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Latest Progress in Floatover Technologies for Offshore Installations and Decommissioning
Alan M. Wang, Xizhao Jiang, Changsheng Yu, Shaohua Zhu, Huailiang Li, Yungang Wei
Installati

on Division, Offshore Oil Engineering Co., Ltd., Tanggu, Tianjin, China

ABSTRACT
This paper presents a comprehensive overview of various floatover technologies based on the latest advancements in offshore installation and decommissioning technology. Each floatover methodology is briefed and categorized into specifically defined divisions in a system of classification, including mechanical and non-mechanical schemes, single-barge, catamaran-barge and twin-barge schemes, etc. The presentation of these various floatover technologies will reveal the floatover history and evolution, the advantages and disadvantages of different methods, as well as the promising prospect of their wide applications in installation and decommissioning of integrated topsides onto and from various fixed and floating substructures.

including jackets, gravity base platforms, tension leg platforms, semisubmersible platforms, and even spars lately. The floatover technology is an offshore topsides installation method that lets large platform topsides be installed as a single integrated package without the use of a heavy lift crane vessel, i.e. modular lifting installation. This allows the integrated topsides to be completed and pre-commissioned onshore prior to loadout, thus eliminating the substantial costs associated with offshore hook-up and commissioning. For the past two decades, the floatover technology has advanced so much from the conventional “Hi-Deck” scheme with leg mating units to numerous floatover techniques with active/passive load transfer systems and different configuration of floatover barge(s), thus providing an installation solution that can accommodate a wide range of topsides sizes and seastate conditions. These floatover techniques of every hue include the use of the smart-leg technology with active hydraulic devices to neutralize vertical impact, the versa-truss boom technology with A-frame booms and multi-winching operations, the strand jack lifting technology, or the hydraulic jack lifting technology to raise floatover decks to the required in-place elevation at offshore sites. In addition, single floatover barge, catamaran barge, or twin barges have been used to meet the different configuration of substructures, which include future floatover technology of SeaMetric’s TML technique with twin-barge configuration using TML lifting beams with ballast tanks and buoyancy tanks and Pieter Schelte’s single lifting technique with catamaran configuration using hydraulically operated lifting clamps, and so forth. A comprehensive overview of present floatover technologies based on the latest advancements in offshore installation and decommissioning technology is presented hereinafter. The systematic category of various floatover technologies defines the two major flaotover methods, that is, the mechanical method when using active load transfer system and/or separation system and the non-mechanical method when using passive load transfer system and/or separation system. In addition, the floatover technologies can be categorized into specifically defined divisions based on the configuration of floatover barge(s), namely single barge scheme, catamaran barge scheme, and twin barge scheme, respectively. The advantages and disadvantages of different floatover technologies are also addressed here.

KEY WORDS: Floatover technology; Hi-Deck, Smart-Leg?; Strand Jack Lifting; TML?; Unideck?; Versa-Truss?. NOMENCLATURE
AHTS DP DSF DSU FPSO GBS GPS LMU LSF TLP TML = Anchor Handling Tow Supply (tug) = Dynamic Positioning = Deck Support Frame = Deck Support Unit = Floating Production Storage Offloading = Gravity Base Substructure = Global Positioning System = Leg Mating Unit = Loadout Support Frame = Tension Leg Platform = Twin Marine Lifter

INTRODUCTION
Various floatover technologies have been developed and successfully applied to offshore installations of integrated topsides onto different fixed and floating platform substructures since the first floatover installation was successfully adapted for the production platform topsides of 18,600 tonnes on the Phillips Maureen Project in 1983. A string of offshore facilities using the floatover concept followed,

PAST, PRESENT & PERSPECTIVE
For the past 27 years, many different kinds of floatover technologies have been developed and successfully applied to offshore installations. The conventional "High-Deck" or topsides floatover methodology was initially introduced by KBR, then Brown & Root, in 1977 at the BP's Magnus Field in the North Sea. The first floatover installation was successfully applied to the 18,600Te integrated topsides on the Maureen Project in 1983, whose mating operation was engineered and performed by KBR UK. Following the Maureen Project's success, floatover technologies, as an effective installation method, have been widely applied to heavier integrated topsides, such as the world-record 28,800Te PA-B gas production topsides offshore Sakhalin Island, and swell dominant conditions and harsher environments, such as West Africa, West Austria, South China Sea, etc. However, a combination of deep water, rough open sea, or swell conditions still pose a challenge to provide a cost effective solution in offshore installations. A dozen of mechanical or non-mechanical floatover technologies with different configurations of single barge, twin barges, or catamaran barge have been developed for various fixed and floating substructures in challenging environments. There are a number of reasons why the floatover method is becoming the preferred installation method for integrated topsides, rather than using heavy lift vessels. The availability of such heavy lift vessels is very limited. Waiting for one suitable crane vessel to come online can cause significant project delays. Since the majority of heavy lift vessels are typically home-based in European waters, the mobilization and demobilization costs can be too costly for projects in Asian-Pacific waters. Only a handful of crane vessels have the capacity to carry out large heavy lifts, and their day rates are very high. In addition, different from modular installations, the primary objective with floatover installations is to minimize costly hook-up and commissioning periods offshore. This allows freedom of equipment layout within the deck compared to modular lifting designs, and also completion of testing and pre-commissioning onshore. The result is a significant reduction in overall development cost through a shorter offshore commissioning phase without using expensive, heavy lift crane vessels.

