civil engineering: as bridges, water breakers and floating docks. – Energy: as storage facilities for oil and natural gas, along with wind and solar power plants. – Military and intervention: as military and emergency bases. – Recreation and residential areas: as casinos, amusement parks, housing, floating hotels and even entire floating cities. – Floating farms. 1.4. Types Some authors (Andrianov, 2005; Watanabe et al., 2004a; Wang and Tay, 2011) classify VLFS into two types according to their geometry: I. Pontoons. II. Semisubmersibles.
Varied in their format, the MOB has received the greatest amount of attention. Although some authors (Andrianov, 2005; Watanabe et al., 2004a) consider floating platforms and rigs of the offshore industry as VLFS, there is a strong justification not to. They do not exceed 1000 m in length, as specified by the leading researchers in the field who defined the term VLFS (Suzuki et al., 2006). 1.5. Historic evolution The idea of the VLFS first appeared in the modern world after the industrial revolution in the form of the floating island described in the nineteenth century by the French novelist Jules Verne (Verne, 1986). However the first one to be promoted in serious terms was the Armstrong Seadrome from 1924 (Popular Science, 1934; Wang and Tay, 2011). Its stability was demonstrated in tank tests and different versions of these platforms were put forward until Armstrong died in 1955. Although this proposal was rejected, industries and the academic community have started to carry out research into VLFS technologies (Wu, 1984). Research into VLFS in the last decade was carried out in two major projects. There was the Mega-Float from Japan, a typical example of the pontoon-type VLFS. Its counterpart in the US was the Mobile Offshore Base (MOB), as the main representative of the offshore type. Other efforts have been made, like the Pneumatically Stabilized Platform or the Versabuoy, discussed below, or the Euphlotea (Englund, 2008). Milestones in the development of VLFS are listed in Tables 2 and 3 below. Although both the MOB and Mega-Float programs were initiated and carried out independently, their underlying scientific principles and technological goals were structured in a similar way. Furthermore, their research objectives coincided.
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Table 2 Milestones in the development of technologies for VLFS in the US (Suzuki et al., 2006). In the United States 1924–1955 1942–1944
The Seadrome by Armstrong and related concepts Flight Deck of Civil Engineers Corps of the US Navy-SOCK Project 1963C–130 Takeoff and landing demonstrations on the USS Forrestal 1960's–1970's Research Laboratory/University of the US Navy 1989–1996 Research sponsored by the NSF 1991 First International Workshop of the VLFS-University of Hawaii 1993–1996 Marine Technology Platform Program DARPA 1997–2000 Scientific and Technological Program of the Mobile Offshore Base-office of Naval Research, ONR
Fig. 4. Mega-float: schematic arrangement of elements. Source: Watanabe et al. (2004a).
Table 3 Milestones in the development of technologies for VLFS in Japan (Suzuki et al., 2006). In Japan 1950's 1960's 1973–1974
Floating Cities concept in architecture and urban design Puppet drama “Hykkori Hyoutan Jima” Floating Airport Proposed for Kansai International Airport Phase construction, the structure of semi-submersible 1975 International Ocean Exposition in Okinawa-Aquapolis 1988 Kamigoto Oil Reserve: 390 m 97 m 2 m 5 units 1994 Proposed Floating Landing Phase 2 Construction of Kansai International Airport, pontoon type floating structure 1995 Mega-Float/Technological Research Association of MegaFloat 1995–1996 TRAM Experimental Phase 1: 300 m 60 m 2 m 1996 Shirashima Oil Reserve: 397 m 82 m 2 m 8 units 1997–2001 Experimental Phase 2 TRAM: 1000 m 60–120 m 3 m Experiment of landing and takeoff 2001–2005 RþD by the Shipbuilding Research Centre Proposed Airstrip Haneda International Airport. Combination of old type and semisubmersible Pontoon
Fig. 5. Mega-float: structural design of the pontoon. Source: Shipbuilding Research Centre of Japan (2011).
Construction is carried out with modules made on land. Between 100 and 300 m in length, these are assembled at sea with welds (Tori et al., 2000) (Figs. 5–8).
2.3. Stages of the research program
2. Mega-Float The Japanese Mega-Float program, conducted by the Technical Association of 17 shipbuilders and steel makers known as the Technological Research Association of Mega-Float (TRAM), was established in 1995 to carry out a joint research and development project to create a large-scale floating structure. 2.1. Concept
In Phase 1, a 300 m 60 m 2 m structure was built. In Phase 2, which began in 1998, TRAM initiated studies to build a model of a Mega-Float airport. It was to be 1000 m in length, 60 (120) m in beam, and 3 m in depth in Yokosuka (Tokyo Bay) to experiment with landing and takeoff of small airplanes. This airport was completed in 1998 and is currently the only Mega-Float that has been built. From these studies, TRAM concluded in 2001 that it was feasible to build a 4000 m long runway for takeoff and landing at Tokyo International Airport (Haneda). The Department of Floating (Mega-Float) Structures is now developing this technology in the Shipbuilding Research Centre of Japan (SRCJ).
As shown in Fig. 4, a Mega-Float: – Is a very large floating pontoon structure. – Has facilities for mooring/anchoring to keep the floating structure on site. – Has an access bridge or floating road to access the floating structure from land. – Has a breakwater if the significant wave height is greater than 4 m. This is to reduce wave forces impacting on the floating source.
2.4. Application In addition to the real life example of the floating coastal airport, several studies were also conducted under the TRAM project. These were in response to the need for floating bases with many other functions to provide port, logistic and recreational facilities. In terms of design, the Mega-float is better for benign conditions than the Mobile Offshore Base (MOB). Moreover, its floating surface is exploited the way land is used.
2.2. Constructive system 2.5. Technical challenges The pontoon has a structure similar to that of a steel ship hull. Proven to be strong and reliable, yet lightweight, this type of construction has long been used in shipbuilding.
From a technical point of view, there are still many challengesmainly hydrodynamic- in attempting to understand the
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Breakwater
Tugging of a floating unit
Mooring Assembly of a floating unit Firm fixation, with welding
Fig. 6. Mega-float assembly process. Source: Tori et al. (2000).
Fig. 7. Mega-float: experimental Phases (left) and Phase 2 (right). Source: Suzuki et al. (2006).
Fig. 8. Mega-float: proposed container terminal, left, and renewable energy plant, right. Source: Suzuki et al. (2006).
Fig. 9. Overall response time under a static concentrated load for (a) conventional ships and (b) VLFS, Source: Suzuki et al. (2006).
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movements of these structures. The greatest of these challenges is that, due to its large size, it is not possible to model this structure as a rigid body. Thus it is necessary to allow for tolerances to flexion movements. The simplest model for a VLFS is a Floating Elastic Plate. Indeed, most of the research on this model has had VLFS in mind (Suzuki et al., 2006; Andrianov, 2005). A further problem related to this structure's dimensions is the complexity of the computer calculations required (Watanabe et al., 2004b; Wang and Tay, 2011). Fig. 9 below shows the differences in behaviour between a conventional vessel with a rigid body and a pontoon type VLFS with an elastic one when a concentrated load is applied.
3. Mobile Offshore Base, MOB 3.1. Introduction A Mobile offshore Base (MOB) was proposed under the VLFS Mobile Offshore Base Science and Technology program to support military operations wherever conventional land bases are not available. The MOB program was sponsored by the US Office of Naval Research from 1997 to 2000 with a budget of $35M. It was an open program that used international trade experts from 26 companies, 16 schools and 11 government agencies (Palo, 2005).
