Environmental statement - Flyndre and Cawdor - Maersk Oil

Flyndre and Cawdor Environmental Statement
Section 3 Environmental Baseline
that are adsorbed to sediment or organic matter is poorly understood. However, in general terms
prolonged contact between hydrocarbons and sediment may result in stronger bond formation and a
subsequent reduction in bioavailability (Van Brummelen et al., 1998). This phenomenon is referred to
as ‘ageing’, and is especially important for sediments with historic contamination such as prolonged
discharge of drill cuttings or produced water.
3.3.2. SEDIMENT CHARACTERISTICS (SEDIMENT TYPE, TOTAL ORGANIC MATTER & TOTAL ORGANIC CARBON)
The distribution of seabed sediments within the CNS results from a combination of hydrographic
conditions, bathymetry and sediment supply. Sediments classified as sand and slightly gravelly sand
cover approximately 80% of the CNS (Gatliff, 1994). These sandy sediments occur over a wide range
of water depths, from the shallow coastal zone down to about 110 m in the north and to below 120 m
in isolated depths to the south and west. The carbonate content of the sand fraction is generally less
than 10% (Gatliff, 1994).
The sediments along the pipeline route consisted principally of silty sand with little variance along the
pipeline corridor, this is illustrated in Figure 3‐7. There are a number of patches of silty sand that are
found in section 2 of the pipeline route survey (heading towards the Clyde platform). Shown also in
Figure 3‐7 are three side scan sonar outputs, two which illustrate an area of higher seabed reflectivity
that corresponds with silty sand with numerous exposure of gravels and clay (in areas closer to the
Janice and Clyde platform), the other side scan sonar image is representative of the majority of the
pipeline route being flat and relatively homogenous. The seabed images that were collected along
the pipeline route corridor illustrate the presence of fine sand, with occasional shell fragments in the
majority of stations sampled.
Sonar scan data from the pipeline route survey revealed a superficial covering (≈ 1 m) of fine to
medium silty sand with occasional scattered shell fragments and possible outcroppings of clay. In the
southern region of the proposed pipeline route there is evidence of coarser materials such as gravel,
cobbles and coarse sand (Gardline, 2011). The sediments can exhibit signs of disturbance from
previous drilling activity, as was shown in the side scan sonar results which can clearly show the spud
can depression associated with the drilling of the Flyndre well (Figure 3‐6). The results of this suggest
that any significant physical disturbance is likely to be visible for several years due to the lower levels
of sediment movement in comparison to shallower waters of the Southern North Sea.
Figure 3‐6 Illustration of the spud can depression from the jack‐up drilling rig at the drilled Flyndre
well using an image obtained from side scan sonar data.
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