PUBLIC INFORMATION SERIES
REPRESENTATIONAL PLANNING, ENGINEERING, ENVIRONMENTAL & TECHNOLOGY EXHIBITS
PRESENTATION 2012
TENSION LEG MOORING & ARTIFICIAL REEF SYSTEMS
OFFSHORE INTERNATIONAL AIRPORT PLATFORM PROGRAM
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Undersea mooring of human-engineered floating structures include a large number of offshore oil and gas platforms and, since 2008, a few floating wind turbines. Two common types of engineered design for anchoring floating structures include tension-leg and catenary loose mooring systems."Tension leg mooring systems have vertical tethers under tension providing large restoring moments in pitch and roll. Catenary mooring systems provide station keeping for an offshore structure yet provide little stiffness at low tensions."A semi-submersible is a specialised marine vessel with good stability and seakeeping characteristics. The semi-submersible vessel design is commonly used in a number of specific offshore roles such as for offshore drilling rigs, safety vessels, oil production platforms and heavy lift cranes. |
The terms semisubmersible, semi-sub or just semi are also generally used for this Offshore International Airport Platform design.
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A semi-submersible obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the good stability of the concept, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck. |
With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must be carefully trimmed to maintain stability. Unlike a submarine or submersible, during normal operations, a semi-submersible vessel is never entirely underwater.
A semi-submersible vessel is able to transform from a deep to a shallow draft by deballasting [removing ballast water from the hull], and thereby become a surface vessel. The heavy lift vessels use this capability to submerge the majority of their structure, locate beneath another floating vessel, and then deballast to pick up the other vessel as a cargo.
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TENSION LEG PLATFORM
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A type of floating production system, tension leg platforms [TLPs] are buoyant production facilities
vertically moored to the seafloor by tendons.
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While a buoyant hull supports the platform's topsides, an intricate mooring system keeps the TLP in place. The buoyancy of the facility's hull offsets the weight of the platform, requiring clusters of tight tendons, or tension legs, to secure the structure to the foundation on the seabed. The foundation is then kept stationary by piles driven into the seabed.
The tension leg mooring system allows for horizontal movement with wave disturbances, but does not permit vertical, or bobbing, movement, which makes TLPs a popular choice for stability, such as in the hurricane-prone Gulf of Mexico.
The basic design of a TLP includes four air-filled columns forming a square. These columns are supported and connected by pontoons, similar to the design of a semisubmersible production platform. Nonetheless, since their inception in the mid 1980s, TLP designs have changed according to development requirements. Now, designs also comprise the E-TLP, which includes a ring pontoon connecting the four air-filled columns; the Moses TLP, which centralizes the four-column hull; and the SeaStar TLP, which includes only one central column for a hull.
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TECHNICAL CREDITS
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Larger TLP's will normally have a full drilling rig on the platform with which to drill and intervene on the wells. The smaller TLPs may have a workover rig, or in a few cases no production wellheads located on the platform at all.
The deepest Tension Leg Platforms [TLP] measured from the sea floor to the surface [estimated] are:
4,674 ft. [1,425 m] Magnolia ETLP. Its total height is some 5,000 feet [1,500 m].
4,300 ft. [1,311 m] Marco Polo TLP
4,250 ft. [1,295 m Neptune TLP
3,863 ft. [1,177 m] Kizomba A TLP
3,800 ft. [1,158 m] Ursa TLP. Its height above surface is 485 ft. [148 m] making a total height of 4,285 ft. [1,306 m].
3,350 ft. [1,021 m] Allegheny TLP
3,300 ft. [1,006 m] W. Seno A TLP
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BALLAST & TENSION LEG MOORING STUDIES · CUT SECTION 0.00 - 6.55
SUBSEA GEOTECHNICAL ENGINEERING
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In subsea geotechnical engineering, seabed materials are considered a two-phase material composed of 1: rock or mineral particles and 2: water. Structures may be fixed in place in the seabed—as in piers, jetties, or fixed-bottom wind turbines—or may be floating structures anchored to remain in a sea-surface position that remain roughly fixed relative to its geotechnical anchor point.Geotechnical engineers perform geotechnical investigations to obtain information on the physical properties of soil and rock underlying [and sometimes adjacent to] a site to design earthworks and foundations for proposed structures, and for repair of distress to earthworks and structures caused by subsurface conditions. |
A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves in-situ testing [two common examples of in-situ tests are the standard penetration test and cone penetration test. In addition site investigation will often include subsurface sampling and laboratory testing of the soil samples retrieved. The digging of test pits and trenching [particularly for locating faults and slide planes may also be used to learn about soil conditions at depth. Large diameter borings are rarely used due to safety concerns and expense, but are sometimes used to allow a geologist or engineer to be lowered into the borehole for direct visual and manual examination of the soil and rock stratigraphy.
