Deepwater Floating Production Systems in Harsh Environments Offshore Norway
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 24511, ’Deepwater Floating Production Systems in Harsh Environments: A Look at a Field Development Offshore Norway and the Need for Technology Qualification,’ by T.S. Meling, Statoil, prepared for the 2013 Offshore Technology Conference Brasil, Rio de Janeiro, 29-31 October. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission.

Deepwater field developments are regarded as standard technology in several areas of the world, but in harsher environments, extreme design loads and increased fatigue loading become more challenging. In particular, risers and mooring solutions are vulnerable to loading conditions in harsh environments. Development solutions that work well in more-benign environments may not work as required in deepwater harsh environments such as those offshore Norway.

Introduction

Floating production systems in harsh environments have long been in operation, but have been limited to water depths of 400 to 500 m. Deepwater floating production systems have been in operation in benign environments such as offshore Brazil, west Africa, and the Gulf of Mexico (GOM) for almost 2 decades. After Hurricanes Katrina and Rita, the design level for the GOM has been increased, and one can argue that the GOM is no longer considered a benign environment. But tropical-storm areas such as the GOM have significantly less dynamic loading in general than harsh-environment areas such as offshore Norway.

Currently, Statoil and its partners OMV and ConocoPhillips have sanctioned the gasfield development Aasta Hansteen in 1300-m water depth in the Norwegian Sea. Field developments are moving into deeper waters in some of the harshest conditions in the world.

Concept Selection: Deepwater Floating Production in Harsh Environments

Aasta Hansteen is a rich gas field located 300 km from shore. It consists of three reservoirs, and the drainage strategy is pressure depletion. Dry trees offer no benefits; therefore, the field will be developed with subsea trees, seven in total, tied back to a floating production facility. There is also a limited amount of condensate present.

The plant processing capacity is higher than needed because the development opens a new area; the production facility will be used as a hub. The treated gas will be exported through a new approximately 500-km-long pipeline to Nyhamna for final processing and further export (Fig. 1), and the produced water will be cleaned and disposed of at sea. However, the small volumes of condensate caused some challenges for concept selection. Economically, the condensate is more of a burden than a value because a solution is required for the limited production. Export through the pipeline was not feasible from a flow-assurance standpoint; and, because the closest existing facility with storage capacity is 150 km away, a separate condensate- export pipeline was not attractive either. Therefore, local storage of 25 000 m3 of condensate and offloading was required at the lowest possible cost.