Gas exploration beyond the shelf break: An oceanographic challenge

Abstract

Norway's second largest gas field, Ormen Lange, is located 140 km west off Kristiansund at an unprecedented depth when it comes to exploration. It will be the first Norwegian project beyond the shelf break. Exploration and development of the field is thus a challenge. An important issue during the planning stage is to understand the current conditions and hydrography of the site. This is especially important regarding pipeline design, deployment and operations. A complicating factor for estimating design currents is the extreme roughness of the local topography. Submarine slides have produced escarpments and sea mounts with height variations of up to 100 m. The hydrography seems to be equally complex; in situ moorings have revealed strong variations in current speed and temperature close to the seabed.

A variety of numerical experiments have been and are being set up in order to recapture and if possible forecast the observed variability. The results show that the flow is influenced by the inflow of Atlantic Water, tides, atmospheric forcing and by flow of water masses inside the Norwegian Sea basin. The variability near the seabed at Ormen Lange is strongly influenced by the local topography and the stratification. Realistic model studies therefore require high resolution models for the Ormen Lange topography connected to basin scale models. The models must be non-hydrostatic and the stratification realistic to enable realistic estimates of extreme events.

Introduction

Processes related to the shelf edges and shelf slopes have so far been of great scientific importance because of their role in the exchange of matter and energy between the shelves and the deep oceans. From the Norwegian offshore industries point of view, this research has been of limited interest since the oil and gas fields have been located on the continental margins at depth less than 300 m. But when Norsk Hydro in 1987 found a significant gas field, Ormen Lange (OL), in the core of the Storegga slide, the need for knowledge about continental slope and continental shelf break processes arose. The combination of large depths, between 800 and 1100 m, low temperatures, below 0 °C, and extremely rough topography represent great challenges for the development of the field.

The general opinion has been that currents in the deep oceans are fairly weak. However, so far the focus has been more on the general flow and mean transports rather than on peak events in oceanography (Seidler et al., 2001). Observed time series from OL at 800–1100 m depth showed large oscillations in the density interfaces and that velocity peaks may exceed 50 cm/s (Eliassen et al., 2000), see Fig. 1 for an example. Since rapid changes in the flow are seen, and the recorded velocities are time filtered, the true maximum velocities may be underestimated at present (Berntsen et al., 2001).

Regarding pipeline design, deployment and operations, the understanding of the current condition and hydrology at OL is crucial for a successful operation of the gas field. Accurate observations and numerical models are therefore important tools. Recordings of time series from OL (OCEANOR, 2000a, OCEANOR, 2000b) and the Svinøy section (Orvik et al., 2001, Skagseth and Orvik, 2002) form the basis of the observations. When it comes to numerical models, several experiments with different grid resolutions are performed and reported in different technical reports (Avlesen and Berntsen, 2001, Alendal, 2002, Berntsen et al., 2001, Berntsen and Furnes, 2002, Eliassen et al., 2000, Eliassen and Berntsen, 2000, Heggelund and Berntsen, 2000, Heggelund and Berntsen, 2001, Sørflaten and Berntsen, 2001, Thiem et al., 2001, Thiem and Berntsen, 2002, Thiem et al., 2002a, Thiem et al., 2002b, Thiem et al., 2002c, Vikebø et al., 2001a, Vikebø et al., 2001b, Vikebø et al., 2001c). The numerical experiments are based on the Bergen Ocean Model (BOM) (Berntsen, 2000) and the general circulation model developed and maintained at MIT (MITgcm) (Marshall et al., 1997a, Marshall et al., 1997b). Both models allow non-hydrostatic simulations.

The time mean current at OL is strongly dominated by the Norwegian Atlantic Current (NAC). The mean current is approximately 30 cm/s close to the shelf edge. At 800 m depth the mean is approximately 5 cm/s, see Orvik et al. (2001). Tidal effects are weak, i.e., less than 5 cm/s at these depths, but increasing towards the shelf edge. In the upper 100 m the flow is strongly affected by the atmospheric pressure and wind forcing. At intermediate depths the flow is mainly aligned with the general topography, while the flow in the bottom layer is strongly affected by the roughness of the topography and is much more directionally unstable (Eliassen et al., 2000).

The extreme events are driven by strong pressure gradients. That is, strong atmospheric low pressures and/or internal pressure gradients at fronts between warmer Atlantic Water (AW) and colder Norwegian Sea Arctic Intermediate Water (NSAIW). Along the shelf slope at OL, steepening of the isopycnal separating AW and NSAIW may occur due to strong Ekman veering during storms and/or approaching internal density fronts (Vikebø et al., 2001a). Fig. 2 is a rough sketch of the water masses along the shelf off mid-Norway. During such events the density surfaces tend to under/overshoot their equilibrium level, and as the forcing weakens, the suppressed water may run up/down along the shelf slope. During these events, peak values in the velocities are often found. If the event is a strong run up event colder and heavier water masses are lifted up on the shelf. As the wave retreats off shelf, some of the heavier water may be left atop of the shelf separated from the water mass it originated from. This high density on shelf water mass follows the general flow along the shelf, and may later on, flow down cross shelf canyons creating gravity currents. In the OL area, findings indicate that gravity currents are sometime present. Flow down the shelf can also be set up by sediment-laden currents (Simpson, 1987). After front passages the oscillations of the thermocline may continue for days. Prediction of waves and currents is thus an important goal for the OL research. Since the water masses are accelerated by pressure gradients, a forecast is therefore dependent on the ability to measure and predict pressure differences in the atmosphere and in the ocean. For the atmospheric, low pressure forecasts are routinely produced and these are trustworthy up to one week ahead. For internal oceanic pressure gradients, an array of current meter and hydrographical moorings surrounding OL is necessary to capture incoming fronts.