Abstract

Large cost reductions can be achieved by utilizing buried and coated flowlines. Subsea flowlines in liquid dominant production systems are prone to difficulties with regard to flow assurance issues. Thermal solutions to keep the produced fluid at a temperature high enough to avoid wax formation and avoid the risk of hydrate blockage can be prohibitively expensive. Flowlines are probably the largest single cost item, apart from drilling, in a subsea production system. This paper will disclose and review the results of studies into insulating properties of super-saturated deep-water soils. It will explain how buried and coated flowline systems can achieve results similar to that obtained with more expensive Pipe in Pipe (PIP) systems. It will show how system performance is improved over that of Pipe in Pipe systems and it will explain the operational methods of deep and ultra-deep water burying equipment.

Subsea developments that have been shelved because of high costs may become viable by use of buried and coated systems. Improved system performance and cost reductions can be achieved using buried andcoated systems.

Introduction

The application of buried and coated flowlines can make developments that currently require pipe in pipe flowlines up to 25 miles in length economically more attractive. This is achieved by understanding the insulating properties of subsea soils and how burial can be achieved. Economics and cost drivers of buried solutions will be reported. Thermal performance will be shown to be similar to that of more expensive systems.

  • Historically Flowlines in excess of 12 to 15 miles for Oil/Multiphase service gravitate to PIP solutions.

  • For Subsea step-out systems, flowline costs are probably the greatest single dollar ($) value second to drilling.

  • Buried and coated Flowline systems give similar performance to PIP solutions. Step-out distances in excess of 25 miles can be achieved for oil dominant systems.

  • The cost of burial and coating is potentially 40% to 50% of that of PIP.

  • Recent investigation and measurement of deepwater soils has shown that the thermal conductivity (k) value of such soil has beneficialthermal properties beyond what is normally assumed.

The benefits of buried and coated (B&C) flowlines ascompared to pipe-in-pipe (PIP) systems may be described in terms of potential cost savings, an increase in allowable transient shutdown times, extension of the "no touch" period, and longer duration before waxappearance and hydrate formation.

Investigation has shown that;

  • B&C (buried & coated) OHTC (Overall Heat Transfer Coefficient) or U value can be close to that of PIP.

  • Burial systems have much longer transient cool down times than PIP

  • OHTC marginally dependent upon burial depth, after ~4 Flowline diameters little gain.

  • Deepwater clays thermal conductivity is unaffected by remoulding (If done correctly)

  • Trench back-fill soil samples indicate low probability of void spaces

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