Abstract

This paper discusses the area of long subsea tie-back developments, and what the limiting factors and related technology barriers are related to this area. The paper starts by providing a brief overview of the Snohvit subsea development in the Barents Sea, which holds the current world record in tie-back distance, from an all-subsea development back to shore. On the basis of the technical solutions for the Snohvit development, the paper goes on to discuss what the technical implications would be if the tie-back distance was extended to a level of about three times that of the Snohvit development, thereby making the system able to accommodate ultra-long step-out distances, including even some of the most remote E&P prospects currently known to the industry.

The paper concludes that step-out distances of these magnitudes would be technically feasible, based on use of technology solutions from the Snohvit development and more or less conventional subsea technology building blocks. An important exception from this, however, would relate to the area of providing high levels of subsea electrical power over long distances. For many all-subsea developments with long step-out distances, subsea high voltage power distribution will be required to provide sufficient power to support subsea gas compression schemes. The required power levels and transmission distances far exceed the current capabilities of the industry in this regard. Therefore, the feasibility of such field development concepts will necessitate development and implementation of technological solutions to meet this challenge.

Introduction

The Arctic region is an emerging frontier area for the offshore E&P industry, with estimates to hold up to 25% of the world's remaining undiscovered oil and gas reserves. A number of significant offshore discoveries have already been made in areas such as Eastern Canada, Northwest Russia and the Barents Sea. Further to the east, areas such as the Kara Sea also hold a large potential for future E&P activity.

The remoteness of many of these locations, combined with the harsh environment and ice conditions prevailing in the Arctic region, means that Subsea to Shore development solutions with the subsea wells tied back directly to an onshore facility in many cases can offer significant benefits over platform or floater based development concepts. In some cases, Subsea to Shore development solutions may even be a strict necessity for the feasibility of the development.

Many of the prospective assets in the Arctic region are located far from the nearest shore, and the industry-wide efforts to meet the associated technical requirements such as management of multiphase flow and provision of remote control over long distances represents a frontier area within the current subsea technology development.

The Snohvit development, located in the Barents Sea and operated by Statoil, can be said to represent the current state-of-the-art in long step-out technology for subsea developments, and holds the current world record in tie-back distance to shore. The Snohvit development is based on an all-subsea development concept, and includes the implementation of several novel technologies and first-offs, including multiphase flow management over ultra-long distances and subsea reinjection of associated carbon dioxide. The subsea production system is supplied by Vetco Gray, and includes a number of technological achievements related to long step-out power supply and broadband fibre-optic data transmission. These technological achievements, born out from extensive development and qualification work, has moved the technology boundaries forward and has contributed to significantly increase the industry's capabilities within ultra-long subsea tie-backs.

The Snohvit Subsea System

The Snohvit development solution is based on an all-subsea production system, controlled remotely from shore through one single electro-hydraulic umbilical, and with the produced gas piped back to the onshore LNG plant through one single multiphase pipeline.

When the gas arrives onshore, the associated CO2 is separated from the gas, and is then sent back offshore in a separate pipeline for reinjection back into the reservoir. In this fashion, emission of greenhouse gasses is minimized.

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