The Dalia SPS Deep Water Project is requesting the most extensive use of thermal insulation until now on SPS equipment for hydrate control during cool down scenarios. The complex processes of insulating the Dalia X-Tree are presented, including insulation of the X-Tree spool, a "Dog House" around the Flow Control Module and a second "Dog House" around a connector. This work illustrates the importance of correct use of numerical simulations in balance with a practical sense of what is possible to manufacture. It is also showing the challenges of verifying a complex thermal system where deviations between the test setup and a real production scenario must be accounted for. In the early phase, the thermal insulation design is established by use of numerical simulations. This provides theoretical models which are adjusted to fit the real and detailed complexity of the components on the X-Tree. A full scale cool down test of the X-Tree is used to verify the design and the numerical models.
The Dalia Field is located in the Gulf of Guinea, in block 17 offshore Angola. The distance to shore is around 230 km and the water depth varies from 1250 m to 1400 m. The Dalia development scheme shown in Fig 1 is framed around a Subsea Production System (SPS), an extensive Umbilicals and Flowlines network incorporating the latest innovative riser technology, and an FPSO to support the processing, storage and offloading functions. One of the technical challenges for Dalia Project is the thermal insulation of the SPS production equipment. Indeed the combination of a low energy reservoir (50°C and 235 bar), a heavy crude oil (23°API) sensitive to hydrate formation and a large subsea network to operate/preserve leads to flow assurance requirements above the oil industry experiences in a deepwater environment.
Thermal behavior of the production system of Dalia is of primary importance in order to:
avoid hydrate formation both in transient states (shut-down/restart) and flowing conditions.
improve productivity, as lower temperature implies higher viscosity which jeopardizes the well productivity and is already high at reservoir temperature.
decrease heating requirements on topsides, as the process requires the fluid to be heated at high temperature to reach the commercial specifications.
The hydrate management plan relies on three key rules:
The hydrate prevention procedure might be performed with the FPSO in shutdown condition, in which case the main power generation will be unavailable.
No part of the subsea system, apart from production tubing, is allowed to enter into the hydrate formation domain for both normal and degraded conditions.
No continuous inhibition by chemical injection in routine operation. The large volume of produced water expected after 5 to 7 years of production makes inhibition by chemicals not cost effective. Moreover supply and environmental considerations are not in favour of large chemical injection.
Fig 2: Dalia hydrate formation domain (Available in full paper)