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Gravity-based structure

A gravity-based structure (GBS) is a massive concrete foundation placed directly on the seabed to support offshore installations such as oil and gas platforms or wind turbines, with stability achieved primarily through its substantial weight and ballast rather than piles or anchors.^[1] These structures typically consist of large, reinforced concrete caissons or cells that form a base capable of withstanding extreme environmental loads, including high waves exceeding 25 meters and winds up to 200 miles per hour.^[2] GBS designs often incorporate integrated storage compartments for oil or other fluids, enabling operations in water depths generally ranging from 70 to 300 meters, though they are most common in 100–200 meters where soil conditions allow self-weight penetration.^[1]^[3] The concept of GBS emerged in the mid-20th century as offshore exploration expanded into harsher marine environments, with the first major installation being the Ekofisk platform in the North Sea, Norway, completed in 1973.^[1] Over the following decades, approximately 50 significant GBS systems were constructed worldwide, primarily for the oil and gas industry, driven by the need for reliable foundations in areas lacking suitable subsea pipelines for immediate export.^[1]^[4] Construction typically occurs in dry docks using slipforming techniques to pour vast volumes of high-strength concrete—often exceeding 1 million cubic meters—with reinforcement densities up to 600 kg per cubic meter to resist tensile forces from waves, icebergs, or seismic activity.^[1] Once built, the structure is floated to site, ballasted with water to sink onto the seabed, and then de-ballasted to achieve the required elevation and alignment.^[2] Notable examples include the Brent Field platforms (Alpha, Bravo, Charlie, and Delta) in the UK North Sea, each weighing approximately 300,000 tonnes and featuring 3-4 cylindrical concrete legs up to 165 meters tall connected to 64 oil storage cells.^[2] The Troll A platform, installed in 1995 off Norway, stands as the tallest GBS at 472 meters (1,549 ft) and was the heaviest object moved by humans at the time, displacing 1.2 million tonnes during towing.^[1]^[5] More recently, GBS technology has been adapted for renewable energy, such as in offshore wind farms where gravity foundations support turbine bases in transitional water depths, as seen in early European projects like Middelgrunden and Vindeby.^[6] Decommissioning poses unique challenges, often favoring in-situ removal of topsides while leaving the base intact due to the immense scale and environmental risks of refloating, as evidenced by precedents in the North Sea.^[2]

Fundamentals

Definition and Principles

A gravity-based structure (GBS) is a large, heavy support foundation, typically constructed from concrete, placed directly on the seabed to bear the weight of offshore installations such as platforms or turbines. These structures derive their stability primarily from their own mass, which applies vertical pressure to the seabed, enabling them to withstand environmental forces like waves, currents, and ice without relying on deep pile foundations.^[7]^[8] In contrast to tension-based systems like jacket structures with piles, GBS anchor through soil compression under their weight and frictional resistance at the base-soil interface, eliminating the need for tensile anchoring elements.^[7] The foundational physics centers on gravity-induced resistance to loads, hydrostatic stability for upright positioning, and buoyancy control to facilitate deployment. During towing and installation, the structure maintains self-buoyancy via sealed compartments, allowing it to float at a controlled draft before controlled ballasting transitions it to a fully gravity-dependent state on the seabed.^[7]^[9] Key to operational mechanics is the balance of moments for overturning stability, where the restoring moment from the structure's weight counters the overturning moment from horizontal forces. This principle is captured by the condition M_{\text{restore}} = W \times b \geq M_{\text{overturn}} = F \times d, with W as the total submerged weight, b as the base width lever arm from the center of gravity to the edge, F as the applied horizontal force, and d as the force's vertical lever arm. Buoyancy management further ensures hydrostatic equilibrium, with the metacenter positioned above the center of gravity to prevent capsizing during floating phases, while post-installation gravity dominance provides resistance to sliding via friction coefficients typically exceeding 0.3 on prepared seabeds.^[7]^[9]

