Tender-Assisted Drilling In The North Sea
Authors: Christiansen P.E. , Cuvillier Gerard , Hicks N.A. 
Published: October 2008

ABSTRACT
Fields with relatively few wells or imposed restrictions on topsides weight can benefit significantly from Tender Assisted Drilling (ZAD) mode of operation. In particular, marginal or satellite field developments can benefit economically by the use of the TAD concept for Platform drilling.
This paper will focus on the use of TAD in harsh environment conditions such as the North Sea, and will review the technical aspects of conversion of existing semisubmersible drilling units to tender assist role to achieve the required performance in terms of operability and safety.
To illustrate the various aspects of TAD mode of operation, reference will be made to the development of Total Oil Marine's Dunbar Field where the Sedco 706 semisubmersible is selected as the Tender Support Vessel (TSV).

INTRODUCTION
Current depressed day rates for drilling semisubmersibles caused by a downturn in exploration drilling activity has created a unique opportunity for low cost development of marginal fields. Most obvious is the conversion of a drilling semisubmersible rig to a floating production facility. However, less obvious but frequently more economical is the use of the vessel for a TAD mode of operation in combination with a fixed platform development.
Making use of the drilling vessel's accommodation, power generation, mud pumping, cleaning and storage facilities etc. can reduce the Platform topsides weight by up to 3000 tonne [1], which in turn may make a Ml installed jacket feasible saving an additional 2500–3500 tonne of jacket steel [2] [3]. Such weight and associated cost savings may comfortably offset the extra cost of conversion of the drilling rig for TAD and vessel day rates during the drilling campaigns illustrated in Figure 1.
Additionally the use of TAD with lead to reduced future operating costs as less Platform maintenance work is required. The reduced Platform accommodation capacity possible with TAD may also be a potential major operating cost saving and is definitely a significant contribution to increased Platform safety.
Whereas TAD has long been common place in calm seas, it was not until the 80's that it emerged as a technique valid for harsh environment conditions like those prevailing in the North Sea, where two types of tenders may be considered:A jack-up operation, also referred to as "skid-off" mode.A semi-submersible vessel.
The current differential in dayrates between jack-up rigs and semisubmersible rigs in combination with the faster mobilisation and demobilisation possible with a semisubmersible favour the latter.
This paper will describe the use of a semisubmersible for TAD operation in harsh environment such as the North Sea. To illustrate the particular problems and possible solutions the paper will make reference to the Dunbar Field development.
The Dunbar Field is located approximately 22 km South of the North Alwyn Field in the northern part of the UK North Sea. The Dunbar development comprises 2 separate hydrocarbon accumulations: The Dunbar Field, which is developed with a freed Platform, and the subsea developed Ellen Field located approximately 9 km from Dunbar and connected with subsea flowlines to the Dunbar Platform.