Breaking new ground (or old ground in new ways)

August 9, 2010

Subsea trenching is not seen as one of the offshore industry's most glamorous or challenging activities – what could be more straightforward than digging in the dirt? In truth, however, it is far easier to lose money trenching than to make it, and success can only be achieved through a combination of understanding, expertise, experience and organisation. Dr Steve Agar of IHC Engineering Business explains why.

A vital element in subsea trenching is having the right tool for the job, and this is where development of the technology and understanding of how to apply it effectively plays an important part.


For those who have any familiarity with digging trenches, it might be said that there is nothing new under the sun (or under the water at least). We have, however, seen a whole new generation of equipment enter the market in the last year. These not only seek to provide better and more cost-effective solutions to old and well-established problems, but to respond to the ever widening boundaries of subsea pipeline and cable installation.

While the main thrust of development has undoubtedly been ‘deeper, bigger, more powerful', it has also to be increasingly ‘smarter'. There remain a limited number of things one can do to the seabed to cut it, move it and then put it back where it was to protect increasingly high revenue-earning cables and pipelines. We can play around at the edges of the three main trenching methods – towed ploughs, water jetting, and mechanical cutting – but gains are really made where attention is focused on concentrating power, effort and resources where and when they are most needed. Overall cost-effectiveness of the whole operation, and satisfying the underlying business drivers of the trenching contractor and its client, has to be the ultimate consideration.

The latest generation of equipment is particularly aimed at meeting the need for trenching bigger, heavier pipes (up to 60in diameter), operating in known difficult areas where field developments are proceeding apace (such as Australia's North West Shelf with its problematic seabeds), and satisfying the requirements of the growing demand in the offshore wind and subsea power cable markets.

Constraints and compromises
As ever with trenching, the optimum solution will depend on the size and weight of pipe or cable to be buried, the type of seabed and the depth of trench required. Not only are there ‘horses for courses', but the trenching contractor and its client have to be aware that the course itself might change from what was anticipated at the start of the race.

At their simplest, water jetting machines fluidize the seabed soil allowing the product to be buried to sink into the resulting dense fluid mixture, or for this ‘slurry' to be sucked or pumped out, leaving an open trench. This technology works effectively only with sands and the softer clays. For harder clays (and 50kPa is hard in this respect) the jetting pressure (and hence power) has to be high in order to actually cut or break up the body of soil, and the dense solids in the resulting mixture may not always easily be sucked out as they fall to the bottom of the trench.

Likewise gravels are not easily excavated, and cobbles and boulders have an annoying tendency to block up suction tubes being used.

The limiting soil conditions for jetting are relatively well-understood, and the application of more power is primarily to increase productivity (or burial depth) in those soils that are recognised as ‘jettable' rather than to allow trenching in nonfluidizable soils.

Towed ploughs have become established as the preferred tool for trenching rigid pipelines. This is because they are:

  • simple;
  • robust;
  • reliable;
  • appropriate for a major earthmoving task;
  • environmentally friendly; and
  • cost-effective.

 

Ploughs also have the inherent ability to cut varying soils effectively (from very soft clays and silts to dense, impermeable sands and very hard seabeds including stiff clay, boulders, ‘hard-pan' and fractured rock). A well-designed pipeline trenching plough will produce a stable, open trench and its cable plough counterpart provides excellent backfill with minimal soil disturbance. Both types maintain required trench depth in various seabed conditions, including coping with the unexpected. The key design features, and consequent performance, of towed ploughs are well-established, well-proven and well understood.

Ploughs, of course, have their own limitations, particularly in requiring a powerful support and towing vessel, and in coping with rock seabeds. Here the self-propelled mechanical trencher has its particular part to play. There are some seabeds where there is little choice but to machine a trench by means of a driven chain or wheel carrying an array of picks to break up the seabed material and paddles to transport it out of the trench. More power equates to more force available at the cutting face, and more chance of excavating the cut material effectively, usually through the use of dredge pumps or eductors.

Achieving and maintaining the optimum balance between the combination of cutting and dredging requires significant skill, experience and understanding of the processes and inevitable compromises involved. Higher power should equate to higher productivity, but with that come the problems of wear and reliability. Long experience has also shown that mechanical trenchers always struggle in variable seabed conditions where combinations of rock or very hard clays, that may included sizeable cobbles or boulders, occur along with sands and softer surface layers that can lead to premature trench infill and loss of manoeuverability.