Historical Development

The development of gravity-based structures (GBS) emerged in the early 1970s, primarily driven by the demands of North Sea oil exploration, where harsh environmental conditions, including severe waves and deep waters, necessitated stable, fixed platforms resistant to dynamic loads. Concrete was introduced as a viable material for offshore construction during this period, offering advantages in durability and fabrication over traditional steel in challenging seabed conditions. The International Association of Oil & Gas Producers notes that these factors prompted the oil and gas industry to pioneer concrete structures in the North Sea, marking a shift from earlier steel-dominated designs.^[10] Key milestones began with the world's first concrete offshore oil storage tank, Ekofisk 2/4 T, installed in 1973 by Phillips Petroleum in the Norwegian sector of the North Sea, developed by Norwegian Contractors as an early prototype for gravity-stabilized systems. This was followed by the adoption of the Condeep design—a concrete gravity base with multiple legs and a storage cellar—pioneered by the same Norwegian group. The first major Condeep production platform, Beryl A, was installed in 1975 by Mobil in the UK sector of the North Sea, becoming the inaugural concrete GBS in UK waters and demonstrating the feasibility of integrated drilling, production, and accommodation on a single structure weighing approximately 200,000 tons. These projects, supported by Norwegian engineering firms, established GBS as a reliable solution for early North Sea developments.^[11]^[12] The 1970s oil crises, particularly the 1973 Arab oil embargo, accelerated research and development in offshore technologies by heightening the urgency for energy independence and expanded production capabilities, leading to increased investment in innovative platform designs like GBS. In the 1980s and 1990s, GBS evolved for deeper waters, reaching up to 300 meters, with advancements in concrete mix and construction techniques enabling larger, more resilient structures. A landmark example is the Troll A platform, installed in 1995 by Statoil (now Equinor) in the Norwegian North Sea at a water depth of 303 meters; at 472 meters tall and with a base weighing 656,000 tons, it remains the tallest and heaviest GBS ever constructed, processing natural gas from the Troll field.^[13]^[14] Entering the 21st century, GBS designs incorporated subsea tie-backs to connect remote wells to central platforms, enhancing efficiency in marginal fields without requiring additional surface infrastructure, as seen in projects like the Hebron GBS off Newfoundland installed in 2017. Decommissioning practices advanced post-2010, particularly in regions like the Gulf of Mexico, where regulatory frameworks from the Bureau of Ocean Energy Management guided the removal or partial reefing of aging structures, with over 3,000 platforms decommissioned since 1985 to ensure environmental compliance. In the 2020s, sustainability efforts have focused on repurposing GBS for renewable applications, such as adapting North Sea platforms for offshore wind or hydrogen production, aligning with global energy transition goals. For instance, the Fécamp offshore wind farm in France utilized 72 gravity-based foundations, completed in 2022, marking a significant application in renewable energy.^[15]^[16]^[17]

Design and Construction

Structural Components

Gravity-based structures (GBS) primarily consist of a robust base that anchors the structure to the seabed through its own weight, supported by integrated components designed for stability and functionality in marine environments. The core of a GBS is the base, typically a large caisson or skirted foundation that penetrates the seabed to provide resistance against uplift and lateral forces. This base is often cylindrical or multifaceted, with skirts extending downward to embed into the soil, enhancing frictional and suction anchorage. Above the base, storage cells are incorporated, serving dual purposes as compartments for oil or liquefied natural gas in operational phases and as ballast tanks during installation. These cells are arranged in a honeycomb-like pattern to distribute weight evenly and contribute to the overall structural integrity. The topsides of a GBS include processing decks that house equipment for drilling, production, and utilities, elevated above the base to protect against wave impacts. Risers, which connect subsea wells to the topsides, are integrated via conductor pipes or riser slots embedded in the base walls, allowing for secure fluid transport while minimizing dynamic stresses. In multi-legged configurations, common for deeper waters, GBS designs typically feature three to four legs, each comprising hollow concrete columns that interconnect the base and topsides, providing enhanced stability against torsional and overturning moments. For instance, the Troll A platform employs four legs to distribute loads effectively in soft seabed conditions. Dome-shaped or conical bases are frequently adopted to reduce hydrodynamic forces by streamlining flow around the structure, thereby lowering wave-induced pressures and fatigue risks. Integration of these components relies on a ballast system to achieve fixation, where seawater is sequentially filled into the base and storage cells post-towing to site, increasing the structure's weight by over 90% of its total mass for self-installation without additional piling. This process involves controlled flooding to maintain stability during submergence, often monitored via inclinometers to prevent tilting beyond 1-2 degrees. In hybrid designs, supplementary mooring lines may connect the base to anchors for added restraint in extreme conditions, though the primary reliance remains on gravitational stability. Ballast management also allows for future decommissioning by pumping out water and refloating the structure. Structural analysis of GBS components employs finite element modeling (FEM) to evaluate stress distribution under wave and current loads, simulating the entire assembly as a composite system. This approach discretizes the base, legs, and topsides into elements to predict deformations and ensure safety factors exceed 1.5 for ultimate limit states. A fundamental aspect of this analysis is the calculation of bending stresses in legs and base walls, given by the equation:

