NPA FINAL REPORT – OFFSHORE OIL & GAS
6.0 TRENDS IN STRUCTURES FOR THE OFFSHORE OIL &
GAS INDUSTRY
Bottom Supported Structures
Converted Jack-up Barges
Jack-up Rigs
Fixed Leg Towers
SPARS
Semi-Submersible
Floating, Production, Storage and Offloading
Topsides
Mooring Facilities
Sub-Sea
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NPA FINAL REPORT – OFFSHORE OIL & GAS
6.0
TRENDS IN STRUCTURES FOR OFFSHORE OIL & GAS
DEVELOPMENTS
The marine environment in the area of an offshore hydrocarbons reservoir, largely
dictates the type of structure that will be required above the ocean floor for
exploitation purposes. In the early days of the offshore oil & gas industry, reserves
sufficient to meet market demand were exploitable in relatively shallow and calm
waters.
A Platform is a generic term used in the Oil & Gas Industry to depict an integrated
fixed or anchored facility in the sea, from which drilling and exploitation is made
possible. Topsides, as they are known comprise oil or gas handling, treatment and
stabilising facilities together with living quarters for operators. The topsides are
supported on
 “jackets” which sit on the sea bed anchored by piles or
 a floating structure, supporting the mass of the topsides.
Bottom Supported Structures
The following types of support structures comprise the present day market for
offshore fabrication and assembly.
Converted jack-up barges – Used only in remote areas and in very shallow waters.
Jack-up barges can be constructed very easily in shipyards.
Jack-up Rigs – The topsides of the rig has legs which are cranked or winched down
to the sea bed, lifting the topsides deck out of the water for drilling or production
purposes. The market for new rigs fluctuates year by year, but refitting/upgrading of
existing rigs provides a steady source of business for several yards.
Fixed leg towers – Can be used in water depths up to 300 meters, although most
commonly in depths less than 150 meters. Within this category there are 4-leg, 6-leg
and 8-leg towers. Compliant towers are commonly used in the relatively calm waters
up to 300metres. Fixed towers of up to 180 meters are to be found in the more
hostile environments such as the North Sea or the Southern Coastline of South
Africa.
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NPA FINAL REPORT – OFFSHORE OIL & GAS
Within this category are “minimal structures”, whose decks are supported by a
single, unbraced or pile braced caisson. Minimal structures are used in water depths
less than 50 metres.
Tension Leg Platforms – TLP’s are used in water depths greater than 200 metres.
They consist of a floating steel deck structure, which is anchored to pile heads on the
sea bed by means of long pipes. The pipes are always kept in tension and thus can
be flexible without risk of buckling collapse failure.
SPARS
A SPAR is a vertical floating steel cylinder, attached by means of cables to anchors
placed on the sea bed more than a kilometre distant. SPARS are becoming common
for very deep water activities. Facilities similar to those required for a jacket assembly
are required, potentially without the expense of as many high capacity crawler
cranes.
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NPA FINAL REPORT – OFFSHORE OIL & GAS
Floating Structures
Floating Structures
Semi-submersibles
Semi-subs are used both for exploration drilling and as floating production systems
(otherwise known as SFPSs). As with jack-ups, the market for new semi-sub drilling
platforms is limited, but refits and conversions are ongoing. Fabrication requirements
notwithstanding, a South African location for final assembly of a semi-sub, could
present some problems, where sufficient water depth, in calm conditions, close to the
fabrication yard, yet well clear of shipping channels, might be difficult to find.
Saldanha might be an option in such circumstances.
Typical Semi-Submersible Drilling Rig
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NPA FINAL REPORT – OFFSHORE OIL & GAS
Floating Production Storage & Offloading (FPSO)
The term “FPSO” embraces any type of floating production, storage and offloading
system, including purpose built vessels, tanker/barge conversions, semi-submersible
platforms, tension leg platforms, and spars. However, the term is used most
commonly to describe a ship-shape vessel with on-board process plant and storage
capacity, which has been newly constructed or converted from an existing vessel. In
the case of a new, purpose built FPSO, the approach to date has been to construct
the hull or “ship-shape structure” in shipbuilders yard, and thereafter move the ship to
another country location for fitting out with topsides. This location is normally an
established module yard, where process and utilities units, living quarters etc have
been pre-fabricated and assembled in readiness for arrival of the ship. The same
principle is applied to existing hull conversions, although some conversions have
been carried out at the same locations as module assemblies.
A typical Floating Production Storage and Offloading System
Topsides
Topsides are best defined as anything above the water line which is supported
directly or indirectly by a Jacket, SPAR, TLP, Semi-Sub, or FPSO, including decks,
process modules (oil/gas separation etc), utilities modules (power generation, water
treatment etc), living quarters, the drilling structure, helicopter pad, flare stack, crane
unit.
Such topsides are common to virtually every type of offshore installation and will
continue to be so regardless of technology developments sub-sea. Most Fabrication
Yards will accommodate both structures and topsides. These Yards also assemble
modules for onshore oil & gas projects.
Decks
Deck structures generally sit on top of a jacket, or TLP, or Spar, in turn
supporting the modules, living quarters etc. Some decks, or module support
frames (MSF’s) as they can be known also contain equipment, tanks, piping
etc, effectively becoming modules in themselves. A deck structure might be
described as easier to fabricate/assemble and load out than a jacket, or spar,
which physically and dimensionally would normally be the case, but materials
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NPA FINAL REPORT – OFFSHORE OIL & GAS
of construction can be of equally high specification and some of the subassemblies equally complex.
The Mossgas Deck (MSF) being loaded out in Durban
Modules
A true module comprises a steel box frame, varying in size, containing
pressure vessels, pumps, piping, electrical equipment and cabling,
instruments and instrument tubing, forming part of a process or utilities
system. A module is engineered and designed to industry standards, the only
difference between a unit of modularised plant, and a section of conventional
plant, being the compactness, which must be inherent in the design. Similarly,
skills required to assemble a module, such as steel erection, equipment
installation, piping erection, cabling etc., are no different to the norm, but
planning/scheduling of the work has to be of a higher standard than normal.
Modules are often inspected, tested and precommissioned whilst still in the
Assembly Yard, being transported as ready-to-operate individual units.
Mossgas Modules at Charl Malan Quay Port Elizabeth
Living Quarters
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NPA FINAL REPORT – OFFSHORE OIL & GAS
Living quarters for an operational offshore facility can be located either on the
main processing platform, or on an adjacent structure, interconnected by a
bridge. In some cases, permanent accommodation is provided by a semisubmersible nearby, or by a “Flotel”. The latter are used extensively during
hook up and commissioning phases. Quarters can comprise either a number
of individual modules, of “ski cabin” proportions, each providing a component
of living requirements (sleeping, eating, recreation etc) or a full sized module
containing all requirements in one unit. Fully integrated living quarter modules
are extremely complex, due to the multitude of services and finishes involved,
requiring considerable hands-on experience.
The Mossgas Accommodation Module
Other topsides structures
Drilling derricks, flare booms, helicopter decks and bridges to adjacent
platforms, together with a miscellany of smaller items, all go into making up
topsides and all of which must be fabricated/assembled in a yard somewhere.
The Mossgas Drilling Derrick on Load Out
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NPA FINAL REPORT – OFFSHORE OIL & GAS
Mooring Facilities
Various types of buoys are required offshore including, other than for mooring
purposes, flare buoys, well head control buoys.
Sub-Sea
With advances in sub-sea completion technology, demand for complex sea bed
modules has increased and many companies are specialising in fabrication and
assembly of same.
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