9.2.1
Baseline Data Sources
This section principally references the following primary data source:
Gardline Environmental (2011). Alma Field Development Site Survey: Environmental Baseline & Habitat Assessment Survey. Ref 8602
9.2.2
Existing Baseline
During the 2011 Alma site survey fifteen stations were selected for investigation with the camera system and for sampling with a 0.1m2 day grab. Two stations were selected close to the proposed northern and southern drill centres (four stations in total), two stations close to the proposed FPSO riser base location and two stations along each of the proposed flowline routes between the drill centres and FPSO riser base. A further five stations were positioned in order to ground truth the sediment conditions in the area.
Fauna observed during the environmental baseline survey (GGL 2011) included cnidarians (anemones), echinoderms (starfish and sea urchins), crustaceans (hermit crabs Pagurus bernhardus) and fish species including (but not limited to) flatfish. Visible fauna from Day grab samples included sea urchins, bivalve molluscs and annelid worms.
The macrofauna was dominated by the polychaete worm Paramphinome jeffreysii, which was present in every sample and accounted for c.18% of all individuals identified. The bivalve Kurtiella bidentata was more abundant at some stations, however, generally it is has a patchy distribution and was absent at several stations sampled. Univariate statistics suggested that there was a lack of dominance structure in the area, and that the faunal community was generally rich and evenly distributed.
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A total of 12,187 individuals from 216 taxa were recorded in the retrieved samples. Of the individuals recorded, 2,367 (22% of the total) were juveniles from 26 taxa (12% of all taxa recorded). The dataset can be divided into five major taxonomic groups; Annelida (Polychaeta), Arthropda (Crustacea), Mollusca, Echinodermata and “Other” (made up of taxa from Cnidaria, Platyhelminthes, Memertea, Priapulida, Sipuncula, Phoronida, Hemichordata and Chordata) (GEL 2011). Table 9-2 shows the contributions of the gross taxonomic groups.
Table 9-2: Contribut ions of th e gross taxonomic g roups
Group Individuals Taxa Abundance Proportional Contribution (%) Abundance Proportional Contribution (%) Annelida (Polychaeta) 6251 51 101 47 Arthopoda (Crustacea) 508 4 41 19 Mollusca 2492 20 47 22 Echinodermata 2533 21 15 7 Others 403 3 12 6 Total 12187 100 216 100 Source: GEL 2011
Twenty taxa (9% of total recorded) were present at every station sampled, and nine (4%) were present in every sample taken. Of the nine, four were polychaetes, one was molluscan, three were echinoderms and one was “Other” (Nemertea). The most abundant species across the survey area was the polychaete P. jeffreysii which is tolerant of hydrocarbon contamination (Olsgard and Gray, 1995). The presence of this species cannot be assumed to be as a result of contamination across the survey area; however the elevated numbers seen at sample station ENV7 and ENV12 may be related to contamination at these stations. Amphiuridae sp. (juv) appeared to show decreased abundance at sample station ENV12, which suggests that the elevated hydrocarbon and metal concentrations at this station were sufficient to cause a change in abundance of some of the more sensitive species (GEL 2011). Table 9-3 shows the top 10 species seen during the survey.
Table 9-3: Species ranking
Rank
Species/Taxon Total rank
score Fidelity Total abundance Score Abundance 1 1 Paramphinome jeffreysii 141 0.94 2182 2 2 Amphiuridae sp. (juv) 136 1.01 1937 3 4 Galathowenia oculata 104 0.86 721 4 5 Pholoe assimilis 80 0.76 635 5 3 Kurtiella bidentata 70 0.77 1057 6 6 Vitreolina philippi 66 0.88 438 7 7 Nemertea sp. 33 0.55 264 8 10 Trichobranchus roseus 33 0.72 219 9 8 Amphiura filiformis 28 0.94 246 10 12 Eudorellopsis deformis 26 1.70 196 Source: GEL 2011
The survey found that overall community structure was comparable to that found in other surveys carried out in the area previously. The community was dominated by polychaetes and other typical CNS species were common GEL 2011).