\sigma = \frac{M y}{I}

where \sigma is the normal stress, M is the bending moment from hydrodynamic or gravitational loads, y is the distance from the neutral axis, and I is the moment of inertia of the cross-section. Such modeling verifies that maximum stresses remain below material yield limits under design storms, guiding component sizing for longevity.

Materials and Methods

Gravity-based structures (GBS) primarily utilize high-strength concrete with compressive strengths typically ranging from 50 to 70 MPa to withstand the substantial hydrostatic pressures and structural demands encountered in offshore environments.^[18]^[19] This concrete is reinforced with high-yield steel rebar, providing tensile strength and ductility while maintaining a minimum cover of 40-50 mm to protect against corrosion in submerged and splash zones.^[19] For instance, the Hebron GBS employed concrete with a 65 MPa compressive strength and a water-to-cementitious materials ratio not exceeding 0.4 to ensure durability and low permeability.^[18] Secondary materials, such as specialized grout, are applied at interfaces between concrete elements and steel components to enhance bonding and load transfer, particularly in skirt foundations or modular joints.^[20] Construction of GBS begins with dry-dock assembly, where the base slab and initial caisson sections are cast in controlled environments to facilitate precise forming and curing.^[18] Vertical elements, such as walls and shafts, are then erected using slipforming, a continuous pouring technique that employs hydraulic jacks to incrementally raise formwork as concrete sets, enabling seamless, monolithic construction at rates of several meters per day.^[18]^[19] This method was pivotal in projects like Hebron, where slipforming completed over 50,000 m³ of concrete in walls up to 153 feet tall within 34 days.^[18] Upon completion of the primary structure, the GBS is floated out of the dry dock, de-ballasted for stability, and towed to the installation site using multiple tug vessels over distances of 300 nautical miles or more.^[18] Installation involves a meticulously controlled ballasting sequence using seawater to achieve precise positioning and foundation stability on the seabed.^[18] Towing voids are first de-ballasted to optimize buoyancy and trim during transit, followed by progressive flooding of compartments to lower the structure onto the seabed, where skirts penetrate 1-2 meters into the soil for enhanced lateral resistance.^[18]^[21] In the Hebron installation, 500 mm deep steel skirts facilitated penetration through the upper soil layer, with final ballasting adding solid materials like iron ore slurry to reach an on-bottom weight exceeding 500,000 tonnes.^[18] Quality controls during fabrication and installation emphasize structural integrity and longevity, including rigorous watertightness testing of storage cells and compartments through hydrostatic pressurization to verify leak-proof performance under operational loads.^[18] Corrosion protection for embedded steel reinforcement and external components relies on cathodic systems, such as sacrificial anodes or impressed current methods, combined with epoxy coatings to mitigate electrochemical degradation in saline environments.^[22] For arctic deployments, adaptations include ice-resistant skirts with reinforced profiles and increased wall thicknesses to resist lateral ice forces, ensuring penetration and stability in frozen seabeds without compromising overall ballasting efficiency.^[23]^[24] These measures adhere to standards like DNVGL-ST-C502 and ISO 19900 for offshore concrete structures, incorporating nonlinear finite element analysis to validate performance.^[18]

