EnQuest has investigated a range of development options for the following project elements:
1)
Production System2)
Drilling rig3)
Production flowlinesThese are tabulated below, (Table 3-2) highlighting advantages and disadvantages with respect to technical, economic and environmental considerations.
The FPSO was selected as the best production option. For a small development with a relatively short field life such as Alma, the ability to re-use an existing production facility is a key factor - on both economic and environmental grounds. Having selected the FPSO option, the Uisge Gorm was selected as the most suitable for the following reasons:
Alma requires a similar mooring configuration and the vessel has previously worked within 15km from the proposed Alma location
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
Recent North Sea service and maintenance records are up to date for recent production operations
Large range of capacity handling
Can install produced water reinjection (PWRI) with minimal oil in water (OIW) discharge, benefiting environment
Potential for a change from diesel power generation, providing environmental improvement
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
REPORT REF: P1459BA_RN2525_REV0 3-6 21/07/2011
Table 3-2: Pros and cons or dif ferent development scenarios
Option Pro Con Decision 1A: FPSO A limited number of FPSOs are available for deployment
Provides an integrated storage and offloading system Modification required are more economic that other available options e.g. new platform
FPSOs fit for expected field life
Using an existing FPSO is cheaper than a new build FPSOs considered have proven track record in the UKCS Minimal seabed disturbance from installation
Can be easily redeployed at end of field life
Age of available FPSOs / fatigue life Substantial modifications required
Tanker offloading system potentially higher risk of oil spills than export pipelines
Selected
1B; Petrofac OE&O FPF-1
Readily available
Would provide consistent Duty Holder across all EnQuest Assets Would be suitable for deployment to NNS at end of field life Established contracting strategy with Petrofac Offshore Engineering and Operations (OE&O)
Would require substantial replacement of a number of systems e.g. process system, accommodation and utilities, riser connection system, helideck, primary and secondary steelwork, mooring system
Fatigue and integrity issues
No crude storage / offloading capabilities so would require export routes
Limited pipeline export routes
Rejected
1C: New platform
Systems designed specifically for Field New facility and equipment designed for field life Latest technology designed into build
Definitive build cost Lower OPEX costs
Limited pipeline export routes
Utilisation of existing facility and avoidance of new build environmental impacts
Larger seabed footprint than FPSO
Jacket structure would require substantial piling operations Substantial costs incurred with build and installation Would require decommissioning at end of field life Higher CAPEX
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
REPORT REF: P1459BA_RN2525_REV0 3-7 21/07/2011
Option Pro Con Decision 1D: Subsea
tie-back
No requirement for oil storage
No requirement for tanker mooring and offloading facility Lower OPEX costs associated with tanker
Avoidance of potential environmental impacts of crude transfer and tanker fuel oil
Avoidance of known seabed hazards from previous operations in field
Limited opportunities to tie into pipeline export routes. Closest facility is 19km
Would require decommissioning at end of field life
Potentially larger seabed disturbance with longer export route Additional costs associated with pipeline crossings
Identification of clear pipeline routes difficult due to large amount of seabed hazards from previous production operations Would incur environmental impacts from pipelay trenching and installation vessel activity.
Rejected
2A: Semi- Submersible Drilling Rig
Less weather dependant during positioning on site
More scope for moving rig but maintaining same anchor pattern – less seabed disturbance. For example, moving to allow subsea infrastructure to be installed, moving rig if subsurface philosophy changes, ability to drill six wells on same anchor pattern Easier to run horizontal xmas trees
Better selection options- at least two rigs are known to be available
Current drilling team has extensive knowledge of semi submersible drilling operations
More waiting on weather during operations
Moving to new wellhead location (skidding rig) on same anchor pattern dependent on weather
More expensive day rates
Potentially larger seabed footprint if consider scour marks from anchor catenary as well as anchor mounds
Selected
2B: Heavy Duty Jack-up Drilling Rig
Lower operation day rates
Potentially lower waiting on weather once located on individual well
Potentially smaller seabed footprint (only area of spud cans)
Lower lifting and storage capacity
Movement between individual wellheads is limited and may require full rig move outside of footprint – could lead to greater seabed disturbance that a semi-submersible
Rig availability tighter than for semi-submersibles Spud can disturbance could affect future rig locations
Would require additional geotechnical site investigations for spud can placement which could not be covered under Alma surveys in 2010
Riser tensioning capacity is greater in deeper water Jack-up may experience problems with horizontal xmas trees
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT
REPORT REF: P1459BA_RN2525_REV0 3-8 21/07/2011
Option Pro Con Decision 3B: Buried
flowlines (water injection)
Greater protection for flowlines – no additional protection such as rock would be required except for mattressing and grouting at trench transition areas
Conventional / proven solution
Option to surface protect spans which cannot be buried due to existing subsurface obstructions
Larger seabed footprint
If select low temperature option for flowlines then flowlines could not be protected or buried
As the flowlines would run quite hot, there would be the requirement for large cooling spools if trenched
Harder to decommission would likely leave in-situ at end of field life
Higher mobilisation costs for installation as would need more vessels and equipment
Higher risk of subsurface obstructions during trenching which would need to be micro-routed
Selected
3A: Surface laid flowlines (production)
Ease of installation – range of installation vessels available. Benefit as compact field layout with possible drilling rig on site during installation
Lower mobilisation costs for installation Potential of re-use / decommissioning easier Conventional / proven solution
Risers only solution available with FPSO Minimal seabed disturbance
Lower risk of subsurface obstructions because no trenching
Greater risk of damage Procurement costs
May require dropped object protection e.g. rock material, concrete mattresses which will increase seabed footprint
Less than 1m of silty sand as surface layer. May sink into sand.
ENQUEST HEATHER LIMITED ALMA FIELD DEVELOPMENT