3.2. Concept The MOB consists of a modular floating base that can be deployed to an area, thereby providing a landing/takeoff area and maintenance, as well as supply and other advanced logistical, support operations. With the MOB concept, the US could have a base anywhere in the world within one month. Designed to have virtually unlimited capacity, it was envisaged by its creators as more than just a floating landing zone. The size of a city, the structure would also be a floating military base where ships could dock, as shown in Fig. 10. After some time, however, it was decided to make this a multiunit structure with several self-propelled, semisubmersible type modules that could be connected/disconnected as required:
Fig. 10. Conceptual illustration of Mobile Offshore Base. Source: Popular Mechanics (2003).
– – – –
Length of each module: 220–500 m. Width of each module: 120–170 m. Number of modules: three to five. Total length of the module when completely aligned: up to 2 km.
These modules were connected/disconnected according to the requirements:
Transit: while in transit between operating locations, the mod
ule is de-ballasted and travels with the pontoons on the surface like a catamaran or any other semisubmersible vessel. Operation: when it reaches its destination, the pontoons are ballasted/submerged below the surface, minimizing the dynamic movements induced by the waves. At this point, the modules are joined. Other particulars:
Fig. 11. MOB, schematic view of the real proposal. Source: Girard et al. (2001a).
– The operational decks are located above wave crests and have a sufficient air-gap. In this way, waves never reach the deck. On-Shore Computer: Supervision Layer Maneuver Coordination Layer Sensor Fusion
Fig. 12. Simulations with three MOB modules in Berkeley's tank test. Source: Girard et al. (2001b).
Module 1
Module 2
Module 3
Stability and Control
Stability and Control
Stability and Control
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– The columns provide structural support and hydrostatic stability against overturning. – Alignment is maintained by means of Dynamic Positioning (DP) thrusters, DP connectors or a combination of both. With these data, it can be concluded that, in terms of geometry and operation, it is similar to the semisubmersible offshore units such as offshore semisubmersible flotels (Lamas Pardo and Carral
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Couce, 2011). In fact, one of the MOBs proposed by Aker Maritime ASA used offshore industry standards from Det Norske Veritas (DNV) (Rognaas et al., 2001) (Figs. 11–14). The MOB program ended in 2000, affirming the technical feasibility of the idea. In year 2001, the Institute for Defence Analyses estimated costs of between $5 and $8 billion for the MOB (approx. $1500 million per module). It was therefore less cost effective than alternative solutions, such as a combination of aircraft carriers and support vessels for logistical issues (Pike, 2005).
3.3. Operational requirements The MOB would be designed to meet operational and survival conditions as follows (Remmers, 1999):
Operating conditions: these would correspond to a significant Fig. 13. Tests to evaluate hydro elastic connectors. Source: Popular Mechanics (2003).
wave height of about 1.9 m, and concentrated wave periods of about 9 s. Later versions were even more ambitious, aiming for 6 m, as discussed below. Survival conditions: The MOB would be required to bear wave loads falling in the range of a significant wave height of 16 m and periods in the range of 20 s.
3.4. Propulsion system and DP alignment As mentioned, a dynamic positioning (DP) system would be employed to keep each module properly oriented. Tests and simulations with real models, along with virtual tank tests, had shown good performance with this system (Girard et al., 2011a, 2011b; Borges de Sousa et al. 2001). A Multi-Module Control Dynamic Positioning System (MMDPCS) was developed for this purpose. In turn, these thrusters would serve to propel the module in transit.
3.5. Connectors
Fig. 14. Load Transfer System (LO/LO) between a MOB and a ship. Source: Goodwin and Bostelman (1998).
Together with dynamic positioning, connectors were developed to keep the modules together. In another section of the text, a pair of these from two of the proposed MOB will be reviewed.
Independent Semisubmersible Modules
Semisubmersible Modules with
Rigidly Connected Semissubmersible
Bechtel National Inc.
Flexible Hinges
Modules
Kvaerner Maritime (Seabase Inc.)
Brown and Root Inc.
Fig. 15. Other proposals for a Mobile Offshore Base. Source: Pike (2005).
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3.6. Load Transfer System The MOB would also be equipped with cranes to lift containers to/from ships docked alongside. The Division of Intelligent Systems of the National Institute of Standards and Technology established requirements. The MOB concept needs to be developed to guide the design of the load transfer between the MOB and the supply vessels (Goodwin, 1998). 3.7. Proposals In that period, MOB design proposals were meant to reduce costs with a variety of material, including steel, concrete, or a combination thereof. Its form would also be varied with singlehull barges, catamarans or semi-submersibles. In the following sections two of them are presented: – Joint Mobile Offshore Base, by McDermott (Babcock and Wilcox). – Hybrid Mobile Offshore Base, by Aker Norwegian Contractors AS. In Fig. 15, three other proposals can be seen. 3.8. Hybrid Mobile Offshore Base, by Aker Maritime ASA The Hybrid Mobile Offshore Base of Aker Maritime ASA (Aker Solutions Group) is a hybrid MOB concept. It consists of a concrete base and steel deckhouse. In these studies two different types of MOB were developed: a. A semisubmersible concept comprising four identical modules. The total length is approximately 1525 m. For this concept, the following types were looked into: 1) pier, 2) barge, 3) vessel, 4) catamaran and 5) semisubmersible type. b. The second concept is a single structural unit consisting of a central concrete 890 m in length with a steel cantilever body of 317 m on each end. The total length of the unit is 1525 m. Concepts 5) and b) are hybrids made with high strength, light weight LWC60 degree aggregate concrete in the steel hull and superstructure deck. It was concluded that fatigue is a major pitfall for a concrete hull with a design life of 100 years (Figs. 15–20). The design criteria informing the design stage to establish the best alternative solutions were those found in Table 4: The concept a.5) hybrid, with a semisubmersible LWC60 grade LWAC light-weight aggregate concrete for its hull and steel for the superstructure, was selected as the most appropriate concept due to the following advantages: 1) The geometry of the semisubmersible hull: a. A semisubmersible has better seakeeping than other structures (Lamas Pardo and Perez Fernandez, 2011). b. The concept can be optimized to have a relatively high speed transit draft (Lamas Pardo and Carral Couce, 2011). 2) Because of its hybrid structure: a. A steel superstructure is lighter than concrete and therefore favourable with respect to depth and hydrostatic stability. 3) The advantages of high performance LWC60 grade concrete (HPC) are (Peters, 2000; Lamas Pardo and Perez Fernandez, 2013):
Fig. 16. The five concepts studied by Aker Maritime ASA. Source: Rognaas et al. (2001). a.1) Pier type, a.2) Barge Type, a.3) Vessel Type, a.4) Catamaran Type, a.5) Semisubmesible Type (columnstabilized), and b) Single Module with cantilevered body.
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Fig. 17. MOB semisubmersible hybrid concept: four modules together (left) and a single module (right), Source: Rognaas et al. (2001).
Fig. 18. Central and lateral connectors, plan view. Source: Rognaas et al. (2001).
