Fields are developed with platform and production equipment, Full Well Stream (FWS) pipeline to the FPSO and Gas Lift (GL) pipeline from the FPSO back to field. Production will flow to the FPSO and existing compressors on the FPSO will supply all fuel gas, instrument and gas lift gas required for field. The pipelines tie-in to the existing FPSO full stream pipeline and gas pipeline. The subsea tie-in will consist of a pipeline end manifold (PLEM), which will be designed to enable minimum interference of the production from the incoming production well.
Scope of Study
The purpose of this study is to optimise the offshore pipeline route for the Field Development. The considerations will be the platform approach to well and the subsea tie-in to the existing FPSO pipelines.
The offshore pipeline system will consist of the following:
The offshore pipeline system will consist of the following:
- Full stream pipeline and gas pipeline from the platform to the subsea tie-in to the existing pipeline system ;
- PLEM consisting of the subsea tie-in for FWS piggable wye-connection and gas tee-connection;
- FWS and GL offset spools from the existing pipeline system to the new PLEM, and from the new PLEM to the existing PLEM.
Considerations made in optimizing the field route selection are as follows:
- Riser location on the platform to mitigate the likelihood of damage to the risers / pipelines from load-out activities and drilling operations.
- Riser to be clamped to the inside of the jacket leg, to accommodate the jacket bracing design requirements
- Pipeline / riser interface orientation at the bottom of the jacket shall be chosen to provide sufficient clearance from the jacket bracings and jacket leg.
- Offset spools designed to allow for end expansion, as the FWS pipeline / riser have high design temperature
- Minimum straight section of from the pipeline flange connections to the offset spools.
- Minimum radius of curvature for the full stream pipeline and Gaslift pipelines respectively to allow a direct route from the platform to the subsea tie-in.
- Minimum spacing between the FWS and Gas Lift pipelines to allow for lateral deflection of the pipelines as a result of on-bottom stability.
- PLEM shall be designed to accommodate space for piggable-wyes for the FWS tie-in and barred-tees for the Gas Lift tie-in
- Location of the new PLEM to allow for the expansion in the pipelines.
The design data presented in this section comes from the pipeline system design basis and the offshore survey. The survey shall be performed for a corridor along the straight line route from the platform to the existing PLEM leading into the risers to the FPSO. The straight line length between the platform and PLEM shall be determined.
Minimal deviation from the straight line surveyed route shall be adopted to accommodate the platform approach and the subsea tie-in at the PLEM. Any deviations from the surveyed route shall be chosen so as to remain within the survey corridor.
The offshore survey shall check that the seabed within the vicinity of the proposed pipeline route from the Platform to FPSO is flat with no bathymetric obstruction. The proposed pipeline route survey corridor shall be free from any abrupt bathymetric changes that could have adverse effects on the pipeline engineering design and laying activities.
The offshore survey shall check route corridor sonar reflectivity and silt type. Seabed features shall check if within a 50 m corridor of the surveyed route were individual pockmarks, trawl scars and anchor scars. The pockmarks are described as moderate but shallow seabed depressions, most are roughly circular in shape, typically 10 – 20 m in diameter and 0.5 – 1.5 m deep. Offshore Geo Surveys suggest that the pockmarks have formed from defluidisation process of the very soft to soft uppermost sediments due to natural sediment compaction and the pressure introduced from the massive water column. Numerous trawl scars were observed scattered throughout the survey corridor indicating the presence of high fishing activities within the region.
The ideal offshore pipeline route is a straight line, which corresponds to the shortest distance between the start and end points of the pipeline. Platform to FPSO pipeline route configuration will be based on the following considerations:
- Platform approach
- Subsea tie-in
- Minimum radius of curvature
- Geotechnical data
- Work over / drilling activities
- Platform load-out activities (crane position)
- Fishing activities.
A minimum straight length 10 times the water depth is used before commencing any curve along the proposed route. This will ensure that the pipeline section upstream of the curve has enough lateral resistance to maintain its position without moving when the lay barge executes a route turn. The minimum pipeline route radii of curvature are calculated and are used as guidelines to ensure that the equivalent stress and residual stress levels at the curve are within acceptable limits.
Where the pipelines are installed adjacent/parallel to an existing pipeline a minimum separation of 30 m shall be maintained, except at the platform approaches.
Based on the basic requirements by drilling, ample space should be provided for drill barges to set-up and drop anchors. No clash exists with pipeline routes. Same for the foot prints of the jack-up rig; no interface exists between pipelines and jack-up foot prints. Pipeline routes shall be selected to provide full access of drilling rig to one side. The drilling rig access platform face has to be left clear over a sector of 180 degrees to provide unhindered access to jack-up drilling rigs. Pipelines are not to encroach into this area.
The geotechnical data shall check if presence of fishing activities in the region. Scarring of the seabed along the surveyed route corridor suggest that the use of trawlers which could damage the pipelines on the seabed. Thus, re-routing of the pipeline will not avoid fishing activities.
Relevant authorities shall be notified of the presence and location of the pipelines to be marked on the appropriate admiralty charts. Exclusion zones around the platform and FPSO should be considered in the operator specifications.Further to operator specifications for policing the exclusion zones, sufficient mechanical protection of the pipelines will need to be designed such as; subsea flange protector and PLEM protective cage.
Platform Loading Activities
The loading activities for the platform will be taken into consideration for the pipeline platform approach and riser location. This attention needs to be made in order to mitigate the likelihood of impact to the pipeline / riser system as a result of a dropped object. Consideration for loading activities shall be made with reference to the positioning of the topside crane and its proposed trajectories. Also, particular attention to the docking location of the loading supply vessels that will dock alongside the platform will be made.
The minimum radius of curvature for submarine pipeline route selection is governed by the following two factors:
- Residual Tension Criteria: The curve radius which can be maintained in equilibrium during installation
- Equivalent Stress Criteria: The equivalent stress in the pipe wall during operation.
This method estimates the minimum curvature radius by approximating the ability of the soil to maintain the radius of curvature of the pipeline in relation to the residual on bottom lay tension.
The longitudinal stress produced in a restrained pipeline subject to expansion or contraction, bending moment due to curvature and span is computed from the following equations:
The bathymetry and geotechnical data shall investigate if a straight pipeline route from the platform approach to the subsea tie-in location is achievable. The optimization considerations for the pipeline route will be made for the platform approach and the subsea tie-in to the existing pipeline system.
The Puteri pipeline system tie-in to the Penara pipeline system shall be to the northwest of the platform. To mitigate the likelihood of impact to the pipeline from dropped objects resulting from loading operations and/or drilling operations, it is recommended that the risers be placed opposite to the loading zone and drilling rig approach zone.
The orientation of the pipeline / riser interface has been considered to allow sufficient clearance from the jacket bracings and jacket leg. Allowances for rotation of the risers during jacket installation and flanges connections for divers have been made.
Expansion offset spools shall be required for the FWS pipeline / riser interface and the pipeline / PLEM subsea tie-in. Although the expansion for the gas lift pipelines shall be insignificant, offset spools shall still be required to accommodate the tie-ins. The offset spools will be optimized based on the end expansion analysis, and the riser design. All bends in the offset spools shall have 90° angles.
The pipeline will be subject to lateral deflections in accordance with the on-bottom stability of the pipelines As such, the FWS and Gas Lift pipelines will need to be spaced a minimum distance apart. The pipelines shall maintain a straight section for a minimum length.