P.3 ~ Next-generation stimulation vessels meet today's performance demands

Today's preferred method of offshore vessel-based stimulation involves multi-zone treatments of horizontal wells – a technology first perfected in the North Sea. A vessel operating in the modern market must perform large-volume, low-rate matrix acid treatments, high-rate Paccaloni-style matrix acid treatments, acid fracturing, and high-concentration proppant fracturing. Furthermore, a vessel must be able to perform treatments at a rate of at least 60 bbl/min.

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High-pressure surface lines

A safer, more compact alternative wing and thread-style hammer unions has been implemented for the new generation of vessels. The rigging-up and -down of high pressure lines is one of the most hazardous operations whether offshore or on land. The North Sea leads the industry in replacing these, and the new generation vessels are the first to eliminate both 3-in. and 4-in. high-pressure hammer unions. The high-pressure "hammerless" connections are durable, leak-resistant, and have superior vibration resistance. They are also faster to disassemble. This pipework features universal connections, eliminating male and female ends and preventing unbalanced flow wear.

The liquid additive system is one of the main areas to benefit from increased automation, redundancy and reliability. Stainless steel piping delivers additives to the process streams. Three-way ball valves direct the additives to the injection points. Additives are stored in stainless steel tanks. The loadings for the additives are programmed into the treatment control systems prior to the job. During the treatment, Coriolis-effect flow meters are monitored by the control systems. Continuous adjustments to the speed of the chemical pumps provides accurate loading.

Marine systems

Increasingly tight regulations in the North Sea mean that the latest-generation stimulation vessels need the ABS Notation for Protected Oil Tank (POT) or equivalent, which insures compliance with MEPC/141 (54), Regulation 12A (the so-called "clean class" requirement). The most important consequence of this classification is that there is no cargo oil, ship's fuel, or ship's lubricating oil tanks against the vessel's hull. This way superficial damage to the vessel will not risk oil pollution.

Another requirement is that the new vessels be fitted with a ballast water treatment plant according to MEPC188 (60). This prevents the spread of aquatic invasive species.

Most stimulation vessels are based on hulls designed to be platform supply vessels (PSVs). The last 30 years has seen innovations to PSV hull design to maximize hydrodynamic efficiency. Modifications from prior hull designs are based on analysis and testing to achieve the most effective hull form possible, while maintaining the capacity for high deadweight that is required for effective PSV (and well stimulation) operations.

The propulsion system design criteria are reliability, supportability/maintainability, low environmental impact, and accurate DP capability.

The propulsion system comprises four main elements:

Control systems.These are comprised of an integrated bridge, alarm and monitoring systems, power management, thruster controls, dynamic positioning, position reference systems and remote monitoring.

Diesel generator sets.The modern generator set is typically IMO II or EPA Tier 2 compliant, as required, and comprises the engine and an independent, two-bearing, water/air-cooled, medium voltage (MV) alternator, mounted on a common fabricated frame. The generator sets are isolated from the vessel structure using anti-vibration mounts. MV is usually used for the power electric systems. Copper conductors and windings are typically used for all applications.

Drives. The latest North Sea stimulation vessel incorporates fixed pitch propellers (FPP). Variable-frequency MV drives and switchgear are used to control all thrusters, so that thrust and direction of thrust are controlled by varying the speed of the propeller, not by varying the pitch of its blades. The drives are water-cooled, active rectifier type, without transformers.

Thrusters. The latest generation of North Sea stimulation vessels typically combine main propulsion propellers (standard or azipod), stern tunnel thrusters, bow tunnel thrusters, and drop-down thrusters. Key in the vessel's ability to maintain station under harsh conditions, the number and type of thrusters is as important as the amount of thrust each produces.

Typically, the vessels are classed to the ABS DP-2 SKP (Station-Keeping Performance) standard or equivalent, with the environmental parameters equivalent to DNV ERN 99.99.99. This ensures effective DP performance in the North Sea in the "as low as practically possible" risk category.

The next-generation vessel design integrates environmental protections from the outset. The vessels are typically zero-discharge with large holding tanks for gray water, wash water, and Modern hulls are designed to significantly reduce drag. The marine and stimulation systems of the new vessels have fuel efficiency and lower emissions designed in, providing both economic and environmental benefits.•

Acknowledgments

The authors thank the management of Baker Hughes for permission to publish this technical paper. The technical details and illustrations of the high-pressure lines were included with the kind permission of Kevin DeLaat at Weir SPM. The authors are grateful for the help and advice from Gary Rook of Edison Chouest. Many thanks are also due to Blake Burnette for his valuable input to completing the paper as well as to Stephanie Weiss for her assistance with the preparation of the final manuscript. The foregoing is excerpted from a Baker Hughes presentation at the SPE Annual Technical Conference and Exhibition.

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