Fig. 20. Non-Linear Compliant Connector, Source: Popular Mechanics (2003). Table 4 Criteria for the MOB design. Total minimum size
1525 m 152.5 m
Draft in shallow water transit 15 m approx. Transit speed 10 knots approx. Design life 40 years minimum Acceptable limits for air operations in State of the sea 6; Hs¼ 6 m landing/takeoff of aircraft Limits of survival conditions Maximum pitch angle between modules: 1.5% (0.86°) Hs¼ 15 m Structural strength of the set Airgap ¼ 25 m
Modules can be transferred from the operating draft (36.5 m) to
Fig. 19. (a) Joint Mobile Offshore Base with five modules. Source: Popular Mechanics (2003), (b) Joint Mobile Offshore Base module. Source: Menard and Mills (1998).
a. Greater resistance to fatigue. b. Low maintenance costs. c. Robustness against accidental loads. 3.8.1. Semisubmersible concept The selected hybrid concept consists of four modules interconnected in operating conditions and the required 1525 (m) long runway. Main features of each module are outlined in the Table 5 : Other features:
the transit draft (15.7 m) in 31 h. The reverse operation takes 11 h. The area within each circular column is a separate floating compartment. The area in the pontoons between the columns is divided into two compartments to provide adequate intact and damage stability
3.8.1.1. Connectors. The one central and two lateral connector systems between the four modules () is designed to absorb axial forces, shear horizontal and vertical forces, and torsional moments or roll. However, it makes both pitch and yaw motions possible. To reduce the forces on the connectors:
Wave directions are limited to745° relative to the axial MOB axis.
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4.1. Operational concept
Table 5 Main features of the semisubmersible hybrid MOB. Main features of each module Overall Length Beam (width) at the level of steel decks Draft during aircraft operations Draft, in transit during self-propelled transport, i.e., on the pontoons Aircraft landing/take off strip width, located on one side of the top deck to allow for parking, loading and unloading on the other side Power of each of the eight (8) propellers Speed
381.0 m 152.5 m 36.5 m 15.7 m 61.0 m
6 MW 8–10 knots
Outside the operation situations, with significant wave height
The PSP uses indirect displacement, relying on trapped air pockets that move water. Air pressure acting on the underside of the deck provides the primary buoyant force. The water in each cylinder moves up and down, and the air pressure in the trapped air space changes. These spaces are connected by lines and pneumatic valves. Thus, changes in pressure cause air movement between the cells. This dampens the waves and distributes their forces to reduce peak loads on the structure (McMillan, 2002). If an air turbine is hooked up to these lines, a wave generator is created. This principle had already been developed since thirty years ago for the “wave pump”; its description was first published in 1979 in the Journal Sunnmørsposten, Norway (OWWE Ltd).
47.5 m, the modules are disconnected so that each one works individually. For the coupling process, the system that was used had already been tested in the “Troll A” offshore project in the North Sea: ball and socket.
3.9. Joint Mobile Offshore Base, by McDermott The Joint Mobile offshore Base (JMOB) was a concept developed in the late 1990s and patented (Menard and Mills, 1998) by McDermott Technology Inc. It consists of five semisubmersible steel propelled ships and a mile (1800 m)-long track connected with non-load bearing structures, the Nonlinear Compliant Connector. A lightweight, collapsible drawbridge allows for transfer between modules. 3.9.1. Nonlinear Compliant Connector McDermott Technology Inc designed the Nonlinear Compliant Connector (NCC), a connection system specifically meant for the MOB. The hinged connection makes movement between two semisubmersible modules possible. Its aim is to minimize the forces interacting between two platforms and, at the same time, to prevent longitudinal movement in the forward/reverse, drift and heave (surge, sway and heave) positions. This is thanks to the relatively large rotations in roll, pitch and yaw. By implementing connectors that act as shock absorbing systems, rather than fixed or rigid connectors, fewer fatigue problems may affect the superstructure. With these relatively large rotations, the system was unfortunately not suitable for an airport in which civil aircraft operated.
Fig. 21. Module of cylinders in tank test and cylinder component. Source: Float Inc. (2006).
4. Pneumatically Stabilized Platform, PSP A VLFS created for the ocean must attenuate the waves. Semisubmersibles structures, like the MOB, are ideal for this situation. Nevertheless other technologies are also being developed. The Pneumatically Stabilized Platform, (PSP), is one of the most interesting ideas in recent years. The PSP, designed by Float Inc. (San Diego, USA), is a type of platform comprising a number of cylindrical components packaged in a rectangular shape to form a single module. Each cylinder is sealed at the top and open to the sea at its base; it contains air slightly above atmospheric pressure. The PSP is like a platform in that it can handle loads and attenuate the wave. It is built in concrete, very modular and easily configurable (Figs. 21–24).
Fig. 22. PSP, assembly operation. Source: Float Inc. (1999).
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intermodal land-sea-air transportation centre. However, in 2003, it was rejected for being too expensive, so it is still in the design stage. The authors claimed that these floating intermodal centres were the best alternative to continuing to build new ports and airports on the coast. Figs. 23 and 24 show two proposals from Float Inc. for an extension of the San Francisco airport and an offshore port.
5. Versabuoy
Fig. 23. Proposal of Float Inc. for the floating airport. Source: Float Inc. (1999).
The Versabuoy system is a patent from Versabuoy International (Urbana, Illinois, USA). Its structure takes a similar form to that of a spar type offshore platform. The platform is basically a rigid lattice structure supported by buoys that are moored to the seabed by taut lines through taut mooring. With the articulated connection between the structure and buoys, independent rotation can be induced by wave action (Versabuoy International). Fig. 25. Spar type platforms are well-known for their good behaviour in motion, with virtually no reaction to waves. The system has been designed for fabrication and modular assembly, allowing for expandability. This would be excellent for a VLFS. However, the system requires tight mooring lines and it is subjected to considerable vertical forces that need to be dealt with. Doing so does not ensure success. Nor is it an easy task.
6. Conclusions 6.1. Advantages and disadvantages of each VLFS design. The VLFS have two main advantages over traditional solutions for land reclamation: cost and low environmental impact. Both are explained below:
Fig. 24. Proposal of Float Inc. for the offshore harbour. Source: Float Inc. (1999).
1. Cost: floating options have a lower cost when water depth start to be considerable. Studies with floating docks (Fousert, 2006) established the starting point as 30 m. Floating designs make it easier and quicker to carry out: a. The construction stage, so that economic benefits can be reaped from the start. b. Dismantling, in case the reclaimed sea area is required in the future (Wang et al., 2008). c. Expansion, as they are modular systems. 2. Environmental impact: the VLFS are friendly environmental (Wang et al., 2008; Riyansyah et al., 2010), because: a. They do not damage ecosystems. b. They do not interrupt marine currents. c. Permanent structures are not installed over the sea bed. Moreover, VLFS enjoy other advantages:
4.2. Modularity and construction When the structure is being built, each cylinder is launched to the water with a slab on its top. Post-tensioned cables are used to help assemble the components to make up larger modules. These modules can then be joined with others to form a complete structural platform. This modularity is crucial to the design).
The structures are protected from seismic impacts (Wang et al.,
4.3. Application The PSP was originally designed so that an airport could be built for San Diego (California) in the Pacific Ocean, three miles offshore. In turn it would serve as a port for large ships, connected by a tunnel to the coast. It would therefore be a truly
2008; Riyansyah et al., 2010) since they are inherently isolated from the base. They remain unaffected by differences in soil consolidation. Their position on the water surface is constant and unaffected by tides. Small ships and boats can dock more easily in any sea conditions. When they are located in coastal waters, other marine facilities for recreational activities like water sports may be raised around the structure. If offshore mega-ports were created, larger vessels could pass through. These economies of scale would mean savings. In turn, more modest ports would benefit from the traffic that these vessels would discharge into the mega-ports. These loads would then be
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Fig. 25. Layout and articulated connections in Versabouy design. Source: Versabuoy (2011). Table 6 Advantages and disadvantages of the different types of VLFS.
Mega-Float
Advantages
Disadvantages
Process for manufacturing and assembly easy and inexpensive.