Key recent developments, therefore, not only include more powerful machines, but also different ways of getting the best out of a combination of equipment and vessels.

The new leviathans
While different contractors favour different approaches, the trenching headlines are stolen by the new ‘mighty beasts' that have entered the market.

Saipem has a long and successful history of trenching pipelines with towed ploughs. The latest additions to its fleet are the trenching plough, PL3, and its companion backfill plough, BPL3. These were delivered from IHC Engineering Business's works in 2009, and are initially destined to trench on Saipem's flagship NordStream project, with campaigns planned through to 2012. While PL3 calls on the design heritage of earlier ploughs, it brings increased capability and performance into the market. It has been estimated that this latest-generation plough can move over 5000t of soil per hour.

PL3 is designed to allow 400t vessel pull to be applied to cutting the trench, and sized to produce a stable 2.5m deep open trench without modification. It can safely handle pipes from the smallest 3in MEG lines up to 60in export lines, requiring pipe support loads up to 200t, in water depths of up to 1000m, in keeping with Saipem's primary markets of major field and trunkline development.

Although having a larger pull force capacity than its predecessors, PL3 also incorporates a full multipass capability to allow deep trenching in the hardest conditions by taking a number of shallower cuts until full depth is obtained. This feature not only allows the number of passes to be planned to suit the available towing vessel pull force and required rate of progress, but also allows immediate rectification of any areas where trench depths are not as anticipated due to unexpected conditions or incidents or to meet stringent out-ofstraightness criteria.

The resulting plough weighs in at around 210t in air, but is still capable of operating in the softest seabeds by means of additional air buoyancy tanks and extended share support surfaces to reduce ground bearing pressure.

The accompanying backfill plough, BPL3, represents a real novel development in this type of equipment. The plough has to span over 22m to push the spoil from the deepest trenches back over the pipeline. As with earlier designs, the mouldboards fold hydraulically to reduce the plough width sufficiently to pass through the legs of an A-frame handling system. Unlike earlier ploughs, however, BPL3 has its forward groundfollowing skids mounted on these mouldboards so that they are not in the trench, in close proximity to the pipeline, thereby almost completely removing the risk of potential damage to the pipe during backfilling operations. This feature is also an element in moving the centre of gravity of the machine further forward, thereby reducing the required outreach of the A-frame and allowing the plough to be deployed and recovered ‘forward-facing'. This provides a significant improvement to handling operations whereby the need to rotate the plough through 180° whilst it is suspended beneath the support vessel is removed.

While comparable in weight to the Saipem PL3 plough, CTC's RT-1 rock trencher uses its own installed power, rather than the tow force of the vessel, to cut through the hardest seabeds. Specifically designed to go beyond the capability of a towed plough in rock and very hard ground, its 2.35MW of installed power is equivalent to the output from a wind turbine with 90m diameter blades. This power has to be distributed and controlled between the three heavyduty cutting chains producing a 2m deep trench with 45° side slopes, four hydraulically-driven dredge pumps to remove the cut spoil and two 350kW jetting pump sets.

The programme to develop RT-1 may have taken over three years, including small-scale prototype cutting trials, but CTC now has real prospects of being able to tackle one of the major obstacles to trenching trunklines in Australian waters – variable seabeds consisting of very hard calcarenite outcrops, areas often overlain with sandy materials and where cemented layers of rock may cap underlying sand.

The trencher was trialled and successfully trenched a piggy back trunkline in the North Sea in 2009 before being shipped down to Australia. A further trial in the North West Shelf area demonstrating its ability to cut in a range of difficult rocky conditions is understood to have been successful, with CTC now anticipating commercial trenching work later this year.

Cable compromise
The burial of cables provides different challenges, and requires different machines, although not monsters of the scale of PL3 and RT-1. However the growth in the offshore wind and subsea power cable markets has driven similar ‘bigger and better' developments in burial equipment. As with rigid pipelines, towed ploughs have long been recognised as providing the most cost-effective tools for protection of long cable routes (a lesson hard-learned in the submarine telecoms market) and are now proving themselves equally capable of providing high reliability and low risk for protecting larger, heavier power cables.

