P.2 ~ Continued - Careful planning precedes successful platform installation at Ekofisk
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Topside dynamic lift analysis
The dynamic lift analysis of the 2/4L topside covers the range from pre-rigging to the pre-set-down hovering ( i.e. bottom of the topside stabbing guides 1.5 m above the jacket legs) conditions. The analysis in time domain was performed using LIFSIM software (developed by MARIN). The hydrodynamic properties for theS7000 were supplied by MARIN, while those for the transportation vessel Osprey were derived using software MOSES. FORSIM software was used to generate wave loading time history.
The analysis was carried out with the following assumptions:
- 1. Sea states with irregular waves of 0.5 to 2.0 m significant height and 4 to 8 second wave peak period (5.7 to 11.3 m wave zero crossing period). Regular waves of height 0.25 to 1.0 m and period of 8 to 12 seconds were also considered to represent swell seas.
- 2.S7000 at lift-off draft
- 3. The inherent structural damping properties of the rigging
- 4. Lifting grommet lengths selected such that the topside would remain nominally level once lifted
- 5. Neither wind nor current considered in the simulation because its dynamic effects are negligible and the steady force they produce would be counteracted by the DP system
- 6. The lift was divided into pre-lift, pre-tensioning, and lift-off stages, during which the Osprey was moored toS7000.
In pre-lift, hooks are lowered to pick-up slings. The relative motion between the crane hook and the point on the laydown platform should be no more than +/- 1.5 m horizontally and +/- 0.75 m vertically.
In pre-tensioning, the topside load is transferred by a combination of ballastingS7000 and picking up crane blocks. Limiting criteria for this stage included:
- No separation more than 100 mm between the topside and its supports
- Maximum dynamic load on grillage of 6,500 metric tons (row B & G)
- S7000 roll and pitch less than 1°
- Boom tip heave motion less than +/-1 m
- No slack slings.
In the lift-off stage, the dynamic hook and sling load were not to exceed maximum allowable dynamic loads and no slings to go slack. In case of re-impact between the topside and transport vessel supports, vertical load limit of 6,500 metric tons to the supports would apply.
During post-lift, the topside would be hanging from the crane hooks and ready for the final position on the jacket. Dynamic interface betweenS7000 and topside was a prime concern in this stage. Further, in this stage, there was to be no vertical distance less than 1.0 m between the topside and the HTV Osprey and no horizontal distance less than 5.0 m between the side of topside and the bow of S7000. While the topside is hovering above the jacket legs, there would be no horizontal distance less than 3.0 m between the side of topside and the bow of S7000, and the horizontal radial distance between the stabbing guides of the jacket and topside could not be greater than 0.62 m for at least 90% of the time (30 minutes).
7.S7000 pitch/roll motion should not exceed +/- 1° and the boom tip heave motion should be less than +/- 1 m.
The following conclusions were derived from the dynamic analysis:
- 1. Most favorable heading for the lift was beam (port/ starboard) seas toS7000 and most unfavorable was longitudinal seas (head/ aft) to S7000. Most favorable wave heading can be chosen for S7000 with DP mode.
- 2. 75% pre-tension was optimal. At this stage, operability was governed by premature lift off and exceeding rigging utilization.
- 3. Parallel separation between topside and HTV was achieved with the final sling lengths adjusted for the expected offset of COG from the lift points and the pre-ballasting of the HTV for any expected residual tilt for topside during lift-off.
- 4. The pre-hook up stage was generally most challenging, followed by lift-off. There was good operability for both stages in the most favorable headings.
- 5. The vertical/ horizontal gap between the hook and the point beneath on the laydown platform were the overall governing criteria. The rigging utilization is also critical to lesser extent especially in higher sea states.
- 6. Based on topside stabbing on the jacket analysis, it was concluded that the topside could be safely installed on the jacket. Longitudinal (head/aft) and beam toS7000 were the most favorable heading and quartering to S7000 was the worst heading.
- 7. The main limiting criteria were the horizontal movement of the primary stabbing guides above the jacket legs.
- 8. The reduction in operability was only in highest periods due to the increasedS7000 pitch motion.
Weight and geometry limitations were determined based on the lifting capabilities of theS7000 cranes. These limits were used to determine the maximum allowable lift weight for different COG ranges and battery limits for dimensions. The limiting parameters were communicated to all involved parties who needed to strictly control these limits. Controls in place during the project management included an accurate and detailed weight and COG monitoring database based on the PDMS model issued monthly to all parties.
The topside was weighed two times and the weighing results reconciled against the weight control database. The weighing results were in agreement with the weight and COG from the predictions established based on the weight control report. Contingency plans were established for weight shedding by removal of items, if required, following the final weighing and COG location calculations prior to sail away. It became evident that the helideck needed to be installed as a separate lift. The helideck was modified to be transported in situ, removed from the topside and back-loaded onto theS7000 deck upon arrival in Norway, and installed after the topside lift.
Conclusion
With execution of the engineering, planning, and project management as described, the offshore lifting operations were safely executed as planned without incident.
A final interesting aspect of the offshore installation was the long transit made byS7000 once the 2/4L topside had been lifted from the HTV. To better control mooring operations, it was agreed that the HTV would use a bow anchor. Since the Ekofisk field is covered with seismic cables, the permitted anchorage was approximately two nautical miles from the installation location. This resulted in a transit with the topsides on the hooks lasting over two hours.
Acknowledgment
The authors acknowledge permission to publish this article from ConocoPhillips Skandinavia AS and co-venturers, including Total E&P Norge AS, Eni Norge AS, Statoil Petroleum AS, and Petoro AS.