Suitable only for benign conditions in places like inlets and bays. Low mobility. Ingress of water on deck ("green water" effect). Elastic Flat Plate theory only cursorily developed. Payload is limited, as is the case with all semisubmersible vessels. Large internal movements: danger of fatigue in the structure. Connector technology still experimental. High construction and operational costs. Experimental technology in its most basic principles: indirect displacement. Joining technology with tensors in need of extensive further development and study. Low mobility.
Unlimited size (modular). Capacity of positive load. Mobile Offshore Base (MOB)
Pneumatically Stabilized Platform (PSP)
Versabuoy
Mobility. Suitable for all types of water: -Deep and shallow waters. -Benign and harsh (conditions good behaviour at sea). Manufacturing and installation process simple and inexpensive, although not as simple as that of the Mega-Float. Suitable for all types of water, although inferior to the MOB in extremely harsh water. Unlimited size (modular). Low or almost zero maintenance. Great reduction in movements induced by waves. Possibility of being expanded. Modular system and assembly.
shared out among smaller vessels on their way to ports much closer to their final destination, thus increasing short-sea-shipping. The Table 6 summarises the advantages and disadvantages of each of the VLFS presented in the paper. 6.2. Comparison of each VLFS design with structures currently used for coastal and offshore development. It is interesting to note that there are similarities between the platforms being studied for existing accommodation vessels or flotels (Lamas Pardo and Carral Couce, 2011) and the VLFS, still in experimental phase. These are outlined in the following table:
Therefore the accommodation vessel or flotel industry uses the same basic engineering principles that are behind the VLFS being developed. One concern for the future of VLFS is that the area required for any floating airport or harbour- their main use-is bigger. New research studies must be developed on issues such us: hydroelastic response, structural integrity (functionality and safety criteria) and drift forces for mooring system design (Wang and Tay, 2011). Moreover, new technologies must cover the following aspects:
Large vertical forces. Mooring system of complicated lines. Without mobility. Experimental technology.
mooring system design (Wang and Tay, 2011), methods for mitigating the hydroelastic response (Watanabe et al., 2004b; ISSC, 2006; Gao et al, 2011; Kim et al., 2014)) and connector designs, (Riyansyah et al., 2010; Gao et al., 2013). These will have to complement the basic engineering principles already in use.
6.3. Final conclusion In this article, the conceptual design for semisubmersible structures made in concrete, such as the hybrid MOB from Aker Maritime ASA, stands out among all the other possibilities. “Semisubmersible” and “concrete, will be key points for Very Large Floating Structures in future offshore developments. Indeed, concrete will continue to be used in marine structures (Lamas Pardo and Perez Fernandez, 2013). In terms of class and regulations, there is still an unresolved issue (Kim et al., 2014). It must be determined if VLFS are considered a vessel, offshore facility or something else. How this is resolved depends on the various international regulatory bodies, like the IMO and other bodies involved with environmental and maritime concerns. Of course, each VLFS will have its own features and purpose and this will influence classification.
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The cost of shore facilities is soaring, while there is growing concern for the coastal environment. It seems natural that the
Inner waters (coves, bays) – benign conditions
Floating structure that best suits the purpose
Floating hotel
Very Large Floating Structure VLFS
Pontoon
Coastal flotel (Coastel)
Mega-Float
Offshore watersintermediate conditions
Monohull with dampening system
Monohull offshore flotel with anti-
Pneumatically Stabilized Platform, PSP
rolling system
Offshore waters – harsh Semi-submersible conditions
Offshore semisubmersible flotel
option of offshore floating logistics facilities for ports and airports will become more attractive. In the 21st Century there will undoubtedly be significant developments in the field of VLFS: both with previously considered solutions and completely different concepts.
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Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Review
A review of Very Large Floating Structures (VLFS) for coastal and offshore uses Miguel Lamas-Pardo a,1, Gregorio Iglesias b,2, Luis Carral a,3 a b
University of Coruna, 15403 Ferrol, Spain University of Plymouth, PL4 8 AA Plymouth, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2014 Accepted 4 September 2015 Available online 22 October 2015
Very Large Floating Structures (VLFS) have sparked tremendous interest and been the focal point of several articles. The Megafloat is particularly well known for coastal use. The aim of this article is to review the concept of VLFS, showing how they are deployed for both coastal and offshore areas. For these offshore areas, the MOB project (Mobile Offshore Base) is the design that has been most fully developed. Although the Megafloat has been widely studied, attention should also be given to other VLFS for offshore purposes. Among these is the MOB mentioned earlier, as well as other VLFS, including the Pneumatic stabilized platform (PSP) or Versabuoy. These floating structures have been designed in response to logistic developments, mainly to create floating harbours and airports, both on the coast and offshore. They have a wide variety of functions. After providing an overview of each VLFS, the different models will be compared. Their advantages and disadvantages will be assessed according to the depth in which they work and their proximity to the coast. Another comparison is then made between the VLFS and other floating structures that have already been in use on the coast and offshore: pontoons, barges, ships and semisubmersible platforms. It must be added that all of the VLFS are only at the design stage, with the exception of the MegaFloat in Tokyo Bay, the only manufactured VLFS in existence.. These projects have not been carried out. Nevertheless, they have inspired research on behaviour-related problems in VLFS design. One area in particular is hydroelasticity. For coastal waters, the increase in costs of real estate and the sensitivity towards the protection of coastal areas will have an impact on the development of these structures in the 21st century. Their use in open ocean water- offshore- requires further studies in order to lower the costs and to offer more reliable solutions. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Very Large Floating Structures Mobile Offshore Base Floating harbour Floating airport Maritime urbanism
Contents 1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 1.1. The VLFS: responding to the sustainable development of the sea in the 21st Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 1.2. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678 1.3. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.4. Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.4.1. Coastal VLFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.4.2. Offshore VLFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 1.5. Historic evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679 Mega-Float . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.1. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680
E-mail addresses: [email protected] (M. Lamas-Pardo), [email protected] (G. Iglesias), [email protected] (L. Carral). Tel.: þ34 649033643. Tel.: þ44 1752 586 131. 3 Tel.: þ34 609 224 026. 1 2
http://dx.doi.org/10.1016/j.oceaneng.2015.09.012 0029-8018/& 2015 Elsevier Ltd. All rights reserved.