Burial specifications for export cables have, rightly or wrongly (depending on one's view of the correct compromise between cost and risk) established 3m as a ‘standard'. This has necessitated the development of a new purpose-designed plough to accommodate cables with bend radii in excess of 3.5m and diameters up to 280mm, specifically suited to the ‘beach to farm' burial requirement, yet with sufficient strength to withstand the expected pull force in a wide range of seabed materials.


The tool of choice for most projects has become IHC Engineering Business's Sea Stallion 4 plough, which in its latest version provides a trench depth of up to 3.3m without the need to rely on sinkage or additional water jetting, and can withstand a maximum peak pull force of 180t. Its aggressive forward-raked share cuts the soil with minimum tow force while maintaining trench depth by clever design that exploits the equilibrium of the cutting and pulling forces and its own weight (of about 30t).

The Sea Stallion 4 illustrates how development in this case really is an example of evolution rather than revolution. The latest ploughs maintain the basic and proven design principles based on theoretical understanding combined with practical experience whilst utilising the latest design tools and technologies to minimise weight and provide ease of operation, reliability and robustness.

The new approaches
Whilst new equipment is what usually springs to mind in terms of development, we are also seeing significant changes in the approach being taken to the provision of a cost-effective trenching capability.

For example, Saipem has not only invested in more capable ploughs, but took the radical step of specifying a new support vessel with trenching as its primary purpose. Farstad's Far Samson, voted ‘ship of the year' for 2009, combines an unprecedented 426t of bollard pull with sufficient deck space to carry both the PL3 and BPL3 ploughs simultaneously. This allows trenching to 2.5m in harder conditions, for it to be completed in a single pass more frequently, and to avoid the need for a time- and fuel-consuming return to port to change between ploughs. The vessel is also equipped with a heave-compensated lift winch to increase the window for plough handling operations and deployment over the pipeline.

Canyon Offshore, likewise, identified that it could offer a more cost-effective service by mobilising a spread of equipment that allowed both soft- and hard-ground trenching from the same, relatively small, support vessel. It has particularly targeted the field development and step-out work where pipelines are smaller diameter and shorter lengths. In order to complement its existing jetting vehicle, T750, it specified a mechanical trencher for conditions which could not be jetted. Last year Canyon took delivery from EB of the i-Trencher, a 90t machine with a combination of mechanical chain cutters and dredge pumps capable of cutting, and subsequently backfilling, a trench 2m deep and 1m wide. The i-Trencher and its dedicated heave-compensated handling system was mobilised onto Island Pioneer alongside T750, providing on a single vessel a set of tools that can be used to cover the full range of seabed conditions common in the North Sea.

This flexibility, where the vehicles can readily be adapted for different projects and products, represents a further development in the market, as it has now allowed Canyon to expand the spread beyond its primary market sector of pipelines and umbilicals and undertake trenching operations on the BritNed power cable where seabed conditions include both sands and stiff clays.

Where to next?
As with all technologies, trenching developments will inevitably be driven by market demands, and trenching remains a global, diverse and expanding market. IHC EB is currently investigating several areas where new equipment or techniques will be required to meet new challenges, including:

  • very deep trenching to provide pipeline protection against iceberg scour in frontier areas of development such as offshore Newfoundland and Greenland;
     
  • incorporation of powered rock-breaking technology into towed ploughs; and
     
  • new vehicles and methods for the burial of inter-array wind turbine cables to improve the timeliness of hook-up and reduce the need for vessel anchor movements in confined locations.

 

All markets have their ups and downs, but what we can be certain of is that new trenching requirements will continue to arise. We can also be confident, however, that we can continue to dig deep into the resourcefulness, experience and expertise of equipment suppliers and operators to meet these demands. OE

About the Author
Dr Steve Agar has been closely involved with the design and development of subsea trenching machines for nearly 30 years, first with SMD and then later as one of the founders of The Engineering Business. Since EB was acquired by the IHC Merwede Group in 2009 he has continued to be involved with business and technology development and says he has ‘absolutely no plans to write a book'.



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