DNV Launches a Design Framework for Floating Structures in Ice

Tue, 04 December 2012

In order to ensure a common, transparent and documented approach to achieving acceptable safety levels for offshore structures in cold-climate regions, a DNV-led joint industry project (JIP), ICESTRUCT, has since 2009 worked to develop a designer-friendly and reliable framework based on the ISO 19906 Arctic Offshore Structure standard.

Per Olav Moslet, Arctic technology research programme director at DNV explains that “The governing design loads for offshore structures in Arctic areas are usually based on interaction with ice, and it is very important that these loads and their effects are treated consistently. Due to the lack of a common industry approach for floating structures in ice, it has previously been difficult for designers to establish the appropriate design loads effects.”

“Because of its nature, ice can generate considerable loads, and structures designed for Arctic operations may look different to structures in open seas. However, ice loads and associated load effects should be treated in the same way as any other environmental load when designing a structure since, in principle, an Arctic offshore structure is no different from any other offshore structure when it comes to assessing adequate structural strength,” he says.

This has JIP has developed a methodology for determining ice load effects. Rather than having a specific custom-made Arctic design practice for ice loads, the methodology developed in the JIP is consistent with existing methods for determining other environmental load effects. Consequently, the existing offshore design practice that has been used for several decades in the North Sea and elsewhere can be used for the design of offshore floating structures in ice.

“The advantage of the new DNV framework is that the same design practice can be used irrespective of the type of structure and environment – Arctic or open sea. That said, the nature and variability of the ice and its complex interaction with structures need to be taken into account,” Moslet says.

Further advantages are:
- A recognisable approach for offshore designers who are familiar with conventional open water design practice.
- The designer of an Arctic offshore structure who has no specialised knowledge of ice mechanics is provided with a basis for determining characteristic ice load effects.
- Adaptable to all structure types.
- The designer is not required to actually perform probabilistic analyses, since the framework provides simplified deterministic solutions that take uncertainty into account.

Broad industry cooperation
Since 2009, the ICESTRUCT JIP has focused on developing designer-friendly methods for determining characteristic loads and load effects on fixed and floating offshore structures in conformance with the ISO 19906 standard. However, the ISO 19906 standard does not provide any guidance on the design of floating structures in ice, so the results of this JIP are considered to be a contribution to the further development of design standards and best practices in the Arctic offshore design community.

The JIP has received wide industry support and sponsorship from oil companies, yards and engineering companies, including Transocean, Shell, Statoil, ENI, Repsol, SBM Offshore, Daewoo Shipbuilding and Marine Engineering, Hyundai Heavy Industries, Multiconsult, Keppel Offshore and Marine, Marin, Huisman Equipment and Dr. techn. Olav Olsen. In addition, valuable work-in-kind contribution have been provided by several key international universities and companies such as Prof. Ove T. Gudmestad, Prof. Karl Shkhinek, Aker Arctic and the Hamburg Ship Model Basin (HSVA). The project ends in December 2012.

Fundamental principles: Environmental design contours
The framework is based on the use of environmental design contours that define a set of ice states. The designer must determine the maximum load effect arising from the contour ice states. Predetermined, tabulated factors can then be used to scale from the maximum load effect to the characteristic load effect. This is a conventional methodology used for other offshore designs, and its introduction to Arctic offshore design practices simply represents a shift in Arctic design philosophy in line with that of the rest of the offshore engineering community.

Standard offshore structure design practices build on the concepts of a characteristic load effect and a characteristic structural resistance (or capacity) separated by a safety margin using safety factors, which ensure that the specific design achieves the required structural reliability. The characteristic load effect should not be exceeded more than once during a reference period, often called the return period. The design equation takes uncertainties into account, based on results from probabilistic models of the environmental conditions and interaction processes. Hence, the uncertainties are taken into account in a systematic and well-proven way, leading to a design with the desired reliability.

These concepts have previously not been applied properly to the design of floating structures in ice. This is mainly because no systematic probabilistic modelling has been carried out for these structures. This is also the reason why there are few references in standards and best practices to “characteristic load effects” for floating structures in ice. Until now, there has been no industry guideline on how the designer should determine relevant characteristic ice load effects.

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