After a major storm crippled production from Liuhua field, a major life extension project and new umbilicals from JDR Cable systems renewed an aging property; Victor Schmidt explains.
Recently, JDR Cable Systems deployed several subsea production umbilicals and power cables for electrical submersible pumps (ESP) in a South China Sea, subsea-tieback project for China National Offshore Oil Corporation (CNOOC). The project was part of a much larger field-life extension effort by CNOOC for the Liuhua 11-1 development (LH 11-1), which included the tieback and start-up of the Liuhua 4-1 oil field (LH 4-1).
The larger project extended LH 11-1’s working life by 15 years and expanded the capacity of the FPS Nanhai Tiaozhan and FPSO Nanhi Shengli to new production from development of the LH 4-1 field. LH 4-1 produces to the LH 11-1 FPS through an 11km, 18in.-diameter pipeline.
The LH 11-1 development is in the South China Sea is 200km southeast of Hong Kong and produces heavy crude from 25 wells in 310m water depth. The main reservoir is 914m below mudline. The field began producing in 1996 and was initially designed for a field life of 10 years.
The FPS Nanhai Tiaozhan semisubmersible provides power to downhole ESPs and engineers onboard control the subsea equipment and production rate of the field. The heavy API-gravity crude is moved by the ESPs to the FPSO Nanhi Shengli through two 13.5in.-diameter production pipelines.
Liuhua11-1 field was developed and produced until 2006, when a major tropical cyclone, Chanchu, broke three flexible risers and six of the ten mooring lines. After a riser repair and recovery project, and drydock for life extension, the FPSO Nanhi Shengli was put back into service and reconnected. It was then that Liuhua 4-1 field could be brought into production through a tieback to the Liuhua 11-1 FPS.
LH 4-1 field is about 11km northwest of LH 11-1 in 260-300m water depth and about 215km from Hong Kong. The field was discovered in 1987, but could not be developed due to technical limitations at the time. The field is an eight-well development; seven horizontal wells produce from a reservoir at 1622m below mudline, and one well produces from two zones at 900m and 1465m. Future expansion can accommodate four additional wells, which, when drilled and completed, will produce through a separate manifold.
The field is controlled by a composite electro-hydraulic system tied-back to the FPS, from which control signals regulate hydraulic power and chemical injections. Each well has dual ESPs for redundancy; production can continue if an ESP fails. This capability delays the expense of a semisubmersible intervention to replace failed equipment. Production is pumped 11km through an 18in.-diameter pipeline to the main LH 11-1 manifold, where the crudes are comingled and sent to the FPSO through the two existing pipelines.
This arrangement requires variable frequency power to individually control each ESP and balance output among the wells. The FPS was also upgraded as part of the life extension project to provide the necessary power and controls for the new wells. The LH 4-1 field uses a high-power changeover switch to control the ESPs, so that manual switching via ROV is unnecessary. This field is the first subsea development to use this technology, according to CNOOC.
JDR was contracted by CNOOC to develop and manufacture more than 42km of subsea production umbilicals and ESP power umbilicals for LH 4-1. The oil field includes one manifold for the eight production wells, jumpers, and an 18in.-diameter subsea pipeline. JDR supplied three, 14km ESP power umbilicals, one 14km main production control umbilical, and a bridging manifold control umbilical. Each ESP power umbilical included three, 3-phase power circuits to electrify the eight wells and provide a redundant spare circuit. This nine-core-style power umbilical reduces the number of risers needed, lowers complexity, and minimizes project cost. The umbilical bundles were built on JDR’s vertical lay-up machine; thee 60 tonne/bobbin capacity reduced the number of core splices required.
The main thermoplastic umbilical was selected to maximize system reliability and minimize project cost. It was optimized for hydraulic control system response and for delivery of injection chemicals. The cross-section was designed with the drag diameterto- weight ratio necessary to meet dynamic mode requirements. The contract included dynamic riser design; modeling FPS movements during workovers of the existing LH 11-1 wells directly below the semisubmersible.
The main and power umbilical risers were optimized to provide fatigue resistance for the system’s design life.Riser engineering had several components requiring tension/angle plots for bend stiffener design, FPS hang-off loads, touch-down point location and arc-length, buoyancy module distribution, riser tension across the umbilical length, and minimum-maximum, sag/ bend clearance.
Hardware delivered included flexible pull-in heads, hang-off bodies and split clamps, dynamic bend stiffeners, buoyancy modules, riser base clamps, and subsea termination units.
The Hartlepool, UK facility was chosen for the LH 4-1 project so that all the umbilicals could be manufactured concurrently. To ship the umbilicals to China, JDR had a heavy-lift vessel fitted with a temporary, 2,000-tonne carousel to carry the load. The single carousel held all four umbilicals along with topside hardware, dynamic bend stiffeners, and subsea terminations. Upon arrival, the umbilicals were transferred to another vessel for installation. LH 4-1 field produced first oil in July 2012. OE