Seabed

Hydrocarbons are of a lower density than formation water. Thus, if no mechanism is in place to stop their upward migration they will eventually seep to the surface. On seabed surveys in some offshore areas we can detect crater like features ( pock marks ) which also bear witness to the escape of oil and gas to the surface. It is assumed that throughout the geologic past vast quantities of hydrocarbons have been lost in this manner from sedimentary basins. 

The time taken to complete a base line study and EIA should not be underestimated. The baseline study describes and inventorises the natural initial flora, fauna, the aquatic life, land and seabed conditions prior to any activity. In seasonal climates, the baseline study may need to cover the whole year. The duration of an EIA depends upon the size and type of area under study, and the previous work done in the area, but may typically take six months. The EIA is often an essential step in project development and should not be omitted from the planning schedule. 

If compaction occurs as a result of production careful monitoring is required. The Ekofisk Field in the Norwegian North Sea made headlines when, as a result of hydrocarbon production, the pores of the fine-grained carbonate reservoir collapsed and the platforms on the seabed started to sink. The situation was later remedied by inserting steel sections into the platform legs. Compaction effects are also an issue in the Groningen gas field in Holland where subsidence in the order of one meter is expected at the surface. 

Tension leg platforms (TLP) are used mainly in deep water where rigid platforms would be both vulnerable to bending stresses and very expensive to construct. A TLP is rather like a semi-submersible rig tethered to the sea bed by jointed legs kept in tension. Tension is maintained by pulling the floating platform down into the sea below its normal displacement level. The legs are secured to a template or anchor points installed on the seabed. 

Typically, a Subsea Field Development or Subsea Satellite Development would consist of a cluster of special subsea trees positioned on the seabed with produced fluids piped to the host facility. Water injection, as well as lift gas, can be provided from the host facility. Control of subsea facilities is maintained from the host facility via control umbilicals and subsea control modules. 

The template will be constructed and fitted out at a fabrication yard and then transported offshore to the drilling location. The template is lowered to the seabed using a crane barge or, if small enough, lowered beneath a semi-submersible rig. Prior to drilling the first well, piles are driven into the sea bed to hold the template in place. 

If one or more clusters of single wells are required then an Underwater Manifold System can be deployed and used as a subsea focal point to connect each well. The subsea trees sit on the seabed around the main manifold (compared to the template). 

With the electro-hydraulic system the signals, power and hydraulic supplies are sent from a Master Control Station (or MCS) on the host facility down Control Umbilicals (Fig 10.36) to individual Junction Boxes on the seabed or subsea structure. 

In areas where seabed relief makes pipelines vulnerable or where pipelines cannot be justified on economic grounds, tankers are used to store and transport crude from production centres. The simplest method for evacuation is to pump stabilised crude from a processing facility directly to a tanker. 

The basic aim of a decommissioning programme is to render all wells permanently safe and remove most, if not all, surface (or seabed) signs of production activity. How completely a site should be returned to its green field state, is a subject for discussion between government, operator and the public. 

All pipelines will be circulated clean and those that are buried, or on the seabed, left filled with water or cement. Surface piping will normally be cut up and removed. Flexible subsea pipelines may be reeled-in onto a lay barge and disposed of onshore. 

Tension leg and floating platforms can easily be released and towed away for service elsewhere, which is cheap and attractive. In the case of the fixed platforms, the topside modules are removed by lift barge and taken to shore for disposal. Gravity based structures can in theory be deballasted and floated away to be re-employed or sunk in the deep ocean, and steel jackets cut and removed at an agreed depth below sea level. In some areas jackets are cleaned and placed as artificial reefs on the seabed. The... 

In addition to the reported economic reserves, there are substantial nickel resources which could be amenable to mining and refining once appropriate technology becomes available. The single largest such resource is seabed nodules which contain ca 1% nickel and which could represent up to 800 X 10 t of nickel (see Ocean rawmaterials). 