type or GBS type that favors the conventional floatover method. In 2009 four floatover installations were successfully carried out in Bohai Bay alone where three integrated topsides ranging from 6500Te to 11,000Te were installed onto jacket substructure by a conventional “HiDeck” installation and one 3000Te topsides was installed directly onto pre-installed piles in an extremely shallow water by the strand-jack lifting floatover scheme. The 21,800Te Lunskoye-A (LUN-A) gas production topsides and 28,800Te Piltun-Astokhskoye (PA-B) gas production topsides were successfully installed onto concrete GBS structures in the Sea of Okhotsk, northeast of Sakhalin Island in June 2006 and July 2007, respectively, setting a new record as the industry's heaviest floatover deck installation, although a 39,000Te Hibernia topsides was installed onto a GBS using a twin-barge configuration in protected waters offshore Newfoundland in early 1997. Nowadays the topsides weight does not significantly affect the floatover procedures or systems. See Fig. 1 for an example. In the early of 1980s two North Sea projects, i.e. Phillip's Maureen and Conoco's Hutton, placed integrated topsides on steel GBS and TLP substructures in relatively sheltered areas and inshore shallow locations. Recently floatover technology can be employed from shallow water to deep water in swell conditions or harsher random waves. Moreover, the floatover substructures can cover almost all types of existing fixed and floating systems, including jackets, GBSs, TLPs, SEMIs, compliant towers, and spars, except FPSOs. The primary design concerns are fixed platforms or floating platforms with a secondary emphasis on shallow water or deep water, as well as benign environments and harsh sea conditions. The installation engineering scope-of-work comprised conceptual design, engineering and planning the entire operation, including loadout, seafastening, transportation and installation. Perhaps even more important in terms of ultimate cost savings for the client is early involvement during the conceptual design phase. Early design decisions for the float-over method can generate considerable savings further down the line. By being involved during the conceptual and detailed design phases, naval architects and structural engineers can provide invaluable input before construction begins. This minimizes the need for costly changes later on. Detailed planning for topsides transportation and subsequent installation also enables hook-up and commission operations to begin earlier. Originally conceived to address the problem of making heavy lifts in remote locations, floatover techniques are increasingly being applied to smaller and smaller topsides. Even in regions where suitable crane vessels are available, specifying an integrated topsides for a floatover installation opens the market to those contractors without access to such crane vessels, thereby providing a degree of additional competition during project tendering. The state-of-the-art technology of floatover installations will be further developed to improve workability, reduce structural requirements, as well as standardize to avoid the early commitment. Refer to Seij (2007) and O’Neill (2000) for details.

FLOATOVER PROCEDURES
Typical floatover operations may be divided into the following major stages: Loadout: Upon weighing, the integrated topsides will be jacked up by a mega jacking system of hydraulic cylinders or lifted by strand jacks before a tall DSF/LSF can be inserted under the topsides prior to loadout operation. Topsides may be skidded onto pre-selected floatover barge longitudinally, or laterally if longitudinal strength is limited, via a pulling system of strand jacks or Self Propelled Modular Transporter (SPMT) trailers with a sophisticated ballast spread.

Fig. 1: Floatover Installation of Lun-A Topsides with T-Shaped Barge Traditionally the floatover method is particularly suited to conditions found in the shallow and benign water area, such as Bohai Bay, China, refer to Liu et al. (2006), and offshore Sakhalin Island, Russia. Therefore, the substructure design tends to be a conventional jacket

Transportation: Once completing the seafastening and floatover preparations, and most of all meeting the sailaway criteria, the barge laden with the topsides sails from fabrication yard to offshore site. The floatover tiedown design is unique and usually consists of two different sections, i.e. the first section connecting the topsides and DSF around DSUs, which will be removed prior to mating, and the second section connecting DSF and barge deck, which will remain until deck cleaning. Where a twin-barge configuration of transportation is required, such as for spar platforms and narrow compliant towers, special transportation and seafastening design should be developed to meet different requirements of rigid, flexible, and even hinged connections between twin barge and the topsides. Pre-Floatover Preparations: Upon arrival at site, the barge is connected with a pre-installed docking/positioning mooring system via AHTS tugs. While in stand-off position, pre-floatover preparations are performed including set up and function test of GPS positioning monitoring system, motion monitoring system, environmental measure system, soft-line rigging preparations, barge and substructure preparations, and so on. Docking Operation: By operating mooring winches and/or positioning AHTS tugs, the barge will be positioned and aligned with the substructure slot. For a configuration of twin barges or a catamaran barge, the barge(s) will be positioned and aligned with the substructure in middle. With the help of soft-line rigging arrangement, the barge(s) will be docked inside the substructure slot or around the substructure when twin barges or a catamaran barge is adopted. One main towing tug can be used for docking operation while workboats or zodiacs may be used for soft-line handling. Mating Operation: Upon aligning stabbing cones with support receptacles, the barge will be ballasted or active hydraulic devices will be used to transfer the topsides load from the barge onto the substructure. The load transfer system generally comprises different sets of multi-stiffness, multi-stage LMUs, which are self-contained and designed for each leg with different stiffness based on the leg load transfer at different stage. The load transfer systems are basically same for fixed or floating substructures. The major difference is that the stiffness required for floating substructures is dominated by small relatively small water-plane area of substructure and their free floating motion characteristics. Special multi-stiffness, multi-stage units may be required when large relative motions between deck and substructure are predicted. Many different kinds of mechanical devices have been invented to facilitate the load transfer system, thus minimizing the impact load during mating. Depending on the site condition and installation window requirement, typical limiting sea states for floatover operation are given as follows: Head Seas Wave Height (Hs) Wave Period (Tp) 1-Min Mean Wind Speed at EL(+) 10m Surface Current 1.5m 5 - 10 sec 10m/sec 1.5m/sec Beam Seas 0.8m 4 - 7 sec 10m/sec 1.5m/sec Quartering Seas 1.2m 5 - 8 sec 10m/sec 1.5m/sec

transfer process until separation occurs. There is no steel to steel contact during separation while the elastomeric units absorb incidental vertical and lateral contact energy. Active separation devices may be employed. Some of the active separation devices may provide exciting separation event, or even explosive separation event. The basic separation system is the same whether for fixed or floating structures, subject to the same multi-stiffness usage as LMUs.

PRIMARY EQUIPMENT SYSTEMS
The equipment systems required for the floatover operations have varied functions and applications. Each equipment system provided is designed to ensure that the overall operation is executed in a safe, timely and efficient manner, while complying with all contractual obligations. The design of these critical installation devices plays a crucial role in ensuring successful floatover operations. The following provides a summary overview of the primary systems: Floatover Barge(s): Upon loadout, the barge will transport the topsides to site and floatover install the topsides onto a pre-installed fixed substructure offshore or a floating substructure in place or inshore. AHTS/Harbor Tugs: The positioning tugs including AHTS and harbor tugs work with a mooring system and a soft-line winching system to form a positioning spread, thus providing longitudinal and lateral pull control during docking and undocking. AHTS tugs are also used to preinstall the mooring system and to hook up the pre-installed mooring lines with mooring winches upon arrival of floatover barge(s). AHTS can also work as a positioning spread to position floating substructure during floatover installation. DSF/LSF: The topsides will be placed on a high transportation frame, normally a truss frame, for its journey to the offshore site. This frame together with the existing height of the barge, i.e. freeboard, will allow the stabbing legs of the topsides to clear the top of the LMUs, if preinstalled on the substructure, immediately prior to mating the two structures. Docking/Positioning System: In shallow water a spread mooring system equipped mooring winches on barge deck, in combination with soft line positioning winches also on barge deck and positioning AHTS tugs, can function adequately to perform barge approach, initial entry, docking and undocking operations. In deep water precise positioning AHTS tugs in combination with soft line winches may be adequate. The soft line positioning winching system is mainly used to suppress surge and sway motions within the slot. When DP vessel(s) are used in floatover installation, such docking system may be eliminated. LMU & DSU: LMUs are designed to buffer the impact load between the support receptacles and the mating cones during mating while DSUs are used to buffer the impact load between the DSF and the integrated topside during separation. LMU makes soft initial contact and reduces relative motions before engaging to increase stiffness for final load transfer. LMUs are specialized leg and deck mating units that act as shock absorbers as the vessel is ballasted down and the topsides load transfers from the deck support structures onto the substructure. The units are custom designed for each leg of the deck to balance deck load through load transfer and motion compensation. The heave stiffness of each leg is designed to meet the exacting stiffness and deflection characteristics required. Additionally, the load transfer units are designed to have the proper stiffness to absorb initial impact energy and any unsuppressed surge and sway energy due to environmental forces. The design of the units has been developed over two decades of experience and employs exacting elastomer mixing, molding, and

