Self Installing Gravity Platform Concept
Authors: Waddell J.W. 
Published: October 2008

Introduction

A new platform concept is described whereby both the substructure and deck can be installed without the use of a heavy lift crane vessel. The novel idea is to use the buoyancy of the substructure to elevate the deck. The substructure is a gravity design that is divided into a lower bottom-founded base section and an upper movable buoyancy section. The buoyancy section lifts the deck by means of vertical tubulars (piles) that slide through guides attached to the lower base section.

The lifting concept is used in conjunction with a deck that is configured with "drop-down" leg sections that can be independently lowered to engage with the top of the substructure. This allows the deck to be transported conventionally, at low level.

The overall concept is essentially an alternative floatover design that is called "Lodeck" to differentiate the design from conventional floatover concepts that are transported high above barge deck level. The paper describes the main elements of the design together with estimated cost differences with conventional floatover designs and lifted decks.

Review of Self-Installed Bottom-Founded Platforms

The cost of platform installation by Heavy Lift Vessel (HLV) is a major component of total investment costs, measured in $tens of millions for a major facility. It is not surprising therefore that the industry has long focused on designs that avoid or minimise dependency on HLV's such as gravity base structures, production jack-ups and floatover solutions. Crane vessel installation still predominates for jacket supported platforms although decks installed by floatover date back to the early 1980's (Ref 1). The development of "bucket" foundations for jacket structures, in lieu of piles, in the early 1990's now provides the potential for complete platform installation without HLV cranage (Ref 2). Installation by crane vessel remains the baseline solution that other installation options are measured against. The lifting method will be one of the primary selection criteria, along with other key economic factors such as minimising Hook-Up and Commissioning (HUC), involving considerations such as:HLV Availability - site remoteness, other project demands, etc.HLV Capacity - deck or substructure cannot be lifted. HLV Suitability - shallow water, etc.Installation Risk - purpose built crane installation verses relatively untried self-installation methods

These issues are well understood although the selection of platform type and installation method are not always easily assessed in a market where crane vessel owners are adjusting to competition from an ever increasing variety of offshore solutions, notably the growing use of floaters. History suggests that HLV installation will continue to predominate but there is a growing interest in self-installed platforms, not only in remote regions but also in the North Sea and Gulf of Mexico where most of the world's largest cranes operate. The history of self-installation techniques is briefly reviewed to provide an insight into likely future trends.

Crane Installed Steel Jackets with Offshore Deck Floatover. Floatover methods relate to deck mating onto a pre-installed jacket which is generally configured with a slot that allows access for a transportation barge carrying the deck.