Core Drilling Using Coiled Tubing From a Riserless Light-Well-Intervention Vessel
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 179086, “Successful Core-Drilling Project Using Coiled Tubing From a Riserless Light-Well-Intervention Vessel in a Norwegian Fjord,” by Susana Rojas, SPE, and Michael Taggart, SPE, Baker Hughes, and Per Buset, Island Offshore, prepared for the 2016 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, Houston, 22–23 March. The paper has not been peer reviewed.
Coiled-tubing technology is commonly used in well servicing to deliver effective solutions in the oil and gas industry. However, coiled tubing is extremely versatile and may be used to deliver cost-effective solutions in many applications outside of well intervention. This paper details the planning, design, and execution of a project involving the core drilling of three wellbores using coiled-tubing directional drilling along the future Rogfast tunnel route on the Norwegian coast. The project obtained core samples to verify rock quality before tunnel drilling.
Introduction
The Norwegian government is enhancing the connectivity of its road network by building a road connecting Stavanger to Bergen. This route includes a 25-km tunnel along the future Rogfast route on the Norwegian coast. To make tunneling operations more efficient, verification of rock quality ahead of tunnel construction eliminates the requirement of drilling pilot holes ahead of the main tunneling machinery. Eliminating the pilot-hole requirement optimizes the tunnel route and significantly reduces tunnel construction time. Therefore, coiled tubing was deployed from a light-well- intervention vessel and drilling and coring operations were performed along the route, enabling rock samples to be collected before tunnel construction. This required surface and subsea coiled-tubing injectors that could be controlled simultaneously and independently. The system is deployed from a light-well-intervention vessel without the use of a riser.
Objectives
The subsea road tunnel will be the world’s longest and deepest tunnel; its deepest point is approximately 385 m beneath sea level. It shortens the travel time between Stavanger and Norway’s second largest city, Bergen. The project required the drilling of four pilot holes along the route before drilling the tunnel, to obtain core samples and verify rock quality.
The project required several new technologies and techniques to enable the collection of the rock samples:
The world’s first coiled-tubing operation simultaneously and independently controlling surface and subsea injectors, deployed from a riserless light-well-intervention vessel
Deployment and application of a new subsea connector and guide base
Deployment of a 30-m, 5?-in.- outer-diameter bottomhole directional-drilling assembly
Drilling and core sampling of a 248-m borehole, with a maximum deviation of 55°
Project-Specific Equipment
During the planning stage, vessel modifications were identified that included an intervention tower installed over the moonpool.
An injector parking stand supported the subsea coiled-tubing injector during the surface rig-up phase. The frame was designed with walkways allowing safe access to the injector heads during maintenance.