Separation & Undocking Operation: Having transferred the topsides load, the barge continues ballasting until safe clearance between the topsides structure underside and the DSF upside has been achieved. Then the barge will be withdrawn from the substructure slot. DSU is a conventional passive elastomeric separation unit which is designed to provide an increasing gap between DSF and topsides through the load

bonding techniques in the fabrication. Fendering System: In general three types of fendering systems should be provided for docking and undocking operations, i.e., sway fenders, surge fenders, and stern guide fenders. The sway fenders can be installed along the barge sides or on the substructure slot insides to protect barge and substructure from direct impact while a minimum transverse clearance may be used to limit lateral movement of the barge and align the LMU mating cones and support receptacles transversely. The surge fenders work as longitudinal stoppers to align the LMU mating cones and support receptacles longitudinally and also used to prevent direct impact between support legs and fender system at final position. The stern guide fenders are constructed to assist the initial docking of the barge into the structure slot to smooth the initial entry and also protect the structure legs. Positioning Monitoring System: A DGPS positioning monitoring system shall be set up in the operation control room located on barge deck. Throughout docking, mating and undocking operations the relative position between barge and substructure shall be continuously monitored by a GPS positioning system and visual observation. Motion Monitoring System: Throughout the floatover operation the barge motions will influence the ability to complete the floatover activities, in particular, the air gap between stabbing cones and support legs, and the mating load during transfer operation. The six-degrees-offreedom motions, i.e. roll, pitch, heave, sway, surge and yaw, should be continuously monitored, especially the motions of the stabbing cones and the deck support points throughout the floatover operation. Environmental Measure System: An environmental measure system will be employed to continuously measure wind, waves, currents, as well as tidal elevation throughout the entire operation. Rapid Ballast System: During mating, a rapid ballast system may be required to transfer topsides load and ballast barge down to achieve safe clearance between DSF and deck in a timely fashion, normally in four to six hours, depending on tidal cycle and range. A precise ballast monitor and control system may also be located on barge deck.

Versatile and forgiving floatover technologies have been employed to install integrated topsides onto various fixed or floating substructures from shallow water to deep water in benign environment, swell conditions, or harsh random waves. Each combination of these conditions may cause different challenges and concerns in development of applicable floatover techniques. Up to now more than a dozen of floatover techniques have been developed for various fixed and floating substructures, in shallow water and deep water with benign sea state or harsher sea state. Each of these floatover methodologies will be briefed while their advantages and disadvantages will be addressed hereinafter. The various existing floatover technologies can be systematically categorized into the following techniques.

Conventional Hi-Deck Technique
So-called conventional Hi-Deck technique is a non-mechanical method with a single large barge configuration, which was originally developed by KBR in 1983. The topsides will be placed on a high DSF for its voyage to the offshore site. Upon arrival at the site the floatover barge will be ballasted to a minimum draft with even keel and even heel. The topsides situated high on the top of DSF, together with the existing freeboard height of the barge, will allow the stabbing legs of the topsides to clear the top of the LMUs, if pre-installed on the substructure, immediately prior to mating the two structures. The floatover barge will be brought between the substructure slot with a combination of positioning tugs, mooring lines, and soft-line winches at an approaching speed of 3-5m/min. As Fig. 2 shows, the concrete GBS has been designed to allow 4 meters clearance between the barge sides and the GBS shafts. Eight mooring lines are attached to the barge as the barge progresses through the GBS. With the aid of fendering system and soft-line winching system, these mooring lines will be used to hold the barge steady in its final lowering position.

CLASSIFICATION OF FLOATOVER TECHNOLOGIES
The floatover technologies have become more and more common in recent years. However, so far there is no strict definition and clear categorization of various existing floatover technologies. This paper intends to categorize these technologies into specifically defined divisions in a system of classification to clarify basic concepts of floatover techniques, thus benefiting further development and applications. In general, all the floatover technologies can be divided into two large systematic categories, namely, mechanical methods and non-mechanical methods. The mechanical method is defined as when mechanical devices are employed as active load transfer systems and/or active separation systems. The non-mechanical method is defined as when passive LMUs and DSUs are used as passive load transfer systems and passive separation systems, where these passive load transfer systems are actuated mainly by ballasting down floatover barge(s) and/or via falling tides. According to the floatover barges, the floatover technologies can be also classified as single-barge methods, catamaran-barge methods, and twin-barge methods based on the configuration of barge(s) used in floatover installations. In addition, the twin-barge methods can be further divided into the rigid connection method, the flexible connection method, and the hinged connection method based on the connection types of the deck support frame supporting astride on the twin barges.

Fig. 2: Illustrative Mating between 11,500Te Malampaya Topsides and CGBS in 90m water depth offshore Philippines, South China Sea Upon aligning the stabbing cones with the leg receptors and removing all the tiedowns around DSUs, ballasting operations will commence. The weight of the topsides will be progressively transferred onto the LMUs by lowering the barge away from the topsides. To reduce the initial impact between the deck and the substructure, elastomeric pads will be included in the LMUs. Ballasting of the barge will continue with the use of a rapid ballast system, say to produce a peak load transfer rate of 30,000 tonnes per hour for the 11,500Te Malampaya Topsides. When the deck weight transfer has been completed the barge will be towed from between the shafts of the GBS.

When the transportation barge, together with the DSF, are clear from the substructure slot the final mating operation will commence. Sand jacks located in the LMUs will be activated to lower the deck and allow the deck legs to come into steel-to-steel contact with the substructure legs if non steel-to-steel LMUs are employed. The contact points will then be welded together to form the one permanent structure of the platform. Before departing from the platform, the construction support vessel will perform post installation surveys including the deployment of a ROV for underwater surveys.

