Stimulating vessel investment

Baker Hughes' Blue Orca in the Southern North Sea. Photo Courtesy of Baker Hughes

In the early 1980s, the North Sea saw a spurge in stimulation vessels by the big three oilfield services contractors. Three entered the market over two years. Until October, two-thirds of that same fleet remained in place. Elaine Maslin looks at a new vessel on the market.

Nearly thirty years after it brought the first purpose-built stimulation vessel into the North Sea, Baker Hughes has returned to the market.

In October last year, the Blue Orca arrived in Denmark, after a successful journey across the Atlantic from shipbuilder and vessel operator Edison Chouest Offshore’s yard in Louisiana. By mid-November, the vessel had carried out its first acid job for Maersk Olie og Gas, offshore Denmark.

The vessel, ordered on the back of a non-exclusive contract with Maersk Olie og Gas, and built to meet a number of Maersk’s requirements, was the eighth in Baker Hughes’ now nine-strong stimulation fleet, with further vessels due.

It is one of the highest-capacity stimulation vessels globally, designed to treat a large number of stages in wells, which can be up to 12-18 in the North Sea, and multiple treatments in one mobilization.

It has 2.5million lbs (1134-tonne) proppant storage capacity, in top tanks, and lower sand silos, which are divided into four compartments, for multiple sand types. It has eight main acid storage tanks, able to hold up to 180,000 US gals concentrate, or up to 414,300 gals at 15% dilution, and 24 liquid additive storage tanks, able to hold flammable fluids, and with remote monitoring and high level alarms.

Proppant delivery is via gravity feeds to two blenders, each with two 12-in.-diameter and one 8-in.-diameter horizontal sand screw. The 12-in. screws are able to deliver between 250-10,800 lbs/min sand, and the 8-in. between 76-2000 lbs/min. The Blue Orca has five, 2750 hydraulic horsepower (HHP), Gorilla pumps, and two, 650 HHP, pumps. 5½-in. fluid ends provide a maximum working pressure (MWP) of 15,000psi (1035bar) at 17.7bbl/ min.

Two, aft-mounted, 400ft Coflexip hoses, rated to 15,000psi MWP, with hydraulic quick disconnects, provide the connections to platform or rig-based wellheads.

The vessel is compliant for the UK, Dutch, Danish and Norwegian sectors, and could also work in the Mediterranean, offshore West Africa, and occasionally North Africa and in the Adriatic, says Tony Martin, director, offshore stimulation, Baker Hughes.

Baker Hughes, whose previous North Sea-based stimulation vessel, the Vestfonn, left the region in 2007, believes the investment and move back into the basin is worthwhile.

Martin says the company expects an increase in stimulation activity in the North Sea over a 4-5 year period, and that the higher level of activity will continue. John Clark, business development manager, pressure pumping, Baker Hughes, said: “We see the stimulation market in the North Sea increasing year after year, with 2015-2016 in particular being extremely busy.”

The Blue Orca’s design was based on the past 30 years’ experience, and what Baker Hughes thinks the vessel will need to be able to deal with in the next 30 years, including reservoir pressures and temperatures. Today’s demands include being able to perform large-volume, low-rate matrix acid treatments, high-rate Paccaloni-style matrix acid treatments, acid fracturing and high concentration proppant fracturing. As a minimum, it needs to be able to pump at 60 bbl/min, with higher rates preferable for acid fracturing, and must mix all fluids on-the-fly with sea or fresh water, Martin says. There has also been a small but increasing demand for treatments in high-pressure, high-temperature formations, he says.

A key consideration was redundancy, Clark says. “If anything goes down, there will always be at least one back up,” he says. “We should never have to stop a job because we were not able to switch pump.” The blending and mixing process systems have full redundancy, he says, and stimulation and power plant systems run independently from the marine power systems.

The blending system allows for acid, water and additives to be blended, without going through an open tub, creating a closed, automated system, with a maximum delivery rate of 80 bbl/min, to minimize potential spills or releasing hazardous fumes. It will be able to work in automated batch mode, automated continuous batch mode, and continuous mix, all through a pre-programmed computer control system, which also continuously monitors the rates and adjusts them accordingly. Concentrations are monitored using a Coriolis-effect mass flow meter, with radioactive densimeters to measure the blended acid concentration.

The Blue Orca's touch screen control room. Photo courtesy of Elaine Maslin

The sand proportioning system is also computer-controlled and fully preprogrammable. Blender tubs are directly under the silos to simplify the sand delivery process. Pneumatic conveying delivers proppant from below deck storage to the above deck proppant silo, and it is possible to return proppant back to the below deck tanks.

The gel blending plant on the vessel uses dry-on-the-fly technology, which means oil-based slurried polymer concentrates are not needed. This system uses a full automated system, to control the addition of dry guar, or other gelling agents, into the fluid stream. High-energy mixers shear the polymers for hydration. The viscocity of the gel is monitored inside the vessel’s onboard laboratory, with real-time pH, temperature and viscosity data recorded and displayed via the control system.

