Foundations for Offshore Structures
Offshore structures are the structures secured on the sea bottom through deep foundation
systems, built as gravity type structures, or as floating production facilities. They are large
structures with facilities for well drilling to explore, extract, store, and process petroleum and
natural gas that lies in rock formations beneath the seabed. The most basic requirement for
offshore exploration activities is a platform/structure from which the whole operation can be run.
Site investigations for offshore structures are necessary to acquire data that will facilitate
successful foundation design, site or route selection, choice of foundation type, dimensioning,
installation and operational integrity of the proposed structure (ISSMGE, 2005). Geotechnical
parameters are developed from laboratory tests performed on piston samples or from highquality push soil samples and from in situ test data collected from cone penetration tests with
pore pressure measurements or field vane, T-bar and ball probe tests.
Offshore structures are generally classified into two categories, which are Fixed Structures and
Floating Structures.
Fixed Structures
Fixed Structures are bottom or deck supported by legs directly connected onto the seabed
foundations. They can be tubular steel jackets, concrete or hybrid gravity structures. Steel jackets
are primarily pile supported, while gravity structures achieve stability by virtue of their immense
structural weight and large diameter base. Additional stability may be provided by use of base
skirts, which penetrate several meters into the seabed. However, the limitation of this kind of
structures is related with economic issues; therefore, they are not used in water depths greater
than 500 m. Fixed structures are further sub-categorized as Steel jacket, Concrete gravity
structure, Tower foundation, Compliant tower and Monotower.
Steel jacket - The Steel template type structure consists of a tall vertical section made of tubular
steel members supported by piles driven into the sea bed with a deck placed on top, providing
space for crew quarters, a drilling rig, and production facilities. The fixed platform is
economically feasible for installation in water depths up to 500m. These template type structures
will be fixed to seabed by means of tubular piles either driven through legs of the jacket (main
piles) or through skirt sleeves attached to the bottom of the jacket. The principle behind the fixed
platform design is to minimize the natural period of the structure below 4 seconds to avoid
resonant behaviour with the waves (period in the order of 4 to 25 seconds. The structural and
foundation configuration shall be selected to achieve this concept.
Figure 9.1. Traditional Large Steel Jacket Plate
The tower is also a welded tubular space frame with three or more near vertical tubular chord
legs with a bracing system between the legs. The tower provides support for the topsides,
conductors, risers and other appurtenances. A tower foundation usually includes cluster piles
which are inserted through and connected to sleeves around the corner legs at the base of the
structure (Figure 9.2). Additional piles, called skirt piles, can be inserted through and connected
to sleeves at the base and along the perimeter of the structure. As an alternative to piles, a tower
can be supported by another foundation system that supports it at its base, such as bucket
foundations. Cluster and skirt piles, or other foundation system, anchor the structure and transfer
both vertical and horizontal actions to the seabed. The global behavior of a tower is that of a
vertical cantilever with all actions being transferred to the foundation system at the base of the
tower.
Figure 9.2. Foundation Tower
A compliant tower is a flexible structure with flex elements (principally flexible legs or axial
tubes) to control mass and stiffness characteristics to mitigate the effects of periodic wind, wave
and current forces. Natural periods are usually greater than 25 sec so they are generally well
outside wave periods. They consist of a narrow, flexible tower and a piled foundation that can
support a conventional deck for drilling and production operations. Unlike the other fixed
platforms, the compliant tower withstands large lateral forces by sustaining significant lateral
deflections, and is usually used in water depths between 300m and 600m. Compliant towers are
relatively slender compared with jacket/tower structures but have higher installation costs and
use a considerable amount of steel. They can support a large number of wells (> 40), with dry
trees, and can sustain a large payload
Figure 9.3. Compliant Tower
A monotower is a fixed structure in which the whole structure, or at least the upper part of the
structure, consists of a single vertical column (tubular or framed) that carries the topsides.
Where the monotower consists of a single vertical column over its full height that continues into
the seabed as the foundation pile, this is described as a ‘free-standing caisson’ or simply
‘caisson’. A ‘braced caisson’ is a monotower where the lower part of the column is laterally
supported by one or more inclined braces between the column and one or more foundation piles.
Monotowers are designed to be unmanned or not normally manned.
Figure 9.4. A Monotower
Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is
remote. These platforms are very common in areas with strong seabed geological conditions
either with rock outcrop or sandy formation. Some part of North Sea oil fields and Australian
coast, these kind of platforms are located. The concrete gravity platform by its name derive its
horizontal stability against environmental forces by means of its weight. These structures are
basically concrete shells assembled in circular array with stem columns projecting to above
water to support the deck and facilities. Concrete gravity platforms have been constructed in
water depths as much as 350m.
