ABSTRACT

The installation of Exxon's Lena guyed tower set two new offshore records when the longest one-piece jacket was positioned on the seafloor and eight main piles were driven to 560 feet. The overall installation of ti1e guyed tower jacket, decks, and pipelines required a variety of offshore installation procedures and extensive project planning. To minimize the potential for damage to the tower's guying system, an unconventional derrick barge mooring system was deployed at the installation site. A special pile latch assembly was developed to assist in installing six torsion piles. Installing 58 conductor strings required efficient handling of over 800 pieces of conductor pipe. Following deck installation, two pipelines were installed through 1000-foot J-tubes using the direct pull method and a dynamically positioned reel ship.

INTRODUCTION

The installation of the jacket, decks, and pipelines for Exxon's Lena guyed tower utilized a combination of conventional and special installation procedures and equipment. This paper focuses on tower derrick barge mooring, jacket ballasting and positioning, and main pile, torsion pile, conductor, deck, and pipeline installation. Additionally, the paper reviews the extensive project planning that was required to develop effective procedures and organize offshore operations.

INSTALLATION OVERVIEW

The primary construction vessels used to install the Lena guyed tower included Brown &. Root's 2000-ton capacity derrick ship, Ocean Builder I, Brown $. Root's dynamically positioned 2000-ton capacity derrick barge, Atlas I, and Santa Fe's dynamically positioned Pipe laying reel ship, Apache.

Prior to jacket launch, the Atlas I installed the first four guylines. The Ocean Builder I was moored at the tower site with the ends of these guylines secured to the port side. After jacket launch, the Ocean Builder I positioned the jacket, installed piling, and began conductor installation, while the Atlas I was installing the remaining 16 guylines. Following guyline installation, the Ocean Builder I was released, and the Atlas I was moored at the tower site to complete conductor installation and set decks. The Apache installed two pipelines while the Atlas I was setting drilling rig packages and completing miscellaneous activities.

TOWER DERRICK BARGE MOORING

Throughout the guyed tower installation, a derrick barge was moored at the tower site directly over the tower's guying system. A special tower derrick barge mooring system was required to minimize the potential for guyline damage. This involved careful consideration of methods to deploy and retrieve mooring lines, since slack mooring lines could not be permitted to contact guylines. Additionally, the mooring system had to accommodate two types of derrick barges and three mooring locations.

The resulting mooring system consisted of 12 preset moorings at the locations shown in Figure 1. The locations were chosen to minimize the number of times the Ocean Builder I mooring lines had to be moved to allow guyline installation by the Atlas I.

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