Generation Enhancement of Semisubmersible Drilling Units
Authors: Gavankar P.J. , Rammohan Vissa J. 
Published: October 2008

The deep water drilling boom of the recent years has led to some exciting challenges for the offshore drilling contractors. So far, the marketplace has not been mature enough to launch an extensive new-building program for semisubmersibles. Contractors have had to reach deep into their existing rig fleets to identify suitable candidates for deep water conversion. Second and third generation units have been undergoing extensive upgrades in an effort to meet the current day requirements for deep water drilling operations. This paper presents some basic considerations for such conversions addressing logistics, economics, schedule constraints, functional requirements and their interdependencies.

Introduction

After having gone through some tumultuous times over the 1980's and early 1990' s, the offshore drilling contractors have once again begun to enjoy the soaring day rates and the nearcapacity utilization of their offshore drilling fleets, particularly the units suitable for deep water drilling such as the column stabilized semisubmersibles. After the early 1980's, there has been a virtual freeze on any active new-build program for semisubmersibles. Rather than a strategy for future business environment, this was merely a reaction to the glut of semisubmersibles which caused serious damage to their day rates and hence to the drillers' bottom lines. This was not good news for the industry in general when the boom came back in the early 1990's and the operating companies had the drillers scrambling for deep water drilling units. The operators' needs were immediate while a newly built semi takes anywhere from 18 to 24 months to be available to spud a well. Additionally, although the day rates were going up, they were nowhere close to be able to justify the huge capital investment needed for a serious new-build program. This led to some creative thinking on the part of the drillers, both financially and schedule wise. Most drilling companies had some old rigs, dubbed second and third generation units (which were not generating any significant revenues) which would not be directly usable for deep water drilling but with some upgrade, had the potential of turning into cash machines again.

Candidates for conversion

Drilling contractors came to a rapid realization that not every second or third generation semi was a candidate for a successful conversion. Some units were more conducive to the conversion process than others and there were some which would be totally unsuitable. It was then a critical task to identify the candidates and rank them in terms of their adaptability to use as deep water units. Some of the major factors to be considered were:Age and general condition which would dictate the cost of renewal and life extension of various systems and structural elements which could work in deep water without replacement.Maintenance history and availability of as-built technical data without which engineering for conversion would be virtually an impossible task.Available free deck space for new equipment and systems. If new deck space has to be created, extensive structural and consequently stability modifications would be required leading to additional costs.