Hybrid Piled/Gravity Platform for Harsh Environments
Authors: Waddell J.W. , Pearce Fred , Stibbs Harold 
Published: October 2008

The paper describes a 'hybrid? gravity base/steel pi1ed offshore substructure designed for severe ice and seismic environmental conditions. The structure comprises a steel piled jacket within a gravity base 'shield?. The gravity base element comprises a simple raft, of steel or concrete construction, with a central monopod leg. The steel piled element comprises a ring of piles within the central leg. During sea-transport to the offshore site the ring of piles is supported by the gravity structure. After the piles are driven, the connection is severed so that the jacket (ie the piles) and gravity base shield function as separate structures.

The topsides is supported only on the ring of piles. Ice loads are carried entirely on the gravity base shield.

The novel elements of the concept are protected by a number of patents.

INTRODUCTOIN

Numerous structural solutions have been used in ice-infested waters including gravel islands, caisson retained islands, steel jackets and gravity base structures. Concepts, too, are numerous, many of which are variation derivatives of the types noted above.

Many offshore areas where ice loads prevail are also earthquake sensitive regions. Examples of interest to the hydrocarbons industry are Cook Inlet, the Sea of Okhotsk and the Arctic Ocean.

The hybrid structure provides an innovative solution for satisfying the conflicting design requirements when both ice and seismic conditions can occur. For severe ice loading it is necessary to provide a massive structure with large stiffness. Such a structure is often unsuitable for seismic conditions because of susceptibility to high dynamic excitation. By supporting the topsides on the inner ring of piles and not on the gravity base a number of benefits are achieved including:Enhanced safety and redundancy.Reduction in deck excitation forces.Conductors isolated from dynamic response of gravity base.Reduced gravity base dimensions.Reduced sensitivity to soil liquefaction.

DESCRIPTION OF HYBRID STRUCTURL

The structure has the outward appearance of a gravity base monopod structure, as shown in Figure 1, View A. Sectional views show the arrangement of the ring of piles within the central monopod leg. The annulus between piles and gravity base leg is shown nominally as 1500mm, although greater separation can readily be provided.

The concept has currently been developed to suit water depths in the range 30m to 45m. In principle the concept is applicable to deeper waters.

Gravity base dimensions are primarily related to water depth, horizontal loads, soil strength, on-bottom weight and floating stability criteria. For the hybrid concept in 30m to 45m waters, base dimensions are typically of the order 100m × 100m for medium strength soils and horizontal loads around 300 MN. Perimeter skirt piles can be used to carry a proportion of the lateral load, providing a means of reducing base area. This will generally be economic if horizontal sliding governs design.

The transition between the gravity base raft and leg can be configured as a cone or as a simple stepped annulus (collar) as shown in Figure 1.