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2.2. Constructive system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.3. Stages of the research program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.4. Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2.5. Technical challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 3. Mobile Offshore Base, MOB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 3.2. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 3.3. Operational requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.4. Propulsion system and DP alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.5. Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.6. Load Transfer System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 3.7. Proposals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 3.8. Hybrid Mobile Offshore Base, by Aker Maritime ASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684 3.8.1. Semisubmersible concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 685 3.9. Joint Mobile Offshore Base, by McDermott . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 3.9.1. Nonlinear Compliant Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 4. Pneumatically Stabilized Platform, PSP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 4.1. Operational concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 686 4.2. Modularity and construction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 4.3. Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 5. Versabuoy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 6.1. Advantages and disadvantages of each VLFS design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687 6.2. Comparison of each VLFS design with structures currently used for coastal and offshore development.. . . . . . . . . . . . . . . . . . . . . . . . . 688 6.3. Final conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689
1. Introduction
1.2. Definition
1.1. The VLFS: responding to the sustainable development of the sea in the 21st Century
A Very Large Floating Structure (VLFS) or Very Large Floating Platform (VLFP) is a unique concept of oceanic structure that embraces a range of unprecedented parameters, as shown in the table below (Suzuki et al., 2006): Their size and flexibility require special consideration in terms of design, analysis, construction, assembly and operation. Apart from the parameters shown in Table 1, the VLFS also have the following characteristics:
In recent years, the demand for developable land around the coastal cities has increased significantly, for residential purposes as well as industrial and logistic uses (Suzuki et al., 2006; Wang and Tay, 2011). Highly congested areas are in need of further expansion. Nevertheless, the land available on the coast is often fully developed; the sea is the only option for expansion. Dubai and Singapore currently stand out in terms of coastal development. Similarly, floating houses already exist in Holland. With more than half of Holland's land surface below sea level, people from this country have proposed the concept of floating towns. These include green houses, shopping centres and floating residential areas (Fig. 1). There are also plans to develop floating airports and ports both near the coast and in the open sea. The latter is mainly for strategic reasons. In cities, floating farms may make it possible to provide arable land and food products for a growing human population. At the same time, the integrity of the ecosystem is maintained (Wang and Tay, 2011). The New York Sun Works Center has built a sustainable engineering science barge on the Hudson; this shows that city gardens can be developed on a floating structure in a sustainable way. Along similar lines, salmon-producing countries, such as Norway, the US, Canada (Fig. 2) and Chile, have offshore farms to ensure a constant supply of fresh fish (Per Heggelund, 1989) With this technology, humans can populate the ocean surface. Proposed by the architect Vicente Callebaut Bélgica (Fig. 3), the Lilypad Floating Ecopolis (Wang and Tay, 2011) is a visionary project to accommodate urban populations on an island in the shape of a water lily. Pernice (2009) proposes other ideas for floating cities. However, the size of an airport and/or port is huge compared to that of the existing floating structures, such as pontoons, barges, ships and offshore platforms. Hence, the concept of Very Large Floating Structure (VLFS) comes into its own (Zhang et al, 2015).
Long design life: 50 years for the MOB and 100 years for a MegaFloat Float (Suzuki et al., 2006)
Low maintenance costs. Durability and resistance to fatigue, key concepts in VLFS for material selection, design and manufacturing.
Fig. 1. Visionary semi-aquatic town in the Netherlands, Source: Wang and Tay (2011).
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However, as there are many types of VLFS, it makes more sense to classify them by location: I. Coastal VLFS. II. Offshore VLFS. 1.4.1. Coastal VLFS These can only be used in calm waters- often within a bay, cove or lake-close to the coastline. Coastal VLFS proposals take on the pontoon design, as its simplicity is suitable for calm water areas. These large floating pontoons have been called MegaFloats. However, due to their small draft in relation to their length, they are also referred to in literature as “mat-like” VLFS. Their structure is simple, with a trunk that offers high stability, low manufacturing/maintenance costs and easy repairs. Compared to other types of marine structures, they are highly flexible, so elastic deformations are more important than their movement as a rigid body. Thus, hydro-elastic analysis is key to the design of pontoon-type VLFS; most VLFS studies focus on this analysis: (Mamidipudi and Webster, 1994; Yago and Endo, 1996; Utsunomiya et al, 1998; Kashiwagi, 1998; Ohmatsu, 1999; Wang and Tay, 2011; Kim et al., 2014).
Fig. 2. Salmon farms at Vancouver, Canada. Source: Wang and Tay (2011).
1.4.2. Offshore VLFS In the open sea, the wave height precludes pontoon type VLFS. Alternative geometries for offshore VLFS are therefore proposed. At first glance, the semisubmersible type seems the most appropriate. This paper deals with the most representative examples: – Mobile offshore Base (MOB). – Pneumatically Stabilized Platform (PSP). – Versabuoy.
Fig. 3. Lilypad floating ecopolis. Source: www.vincent.callebaut.org.
Table 1 VLFS parameter ranges. Parameter
Minimum
Maximum
Length Displacement Cost
1000 m 106 tons $5000 million
10,000 m 107 tons $15,000 million
1.3. Applications VLFS are designed primarily for floating airports and ports, for calm waters on the coast or on open sea. However, there could be other uses, including: – Civil engineering: as bridges, water breakers and floating docks. – Energy: as storage facilities for oil and natural gas, along with wind and solar power plants. – Military and intervention: as military and emergency bases. – Recreation and residential areas: as casinos, amusement parks, housing, floating hotels and even entire floating cities. – Floating farms. 1.4. Types Some authors (Andrianov, 2005; Watanabe et al., 2004a; Wang and Tay, 2011) classify VLFS into two types according to their geometry: I. Pontoons. II. Semisubmersibles.
Varied in their format, the MOB has received the greatest amount of attention. Although some authors (Andrianov, 2005; Watanabe et al., 2004a) consider floating platforms and rigs of the offshore industry as VLFS, there is a strong justification not to. They do not exceed 1000 m in length, as specified by the leading researchers in the field who defined the term VLFS (Suzuki et al., 2006). 1.5. Historic evolution The idea of the VLFS first appeared in the modern world after the industrial revolution in the form of the floating island described in the nineteenth century by the French novelist Jules Verne (Verne, 1986). However the first one to be promoted in serious terms was the Armstrong Seadrome from 1924 (Popular Science, 1934; Wang and Tay, 2011). Its stability was demonstrated in tank tests and different versions of these platforms were put forward until Armstrong died in 1955. Although this proposal was rejected, industries and the academic community have started to carry out research into VLFS technologies (Wu, 1984). Research into VLFS in the last decade was carried out in two major projects. There was the Mega-Float from Japan, a typical example of the pontoon-type VLFS. Its counterpart in the US was the Mobile Offshore Base (MOB), as the main representative of the offshore type. Other efforts have been made, like the Pneumatically Stabilized Platform or the Versabuoy, discussed below, or the Euphlotea (Englund, 2008). Milestones in the development of VLFS are listed in Tables 2 and 3 below. Although both the MOB and Mega-Float programs were initiated and carried out independently, their underlying scientific principles and technological goals were structured in a similar way. Furthermore, their research objectives coincided.
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Table 2 Milestones in the development of technologies for VLFS in the US (Suzuki et al., 2006). In the United States 1924–1955 1942–1944
The Seadrome by Armstrong and related concepts Flight Deck of Civil Engineers Corps of the US Navy-SOCK Project 1963C–130 Takeoff and landing demonstrations on the USS Forrestal 1960's–1970's Research Laboratory/University of the US Navy 1989–1996 Research sponsored by the NSF 1991 First International Workshop of the VLFS-University of Hawaii 1993–1996 Marine Technology Platform Program DARPA 1997–2000 Scientific and Technological Program of the Mobile Offshore Base-office of Naval Research, ONR
Fig. 4. Mega-float: schematic arrangement of elements. Source: Watanabe et al. (2004a).
Table 3 Milestones in the development of technologies for VLFS in Japan (Suzuki et al., 2006). In Japan 1950's 1960's 1973–1974
Floating Cities concept in architecture and urban design Puppet drama “Hykkori Hyoutan Jima” Floating Airport Proposed for Kansai International Airport Phase construction, the structure of semi-submersible 1975 International Ocean Exposition in Okinawa-Aquapolis 1988 Kamigoto Oil Reserve: 390 m 97 m 2 m 5 units 1994 Proposed Floating Landing Phase 2 Construction of Kansai International Airport, pontoon type floating structure 1995 Mega-Float/Technological Research Association of MegaFloat 1995–1996 TRAM Experimental Phase 1: 300 m 60 m 2 m 1996 Shirashima Oil Reserve: 397 m 82 m 2 m 8 units 1997–2001 Experimental Phase 2 TRAM: 1000 m 60–120 m 3 m Experiment of landing and takeoff 2001–2005 RþD by the Shipbuilding Research Centre Proposed Airstrip Haneda International Airport. Combination of old type and semisubmersible Pontoon
Fig. 5. Mega-float: structural design of the pontoon. Source: Shipbuilding Research Centre of Japan (2011).