Smart-Leg Technique
The first successful shockless Smart-Leg System was completed offshore Nigeria in June 1997 by McDermott-ETPM for the 4,100Te topsides of Ekpe Gas Compression Platform located in a water depth of 50 meters. The barge preparations were done in 3 days while the floatover operation only lasted about 6 hours. The Smart-Leg Technique is a mechanical method with a single barge configuration. This technology was successfully used in November 1996 to position a 192m long and 8000Te heavy bridge spans crossing the strait between Prince Edward Island and New Brunswick in Canada.

ETPM developed and patented the Smart-Leg System, refer to Labbé (1998) and Seij (2007) for details, which uses active hydraulic jacks to neutralize the vertical movements of the barge and to transfer the deck weight from the cargo barge to the piled jacket structure. Upon docking, the jacks activate the mating of each deck leg onto the corresponding jacket pile by deploying extension pipes. The activation mechanism is controlled by non-return check valves located between each jack and gas accumulator. The action of activating the valves only during rising period of the deck legs will smoothly lock the deck at the peak height of the motion and take place at the precise time when the deck leg vertical speed is zero, therefore eliminating the kinetic energy and the risk of impact. The deck mating is completed in just a few seconds, less than the swell period. Rather than using LMUs to absorb the shocks, retrievable jacks are used to progressively freeze the motion of the cargo barge. This allows a deck to be installed in significant swell conditions common to offshore West Africa. The acceptable swell limit is usually no more than 2.8 meters high in a long period of 15 seconds. Smart Fins and Smart Fenders are also developed to restrict the motion of the barge in sway, surge, and yaw. The Smart Fins and Smart Fenders are deployed and recovered by hydraulic rams installed on barge deck. Smart Fins are equipped with hydraulic shock absorbers to establish contact at the four corner legs, thus reducing the surge to under 25mm excursion. Then the Smart Fenders will be activated to progressively eliminate the sway. Upon aligning, the Smart Leg System can be activated to start the deck mating. After partial load transfer occurs as a result of locking of Smart-Leg Jacks, say 50% load transfer, the remaining load transfer can be achieved by ballasting the barge and further jacking up the deck to the point where the Smart Shoes can be actuated to collapse with explosive split nuts, thus yielding a 2.7m undocking clearance. The Smart Shoes are special deck support two AFrames with sliding bearing pads adapted to fit onto skid rails. Refer to Figs. 3a and 3b for details. The major disadvantage of this scheme is lack of controlling the final elevation of the topsides and its levelness since the Smart Leg is activated by the vertical motion of the barge when the deck leg rises to its highest position before falling in the swell. In addition the Smart technology is based on complex active mechanism and do not allow single failure, less reliable compared with passive LMUs and DSUs.

Fig. 3a: Hydraulic Jack Assemblies Accommodated in Deck Legs right above Jacket Legs

Strand-Jack Lifting Technique
The strand jack lifting installation was first developed by JGP in July 2000 to jack up a 300Te topsides more than 30m for installation of Millom West gas platform in the Irish Sea. Following the successful installation, this strand jack lifting technique was successfully applied to installations of four integrated topsides ranging from 2,000Te to 6,200Te in 2002, 2003, 2008, and 2009, respectively, for the Apache /PetroChina Zhaodong Project. The field site is in extremely shallow near-shore waters in western Bohai Bay with a water depth of 1.78 m as per chart datum and 4.2 m referenced to the local mean sea level. This mechanical lifting method can position topsides at a very low level on barge deck, which only depends on fabrication requirement, thus improving the barge stability and reducing the size of floatover barge significantly. Therefore no jacking up of the topsides is required prior to loadout. During deck mating, the supporting legs will be first lowered to make initial contact with the pre-installed piles. This requires less or no ballasting for deck mating, eliminates rapid ballast system, and also eases the dredging requirement of the installation basins. Most importantly, the use of strand jack lifting scheme reduces the initial contact impact significantly, just in the magnitude of the weight of the support leg. This eliminates the use of passive LMUs and

Fig. 3b: Smart Shoes with Explosive Collapsible Mechanism

only requires simplified DSUs. The lifting height mainly depends on the length of strands and supporting legs. In addition, the in-place deck elevation can be controlled precisely by strand jacks up to a few millimeters. The discrepancy between the in-place elevations of the two adjacent platforms was successfully controlled within 6 mm. Fig. 4a shows the general layout of strand jack system on the topsides while Fig. 4b show the post-floatover installation of a 6200Te topsides.

wires. Lower the 4 inner legs onto the pre-installed piles and transfer the leg reaction load pre-defined by a finite element analysis to the 4 inner legs by using hydraulic collar jacks. Weld the shim blocks between the 4 inner leg sleeves and the 4 inner legs before decommissioning and removing the hydraulic screwed locking collar jacks and strand jacks from the 4 inner legs.

UNIDECK Technique
The Unideck technology was engineered by Technip for the installation of topsides by floatover and hydraulic jacking technique, a mechanical and single-barge scheme. The Unideck technigue is entirely reversible during installation and is particularly well suitable for benign environments and in long period swell conditions such as West Africa. So far there have been 12 successful floatovers completed, including 3 in West Africa. The EAP GN Topsides is the largest topsides installed by the Unideck technique in the swell condition offshore Nigeria. The floatover installation of the 18,000Te topsides was executed in early November 2005, during the West African installation season running from early November to end of March.

Fig 4a: General Arrangement of Equipment Layout on Deck

Fig. 5a: Hydraulic Jacks Can Elevate and Quickly Lower the Deck

Fig. 4b: Strand Jack Lifting Installation of 6200Te Topsides in extremely shallow water of Bohai Bay for Apache Zhaodong Project The strand jack lifting sequences may be described as follows: The floatover barge starts docking operation during rising tide with the help of a conventional docking system. Upon docking, aligning the stabbing cones of support legs with the pre-installed piles or the jacket piles. After removing all the tiedowns, lower the 4 outer legs by about one foot a time one by one, otherwise it is difficult to insert overcurved strand wires through fixed anchors in a timely fashion. If necessary, simultaneously ballast the barge down to facilitate the deck mating. Continue lifting the topsides from the barge to the final design elevation at a speed of 0.3m per minute. Undock the barge once there is an adequate clearance. Level the topsides at the in-place elevation using strand jacks. Weld the shim blocks between the 4 outer leg sleeves and the 4 outer legs before decommissioning and removing the strand jacks from the 4 outer legs upon relieving topsides load from the strand

Fig. 5b: Typical Support Structure with Cylinder Jacks Elevated The Unideck technique enables short-time installation duration suitable to the swell conditions and ensures a safe installation operation. The typical sea condition is a significant wave height Hs = 1.5m in a period of 10 sec or Hs = 1.2m in a period of 14 sec. The technique combines ballasting and jacking to improve the stability of the barge during transportation. The active hydraulic system is used to achieve the docking clearance by jacking up before entering the slot and the initial 50% load transfer by lowering jacks within duration of only one minute. The mating operation will be completed by ballasting down barge and

extending jacks 2nd time. When the 75% load transfer is completed, a rapid ram retraction is performed within one second to complete the 100% load transfer, thus achieving an instant 1.5m gap for safe undocking. Refer to Figs. 5a and 5b and Tribout (2007) for details. Technip also developed the Floatover High Air Gap (FOHAG) concept derived from the Unideck and TPG 500 technologies. It allows deck floatover installation applicable to the high air-gap areas, such as Canada and Sakhalin Island, where platforms are exposed to large amplitude waves or cyclonic conditions like in South East Asia. During installation, the deck is elevated well above the air gap and positioned above the substructure and lowered down in place.

