Adaptation of Fixed Platform Rig Instrumentation to Floating Vessels
Authors: Ruhe Robert W. , Griffin Phil W. 
Published: October 2008

Recent emphasis over pollution control coupled with the increasing use of floating vessels for oil and gas exploration in deeper water places even more responsibility on the drilling crews. Rig instrumentation has proved to be of tremendous value to the crews in the performance of this duty.

Almost all instrumentation designed for use on drilling rigs relates to a fixed derrick operation with the various items which are being measured or monitored being the variables. Floating vessels which respond to wave action add a unique set of problems to this existing instrumentation. The use of bumper subs necessary to maintain weight on bit, the use of riser pipe and slip joints to allow mud returns back to pits located on the vessel, and the roll, pitch, and yaw of the vessel itself all contribute to the problems encountered in the adaptation of fixed derrick instrumentation to floating vessels.

Most of the existing instrumentation can be adapted to floating vessels if the new problems are properly considered and understood. Certain instrumentation cannot be adapted in its present form and major redesign will be necessary. Nevertheless, the required variables can be measured and/ or monitored on floating vessels and the necessary modifications are within present capabilities.

INTRODUCTION

In August of 1969, the United States Department of the Interior issued revised orders concerning drilling and exploration operations on the outer continental shelf. These revised orders spelled out in specific terms those variables which would be continuously measured and monitored. These include the measurement of surface mud pit volumes to determine gains and losses, measurement of mud volume required to fill the hole while tripping, and comparing mud returns from the hole with mud pumped into the hole as measured at the mud pump discharge to determine gains or losses.

In addition to those variables specified in the Department of the Interior's orders, drilling engineers require additional knowledge of other parameters such as hook load or weight on bit, rotary speed, pump pressure, rate of penetration, rotary torque, etc. In the event of a kick, shut-in drill pipe pressure, circulating rate or pump strokes, circulating drill pipe pressure, and casing pressure are also required. When blowout preventers and chokes are installed on the ocean floor, these measurements are even more necessary and must be taken remotely. This requires more sophisticated and complex instrumentation where absolute reliability is all important.

None of these measurements or instrumentation is particularly complex on a fixed derrick or platform rig. But efforts to transfer this same instrumentation to an unstable floating vessel without first considering wave action and the resulting motion transferred to the vessel will produce data which are extremely difficult to interpret.

DESCRIPTION OF PROBLEM

A great deal has been written and reported in various industry publications and technical journals on the effects of wave action and water motion on the stability of floating vessels. Thus, it is not the intent in this paper to report on them in detail.