Water jetting is now a well-established subsea approach and the technology has evolved significantly in the 30 years since the first mass-flow excavation (MFE) tool reached the market. David Morgan looks at the latest such development: the Marin Group Evo Claycutting system, designed by Nick Sills and George Stroud, the duo behind many a MFE and claycutting landmark including the pioneering Remote Underwater Excavator, the JetProp, HydroDigger and SeaVator as well as four earlier generations of claycutter.
Claycutting was successfully proved as a fully viable excavation concept in 1994 when Nick Sills and George Stroud employed their second-generation prototype near the Midgard field on Norway’s Haltenbanken in trials for Saga Petroleum (OE August 1994). The seabed here is predominantly clay and pitted by numerous valleys created many thousands of years ago by iceberg grounding. For Saga, this meant that – even with an optimised route – extensive remedial work was necessary at a significant number of pipeline crossings to reduce critical free-spanning.
The prototype Claycutter was used in conjunction with the recently-developed JetProp MFE tool (OE June 1994) to pretrench through the shoulders of iceberg ploughmarks and scarring to create an 80m corridor 7m wide by 4m deep, taking the length of free-spanning to the 70-100m range acceptable for a large pipeline. Sills and Stroud applied a two-stage jetting process, using the claycutter to break up the stiff clay and the JetProp to blow away the debris. Both tools were deployed by drillstring in 210m of water from the drilling vessel Bucentaur and driven by a 4000hp high-pressure pumping spread.
What differentiated the Claycutter from other jetting devices then available – such as the JetProp – was its ability to cut into the surface from above rather than excavating horizontally. The value of this innovation was borne out at Midgard, where the Claycutter and JetProp were run for 48 hours and eight hours respectively, between them moving around 2000m3 of firm clay at a rate of 70t an hour.
Building on what Midgard had confirmed, Sills and Stroud returned to Norway 10 years later, this time to play a key role with their third-generation Claycutter in the deepwater Ormen Lange project. Here the obstacle was created by the Storegga slide – the Ormen Lange gas field lies around 2000m below the seabed adjacent to the 300km-long slide scar.
The excavation task Sills and Stroud faced involved removing some 15,000m3 of seabed soils from the challenging irregular terrain through which Hydro’s lines had to pass. Because of the water depths – ranging from 200m to 870m – using the sequential Midgard approach was not viable as swapping out the Claycutter for the MFE tool would involve a 14-hour turnaround. So they instead configured the Claycutter and JetProp to excavate together in a single pass-through, an approach proved in an offshore trial funded by Hydro in August 2004.
For Ormen Lange, Sills and Stroud also increased the pumping power, delivering 10,000hp and 200bar water pressure from the deck of the intervention vessel Island Frontier. A single drill pipe was used to support the dual-function system, with the Claycutter slung on chains below the JetProp. The combination worked for 170 days at 44 locations in 2005, and a further 15 locations over 50 days in 2006.
The next version of the Claycutter was successfully deployed in dual and solo operation in subsequent Ormen Lange phases and Stills and Stroud have drawn on all that they learned in Norway and from other complex projects to deliver the technology’s fifth generation.
Sills says: ‘One of the key lessons from Ormen Lange was that the dualfunction approach is not effective in all situations and it can sometimes actually be inefficient to use the MFE tool with the Claycutter. With the Evo Claycutting system we retain the ability to cut and wash in a single pass-through, but by using a single, much more efficient, versatile and compact tool.’
The versatility comes from an innovative ‘plug and play’ capability which allows easy subsea configuration. ‘We realised that the most attractive way for our clients to achieve better productivity and greater results would be to have a tool that can be easily reconfigured in situ,’ adds Sills. ‘With the Evo this can be done by ROV intervention or remotely from the surface, at any depth below 2m. There is no longer any need to recover the tool to the surface.’
He and Stroud also recognised that being able to use the Evo with a broad spread of pumping options would further enhance the Claycutter’s ‘plug and play’ appeal. It has accordingly been designed to work with centrifugal and triplex pumps, including rig pumps, flooding pumps, injection pumps, and well intervention pumps. Additional versatility is delivered with the option to deploy by drillstring, high-pressure hose, Coflexip Dual systems, and electronic pumping systems, and to operate via a dedicated handling tower or system, by wire, by tubular system, or subsea via ROV.
At the heart of all claycutters lies the jetting technology, and Stills and Stroud have been researching, developing and refining this for nearly 20 years. They first introduced a cutting head for soft soils, then one for medium soils, and finally one for compressed soils. With the Evo they introduce the Bladejet, an entirely new design claimed capable of delivering a more efficient cut into clays – from 0kPa right up to 500kPa – through greater cutting ability, reduced wash-out and less impact damage.
To illustrate the new tool’s capability, Sills refers back to the Midgard trials. ‘There we were shifting the equivalent of a lorry-load of clay every eight minutes. If we repeated the operation today, we would move 35,000m3, or anything up to four lorry loads every eight minutes.’
The new claycutter forms part of the integrated Evo Excavation System, a spread of MFE, claycutting and other complementary equipment operated by Marin Group company Marin Subsea, which is based near Aberdeen. Sills is a director of Marin Subsea, and Stroud is Marin Group CEO. OE