A semi-active Amplemann system is developed to combine one central passive jack and six active jacks to separately withstand the static and dynamic loads, respectively, and therefore reducing the power requirements of this operating system. The central passive cylinder will be set to such pressure equal to the static reaction load while the six active hydraulic cylinders will take the dynamic loads, see Fig. 6a for details. Floatover installation requires a minimum of four Amplemann systems and four conventional passive hydraulic jacks to provide eight supports for 12,000Te topsides with one redundancy in case of single Amplemann system failure, see Fig. 6b for details. The geometry of the Amplemann system will have a large effect on the system performance. This technology is still subject to the financial feasibility and technical reliability of complex mechanism and complicated control/monitor system. Refer to Gerner et al. (2007).

Ampelmann System Technique
The core technology of the Ampelmann System is a motion compensation platform that allows easy, fast and safe access from a moving vessel to offshore structures, even in high waves, to eliminate crew transfer by crew baskets, swing ropes and boat landings. The technology was developed in early 2005 by the Delft University of Technology and the Ampelmann Company.

Twin-Barge Technique
In November 2006, the twin-barge floatover technology was applied to install the 3,400Te Kikeh topsides onto the first-ever spar outside of the Gulf of Mexico for the first time in open waters. The installation site is located in 1320m water depths offshore East Malaysia, South China Sea. The twin-barge concept was developed by Technip with the topsides resting astride on two identical barges. The twin-barge configuration is centered above the submerged spar hull which is anchored at its final in-place site. Refer to Fig. 7 and Edelson et al. (2008) for details.

Fig. 6a: Semi-Active Ampelmann System with Six Active Hydraulic Cylinders and One Central Passive Cylinder

Fig. 7: Twin-Barge Configuration with Positioning Spread Approaching toward Pre-Installed Spar Hull The significance of Technip’s success of this world first operation is that the twin-barge technique can also be used for future large deck integrations well beyond lifting capacities. Different from the single barge method, the twin-barge floatover procedures as described below. Upon hooking up the 10 semi-taut mooring lines, the spar hull is ballasted by both solids and variables to achieve a freeboard of 4.3m prior to mating. The topsides was loaded onto a single transportation barge at the fabyard in Johor Bahru, Malaysia and then towed to the offload operation base in Labuan, East Malaysia. The twin floatover barges are equipped with deck supporting frames and maneuvering/positioning equipment including double and single drum winches, HPUs, generators, compressors, storage and operation control containers, etc., on the limited deck space at the mobilization facility in Singapore, and then towed to the base in Labuan.

Fig. 6b: Deck Layout with Four Semi-Active Ampelmann Systems and Four Passive Supports The Ampelmann System is a platform supported on 6 hydraulic cylinders, capable of compensating the barge motions in six degrees of freedom. The topsides are placed on multiple Ampelmann systems. By measuring the vessel motions, the six hydraulic cylinders can extend or retract in real time by a monitor and control system, thus maintaining the topsides in very small motions related to fixed substructures, but not floating substructures. The barge can move freely in between the slot while the topsides are held in virtually still. This results in that the barge will transfer no horizontal forces onto the substructure. As a result, the fendering system may be simplified significantly while LMUs and DSUs can be totally eliminated.

The floatover barges are positioned under topsides either side of the transportation barge one at a time, where additional four tubular members provide diagonal supports to the vertical stanchions at the barge sideshells. The topsides will be transferred from the transportation barge to the twin barges by ballasting the transportation barge beneath the topsides and deballasting the twin barges, thereby transferring the topsides load completely onto the floatover barges before removing the middle barge. Seafastening is installed to make the topsides and floatover barges into a rigidly connected catamaran. The twin barges are towed from Labuan approximately 110 km to the Kikeh site via a single main tug connected to primary tow bridles to minimize splitting loads on the deck support frames while additional tug connected to temporary stern bridles to improve directional stability and to tail the catamaran for a sharp turn. Upon arrival, the catamaran is hooked up to two pre-installed moorings of the Tender Assist Drilling unit, one to each floatover barge, to act as brakes and pull-backs. Eight maneuvering/positioning soft lines are rigged with the twin barges to the spar hull. The docking system pulls the catamaran over the spar hull until the stabbing cones on the topside align with the LMUs on spar hull legs. The topsides load transfer is completed by deballasting the spar hull as fast as possible until the full weight of the topsides has been removed from the floatover barges and then continues deballasting until the freeboard reaches storm safe level. The floatover barges disengage from the soft lines and are towed clear of the topsides underside, and then disconnected from the preset moorings before demobilization. It should be pointed out that there is a conflict between local barge stability and structural concerns when designing the bridging frame supporting astride between twin barges. The twin barges can be rigidconnected to function as a fixed catamaran, but substantial structural stresses are imparted to the deck and the DSF. The deck may be hinged onto the two independent barges, but then local barge stability may be lost or limit deck transportation weight. Special consideration should be taken to provide virtually infinite local barge stability without significantly imparting the strength of deck and DSF.

truss technique is that it eliminates the need for the open slot during docking while reducing the float-over support truss height and therefore improving transport barge stability. One center transport barge (400’×99.5’×20’) and two twin barges (250’×72’×16’), or called outrigger barges, were used to install all the three decks in less than three weeks utilizing the same set of Versa-Truss system. The installation system consists of three pairs of Versa-Truss booms where the base of each boom is secured to the longitudinal centerline of the outrigger barge deck utilizing pre-fabricated load spreader bars and heel pins. The tip of each boom will be inserted into a specially designed pin installed at the upper deck edge of the topsides. The topsides are lifted using an operation procedure combining the barge ballasting and the winch wire tensioning, where the winch wires are attached to each of the two outrigger barges through the use of the winches located on each of the outrigger barge deck at the base of the booms. Tensioning of the winches effectively increases the inclination angle of the booms, thus transferring the deck load from the center barge to the outrigger barges and finally lifting the deck off the center barge. This active lifting system eliminates passive LMUs and DSUs. The tri-maran and catamaran short tow configurations have no need of the substructure slot and less requirement of water depth, but they are vulnerable to rough seas.