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Variation in fauna present is likely to be a result of natural variation in water depth and sediment size coupled with the effects of the elevated hydrocarbon concentrations seen at some stations. However, the survey did not find consistently significantly differences between contaminated and uncontaminated samples.
No habitats or species of conservation significance under the UK’s Offshore Marine Conservation (Natural Habitats, &c.) (Amendment) Regulations 2010 or EC Habitats Directive – Annex 1 species were observed.
The photographs below (Figure 9-1) are representational of the seabed conditions observed during the baseline survey.
Figure 9-1: Seabed photographs of the Alma development area
Source: GEL (2011)
9.2.3
Potential Impact Identification
The EIA identified that during the project life cycle the activities listed in Table 9-4 have the potential to interact with the benthic community.
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Table 9-4: Benthic commu nities – potential impact identifi cation
Project Activity Aspect Potential Impact
Construction
Physical presence and
movement of vessels Anchoring
Physical damage to individuals Smothering
Drilling of wells
Discharge of cuttings Physical damage to individuals
Smothering Discharge of chemicals (including WBM)
Potential toxic effects Discharge of reservoir hydrocarbons
Installation of flowlines
Discharge of chemicals (including WBM) Potential toxic effects Physical presence of subsea infrastructure
and flowlines
Habitat loss
Physical damage to individuals Smothering
Habitat creation Trenching and backfill
Concrete mattressing and rock placement
Installation of FPSO Anchoring Physical damage to individuals
Smothering Production
Physical presence, operation and maintenance of FPSO
Discharge of produced water
Potential toxic effects Discharge of chemicals
Accidental Events
Overboard loss of equipment
or waste Dropped objects Physical damage to individuals
Chemical / hydrocarbon release (< 1 tonne)
Diesel, crude or chemical spill (including OBMs)
Smothering
Potential toxic effects Chemical / hydrocarbon
release (1-10 tonnes) Chemical / hydrocarbon release (>10 tonnes)
In general the potential impacts identified above fall into two categories:
Impacts from chemical discharges e.g., lethal (direct) toxicity and sublethal (indirect) effects
Impacts resulting from physical disturbance e.g., smothering, damage to individuals or habitat loss
The EIA concluded that there will be an interaction between the project activities and the benthic community but that any resulting impacts will be restricted to the project area. Changes to the baseline may only just be noticeable (i.e., sensitivity of low) but the duration of most impacts will be short- term (up to five years), the exception is the permanent siting of structures on the seabed which will have a long-term effect. The likelihood, spatial extent, frequency, duration, sensitivity, recoverability and significance of the impacts have been assessed in Sections F of Appendix A2 and A3 and Section D of Appendix A4.
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9.2.4
Mitigation Measures
Measures outlined in Sections 8.3.4 and 8.4.4 adopted to reduce and/or eliminate the toxic impacts of the development on water quality and the footprint of the development on the seabed will also mitigate the potential impacts on the benthic community. These are not repeated here but are listed in the previous sections and summarised in Table 12-2 and in Sections F of Appendix A2 and A3 and Section D of Appendix A4.
9.2.4.1 Physical disturbance
The main impact of construction activities on benthic communities results from physical disturbance to the habitat either as a result of direct removal of the habitat or smothering of individuals and/or the habitat. The Alma development will disturb approximately 0.04km2 of seabed. Disturbance will occur by a number of mechanisms:
Positioning of structures on the seabed, such as the manifold, drill rig anchors, well head xmas trees, concrete mattressing and rock protection Flowline and umbilical installation e.g., trenching and backfill
Discharge of drill cuttings
In all instances the direct result will be mortality of flora and fauna within the impact footprint. As demonstrated in Section 9.2, the species identified in the area are typical of the CNS, and no rare or protected species were identified in the environmental baseline surveys. The main species present in the project area are known to be generally tolerant of increased levels of suspended sediments and low levels of smothering but are generally intolerant of displacement (GEL 2011). This suggests that individuals on the periphery of the project footprint and to some extent within the direct footprint may show some tolerance to disturbance particularly if the disturbance is short-term in nature. However, species individuals will experience mortalities directly within the project footprint and in particular along the flowline corridors. Disturbance from the majority of project activities will cease within a few days, or at most within a few months, allowing for recovery and recolonisation of disturbed areas to commence almost immediately.