A variety of soil samplers exist to meet the needs of different engineering projects. The standard penetration test [SPT], which uses a thick-walled split spoon sampler, is the most common way to collect disturbed samples. Piston samplers, employing a thin-walled tube, are most commonly used for the collection of less disturbed samples. More advanced methods, such as ground freezing and the Sherbrooke block sampler, are superior, but even more expensive.
Atterberg limits tests, water content measurements, and grain size analysis, for example, may be performed on disturbed samples obtained from thick walled soil samplers. Properties such as shear strength, stiffness hydraulic conductivity, and coefficient of consolidation may be significantly altered by sample disturbance. To measure these properties in the laboratory, high quality sampling would required. Common tests to measure the strength and stiffness include the triaxial shear, unconfined compression test.
Surface exploration can include geologic mapping, geophysical methods, and photogrammetry, or it can be as simple as an engineer walking around on the site to observe the physical conditions at the site. Geologic mapping and interpretation of geomorphology is typically completed in consultation with a geologist or engineering geologist.
Geophysical exploration is also sometimes used; geophysical techniques used for subsurface exploration include measurement of seismic waves [pressure, shear, and Rayleigh waves], using surface-wave methods and/or downhole methods, and electromagnetic surveys [magnetometer, resistivity, and ground-penetrating radar].
ARTIFICIAL REEF DEVELOPMENT STUDIES
OFFSHORE PLATFORM TENSION LEG MOORING SYSTEMS
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Artificial reefs have been created with many materials, including sunken ships, old cars, armored vehicles and even tires, the electro-accretion process builds a reef that is closest in composition to the natural reef.In addition to artificial reefs, this process could be incorporated in mid-ocean and deep-sea structures. These possibilities include wind farms, out of sight of land, or artificial islands for resort and recreational use.One of the more interesting possibilities is specific-built artificial reef structures configured upon the sea bed mooring system of a floating structure in deep water. An artificial ecosystem could be developed, providing the basis for sustainable fish-farming and mariculture.Building an artificial mooring bed reef initiates the ecosystem. Adding real coral parents a seafood pyramid. With such a structure, the food pyramid could enhance economically useful mariculture members from kelp to sardines, groundfish, tuna and cod. |
OPLATArtificial ocean structures could become an important part of maintaining biodiversity, while at the same time providing new locations to practice aquaculture and mariculture for the purpose of growing food and other oceanic products.
TECHNICAL DATA SOURCE CREDIT
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GENERAL DISTRIBUTION OF COLD-WATER CORALSDeep-water corals are found globally, from coastal Antarctica to the Arctic Circle. In northern Atlantic waters, the principal coral species that contribute to reef formation are Lophelia pertusa, Oculina varicosa, Madrepora oculata, Desmophyllum cristagalli, Enallopsammia rostrata, Solenosmilia variabilis, and Goniocorella dumosa. Four of those genera (Lophelia, Desmophyllum, Solenosmilia, and Goniocorella) constitute the majority of known deep-water coral banks at depths of 400 to 700 m [Cairns and Stanley, 1982].Deep-water corals are similar in some ways to the more familiar corals of shallow, tropical seas. Like their tropical equivalents, the hard corals develop sizeable reef structures that host rich and varied invertebrate and fish fauna. However, unlike their tropical cousins, which are typically found in waters above 70m depth and at temperatures between 23° and 29° C, deep-water corals live at depths just beneath the surface to the abyss [2000 m], where water temperatures may be as cold as 4° C and utter darkness prevails. |
Deep-water corals range in size from small solitary colonies to large, branching tree-like structures, which appear as oases of teeming life surrounded by more barren bathymetry. The gorgonians [sea fans] also range from small individuals to those with tree-like dimensions. The gorgonian, Paragorgia arborea, may grow in excess of three meters in length [Watling, 2001]. Growth rates of branching deep-water coral species, such as Lophelia and Oculina, range from ~ 1.0 - 2.5 cm/yr, whereas branching shallow-water corals, such as Acropora, may exceed 10-20 cm/yr. Using coral age-dating methods, scientists have estimated that some living deep-water corals date back at least 10,000 years [Mayer, 2001].
However, little is known of their basic biology, including how they feed or their methods and timing of reproduction.
TERRITORIAL WATERS EXHIBITTerritorial waters, or a territorial sea, as defined by the 1982 United Nations Convention on the Law of the Sea, is a belt of coastal waters extending at most twelve [12] nautical miles [22 km; 14 miles] from the baseline [usually the mean low-water mark] of a coastal state.The territorial sea is regarded as the sovereign territory of the state, although foreign ships [both military and civilian] are allowed innocent passage through it; this sovereignty also extends to the airspace over and seabed below.The term "territorial waters" is also sometimes used informally to describe any area of water over which a state has jurisdiction, including internal waters, the contiguous zone, the exclusive economic zone and potentially the continental shelf.
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