Towed Production Systems for Economic Field Development
Authors: Rahtz J.Q. , Chell Kevin 
Published: October 2008

INTRODUCTION

Future development of most offkhore hydrocarbon deposits will focus upon the need to develop smaller and more remote fields. The economic viability of such fields depends not only on a stable or rising oil price, but also on reducing capital expenditure and lowering operating costs. The drive for cost effective development of marginal fields has provided the impetus to use subsea production systems, usually tied back to an existing host facility.

However, operating expenditure for subsea wells is generally higher than for topside wells due to therequirement to charter vessels in order to perform any maintenance or inspection work. Recent developments in the use of through-flowline (TFL) tools, and the increasing reliability of subsea equipment will lead to reductions in operating expenditure, but the main economic benefit of subsea projects lies in lower capital expenditure, which hasa major influence on the economic viability of field developments.

The application of towed production systems (?ITS) to field developments focuses upon this reduction in capital expenditure, through the incorporation of the subsea components into a single integrated towed pipeline and structure assembly.

Installation of Submarine Pipelines Usingthe Controlled Depth Tow Method

Although the practise of towing pipelines is well established it was not until 1980 that the first pipeline bundles encased in carrier pipes were fabricated and towed to the installation site in Conoco?s Murchison field using the controlled depth tow method (CDTM). Since this pioneering start CDTM and other tow methods have been developed and have become established and acceptable techniques for the installation of pipelines as part of a field development (see Table 1) details the pipeline bundle projects to date.

The installation of pipeline bundles is feasible using any one of the following towing methods:

Bottom Tow

As the name suggests the bundle is in contact with the seabed during the tow to the field. This method requires selection of a tow route which is clear of seabed obstacles, and is therefore more suited to areas in which the subsea infrastructure is limited in development. Existing pipelines can be crossed provided specific preparatory measures are taken in advance. The on-bottom weight of heavy bundles can be reduced either by the addition of buoyancy tanks or by the use of a larger carrier pipe.

Off-bottom Tow

This method has the advantage that contact with the seabed during towing is avoided, which yieldslower net towing forces and greater maneuverability. The bundle is towed at a fixed distance just above the seabed. The correct profile is achieved through a combination of buoyancy and free-hanging chains. The chains remain in contact with the seabed during toting.

Surface Tow

Bundles installed using this method have sufficient buoyancy to float on the surface, thus avoiding allseabed contact during the tow. A significant disadvantage of this technique is the influence of surface waves and current on the bundle. Further the controlled lowering of surface tow bundles in deepwater may pose problems.