Dr.Eng. Rudi W. Prastianto
Outline 1. Type ype of Deck Deck Conc Concep epts ts 2. Deck Deck Str Struc uctu turral Sys Systtems ems 3. Factor actorss aff affecting ecting Offs Offshor hore e Platf Platform orm Deck Deck Types
1. Type of Deck Concepts
a). The Integrated Deck •
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An integrated deck may be divided into a number of levels and areas depending on the functions they support. If the dry deck weight and geometry are within the available crane capacity an integrated deck configuration may be considered.
a). The Integrated Deck (cont. …) Typical levels of the Integrated Deck are: Main (upper) deck: supports the drilling/production systems and several modules (drilling, process, utilities, living quarters, compression, etc.). Cellar deck: supports systems that need to be placed at a lower elevation and installed with the deck structures (e.g. pumps, some utilities, pig launchers/receivers, Christmas trees, wellhead manifolds, piping, etc). Additional deck levels, if needed. For example, if simultaneous drilling and production operations are planned, some process equipment may be located in a mezzanine deck . •
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The Integrated Deck (An Example: Topside layout of Diana SPAR Design) •
The Upper Deck (is called the “Drilling Deck”): •
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The production and temporary quarters buildings, drill rig, chemical tote tank storage, and communications & radar satellite dishes.
The mid-level (Mezzanine) Deck (is called the “Production Deck”): •
The majority of oil and gas separation, processing, treating, compression equipment, power generation equipment, the MCC/Control Room and many of the utilities.
The Integrated Deck (An Example: Topside layout of Diana SPAR Design) •
The lower “Cellar Deck”: •
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Other utility systems (cooling water, fresh water, firewater, flare scrubber, etc.) as well as oil and gas sales meters, pipeline pig launchers and receivers, manifolds and shutdown valves.
A Sub-cellar Deck which is a partial deck suspended below the cellar deck could also be installed usually small, it could be designed to withstand impact from the wave crest and transport the lateral loads to the rest of the structure: –
to contain the gravity drain sumps and pumps.
b). The Modular Deck
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A modular deck a number of pieces and modules depending on the functions they support and available installation equipment. Modular deck system will be used, when:
The deck weight or geometry exceeds the available crane lift capacity, or
Modules containing some of the production equipment (e.g. a compression or pumping module) will be added at later dates.
b). The Modular Deck (cont. …) The typical modular deck components are: a) Module Support Frame (MSF):
A space frame (a system of horizontal girders and vertical and diagonal braces) supporting the modules and transferring their load to the jacket/tower structure.
Be designed to envelope a number of platform facilities (e.g. the storage tanks, pig launching and receiving systems, metering/proving devices and the associated piping systems).
b) Modules provide a number of production and life support systems.
Various of the Modules •
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Living quarters module generally supporting a heliport, communication systems, hotel, messing, office and recreational facilities, Utilities module generally supporting power generation and electrical and production control systems, including a control room, Wellhead module generally supporting the wellheads, well test and control equipment, Drill rig module containing the drill tower, drawworks, drillers and control rooms, drill pipe and casing storage racks and pipe handling systems. Drill rig module is located over and supported by the wellhead module.
Various of the Modules (cont. …) •
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Production module containing the oil/gas/water separation and treatment systems and other piping and control systems and valves for safe production, metering and transfer of the produced liquids and gas to the offloading system. A compression module may be added, if gas compression for injection to the formation and/or highpressure gas pumping to shore is needed. Since compression may be needed at later production stages, this module may be installed on the deck at a later date or on a nearby separate platform (generally bridge connected to the deck). Similarly, water injection and pumping modules may be added if these functions are needed at later field development stages.
c). Hybrid Deck •
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Hybrids of modular and integrated deck. Some of the utility, wellhead and/or process equipment and storage may be placed inside the MSF, which would be installed in a manner similar to an integrated deck, followed by the installation of modules on its top.
Note •
In general, Integrated Decks result in more efficient and lighter structural systems since additional module steel, which is only needed for installation reasons, is avoided.
2. Deck Structural Systems •
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The main function of the Deck Structural System provide a suitable horizontal area above the water surface where the drilling and production operations are performed. The Deck Framing transfers the loads from equipment supplies and operations exerted on the deck coverings to the deck legs, which in turn transfers loads to the jacket legs and the foundation.
2. Deck Structural Systems Deck structural component sizing
Estimation of the deck loads (e.g. the drilling rig, production equipment, live load, supplies, etc.)
Initial deck sizing (deck plate and beam sizes) upper bound distributed loads representing the deck equipment and supplies may be used. (main deck = 500 1000 psf loads, mezzanine and cellar decks = 300 500 psf loads)
For detailed design (computer model) more accurate equipment and operational loads are used.
