PATROL VESSELS Congress appropriated funds for the
design, build, and test of Sealion technology demonstrator in 2000, and an Integrated Product Team (IPT) was formed, bringing together the designer, the builder, and the end-user. Oregon Iron Works was selected to build Sealion based on its experience with similar craft, and with Alligator, and for its expertise in precision fabrication and ability to work successfully in a prototype and developmental environment. Construction of the craft began in October
of 2001 when the frames for the craft were cut using Numerically Controlled (NC) burn machines. The construction phase lasted 18 months, with builder and acceptance trials starting in December 2002. Acceptance testing was conducted on the US West Coast under the direction of the Government test team from the NSWC, and after acceptance, a demonstration was held for the US Navy at the acceptance test site, before the craft was moved to Cape Disappointment in Washington State for technical testing. On completion of these, the craft was delivered to the US East Coast and is now maintained and operated by the NWSC. The hullform chosen for Sealion was based on
a slightly larger version of Alligator, which had demonstrated superior seakeeping performance during testing. The hull structure was designed using 5000-series aluminium for plating and scantlings, and loads were derived using the Carderock division’s in-house methods for structural design of high-speed craft. The bottom structure was designed using empirical methods, including Heller-Jasper and Allen- Jones equations for predicting accelerations, with Hoggard-Jones method for equating acceleration to a quasi-static design load for sizing scantlings. The aft side structure, topside decks, and transom loads were derived as a function of hydrostatic pressure with a load factor. The forward side structure was based on hydrostatic pressure and hydrodynamic pressure. Internal tanks were designed to withstand hydrostatic pressure. The performance prediction of the craft was
based on the full scale testing of Alligator craft and on model testing conducted by Stevens Institute of Technology, in support of the Alligator project, and full-scale test data obtained by the CCD test team was compared against the predicted data as well as the model data, to develop correlation allowances for use on the larger variant. As Ms Grimsley Speirs explained, the
topsides design for Sealion was modified compared to Alligator, the reasoning behind this being threefold. Firstly, the operators wanted to combine crew and passenger seating into one space to facilitate mission planning as well as personnel comfort including space, climate control, and other habitability issues. Secondly, the operators desired an engineroom large enough to walk through, and the Alligator arrangement required crewmen to exit the crew space onto the weather deck and open a hatch to access the engines, which would not be optimum in bad weather. Thirdly, the US Navy also wanted to investigate modular payload capability, using an enclosed payload space. The propulsion plant selected for Sealion
consists of twin 12V183 TE 93 MTU diesels of 1136hp each and twin A40 series Kamewa
WARSHIP TECHNOLOGY MAY 2007
waterjets, a package which has worked well in the past on other US Navy platforms, providing a good balance of power to weight. The most significant challenge to the craft from
a systems and arrangement standpoint is the fact that the craft is a semi-submersible vehicle and, as a result, a significant portion of the internal volume – approximately 50% – is dedicated to ballast tanks, and this being the case, the selection of components that balanced reliability, capability versus size, and installed accessibility for maintenance was a challenge. ‘Many of the craft systems were actually located
within the ballast tanks, so provision had to be made to ensure these components were watertight as required. Additionally, the structural layout of bulkheads and scantlings was designed to ensure that ballast tanks could be properly vented to avoid air bubbles from getting trapped within the tank and reducing the attainable ballasted draft,’ Ms Grimsley Speirs explained. The overall approach to the construction
of Sealion was to build a quality prototype that could withstand hundreds of hours of high-speed operation in rough weather with minimal degradation or damage to the hull and equipment. Acceptance testing was conducted on the
craft in January of 2003, during which Sealion met or exceeded all requirements set forth by the IPT at the beginning of the programme, and was accepted by the US Navy. In addition, the craft was delivered under budget and ahead of schedule. After delivery, the craft underwent calm water trials, including speed-power and turning and manoeuvring tests, before it was transported to Cape Disappointment to undergo additional technical trials, all calm water trials having been completed successfully. During technical trials at Cape Disappointment,
the craft underwent rough water trials for the first time. The vessel was designed to be fully operational in a Sea State 3, and operate in a survival mode (maintain headway and steerage) in a Sea State 5, and during rough water trials, the craft was fully operational in SS5. Due to the fact that the sea state was consistently
above the design condition, the testing sequence was cut short to save funds, and rough water testing continued on the east coast in the fall of 2004. Follow-on testing in the spring of 2005 included side-by-side operations in rough water, with other high-speed US Navy assets to compare the performance of Sealion with other designs. Sealion’s motions were less than predicted
accelerations, and less than half the measured accelerations for a comparable planing hull, even though Sealion is able to maintain a higher speed in a higher sea state. These results have warranted further investigation of the craft’s performance in a wider range of sea conditions, both in full-scale trials and model testing at David Taylor Model Basin in Bethesda, Maryland, the results of which Ms Grimsley Speirs says will be published in future papers. After technical trials were concluded at Cape
Disappointment, the vessel was transported to Ft Monroe in Hampton, Virginia, the site of Carderock Division’s test facilities. The craft is currently home-based there for on-going operational experimentation technical evaluation, and Chesapeake Bay – with its adjoining major and minor rivers, coastal areas, and access to
the deep waters of the Atlantic Ocean – would provide a very broad array of test areas in which test team can perform a wide range of testing, experimentation, and exercises for littoral operations. Lessons learned from around 500 hours of craft
operation to date are being incorporated into the design of the second, slightly longer craft, Sealion II, and additional information was expected to be gained during technical testing in late 2006.
As Ms Grimsley Speirs also explained,
the success of the Sealion programme has resulted in several follow-on efforts:
• Modularity: this effort will investigate modular payloads for Sealion via the design, build, and test of a mission module. This module was not mission specific, but did investigate and demonstrate solutions for the technical challenges of a modular payload bay, including system interfaces such as hydraulic, fuel, sea water, air, and electrical . It invest igated and demonstrated structural boundaries and the transmission of forces from the craft to the module. Studies and limited demonstrations were also conducted in areas of the logistics chain, including transporting, storing, and interchanging modules to suit different mission requirements.
• Craft Integrated Electronics Suite (CIES): this was designed by Azimuth Inc and includes a hardware and software suite that offers small craft operators an integrated interface to all onboard systems. The system is designed in a modular fashion to enable tailoring to a variety of craft and missions. The system offers integration of: Craft Systems; Navigation/Charting; Sensor Systems; Intercom/Communications (voice and data); Video Acquisition; Situational Awareness; and a Gigabit Local Area Network (LAN). The CIES software is designed from the ground up as a distributed system to support full system functionality at each workstation, and provide truly integrated access to mission oriented data. The user interface is designed to operate with a variety of input devices and provide ease of use in rough environments, and the entire software suite is designed to accommodate the future integration of additional capabilities.
•
Interface with manned and unmanned systems: current and ongoing efforts are planned to interface Sealion with a variety of small manned and unmanned systems, including Unmanned Surface Vehicles (USV), Unmanned Aerial Vehicles (UAV), and Unmanned Underwater Vehicles (UUV).
Lessons learned and technology
advancements in the areas of weapons, sensors, and control have been incorporated into the follow-on craft, Sealion II, which was delivered towards the end of August 2006.
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