A Pilot Subsea Completion Installation in the Ninian Field, U.K. North Sea
Authors: Seligman P.R. , Scott A.G. 
Published: October 2008

A project to design, manufacture, test, install, commission and operate a single well prototype subsea completion of a suspended appraisal well on the Ninian Unit has been successfully accomplished. This was carried out over the period September 1976 through February 1980.

The main objective of the project was to evaluate prototype subsea equipment for both producing and injection wells which could be needed in the future to supplement fixed platform locations. The opportunity was taken to specify a number of designs which would have merit if a large supplemental programme was ever required.

The paper describes the now installed system of wellhead, flowlines and platform facilities. The system was tested on land by subsystems, systems from separate suppliers and finally as an integrated system. Installation of wellhead equipment from a semisubmersible and of flowlines by a layship was conducted simultaneously towards the end of 1979.

The various project phases are discussed and the results presented.

INTRODUCTION

The Ninian Unit in UK Blocks 3/3 and 3/8 as shown in Figure I contains in excess of 1 billion barrels of recoverable reserves. It is in the Northern north sea with a water depth of about 450'. The field development programme approved provided for platforms in the northern, central and southern sectors of the field as well as interconnecting field pipelines and a pipeline to shore. Platform slots allow for drilling a maximum of 109 wells.

As it was recognised that there might be a need to supplement the wells drilled from the platforms, it was decided to develop a specification and tender for subsea completion systems to quantify the state of the art, designs and costs. The equipment was to cater for new wells or reentry completion of suspended appraisal wells, either producers or injectors. Bids were received, reviewed and the capability to complete subsea to specification verified.

The final decision was made in early 1977 to contract for a single well completion to permit first hand design, manufacture, testing, installation and operating experience. As firm needs were unknown, it was decided to include requirements to cater for the most complicated case and therefore a diverless design with through flowline (TFL) maintenance capabilities was justified. To gain knowledge on systems suitable for complicated single wells, manifolds and templates, multiplexed production controls were specified. A split tree to provide additional safety during pull-ins and maintenance operations, and wellhead protection against fishing equipment and small anchors were agreed.

Consideration was given to drilling a new well but review of the suspended appraisal wells determined several were suitable for re-entry and completion. Based on its low profile, downhole configuration (9 8" casing to total depth), and distance from the Central Platform which would give a good test of TFL tools, as well as minimising interference with other field facilities, Well 3/3-5A was selected.

The project having been initiated saw the engineering group, which was originally set up only to conduct feasibility studies and other preliminary work, follow through from design to commissioning.