Economic Analyses For Installation Of Offshore Pipelines
Authors: Brown R.J. , Hirsch A.S. 
Published: December 2007

Offshore pipelines may be installed by several methods including lay barge, reel barge, bottom tow/pull and combinations of these. Technical advantages and limitations of each method are presented as well as their impact on the cost presented as well as their impact on the cost of the installation.

Introduction

The most common method for the installation of offshore pipelines makes use of the lay barge. As costs of installation, for example in the North Sea, have escalated to a point where pipelines there now cost between 1 and 2 million dollars per mile, it behooves the pipeline owner to consider alternatives to the lay barge if these alternatives are technically feasible and offer a cost savings.

For non-weight-coated lines of less than about 12 in. in diameter a reel barge may be a possible alternative. It can store several miles of pipe on board therefore eliminating extensive need for re-supply. Since the pipe has already been welded on shore there is less pipe has already been welded on shore there is less need for joining at sea which is a significant manpower and time expense. However, shore welding time, facilities and manpower are necessary for preliminary pipe preparation.

The bottom tow/pull method may have its most attractive applications for short lines either in an area where weather on the surface prohibits the use of surface vessels to lay pipe because of excessive motion or in areas where the depth dictates the size of equipment required to support the weight of the pipe or where fixed costs of mobilization/demobilization, stinger and often-times, minimum commitment times in excess of what is strictly required, involve a very high cost per unit length of pipeline. A good example of the latter is the recent installation of a 36 in. pipeline in Statfjord field by bottom tow. Bottom pipeline in Statfjord field by bottom tow. Bottom tow also minimizes the chance of anchor damage by lay vessels in an area already populated by existing pipelines. pipelines. Other concepts that are in the testing stage are surface tow and sub-surface tow which will not be covered here.

The choice of lay barge over reel barge or bottom tow is often the best alternative, but even in this case no two barges, and particularly no two hull shapes, have the same characteristics. The inherent differences can impact the total cost of the job both in day rate and production rate. The purpose of the paper is to enumerate and evaluate the advantages and limitations of each system and, perhaps, offer a guideline for the choice of alternative installation methods.

LAY BARGE

Historically the lay barge has been the accepted method for installation of offshore pipelines. The Gulf of Mexico was the first area where sizeable interest was shown in offshore exploration and the first lay barges were ideally suited to the typically calm weather conditions encountered here.

When pipeline installation in the North Sea started in 1966, the engineers and constructors entered an entirely new area characterized by adverse environmental conditions. Pipe laying equipment and methods developed in the Gulf of Mexico were transplanted to this new area with relative success. The first submarine pipeline in the North Sea extended from the West Sole field in the southern U.K. sector to Easington, U.K. The deepest water encountered in this area was 105 ft.

One of the first offshore oil reserves with commercial potential was the Ekofisk field in the Norwegian sector. With its construction in 1974 it was the deepest line (312 ft. of water) at that time. The development of the North Sea has continued in a northerly direction where water depths have increased and more hostile environmental conditions exist. The Flags line from the Brent field to St. Fergus passes through areas of 525 ft. water depth and lies passes through areas of 525 ft. water depth and lies in an area where the effective working time for a conventional barge may only be a total of one month in a whole construction season.