Applications

Offshore Oil and Gas

Gravity-based structures (GBS) serve as fixed platforms primarily for drilling, production, and storage in offshore oil and gas operations, particularly suited to water depths ranging from 100 to 300 meters where seabed conditions provide adequate support for their massive concrete bases.^[25]^[26] These structures rely on their weight—often exceeding 300,000 tonnes—to maintain stability against environmental loads, enabling reliable hydrocarbon extraction in harsh marine environments like the North Sea and Canadian offshore regions. In the North Sea, GBS constitute a notable portion of fixed installations, with 27 such platforms in the OSPAR Convention area supporting long-term field development.^[2]^[27] Prominent case studies illustrate the application of GBS in challenging hydrocarbon extraction settings. The Hibernia platform, developed in the 1980s and installed in 1997 off Newfoundland, Canada, features a approximately 600,000-tonne GBS—with the total platform weighing about 1.2 million tonnes—designed for ice resistance on the Grand Banks, where it withstands iceberg impacts through a robust concrete caisson and protective skirt.^[28]^[29]^[30] In the Gulf of Mexico, while pure GBS are less common due to softer sediments, hybrid designs incorporating GBS elements with steel components have been explored for deepwater stability, as seen in conceptual assessments for compliant systems that blend gravity anchoring with flexible towers to handle extreme depths up to 600 meters.^[31] These examples highlight GBS adaptability to regional geohazards, such as ice in Arctic-adjacent waters or hurricanes in the Gulf. Key adaptations in GBS for oil and gas include integrated storage tanks within the base, capable of holding up to 1.3 million barrels of crude to facilitate on-site processing and offloading via shuttle tankers, reducing dependency on subsea pipelines.^[28] Wellhead protection is another critical feature, with the structure's broad footprint and reinforced skirts shielding subsea infrastructure from vessel collisions or drifting ice, ensuring operational safety in high-traffic extraction zones.^[29] GBS platforms typically operate for 25 to 50 years, aligning with reservoir lifespans, after which decommissioning becomes necessary under international guidelines. The Brent Spar incident in the 1990s, involving a North Sea storage buoy (not a GBS) proposed for deep-sea disposal by Shell, sparked global controversy led by Greenpeace, resulting in its onshore dismantling and the establishment of stricter OSPAR regulations for offshore structure removal to minimize environmental risks—these regulations also apply to GBS.^[32]^[3] This event underscored the need for sustainable end-of-life strategies, influencing modern practices like partial removal and topsides reuse.

Renewable Energy Installations

Gravity-based structures (GBS) serve as stable foundations for fixed-bottom offshore wind turbines, particularly in water depths ranging from shallow to mid-depths up to 60 meters, where soil conditions allow for self-weight anchoring without extensive piling. These structures leverage the inherent mass of concrete to resist overturning moments from wind and wave loads, providing a cost-effective alternative to driven piles in suitable seabeds. Unlike floating systems, GBS enable direct seabed contact, facilitating easier installation and maintenance for turbine arrays. A key design feature in GBS for wind applications is the adaptation of monopile-like gravity bases, which are wide, caisson-style foundations filled with ballast to enhance stability and incorporate scour protection layers, such as rock armor or geotextiles, to prevent seabed erosion around the base. More recent examples include the Fécamp Offshore Wind Farm in France, which utilizes concrete GBS for its 71 8 MW turbines in water depths of 25-35 meters and became operational in phases starting 2023, demonstrating the scalability of this approach for multi-gigawatt projects.^[17] Cable routing is integrated through dedicated conduits within the GBS, minimizing seabed disturbance and protecting electrical exports from environmental hazards. Additionally, these structures address turbine-specific dynamics through vibration damping, ensuring the system's natural frequency avoids resonance with operational loads via the relation f_n = \frac{1}{2\pi} \sqrt{\frac{k}{m}}, where k represents structural stiffness and m the effective mass. Emerging trends in GBS applications include hybrid installations combining offshore wind with wave energy converters, optimizing shared foundations to reduce overall costs and environmental footprints in coastal zones. Post-2020 developments in Europe highlight this growth, underscoring GBS viability in seismically active areas through reinforced concrete designs that accommodate lateral loads.

