7 ACCIDENTAL EVENTS
7.2 P ROBABILITY OF A CCIDENTAL E VENTS O CCURRING
Typically the likelihood of accidental events occurring is minimised through legislation governing the industry, emergency shut-down procedures and multiple control and mitigation measures consistent with industry best practice across the project life cycle.
Events are possible during both the construction and production phase. The most frequently expected type of spill would be a small (<1 tonne) spill of oil or chemical from the rig or FPSO during bulk transfer to/from the facilities, leakage or during use or storage.
Three possible incidents have been identified within the project scope as potential sources for a major spill of hydrocarbons (>100 tonnes). These are considered to be the worst case scenarios possible:
1)
Loss of diesel inventory from the FPSO and tanker through collision – 2,400m3 (2,016 tonnes) from the FPSO and 3,430m3 (2,881 tonnes) from the export tanker2)
Loss of crude oil inventory from the FPSO and tanker through collision – A maximum of 94,500m3 (81,0901 tonnes) from the FPSO and a maximum of 100,000m3 (87,000 tonnes) from the export tanker (note: neither vessel will be full at the same time)3)
Loss of diesel inventory from the drilling rig – 1,665m3 (1,399 tonnes) The probability of an extensive release of hydrocarbons occurring on this project, during construction and/or production, has been assessed below.7.2.1
Construction and drilling
Historical spill data in the UK is collated by both the DECC and the Maritime and Coastguard Agency (MCA). The DECC collates incident reports from offshore oil and gas installations, including drilling rigs, whereas the MCA collates incident reports from UK vessels, including those associated with support and construction activities for the oil and gas industry. The MCA data also incorporates data reported to the DECC to provide an overview of all marine incidents. Both sets of data have been used in the analysis below during the 20 month Alma construction period, during which the following vessel activity is envisaged:
Drilling rig – 447 days
Standby vessel to support rig – 447 days Anchor handling vessel – 112 days
Vessels for flowline installation – combined total of 70 days on site 112 visits by supply vessels over 16 months
The spill risk associated with these activities can be divided into two main categories:
a) Offshore support vessels (e.g., standby, construction and supply vessels)
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
Available data from the MCA (ACOPS reports) has been reviewed for years 2000 to 2009 (ACOPS 2001-2010) to establish the probability of a spill occurring from an offshore support vessel (category a) above). However, there are only two reported incidents which suggests, there is either little reported data for construction related incidents or the percentage of incidents is very low. Given the lack of data, an estimation of the likelihood of a spill from a support vessel occurring per annum would not be statistically valid.
Historic data held by the DECC are more descriptive and can be used to establish the probability of a spill during drilling operations (category b) above). Records for spills from mobile drilling units have been analysed for the period 2003 to 2007. Excluding very small spills (<0.1 tonnes), data for this period shows there were 32 spills from drilling rigs. Of these, 26 were less than 1 tonne and one was over 10 tonnes (13.4 tonnes oil based mud in 2006). Over the same period it is estimated that there were approximately 1,100 wells drilled from mobile drilling units such that the probability of a release of greater than 0.1 tonnes is approximately 3% per well. At Alma there will be up to eight wells drilled which gives a 24% likelihood of a spill greater than 0.1 tonnes. There is not enough data for the larger spills (>10 tonnes) to estimate the likelihood of such a spill statistically although it can be inferred from the data that such spills are very rare
7.2.2
Production
During the project life cycle there is the potential for a loss of containment integrity of the oil production flowlines leading to a release of approximately 81m3 of crude oil. The report “Riser and pipeline release frequencies” by the OGP (2010) states that failures in a pipeline may occur as a result of:
Loads exceeding pipeline critical loads, usually resulting in an isolated incident
Gradual weakening of the pipeline over a period of time.
The primary causes of pipeline failures are critical loads that may lead to an isolated incident include loads from trawl boards, ship anchors or subsea landslide. The second cause of pipeline failures are mechanisms which act over time include corrosion, open spans causing fatigue and buckling (OGP 2010).
Table 7-1 provides the recommended pipeline failure frequencies for the UK offshore oil and gas industry based on the above potential causes.
Using this table it has been calculated that the frequency of the production flowline failing in open sea is 0.00125 times per year, the frequency of it failing as a result of external damage is 0.0001275 times per year and the frequency of the risers failing is 0.0036 times per year.
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
Table 7-1: Industry riser and pipelines failure fr equencies
Pipeline Category Failure frequency Unit
Subsea pipeline: in open
sea Well stream pipeline and other smallpipelines containing unprocessed fluid 5.0 x 10-4 Perkm/year Processed oil or gas, pipeline diameter
≤24 inch 5.1 x 10-5
Per km/year Subsea pipeline: external
loads causing damage in safety zone
Diameter≤16 inch 7.9 x 10-4 Per year
Flexible pipelines: subsea All 2.3 x 10-3 Per
km/year
Risers Steel – diameter≤16 inch 9.1 x 10-4 Per year
Flexible 6.0 x 10-3 Per year
Source: OGP (2010)