Marine construction

Marine construction is the process of building structures in or adjacent to large bodies of water, usually the sea. These structures can be built for a variety of purposes, including transportation, energy production, and recreation. Marine construction can involve the use of a variety of building materials, predominantly steel and concrete. Some examples of marine structures include ships, offshore platforms, moorings, pipelines, cables, wharves, bridges, tunnels, breakwaters and docks. Marine construction may require diving work, but professional diving is expensive and dangerous, and may involve relatively high risk, and the types of tools and equipment that can both function underwater and be safely used by divers are limited. Remotely operated underwater vehicles (ROVs) and other types of submersible equipment are a lower risk alternative, but they are also expensive and limited in applications, so when reasonably practicable, most underwater construction involves either removing the water from the building site by dewatering behind a cofferdam or inside a caisson, or prefabrication of structural units off-site with mainly assembly and installation done on-site.^{[citation needed]}

Environmental influences

Some aspects of the marine environment that complicate construction:

Distance from permanent facilities causes logistical problems for provision of materials, equipment, power supplies, and accommodation.<ref name="Gerwick 2007" />^{: Ch 1.2}
Hydrostatic pressure due to depth in the water column. Hydrostatic pressure is linear with depth and increases at approximately 1 bar for every ten metres of depth.<ref name="Gerwick 2007" />^{: Ch 1.3}
Temperature of the water and the air above it. Sea surface temperature can vary from a minimum of −2 °C (28 °F) to a maximum of about 32 °C (90 °F). Higher surface temperatures decrease rapidly with depth, and generally reach a steady-state of about 2 °C (36 °F) by about 1,000 metres (3,300 ft), though on Australia's Northwest Shelf, water and seabed temperature may exceed 30 °C (86 °F) at a depth of 250 metres (820 ft). Air temperatures may vary much more, and though the water has a moderating effect, the air temperature more variable and may be much more extreme.<ref name="Gerwick 2007" />^{: Ch 1.4}
Seawater and splash zone chemistry<ref name="Gerwick 2007" />^{: Ch 1.5}
Biofouling<ref name="Gerwick 2007" />^{: Ch 1.5}
Currents cause drag loading on structures and equipment. There are several different types of currents in the seas. Categorised by their cause, these include geostrophic, tidal, wind-driven, oceanic circulation and density, as well as those caused by river discharge. River currents are driven mainly by gravity, but differ laterally and vertically. The highest velocity in river currents is usually near the outer bank in a bend, and hey also locally increase around obstructions to their flow. Current may also spin off eddies, and their lateral boundaries may vary considerably over the short term. There may also be short or long term vertical components in some areas, the upwellings and downwellings often associated with bottom topography and winds in coastal areas. Currents may be stratified vertically, even to the extent that they may flow in opposing directions at the surface and at the bottom. Their influence on underwater construction is a consequence of the drag forces induced and the transport of unconsolidated sediment, particularly by scour, both during and after construction. Water passing over an immersed surface can also cause lift forces, proportional to the square of the velocity, which can complicate operations, so the variations of instantaneous velocity induced by waves can further complicate matters.<ref name="Gerwick 2007" />^{: Ch 1.6}
Waves are a further environmental influence near the surface, which cause perturbations in all six degrees of freedom on a floating object, and can exert large forces on a fixed object. They are a major cause of delays and reduced operating efficiency, and may be the major design criterion for fixed structures. Tsunamis are a class of wave which occur relatively seldom but can have a devastating effect due to the large amounts of energy they can carry over long distances at high speeds. The depth to which a wave has significant motion is a function of wave length, and to a lesser extent, on wave height.<ref name="Gerwick 2007" />^{: Ch 1.7}
Wind
Tides and storm surges
Rain, snow and fog reduce visibility, but with radar and precision positioning systems they are not as serious a hazard as in the past. Nevertheless they may cause delays in some operations. In heavy rainfall appreciable quantities of water may enter unprotected openings, and must be removed before it has adverse effects on stability due to free-surface. Fog may be of the summer fog type which forms some distance above the water and may leave a narrow band of clear air just above the water, or winter type, where there mat be dense fog just above the water but clear skies a short distance up. Rain, fog and snow may prevent helicopter operations, which generally require good visibility for landing. Snow may present the additional problem of removal to prevent accumulation. Spray caused by wind and waves can also dump large amounts of water which must be drained, and in colder conditions may freeze and build up a burden of ice. Severe spray may prevent people from working on deck. Atmospheric icing , also known as black ice can occur in sub-Arctic regions, when the air is humid and the temperature is low. Ice forms directly on cold surfaces and can accumulate rapidly, adding topside weight and increasing wind drag. Icing and frozen spray may occur in the same regions and in similar conditions. Lightning is generally not a major problem to steel and concrete structure with adequate lightning protection, but is a hazard to personnel working in high exposed areas.<ref name="Gerwick 2007" />^{: Ch 1.10}
Sea ice and icebergs<ref name="Gerwick 2007" />^{: Ch 1.11}
Earthquakes and tsunamis
Scour<ref name="Gerwick 2007" />^{: Ch 1.14}
Siltation<ref name="Gerwick 2007" />^{: Ch 1.15}