Fig. 8a: Stage 5: Pre-Setdown onto the Jacket

Versa-Truss System Technique
The versa-truss technique is a mechanical floatover method with twinbarge configuration and hinged connection. This system was successfully applied in shallow water of Lake Maracaibo, Venezuela in 1999 to accomplish the installation of the three main complex decks ranging from 4,200Te to 5,400Te. KBR performed detailed design and Crowley provided installation services to the Chevron LL650 Project. Deck lift weights and bridge clearance restrictions precluded the use of conventional heavy lift vessels for the project while extremely shallow water prevented floatover operations with conventional High-Deck techniques. Crowley selected Versa-Truss, now renamed as Versabar, to provide new lifting systems to accomplish the installations. This proven technology was also used to decommission two decks weighing 180Te and 1,200Te in Gulf of Mexico. Chevron determined that it was most economical to fabricate the large integrated topsides directly on a barge. Because of the restricted clearance access to the installation site, Versabar was tasked with developing a load-spreading system to support 6,000Te topsides on six points without increasing the height of the structure more than 36”. This was accomplished by developing an internal load spreading structure to meet the fabrication constraints. The advantage of versa-

Fig. 8b: Versa-Truss Installation in Lake Maricabo, Venezuela Crowley and Versabar jointly developed the ballast and mooring plans, as well as defined the lift sequences in the following six stages: Stage 1: The outrigger barges were brought alongside the center barge one at a time for connecting boom tips with the deck and winch riggings. At this instant, 100% deck load acts on center cargo barge. Stage 2: Then tension up to transfer 30% deck load on the outrigger barges while 70% deck load remaining on the center barge in stable Trimaran configuration for short-distance tow to approach the site.

Stage 2a: If required, 40% deck load remained on the center and 60% deck load transferred on the outrigger barges in survival condition set for tow to safe havens or standby offshore. Stage 3: When approaching to the site 40% deck load shall be transferred onto outrigger barges via further paying in the tension winching rigging lines for preparing removal of the center barge. Stage 4: Further tensioning up for a total lift and then ballast down the center barge to achieve a sufficient clearance for undocking the center barge and avoid any potential impact. Stage 5: Position the catamaran and hook up the pre-installed mooring lines and positioning soft lines. Upon docking in a catamaran configuration, align the mating cones with the substructure legs with soft-line positioning winches and spread mooring lines. See Fig. 8a for details. Stage 6: Lower the topsides by paying out the tension winching riggings until the final setdown, that is, 100% deck load are transferred onto the substructure. Upon confirming the full load transfer, then disconnect all the riggings prior to undocking. Note: the minimum boom angle shall be determined for the initial contact while maintaining 100% deck load on the outrigger barges. Fig. 8b shows the load transfer stage.

on the topsides, open the hatches and dump the drop tanks within 10 seconds so that the lifting arms will be lifted to touch the topsides lifting points, meanwhile approximately 10% of the load is transferred. This is to ensure that the arms don’t slam against the topsides, thus tightly connecting together vertically and horizontally. When both TMLs are connected to the topsides and the drop tanks are emptied, set DP system on stand-by. Pump water from the buoyancy tanks to the ballast tanks until 90% of the topsides load is transferred to the TML lifting arms. Meanwhile the drop tanks are refilled. Prior to a total lift, remaining weight on each leg is checked and adjusted if necessary. The hatches on the drop tanks are fully opened so that the tanks can be emptied in about 10 seconds to ensure that all the arms lift the topsides simultaneously by approximately 2.5meters above the support grillage. See Fig. 9b.

TML System Technique
SeaMetric developed the TML system to principally undertake lifting operations related to decommission and installation of offshore structures. The TML system consists of two DP3 semisubmersible heavy transport vessels (HTVs), each equipped with 4 thrusters, 2 center skid rails, one outer skid rail and one inner skid rail on deck. The HTVs have submersible capabilities up to 20 meters below water surface. The vessels are approximately 140m long, 40m wide and 10.75m deep. The maximum operating draft is 8m. The TML system is equipped with 4 sets of lifting arms per vessel as standard. The maximum static lifting capacity is 20,000 tonnes in fork-lift mode, that is, 2,500 tonnes per lifting arm. When performing floatover operations in Hs = 2.0m the lifting capacity is reduced to approximately 18,000 tonnes. The TML system is based on creating a lift force by shifting ballast water from one side to the other on balanced lifting arms located on two HTV vessels. The telescopic lifting arms hinge-supported at the HTV centerline have two combined buoyancy/ballast tanks and two drop tanks on one side, and two ballast tanks on the other side. Both ballasting pumps and quick ballast evacuation by gravity discharge can be used. Refer to Fig. 9a for details. Similar to the Versa-Truss System, the TML System is a mechanical floatover method with twin-barge configuration and hinged connection. This DP3 system eliminates the conventional docking system and the fendering system while the active TML lifting system also eliminates passive LMUs and DSUs. In addition, the twin-barge configuration has no need of the substructure slot and less requirement of water depth. Due to hinged connection, the local barge stability should be investigated to meet the MWS requirements. The following TML floatover procedures can be completed within 16 hours. Upon completing pre-floatover preparations, the HTV moves towards the transport vessel in individual DP 3 mode one at a time and is positioned adjacent to the transport vessel. When the short range relative position is activated, the HTV with TML system move slowly towards the topsides and align with the topsides lifting points. When all forktips are fully engaged with the guide pins, the remote operated hydraulic claw is closed and the arm is fixed in the horizontal plane. Then set the DP mode to joystick. Upon adjusting the lifting arms about 0.5m below the lifting points

Fig. 9a: TML Lifting Arm with Ballast & Buoyancy Tanks

Fig. 9b: Deck Transfer from Cargo Barge to HTVs prior to docking Set the DP in a tandem Master-Slave mode. The TML system with the lifted topsides approaches towards the pre-installed substructure. Upon aligning the stabbing cones with the support legs, the valves at the bottom of the ballast tanks are opened to empty the tanks, meanwhile the buoyancy tanks and the HTVs are ballasted down to maintain approximate same vessel draft during the load transfer. When all loads are transferred, set DP to joystick mode. When the arms are clear of the topsides, the claws on the arms are opened and the HTVs undock. The TML lifting arms will be tilted upwards about 8? and then all the lifting arms and buoyancy tanks will be seafastened prior to demobilization.

