Steve Sasanow
Special to Offshore
By the time the current group of long-distance tieback (LDT) projects - Snøhvit, Ormen Lange, and phase two of West Delta Deep - have been completed, the novelty of the LDT may have just about worn off.
The next group of projects is not dissimilar in technical specifications to what has come before. Shell expects to finally get Corrib moving again after several years of wrangling with the Irish authorities about the location of the onshore gas terminal. This is a 70-km subsea-to-beach development in 350 m of water. What may tax the offshore construction element are the wild weather conditions in that part of the Atlantic.
Meanwhle, ChevronTexaco has moved forward with the Gorgon development offshore Western Australia. It has awarded pre-front-end engineering and design work on the LNG plant to a US/Australian joint venture including KBR and Clough. It has also received permits from the Mexican government for its proposed offshore regasification facility near Tijuana. This is also an approximately 70-km LDT, with its initial phase wells in waters of no more than 200 m. But there is a deeper water section, and ExxonMobil is known to be currently doing conceptual engineering work on its deepwater (1,300 m) Io/Janscz prospects that are just beyond Gorgon.
Reliance is still trying to come to terms with exactly what it wants to do at its massive D-6 Dhibuhai offshore gas complex in the Bay of Bengal. The reserves are estimated at 175-235 bcm. The complex is around 45 km from shore, but the waters reach 1,400 m. The suggestion is that up to 10 subsea manifolds and 100 subsea wells will be necessary to produce the entire network of gas structures.
The first phase could be 34 wells producing 40 MMcm/d. The suggestion that this field could be in production by August 2007 stretches credulity, though, as no contracts have yet been placed.
If one wants to get excited about the next quantum leap in LDT, then it is necessary to talk about the massive Shtokman field in the Russian Arctic. With reserves of 3.2 tcm - that is 113 tcf - it is more than twice the size of Norway’s Troll field. And at more than 500 km offshore, it will present a new scale of technology challenge.
Both Norsk Hydro and Statoil, operators of the Ormen Lange and Snøhvit fields in home territory, are expected to play some part in this development. Hydro, for its part, has already been consulted on the subsea technology aspect of such an LDT. As with most long-distance projects, the control system and flow assurance will be the key issues. The latter is of a different scale from Ormen Lange. There will be no slug traps related to an undulating seabed. The route from Shtokman to shore is relatively flat, but the delivery of methanol over such a long distance will present a technology and cost issue.
Sometime in the future, offshore compression will be required. It seems likely that by the time Shtokman would require this technology, subsea gas compression will have already been put to use at Ormen Lange and possibly elsewhere.
Fiber optics for communications will be a necessity, and the development of repeaters will be required to transmit signals that distance. Even though the tieback distance at Snøhvit is 145 km, more than three or four repeaters would be necessary to guarantee reliable communications.
If standard hydraulics are used, an open-ended system would have to be employed with either pressure intensifiers or a local HPU for the downhole safety valve. The alternative would be to go for an all-electric system, and Hydro has reported that it has already been in contact with Cameron about its Cameron DC system.