Geotechnical aspects

The geology of the seabed has a strong influence on almost any marine structure. The seabed is the substrate on which the structure must stand, and both the morphology and the material affect the design and construction. It is therefore necessary for accurate and reliable geological surveys to be made before a construction project can be started. There have been developments in sampling of seabeds but some soils remain difficult to analyse and sampling may not produce results as accurate as would be desired. In-place strength may be greater than conventional sampling methods indicate, and sampling methods may not recover and identify critical constituents due to insufficient samplung. Failure to identify potential problems can lead to delays and cost overruns. Many marine structures cover extensive areas, and the soil properties may vary considerably. Cost and time constraints may make it difficult to gather sufficient samples from borings to fully describe the substrate. Other methods for remote substrate analysis may identify potential variations for closer examination.<ref name="Gerwick 2007" />^{: Ch 2.1}

Bottom material is often sedimentary, and in deeper water, may range from extremely fine, low density silts to loose gravels, to dense, highly compacted sands. Granular sediments may be subject to liquefaction if strongly disturbed, as by earthquakes, cyclic impact of storm waves, or crushing by sea ice.<ref name="Gerwick 2007" />^{: Ch 2.2} When this occurs the soil can behave like a dense liquid. This can also happen during some construction processes, such as piledriving.<ref name="Gerwick 2007" />^{: Ch 2.3} The presence of large boulders in glacial till deposits can give a misleading impression of bedrock, hiding the further extent of softer sediment below.<ref name="Gerwick 2007" />^{: Ch 2.5}

In arctic regions, permafrost can form an unreliable foundation, and deeply buried clathrates can be a problem when drilling, but are usually too deep to be a problem for construction.<ref name="Gerwick 2007" />^{: Ch 2.7}

Methane can occur at shallow depths in delta sediments with significant quantities of organic matter, and in arctic silts. These van reduce the shear strength of silty and clay soils. A sudden, large release may temporarily reduce water density sufficiently to cause vessels or drill rigs to sink, and can cause an explosion or fire at the surface.<ref name="Gerwick 2007" />^{: Ch 2.10}

Clays may initially support a steep slope when excavated, but are subject to creep and sudden large-scale collapse when subjected to shock loads or vibration.<ref name="Gerwick 2007" />^{: Ch 2.11}

Unconsolidated sand – seasonal shifts.<ref name="Gerwick 2007" />^{: Ch 2.13}

Underwater sand dunes can form where there are strong currents, which may move with the currents, as sand is lifted by flow over the back of the dune, and dropped at the front. This can be a problem when laying pipelines, and may require deeper than usual burial.<ref name="Gerwick 2007" />^{: Ch 2.14}

Ecological and societal impact

Topics of concern:

Oil and petroleum products^{[citation needed]}
Toxic chemicals^{[citation needed]}
Contaminated soils^{[citation needed]}
Construction wastes^{[citation needed]}
Turbidity^{[citation needed]}
Sediment transport, scour, and erosion^{[citation needed]}
Air pollution^{[citation needed]}
Disturbance and destruction of marine habitats
Noise^{[citation needed]}
Impact on traffic^{[citation needed]}
Protection of existing structures
Liquefaction of soils^{[citation needed]}
Third party safety^{[citation needed]}
Archaeoligical impact. Early civilisations often left traces of their presence in coastal areas and along river banks, which may be found during construction work.^{[citation needed]} Legislation will generally provide guidance on how to manage such sites when they are discovered.^{[citation needed]}

