Jeremy Beckman
Editor, Europe
Most building blocks are now in place for Rhum, BP’s first high-pressure, high-temperature development project in the North Sea.
The hostile reservoir conditions demanded lateral thinking across the entire offshore production chain, with design and installation of the export pipeline providing one of the biggest challenges. The 44-km distance to BP’s Bruce processing complex to the south makes this one of the longest HP/HT subsea tiebacks attempted anywhere.
Rhum is located 380 km northwest of Aberdeen in UK block 3/29, in a water depth of around 110 m. Recent appraisal work suggests the field contains 800 bcf recoverable, constituting the largest unexploited gas accumulation in UK waters.
BP’s first encounter with the field came in 1972, with a well plugged and abandoned before reaching total depth.
What the UK Department of Trade and Industry classifies as the discovery well was drilled in 1977, according to Rhum Project General Manager Gerry McGurk. “The depths for these wells - 4-5-km - weren’t terribly unusual even then. But the industry wasn’t ready for HP/HT at that point. You have to bear in mind that this was very early on in the North Sea’s history, a time when wildcats were being sunk everywhere.”
During the mid-1980s, the industry was avoiding HP/HT prospects altogether, due to technical constraints in drilling and testing, let alone production, McGurk says. The advent of 15,000-psi BOP equipment at the end of the decade made the drilling aspect safer, but there was still little progress in HP/HT techniques for some time.
The Castoro Sei laying the Rhum export pipeline at the approach to the Bruce CR platform.
“Within BP, from the 1977 discovery through to our next well on Rhum in 2000, there was a lot of in-house thought and consultation, mainly related to technology and timing for the development. Even when we started front-end engineering in 2002, on the back of two recent appraisal wells, many people still thought this project would be a real stretch.”
Fluid issues
The Rhum reservoir, located at a depth below the seabed of around 4,700 m, comprises an Upper Jurassic submarine fan with low density turbidite sands. It appears to be a largely connected structure, with no serious compartmentalization. For its depth, the reservoir is over-pressured, which should help well productivity. Rhum’s fluids are mainly gas, with a small proportion of condensate. However, the gas is corrosive and sour, containing around 6% CO2 and 10 ppm of H2S, both of which have a major bearing on the development.
BP as operator and IOC UK (a subsidiary of the National Iranian Oil Co.) are 50-50 partners in the project. Odfell’s semisubmersibleDeepsea Bergen has been contracted to drill two wells and complete three for the first development phase. One is a re-completion of appraisal well 3/29a-4, drilled in 2002, which flowed 45 MMcf/d. As Rhum builds up to peak output, the partners will analyze well and reservoir performance data to determine whether further wells are needed under a second development phase.
“We have engineered these wells intentionally to be vertical to avoid adding further complications,” McGurk explains. “Partly, this has to do with avoiding casing wear, which is one of the problems reported from previous deviated HP/HT wells.”
TheDeepsea Bergen was brought over from Norway following modification and maintenance to bring it up to standards required under UK legislation for HP/HT work. One of the attractions of the rig was that the 15K subsea spool trees supplied by Cameron for Rhum. Normal 10K spool trees weigh 30-40 tons, but Rhum’s are 70-80 tons. Not many rigs can handle this kind of load.
“This also happens to be the first application of these trees anywhere for Cameron,” McGurk says. “We have been working with them on this project since 2002-03. It has entailed an extensive design and qualification program on their part. One advantage of subsea spool trees is that they make workovers easier.”
TheDeepsea Bergen started drilling in July 2004, and the two new wells have both been drilled successfully, with the second reaching TD this February. “We’re now at the completion phase for all three wells,” McGurk says, “and we expect this program to continue through the end of the year. The completions present one of our biggest challenges on this project. There are very few subsea HP/HT completions anywhere, so industry knowledge is limited. It has therefore demanded a lot of contingency planning on joint operating procedures between Odfjell and the service companies - much more than for a normal 10K development.”
Pipeline protection
Before selecting Bruce to receive and process Rhum’s production, the partners also considered a fixed or floating production platform exporting directly to the Frigg gas trunkline system. Economics ruled in favor of the subsea option, despite the much higher technical risks. Managing pressure through the 44-km long, 22-in. OD export pipe was one of the main issues. The proposed pressure management technique would involve the first seabed application of a fully-rated high-integrity pipeline protection system (HIPPS) in the region.
To accommodate wellhead shut-in pressure of 1,790 barg, the HIPPS - designed and supplied by ABB Offshore Systems - is incorporated into the production manifold. The system comprises a series of high-reliability valves contained by pressure sensors at an upstream location. The valves will automatically close on detecting set pressure levels.
Pressure must always exceed a pre-determined lower set point for flow assurance purposes, while also remaining at a level lower than the main export pipeline’s design pressure of 210 barg. The HIPPS is designed to prevent the main export pipeline from experiencing the wellhead shut-in pressure, which would cause a line failure. The system is further protected with a fortified riser and pipeline section at the Bruce platform.
“Building the manifold itself was quite a challenge, in terms of the design and materials selection,” McGurk says. “One of the risks with the metals we have selected is hydrogen-induced stress cracking. This can be quite prevalent on installations involving high levels of chrome steel, due to the reaction of anodes subsea with the metals.”
R&M in Dundee built the manifold, which is equipped with spare slots for any future wells.
