An alternative to traditional trenching

As industry moves into deeper waters, Sonsub, a Saipem Group company with extensive experience in subsea engineering and remote intervention, responded to the challenge by developing a trenching system that is more flexible and cost-effective than traditional subsea cable-maintenance systems.

Named Centaur, the system combines the company’s 3,000-m- depth- rated Innovator ROV and a tracked trenching skid – the Innovator Trenching Skid. This development program is funded by the European Union and is an evolution of the company’s existing technology. The breakthrough lies in the system’s ability to mate and un-mate underwater, i.e., to perform as seabed or free-swimming tractor, thus enabling various survey, maintenance and burial tasks.

An ROV docked to a trencher.

System design. Conceptual de-sign requirements of the new system are the result of analysis of available equipment on the market. Traditionally, trenching equipment can be classified in two main categories: purpose-built trenching equipment and ROV-deployed work packages. Both have documented advantages and disadvantages.

Enhanced productivity and simple design are significant benefits of purpose-built trenchers. Enhanced productivity is achieved by allocating more power resources to specific functions. Design is simple and linear, because purpose-built trenchers have a specific objective for every project. However, these systems also present significant weaknesses.

For example, while burial performance can be optimized at a price, flexibility is often compromised to the extent that support equipment, such as an ROV, may be required to successfully complete operations. Further, purpose-built trenching equipment imposes stricter host-vessel specifications. An expensive vessel and a comprehensive package are, thus, necessary to accommodate bulky surface equipment such as heavy deployment systems, power-supply units and numerous containers used for vehicle support.

As an alternative to purpose-built trenchers, the market offers numerous ROV-deployed trenching skids, but only where seabed conditions permit. During conventional ROV operations such as survey and manipulator intervention, ROV-deployed trenching skids offer the advantage of maximum flexibility.

The disadvantage, however, is restricted performance, because work-class ROVs are not designed to meet the demands of trenching activities. Limited in power and stability, ROVs are not configured to overcome varying sediment conditions. Additionally, since multi-task operations cause significant amounts of non-productive vessel time during ROV reconfiguration, the end result is increased operating costs.

Weaknesses of both purpose-built trenchers and ROV-deployed trenching skids have now been minimized without compromising their advantages. The new system has a unique subsea-docking system featuring unlimited subsea mating of the ROV with the trenching skid. To provide clients with an effective multi-role subsea solution, the ROV is configured with the system standard manipulator package on one end and a complete survey suite on the other. This combination offers clients real-time flexibility, whereby product lay support, pre- or post-lay survey, manipulator intervention and trenching activities can be accommodated from depths of 20 to 2,000 m (66 to 6,600 ft) without ever reconfiguring the ROV.

During conceptual design, the company recognized that the system’s innovative advantages must not compromise the ultimate objective – trenching performance. Optimizing trenching performance is, thus, based on placing the necessary weight on a traction system designed to deliver the required stability and bollard pull to promote consistent burial performance. A passive water-jetting system achieves trench formation by means of two independent trenching tools.

On mating, a power-sharing solution was devised to divert power from the ROV to the trenching skid, thus maximizing the power budget available to the jetting system. This was further enhanced by an additional power unit, which directly drives the jetting system via an underwater mateable connector. As a result, the total system maintains a 300-hp power budget. The system advantages over alternative ROV-deployed trenchers are unquestionable. All available energy is delivered to the jetting system to optimize burial performance rather than waste it on ROV propulsion.

Operation. Considerable design emphasis has been given to developing operator aids in an effort to maximize efficiency. The trenching skid has a self-contained control system that enables data acquisition and monitoring diagnostics during online operations. Real-time information of individual track speeds, ground speed, jetting-tool position and jetting-module supply efficiency are a few examples of information available to the operator and the client with online data-recording facilities.

Available automation modes include cable-tracker steerage for trenching and auto-heading track control for "rapid" transit mode to minimize set-up time. Auto-modes are intended to optimize efficiency during trenching operations and minimize the potential for human error. However, the operator always has the option to select manual control.

The system’s operating principle offers comparable performance to purpose-built trenchers without the added burden of deployment and support equipment. The trenching skid is compatible with all of the company’s eleven Innovator ROVs. Additional equipment is limited to a 2,000-m deployment winch and one, combined, power and spares container. Deployment is achieved via vessel crane or an A-frame of convenience. The deployment winch is only required for deeper waters that the vessel equipment cannot handle.

Conclusion. A cost-effective improvement for trenching operations has been found. The most obvious advantages are reduced mobilization time, limited vessel requirements and lower cost. The company has submitted a patent application in the UK for the system. Worldwide patent protection will be applied for in the coming months.