INTRODUCTION

This chapter discusses the variability of tie-in methods and equipment applicable for current and future deepwater installation The tie-in of a pipeline to a wellhead or template involves many aspects of subsea engineering, ranging from pipeline installation methods through ROV operations to the architecture of the template itself This chapter attempts to give a reader concerned with template design a better understanding of the problems facing the pipeline engineer in deep water. It also presents current proven limitations for various aspects of diverless tie-ins and provides a checklist for the template designer covering his design elements of the problem It is the authors' contention that a "standard tie-in module" is not a practical piece of equipment for the industry to develop because of the variability of the problem rather, the key common aspects of tie-ins should be identified and equipment/techniques developed and proven that can be assembled to tackle a variety of configurations This chapter attempts to identify these key aspects to assist designers of future development programmes

THE DEFINITION OF DEEP WATER

Deep water is best defined as water beyond the reach of diver intervention The point at which deep water begins is therefore not fixed, it depends partly on technology, partly on physiology and partly on the attitudes of diving companies, operators and regulatory bodies

Improved equipment and gas mixed are enabling step-by-step increases m diving depths, but as they do so the physiological difficulties become increasingly severe, and a limit upon working depths must inevitably be reached. The influence of attitude is demonstrated by the differing diving limits which apply in different parts of the world Typical limits for different classes of diving are shown in Fig 1.

By the above definition, very few deepwater flowline tie-ins have been performed A larger number of tie-ins have been performed using equipment suitable for deep water, but usually with divers present or at least with the knowledge that they could be called in if necessary This is partly due to understandable caution about using new technology without any back-up and partly because, by the tune diverless techniques are developed and proven, advances in diving have caught up with the previously unreachable water depths However, the situation is changing as confidence grows in diverless techniques and as the inevitable physiologcal limits are approached.

The definition of deep water can be further refined to that of very deep water that is water beyond the design depths of existing equipment Broadly speaking, this starts at twice the depth of deep water (currently 500–600 m)

At such depths the limitations are again physical, but in this instance they relate to the equipment, m particular umbilicals and control systems

EXISTING PRACTICE

The majority of existing flowline tie-ins employ spoolpieces The two principal advantages of spoolpieces are that

  • they can absorb some misalignment of the flowline

  • they absorb longitudinal expansion of the flowline

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