The concept of recovery of biological resources is not easy to define as community composition can vary over time, even in areas that remain undisturbed. A key factor to be taken into consideration when determining whether a community is identical in species composition and population structure following cessation of impact, is whether the biodiversity would have remained stable over that period in the absence of disturbance. Furthermore, the rate of recovery depends on several factors, including:
Levels of natural disturbance Coarseness of sediment Water depth
Type of benthic community that is disturbed
The types of benthic communities that typically colonise coarse sediments that are subject to natural disturbances by currents have much faster recovery rates
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environments. Conversely, fine sediment communities, particularly those with slow growing long lived fauna, usually have slow recovery times.
Mixing of surface sediments can result in a change in biological community type e.g., infill of depressions can result in detectable faunal difference on a local scale. However, the area impacted is minor when compared to the extent of the CNS and physical disturbance from other activities such as trawling.
The majority of CNS seabed species have short life spans (< few years) and relatively high reproductive rates, indicating the potential for recovery within 5 years. Mobile organisms are less at risk as they are able to avoid the area of disturbance.
The benthos in the project area is typical of the wider CNS and consequently species that inhabit the area tend to recover quickly after disturbance. The process of recovery is dependent on the species within the surrounding area, their respective life cycle characteristics and mobility. The primary mechanisms for recolonisation of an impacted area are:
Individuals migrate into displaced sediments from adjacent areas Settling out of eggs and larval stages from the water column
The proposed development is located within an area of previous drilling activity. The development area is sufficiently homogenous that any localised losses are unlikely to affect the integrity of the community as a whole. In general, recolonisation is expected to occur quickly with the initial appearance of opportunistic species such as polychaetes of the spionid family; followed by a progression over time to a more stable and diverse community, better representing the current conditions.
At present there is very little available information on the rate of recovery of biological resources following the installation of pipelines/flowlines compared with the impacts from activities such as dredging and trawling. Physical disturbance will be the dominant mechanism of ecological disturbance from the jet trenching installation technique. The fluidisation of sediments may cause
some fatalities of infauna, although this will be limited to the width of the fluidised sediment (typically only a little wider than the outer diameter of the pipeline). The deposition of suspended sediments may smother sessile species and filter feeders along the pipeline corridor, but the extent of this impact will be limited to a footprint no wider than 10m and the intensity of the impact will decrease with distance from the flowline. The recovery of benthic communities will partly depend on the community type present. As discussed above the benthic community associated with the Alma development area is typical of the wider CNS and is characteristic of a dynamic environment. To a certain extent, species are used to a degree of smothering caused by natural sediment movements and as such exhibit relatively quick recovery times.
The surface sediments at the development consist of <1m thickness of very loose to loose silty shelly sands (with a varying degree of gravel and shells) over firm to very stiff sandy gravelly clay. Both the sand and clay will be suspended. The sand is likely to settle out of suspension soon after the disturbance close to the trench, potentially smothering sessile fauna in the immediate vicinity. The finer particle size of the clay (0.002mm) will remain in suspension, potentially for days, and will be transported by currents away from the flowline routes. These will gradually settle out, becoming deposited over a
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wider area. The layer of deposited clay material is likely to be very thin, possibly undetectable. Although the suspended clay is unlikely to smother sessile species the increase in turbidity may clog the filter feeding mechanisms of sessile species
Furthermore, the altered sediment characteristics within the area affected by trenching may affect recolonisation of the impacted area. Although the sediments are likely to be uncontaminated they will contain a higher proportion of sandy gravelly clay from the Forth and Fisher Formations compared to the surrounding principally fine sand sediments. Recolonisation should be relatively quick, but it may be impeded by the change in sediment type. However, it will occur through the mechanisms discussed above.