“Pumps have not changed much over the years, but instrumentation and controls are much more advanced now,” Martin says. “We used to have a wall of the control room covered with a process flow diagram, switches and a panel of hundreds of valves that needed three people to operate. Today, controls look like something out of a science fiction movie. They are remotely operated by a small number of people in the control room via touches screens, and can even be operated from onshore.”

The entire stimulation system is controlled from inside the control room. Within the vessel’s monitoring system is a data acquisition system, which gathers data and transmits it to the data analysis software, which supports remote satellite monitoring, from onshore. A batterypowered wireless data acquisition skid can be positioned on a platform or rig to transmit data back to the vessel using wireless LAN communications.

The automated systems eliminate the need for staff to work in hazardous environments around the rotating or high-pressure equipment under normal operations. The marine systems, have been built to be able to operate in the North Sea’s harsh conditions, including a 10m swell, or 40knot wind. It has a drop-down, electrically powered azimuth thruster, able to rotate 360° for station keeping. The marine power systems are separated from the treatment and pumping systems.

The vessel has enclosed external walkways, and single-man cabins. The vessel works with two crews, for 24-hour operation. Support crews also arrive at the rig or platform before the vessel, so there is no need to transfer personnel from the vessel to the rig.

The vessel undertook its first job for a North Sea operator in December, to improve the injection profile of a water injector well, which supports reservoir pressure. The job consisted of three stages of a viscosified 15% HCL with two stages of Baker Hughes’ diverter system, Enhanced Acid System. Pumping rates were 4-12 bbl/min. Pressures were 3000- 4000 psi, using about 1200 HHP. A step rate test (SRT) was performed before and after the treatment. The SRT after treatment showed much less surface pressure, compared to before the treatment.

The vessel was on location for about 24 hours and saw zero HSE incidents.

The North Sea stimulation vessel fleet

The Skandi Fjord

The stimulation market in the North Sea began in earnest in the 1960s, with temporarily-placed, skid-mounted, limited scale, equipment on platforms. According to 1984 SPE paper 12993-MS, stimulation operations from a vessel were first carried out in 1980, in the southern North Sea from a small Arabian Gulf vessel. This led to the conversion of two large (at the time) North Sea supply vessels into dedicated stimulation vessels, operating in the northern and southern sectors. Their capability was quickly found wanting, specifically around pumping, batchmixing and proppant-carrying capacity. Maximum pumping power on both vessels was 4000-5000 horsepower (hp) and quality control and job monitoring were also limited.

Baker Hughes, Schlumberger, and Halliburton, soon stepped in to fill the gap. Baker Hughes brought the 1983-built 82.3m-long Vestfonn on to the market in 1986.

In 1984, Schlumberger introduced to the market the 74m-long Big Orange XVIII, on a long term contract from owner Tracer Offshore ANS, and managed by Wilhelmsen Ship Management.

The following year, Halliburton contracted the 88m-long Skandi Fjord, owned by DOF Subsea and managed by DOF Management AS. The three vessels were the largest of their type at the time, and remain so, Martin says, in paper SPE 168243. The requirements of the stimulation treatments far exceeded anything required in other parts of the world.

These requirements entailed mobilizing the world’s largest capacity to store and blend raw hydrochloric acid, and the ability to store and blend unprecedented quantities of proppant.

The Vestfonn, which left the North Sea in 2007, and has been working offshore India since, has 10,000- and 15,000-psi (68.9- and 103.4-MPa) pumps, supported by 13,400-hydraulic pumping capacity.

It has 180,000-gallon raw acid storage capacity and more than 1.2 million poundmass (lbm) (544,310.9 kg) for proppant.

The Big Orange VIII, remains in the North Sea. It has 4800- and 10,000-psi (at 58- and 70 barrels per minute (bpm) pumps, supported by 12,000-hydraulic pumping capacity. Schlumberger offers pumping pressures of up to 15,000-psi on request.

Big Orange VIII has 180,000-gallon raw acid storage capacity and 15,400cu ft total proppant storage capacity.

The Big Orange VIII

The Skandi Fjord is understood to have just been decommissioned. It also had 180,000-gallon raw acid capacity. Proppant storage capacity was more than 2 million lb. The Skandi Fjord’s maximum pumping pressure was 15,000-psi, supported by 10,400 hydraulic hp.

Apart from the short-lived introduction of the Western Renaissance, in 1993 (it left the North Sea in 1994, and was converted into a pipelay vessel after working in the Gulf of Mexico from 1994-1995), the departure of the Vestfonn to India in 2007, and the occasional use of modular stimulation packages on supply or support vessels, the stimulation vessel fleet in the North Sea saw little change until 2010.

Shortly after the Vestfonn left for India, a new company, StimWell Services, was launched out of Great Yarmouth, the Vestfonn’s former base. StimWell launched the 86m-long Island Patriot, a conversion, in 2010.

The Island Patriot, which is owned by Island Offshore and managed by Island Offshore Management, started a fouryear contract with BP, primarily for use on the Valhall field, in 2010. Island Offshore is a 50:50 joint venture between Edison Chouest and Ulstein Group.