Subsea stations could reduce cost,
loads of long-distance umbilicals
Total Exploration & Production
Subsea production requires injection of various chemicals and hydrate inhibitors conveyed from the topside production fa- cilities to the subsea equipment via umbilicals. Chemical/ methanol injection tubes enclosed within the umbilicals are designed to carry their products at a pre-determined fow
rate and injection pressure.
Main parameters governing the composition of the umbilical (the
size and number of hydraulic tubes) are tieback length (managing the
pressure drop caused by frictional losses in the tubes); fow rate, especially for methanol or low dosage hydrate inhibitor (LDHI); and the
pump discharge availability at the topsides, typically 345 bar ( 5,000 psi).
However, to accommodate the longer-distance tiebacks now in the
planning stage – over 30 km ( 18. 6 mi) for oil felds and 300 km (186 mi)
for gas felds in remote regions – the umbilical will have a considerably
larger and more complex cross-section. But, this raises issues:
Manufacturability. Umbilical assembly machines’ handling capabilities for stainless steel tubes and other umbilical components are limited
by diameter and number of bobbins. Multiple tubes can also lead to
large and complex terminations for the tie-in to subsea facilities. A wider umbilical outside diameter, with its associated weight and stiffness,
will make it harder, and possibly unfeasible, to manage the umbilical
High procurement cost. This is due to the number of stainless steel
tubes required and the testing needed to qualify the umbilical section.
Installation. This may prove diffcult due to the increased outside
diameter and stiffness and size of the terminations, and the number of
reels or carousels needed to install the umbilical sections may prove
impossible to accommodate on a standard pipelay vessel reel or carousel. The weather window is another factor. All these issues may cast
a doubt over the project’s technical and economic feasibility.
To reduce the umbilical cross-section and mitigate these various issues, Total and Doris propose a subsea station at various points along
the route of the tieback for chemical storage and injection. The main
goal is to locate these functions close to the subsea processing equipment in which the chemicals are to be injected. This would dispense
with the need for chemical lines in the umbilical altogether, reducing its diameter and size and therefore lowering its weight, leading to
lower procurement and installation costs. The concept also eliminates
the need for a chemicals skid onboard the topsides facility, and allows
improved local control over chemical injection.
Equipment and systems
The subsea station comprises subsea storage tanks; chemical
injection pumps with fow control devices; piping for distribution of
chemicals from the chemical storage tanks; a subsea control module
for valve actuation, data transmission, and pump control; an electri-
cal distribution module to supply the high fowrate chemical pumps
with high voltage electric power; structure and foundation. Chemical refll operations would be performed periodically.
For oil feld tieback developments, two types of chemicals are
typically injected. Corrosion inhibitor, scale inhibitor, biocide and
demulsifer are injected continuously into production lines, normally at rates of 6-20 l/hr. The exception is biocide, injected in batch
mode and normally for 5 hr/week at 180 l/hr. Hydrate inhibitors,
i.e. methanol and LDHI, are injected into wellheads, fowlines and
jumpers, but only during well shutdown and/or start-up operations
in order to avoid hydrate formation. They are typically injected at a
much higher fow rate of 5-25 cu m/hr ( 1,321-6,604 gal/hr).
To accommodate these two injection modes and the varying
amount of chemicals to be stored, two separate subsea stations
would be optimum. One is a chemical injection station for storing
chemicals applied continuously; the other is a shutdown/re-start
station for storing hydrate inhibitors.
For gas feld tieback developments, corrosion inhibitor, scale inhibitor, biocide, and monoethylene glycol (MEG) are all injected continuously. MEG, however, which must be injected into the gas stream
at rates of up to several cu m/hr to prevent hydrate formation, the
quantities involved render subsea MEG storage unfeasible.
Total and Doris applied the following philosophy for all aspects of
the subsea station design:
• The subsea system must be modularized so that components re-
quiring maintenance and replacement such as injection pumps,
storage tanks, and electrical distribution modules can be re-
trieved to surface, replaced and tested, either as part of a module