Abstract

A number of engineering and operational considerations are addressed during the planning stage of a deepwater operation. Most Deepwater operations occur in benign environments such as the Gulf of Mexico and West Africa. The subject well was drilled in the United Kingdom on the Western edge of the North Atlantic in an area of potentially severe metocean conditions. This paper summarizes the comprehensive analysis that was performed for a deepwater drilling operation utilizing a dynamically positioned (DP) drillship in this area. This analysis more accurately defined the guidelines required to operate the rig safely under anticipated Northern Atlantic sea state conditions.

Introduction

The subject well was located 134 miles west of Scotland in 6189' of water in the North Atlantic (See Figure 1) and is the water depth record for wells drilled in the United Kingdom. A dynamically positioned (DP), deepwater drillship was the best choice for this operation due to water depth and rig availability. Operating any deepwater well in a potentially hostile environment presents some unique challenges to both the rig contractor and operator. Utilizing a DP drillship under these circumstances added significant complexity to the situation and understanding the response of the rig and rig systems to its environment was critical to the success of the operation.

Conoco adopted a comprehensive analytical approach for defining the operating parameters for this rig on this location.

  • Metocean data was collected and compared to data on vessel motion characteristics to create a realistic model of vessel systems response for this specific location.

  • The riser system was designed based on a set of parameters agreed between the operator and contractor.

  • Key riser system response times were reviewed, and all differences in philosophy between operator and contractor were identified and resolved. Mechanical solutions to reduce system operational sequence times were then implemented.

  • Well Specific Operating Guidelines (WSOG) for rig and location were created jointly between the operator and contractor.

This paper will define and detail the approach that the company's engineering team utilized to identify and address the engineering and operational issues specific to the well. This paper provides representative results from these analyses, and does not provide quantitative details. The authors assume that the reader is familiar with analysis of risers, DP systems, and vessel motion.

Vessel Operations — Data for Environmental Conditions

The weather and sea states in this area can be severe. In order to analyze the conditions in this location accurately, a specific model of the block's metocean condition was created. Current data was collected on the location over a yearlong period using a data collection buoy deployed in 1998–1999. Wind and wave data from a Hindcast forecast for the immediate area was obtained from the Ness Extension Hindcast (NEXT) database. All data was compiled into an environmental model of the location covering a typical yearlong period.

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