Articulated Towers for 1000-m Water Depth With Rigid Seabed Connection
Authors: Stewart William P. , Hodgson Trevor 
Published: December 2007

Deepwater oil and gas production concepts are reviewed, and a new concept based on a compliant articulated tower is presented. The means of avoiding a seabed articulation are discussed. Alternative outline structural configurations and cost data are given for water depths to 1000 m.

Introduction

As structures have been designed for ever deeper and more exposed waters to exploit the world's hydrocarbon resources, the cost of such structures has risen dramatically. Reasons for cost increases are closely associated with the increasing size and weight of such structures and with difficulty of installation in harsh offshore environments. It becomes economically unattractive to extrapolate conventional steel jackets to water depths greater than 400 m. There are no proposals for conventional platforms in water depths of 1000 m, although there are alternative structural forms. These second generation offshore structures, although fixed in one geographic location, are compliant and able to move with the waves. Forces thereby are reduced, enabling a reduction in structural weight and a consequential reduction in costs.More than $2 million have been spent by the DSPS (Deep Sea Production Systems) consortium of companies on an R and D program on Production Systems) consortium of companies on an R and D program on seabed hydrocarbon production. This work included the design of a multirole deepwater structure to support power generating equipment, to restrain a shuttle tanker, to support pumping and ancillary equipment, and to support numerous service risers and heavy power cables serving the subsea production plant.Various alternative structures were examined in water depths to 1000 m in an extreme North Sea-type environment with payloads in the range of 3000 to 4000 Mg.

Concept of Compliant Structures

It is possible to divide fixed deepwater structures into two categories - those with their fundamental natural periods being shorter than the predominant wave periods and those with their fundamental natural periods being longer.Examples of the first category of structures are conventional steel jackets. In 100-m water depth, a conventional steel platform has a fundamental natural period of around 2 seconds. In 300-m water depth, this period has increased to between 4 and 6 seconds, depending on topside load and jacket stiffness. Designers attempt to minimize this period to minimize dynamic excitation of the structure and consequent fatigue damage.An alternative solution in deep water is the other category of structures, which have long natural periods and are generally compliant. For example, a structure to be installed in the Gulf of Mexico in 1983 in 300-m water depth with a natural period of 30 seconds is Exxon Corp.'s guyed tower. The jump to the low-frequency side of the wave spectrum, in the case of the guyed tower, is made by reducing the amount of steel in the structure by reducing its cross section, thereby making it more flexible. Lateral restraint at the top of the guyed tower is provided by guys, each with a 180-Mg clump weight at the seabed before termination at an anchor pile. This is sufficient to prevent overstressing of the tower legs in heavy seas but allows a compliance of the order of 12-m excursion under design wave conditions (without an articulated joint). The guyed tower is, like a conventional platform, bottom supported.

JPT

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