Rig Modifications Enhancing Capabilities of Third Generation Semi's
Authors: Grant Paul , Pangarkar Sunil 
Published: December 2007

IADC Members

The drilling and/or completing of exploration, appraisal and development wells in today's business environment places increasing demands on an ageing Semi-Submersible fleet. In respect of development drilling, operating companies require more complex wells to be brought on line in a timely and efficient manner such that the production cost per barrel is minimised. The need to enhance the capabilities of existing Semi-Submersible units in order to meet these needs has never been greater. It is important that Operating Companies and Drilling Contractors align their respective goals both technically and commercially in order for both to remain competitive within their respective markets, and meet future needs.

This paper describes:*The change in the type of wells drilled in the North Sea over the past few years, and also the type of wells that will be required in the future.*The age and capabilities of the existing UK Sector semi-submersible fleet.*The change in market availability of semi-submersible units in the UK Sector.*Rig characteristics required for the enhancement of current and future performance.*The importance of Drilling Contractor and Operating Company goal alignment for performance improvements.*The upgrading of a third generation semi-submersible with (a) active heave compensation (b) 7500 psi circulating system (c) enhanced solids control capabilities, and the associated benefits plus rational behind these upgrades.*Annual savings in the region of $4,740,000 can be achieved with the above upgrades, depending on the daily operating costs.

Introduction

Over the past 10 years, there has been a dramatic change in the type of operations performed by semi-submersible drilling units in the North Sea, with a far greater focus on directional subsea development projects along with deep water and HPHT wells, as opposed to the conventional exploration and appraisal wells. There is an increasing trend towards bringing previous discoveries which were considered to be marginal from an economical prospective, on line by using new subsea technology coupled with reduced drilling costs through performance improvements. This change in activity coupled with a steady rise in the average age of the existing fleet of rigs forces us to ask the question, "How suitable are these existing units in terms of performance optimisation for the type of work that is currently being undertaken ?" More importantly as an industry we must be asking ourselves the question, "How can we improve performance?" A study was initiated into reviewing the potential performance improvements that could be achieved through the upgrading of an existing semi-submersible unit. These potential performance improvements were also viewed in light of the rig type and the work programme that was to be executed.

This paper includes a case study of the equipment upgrades planned for the Sedco 714, and also presents an example of how such upgrades, along with the selection of an appropriate unit, can satisfy the required performance improvements that the existing market place demands.

North Sea Activity

There has been a dramatic rise in Semi-submersible rig activity in the North Sea over the past 2 years, due to a number of factors including changes in technology, taxation and oil demand - see Figure 1.

Semi-submersible activity has shifted towards development drilling in the past few years - see figure 2.

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