ABSTRACT

This paper describes the design and operating experience of the propulsion system of the first self-propelled classed semi-submersible drilling rig SEDCO-702. The design parameters are presented for a propulsion operation. The important fabrication details are outlined and the sea trials described. Model studies, mooring analyses and dynamic stationing simulation studies are discussed. The performance and operating experience of the sea trials are reported. The certification, manning and licensing of this first U. S. Flag self-propelled semi-submersible are presented.

INTRODUCTION

There have been a number of offshore drilling units constructed in the past that were capable of self-propulsion but the majority of these units were drill ships. The SEDCO-702, shown on Figure I, is one among many semi-submersible units presently operating or under construction with a self-propelled classification. The primary purpose of this paper is to briefly outline the advantages realized by adding propulsion to a semi-submersible and to compare sea trial results with predictions regarding maneuverability for the SEDCO 700 type units.

PROPULSION ADVANTAGES

Propulsion adds a new dimension to the semi submersible type rig with increased mobility only being one of many advantages. Increased safety, mooring assistance, and better power utilization are other features which narrow the competitive gap which once existed between the drill ships and the semi-submersible.

MOBILITY

A semi-submersible type rig offers many operating advantages compared to other type of drilling units. Until recently, transit speed was a major disadvantage of the semi-submersible. Traditionally, "semis" have been towed as a barge with the average speed being 2.5 knots. Consequently; mobilization over long distances with this type of unit was expensive compared to a drill ship which can move at an average speed of 10 - 12 knots. Today a semi equipped with propulsion and assisted by a tug can easily move at 10 knots. Without the assistance of a tug, these drilling units can transit at 7 knots. A typical mobilization to the North Sea from the Gulf of Mexico is 20 days for a ship and 60 days for a towed semi-submersible which has no propulsion. Propulsion on a semi reduces the time to about 26 days when assisted by a tug.

The propulsion units on the semi-submersible are of two types:

  1. in-line main propulsion and

  2. animating thrusters. These two types are illustrated in Figures 2, 3 and 4. Inline propulsion offers all of the earlier mentioned advantages except mooring assistance. Animating thrusters provide mooring assistance through the capability of being oriented into maximum environmental forces. Consequently, for deep water applications, SEDCO operators has elected to select units with animating thrusters.

MOORING ASSISTANCE

As previously mentioned, utilization of propulsion (thruster type) provides mooring assistance. Some storms generate enough force to yield mooring line tensions of 500,000 pounds or greater. It is during these situations that azimuthing thrusters can be baised to reduce tension to safe working levels. Figure 5 illustrates a "mooring assist" application for thrusters. Figure 6 graphically illustrates the mooring assistance for a SEDCO 700 type semi-submersible when subjected to various wind loads.

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