Condeep built
The Ninian Platform
Figure 9.5. Concrete Gravity Platforms
Floating Structures.
Floating structures must have enough buoyancy to support the weight of the turbine and to
restrain pitch, roll, and heave motions within acceptable limits. The most important loads to
consider are wind turbine thrust, wave loads, wind turbine torque and drift forces. There are
some key differences in the load characteristics of floating wind turbines to that of floating oil
rigs. While floating oil rigs and payload and wave driven, floating wind turbine loads are
primarily wind-driven overturning moments.
Many floating concepts are easier to construct and install than fixed structures – the
decommissioning process is easier as well. However, floating support structures have several key
challenges to overcome, one of which is turbine stability. Floating structures must be designed in
a way that they can support wind turbines in the megawatt class, while minimizing turbine and
wave-induced motion. The added complexity in the design process, including understanding the
coupling between the support structure and the wind turbine is another hurdle the industry must
overcome. The common types of floating structures are:
A tension-leg platform (TLP) or extended tension leg platform (ETLP') is a vertically
moored floating structure normally used for the offshore production of oil or gas, and is
particularly suited for water depths greater than 300 m and less than 1500 m. Use of tension-leg
platforms has also been proposed for wind turbines.
The platform is permanently moored by means of tethers or tendons grouped at each of the
structure's corners. A group of tethers is called a tension leg. A feature of the design of the
tethers is that they have relatively high axial stiffness (low elasticity), such that virtually all
vertical motion of the platform is eliminated. This allows the platform to have the production
wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the
seafloor. This allows a simpler well completion and gives better control over the production from
the oil or gas reservoir, and easier access for downhole intervention operations.
Figure 9.6. Tension leg platform
Spar (Single Point Anchor Reservoir) is a type of floating oil platform typically used in very
deep waters, and is named for logs used as buoys in shipping that are moored in place vertically.
Spar production platforms have been developed as an alternative to conventional platforms. The
deep draft design of spars makes them less affected by wind, wave and currents and allows for
both dry tree and subsea production.
A spar platform consists of a large-diameter, single vertical cylinder supporting a deck. The
cylinder is weighted at the bottom by a chamber filled with a material that is denser than water
(to lower the center of gravity of the platform below its centre of buoyancy, providing stability).
Additionally, the spar hull is encircled by helical strakes to mitigate the effects of vortex-induced
motion. Spars are permanently anchored to the seabed by way of a spread mooring system
composed of either a chain-wire-chain or chain-polyester-chain configuration.
There are three primary types of spars; the classic spar, truss spar, and cell spar. The classic
spar consists of the cylindrical hull noted above, with heavy ballast tanks located at the bottom of
the cylinder.
Figure 9.7: Mad dog Spar Platform
A semi-submersible foundation is a type of floating wind foundation that is partly submerged
to provide station keeping and stability. It typically comprises a submerged hull typically
pontoon-type columns joint together with the foundation by large tubulars. The benefit with the
semi-sub is to avoid large wave loads by having the smaller tubulars in the splash zone.
Figure 9.8. Semi-Submersible Platform
A Floating Production Storage and Offloading (FPSO) unit is a floating vessel used by the
offshore oil and gas industry for the processing of hydrocarbons and for the storage of oil or
natural gas. Ship-shaped FPSOs are by far the most widely used type of floating production unit,
accounting for over 100 deployments globally. As operators are moving into deeper waters,
FPSOs are continuing their popularity as the most cost-effective units. Design, maintenance and
inspection of ship-shaped FPSOs have drawn upon the extensive experiences from tankers. The
major difference between a trading tanker and an FPSO moored at a specific location is that dry-
docking for inspections would be more complicated, and usually not possible, for FPSOs that
need to be stationary at the offshore site.
Figure 9.9. Floating Production Storage and Offloading (FPSO) Platform
Foundations for Telecommunication Tower
Telecommunication towers are multistory structures usually constructed of steel. The height is
much higher than the length and width of the structure. Therefore because of the height, it is
usually subjected to wind load and little axial load. Because of the wind load, they are subjected
to horizontal forces which tend to overcome the telecommunication towers. The foundations are
usually subjected to uplift loads.
For stable tower structure, the restraining moment due to self-weight of the structure and soil
must be greater than the overturning (uplifting) moment due to horizontal and uplift loads,
otherwise the structure will collapse. Hence, the tower has higher tendency to fail by uplifting rather
than failed by bearing, sliding and other failures.
Figure 9.9. Forces on Telecommunication Tower