EVALUACIÓN DE LA ROBUSTEZ

The relevant issues in connection with these assessments include: • Disruption of seabed and bottom fauna due to operations • Impacts due to underwater explosions

• Covering seabed with gravel • Reef effects

Alternative IIA – Reefs in-place

Steel Jackets

As described in the previous Section (Section 7.2.3), anchors for lifting vessels may sometimes be deployed during operations to lift and remove the jackets. This might disturb the top 0–2 metres of the seabed surface. The physical effects of such disturbance will be very small, very local in character, and reversible.

Dredging the sediment in connection with cutting the legs/pilings may alter the bottom substrate where soil is removed or dumped. Bottom- dwellers will be buried where the sediments are dumped. The changes to the bottom substrate will occur in a limited area and the bottom will gradually even out. The scale of the physical effects is therefore deemed small.

The legs/pilings can also be severed using explosives. This would cause sediments in the vicinity to swirl up, then settle again in near-by areas. Sedimentation may bury bottom-dwellers in the immediate vicinity. The physical effects are expected to be short-lived and «insignificant».

Use of explosives to cut the legs/pilings will clearly affect living organisms, the scope depending on the amount of explosive, depth, and population of organisms (Larsen 1994). Effects on fish have previously been modelled on several occasions, among others, Larsen 1994, in connection with removal of the Odin installation. Fish with swim bladders, including

cod, saithe and haddock, and larvae and small fish, are most susceptible. Only minor effects can be expected for larger fish, restricted to the area close by the installation. Larger fish are very mobile and can be scared off before blasting. Use of explosives for cutting will therefore be most suitable in the period August–October, since this takes account of fry and small fish being among the life forms most susceptible to explosion and these cannot be scared off. The scope of the physical impacts is deemed «moderate negative». The steel jacket is a structural feature that may attract fish. A range of uncertainties exist regarding how the existence of such structures influences the population of fish. It is unknown whether reefs simply attract the fish in the area, or whether they create a new breeding habitat that increases the overall production (Aabel et al. 1997). The latest research in this field (Soldal et al. 1998) leans towards the conclusion that simple attraction is the predominant effect in the North Sea. Explanatory models supporting greater production are that the reefs cause more individuals to survive, more of them reach reproductive age, and thus the total production of eggs increases. If the fish simply congregate without any increase in biomass, then the attraction process will render these more likely to be harvested.

The total scale of the physical effects arising from use of the steel jackets as reefs in the North Sea is considered «moderate negative» (damage to the seabed habitat), regardless of where the reef is located (in-place or beside the Tank). The overall environmental consequences of the physical effects are deemed in both cases to be «moderate negative».

Ekofisk Tank

If the Ekofisk Tank is used as part of an artificial reef it will be left intact. In the long term parts of the Protective Barrier Wall may collapse and cause concrete pieces to drop off the structure, hitting the seabed. A concrete structure of this type is very enduring, and the date when it will start to collapse is difficult to predict. The upper parts of the Protective Barrier Wall may collapse in a few decades, and more solid parts will exist for several centuries. The effects of falling concrete on the seabed are intense but extremely local, depending on the size of the pieces. The effects will be temporary re-suspension of sediments, and burial of some local bottom-dwellers. The scale will be small and very local.

The total scale of the physical effects resulting from using the Ekofisk Tank as an artificial reef is deemed small. The total environmental impacts resulting from these physical effects are therefore also assessed as «small negative». Overall impact – Physical impacts/habitat effects (Alternative IIA)

The overall physical/habitat impacts of using the steel jackets and Ekofisk Tank as artificial reefs in place is assessed as «small negative».

Alternative IIB – Reef near Ekofisk Tank

No significant differences are expected from this Alternative relative to the foregoing one, Alternative IIA. One can argue that the «reef effect» will be rather larger from a collective reef rather than several dispersed reefs, but available data in the North Sea, and hence our knowledge in this field, is not sufficient to draw a firm conclusion.

