Latest Progress in Floatover Technologies for Offshore Installations and Decommissioning

Latest Progress in Floatover Technologies for Offshore Installations and Decommissioning Alan M. Wang, Xizhao Jiang, Changsheng Yu, Shaohua Zhu, Huailiang Li, Yungang Wei Installation 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. KEY WORDS: Floatover technology; Hi-Deck, Smart-Leg®; Strand Jack Lifting; TML®; Unideck®; Versa-Truss®. NOMENCLATURE AHTS = Anchor Handling Tow Supply (tug) DP = Dynamic Positioning DSF = Deck Support Frame DSU = Deck Support Unit FPSO = Floating Production Storage Offloading GBS = Gravity Base Substructure GPS = Global Positioning System LMU = Leg Mating Unit LSF = Loadout Support Frame TLP = Tension Leg Platform TML = 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, 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. 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. 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 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 “Hi- Deck” 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. 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 Beam Seas Quartering Seas Wave Height (Hs) 1.5m 0.8m 1.2m Wave Period (Tp) 5 - 10 sec 4 - 7 sec 5 - 8 sec 1-Min Mean Wind Speed at EL(+) 10m 10m/sec 10m/sec 10m/sec Surface Current 1.5m/sec 1.5m/sec 1.5m/sec 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 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 pre- install 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 pre- installed 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 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