“At one time, the design incorporated a subsea control manifold alongside, but we opted instead for a single structure occupying a smaller footprint,” McGurk says. “One issue we faced was that if the manifold happened to be too large, there were not many vessels that could install it. Ours weighs 160 tons, and has been successfully installed by theTechnip Constructor.”
Another concern at the time of project sanction was the export pipeline’s metallurgy. With no processing of the wet, sour, and corrosive fluids prior to entering the pipeline, the latter would have to be constructed from corrosion-resistant materials. And introducing a hot system made metal selection even more critical.
“There are a number of ways we could have gone,” McGurk says. “In the end, we opted for a carbon steel line, but with a 316L steel liner. That in turn threw up further challenges, as it would involve welding of dissimilar materials. Quality control of the liner during manufacture and application became really important. And taking that to an offshore environment carried a big risk.”
The export line, designed by JP Kenny, is an insulated 16-in x 22-in. CRA pipe-in-pipe system clad internally with 316L. The design philosophy is to preserve heat, maintain Rhum’s fluids above the hydrate formation temperature, and limit associated energy requirements on the Bruce CR compression platform receiving the fluids.
Thicker-walled sections have been put in at the approach to the Bruce complex, right up to the connection to a newly installed caisson riser on Bruce CR. These will protect the complex in the event of multiple protection failures further down the export line.
This summer, the Saipem laybargeCastoro 6completed installation of this and the dual-laid 3-in. carbon steel line taking hydrate inhibitor from Bruce CR to the Rhum wells, in the form of industrial methylated spirit (IMS). Inhibitor will be needed during start-up of the wells, and for a system cold start.
“The Rhum export pipeline is one of the most complex installations to date in the North Sea,” says McGurk. “The combination of pipe-in-pipe, internal clad liner, and the large number of welds to be executed offshore all led to extensive welding trials alongside the installation barge during the winter season in Holland. These proved invaluable for the offshore execution. Quality assurance and control were particularly important, and inspection techniques were developed and tested during these onshore trials.
We selected a combination of AUT and internal cameras, which were used to good effect.”
Saipem mobilized its global experts to supervise the onshore program and final development of welding procedures. It also assembled the pipe-in-pipe sections: the pipe was manufactured by Sumitomo in Japan and free-issued to Saipem for fabrication into pipe-in-pipe sections by Corus.
JP Kenny also engineered the three 2-km infield flowlines that connect the wells to the production manifold. The flowlines themselves are pipe-in-pipe, but this time fully rated for the maximum wellhead shut-in pressure as they are upstream of the Hipps. They are heavy walled and made from solid 8-in. 25% chrome super-duplex 28-mm internal pipe, with an outer 12-in. carbon steel pipe of 17 mm. Technip’s yard at Evanton in Scotland fabricated the lines. They have since been reel laid by the Apache, the latter’s heaviest pipe-in-pipe installation of this type to date, according to McGurk.
Aker Kvaerner in Moss, Norway, designed and fabricated the main 44-km steel tube control umbilical between the manifold and Bruce CR and the three infield umbilicals, the tubes being supplied by Sandvik in the Czech Republic. Technip’s Normand Pioneer installed the umbilical.
Saipem and Technip have since turned their attention to trenching, backfill and rock-dumping of the various lines, as well as flooding and gauging of the main export line. This work will be followed by further subsea construction, i.e. tie-in of the flowline ends. Subsea activity will continue throughout the current season and right up to the start of production.
Bruce overhaul
“When we sanctioned Rhum, there happened to be an upcoming compression project on Bruce,” McGurk says. “We integrated topsides modification work on Bruce with installation of the new process equipment for Rhum on the Bruce CR platform. This allowed us to optimize space and weight capability on the existing structure.” In July 2004, Heerema’s Hermod barge installed a new 1,900-ton module, comprising two 11-MW gas-turbine driven compressors, coolers, knockout drums and associated control systems, and the new equipment for Rhum.
A brownfield construction program has been running since July 2003 to tie in the two projects to facilities on the Bruce D and production/utilities/quarters platforms. BP also added a new caisson riser, manufactured by Burntisland Fabricators in Scotland, and an emergency shutdown valve was installed on the outside of the CR platform to bring aboard Rhum’s fluids.
This program led to a 47-day shutdown of the entire Bruce production complex last year, the biggest for any BP operation globally.
“Essentially, Bruce now has capacity to take incoming gas from Rhum at design rates of 300 MMcf/d,” McGurk says. “However, the production profile will be different from most gas fields. We’ll ramp Rhum up to its maximum and stay at that level for two to three years.”
The Frigg trunkline also takes gas from Bruce and the Alwyn area. Rhum’s associated liquids, which are relatively low in volume, due to the gas being so dry, will flow through the Forties pipeline system.
Aside from the reservoir challenge, this development for BP is also about maximizing existing local infrastructure such as Bruce and Frigg, McGurk points out. “That means we don’t have to put in new infrastructure that in turn gives us yet another set of assets to manage.”
Lessons learned from this project will also be transferred to future BP-operated high-pressure developments in the area, such as Devenick, which forms part of our plans for gas development in the Harding area.
“Other HP/HT project teams globally - not just BP’s - have also provided invaluable lessons and we hope that what we learn on Rhum can help raise the industry’s capability on these types of technically challenging developments,” McGurk says. “A number of project teams around the world have already shown interest in what we are doing. By working with suppliers to develop new product lines for Rhum, we’re also helping to make these products more marketable elsewhere. The industry will benefit in various ways as a result of this project. Contractors and suppliers can use it as a showcase of new capabilities in this area of technology.”•