Materials and fabrication

Marine construction materials subjected to coarse conditions including corrosion and temperature change. Fabrication can also be complicated by the large scale of some structures, and the need to transport them to the site for modular installation, and possible thermal differences between components and the fabrication and installation sites. The most commonly used materials in marine construction are concrete and steet.<ref name="Stannard 2021" /><ref name="Gerwick 2007" />^{: Ch 4}

Steel

Durability in the marine environment is affected by corrosion, both inside and outside of hollow structures, and can be particularly severe in crevices and cracks. The interior surfaces of steel tanks may also be corroded by liquids and other substances stored in them. The rate of corrosion may be increased by abrasion or erosion, and also by higher temperature, higher oxygen concentration, and the presence of chloride ions. Corrosion is therefore usually most severe in the splash zone.<ref name="Gerwick 2007" />^{: Ch 4.2}

Materials

Steels are measured by the following material properties:<ref name="Gerwick 2007" />^{: Ch 4.3}

Minimum yield strength
Minimum ultimate strength
Minimum elongation at rupture
Notch toughness at low temperatures
Properties in the thickness direction (properties on the z-axis)
Weldability
Fatigue endurance
Alloy composition
Corrosion resistance

Fabrication and Welding

For major projects, coded welders (approved welder, tested against a particular standard) may be stipulated, and non-destructive and mechanical testing may be specified. Design of joints and fabrication methods should take stress raisers and ambient conditions into consideration to minimise the risk of fatigue cracking, and weld preparation and finishing processes should also take this into consideration.<ref name="Gerwick 2007" />^{: Ch 4.2}

Erection and assembly

Coatings and corrosion protection

Steel is subject atmospheric corrosion, splash zone corrosion, crevice corrosion, and other causes, including corrosion due aerobic and anaerobic bacteria interacting. Seawater rates are twice that of freshwater. Coatings may delay the start of corrosion by 10 to 20 years.<ref name="Gerwick 2007" />^{: Ch 4.2}

When reasonably practicable, workshop conditions of suitable humidity and protection from hazardous weather should be used when painting and coating steel structures, and keeping joint surfaces clean for welding. This may require temporary shelter to be erected, and heating or dehumidification may be required.<ref name="Gerwick 2007" />^{: Ch 4.2}

Sacrificial anodes or impressed current cathodic protection are used to protect steel in immersed areas. Anodes must be secured to prevent damage during transport. Effective connection are required between anodes and structures. It is prohibited in en closed spaces or where gas may be trapped because hydrogen may be generated and may produce an explosive atmosphere.<ref name="Gerwick 2007" />^{: Ch 4.2}

Steel coatings may be applied to steel that will primarily be underwater if the coating has the required resistance to cathodic disbondment. Additional protection may be provided by materials like copper nickel or by usage of additional steel to thicken the area allowed for corrosion. Allowances of 0.1 to 0.3 mm per year are typical.<ref name="Gerwick 2007" />^{: Ch 4.2}

Structural concrete

Prestressed and reinforced concrete have been used for several decades for the construction of large offshore platforms, mostly in the North Sea. Concrete is also used together with steel structure in hybrid and composite designs, and cement grout is used on steel platforms to bond piles to the skirts and jacket legs.<ref name="Gerwick 2007" />^{: Ch 4.3.1}

The component materials must be well matched to work effectively. It must be durable when exposes to the sea and air. Careful design and quality control during fabrication help to provide long service with low maintenance. The splash zone is most vulnerable to seawater damage, while the immersed zone and areas below the mudline have few problems. The atmospheric and splash zones have high susceptibility to chloride and carbon dioxide corrosion of the reinforcing steel.<ref name="Gerwick 2007" />^{: Ch 4.3.1}

Mixes and properties

Reinforcement

Prestressing tendons and accessories

Embedments

Conveyance and placement

Curing

Concrete coatings

Construction joints

Formwork and support

Combined steel–concrete structures

Two forms of combined steel and concrete construction have been used to advantage:

The hybrid approach:

Structural steel superstructures supported by concrete substructures,
Structural steel frames supporting exterior concrete walls and slabs,
Steel hinge providing an articulated connection between a concrete base and a steel or concrete column.

