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 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.

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.
  • 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.
  • Temperature of the water and the air above it. Sea surface temperature can vary from a minimum of to a maximum of about. Higher surface temperatures decrease rapidly with depth, and generally reach a steady-state of about by about, though on Australia's Northwest Shelf, water and seabed temperature may exceed at a depth of. 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.
  • Seawater and splash zone chemistry
  • Biofouling
  • 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.
  • 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.
  • 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.
  • Sea ice and icebergs
  • Earthquakes and tsunamis
  • Scour
  • Siltation

    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 sampling. 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.
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. When this occurs the soil can behave like a dense liquid. This can also happen during some construction processes, such as piledriving. The presence of large boulders in glacial till deposits can give a misleading impression of bedrock, hiding the further extent of softer sediment below.
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.
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.
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.
Unconsolidated sand – seasonal shifts.
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.

Ecological and societal impact

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

    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 steel.

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.