Annex D Description of Typical Offshore Transportation and Injection System Components

The offshore transportation and injection system components for CCS are typical of those to be found in the offshore oil and gas industry. While the items described do not represent an exhaustive list, the following description of equipment should indicate concerns with respect to modes of failure and their consequential leakage potential. Damage to components could be catastrophic both to the system and operating personnel. Protective structures are the main line of defence for these assets.

Connections for simple spur lines to CO2 storage complexes to develop manifolds for current and future connections may be part of the design concept for any CO2 network.

Figure D.1 illustrates the simplest and most economical of in-line T-connections for a future distribution take off to a storage sink including a future connection for a supply from another pipeline. In this solution the tees for the future CO2 source and the future storage sink flow line connections are laid as an integral part of the main trunkline. When the storage complex is to be connected, a tie-in manifold (TIM) as shown in Figure D.2 would be installed.

Figure D.3 illustrates an example of an 18" tee that may be installed as an integral part of the main pipeline. In this arrangement it has two 18" valves and a 'double block and bleed' arrangement with chemical injection points, to ensure that all dead legs are filled with methanol or glycol and enable the flushing of future connections.

Figure D.1 Tie-in schematic (option 1)

Figure D.2 Tie-in manifold schematic (option 2)

Figure D.3 Typical tee arrangement

Another potential option in an expanding network is that the initial pipeline terminates at the off-take for a spur line to a storage complex. To allow for future expansion, a TIM is used to terminate the line and provide this future capability by:

a) Commissioning the main pipeline from the onshore booster station to the distribution spur.

b) Connecting and commissioning the 18" flow line to the initial storage sink.

c) Connecting and commissioning a future pipeline connection from another source.

d) Connecting and commissioning the future pipeline from the initial storage sink to a further storage sink.

e) Future inspection pigging between the onshore booster station and future storage sink.

Figure D.4 shows a schematic of this TIM with the initial 'jumpers' shown as solid lines and future jumpers and pigging loops shown dotted. This TIM will have an over-trawlable structure128 and will have a piled foundation. All blind pipe ends will have a double barrier against the CO2 injection stream with 'double block and bleed' arrangements with chemical injection points, to ensure that all dead legs are filled with methanol or glycol and enable the flushing of future connections.

The future connection for the CO2 input from an additional source pipeline has been included in the TIM to simplify the main pipeline installation.

Figure D.4 Tie-in manifold schematic (option 3)

Any subsea components attached to the trunkline or installed subsea over a storage complex will need to be protected from fishing operations. Exposed assets should be surrounded by a structure and then buried in a gravel dump, which will be profiled to permit trawl boards to slide over the top. Figure D.5 shows a typical gravel dump arrangement for an in-line tee, and Figure D.6 shows a typical protection structure for a manifold associated with a subsea injection site. The foundations of the structures are most likely to have piles driven through the four corner columns, but this will depend on the soil conditions at the location of the individual structures.

Figure D.5 Tee protection – typical gravel dump arrangement

Figure D.6 Manifold protection structure

Ideally, the pressure within the CO2 trunkline should enable subsea connections to underwater structures associated with storage complexes without further pressure boosting. However, this presents an additional complication in the ability to install, control and maintain the proposed facilities. Subsea systems should be designed with ease of change-out and replacement of those items that cannot be expected to last the design life without maintenance. The water depths at the location of the various subsea facilities in the UKCS are expected to be suitable for diving, and so this is the preferred option for primary installation. Commissioning and operations should be diver-less, where possible.

Subsea CO2 storage complexes present their own set of problems with respect to controlling, measuring, monitoring and verification of the CO2 stored. The Snøhvit Project in Norway has demonstrated the use of onshore control systems connected to the storage complex through a 165 km electro/hydraulic multiplex control umbilical, but this is near the limit of such a control system. Many proposed offshore facilities may be significantly further from shore than that and may need to be serviced from a closer existing platform. Alternatively, two companies have developed and installed all-electric 'Christmas trees'129 that would enable a longer and more expandable control system, but these systems are relatively new and less proven than a conventional electro/hydraulic system. Another alternative would be the use of a minimum facilities platform130 or buoy system to provide communications and electro/hydraulic control of the subsea systems. This would require additional attention to maintain these surface facilities but the ability to reach offshore further with more reliability may be the deciding factor.

Figure D.7 shows a typical normally unattended platform and Figure D.8 shows an unmanned buoy system.

Figure D.7 Typical unmanned platform

Figure D.8 Unmanned buoy

If CO2 is brought up on to a minimum facilities platform or an existing oil and gas platform or future EOR platform, there should be the ability to isolate the platform from the trunkline. Isolation valves at the in-line tee should be considered. In addition, a subsea isolation valve (SSIV) set in a failsafe shut mode should be located at a safe distance from the platform. This is so it is not subject to damage from dropped objects. Equally, an anchor could interfere with it. At the same time, the distance from the platform to the SSIV should not be so great as to increase the inventory of CO2 within the pipeline in the event of a line break on the platform side of the SSIV. These factors should be considered for each platform, to set the safe distance, but will typically be 100 to 500 metres from the platform.

One other aspect of bringing CO2 up onto a platform, with the correct facilities, is that the pressure of the CO2 can be further boosted prior to injection. This will require additional power, but may be required to get the CO2 into deeper and higher pressure geological formations, for instance in EOR applications.

128 An over-trawlable structure may be defined as one that does not snag the fishing gear causing either the stopping of the vessel or breaking its warp line during over-trawling. The structure should also be able to withstand any loads or impacts experienced during over-trawling.

129 In petroleum and natural gas production, an Xmas tree is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells which are likely to include CO2 injection.

130 Minimum facility platforms are an alternative to subsea systems that allow for the incorporation of a platform extension without the need for additional costly subsea solutions.