VERSATRUSS: An Innovative Alternative in Single-Piece Deck Installations and Salvage
Authors: Thompson Barbara L. 
Published: October 2008

The offshore oil industry demands design engineers and inventors to create innovative solutions to construct, install and operate structures in an unfriendly environment. One of the challenges to installation of the newest and most complex facilities is the ability to install them in one piece, fully integrated and commissioned for service. Another challenge is the salvage of massive North Sea topsides.

VERSATRUSS is a new innovative solution to the single piece installation salvage of topsides and jackets. It employs a set of barges, lift booms, and common multi-part rigging to either lift or set a large deck structure. This paper will describe the components of the system, the procedures for using the system, and the computational approach to estimating the vessel(s) motions. A full scale model test in the Gulf of Mexico will be discussed with computational predictions along with actual results. Conclusions in the feasibility and workability of the system are also discussed.

Introduction

Many operators are seeking to install topsides as completed facilities rather than a modular installations that require extensive hook-up and commissioning offshore. Hook-up and commissioning at the fabrication yard is more desirable due to proximity of materials and qualified labor. Many installation sites are in remote areas of the world with limited infrastructure.

Another offshore construction application is the increasing need for salvage of topsides from older fields in the North Sea. Environmentalist pressure is driving operators to search for methods for single piece removals.

Single piece decks have historically been installed with large cranes or with floataer methods. Both of these methods have proven to be valuable, but each have drawbacks due to cost, environmental constrictions, and the need for specialized topsides or jacket design. The current limits of floatmer or Semisubmersible Crane Vessel (SSCV) are approximately 10,000 short tons. A limited number of these high capacity SSCV's are available throughout the world. These limits produce competition for the use of the SSCV, and thereby, reduce their availability which then affects schedule and costs. Finally, some operators have remote fields located in nearshore waters of less than 25 A. that are completely inaccessible by SSCV. An alternative method of single piece installation would be desirable for these projects.

The alternative method offered in this paper is called VERSATRUSS. Although VERSATRUSS is a unique system, it is a combination of several common offshore construction components and marine equipment. The successful design of a VERSATRUSS system is possible with current design capabilities and software tools for motions and structural analyses. The following features of VERSATRUSS make it an attractive alternative:It has the potential to install/salvage integrated decks or jackets weighing up to 20,000 tons.It is ideal in very shallow water where access via an installation vessel would entail dredging operations or is not feasible.No major modifications are required to the jacket or the deck to suit the VERSATRUSS system.Does not require mobilization or demobilization of a semisubmersible crane vessel (S.S.C.V.) when the deck weight exceeds the capacity of conventional mono-hull crane vessels.