Petrobras: The deep-water riser riddle
The main obstacle facing scientists and engineers at Brazil's Petrobras is designing risers to connect the bottom of the ocean to floating production platforms as upstream operations push into water depths approaching 3,000 metres, says Jacques Braile Saliés, manager of Procap-3000. Tom Nicholls reports
IT WILL not be long before Petrobras needs to deploy technology for producing oil in water depths approaching 3,000 metres. Although it has not yet made any discoveries offshore Brazil in water depths of this order, the Brazilian firm is involved in ventures in the US Gulf of Mexico where many of the systems developed under the Procap-3000 deep-water research programme will be applicable. Water depths at Chinook are nearly 2,700 metres, at Cascade almost 2,500 metres and at St Malo, around 2,100 metres.
With spending on Procap-3000, which started in 2000 and is due to run to the end of this year, expected to reach $0.645bn, programme manager Jacques Braile Saliés is confident Petrobras will be able to produce oil in 3,000 metres of water by next year. One of the principal challenges it must overcome is designing riser systems that are suitable for such extreme water depths.
Flexible risers cannot be used because they would collapse under the water pressure, while steel catenary risers would not be able to tolerate the motion of the floating production system (FPS). Petrobras' Tethered Riser Buoy (TRB), says Saliés, minimises the effect of fatigue on risers caused by platform movement. A buoy is tethered to a deadweight on the seabed with polyester rope and chains, and suspended beneath the surface. Steel catenary risers connect the subsea flowline to the subsurface buoy and flexible risers connect the subsurface buoy to the production platform. The effect is to decouple the motion of the FPS from the bulk of the riser system, allowing steel catenary risers to be deployed.
Tank simulations have been undertaken and a prototype—with a surface area of 27 square metres, a height of about four metres and weight of 250 tonnes—is being built, re-using old buoys, in Rio's Sermetal shipyard, and will be installed later this year in about 600 metres of water in the Campos basin. Following field testing, Petrobras plans to build a buoy for installation in the Roncador field—with a surface area of 40 square metres.
'It is easy to build, competitive on price and gives us another solution for reducing fatigue on risers,' says Saliés. It also reduces the weight exerted on the platform, which means more facilities or processing capacity can be placed on board, or that the platform can be designed with a lower buoyancy requirement—which is cheaper.
Alternatively, the platform itself can be designed to minimise movement. Cenpes, Petrobras' research and development (R&D) unit, is working on a ship-shaped hull designed specifically for use in the Campos basin. Until now, old ships have been adapted for production purposes, 'but in 3,000 metres of water, our concerns about risers are forcing major changes in hull design to reduce the movement of the ship. We are looking to design and build new hydro-dynamic concepts for hulls,' says Saliés.
The reasoning is simple: generally, adapted ships were designed for speed or transportation purposes, but production platforms are stationary, often for decades. 'Speed is not important. We want to minimise movement.'
Saliés adds that Brazilian shipyards are better equipped to build ship-shaped hulls than semi-submersible platforms and, given Petrobras' commitment to local content (in the first half of the year, some 85% of the firm's purchases of equipment and services were done locally), this area of research should yield significant cost benefits.
In more-conventional platform design, Cenpes claims to have made a breakthrough with the MonoBR, a mono-column production platform designed for deep-water use. It has 'a very good response to out-of-plane motions', with 'minimal' response to pitch, heave and roll, making it 'riser friendly'. In addition, says Saliés, Brazilian shipyards can construct the design at competitive rates.
'We have done all the designs and tank tests and it has been approved by Petrobras engineers as a possible solution for future developments.' Both platform concepts could be used in conjunction with the TRB, but this is unlikely. 'We expect their movement to be so small that they will be able to work without the buoy.'
Significant cost-savings have also been achieved during installation. Petrobras' torpedo pile is an alternative to traditional anchors for mooring FPSs. Capable of taking loads of up to 1,000 tonnes, each pile is positioned at a pre-determined height over the seabed and dropped. The momentum generated by the free-fall is sufficient to embed it in the soil to a depth where soil friction is enough to exceed the designed pullout load. The device, says Saliés, is cheaper and easier to install than normal anchors and can be placed more precisely.
Among Cenpes' most recent innovations is the torpedo wellhead—undergoing tests, Petrobras expects to make the first commercial installation around the end of the year. The torpedo, which contains low-pressure housing, a drilling base and 30-inch casing, is dropped to the seabed like a gigantic dart, installing the wellhead in a single shot. 'It enables us to install wellheads without a drilling vessel. When the ship comes to drill the well, low-pressure housing has been installed by a supply boat, saving one-to-two days of rig time. The system takes four hours to install with a supply boat, which cost in the region of $30,000 a day to hire, compared with two days in the case of a rig, typically costing $200,000 a day. It will result in at least $150,000 of savings per installation.'
Cutting lifting costs
Procap-3000 also aims to reduce lifting and development costs of fields in 1,000 metres of water and above. Petrobras aims to cut liquids-lifting costs from $3.41 a barrel in 2003 to $3.05/b in 2010 and, because most of its output is from deep water (about 75%), this is where most of the savings must be made. 'There's a lot of optimisation that we are still able to do,' says Saliés. This is being achieved by, for example, gas-lift optimisation, horizontal drilling and long tie-backs.
With cost-reduction in mind, Petrobras, in a partnership with ABB, FMC and Kværner, is working on subsea water separation. With injection of a barrel of water required to produce a barrel of oil, considerable energy is expended on water treatment—water is cleaned at the surface and pumped back into the sea. 'Our idea,' says Saliés, 'is to have subsea water-separation systems to separate water from oil and gas. The water would then be reinjected into the reservoir or any suitable reservoir and very little water would be brought to the surface with the hydrocarbons. This means the water does not need to be treated to the same standard as before.' This will reduce costs and free up space on the FPS, increasing recovery factors.