For these structures, the main problem is the working of the joint under cyclic-dynamic loads, which can frequently be managed by pre-stressing the connection. Care must be taken to ensure that the stresses set up by this method are of the correct type to suit each material, and that the bearing loads are evenly distributed to avoid bursting stresses.<ref name="Gerwick 2007" />^{: Ch 4.4.1}

The composite approach:<ref name="Gerwick 2007" />^{: Ch 4.4.2}

Typical examples include:

A concrete deck is connected to a steel beam by casting it over studs welded to the beam top surface. The concrete takes compression, and the steel carries tension and transverse shear. This system is commonly used in bridge construction.<ref name="Gerwick 2007" />^{: Ch 4.4.2}
Special shear connectors in the form of transverse bars or perforated vertical plates, are welded to the steel.
Two steel plates, spaced apart and tied together by steel webs, bars, or bolts, are filled with concrete. The concrete distributes local loads and carries out of plane shear loads.<ref name="Gerwick 2007" />^{: Ch 4.4.2}

This type of composite construction appears to have advantages where offshore structures must withstand high local impact forces, such as ice or ship collision, and for the walls of concrete structures where cracks and leakage are unacceptable and difficult or impossible to repair.<ref name="Gerwick 2007" />^{: Ch 4.4.2}

Synthetic and composite materials

Titanium

Titanium has high strength and is very resistant to corrosion, but is also very expensive, so it tends to be used in highly-corrosive environments or as cladding.<ref name="Gerwick 2007" />^{: Ch 4.6}

Rock, sand, and asphaltic-bituminous materials

Equipment

Some classes of major equipment frequently used in marine construction:

Barges<ref name="Gerwick 2007" />^{: Ch 5.6}
- Crane barges<ref name="Gerwick 2007" />^{: Ch 5.7}
- Revolving derrick barges<ref name="Gerwick 2007" />^{: Ch 5.8}
- Semi-submersible barges<ref name="Gerwick 2007" />^{: Ch 5.9}
- Jack-up barges<ref name="Gerwick 2007" />^{: Ch 5.10}
- Launch barges<ref name="Gerwick 2007" />^{: Ch 5.11}
- Catamaran barges<ref name="Gerwick 2007" />^{: Ch 5.12}
Pipe-laying ships and barges<ref name="Gerwick 2007" />^{: Ch 5.14}
Dredgers and hopper barges<ref name="Gerwick 2007" />^{: Ch 5.13}
Platform supply vessels<ref name="Gerwick 2007" />^{: Ch 5.15}
Anchor handling vessels<ref name="Gerwick 2007" />^{: Ch 5.16}
Heavy-lift ships<ref name="Dockwise 2010" />
Towboats<ref name="Gerwick 2007" />^{: Ch 5.17}
Drilling vessels<ref name="Gerwick 2007" />^{: Ch 5.18}
Crew boats<ref name="Gerwick 2007" />^{: Ch 5.19}
Floating concrete plant<ref name="Gerwick 2007" />^{: Ch 5.20}
Tower cranes<ref name="Gerwick 2007" />^{: Ch 5.21}
Diving support vessels

Other equipment typically used in land-based civil and structural engineering appropriate to the materials in use is also used, sometimes without modification, and sometimes modified or specially adapted to suit the working environment.

Marine operations

Commonly marine and offshore operations include towing, mooring, ballasting, lifting and lowering heavy loads, personnel transfer, surveying, inspection, and diving.<ref name="Gerwick 2007" />^{: Ch 6}

Towing<ref name="Gerwick 2007" />^{: Ch 6.1}
Moorings and Anchors<ref name="Gerwick 2007" />^{: Ch 6.2}
Lifting and setting heavy loads<ref name="Gerwick 2007" />^{: Ch 6.3}
Personnel transfer at sea<ref name="Gerwick 2007" />^{: Ch 6.4}
Underwater interventions, diving, underwater work systems, remotely operated underwater vehicles and remote manipulators<ref name="Gerwick 2007" />^{: Ch 6.5}
Underwater concreting and grouting<ref name="Gerwick 2007" />^{: Ch 6.6}
Surveying and navigation<ref name="Gerwick 2007" />^{: Ch 6.7}
Temporary buoyancy augmentation<ref name="Gerwick 2007" />^{: Ch 6.8}