Construction is carried out with modules made on land. Between 100 and 300 m in length, these are assembled at sea with welds (Tori et al., 2000) (Figs. 5–8).
2.3. Stages of the research program
2. Mega-Float The Japanese Mega-Float program, conducted by the Technical Association of 17 shipbuilders and steel makers known as the Technological Research Association of Mega-Float (TRAM), was established in 1995 to carry out a joint research and development project to create a large-scale floating structure. 2.1. Concept
In Phase 1, a 300 m 60 m 2 m structure was built. In Phase 2, which began in 1998, TRAM initiated studies to build a model of a Mega-Float airport. It was to be 1000 m in length, 60 (120) m in beam, and 3 m in depth in Yokosuka (Tokyo Bay) to experiment with landing and takeoff of small airplanes. This airport was completed in 1998 and is currently the only Mega-Float that has been built. From these studies, TRAM concluded in 2001 that it was feasible to build a 4000 m long runway for takeoff and landing at Tokyo International Airport (Haneda). The Department of Floating (Mega-Float) Structures is now developing this technology in the Shipbuilding Research Centre of Japan (SRCJ).
As shown in Fig. 4, a Mega-Float: – Is a very large floating pontoon structure. – Has facilities for mooring/anchoring to keep the floating structure on site. – Has an access bridge or floating road to access the floating structure from land. – Has a breakwater if the significant wave height is greater than 4 m. This is to reduce wave forces impacting on the floating source.
2.4. Application In addition to the real life example of the floating coastal airport, several studies were also conducted under the TRAM project. These were in response to the need for floating bases with many other functions to provide port, logistic and recreational facilities. In terms of design, the Mega-float is better for benign conditions than the Mobile Offshore Base (MOB). Moreover, its floating surface is exploited the way land is used.
2.2. Constructive system 2.5. Technical challenges The pontoon has a structure similar to that of a steel ship hull. Proven to be strong and reliable, yet lightweight, this type of construction has long been used in shipbuilding.
From a technical point of view, there are still many challengesmainly hydrodynamic- in attempting to understand the
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Breakwater
Tugging of a floating unit
Mooring Assembly of a floating unit Firm fixation, with welding
Fig. 6. Mega-float assembly process. Source: Tori et al. (2000).
Fig. 7. Mega-float: experimental Phases (left) and Phase 2 (right). Source: Suzuki et al. (2006).
Fig. 8. Mega-float: proposed container terminal, left, and renewable energy plant, right. Source: Suzuki et al. (2006).
Fig. 9. Overall response time under a static concentrated load for (a) conventional ships and (b) VLFS, Source: Suzuki et al. (2006).
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movements of these structures. The greatest of these challenges is that, due to its large size, it is not possible to model this structure as a rigid body. Thus it is necessary to allow for tolerances to flexion movements. The simplest model for a VLFS is a Floating Elastic Plate. Indeed, most of the research on this model has had VLFS in mind (Suzuki et al., 2006; Andrianov, 2005). A further problem related to this structure's dimensions is the complexity of the computer calculations required (Watanabe et al., 2004b; Wang and Tay, 2011). Fig. 9 below shows the differences in behaviour between a conventional vessel with a rigid body and a pontoon type VLFS with an elastic one when a concentrated load is applied.
3. Mobile Offshore Base, MOB 3.1. Introduction A Mobile offshore Base (MOB) was proposed under the VLFS Mobile Offshore Base Science and Technology program to support military operations wherever conventional land bases are not available. The MOB program was sponsored by the US Office of Naval Research from 1997 to 2000 with a budget of $35M. It was an open program that used international trade experts from 26 companies, 16 schools and 11 government agencies (Palo, 2005).
3.2. Concept The MOB consists of a modular floating base that can be deployed to an area, thereby providing a landing/takeoff area and maintenance, as well as supply and other advanced logistical, support operations. With the MOB concept, the US could have a base anywhere in the world within one month. Designed to have virtually unlimited capacity, it was envisaged by its creators as more than just a floating landing zone. The size of a city, the structure would also be a floating military base where ships could dock, as shown in Fig. 10. After some time, however, it was decided to make this a multiunit structure with several self-propelled, semisubmersible type modules that could be connected/disconnected as required:
Fig. 10. Conceptual illustration of Mobile Offshore Base. Source: Popular Mechanics (2003).
– – – –
Length of each module: 220–500 m. Width of each module: 120–170 m. Number of modules: three to five. Total length of the module when completely aligned: up to 2 km.
These modules were connected/disconnected according to the requirements:
Transit: while in transit between operating locations, the mod
ule is de-ballasted and travels with the pontoons on the surface like a catamaran or any other semisubmersible vessel. Operation: when it reaches its destination, the pontoons are ballasted/submerged below the surface, minimizing the dynamic movements induced by the waves. At this point, the modules are joined. Other particulars:
Fig. 11. MOB, schematic view of the real proposal. Source: Girard et al. (2001a).
– The operational decks are located above wave crests and have a sufficient air-gap. In this way, waves never reach the deck. On-Shore Computer: Supervision Layer Maneuver Coordination Layer Sensor Fusion
Fig. 12. Simulations with three MOB modules in Berkeley's tank test. Source: Girard et al. (2001b).
Module 1
Module 2
Module 3
Stability and Control
Stability and Control
Stability and Control
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– The columns provide structural support and hydrostatic stability against overturning. – Alignment is maintained by means of Dynamic Positioning (DP) thrusters, DP connectors or a combination of both. With these data, it can be concluded that, in terms of geometry and operation, it is similar to the semisubmersible offshore units such as offshore semisubmersible flotels (Lamas Pardo and Carral
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Couce, 2011). In fact, one of the MOBs proposed by Aker Maritime ASA used offshore industry standards from Det Norske Veritas (DNV) (Rognaas et al., 2001) (Figs. 11–14). The MOB program ended in 2000, affirming the technical feasibility of the idea. In year 2001, the Institute for Defence Analyses estimated costs of between $5 and $8 billion for the MOB (approx. $1500 million per module). It was therefore less cost effective than alternative solutions, such as a combination of aircraft carriers and support vessels for logistical issues (Pike, 2005).
3.3. Operational requirements The MOB would be designed to meet operational and survival conditions as follows (Remmers, 1999):
Operating conditions: these would correspond to a significant Fig. 13. Tests to evaluate hydro elastic connectors. Source: Popular Mechanics (2003).
wave height of about 1.9 m, and concentrated wave periods of about 9 s. Later versions were even more ambitious, aiming for 6 m, as discussed below. Survival conditions: The MOB would be required to bear wave loads falling in the range of a significant wave height of 16 m and periods in the range of 20 s.
3.4. Propulsion system and DP alignment As mentioned, a dynamic positioning (DP) system would be employed to keep each module properly oriented. Tests and simulations with real models, along with virtual tank tests, had shown good performance with this system (Girard et al., 2011a, 2011b; Borges de Sousa et al. 2001). A Multi-Module Control Dynamic Positioning System (MMDPCS) was developed for this purpose. In turn, these thrusters would serve to propel the module in transit.
3.5. Connectors
Fig. 14. Load Transfer System (LO/LO) between a MOB and a ship. Source: Goodwin and Bostelman (1998).
Together with dynamic positioning, connectors were developed to keep the modules together. In another section of the text, a pair of these from two of the proposed MOB will be reviewed.