Other Engineering Uses

Gravity-based structures (GBS) find application in marine environments for coastal protection and land reclamation, where their inherent stability from mass allows them to withstand wave forces without additional anchoring. In breakwater construction, vertical caissons—hollow concrete boxes filled with ballast—serve as primary components, placed end-to-end to form barriers that dissipate wave energy and shelter harbors. These structures rely on their weight to resist sliding and overturning, making them suitable for sites with firm seabeds. For instance, composite breakwaters often incorporate rubble mound foundations topped with caisson walls to enhance hydraulic performance and durability against erosion.^[33] Artificial islands and land reclamation projects also utilize GBS-like caissons to create stable foundations in shallow waters. These precast units are fabricated onshore, towed to site, and sunk to form the perimeter or base, enabling controlled sediment infilling for development. A notable example is Belgium's Princess Elisabeth Island, an offshore artificial platform for energy infrastructure, constructed using 23 massive concrete caissons weighing up to 22,000 tonnes each, positioned to form a robust enclosure against marine currents. This approach demonstrates the versatility of GBS in expanding usable land while minimizing environmental disturbance compared to dredging-intensive methods.^[34] In bridge engineering, particularly in seismic zones, GBS elements provide reliable pylon foundations by distributing loads to the seabed and absorbing lateral forces from earthquakes. Caisson foundations, a form of GBS, are sunk to bedrock and filled with concrete to create unyielding supports that mitigate differential settlement and vibration amplification. The Akashi Kaikyō Bridge in Japan, spanning a seismically active strait, incorporates large-scale steel caissons for its towers, each sunk to depths exceeding 60 meters and filled with over 230,000 cubic meters of underwater concrete to ensure stability against strong ground motions. This design contributed to the bridge's resilience during the 1995 Great Hanshin earthquake and subsequent events.^[35] Emerging experimental uses include floating breakwaters adapted for climate resilience, where modular GBS-inspired designs offer temporary wave attenuation in vulnerable coastal areas. These semi-submersible units, often with ballast for stability, can be deployed to protect against rising sea levels and intensified storms without permanent seabed alteration. In Norway, post-2015 prototypes for aquaculture have explored submerged gravity pens, such as flexible gravity-based net enclosures that maintain position through weighted frames, reducing exposure to surface pathogens and enabling operations in deeper waters. These innovations, tested under development licenses, prioritize biosecurity and scalability for sustainable fish farming amid environmental pressures.^[36] A key advantage of GBS in these non-energy applications is their scalability, allowing adaptation from temporary modular installations—such as relocatable breakwaters using lighter caissons—for short-term protection, to permanent heavy-duty structures for long-term infrastructure. Unlike offshore energy platforms requiring tall columns for elevation, these uses often demand lower heights, focusing instead on base width and mass for horizontal stability, which simplifies fabrication and reduces material needs.^[18]

Advantages and Challenges

Key Benefits

Gravity-based structures (GBS) provide exceptional stability in harsh marine environments due to their massive concrete construction, which relies on self-weight to resist environmental loads such as waves, currents, and ice impacts. This inherent stability is particularly advantageous in regions like the North Sea and Arctic waters, where platforms such as the Troll A and Hebron have withstood severe conditions, including water depths up to 330 meters and iceberg collisions, without requiring additional anchoring systems.^[37]^[18] The design of GBS ensures a long natural period of oscillation, typically exceeding common wave periods, thereby minimizing resonance and dynamic amplification under wave action. This feature enhances overall structural integrity against hydrodynamic forces, making GBS suitable for sites with high seismic activity or extreme weather, as the low center of gravity and broad base distribute loads effectively to the seabed.^[38]^[39] Cost-effectiveness is a key advantage of GBS over piled structures, primarily through reduced maintenance requirements and shorter installation timelines. Concrete's durability in seawater minimizes corrosion-related upkeep, leading to lower life-cycle costs compared to steel alternatives that demand frequent inspections and repairs. Additionally, the modular construction allows for towing the entire structure to the site, reducing on-site assembly time and weather exposure risks, which can cut overall project expenses. Potential reusability during decommissioning enhances economic viability, though in practice, full refloating and relocation is theoretical and rarely implemented, with many structures left in place or partially removed.^[37]^[40]^[41]^[2] From an environmental perspective, GBS cause minimal disturbance to the seabed, as they rest directly on the ocean floor without the need for pile driving, which avoids sediment resuspension and noise pollution that can harm marine life. This approach facilitates easier end-of-life removal through de-ballasting and refloating for complete or partial relocation, though actual practices often favor in-situ options to reduce ecological risks. For instance, the process involves displacing ballast water to restore buoyancy, allowing the structure to be towed away intact where feasible.^[42]^[3] The versatility of GBS extends to their adaptability for remote or logistically challenging sites, where heavy-lift cranes are unavailable, as installation relies on flotation and controlled ballasting rather than precise piling operations. With over 40 such structures installed worldwide in the offshore oil and gas sector, primarily in the North Sea and other harsh basins, GBS have demonstrated reliability across diverse soil conditions and water depths up to 300 meters.^[2]^[37]

