New Requirements In Deep-Working Submersibles
Authors: Osterhaug John E. , Swanson Terry L. 
Published: October 2008

The need for a new class of remotely controlled subsea vehicles has arisen, due to the increase in offshore production activities and the accompanying increase in installation, maintenance, and repair tasks involving durations and risks to life which make manned submersibles impractical.

This paper discusses significant new requirements for the Kvaerner trencher and for other vehicles now in the conceptual or design stage. For each requirement, design alternatives are presented, as well as the design approach and implementation employed in the Kvaerner trencher. Finally, the paper summarizes pertinent results from the Kvaerner trencher testing.

INTRODUCTION

Two recognized situations indicate the need for the development of a new class of subsea vehicle, one larger and more powerful than the typical reconnaissance and survey vehicle in common use. First is the need to bury (trench) subsea pipelines and cables over long distances and at significant depths. Second, with subsea oil production moving into deeper water, maintenance and repair functions must be performed on wellheads and risers at depths of 1,000 feet or more. The use of manned vehicles is usually precluded due to the long duration of the task, or the risk to human life. A number of tethered vehicles and support systems have been proposed over the last few years to perform these tasks. Some are in the conceptual stage, others are in development, and at least one system, the Kvaerner-Myren trencher (see references 1 and 2), is approaching operational readiness.

A unique new set of requirements applies to any system capable of performing these tasks: precision navigation and guidance systems are required to maneuver the vehicle to a specific work station. In the case of a free-swimming vehicle, a maneuvering control system is also required. Instrumentation and control are necessary for any such vehicle, including the ability to transmit data between the vehicle and the support vessel. Power must be supplied to the vehicle suitable to the magnitude of the task. Finally, the system must have sufficient reliability to continue working for prolonged periods without maintenance or repair. For each of these requirements, design considerations, approaches, and alternatives are presented below.

PIPELINE AND CABLE BURIAL

Pipelines and cables traversing the sea floor at depths to 2,000 feet may require burial for protection from a variety of hazards. Vehicles that have been proposed to accomplish burial include towed plows and self-propelled vehicles moving on treads or skids, or on wheels gripping the pipe.

Subsea pipelines, which may range up to 40 inches in diameter, are normally laid and trenched in separate operations. This requires that the pipe be located prior to trenching, and that the trenching vehicle be guided to and mated with the pipe.

Power and communication cables which may range from less than 0.5 inches to over 5 inches in diameter, may be laid before or during trenching. If the bottom composition is extremely hard, the trench may be precut; it is then relocated, cleaned, and backfilled as the cable is laid.