Seafloor modifications

The seafloor may be covered with level sediments that have been consolidated over the years and provide a stable platform, but this is often not the case, and the construction site may be uneven, covered with unconsolidated, weak sediment, lie on a slope, or be irregular, with rock outcroppings or scattered boulders.<ref name="Gerwick 2007" />^{: Ch 7.1}

There two ways to manage an unsuitable substrate. In the past, most cases of offshore construction adapted the structure to suit the substrate, but the second method is often used for major land structures and can and has been used for shallow water harbor and coastal structures. Seafloor preparation can also have significant potential advantages for deep water. There is normally time available to prepare the site during the lead time for procurement and fabrication of the structure before installation.<ref name="Gerwick 2007" />^{: Ch 7.1}

Seafloor modifications to improve performance of foundations are intended to provide a stable base of sufficient strength to support the structure and to resist failure and degradation for both a single extreme event and repetitive dynamic loads. In some cases, extensions to the foundations may be placed to protect the structure moving ice or ship collision. These operations may include:<ref name="Gerwick 2007" />^{: Ch 7.1}

The site may be prepared by dredging, levelling and removal of obstructions.<ref name="Gerwick 2007" />^{: Ch 7.3} Soft material may be removed, consolidated or reinforced,<ref name="Gerwick 2007" />^{: Ch 7.6} and high areas of hard material and rock removed.<ref name="Gerwick 2007" />^{: Ch 7.4} Granular material such as crushed rock, gravel, and sand can be placed and leveled as a practicable and economical fill for a reasonably level and uniform support for structures, either across the whole site , in low areas, or as a replacement for previously removed soft materials.<ref name="Gerwick 2007" />^{: Ch 7.5}

Sand piles and stone columns can be used to provide better bearing strength for weak soils like silts and clays, and are installed by drilling or driving a mandrel into the soil into which the coarser material is fed before forcing it out under pressure and removing the mandrel. The process is rapid where only small areas need to be improved. This method increases both bearing and shear resistance and can prevent liquefaction. Injection of cementitious material, following land-based grouting procedures, can be used to displace pore water and give a stronger substrate. The cementing pressures must displace the water yet not cause fracturing of the formation by channelization. Cement particles must be small enough to penetrate the interstices, and a wetting agent can be added to the grout to reduce viscosity.<ref name="Gerwick 2007" />^{: Ch 7.6}

Measures to prevent liquefaction of the substrate<ref name="Gerwick 2007" />^{: Ch 7.7}
Protection from scour<ref name="Gerwick 2007" />^{: Ch 7.8}

Installation of piles

A pile or piling is a vertical or near vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site. There are many reasons why a deep foundation may be preferred, such as large design loads, a poor soil at shallow depth, There are different terms used to describe different types of deep foundations including the pile (which is analogous to a pole), the pier (which is analogous to a column), drilled shafts, and caissons. Piles are generally driven into the ground in situ, and can be made out of timber, steel, reinforced concrete or prestressed concrete.

Harbour, river and estuary structures

The most common type of harbour structure is the quay or marginal wharf intended for loading and unloading containers. Finger piers are commonly used for transfer of petroleum products, and trestle supports are provided for access to loading platforms and wharves.<ref name="Gerwick 2007" />^{: Ch 9}

Pile-supported structures consist of steel or concrete piles, driven into soft clays and sands of the seafloor, and usually carry a deck of reinforced concrete. The piles are spaced from 7 to 10 m apart for economy, with a load bearing capacity of 100 to 250 tons each.<ref name="Gerwick 2007" />^{: Ch 9}

Steel piles are either H-section or tubular, with diameters from 400 to 600 mm. Tubular piles are easier to protect from corrosion. Water depths for container and cargo ships are about 16 m, with a designed pile capacity of about 200 to 400 tons, so in most cases the pile length will be between 30 and 40 m. Larger container ships may need 20 m of water alongside, and may need piles of 40 to 50 m long to adequately support vertical crane load, and 1 m diameter for lateral stiffness. Petroleum terminals need more depth, usually about 23 m, so the piles are larger and more heavily loaded.<ref name="Gerwick 2007" />^{: Ch 9}

