Riser Technology: Connectors Stiffen Caesar Tonga's Resolve

Caesar/Tonga is a joint project operated by Anadarko Petroleum Corp. developed as a subsea tieback to the Constitution spar facility situated 306 km (190 miles) off the coast of New Orleans. The Spar has the capacity to produce 70,000 b/d of oil and 200 MMcf/d of natural gas.

A combined development by Anadarko and its partners, the Caesar/Tonga development comprises the Tonga, Tonga West, and Caesar fields in the Green Canyon area of the Gulf of Mexico (GoM). The first development phase is estimated to have cost US $1.3 billion, including the drilling of four wells. The first three wells were put into operation in March; the fourth well is anticipated to be completed before year-end 2012. Flow and reservoir tests on the first three wells were undertaken during 2Q 2011, with all the wells testing at flow rates of 15,000 b/d of high-quality oil.

The Caesar/Tonga development was the first not only to use a spar floating production facility but also the pioneering application of steel lazy wave riser technology in the GoM.

Riser Issues Arose

The DBSC is lifted onto the platform for installation and fitting. (Photos courtesy of FES International)

However, in January 2011, Anadarko announced there was a mechanical issue on the Caesar/Tonga field project that could potentially have delayed production plans significantly. A hydro-test of the production riser system revealed that it was not working properly.

The fields associated with Caesar/Tonga, located on Green Canyon Blocks 683, 726, and 770, are situated in up to 1,525 m (5,000 ft) water depth. At these depths maintenance and repairs can be a lengthy process causing significant delays to production and increased downtime.

Wellstream, which is responsible for the design and manufacture of the risers for the project, commissioned FES International for the supply of diverless bend stiffener connectors (DBSCs) that would enable another riser to be connected to the well on the seabed while repairs were being made to the existing risers.

Bend stiffeners are important in maintaining the integrity of flexible risers, umbilicals, and other kinds of cables deployed from spar facilities or floating units. They are tapering cylindrical devices that are fixed to the end of the I- and J-tubes through which the riser runs out beneath the platform, and they support the pipe or cable by limiting its movement and preventing it from damaging itself against the mouth of the tube. Bend stiffeners failures can reduce the life of the pipe or cable by a factor of 20 times.

DBSC Design

FES has designed its DBSCs specifically to allow for the quick, efficient, and cost-effective installation of riser and umbilical bend stiffeners in congested or confined areas.

Prior to disconnecting the DBSC male and female connections, the weight of the riser is removed.

Latching mechanisms on the DBSCs have been tailored and manufactured to fit Wellstream's requirement, provid- ing an automatic intervention, which offers safer and more reliable connections.

The installation of DBSCs to a riser can be broken down into five stages. First, the riser/umbilical pulls in the head inserted through the DBSC female unit, then the DBSC male unit approaches and locates the female unit. Once located, the male engages through the latch arms where the riser/umbilical pulls up through the I-/J-tube and into the hang-off position. At this point the male and female units are engaged and fully operational. Underwater operations, principally attaching the connecting bend stiffener to the riser or cable to be pulled in, are performed by ROVs and as a result, the full connection operation can be completed within a matter of hours.

Prior to disconnecting the DBSC male from the DBSC female, the weight of the riser is removed from the latch arms. The riser end termination pull-in head is attached to the recovery deployment winch. The riser is then recovered to allow for removal of the riser hang-off collar and allows for the latch arms to be energized.

Energizing the hydraulic cylinder and rotating the cam ring allows the latch arms on the DBSCs to retract freely past the male half and enable riser/umbilical deployment. Once the riser is fully deployed, the DBSC female latch arm can be de-energized, reengaged, and ready for further use.

Significant testing ensures that all components will operate as needed during installation.

FES tests all of its bend stiffener connector (BSC) units to be sure they can endure the high pressures and environmental conditions of their destinations. All tests assess the integrity of the pressure-retaining parts and ensure the appropriate alignment of the latch, cam rings, and the hydraulic cylinder. The latches need to be able to fully engage and fully retract at the hydraulic cylinders' fully closed and fully opened positions.

The team at FES also performs latch function and pressure testing of the BSC male and female cam latch dog systems to ensure pressure integrity of the hydraulic system.

Load tests also are conducted that subject the BSC male and female assembly to the maximum operation loads to verify structural integrity. Further simulated installation in loaded condition tests are carried out to verify the coefficient of friction between the DBSC male and female units and to check that the male unit pulls in and engages with the female unit when entered at a misaligned angle. These final tests are crucial for checking whether or not the unit will be able to function in the conditions on site.

Riser Restoration

For this project, FES designed a bespoke DBSC in line with Anadarko's specifications. Usually DBSCs are designed with female latching. However, the company manufactured the industry's first male latch DBSC, which can be easily connected to the risers using pressure-operated valves. The male latch is operated by ROV/diver intervention, which allows a much quicker and more efficient method to release the risers and umbilical end terminations from the latching mechanism to the topside hang-off/connection point.