Independent Semisubmersible Modules
Semisubmersible Modules with
Rigidly Connected Semissubmersible
Bechtel National Inc.
Flexible Hinges
Modules
Kvaerner Maritime (Seabase Inc.)
Brown and Root Inc.
Fig. 15. Other proposals for a Mobile Offshore Base. Source: Pike (2005).
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3.6. Load Transfer System The MOB would also be equipped with cranes to lift containers to/from ships docked alongside. The Division of Intelligent Systems of the National Institute of Standards and Technology established requirements. The MOB concept needs to be developed to guide the design of the load transfer between the MOB and the supply vessels (Goodwin, 1998). 3.7. Proposals In that period, MOB design proposals were meant to reduce costs with a variety of material, including steel, concrete, or a combination thereof. Its form would also be varied with singlehull barges, catamarans or semi-submersibles. In the following sections two of them are presented: – Joint Mobile Offshore Base, by McDermott (Babcock and Wilcox). – Hybrid Mobile Offshore Base, by Aker Norwegian Contractors AS. In Fig. 15, three other proposals can be seen. 3.8. Hybrid Mobile Offshore Base, by Aker Maritime ASA The Hybrid Mobile Offshore Base of Aker Maritime ASA (Aker Solutions Group) is a hybrid MOB concept. It consists of a concrete base and steel deckhouse. In these studies two different types of MOB were developed: a. A semisubmersible concept comprising four identical modules. The total length is approximately 1525 m. For this concept, the following types were looked into: 1) pier, 2) barge, 3) vessel, 4) catamaran and 5) semisubmersible type. b. The second concept is a single structural unit consisting of a central concrete 890 m in length with a steel cantilever body of 317 m on each end. The total length of the unit is 1525 m. Concepts 5) and b) are hybrids made with high strength, light weight LWC60 degree aggregate concrete in the steel hull and superstructure deck. It was concluded that fatigue is a major pitfall for a concrete hull with a design life of 100 years (Figs. 15–20). The design criteria informing the design stage to establish the best alternative solutions were those found in Table 4: The concept a.5) hybrid, with a semisubmersible LWC60 grade LWAC light-weight aggregate concrete for its hull and steel for the superstructure, was selected as the most appropriate concept due to the following advantages: 1) The geometry of the semisubmersible hull: a. A semisubmersible has better seakeeping than other structures (Lamas Pardo and Perez Fernandez, 2011). b. The concept can be optimized to have a relatively high speed transit draft (Lamas Pardo and Carral Couce, 2011). 2) Because of its hybrid structure: a. A steel superstructure is lighter than concrete and therefore favourable with respect to depth and hydrostatic stability. 3) The advantages of high performance LWC60 grade concrete (HPC) are (Peters, 2000; Lamas Pardo and Perez Fernandez, 2013):
Fig. 16. The five concepts studied by Aker Maritime ASA. Source: Rognaas et al. (2001). a.1) Pier type, a.2) Barge Type, a.3) Vessel Type, a.4) Catamaran Type, a.5) Semisubmesible Type (columnstabilized), and b) Single Module with cantilevered body.
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Fig. 17. MOB semisubmersible hybrid concept: four modules together (left) and a single module (right), Source: Rognaas et al. (2001).
Fig. 18. Central and lateral connectors, plan view. Source: Rognaas et al. (2001).
Fig. 20. Non-Linear Compliant Connector, Source: Popular Mechanics (2003). Table 4 Criteria for the MOB design. Total minimum size
1525 m 152.5 m
Draft in shallow water transit 15 m approx. Transit speed 10 knots approx. Design life 40 years minimum Acceptable limits for air operations in State of the sea 6; Hs¼ 6 m landing/takeoff of aircraft Limits of survival conditions Maximum pitch angle between modules: 1.5% (0.86°) Hs¼ 15 m Structural strength of the set Airgap ¼ 25 m
Modules can be transferred from the operating draft (36.5 m) to
Fig. 19. (a) Joint Mobile Offshore Base with five modules. Source: Popular Mechanics (2003), (b) Joint Mobile Offshore Base module. Source: Menard and Mills (1998).
a. Greater resistance to fatigue. b. Low maintenance costs. c. Robustness against accidental loads. 3.8.1. Semisubmersible concept The selected hybrid concept consists of four modules interconnected in operating conditions and the required 1525 (m) long runway. Main features of each module are outlined in the Table 5 : Other features:
the transit draft (15.7 m) in 31 h. The reverse operation takes 11 h. The area within each circular column is a separate floating compartment. The area in the pontoons between the columns is divided into two compartments to provide adequate intact and damage stability
3.8.1.1. Connectors. The one central and two lateral connector systems between the four modules () is designed to absorb axial forces, shear horizontal and vertical forces, and torsional moments or roll. However, it makes both pitch and yaw motions possible. To reduce the forces on the connectors:
Wave directions are limited to745° relative to the axial MOB axis.
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4.1. Operational concept
Table 5 Main features of the semisubmersible hybrid MOB. Main features of each module Overall Length Beam (width) at the level of steel decks Draft during aircraft operations Draft, in transit during self-propelled transport, i.e., on the pontoons Aircraft landing/take off strip width, located on one side of the top deck to allow for parking, loading and unloading on the other side Power of each of the eight (8) propellers Speed
381.0 m 152.5 m 36.5 m 15.7 m 61.0 m
6 MW 8–10 knots
Outside the operation situations, with significant wave height
The PSP uses indirect displacement, relying on trapped air pockets that move water. Air pressure acting on the underside of the deck provides the primary buoyant force. The water in each cylinder moves up and down, and the air pressure in the trapped air space changes. These spaces are connected by lines and pneumatic valves. Thus, changes in pressure cause air movement between the cells. This dampens the waves and distributes their forces to reduce peak loads on the structure (McMillan, 2002). If an air turbine is hooked up to these lines, a wave generator is created. This principle had already been developed since thirty years ago for the “wave pump”; its description was first published in 1979 in the Journal Sunnmørsposten, Norway (OWWE Ltd).
47.5 m, the modules are disconnected so that each one works individually. For the coupling process, the system that was used had already been tested in the “Troll A” offshore project in the North Sea: ball and socket.
3.9. Joint Mobile Offshore Base, by McDermott The Joint Mobile offshore Base (JMOB) was a concept developed in the late 1990s and patented (Menard and Mills, 1998) by McDermott Technology Inc. It consists of five semisubmersible steel propelled ships and a mile (1800 m)-long track connected with non-load bearing structures, the Nonlinear Compliant Connector. A lightweight, collapsible drawbridge allows for transfer between modules. 3.9.1. Nonlinear Compliant Connector McDermott Technology Inc designed the Nonlinear Compliant Connector (NCC), a connection system specifically meant for the MOB. The hinged connection makes movement between two semisubmersible modules possible. Its aim is to minimize the forces interacting between two platforms and, at the same time, to prevent longitudinal movement in the forward/reverse, drift and heave (surge, sway and heave) positions. This is thanks to the relatively large rotations in roll, pitch and yaw. By implementing connectors that act as shock absorbing systems, rather than fixed or rigid connectors, fewer fatigue problems may affect the superstructure. With these relatively large rotations, the system was unfortunately not suitable for an airport in which civil aircraft operated.
Fig. 21. Module of cylinders in tank test and cylinder component. Source: Float Inc. (2006).