River structures include locks, low-level dams, overflow structures, and flood walls. Historically, these have usually been constructed behind sheet pile cofferdams to allow the worksite to be dewatered, so that conventional civil engineering construction methods can be used.<ref name="Gerwick 2007" />^{: Ch 9.3}

Coastal structures

Offshore platforms

Oil platform Mittelplate in the North Sea

An offshore platform is usually a large structure with facilities to extract and process petroleum and natural gas that lie in rock formations beneath the seabed. Many platforms also have facilities to accommodate the workers, although it is also common to have a separate accommodation platform bridge linked to the production platform. Most platforms engage in activities on the continental shelf, though they can also be used in lakes, inshore waters, and inland seas. Depending on the circumstances, the platform may be fixed to the ocean floor, consist of an artificial island, or float.<ref name="Ronalds 2005" /> In some arrangements the main facility may have storage facilities for the processed oil. Remote subsea wells may also be connected to a platform by flow lines and by umbilical connections. These sub-sea facilities may include of one or more subsea wells or manifold centres for multiple wells.

Steel offshore platforms

Concrete offshore platforms

Permanently floating structures

Other applications

Submarine pipelines and cables

A submarine pipeline is a pipeline that is laid on the seabed or below it in a trench.<ref name="Dean 2020" /><ref name="Gerwick 2007" />^: p583-585 Some of the pipeline may also be on land.<ref name="Palmer and King 2008" /> Submarine pipelines are used primarily to carry oil or gas, but transportation of water is also important.<ref name="Palmer and King 2008" /> A distinction is sometimes made between a flowline and a pipeline.<ref name="Dean 2020" /><ref name="Palmer and King 2008" /><ref name="Bai and Bai 2010" /> The former is an intrafield pipeline, in the sense that it is used to connect subsea wellheads, manifolds and the platform within a particular development field. The latter, sometimes referred to as an export pipeline, is used to bring the resource to shore.<ref name="Dean 2020" /> Sizeable pipeline construction projects need to take into account many factors, such as the offshore ecology, geohazards and environmental loading.<ref name="Dean 2020" />

Example of a submarine pipeline route: the Langeled pipeline.

Submarine cable is communications or electrical power cable that is laid on or under the seabed.

Topside installation

Arctic structures

Repairing and strengthening marine structures

Removal and salvage

At the end of their useful working life, marine structures may need to be removed or rehabilitated.<ref name="Gerwick 2007" />^{: Ch 20}

<ref name="Gerwick 2007" />^{: Ch 20.1}

Consrtuction

Underwater construction

History

Harbour construction in antiquity<ref>Oleson, John. (2007). The Technology of Roman Harbours. International Journal of Nautical Archaeology. 17. 147 - 157. 10.1111/j.1095-9270.1988.tb00635.x.</ref>
Dykes and land reclamation#History

References

<references group="" responsive="1"><ref name="Bai and Bai 2010" >Bai, Y.; Bai, Q. (2010). Subsea Engineering Handbook. New York: Gulf Professional Publishing. p. 22.</ref>

<ref name="Dean 2020" >Dean, E.T.R. (2010). Offshore Geotechnical Engineering - Principles and Practice. Reston, VA, U.S.A.: Thomas Telford. pp. 338–340.</ref>

<ref name="Dockwise 2010" >"Projects: Thunder Horse PDQ". Dockwise. 2010. Retrieved 23 May 2010.</ref>

<ref name="Gerwick 2007" >Gerwick, Ben C. Jr (2007). Construction of Marine and Offshore Structures (third ed.). Taylor and Francis. ISBN 978-0-8493-3052-0.</ref>

<ref name="Palmer and King 2008" >Palmer, A. C.; King, R. A. (2008). Subsea Pipeline Engineering (2nd ed.). Tulsa, USA: Pennwell. pp. 2–3.</ref>

<ref name="Ronalds 2005" >Ronalds, B.F. (2005). "Applicability ranges for offshore oil and gas production facilities". Marine Structures. 18 (3): 251–263. doi:10.1016/j.marstruc.2005.06.001.</ref>

<ref name="Stannard 2021" >Stannard, Liam (6 January 2021). "5 Amazing Underwater Structures: How Underwater Construction Works". www.bigrentz.com.</ref></references>

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