Catamaran Technique
Several catamaran concepts are developed to perform floatover installations and decommissions on conventional jackets, compliant towers, and spar type floating substructures, etc. Allseas developed and planned to build a multi-purpose dynamically positioned large platform installation/decommissioning and pipelay vessel, Pieter Schelte, which is 382 m long, excluding tilting lift beam protrusion and stinger, and 117 m wide and the largest offshore construction vessel to be ever-built. Pieter Schelte will be classed as DP III and has a transit speed of 14 knots and an accommodation of 571 persons. For offshore decommission and installation operations, the vessel will have a capacity to handle topsides of 48,000 metric tons and jackets up to 25,000 metric tons. In addition, Pieter Schelte will be equipped with a 170m stinger, total tensioner capacity of 4×500Te at 30m/min, and pipelay capacity up to 6” to 68” O.D. pipelines.

Schelte. These clamps have been previously adjusted to the exact dimensions of the platform legs. Whilst the vessel itself is in slight motion due to wave action, all motion of the clamps relative to the platform is eliminated by engaging the active motion compensation system. This enables careful closing of the friction clamps around the topsides legs, the natural strong points, specifically for the lifting operation. With the clamps connected, pre-tension in the lift system is gradually decreased in order to transfer the weight of the topsides from the vessel to the jacket. See Fig. 10b for details. Analyses have shown that in hostile environments such as the North Sea, the motion compensation system is essential on a single-lift vessel to eliminate impact forces on large topsides, due to the giant masses involved. In the absence of such a system, local damage will occur even when the wave height is small and vessel motions are very limited. Due to the large motion compensation capacity in both vertical and horizontal directions of Pieter Schelte’s topsides lift units, the dynamic forces introduced in the topsides during engaging and pre-tensioning are very low, even when working in less favorable sea states. In addition, the ample lift capacity can accommodate normal inaccuracy of the topsides weight and/or the position of the centre of gravity. For platform decommission, the reverse of the aforementioned procedures is applied. Capacities of the lifting systems are the same for installation as for removal. Fig. 11 shows another catamaran GM-Lift concept, a Ushaped semisubmersible, developed by AMEC.

Fig. 10a: Pieter Schelte Docking Operation

Fig. 11: Pre-Docking Operation with GM-Lift by AMEC

Floatover Technique for SEMIs or TLPs
The floatover installation of 24,000Te Auger TLP Deck in 1993 is the first open-sea installation onto a floating substructure in place. Shell only used the floatover mating for the Auger TLP in unsheltered deep water in the Gulf of Mexico, but still selected the module lifting method for subsequent TLPs. In the 1980’s floatover mating of large deck modules were successfully conducted on many large SEMI and TLP hulls, including the Hutton and Snorre TLPs, the Njord semisubmersible, and several Norwegian gravity-based systems. Until recently the majority of these integrated topsides have been installed onto the floating substructures in sheltered bays and waters where sufficient water depth was available to submerge platforms to mating drafts. See Fig. 12 for a recent example of P-52 SEMI deck & hull mating. As mentioned earlier, the twin-barge floatover installation of the 3,400Te Kikeh topsides is the second open-sea installation onto a floating substructure in place, which is also the first floatover application to a spar hull. Load transfer and separation are eased by the free-floating substructure

Fig. 10b: Pieter Schelte Mating Operation Pieter Schelte will be one of the most advanced and heaviest construction vessels in operation, and is due to enter service in 2013. The vessel has a U-shaped slot at the bow and can position itself around a substructure for floatover installations and decommissions, as well as removal of jackets. The catamaran slot is 90m long and 43m wide and has been dimensioned to accept narrow types of substructures such as jackets, compliant towers, and spars, although the connecting structure between the twin hulls can be widened in future in order to fit even the largest jackets. The hulls can also be separated at regular intervals, in order to allow the hulls to be inspected during dry-docking. Refer to Fig. 10a for details of the catamaran structure. Floatover topsides can be lifted and lowered by using the hydraulically operated lifting clamps, mounted on eight beams at the bow of Pieter

and not as important as the fixed substructures. The water plane of the floating substructure acts as a soft spring to absorb impact energy and ease separation. However, the general requirement of a catamaran barge configuration for spars may complicate the transportation process. The increased water depth decreases the effectiveness of conventional mooring and docking systems. Since the free-floating substructure is also subject to motion, the relative motions between the barge and the substructure may further complicate barge docking as well as suppression of relative surge and sway motions. The deep water presents challenges for docking operation, especially in heavy seas or swell conditions. The floating-structure draft can be adjusted to meet the overall requirements of floatover dimensions. The minimum achievable barge draft and the minimum allowable freeboard of the floating substructure determine the height of DSF. Their dimensions are less critical when installing onto a floating substructure. Since small waterplane area yields soft vertical hydrostatic stiffness, the elastic pads may not be required for mating the integrated deck. At least this may significantly simplify the design of LMUs and DSUs. However, the out-of-phase motion of the two floating structures may overstress the floatover system. It is very important to synchronize the barge motion with a floating substructure in plane by optimizing mooring and fender systems, which are able to maintain the integrated deck in alignment with the floating substructure. Numerical analysis of two large floating structures can be a challenge due to complex hydrodynamic interaction between the barge and the floating substructure. Model tests should be conducted to accurately predict the dynamic responses and design loads, thus helping develop the analysis technology and optimizing the floatover moorings and fenders. In addition, the small water-plane area of the floating substructure has another advantage, which eases the deballast requirement for mating and separation.

special barge was devised and constructed to be suitable for the loadout, transportation and installation of the two largest integrated topsides ever floatover installed in open sea. The barge is capable of supporting heavy loadout loads transferred from shore by skid ways and having sufficient stability and ballasting capacity to operate in the various conditions of loadout, transportation, and mating operations. The barge consists of 33 ballast tanks with gravity filling system in order to supply the required ballasting capacity and to minimize the impact loads transferred to the GBS during floatover mating. The T-shaped configuration can be divided into two major portions, one main barge hull and two wing pontoons. The two pontoons are connected to the port side and the starboard side of the aftbody of the main barge hull. Both pontoons have same shape and dimensions. See Fig. 13b for the configuration. The main particulars are given as follows: Particulars T-Shape Barge Length Overall T-Shape Barge Width Overall Forebody Width Moulded Pontoon Length Moulded Pontoon Width Moulded Depth Moulded Values 190.0m 92.0m 45.0m 47.5m 23.5m 12.5m

Fig. 13a: Mating Operation of Lun-A Topsides and GBS
in a water depth of approximately 40m offshore