Limitations and Solutions

Gravity-based structures (GBS) face significant challenges related to their immense scale and material properties, primarily manifesting in high upfront costs and substantial weight requirements. These structures often weigh between 3,000 and 1.2 million tonnes, with notable examples like the Troll A platform exceeding 680,000 tonnes, necessitating specialized heavy-lift vessels for transportation and installation.^[3] This mass, while providing inherent stability, escalates fabrication and logistics expenses, making GBS less competitive in regions without access to advanced marine infrastructure. Additionally, GBS are generally limited to water depths of up to 350 meters due to constraints on concrete's ability to withstand hydrostatic pressures and the practicalities of construction in deeper environments.^[37]^[7] Installation poses further risks, particularly during towing and placement, where narrow weather windows—typically 2 to 4 weeks in favorable seasons—constrict operations in harsh offshore conditions like the North Sea. In soft or unstable soils, excessive settlement can compromise long-term integrity, as the structure's weight may cause uneven foundation penetration without adequate countermeasures.^[43] These issues are mitigated through the use of skirts on the base, which penetrate 5 to 10 meters (or deeper in extreme cases, up to 36 meters as in Troll A) into the seabed to anchor the structure and distribute loads to firmer soil layers, reducing differential settlement by up to 50% in monitored installations.^[44]^[43] To address these limitations, hybrid designs combining GBS with steel jackets or piled foundations have emerged, allowing deployment in deeper waters or variable soil conditions by leveraging the gravity base for primary stability while steel elements enhance resistance to overturning moments.^[45] Digital twins—virtual replicas integrating real-time sensor data with predictive simulations—enable proactive modeling of installation dynamics and long-term performance, helping to reduce risks from environmental variables. For decommissioning, recycling protocols emphasize material recovery through dismantling and processing, with reinforced concrete components potentially crushed for aggregate in new infrastructure, aligned with international guidelines like OSPAR.^[3] Emerging mitigations focus on material and process innovations to lighten structures without sacrificing stability. 3D-printed concrete variants, incorporating lightweight aggregates like cenospheres, are being explored to reduce overall mass while maintaining sufficient compressive strength, potentially facilitating easier towing and broader applicability, though specific applications to GBS remain in development. AI-optimized ballasting algorithms are under investigation to refine weight distribution during installation, using machine learning to predict and adjust ballast in real-time and minimize settlement risks in soft soils. These advancements, though still in pilot stages, promise to extend GBS viability into more challenging environments, including recent adaptations for offshore wind foundations in transitional water depths as of 2025.^[46]

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Jan 8, 2024 · Our digital twin solution integrates (1) a Kalman filter to estimate the structural states based on a linear model of the structure and ...<|separator|>
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New EU rules to reduce, reuse and recycle packaging | News
Apr 24, 2024 · Under the new rules, all packaging (except for lightweight wood, cork, textile, rubber, ceramic, porcelain and wax) will have to be recyclable ...Missing: gravity structures
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Tailoring Light-Weight Aggregates for Concrete 3D Printing ... - NIH
Apr 1, 2023 · In this study, we explore printing lightweight concrete by replacing normal weight aggregate with lightweight aggregates such as cenospheres, perlite, and foam ...Missing: lighter | Show results with:lighter
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Optimization method and experimental research on attitude ... - Nature
Oct 26, 2022 · A ballast water allocation optimization model and its efficient solution algorithm model of the ballast pump and gravity composite ballast ...