Compact de-oxygenation technology is being applied to assist water injection schemes offshore Angola and in the Gulf of Mexico. The process, called Seaject, offers a 75% weight reduction on platform decks compared with alternative de-aeration systems.
Corrosion of wellhead and subsea structures can be substantially eliminated by reducing oxygen concentrations to extremely low levels. Seaject employs one of the most basic chemical reactions to virtually eliminate dissolved oxygen in water streams.
Among the competing de-oxygenation techniques is the Minox process, developed originally by Norsk Hydro and Kvaerner and subsequently marketed by the latter. Other systems rely on vacuum pumps or gas stripping equipment. The two say that Seaject is more compact than any of these, as it does not require a de-aeration tower, booster and vacuum pumps.
Simplified Seaject flow diagram.
Norsk Process first devised the technique in the early 1980s. In the Seaject process, dissolved oxygen is removed from water by adding hydrogen to the water stream, then running the mixture across a palladium catalyst (specially developed and supplied by Bayer in a coarse, spherical bead form). Thereafter, the dissolved oxygen and hydrogen combine to form water, according to the following equation: H2 + 1/2 O2 - > H2O
The warmer the water temperature, the faster the reaction - and therefore, the lower the amount of catalyst required, according to Axsia Serck Baker Technical Director Wayne Evans. This in turn brings down the system cost.
Catalyst beads vary in diameter from 0.3 mm to 1 mm. Water inlet conditions to the reactor vessel must be strictly controlled to ensure that no unwanted reactions take place in the catalyst. Any chemicals added to the system upstream of the reactor vessel must be carefully scrutinized as a possible contamination source.
Development advanced slowly, with initial funding from Conoco, until the breakthrough trial application in 1990 on Shell's Cognac platform in the Gulf of Mexico. Following extensive tests onshore, a pilot plant was installed on the platform to determine whether the system's catalyst would suffer contamination.
No catalyst washing
Following the one-week trial, it transpired that washing of the catalyst would not be necessary. The outlet oxygen concentration remained at non-measurable levels even at loads of 80 bed volumes/hr (the maximum flowrate at which seawater could be delivered).
Two years later, Shell took a full-size Seaject unit (operating at 10,000-15,000 b/d of water) for a further trial on the Bullwinkle platform in Green Canyon Block 65A. This system also performed to expectations. Following 13 months in operation, outlet oxygen concentrations were again mainly negligible with loads of up to 12,000 b/d. During this period, over 1.8 million bbl of water were de-oxygenated, yet the catalyst only had to be backwashed every 3-4 months. Another system was deployed on the Ram-Powell tension leg platform in the Gulf of Mexico in 1994 (this unit began operating recently as water injection came into play).
Despite these results, interest in the technique tailed off due to perceived shortcomings - among them, a higher capital outlay than competing systems. In 1998, Axsia Serck Baker, the Gloucester, UK-based specialist in separation, produced water and water injection process technology, acquired Seaject, and is now working to redress the conception problem as well as refining the process. Two orders already have been secured for water injection applications offshore Brazil and for Chevron's Kuito floating production, storage, and offloading (FPSO) vessel off Angola.
Dissolution method
Depending on water temperature and salinity, oxygen content will be 8-12 g/cu meter, which in turn necessitates a maximum of 22 liters of hydrogen under standard conditions for every cubic meter of water. This figure has suitable allowances for reaction with free chlorine and a small excess to ensure reaction completion. In the Seaject process, electrolysis of sweetwater is employed to yield both hydrogen and oxygen. The latter is vented off while the hydrogen is piped to the mixer where it dissolves into the seawater stream. Hydrogen generation is fully automated and produces hydrogen on demand to minimize the safety aspect of use and storage.
To ensure correct functioning of the system/process, the hydrogen must be dissolved into the water. Small undissolved micro-bubbles will adhere to the catalyst resin and block active areas on the surface. A proprietary jet mixer system is required to ensure dissolution of these minute hydrogen bubbles. Any larger bubbles that should develop are vented to a safe area from the top of the reactor vessel , which is basically a simple pressure vessel. The vessel, which is the primary weight/volume component, can be tailored in shape, according to process space requirements on the offshore installation.
The weight differential between the Seaject and a vacuum de-aeration system deviates dramatically as throughput of de-oxygenated water increases.
According to inlet water quality, the catalyst is simply washed with seawater or washed with sodium hydroxide to remove most of its organic contaminants. Backwashing frequency is monitored by measuring the differential pressure over the catalyst.
Backwash can be initiated automatically by the Seaject's control system (a PLC or DCS, as required). The need for a sodium hydroxide wash will be indicated by an increased outlet oxygen concentration. The washing cycle lasts around 45 minutes. Frequency depends on water quality, and may vary from weeks to months.
Seaject was developed originally to bring down the weight, volume and footprint of de-oxygenation packages. The disparity widens as the water-handling capacity increases.
According to Axsia Serck Baker, a flooded Seaject with throughput capacity of 150,000 b/d for 5°C seawater will weigh less than 50 tons. That compares with 200 tons for a conventional oxygen removal de-aeration tower.
As Evans points out, "weight has a major impact on the deck's steelwork. Shell is keen because if you take their Gulf of Mexico tension leg platforms, they are in relatively deep water, but with relatively small topsides...We also find that on FPSOs, often the crane lifting limit is dictated as being 10 meters high. If the vacuum tower is 20-meters high, that inhibits the swing of the crane. And that also affects the center of gravity on the FPSO."
Hydrogen concerns
Kuito is a deepwater application, with water temperatures of 14-15°C. The main aspect of Seaject which had to be re-engineered for this project was the hydrogen generator - in Norsk Process' time, there was no version available commercially for zone 1 or zone 2 hazardous environments. Axsia Serck Baker managed to get certification from ABS for its Kuito generator, which basically comprises an electrical cell, a couple of separators and control devices. The equipment operates at pressures up to 30 bar, supplying hydrogen on demand, without compression. No hydrogen is stored, so there is nothing to explode, Evans claims. Despite its image problem for offshore installations, hydrogen has a large explosive envelope of 4-96 vol%. Methane, with its high calorific value, is far more dangerous.
The main attraction of the Seaject for SBM, operator of the Kuito FPSO, was its topsides weight-reducing capability. Production is due to start at the field shortly, but the Seaject system will not be commissioned until water injection comes into play shortly. The anticipated injection rate is 100,000-110,000 b/d.
Cost savings
Recently, Axsia was awarded a contract by Petrobras to supply a Seaject unit for a 50,000 b/d seawater treatment for water injection for the Garoupa Project offshore Brazil. Axsia Serck Baker aims to produce six systems per year either for new developments or retrofits. "However, it is quite a tough sell at the moment," Evans admits, due to the perceived cost. When chemicals, cabling and other accessories are separated from a vacuum de-areation system proposal, the Seaject can appear to be 50% more expensive.
"If you add in the major elements such as steelwork to hold the vacuum tower to the FPSO or jacket, plus the cost of the booster pump set to lift water from the vacuum tower (which is not required on the Seaject), plus the need to have a 20 meter high tower, with associated duplex pipework and electrical installation costs, then the alternative technology's costs can double to $900,000. That compares with around $700,000 for a fully engineered Seaject with a fully fabricated equipment skid, which is connected to get zero oxygen straightaway."
A further advantage lies in logistics. There is no need to inject oxygen scavenger or anti-foam chemicals, both of which tend to be in short supply in some of the more remote offshore locations.