The placement of the manifold, concrete mattresses and rock protection material on the seabed will smother individuals within the immediate footprint of the structures. Sedentary species will be particularly vulnerable to burial as they are unable to avoid such disturbances. The area of hard substrate is expected to attract colonisation of certain species that require hard substrate for anchoring points, but it may take longer to establish a community due to the limited larval supplies in the predominantly sandy surrounding areas. However, the area affected is extremely small (0.04km2) in comparison to the considerable extent of sandy sediments in the wider region.
As discussed in Section 6.1.3.4, the worst case total seabed footprint from all drill cuttings piles is 21,016m2. In reality this is more likely to be in the order of 2,992m2 or less, as the wellheads are within 15m of each other, and it is expected that the cuttings piles will overlap, reducing the area of impact. As bottom currents in the development area are expected to be in the region of 0.42ms-1 it is thought that piles will erode and disperse, although cuttings piles in the CNS have been known to remain in place for 5-10 years (UKOOA 1999). However, there is no evidence on the survey data of previous drill cuttings piles suggesting that they will eventually be eroded completely. Between wells there will be a two to three month period for benthic community recovery before cuttings are once again deposited. This cycle of deposition and recovery means that the community will stay in a perturbed state for at least a year and a half until all wells are completed. Thereafter, recovery within five years is anticipated, as discussed above.
Eight anchors are deployed per drill centre within a radius of 1,000m (3,281ft) from the rig. The length of anchor chain depends on rig height, water depth and anchor point location, in this case around 1,000m. Future anchoring would be expected to use similar lay-out and anchor lengths. Anchor chain scars are produced as a result of the laying of the anchor chains on the seabed. Sidescan sonar images of the Alma Field area clearly show the impact of previous drilling operations.
Impacts also occur when the anchors are pulled out. When anchors are lifted clear the anchor fluke levers sediment onto the seabed creating a scar in the seabed and in some cases a mound where clay clasts that have been removed by the anchors are subsequently dumped on the seabed.
The size of the scar or disturbed mound is dependent on the seabed characteristics. Anchor scars are common where seabed sediments or shallow sub-surface sediments are composed of clay. Where clasts are stiff then this will also be conducive to coherent sections of clay being drawn out by anchors and then redeposited on the seabed. Sediments within the Alma Field
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comprise a thin layer of sand over sandy silty clay, which will be conducive to the creation of anchor scars and mounds.
The size of the scars produced as the anchors are withdrawn will depend both on the size of the anchor and how they are withdrawn, as the method of withdrawal can reduce the scarring that occurs.
On removal of anchor chains the soft seabed sediments would be expected to slump into any slight depressions that have been caused, even out the seabed topography. Anchor chain scars do not appear to cause an obstruction to bottom fishing as the survey has shown trawl scars crossing the anchor marks. Laying the anchor chains on the seabed is likely to cause some localised mortality of sessile benthic fauna. However, on removal of anchor chains lateral recruitment of megafauna is likely to occur with several days (FRS pers comm. 2008). The majority of CNS seabed species have short life spans and relatively high reproductive rates, indicating the potential for rapid recovery. The impact of anchors on biology will be similar to that of the chains with individuals within the impact zone of each anchor being crushed as the anchor is laid. Some smothering of individuals could also occur in the immediate vicinity as the anchors are withdrawn.
From this assessment it is expected that anchoring has the potential to have an impact of minor significance on benthic communities.
As a result of the placement of concrete mattress, flowlines and other subsea infrastructure, new habitat will be created for those species that require hard substrate for anchoring. This could lead to settlement of new species and the potential for a localised change in marine ecology (increased localised biodiversity).
The EIA has concluded that there will be a residual impact on benthic communities from physical disturbance to the seabed, which may be noticeable when compared to the baseline. However, the disturbance will be localised to the immediate vicinity of the development. Therefore, the seabed activities that cause physical disturbance have been classed as having a minor residual impact.