Deeper, further, colder

OE staff
Sunday, January 2, 2011

Newly published Polar class vessel rules from French classification society Bureau Veritas are aimed at speeding Arctic and Caspian Sea oil and gas development by facilitating the building of tank, cargo and offshore service vessels which can operate unsupported by icebreakers in very heavy ice. BV also issued specific guidance on ice/structure interaction to help facilitate floating LNG and oil platform construction in high Arctic conditions.

The places where states and oil majors can exploit new offshore oil and gas fields are getting deeper, more remote and icier. 'That is why we continually have to revise and update our tools and rules,' explains Marie-Francoise Renard, head of offshore at Bureau Veritas (BV).

'And of course following the Deepwater Horizon there is more emphasis on verification of offshore projects as a whole. That is where our unmatched experience with FPSO structures can usefully be blended with our experience of asset integrity maintenance and certification in other non-marine industries. We can take a holistic view, which is one reason why we were awarded the verification of the Goliat FPSO for Eni Norge.'

The Goliat field is located in the Barents Sea northwest of Hammerfest, Norway, and the licensees are operator Eni Norge (65%) and Statoil (35%). Renard points out that BV has close links with Hyundai Heavy Industries, the Korean yard contracted to build Goliat's floater which, at 100m diameter, will be the biggest FPSO yet built to the circular Sevan design. 'There are a lot of challenges to high latitude floaters, not least ice,' she says. 'We have done FEED studies on some 14 floating units this year alone. Some offshore projects are for very heavy ice, such as the Russian Arctic Shtokman facility. And for all that ice we had to develop new rules and also a new tool, IceSTAR.'

According to Renard, there are a number of rules and standards for Polar class ships and until now they have been adequate for calculating the reinforcement required for different areas of a ship's or floater's structure for operating in specific areas and ice conditions. 'But the existing rules built on experience are not thorough enough when you are studying floaters and ships to serve them for high Arctic exploitation of oil and gas, and the shipping rules are not sophisticated enough for these newgeneration structures,' she explains.

'We have to be able to do direct calculation of the effects of the ice loads on the structures, and also the mooring systems of the floaters. These units and the tankers that serve them will have to be able to operate safely and keep operating for a high percentage of the time, so the modelling of ice effects has to be accurate. There are manual ways to do this, but it takes time. IceSTAR saves time by mapping ice loads and applying them to an FEM model of the unit's hull structure, adjusting those loads according to the ice drift speed the offshore units will have to withstand.'

The right loads

Getting the right loads to input to the software was a large part of the IceSTAR project. That was done in co-operation with the Russian State Maritime University of St Petersburg using its ice data and experience married with BV's experience of developing modelling tools. At present IceSTAR is an internal tool for BV engineers working on FEED studies on Arctic units and ships, but Renard expects it to be rolled out as an industrial tool during 2011.

According to Renard, where IceSTAR differs from conventional calculations is not just in speed of use, but in the way ice loads on specific structural areas are conceptualised. All the current class and national rules for ice are based on calculating the design load for a glancing bow impact on ice of a specific thickness. Then the impact loads on the fore body are converted to loads on other hull areas using an area factor, which in turn is used to calculate the scantlings for different parts of the structure.

IceSTAR works directly by calculating the ice load for each specific hull area and then through an FEM modeller it can calculate the exact structural response for each area of structure and a range of speeds and types of impact. The different modes for ships include breaking ice head or astern, working in a channel or glancing impacts, and for floating units a wide range of potential ice collisions can be modelled. Those models decide on the structure required for each element, and also underpin the disconnection strategy which will be the last line of defence for floating units threatened by approaching ice.

While ships can move easily, floaters cannot and disconnecting them and moving them out of the way of ice is costly. When drifting ice threatens an Arctic floater then disconnection is the ultimate defence. For each unit there has to be a clear disconnection decision strategy, which will depend on the expected structural impact and also mooring load impact of moving ice of different sizes and speeds. IceSTAR is designed to help develop those strategies, which in practice will depend on guard zones around each floater which when breached will trigger the decision to disconnect.

'With IceSTAR for FPSOs and other floaters the ice is moving, not the unit, and even if the unit can rotate under power, it cannot always determine exactly how the ice will hit it,' notes Renard. 'So we have to be able to directly calculate the effect on the FPSO structure of different types of ice impacting in different places at different speeds. IceSTAR will displace the current extensive calculations necessary and very much speed up the process of design and verification of Arctic floaters across the industry,' she adds.

BV's research engineers used a 20-year service history and damage records for a series of tankers operating in known ice conditions in the Arctic to validate IceSTAR. OE

Breaking the ice

BV's new Rules for the classification of Polar class and icebreaker ships bring together for the first time the requirements for icebreakers and the requirements for Polar class cargo and other service vessels such as oilfield support vessels, says the class society's director of development Philippe Baumans.

'Our Polar rules covered the requirements for ships operating in the high Arctic, accompanied by an icebreaker, and there are separate rules for icebreakers,' he explains. 'But for extracting oil and gas from the Arctic, and also from the very icy and difficult Caspian Sea, there is an increasing demand for vessels which can carry cargo and also ram and break ice unsupported by an icebreaker.

'Under our new Polar rules that is possible, and the owner can choose an icebreaker class from 1 to 7 for the cargo vessel, tanker or platform supply vessel they need, which will reflect the heaviness of the ice that can be dealt with unsupported.'

BV is currently classing a series of offshore vessels specifically for the Caspian Sea and its very heavy ice characteristics. 'We are considering a specific Caspian notation, as although the ice requirements in the Caspian are similar to the Arctic in some ways, in others they differ,' notes Baumans. 'Understanding ice loads and structure in detail is vital to that, as it is for the new generation of offshore floater that will be needed for high Arctic fields such as Shtokman.'

With that in mind BV also published Guidance note NI565: Ice characteristics and ice/structure interactions.

The purpose of this note is to collect and provide data on ice characteristics as well as giving some guidance on the calculations of the forces generated by the ice on ships and offshore structures. As well as indicating some information on the different types of ice and on their mechanical properties, it provides analytical formulae and methodologies to estimate the forces applied on the structures due to ice, with respect to the different modes of failure of the ice.

'We have done extensive work with St Petersburg University and Aker Arctic on ice loads, much of it for the Shtokman project and aimed at clarifying the needs for floating LNG platforms and LNG shuttle tankers,' says Baumans.

In addition to further refining IceSTA R and preparing to make it available to industry, BV also expects to publish new rules and guidance for using podded propulsion in ice during the course of this year.

'That will facilitate the development of double acting tankers and gas carriers,' adds Baumans. 'They will break ice stern first and make ocean transits bow first, maximising the hull form efficiency in each environment.' OE

Categories: Arctic FPSO Software Europe System Design

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