Project Overview
Petrobras contracted Allseas to perform the engineering design, supply of subsea equipment, construction and pre-commissioning of a 20”- 24” gas pipeline connecting the Cernambi/Lula fields to Jacone beach.
The client’s Rota 3 Pipeline crosses the Rota 2 Pipeline in a water depth of 1280m. The original crossing was achieved using two concrete mattress towers, one placed on either side of the Rota 2 pipeline. During a survey during the dewatering stage, it was discovered that the Rota 3 Pipeline had moved from the crossing support mattresses and was resting on the Rota 2 pipeline.
The client required custom-designed sleepers to support the gas pipeline as well as planning and logistics to effectively implement the solution.
MMA's Solution
We designed and fabricated two sleepers that were placed on either side of the Rota 2 Pipeline to create a pipeline crossing. Each sleeper weighed 110 tonnes with dimensions of 26.4m (L) x 4m (W) x 2.2m (H). To accommodate the lateral expansion of the pipeline, both sleepers incorporated a low friction sliding surface on top of the beam and stoppers at each end to retain the pipeline.
The weight of the sleepers, hook capacity and active heave compensation abilities of the installation vessel were key constraints for the client. To satisfy these constraints, our engineers designed 0.5m x 0.5m voids in the base and central beam to reduce the weight.
Another constraint was the bearing capacity of the soft silty soils along the pipeline corridor.
Our engineers specified a heavy-duty geotextile to be cast into the base of the structure, providing a filter barrier at each of the voids to encapsulate the seabed beneath the sleepers, consolidating the soil and increasing the bearing capacity. The concrete reinforcement and lift points were protected by a cathodic protection system to ensure a design life of 30 years.
The massive size of each structure meant the fabrication had to be performed on the quayside ready for loadout onto Allseas’ vessel. We worked with a local partner to cast the sleepers within the country, with the scope including the heavy-lift load-out operations.
Outcome
The sleepers were fabricated on a quayside in Rio de Janeiro and lifted by a crane barge off the quayside and onto Allseas’ vessel for installation. The solution was efficient in supporting the Rota 3 Pipeline where it crosses over the Rota 2 Pipeline. Plus, most importantly, our client was pleased with the result, saying, “We recommend MMA highly for flexible and creative solutions.”
Technical Details
- Crossing Water Depth: 1280m Pipeline Diameter 24”
- Dimensions: 26.4m (L) x 4m (W) x 2.2m (H)
- Dry Weight: 110 metric tonnes
- Cathodic Protection: Zinc Anodes
MMA Innovation / Value Added
Optimised design and fabrication of the sleepers to suit the installation vessel • Maximised bearing capacity by using geotextile filter barriers in voids • Local fabrication to reduce transportation cost and maximise local content
