DNV-led JIP presents new offshore floating structure design method for ice-covered waters

Offshore staff

HOUSTON –An enhanced design framework for floating structures in ice-covered waters has resulted from a DNV-led joint industry project named ICESTRUCT started in 2009.

Per Olav Moslet, Arctic technology research program director at DNV presented the new framework, which represents a shift in Arctic design philosophy, at the Arctic Technology Conference in Houston.

“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,” said Moslet. 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.”

The JIP used existing and established design practices and based its work on the ISO 19906 Arctic Offshore Structure standard. The result is a method to determine ice load effects that is consistent with existing methods to determine other environmental load effects, which means established design practices can be applied to offshore floating structures in ice.

Moslet said other advantages include:

• Use of a recognizable approach for offshore designers who are familiar with conventional open water design practice

• A designer without specialized knowledge of ice mechanics will have a basis for determining characteristic ice load effects

• The method is adaptable to all structure types

• The designer is not required to perform probabilistic analyses, since the framework provides simplified deterministic solutions that take uncertainty into account.

The JIP support is from 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, with additional support from several international universities and companies.

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.

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.

12/04/2012