Type of the Deck Framing Systems 1.
A popular system the loads are transferred through flooring and deck beams to longitudinal trusses made of tubulars and standard or built up beams (Figs. a & b).
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Portal frames with stiffened plate girders directly framing into deck legs (Fig. c).
3.
Hybrids of the above-described deck structural systems: •
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The main load bearing structure (truss lines 1 and 2 in Fig. d) would be K-braced trusses, The transverse wind bracing structure (lines A and B) would be stiffened girders.
Designing the plate and beam flooring systems
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The plate flooring cases (if properly welded to the deck beams) : –
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The deck plate acts as a membrane distributes the horizontal wind and wave loads to the main deck structure. Particularly important for diagonal loadings force the deck to behave as a horizontal portal frame.
If deck plate is not present using horizontal diagonal braces connecting the column tops at each deck level should be considered.
Designing the plate and beam flooring systems (cont. …) •
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The “lowest weight” solution for a specified deck load may not always result in the “lowest cost”.
Factors influence the initial and maintenance costs of the system designed: –
number of cuts,
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ease of fitting,
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length and difficulty of welds &
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ease of maintenance.
Typical Stacked-Beam Deck Flooring System
1. K-Braced Deck Truss System
2. Stiffened Plate Girder Deck
3. Hybrids Deck System
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No major cost difference between the trussed or stiffened plate girder deck structures. The fabrication of the plate girders may pose dimensional control difficulties for construction yards with no plate girder fabrication experience.
Deck Structure Flooring System
1. Stacked Beam Deck System 2. Two-way (Orthotropic) Deck System
1. Stacked-Beam Deck Flooring System
1. Stacked-Beam Deck Flooring System
The stacked deck plate-deck beam-main girder system (fig. a) results in heavier decks but requires much less labour to build it. If the deck width < 80 ft the deck beams could be ordered from the manufacturers as single pieces, assembled stacked over the main deck girder, plated/grated over, lifted and installed over the pre-fabricated deck legs and braces much lower fabrication costs. The deck plate loads are transferred to the deck beams. Deck beam spacing dictated by the wellhead spacing. Beams are generally placed at equal spacing, spanning the space between the main deck trusses (or main deck girders).
1. Deck Buta
1. Stacked-Beam Deck Flooring System A standard GoM platform drilling rig the skid beam spacing dictates the deck leg spacing for a drilling platform or module. Placing the deck legs directly underneath the skid beams facilitates direct transfer of the high rig and drilling loads onto the column tops, resulting in an efficient structural system. Most GoM drilling platform: Have 40 ft skid beam spacing. Use of cantilevers to gain additional deck space Most efficient cantilevers span 1/2 the lengths of the deck spans. GoM platform deck footprints 80 ft by 80 ft four legged and 120 ft by 80 ft eight-legged.
2. A Two-way Framed (Orthotropic) Plate-Beam Approach
The deck plate, deck beam & truss upper (lower) chord are replaced by a continuously welded plate/beam system. The plate carries the direct loads along its span while it functions as top flange of the deck beam and the main girder system. Lightweight but expensive structures because of the extensive cutting, fitting and welding requirements. Popular in deepwater fixed or floating platforms Compliant Tower platforms (CPT), Tension Leg Platforms (TLP), SPARS and Semi-submersible floating vessels (SSV) the deck weight may drive the platform cost.
2. Two-way (Orthotropic) Deck Framing Details
3. Factors affecting
Offshore Platform Deck Types 1. Operational requirements, 2. The fabrication infrastructure, 3. Installation equipment availability.
1. Operational requirements Operational requirements dictate: The general deck size, and Configuration (number of deck levels and their layout, etc.). • •
Example: A need for a fully integrated drilling and production system would dictate vertical and horizontal layering of the deck structure to provide an efficient operation + providing an acceptable level of human and environmental safety.
2. The fabrication infrastructure Fabrication facilities and labour : If fabrication facilities and skilled labor are not available the economics may dictate building deck: in smaller pieces & modules This approach may increases: steel weight; offshore construction time and cost, while the expense of investing in a major fabrication yard may be avoided.
2. The fabrication infrastructure Fabrication facilities and labour: If fabrication infrastructure and equipment are readily available:
Design the deck as an integrated single piece structure, or
as a Module Support Frame (MSF) supporting few large modules.
3. Installation equipment availability
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Deck in smaller pieces & modules assembling these offshore using low-capacity offshore lifting equipment available. The “integrated deck” be installed in site using [Cotrell and Adrian, 2001]: –
High capacity lifting cranes,
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A float-over deck installation approach.
