Clough brings Buffalo Field on-streamProjects and Operations // March 6, 2000
Western Australia's Minister for Mines, Norman Moore, said although Buffalo was a relatively small field, it was an exciting development given its economic recovery rates. He said the field had an anticipated peak production rate of 40,000 barrels a dayand reserves of around 22 million barrels.
"The project presented a range of significant environmental, geo-technical and facility load interface challenges. This demanded fast track changes to the design to meet some unusual site conditions. We were proud to have finished pre-commissioning and clean-up for the first oil to be delivered two days ahead of schedule," Phil Stewart, Clough Offshore Project Manager said.
"The Clough Group motto - 'Challenge is our Tradition' - was well and truly put to the test on the project and our team passed with flying colours," said Stewart.
In October 1998, Clough Offshore was awarded a A$27 million contract for the installation of the Buffalo FPSO Facilities by BHP Petroleum Pty Ltd (BHPP) as a nominated subcontractor to MODEC, Inc of Japan. The Buffalo Field is located 300 nautical milesnorth west of Darwin and is positioned on 'Big Bank', part of the Big Bank shoals.
Stewart said the Clough Offshore fleet vessel Java Constructor played a key role in the installation program, with new-build vessels Maersk Seeker and Pacific Server employed on specific elements of the installation work.
Stewart said the first challenge to be confronted by Clough Offshore during early construction was that the steep, rugged edge to the 'Big Bank' necessitated a re-design of the flexible risers and umbilical. Later installations had to also contend withlateral currents across the edge of the Bank.
"A difficult challenge which the project team had to overcome was the need to position two of the facility's mooring legs on the Bank and four of them off the Bank," Mr Stewart said.
"At times the Java Constructor had to set four anchors in 30m of water and four anchors in 300m of water. Barge movement on the anchors resulted in a significant number of difficult anchor set-ups."
He said the operation was ultimately very successful.
"BHP Petroleum was delighted with the performance of the Clough Offshore project team and the 'can-do' attitude to getting the job done to meet our deadline for the first oil flows," Thyl Kint, BHPP Project Manager said.
"BHP Petroleum has been very happy too with the way the facility has performed since coming on-stream."
Initially the FPSO facility included the FPSO, a converted tanker, two flowlines - being a 10" production line and a 4" gas lift line - and a 6" electro-hydraulic control umbilical connecting the vessel to the Wellhead Platform (WHP).
The WHP was installed by BHPP in mid-1999 on top of 'Big Bank' in 30m of water and the FPSO was to be positioned 2.5 kilometres away, off the bank in 300 metres water depth.
The flowlines were designed as an integrated rigid pipeline to the edge of 'Big Bank' and flexible riser section in free-hanging catenary profile into the FPSO turret. The umbilical was a similar profile, with a static section over the bank and a dynamicsection into the turret.
During the early construction phase it became apparent that the free-hanging catenary profile was not fit for purpose off the steep, rugged edge of the Bank, and that any type of abrasion protection system could not guarantee prevention of damage to theflexible risers and umbilical through the excursion envelope created from the FPSO.
As a result, the configuration was changed to a pliant wave profile for all three systems during the first quarter of 1999, which put the offshore construction work into a fast track program to meet the Ready for First Oil Date by end of 1999.
Clough Offshore assumed the task of designing and fabricating the seabed restraint anchors for the two flexible risers and umbilical following the change in scope, which became a significant challenge to meet the environmental, geotechnical and facilityload interfaces.
The end result was a dual tether gravity base and anchor gravity base system connected by a permanent tether some 140 metres long between the two base systems.
The tether bases were a steel frame and cast tether hook filled with moulded lead. The anchor bases were also steel frames, with four steel anchor blocks, which were installed separately during the offshore installation phase.
The final on-bottom weights for the tether and anchor bases were in the range of 100Te and 250Te, respectively, and provided the complete reaction restraint for the flexible risers to rigid pipeline interface.
Clough Offshore used Java Constructor to install the two flowlines and umbilical, including the seabed restraint systems, rigid pipelay, WHP tie-in, flexible risers and umbilical lay and tie-in.
During the installation of the flexible risers and umbilical, the Java Constructor used two anchor lines connected into the FPSO chain anchors directly below the chain table. This provided a very effective anchor set during the sensitive operation of pulling the flexible risers and umbilical into the riser guide tubes on the FPSO.
The complete operation of pulling the flexible risers and umbilical from the Java Constructor to the FPSO, installing the mid-depth buoyancy, connecting the tether line to the base and making up the seabed Grayloc connection to the rigid pipeline was very successful, claims the company.
This type of operation would traditionally have been undertaken with a DP construction vessel due to the very tight installation tolerances dictated by using flexible riser systems.
The installation of the FPSO mooring system was undertaken with one of Maersk's new 18,000BHP AHTS vessels built in Singapore. The Maersk Seeker completed sea trials before preparing to tow the mooring cargo spread to site and commencing the mooring installation and tensioning operations.
Clough Offshore had modifications made to the Maersk Seeker while it was still under construction, which enabled the vessel to be used for through tensioning of the mooring legs.
The six mooring legs - two on the bank and four off the Bank - were diametrically tensioned to the design proof loads. This was carried out by connecting one leg to a 350Te Smit bracket on the bow of Maersk Seeker, then using the vessel's chain gypsy toapply the proof load through the diametrical opposite leg.
This operation was also successful, says Stewart, even though two of the six legs required in-field adjustment to overcome an anchor positioning problem during the final phase of the work.
The FPSO Buffalo Venture hook-up operations were carried out using the Pacific Server, another new-build vessel, directly out of Japan. The Pacific Server is a DP AHTS and was used to recover the pre-installed anchor legs and pass over to the Buffalo Venture.
The hook up and final tensioning of the FPSO was a lengthy process due to the lateral currents experienced across the edge of 'Big Bank'. These however had been identified early in the pre-engineering phase and subsequently the whole operation was completed within the planned duration.