SWATH – Four Years of Successful Operations


In late 1994, Alan C. McClure Associates (ACMA) was challenged by Brazil’s Petrobras oil company with the task of designing a crewboat that could travel at an average speed of 25 knots through 3-meter seas with 25-knot winds for 4 hours and have less than 5 percent of the passengers experience any seasickness. The assignment included taking full responsibility for every facet of the design and analysis of this unique vessel…from concept through detail design.


In November 1998, ACMA completed its assignment, which included engineering and project management assistance throughout the building process, and delivered the 120-foot Small Waterplane Area, Twin Hull (SWATH) crewboat, STILLWATER RIVER, to Trico Marine Operators, Inc. The vessel was designed to transport up to 250 offshore workers in air-conditioned comfort with a highly reduced incidence of seasickness and to meet stringent interior noise criteria. Propulsion power was provided by twin 4,600-horse power Allison turbines with a fuel capacity which allowed for a 600-mile range, and there were accommodations on board for 6 crewmembers. Following a delivery tow to Macae, Brazil, the vessel went into service in January 1999 under a 5-year contract to Petrobras.


For the past four years, the STILLWATER RIVER has performed extremely well and has exceeded all of the performance requirements that Petrobras originally required in the crew transport service contract.


Adapting To Challenges


Initially, there were some problems with the fuel quality, which had the effect of shortening the life of the turbines. Consequently, the initial operating up-time of the vessel suffered to some degree. This challenge was addressed by using fuel additives and providing additional fuel treatment facilities for the fuel supplied to the vessel. Lessons learned also led to modifying engine maintenance procedures and eventually to the ability to tear down and remove a turbine without dry-docking the vessel, a procedure envisioned in the initial design of the vessel but not thought possible due to unforeseen differences in the turbine design late in the vessel design stage.


One serious incident occurred when the failure of the propulsion system led to the breaching of the engine room shell plating and, subsequently, the flooding of that space. The vessel rapidly took on a heel and trim, eventually reaching equilibrium heel and trim at approximately the same angles as predicted in the original damage stability analysis. The vessel was quickly brought to an acceptable heel and trim condition by counter ballasting and was able to return to port under its own power with no harm to the passengers or the crew.


Since that time, the vessel has been operating with a high degree of up-time and no further incidents of mechanical or other system failure. The vessel has performed transits at speeds of 27 knots in 4.0-meter seas with associated winds on numerous occasions. Passenger comments, from offshore workers to high-level oil company executives, have all been very complimentary, noting how smooth and quiet the ride is, especially when compared to helicopters.


A Proven Resource


Without a doubt, the STILLWATER RIVER has proven a SWATH-type crew transport vessel has a place in the offshore industry. In the immediate aftermath of the (unfortunate) P-36 accident, the STILLWATER RIVER provided rapid transport of equipment and material to the onsite response team. The vessel is capable of transporting up to 22 Mt of cargo (in lieu of passengers) at a sustained speed of more than 28 knots. And, cargo may be carried on the open 01 deck, allowing direct access to offloading cranes. In this case, the STILLWATER RIVER made several round trips from the shore base at Macae to the stricken P-36 before the P-36 ultimately was lost.


As drilling and production units have moved further offshore and their down-time has become more expensive, the SWATH has proven its ability to adapt. A SWATH crewboat can provide fresh crews while maintaining a consistent transit speed, regardless of the sea conditions. A SWATH can also rapidly supply needed materials for rigs that may be down or in peril. In the final analysis, when payload, speed and comfort are a concern, the economics of a SWATH make it an ideal choice when compared to its competition - helicopters and conventional planing vessels. With the operating success of the STILLWATER RIVER, the future of SWATH crewboats for critical-service, long-distance, offshore transportation is now a reality.



About Alan C. McClure Associates, Inc.

Headquartered in Houston, Texas, Alan C. McClure Associates, Inc. is one of the industry’s premier naval architecture and engineering firms, and has been providing a wide variety of design and engineering services to an international clientele for over 28 years. Projects include drilling rigs, floating production systems and support craft for the offshore petroleum industry. ACMA’s array of services also includes project management, legal/arbitration consulting, surveying and negotiations. The ACMA staff and services represent the engineering disciplines necessary to successfully complete projects in naval architecture, marine engineering, electrical engineering, mechanical engineering and engineering mechanics. For more information on ACMA, please visit their website at www.acma-inc.com