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Alternative IIC – Jackets to shore, leave Tank in-place

The consequences due to mooring and cutting the legs/pilings are expected to more or less replicate Alternative IIA. However, as no jackets are to be toppled or placed on the seabed, this Alternative will cause less impact.

No physical impacts are anticipated due to transport to shore or demolition of the steel jackets.

The total scale of the physical effects of removal of the jackets, their transport to shore, and demolition are considered small. The total environmental consequences resulting from these physical effects of removal of the jackets are therefore assessed as «none/insignificant». Use of explosives for cutting jacket piles would result in «small» to «moderate negative» impacts.

The physical/habitat impacts of leaving of the Ekofisk Tank in-place are the same as for Alternative IIA and IIB, i.e., «small negative».

Alternative IID – Jackets to shore, Tank deposited in deep sea The effects from removal of the steel jackets will be as described in Alternative IIC, i.e., «none/insignificant».

To provide the Ekofisk Tank with adequate buoyancy for removal, some of the ballast would have to be removed. Gravel and rock in the ballast voids above the oil storage tanks and in the chambers of the Protective Barrier Wall, about 355,000 m3_{in all, would therefore have to be removed and deposited on}

the seabed. Gravel and rock, as RaKon (1999) has proposed, could be released east and west of the Tank, but this depends on the infrastructure in the vicinity. The gravel would be dispersed over the seabed and, if laid in a layer 2 metres thick, would cover 177,500 square metres (see Figure 37). The bottom in the vicinity is naturally sand, in the main. Dumping of gravel and rock could thus alter the nature of the bottom surface. Structural unevenness would attract some organisms, fish, for instance, and might cause immobile organisms to establish themselves. In addition, the dumping of gravel and rock would bury organisms that live in or on the bottom, and change the fauna in the covered areas. Stone fills over many pipelines in the area are already a feature, and the scale of the physical effects of the gravel and rock is deemed «moderate negative».

The physical effects resulting from an accidental sinking during towing of the Tank would affect a few hundred square meters of the seabed, and are assessed as small. However, the effects would be somewhat dependant on location.

For the scuttling operation, explosive charges would be set, for instance in pipeways going through the Protective Barrier Wall, allowing the structure to take in water. The firing of these charges would clearly affect any marine organisms, and the extent of the impact would depend on the amount of explosive used, the depth, and the presence of organisms nearby. There are six pipeways, and the total explosive needed is estimated at 1080 kg (RaKon 1999). Lethal effects have to be expected on any nearby fish.

Pressures arising as the Tank sinks are expected to cause implosion of the flotation chambers at a depth of about 200 metres. The suction in the water due to the implosion has the potential to kill organisms in the water column near the implosion site. This effect is very local and the scale of the physical effects is therefore assessed as small.

The implosion would cause fragmentation of the concrete structure, which would sink to the bottom with its reinforcement bars and cables. The fragments of the Tank would spread over several hundred square metres and might cover sediments and marine organisms in the vicinity. It is at present uncertain how long a re-colonisation period is required for bottom fauna in deepwater sediments (NERC 1996). Trials run by Grassle (1977) suggest that every physical impact on the bottom will cause lasting changes in the deep water community structure and composition for a minimum of two years. The scale of the negative physical disturbance is deemed moderate.

The total scale of the physical disruption from deepwater disposal is deemed «moderate negative». The total environmental consequences caused by the physical disruption are therefore expected to be «moderate negative». Alternative IIE – Jackets and Tank to shore for demolition and recycling

Environmental effects due to physical disruption in connection with a removal of the Ekofisk Tank are similar to those described for Alternative IID above, i.e., «moderate negative» physical impacts.

Figure 37

Possible location for disposal of Tank ballast on seabed near Ekofisk Center (Rakon 1999)

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No physical impacts are expected to result from demolition, recycling and disposal of the Ekofisk Tank.