7.3.1
Construction
As discussed above, during the construction phase there is the potential for loss of containment from the drilling rig leading to a release of diesel and fuel oil. If this were to happen, a maximum of 1,665m3 (1,399 tonnes) of diesel would be released instantaneously.
This release is smaller than those for production. Modelling has only been undertaken for the worst case spill scenarios, which occur during production and are presented in Section 7.3.2 below. The drilling rig will not be on location at the same time as the export tanker and FPSO and therefore the three inventories could not combine in an incident.
As discussed in Section 7.1.1, in the event that well control is lost the wells will effectively self-kill and modelling of a well blow out has not been undertaken.
7.3.2
Production
During production from Alma the worst case scenario is for full loss of containment from both the FPSO and export tanker due to collision (with each other). If this were to happen, a maximum of 5,830m3 (4,897 tonnes) of diesel and 100,000m3 (87,000 tonnes) of crude oil would be released instantaneously. The 100,000m3 of crude oil represents the maximum (larger) capacity of the export tanker as neither vessel will be full at the same time.
Modelling was commissioned to determine the extent of the spill should a collision occur. The results are presented below and in more detail in Appendix B. It should be noted that all modelling information provided is generic and illustrative only and not intended to be relied upon in any specific instance. This is because in practice any number of variables may impact on an oil spill or other environmental incident and as such should be addressed on an individual basis, taking account of the specific conditions encountered.
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
The scenario was modelled using two types of models:
Stochastic - A stochastic model, also known as a probability model shows the probability of where an oil spill may impact for defined periods of time for a range of prevailing wind directions. The model uses historical wind data to run a series of trajectories for the various wind directions. It then combines the results to produce an overall illustration of the probability of where oil might travel to in the defined period of time. This type of modelling is an important tool for determining the areas of coastline that could potentially be affected by a spill and therefore the best locations to place oil spill response equipment. However, this type of diagram is typically the most misunderstood part of an Environmental Statement or Oil Pollution Emergency Plan. The most important thing to note is that it does not illustrate the extent of an oil spill should a collision occur.
Trajectory - A trajectory or deterministic model are used to predict the route of an oil slick over time and under certain metocean conditions. UK legislation requires two trajectory models are undertaken for each spill scenario investigated by the oil and gas industry; one trajectory using a 30 knot wind blowing towards the nearest stretch of UK coastline; and one trajectory using a 30 knot wind blowing towards the closest international boundary.
Figure 7-1 shows the output of the stochastic model for the instantaneous loss of the 100,000m3 of crude oil from the export tanker. The model output illustrates the extent of the oil spill after 417 days for 12 prevailing wind directions and provides the probability of the spill reaching a particular area. For example, if during the collision the prevailing wind direction was towards the west-north-west, the diagram illustrates that there is a less than 10% probability of oil beaching on the west coast of the Shetland Islands. In this scenario, the probability of the oil also beaching in Norway would be significantly less, probably around 0%. Therefore, depending on the prevailing wind condition at the time of the event there is a 1% chance of oil beaching along the coastlines of the majority of countries bordering the North Sea (Figure 7-1 and 7-2). Modelling indicates that the spill will have beached on the UK coastline within 8 days and 10 hours and on the Danish coastline within 5 days and 12 hours. The spill will have completed dispersed within 417 days.
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
Figure 7-1: Stochasti c mod el run – 100,000m3 instantaneous spill of 38° API crude oil (417 days)
Source: Adapted from OSR (2011)
Figure 7-2: Possible beaching l ocations
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
For a worst case diesel spill, the probability of any beaching occurring is close to zero. The stochastic modelling shows that after 10 hours, all spilt diesel has either evaporated or has been dispersed (Figure 7-3) (OSR 2011).
Figure 7-3: Stochast ic model run – 5,830m3instantaneous spill of diesel
Source: Adapted from OSR (2011)
For the purposes of oil spill response planning EnQuest are required to model, using the trajectory method, the fate of the worst case spill scenarios during conditions when:
A 30 knot wind would blow the spill directly to the UK coastline
A 30 knot wind would blow the spill directly towards the closest international boundary
This type of modelling can provide a guide as to where a spill may beach and what volumes of oil may reach the coastline.
Trajectory modelling (Figures 7-4 and 7-5) indicates that with a 30 knot wind towards the earliest point of impact on the UK coast, approximately 86,393m3 of crude oil would beach approximately 200 hours after the incident along the North Yorkshire coastline (OSR 2011).
In 30 knot wind conditions towards the nearest international boundary, crude oil would head towards the UK / Norwegian median line, crossing it within 5 hours of the incident occurring. On this trajectory it would continue towards the Norwegian and Danish coastlines, crossing the Norway / Denmark median line after 31 hours and potentially beaching on the Danish coast in approximately 130 hours. The volume of oil emulsion beaching is estimated to be 161,742m3 (OSR 2011).
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
Figures 7-6 and 7-7 show that in either scenario (onshore/offshore), no diesel will beach on any coastline. All diesel will either be evaporated or dispersed within 10 hours of the spill occurring.
Figure 7-4: Spill trajectory under 30 knot wind c ondition s towards th e UK coastline
Source: OSR (2011)
Figure 7-5: Spill trajectory under 30 knot wind condit ions towards th e nearest international boundary
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
Figure 7-6: Diesel spill trajectory under 30 knot wind c ondition s towards th e UK coastline
Source: OSR (2011)
Figure 7-7: Diesel spill t rajectory under 30 knot wind co nditions towards the nearest international boundary
ENQUEST HEATHER LIMITED ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT ALMA FIELD DEVELOPMENT