Environmental statement - Flyndre and Cawdor - Maersk Oil

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Flyndre and Cawdor Environmental Statement Section 2 Proposed Development Figure 2‐8 Modular trenching plough capable of simultaneous lay and trench (CTC Marine, 2011). The final decision on the installation method for the umbilicals has yet to be determined. ROCK PLACEMENT Rock placement will be required for pipeline dropped object protection, upheaval buckling mitigation and for pipeline crossings. An initial maximum estimate is that 20,000 tonnes of rock will be required for the development. Three pipeline crossings have been identified along the Flyndre and Cawdor pipeline development which will require additional spot rock placement of approximately 3,000 tonnes each. The final mass of rock required will be calculated by undertaking upheaval buckling analysis based on information provided from post trenching surveys. MATTRESSES Mattresses will be used to provide protection at sections where the pipelines and umbilicals are not trenched e.g. connections with wellheads and manifolds. It is estimated that 250 mattresses measuring 6 m x 3 m will be required covering a total area of 4,500 m 2 . 2.6.8. SUBSEA INSTALLATION SUPPORT VESSELS Various support vessels will be associated with the subsea installation phases of this development. Vessel types, duration and fuel usage of vessel during installation are given in Table 2‐10. 2‐ 14 D/4114/2011
Flyndre and Cawdor Environmental Statement Section 2 Proposed Development Vessel type Duration (days) Working fuel consumption (Te/d) 1 Total fuel use (Te) Survey 40 11 440 Dynamically positioned reel‐lay vessel 25 23 575 Trenching support 40 17 680 Diving support vessel 75 18 1350 Rock dumper 10 15 150 Guard vessel 50 4 200 Supply vessel 20 10 200 Total 3595 1 Source; The Institute of Petroleum 2000. Table 2‐10 Vessel type and fuel usage during installation of subsea infrastructure. 2.6.9. FLOW ASSURANCE HYDRATES The initial flow assurance work suggests the Flyndre and Cawdor hydrocarbons will be susceptible to hydrate formation. The pipeline insulation design will ensure that the temperature profile during normal operation remains above the hydrate formation temperature. For extended shutdowns the hydrate mitigation philosophy is to blowdown the system at the host to a pressure below the hydrate formation envelope at ambient temperature. Hydrate prevention during start up will be by continuous methanol injection at the trees, upstream of the choke and may also be injected into the riser local to the platform. The Clyde platform will provide the necessary storage and injection equipment to enable methanol to be injected at the tree and upstream of wing and master valves. SCALING The need for scale inhibition at Cawdor is currently uncertain and will need to be assessed further once the first well is drilled. However, injection facilities will be provided in the subsea chemical delivery system to allow downhole scale injection should it be required. The Flyndre well is already completed with a downhole chemical injection service installed. SAND Sand is not anticipated from the Cawdor reservoir and is considered unlikely from the Flyndre reservoir. WAX The Wax Appearance Temperature (WAT) is currently estimated at 35 o C and 37 o C for Flyndre and Cawdor respectively. In addition to a low thermal loss design for the pipeline, a wax inhibition delivery system will be provided. The pipeline insulation design will ensure that the temperature profile during normal operations remains above the WAT. D/4114/2011 2‐15
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Environmental statement - Flyndre and Cawdor - Maersk Oil

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