Ice Breaker

2 September 2008 (Last Updated September 2nd, 2008 18:30)

For almost 20 years, StatoilHydro has facilitated oil and gas exports from ice-infested areas. Staff engineer manager Ola Ravndal outlines the challenges faced and the technical developments being made.

Ice Breaker

StatoilHydro has worked on the exploration, production and export of hydrocarbons in the Arctic region since the 1990s. The company’s early work concentrated on the development of oilfields in the Russian Arctic waters from where the main export solutions have traditionally been based on oil pipelines.

As hydrocarbon prospects have moved further away from coastal waters, exports based on ship transport have provided more interesting alternatives. The main challenges with respect to shipping to and from Arctic oil and gas fields are the ice and, perhaps less obvious, the remoteness from existing infrastructure, both for export facilities and supply of consumables.

"Icy conditions are a challenge for Arctic operations."

Examples of particular solutions to these challenges are summed up in two concepts that StatoilHydro has developed for the Arctic areas: the Arctic tandem offloading terminal (ATOT) and the Arctic drillship (ADS). The former concept is mainly designed to maximise ice operability, whereas the latter is designed to be a compromise of maximum ice operability with self-sufficiency for independent operation in remote areas.

ATOT system

The ATOT concept was developed as an offloading terminal in ice-infested waters. The terminal consists of an offloading icebreaker (OIB), which is turret-moored to a position by way of an oil-producing facility located either offshore or onshore. Moored in tandem to the OIB is an Arctic tanker, which loads oil from the OIB.

The ATOT system has two distinct operational modes:

Close tow operation, in which the ice conditions are such that no relative movements between the vessels can be caused by wave actions. This typically occurs when there are heavy ice conditions.

Distant tow operation is used during light or no ice conditions, where waves may cause relative movements between the vessels. Such conditions can damage the vessels unless they are separated by a distance of some 60-80m.

A particular feature of the ATOT system is its ability to enable a fast turnaround during the loading cycle. The reason for this is that sea ice has a tendency to drift with wind and current.

Current drifting is predominantly tidal in nature and is usually oval/circular over time, becoming more elliptical (or flat-ended) closer to the shore. This means that the ice drift changes direction relative to the moored system and with a rate of directional change increasing towards the beginning and end of each tidal cycle.

The ice drift curvature has a tendency to squeeze the train, thereby increasing the mooring forces between the tanker and the OIB. The train is not intended to operate during such rapid drift change and has to disconnect during such conditions.

Therefore, it is an operational goal to perform a complete loading operation within a single tidal cycle when the ice drift is more or less head on (therefore, little rate of change in the ice drift direction).

Offloading icebreaker

The main role of the OIB is to function as a loading buoy for the Arctic tanker, which is principally the same as that of the SBM-type loading buoys. In the case of the ATOT, the OIB also acts as a channel breaker for the Arctic tanker.

The OIB is equipped with four podded propulsors; two at the front of the vessel acting as ice-milling devices, and two at the stern acting as channel-widening thrusters. The combined action of the two sets of propulsors attains lower mooring forces on the anchor lines, reduced size of ice floes underneath the OIB interacting with the mooring lines and less resistance for the Arctic tanker moored at its stern by widening the broken channel in the ice, thereby reducing the icebreaking forces
on the tanker’s hull.

The OIB is moored on the location for relatively long periods at a time. Another OIB, or a general icebreaker (IB), is used as back-up tonnage and for ice management support.

Another IB may serve the role as both supply vessel to the oil field and as ice management vessel as necessary. The OIB is equipped as a multipurpose supply vessel capable of firefighting and oil spill response.

It is further equipped with a small moon pool to enable launching and operation of remotely operated vessels. There is a tendency to go for specialised, single-purpose tonnage generally in the offshore industry, but such specialised vessels are harder to come by in the remote Arctic areas where the distance to the nearest base could be more than 1,000 nautical miles.

