ABSTRACT

This paper, named "Deep Water Tie-in", describes a tie-in concept for large diameter pipelines The concept is developed for use at large water depths where diving is both costly and dangerous. The concept is developed for a typical Gravity Base (GBS) platform situated in 300 metres water depth. The tie-in of pipelines with a diameter of up to 40″ is achieved through a tie-in arrangement system pre-installed in the platform substructure The main components are an atmospheric tie-in tunnel, seal tube and J-tube for pipeline pull cable The paper describes each of these components in more detail before it embarks on a step by step account of the pipeline tie-in procedure. In order to establish design loads on the sealtube and identify the optimum pipeline approach, computer analyses have been carried out It is shown that a curved pipeline approach or a dogleg are both feasible from a load on anchor or stress in pipeline point of view However, a straight pipeline approach causes loads and stresses too high for practical design purposes. In order to evaluate the advantages of the proposed tie-in arrangement, the paper ends with an outline of alternative tie-in concepts

INTRODUCTION

This paper describes a deep water tie-in concept suitable for large diameter pipelines. The concept has been developed by Aker Engineering Bergen A/S (Previous Bergen Engineering A/S) for a typical Gravity Base Structure (GBS) situated in 300 metres water depth. Aker Engineering Bergen A/S has during the last few years performed several studies with regards to the tie-in of pipelines at deep waters

The tie-in of pipelines up to 40″ diameter is achieved through a tie-in arrangement system pre-installed in the platform substructure This so that the tie-in of the pipelines to the platform riser systems can be achieved in a safe and practical manner with emphasis on flexibility and back-up possibilities.

The tie-in of the large diameter pipelines are to be performed within a dry tie-in chamber through a seal tube system constructed as an integrated part of the GBS structure This tie-in arrangement has the following main advantages compared to alternative methods

  • The entire riser is installed within the riser shaft and protected against environmental loads and dropped objects

  • Straight forward inspection and maintenance of the riser and corrosion protection system performed by the platform crew

  • Pipeline or pipeline section pull-in directly from a lay barge.

  • Tie-in weld is performed in dry one-atmospheric Condition

  • No diving required. Pull-in will be assisted by ROV

  • Flexibility with regards to sizing of future pipelines, The pre-installed tie-in arrangement can accommodate a range in pipeline sizes

  • Possible to reverse tie-in in order to change pipelines.

In order to evaluate the advantages of the proposed tie-in arrangement, alternative tie-in concepts have been outlined

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