SWATH Technology for Subsea System Intervention
Authors: Maged Kassem 
Published: October 2008

Small water-plane area twin hull (SWATH) technology is being used for seagoing vessels in the marine construction industry to provide a stable working deck in strong sea and weather conditions. Deck area can be used to support concurrent offshore operations, from saturation diving and remotely operated vehicle (ROV) deployment to installation of smalldiameter (4-inch) pipelines for oil and gas production. A SWATH vessel also transits at a competitive and economic speed in a heavy sea state. For these reasons, SWATH technology is increasingly targeted for deepwater and remote subsea system intervention. The Global Pioneer multi-service vessel is the only semi-submersible SWATH vessel presently marketed for offshore oil field construction. However, the process of re-supplying the Pioneer for deepwater development has been limited to the transit ability of monohull supply vessels. Re-supply by a monohull vessel is generally limited to favorable weather conditions, thereby limiting the work program of the SWATH vessel during winter months.

A three-month reconfiguration of Pioneer was completed in August 2000 to increase the vessel's deck load capacity for supplies and limit the need for re-supply. Certain bow and stern modifications were implemented to reduce drag on the vessel and improve transit speed; and an auxiliary generator was added to the vessel to enhance its continued, independent operation in the deepwater markets of the Gulf of Mexico.

Introduction

Original design for the Global Pioneer specified a 150-ton deck load capacity, limiting the vessel to installation of smalldiameter pipelines no longer than 1.89 miles. In general, the design of a SWATH vessel is similar to that of a semisubmersible drilling rig. A steel frame comprising the work platform is built on columns with two submerged pontoons. The twin-pontoon structure (lower hull) of the Pioneer contains tanks for ballast, fresh water, diesel fuel, and marine systems. It also supports a system of four thrusters controlled by an ASK 4002 dynamic positioning (DP) system. Under its original construction, the Pioneer succeeded in providing a stable work platform with little roll and pitch but could not overcome its dependency on weather-sensitive supply vessels.

Each pontoon of the Pioneer is connected to the upper hull by two watertight columns. The upper hull contains a buoyant inner bottom that extends to the bulkhead deck (2nd deck) and is watertight up to the main deck. The upper deck contains the largest portion of marine and industrial systems.

At maximum load capacity, 150 tons, the Pioneer could not achieve its optimum specified speed of 10 to 12 knots. It was determined that a reconfiguration of the lowers hulls, columns, bow, and stern of the Pioneer would improve the vessel's payload, speed, and performance for deepwater construction.

Technical Specifications

Global Industries, Ltd. constructed the Pioneer SWATH vessel in 1995 based on BSM Joint Venture SWATH technology (Table 1). The Pioneer features a modular design, with a wide deck area and 12-foot deep pontoons for navigation through small and shallow waterways