Jumper/separation concepts could simplify ultra-deepwater planning

May 1, 2006
Downsizing FPSOs to restrain capital costs is a major concern for West African deepwater operators.

Downsizing FPSOs to restrain capital costs is a major concern for West African deepwater operators. Another is maintaining production integrity, as the industry grapples with longer distance, ultra deepwater subsea tiebacks.

Doris Engineering is working on new concepts which could make a material difference in both cases. One concerns modifying jumpers to limit the cost of hydrate prevention; the other involves transferring the separation process to the seabed.

According to Doris’s CEO, Loic des Deserts, the typically 6-in., diameter jumpers linking wellheads to manifolds in deepwater fields are an important part of the subsea flowline network; but they are also difficult to insulate effectively for an appropriate cool-down time outside the hydrate formation zone.

During a shutdown, the oil/gas wellstream that normally flows through the jumper will separate out with gas in the jumper’s upper section, and oil and water in the lower section.

The build-up of trapped gas will have no immediate impact, but during restart operations, the gas will be driven through the cold liquids, and hydrates may form. As a counter-measure, the operator is normally forced to inject large quantities of methanol via the control umbilical. This brings added problems, as methanol itself is a volatile substance.

IFP’s hydrate dissociation test rig.
Click here to enlarge image

Doris’ proposal, which is interesting Total, among others, is to install an electrically heated jumper from the outset of a subsea project, in order to lessen the risk of a hydrate plug causing a loss of production. The electrical heating reduces the risk of hydrate formation, and the requirements for methanol injection can be significantly reduced, in turn allowing the umbilical dimensions to be scaled down.

The heating system can be employed at start-up to limit insulation requirements and, if necessary, to dissociate a hydrate plug in case of an accidental incident.

At present, Doris is working on the concept in association with Emc3, with funding from the French government. As part of the present development, hydrate dissociation will be tested at IFP’s facilities prior to its forthcoming hydrate plug test program.

Seabed study

Doris is also working for Total on concepts for its ultra deepwater discoveries in Angola block 32. Like BP in block 31, Total is leaning toward FPSOs with subsea wells, while also seeking to limit its capital outlay. One solution under investigation is seabed separation of oil and gas, or gas/liquids.

There are numerous benefits, according to des Deserts. “Firstly, you are reducing the amount of equipment needed on board the FPSO; secondly, the constraints on fluids entering the flowlines and risers are much smaller, because the gas is already stripped out. Thirdly, you may be able to operate with a single production flowline instead of a more classic looped flowline arrangement. With a single flowline, you also have more flexibility in placing wells at the optimum location, which in turn reduces development drilling costs.”

OTC recognition

At this year’s OTC Distinguished Achievement Awards, Doris will be recognized for its pioneering work in design and construction of a wide range of offshore facilities. Its current workload continues that trend:

  • Usan deepwater development offshore Nigeria - basic engineering, including subsea, umbicals, flowlines, risers. and FPSO topsides facilities
  • Moho-Bilondo offshore Congo - supporting detailed design of the floating production unit, and EPCI on turnkey basis of the floater’s mooring spread
  • Block 14, offshore Angola - subsea tieback (study) to compliant tower
  • PetroVietnam - subsea facilities for floating system, under subcontract to Berwanger in Houston
  • East Caribbean gas pipeline, taking gas from Trinidad to various islands - pre-conceptual studies
  • South Pars 11 - engineering design for two platforms with onshore separation at planned Pars LNG plant
  • Offshore Angola - subsea well intervention and marginal field development studies for ADC, a consortium of Sonangol, Doris, and Pride International.