Fig. 12: Pre-Docking Operation of P-52 SEMI Deck & Hull Mating in a water depth of 50m inshore

T-Shape Barge Technique
A special T-Shape Barge, TCB-2, was purpose-built to floatover install the 21,800Te Lunskoye-A (Lun-A) Topsides and the 28,800Te PiltunAstokhskoye (PA-B) Topsides northeast of Sakhalin Island for the Sakhalin Phase II Project. By successfully mating the 28,000Te PA-B topsides to its pre-installed concrete GBS base in July 2007, Sakhalin Energy has broken its own world record set by the Lun-A topsides installation in June 2006. See Fig. 13a. The T-Shape barge is an effort to reduce the slot requirement, thus having less impact on design of the substructure and the topsides. This

Fig. 13b: T-Shape Barge TCB-2 prior to Loadout One service vessel and five tugboats were use to control and support the transport barge carrying the topsides on their 3,000 km voyage from the fabrication yard in South Korea. Once arrival at the site, the barge was carefully towed to enter the slot between the four legs of the

concrete GBS. The massive topsides were then slowly and smoothly lowered over the four legs by ballasting down the transport barge. Special large capacity of four LMUs built into the four legs allowed the topsides to achieve a perfect fit and acted as shock absorbers during the initial contact and mating. The noticeable difference of the barge comparing with other conventional cargo barges is the protruded pontoons from its port and starboard at the aftbody outwards. The pontoons are aimed to provide the additional stability while the frontal area of the T-Shape barge also induces additional environmental loads acting on the GBS structure.

trials should be performed to simulate actual docking operation. During approaching and initial docking the DP system is set in a full DP mode to enter the slot with a precise control. Once entering the slot, the vessel will be maneuvered manually by using joystick or direct control of the propulsion and thruster units to complete the docking operation. During the mating the vessel ballasting should preferably commence on a falling tide while the capture radius of the stabbing cones should be maintained by the fendering system and the DP system in a manual mode. Refer to Beerendonk et al. (2008).

CONCLUSIONS DP Vessel Technique
The floatover installations of three integrated topsides in Gulf of Thailand have been performed utilizing the DP II semisubmersible cargo barges Tai An Kou and Kang Sheng Kou owned by COSCO. These three topsides include the 9,000Te Bunga Raya A Deck installed in July 2003 and the 9,500Te Bunga Raya E Deck installed in February 2006 offshore Malaysia, as well as the 6,000Te Rong Doi Deck installed in August 2006 offshore Vietnam. See Fig. 14 for details. The floatover technology has been growing to full stature as the preferred installation and decommissioning methods for offshore facilities. The wide range of its application has covered the strand-jack lifting topsides onto the pre-installed piles in the extremely shallow water of Bohai Bay, just 1.78m as per chart datum, and the twin-barge floatover configuration of the topsides onto the floating spar hull moored in the 1,320m deep water offshore East Malaysia in South China Sea. Through building track records and benefits over modular lifting installations, the floatover technology will be further developed as a reliable means of installing and decommissioning different kinds of offshore platforms. This paper intends to present the state of the art in the floatover technology and systematically to classify various floatover technologies, and therefore clarifying basic concepts of different floatover techniques. This will not only benefit their further development but also reveal the promising prospect of their wide applications in installation and decommissioning of integrated topsides onto and from various fixed and floating substructures.

ACKNOWLEDGEMENTS
Very special thanks to AkerSolutions, ALE/JGP, Allseas, Dockwise, ETPM, Heerema, KBR, McDermott, Saipem, Technip, Versabar, and WorleyParsons for their invaluable floatover experience and expertise, as well as their valuable photograph courtesy. Fig. 14: Mating Operation of Bunga Raya A Topsides with DP Vessel The DP capabilities have the added bonus of maneuverability and thereby eliminating the mooring spread, soft line winches, and positioning tugs, etc. There are no requirements for mooring winches and soft line winches, as well as associated power packs to be placed on the barge deck. The pre-floatover preparations with a DP system are minimal when compared to the conventional moorings. The DP floatover operation without the aid of conventional moorings and positioning tugs can be completed in a much shorter timeframe, say within three hours between docking and undocking, and therefore providing a significant advantage over a barge in weather sensitive operations. Normally a typical mooring assisted floatover operation requires a 48-hour weather window while a typical DP floatover operation only needs a 24-hour weather window. In addition, there is no need for anchoring or pre-installing buoyed moorings where a number of subsea assets, pipelines and other platforms exist and interfere with the spread mooring system. This will not only keep a minimal offshore spread but also simplify operational chain of command and communications, thus minimizing the human errors of tug masters and winch operators. The DP system is designed to counter the environmental forces. This is one of the most effective ways to damp out the low-frequency motion but is designed not to counteract the wave frequency loads which fluctuate at high frequency. Therefore the fendering system is still required to work with the DP system to align the stabbing cones and the LMU receptors in the final position. Prior to docking, the DP system

REFERENCES
Beerendonk, M, Capt. Groves, N, and Palmer, R (2008). "Float-Over Operations Using Dynamically Positioned Vessels," Proceedings of Royal Institution of Naval Architects RINA Paper 10.3940, 12 pp. Edelson, D, Luo, M, Halkyard, J, and Smiley, D (2008). "Kikeh Development: Spar Topside Floatover Installation," Offshore Technology Conference Paper OTC-19639, Houston, Texas. Gerner, FWB, Tempel, JVD, Zoontjes, R, and Groot, HD (2007). "Motion Compensated Float-Over Installation with the Use of Ampelmann Systems," Offshore Technology Conference Paper OTC18640, Houston, Texas. Labbé, JP, Allègre, V, Volker J, and Agdern, F (1998). "EKPE Gas Compression Project - Float-over Deck Installation," Offshore Technology Conference Paper OTC-8612, Houston, Texas, pp 11-18. Liu, LM, Zhang, SF, Fang, XM, Chen BJ, Hao J, and Wang, AM (2006). "Floatover Installation Succeeds for Nan Bao 35-2 Topsides," Journal of World Oil, July 2006, Vol. 227, No. 7, pp 63-69. O'Neill, LA. Fakas, E, Ronalds, BF, and Christiansen, PE (2000). "History, Trends and Evolution of Float-Over Deck Installation in Open Waters," SPE Annual Tecnical Conference and Exhibition Paper SPE-63037, Dallas, Texas. Seij, M, and Groot, HD (2007). "State of the Art in Float-Overs," Offshore Technology Conference Paper OTC-19072, Houston, Texas. Tribout, C, Emery, D, Weber, P, and Kaper, R (2007). "Float-Overs Offshore West Africa," Offshore Technology Conference Paper OTC19073, Houston, Texas.


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