The ocean is host to a variety and quantity of inorganic raw materials equal to or surpassiag the resources of these materials available on land. Inorganic raw materials are defined here as any mineral deposit found ia the marine environment. The mineral resources are classified generally as iadustrial minerals, mineral sands, phosphorites, metalliferous oxides, metalliferous sulfides, and dissolved minerals and iaclude geothermal resources, precious corals, and some algae. The resources are mosdy unconsoHdated, consoHdated, or fluid materials which are chemically enriched ia certain elements and are found ia or upon the seabeds of the continental shelves and ocean basias. These may be classified according to the environment and form ia which they occur (Table 1) and with few exceptions are similar to traditional mineral deposits on land. 

Fig. 1. Global distribution of seabed mineral deposits, where x represents chromite + barite titanium, zirconium, hafnium, and thorium tin I gold, platinum, and silver 3 sand and gravel shell, calcium carbonate gems marine polymetaUic sulfides phosphorites Cl cobalt cmsts S sulfur and B...

Precious Corals. One important deep seabed resource having worldwide distribution is precious coral. The industry extends worldwide, but the richest beds... 

Consohdated deposits are those which occur as soHd masses upon or within the stmcture of the seabeds. These may be removed only by fracturing, fluidi2ing, or dissolving the materials to be recovered. 

Ocean Basins. Known consohdated mineral deposits in the deep ocean basins are limited to high cobalt metalliferous oxide cmsts precipitated from seawater and hydrothermal deposits of sulfide minerals which are being formed in the vicinity of ocean plate boundaries. Technology for drilling at depth in the seabeds is not advanced, and most deposits identified have been sampled only within a few centimeters of the surface. 

Fluid deposits are defined as those which can be recovered in fluid form by pumping, in solution, or as particles in a slurry. Petroleum products and Frasch process sulfur are special cases. At this time no vaUd distinction is made between resources on the continental shelf and in the deep oceans. However, deep seabed deposits of minerals which can be separated by differential solution are expected to be amenable to fluid mining methods in either environment. 

P. Hoagland and J. M. Broadus, Seabed Commodity and Kesource Summaries, Technical Report WHOI 87—43, Woods Hole Oceanographic Institute, Woods Hole, Mass., 1987. 

N. A. Wogman, K. Chave, R. K. Sarem, eds., Inter-Universif Program of Research on Ferromanganese Deposits of the Ocean Floor, Seabed Assessment Program, Washington, D.C., 1973. 

Joly observed elevated "Ra activities in deep-sea sediments that he attributed to water column scavenging and removal processes. This hypothesis was later challenged with the hrst seawater °Th measurements (parent of "Ra), and these new results conhrmed that radium was instead actively migrating across the marine sediment-water interface. This seabed source stimulated much activity to use radium as a tracer for ocean circulation. Unfortunately, the utility of Ra as a deep ocean circulation tracer never came to full fruition as biological cycling has been repeatedly shown to have a strong and unpredictable effect on the vertical distribution of this isotope. 

Seabed trenching from remote-operated vehicles (Figure 7.9)... 

In determining the protection current required, the surfaces of the objects to be protected in the water and on the seabed, as well as those of foreign constructions that are electrically connected to the object to be protected, should be isolated. The protection current densities derived from experience and measurements for various sea areas are given in Table 16-3. In exceptional cases measurements must be carried out beforehand at the location of the installation. Such investigations, however, provide little information on the long-term development of the protection current. By using a suitable coating [4], the protection current density in the early years of service will be only about 10% of the values in Table 16-3. For a planned operational lifetime of 30 years, about 50% of these values is necessary. 

The anodes can either be laid on the bottom, hung between the pillars or attached to the object to be protected. In every case they should be mounted so they can be easily exchanged. Anodes laid on the seabed are installed on concrete sleds or concrete slabs (see Fig. 16-7) so that they do not sink into the mud or become covered with sand. The current output in sand is considerably reduced due... 

Temperature is of particular importance to the performance of anodes, especially when anodes are buried. Anodes may often be used to protect pipelines containing hot products. Thus temperature effects must be considered. Figure 10.14 illustrates the effect of temperature on different anodes in hot saline mud. Al-Zn-In anodes experience greatly reduced capacity in open seawater at temperatures above 70°C (down to 1200Ah/kg at 100°C) and in seabed muds in excess of (900 Ah/kg at 80°C). At... 

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