4. Pneumatically Stabilized Platform, PSP A VLFS created for the ocean must attenuate the waves. Semisubmersibles structures, like the MOB, are ideal for this situation. Nevertheless other technologies are also being developed. The Pneumatically Stabilized Platform, (PSP), is one of the most interesting ideas in recent years. The PSP, designed by Float Inc. (San Diego, USA), is a type of platform comprising a number of cylindrical components packaged in a rectangular shape to form a single module. Each cylinder is sealed at the top and open to the sea at its base; it contains air slightly above atmospheric pressure. The PSP is like a platform in that it can handle loads and attenuate the wave. It is built in concrete, very modular and easily configurable (Figs. 21–24).
Fig. 22. PSP, assembly operation. Source: Float Inc. (1999).
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intermodal land-sea-air transportation centre. However, in 2003, it was rejected for being too expensive, so it is still in the design stage. The authors claimed that these floating intermodal centres were the best alternative to continuing to build new ports and airports on the coast. Figs. 23 and 24 show two proposals from Float Inc. for an extension of the San Francisco airport and an offshore port.
5. Versabuoy
Fig. 23. Proposal of Float Inc. for the floating airport. Source: Float Inc. (1999).
The Versabuoy system is a patent from Versabuoy International (Urbana, Illinois, USA). Its structure takes a similar form to that of a spar type offshore platform. The platform is basically a rigid lattice structure supported by buoys that are moored to the seabed by taut lines through taut mooring. With the articulated connection between the structure and buoys, independent rotation can be induced by wave action (Versabuoy International). Fig. 25. Spar type platforms are well-known for their good behaviour in motion, with virtually no reaction to waves. The system has been designed for fabrication and modular assembly, allowing for expandability. This would be excellent for a VLFS. However, the system requires tight mooring lines and it is subjected to considerable vertical forces that need to be dealt with. Doing so does not ensure success. Nor is it an easy task.
6. Conclusions 6.1. Advantages and disadvantages of each VLFS design. The VLFS have two main advantages over traditional solutions for land reclamation: cost and low environmental impact. Both are explained below:
Fig. 24. Proposal of Float Inc. for the offshore harbour. Source: Float Inc. (1999).
1. Cost: floating options have a lower cost when water depth start to be considerable. Studies with floating docks (Fousert, 2006) established the starting point as 30 m. Floating designs make it easier and quicker to carry out: a. The construction stage, so that economic benefits can be reaped from the start. b. Dismantling, in case the reclaimed sea area is required in the future (Wang et al., 2008). c. Expansion, as they are modular systems. 2. Environmental impact: the VLFS are friendly environmental (Wang et al., 2008; Riyansyah et al., 2010), because: a. They do not damage ecosystems. b. They do not interrupt marine currents. c. Permanent structures are not installed over the sea bed. Moreover, VLFS enjoy other advantages:
4.2. Modularity and construction When the structure is being built, each cylinder is launched to the water with a slab on its top. Post-tensioned cables are used to help assemble the components to make up larger modules. These modules can then be joined with others to form a complete structural platform. This modularity is crucial to the design).
The structures are protected from seismic impacts (Wang et al.,
4.3. Application The PSP was originally designed so that an airport could be built for San Diego (California) in the Pacific Ocean, three miles offshore. In turn it would serve as a port for large ships, connected by a tunnel to the coast. It would therefore be a truly
2008; Riyansyah et al., 2010) since they are inherently isolated from the base. They remain unaffected by differences in soil consolidation. Their position on the water surface is constant and unaffected by tides. Small ships and boats can dock more easily in any sea conditions. When they are located in coastal waters, other marine facilities for recreational activities like water sports may be raised around the structure. If offshore mega-ports were created, larger vessels could pass through. These economies of scale would mean savings. In turn, more modest ports would benefit from the traffic that these vessels would discharge into the mega-ports. These loads would then be
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Fig. 25. Layout and articulated connections in Versabouy design. Source: Versabuoy (2011). Table 6 Advantages and disadvantages of the different types of VLFS.
Mega-Float
Advantages
Disadvantages
Process for manufacturing and assembly easy and inexpensive.
Suitable only for benign conditions in places like inlets and bays. Low mobility. Ingress of water on deck ("green water" effect). Elastic Flat Plate theory only cursorily developed. Payload is limited, as is the case with all semisubmersible vessels. Large internal movements: danger of fatigue in the structure. Connector technology still experimental. High construction and operational costs. Experimental technology in its most basic principles: indirect displacement. Joining technology with tensors in need of extensive further development and study. Low mobility.
Unlimited size (modular). Capacity of positive load. Mobile Offshore Base (MOB)
Pneumatically Stabilized Platform (PSP)
Versabuoy
Mobility. Suitable for all types of water: -Deep and shallow waters. -Benign and harsh (conditions good behaviour at sea). Manufacturing and installation process simple and inexpensive, although not as simple as that of the Mega-Float. Suitable for all types of water, although inferior to the MOB in extremely harsh water. Unlimited size (modular). Low or almost zero maintenance. Great reduction in movements induced by waves. Possibility of being expanded. Modular system and assembly.
shared out among smaller vessels on their way to ports much closer to their final destination, thus increasing short-sea-shipping. The Table 6 summarises the advantages and disadvantages of each of the VLFS presented in the paper. 6.2. Comparison of each VLFS design with structures currently used for coastal and offshore development. It is interesting to note that there are similarities between the platforms being studied for existing accommodation vessels or flotels (Lamas Pardo and Carral Couce, 2011) and the VLFS, still in experimental phase. These are outlined in the following table:
Therefore the accommodation vessel or flotel industry uses the same basic engineering principles that are behind the VLFS being developed. One concern for the future of VLFS is that the area required for any floating airport or harbour- their main use-is bigger. New research studies must be developed on issues such us: hydroelastic response, structural integrity (functionality and safety criteria) and drift forces for mooring system design (Wang and Tay, 2011). Moreover, new technologies must cover the following aspects:
Large vertical forces. Mooring system of complicated lines. Without mobility. Experimental technology.
mooring system design (Wang and Tay, 2011), methods for mitigating the hydroelastic response (Watanabe et al., 2004b; ISSC, 2006; Gao et al, 2011; Kim et al., 2014)) and connector designs, (Riyansyah et al., 2010; Gao et al., 2013). These will have to complement the basic engineering principles already in use.
6.3. Final conclusion In this article, the conceptual design for semisubmersible structures made in concrete, such as the hybrid MOB from Aker Maritime ASA, stands out among all the other possibilities. “Semisubmersible” and “concrete, will be key points for Very Large Floating Structures in future offshore developments. Indeed, concrete will continue to be used in marine structures (Lamas Pardo and Perez Fernandez, 2013). In terms of class and regulations, there is still an unresolved issue (Kim et al., 2014). It must be determined if VLFS are considered a vessel, offshore facility or something else. How this is resolved depends on the various international regulatory bodies, like the IMO and other bodies involved with environmental and maritime concerns. Of course, each VLFS will have its own features and purpose and this will influence classification.
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The cost of shore facilities is soaring, while there is growing concern for the coastal environment. It seems natural that the
Inner waters (coves, bays) – benign conditions
Floating structure that best suits the purpose
Floating hotel
Very Large Floating Structure VLFS
Pontoon
Coastal flotel (Coastel)
Mega-Float
Offshore watersintermediate conditions
Monohull with dampening system
Monohull offshore flotel with anti-
Pneumatically Stabilized Platform, PSP
rolling system
Offshore waters – harsh Semi-submersible conditions
Offshore semisubmersible flotel
option of offshore floating logistics facilities for ports and airports will become more attractive. In the 21st Century there will undoubtedly be significant developments in the field of VLFS: both with previously considered solutions and completely different concepts.
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