Alternative Concepts to Gloryholes for Protection of Subsea Infrastructure in Ice Prone Regions
Authors: Freeman Ralph , King Tony , Zakeri Arash 
Published: December 2011

Marginal field developments on the Grand Banks and other regions may not be economic if conventional protection using gloryholes must be used, and new technology is required. Identification of alternative concepts to gloryholes for protection of subsea facilities against threats from ice keel interaction was the focus of recent research and development activities at C-CORE. The primary criterion for the design of protection structures is that systems are safe and environmentally responsible, thereby preventing hydrocarbon discharge. This paper provides an overview of the risk-based approach for design of gloryholes including geometry and depth requirements as well as associated challenges and uncertainties. It then presents three conceptual alternatives to gloryholes:revised well casing design,protective truncated cone structure installed above the mudline, andsub-seabed protective structure.

The first concept involves devising a mechanical Shear Connection Linkage such that, given an extreme ice keel loading event, displacement of the wellhead system is isolated to a zone near the mudline while maintaining the integrity of the downhole safety barriers. The second concept includes a protection structure at the mudline that sits overtop of a single wellhead system protecting it from direct interaction with an iceberg keel. The third concept is applicable to single and small well cluster systems. It requires relatively smaller but more precise seabed excavation compared to the conventional gloryholes. If further developed, all three alternatives could potentially provide considerable cost savings for specific situations. Concept evaluation is briefly discussed for each alternative in terms of logistics for design, construction and installation based on regional, environmental and geotechnical conditions. Finally, the outstanding technical challenges and knowledge gaps that need to be addressed are identified and discussed. The significance of this work is that marginal field developments not deemed economic using conventional technology may be possible in the future.

Introduction

Over 25% of the world's petroleum reserves are believed to be in arctic regions and other offshore ice-frequented environments. As world energy demand increases, development of oil and gas resources in harsh ice environments is being increasingly considered by industry. The advancement of safe, cost-effective and reliable engineering solutions for subsea infrastructure in ice environments is a key requirement for sanctioning stand alone and marginal field developments. Subsea infrastructure that requires protection from ice keels includes components such as wellheads, production Xmas trees, manifolds, umbilicals, flowlines and pipelines (Randell et al, 2008). One of the major engineering challenges in operating off the East Coast of Canada, and also Greenland, is the presence of icebergs with sufficient draft to interact with the subsea and seabed infrastructure. Figure 1 shows a portion of heavily gouged seabed illustrating both gouge furrows and pits.