For this reason, it is important that all vessels serving the area are as versatile as possible in order to reduce the number of ship movements in or out of these remote areas.

The ATOT system has been subject to extensive R&D work within StatoilHydro and its partners. As part of this development, extensive ice tank testing has been carried out at both the Hamburg Ship Model Basin (HSVA) in Hamburg, Germany, as well as at Helsinki University of Technology in Helsinki, Finland.

The ATOT system has been developed in cooperation with Barlindhaug Consult in Tromsø, Aker Kværner Pusnes in Arendal, Norwegian University of Science and Technology in Trondheim, Norway, and ILS OY in Turku, Finland.

Arctic drillship

The ADS has been developed in a joint cooperation with StatoilHydro and LMG Marin in Bergen, Norway. As the name implies, the ADS is a drillship intended to be suitable for drilling operations in ice-infested waters.

Owing to the remoteness of some of the Arctic field developments, a drilling ship should be self sufficient for long operational periods without needing re-supplies of consumables. This requirement is a size driving factor in the general vessel design.

For ice operations, vessels above a certain size have a tendency to lose operability due to restrictions imposed by parameters such as beam and length of the ship.

In the case of the ADS, a number of features were explored in order to reduce ice forces on the ship. These included ice-milling devices placed at the end of the vessel facing the ice drift, which weakened the ice prior to hitting the hull.

Whereas these appendages gave some effect towards ice force reduction, their value was considered marginal compared with the added operational complexity caused. In the end, only particular features in the hull design were successfully incorporated to counteract some of the penalties imposed by the ship size.

The ADS is intended to be a drillship suitable for water depths ranging from 40m to 1,500m.

In shallow and ice-infested waters, the ship will be turret-moored to its location. At deeper waters (over 250m), the ship will be DP operated (when not operating in ice).

The ADS is designed with a normal bow section suitable for open water operation and for light ice breaking operations. The stern of the vessel has a much reduced rake angle in order to improve its ice-breaking resistance.

The vessel is thus intended for ice operations with its stern facing the ice drift direction.

The vessel is equipped with five podded propulsors: three at the stern and two at the bow section. As for the OIB, the propulsors facing the ice drift direction primarily act as ice milling devices and reduce the average size of those ice floes likely to hit the moon pool area of the ship.

The concept has been subject to extensive testing at the HSVA. It has proven robust against 2m-level ice and ice ridges in excess of 15m depth.

Although these ice conditions are worse than most observed first year ice conditions in ice-infested waters, it cannot be guaranteed that the ship will operate independently.

Ice management

As for both concepts described, there will be a need for ice management when the ice conditions become heavy. The definition of what constitutes heavy ice conditions depends upon the area of operation and local conditions.

In order to improve the operability it would be desirable to have ice management by another auxiliary vessel, to break up ice upstream of the vessel so that the oncoming ice consists of smaller ice floes. This reduces the ice-breaking forces on the ADS and thus improves the operability.

Even though model testing indicates operability in level ice thickness of 2m, real ice conditions include features such as ridges and hummock fields, which vary over the season and with location. It is impossible to mimic all conceivable conditions in a laboratory and for this reason ice management is recommended during the entire ADS operation in ice.

However, it is still reasonable to expect that the ADS may be independently operable in level ice up to some 0.5-0.8m, but until operational experience is gained, such statements should be considered speculative.

The ATOT concept is much more robust and the OIB itself is unlikely to require ice management for its own purpose. Ice management here will be limited to the periods of loading operations.

This will particularly be during tanker operations in way of the OIB prior, during and after loading.

StatoilHydro

StatoilHydro is an integrated technology-based energy company, which focuses on upstream oil and gas operations. It was established in October 2007 following a merger between Statoil and Hydro’s oil and gas activities. Based in Norway, it has over 30 years’ experience of the Norwegian continental shelf, pioneering complex offshore projects under tough conditions.