The paper summarises the results of a study performed by Kvaerner Oil & Gas Field Development and Smedvig Asia Limited investigating the use of tender assisted drilling (TAD) for floating structures. The paper further discusses the general benefits and challenges associated with the use of tender assisted drilling for floating structures and how the CAPEX and OPEX of field developments can be significantly reduced. Results from a study investigating the use of tender assisted drilling with a wellhead TLP in a benign environment will be used for quantifying the benefits and to describe how the challenge can be solved for a specific development. Focus is put on the following :Overall Tender vessel and TLP configuration.Reduction in size of the TLP due to reduced payload requirement. A conceptual design of a TLP with conventional drilling and tender assisted drilling are presented.The challenge related to stationkeeping of two floating structures in proximity and how to connect them ensuring that the distance between them is controlled. Several different configurations are discussed and results from non-linear time domain analyses of the two connected vessels are presented.Safety and operational aspects related to tender assisted drilling are presented. Typical topics covered are drilling operation, installation of drilling and productionFinally cost and schedule estimates are presented. Introduction For any field development one always faces the question whether to use dry or subsea completed wells. There are many arguments to which dry completion offers the most efficient solution considering total capital and operating expenditure.easy and economical well interventionincreased recoverable reserves as a result of more frequent well stimulationsimple flow assurancesimple well control systems The main challenge for dry well completion lies in the geographical extents of the field and the fact that the reservoir must be drained from preferably one central position to avoid investing in several wellhead platform structures facilitating drilling equipment. For deep water development this challenge is even more apparent considering that an increased topside capacity has significant knock-on effects on the entire platform configuration. Therefore, many of today's deep water hydrocarbons reservoirs have been developed with a subsea solution and the associated operational problems and adverse effect on the overall recoverable reserves. A solution to the challenge is tender assisted drilling, which reduces the functional requirement to the wellhead platform and therefore makes the dry completion more competitive from a CAPEX point of view. Tender assisted drilling with self erecting rig capability is a proven technology which in shallow water areas such as Southeast Asia and West Africa has been in use for more than 25 years. Traditionally, the wellhead platforms for which tender assisted drilling are performed have been jacket structures. The benefits transferred to floating structures are expected to be even more pronounced. In deepwater developments dry completion units such as Tension Leg Wellhead Platforms (TLWP), SPARs or Deep Draft Floaters have been shown to be economically attractive.

The paper describes an FPDSO (Floating Production Drilling Storage and Offloading unit) equipped with a drilling plant and a near surface disconnectable drilling riser ("free-standing riser"). Drilling operation takes place through a dedicated moonpool, which is located just aft of the turret area. This allows a 100% clean split between the drilling and station keeping functions with minimum interfaces between two systems, which is a novelty for concepts based on turret moored drilling vessels.The relatively small internal turret makes use of taut leg mooring system and provides a number of slots for import/export risers as well as a swivel/drag chain transfer system. The mooring legs are configured in clusters, typically 120° apart. The vessel may change its translatory position by adjusting top chain sections using winches at the turret, while simple weathervaning or thruster systems perform heading control. The subsea wells are located in the "down wind" sector of the mooring leg clusters, whereas the import/export risers are laid out in the remaining sectors which are normally observed "upwind"The paper describes the operational procedures for the drilling (or well work over) from the vessel. Drilling/work over may take place under conditions when weathervaning is required; featuring the novelty that the drilling/work over riser may stay connected as long as the weathervaning takes place within approximately a 180° sector (+/-90° from prevailing heading). The weathervaning, naturally, has to take place about the axis, which runs through the well/centre and drilling riser. This means that the vessel has to change heading (weathervane) at the same time as it performs a translatory adjustment of its turret position. This is made possible through stepwise, predetermined manipulations of the chain section and some thruster control, if necessary. If weather forecast suggests heading beyond the 180° sector, a disconnection of the drilling riser system is planned for (upper portion onlythe rest remains free-standing in the water), and executed if necessary. After disconnect of the riser the vessel may rotate around the turret in normal FPSO manner and a full n x 360°w eathervaning capability is achieved. Drilling has in this situation been suspended, however, kill and choke lines and BOP controls may still be connected as these may be routed through the turret/swivel arrangement and connected to the top of the submerged free-standing drilling riser. This allows for continuos mud circulation and well control while disconnected.The paper also outlines the subsea wells and seabed arrangement, riser configuration, turret swivel solution, the mooring leg configuration, the drilling plant and the overall FPDSO layout. The potential of the proposed FPDSO design carried out by Kvaerner will be briefly discussed in the paper, like the main features of the FPDSO unit, the status of the technology, and the areas which may need further studies.