Three Hulls = One Ship

The U.K. Ministry of Defense (MOD) has been studying the use of the trimaran hullform for its Future Surface Combatant. As part of its research and risk reduction process an ocean going demonstrator Research Vessel (RV) Triton was procured by QinetiQ, the former U.K. Defence Evaluation and Research Agency. From October 2000 to March 2002 it was used in these risk reduction trials and is now available for hire as a general trials ship. Over the past seven years the U.K. Ministry of Defence (MOD) has explored in some detail the potential benefits of a trimaran.

Hydrodynamic and structures studies by QinetiQ together with design investigations by the U.K.

MOD Director of Naval Architecture and the Surface Ship Concepts Group at QinetiQ have suggested that, for a given requirement, the trimaran could offer: • Improved powering characteristics at high speed leading to a reduction in installed power; • Alternative ship layout options helping to optimise operational effectiveness and the potential to improve survivability; • The ability to adjust the hull configuration to cater for through life changes, which would significantly effect stability; • An upper deck layout and superstructure design that can be more aviation friendly; • The ability to fit larger weapons and sensors higher up with less penalties than for a monohull; and • Greater scope to accommodate alternative propulsion arrangements including independent propulsion in side hulls improving damage tolerance.

Many of these potential benefits will be explored by the design and build of RV Triton, the Trimaran Demonstrator and an extensive series of sea trials.

Trimaran Research The first work was carried out at the University College in London where the U.K. MOD sponsors a department, studying warship design. In the late 1980's as part of their MSc course, students were given the task of designing various trimaran warships. The results of this work were so encouraging that the Director of Operational Requirements within the U.K. MOD decided to fund research work at QinetiQ to confirm the advantages promised by the hullform and also to understand the design constraints imposed on the use of the hullform in warship design.

This work started in 1994 and initially concentrated on the hydrodynamic aspects of the hull design, which was carried out by running small-scale models of typically 26 ft. (8 m) in the tanks at the QinetiQ site at Haslar. The research was successful and the key parameters of main hull length to beam ratios and side hull length and position were identified. While this was progressing computer design tools were developed which predicted the resistance, propulsion, seakeeping and maneuvering characteristics of the trimaran warship.

Once the hydrodynamic design was set, our structural experts at Rosyth started to investigate the structural design requirements. This has been largely based on the development of computer codes in a series of numerical models. These have been used to predict loadings and stresses in different sea states, which have allowed the structural rules for the design of trimaran warships to be established. The final area of research covers survivability where any peculiarities of using a long slender hullform are being investigated through the use of scale model experiments and the development of computer based numerical models.

The results of this research have been most encouraging and to date nothing has been identified which negates any of our perceived advantages. However it is a very brave group that would commit to building a fleet of trimaran warships based on the results of small scale model testing. An interim step was needed and this is the Trimaran Demonstrator — RV Triton.

The Need for a Demonstrator It is unusual to build a prototype warship on this scale, but there is a great deal at stake. Running a fleet of trimaran warships would represent a Significant change from the more conventional monohull, both in terms of the design challenges and the operation of such ships.Perhaps the greatest concern is that of structural design. The critical issue is; exactly what design loads and associated safety factors should be applied?

While most other aspects of design and performance can be predicted adequately by mathematical or small scale physical modeling, structural synthesis relies on knowledge of the loads the ship is likely to encounter.

Warships are inherently lightweight structures to maximize payload in a small platform and to ensure high speed at minimum cost. A conservative approach to the structural design of a trimaran could severely penalize the form when compared to a mono-hull designed to the same requirement. To enable appropriate design criteria to be established an accurate knowledge of the loads which an ocean-going trimaran will experience is required.

Developing the Requirement During the early stages of the project it became clear that it would take about two years of sea trials to gather the structural load data required by the MOD. However, a financial commitment to purchase a vessel of this size with use for just two years was not viable. To justify the procurement of RV Triton, a business case was prepared showing that an acceptable return on investment could be provided by a program of various trials over a 15-year period. RV Triton therefore has two roles. First, as a research tool and subsequently as a trials vessel with flexibility to fit and trial many disparate weapons and commercial systems.

Given the need to produce "frigate like" structural data, the first question to be answered was "how big should the demonstrator be?" This question was put to DNV the classification society that was selected, and the U.K. MOD's structural experts at Rosyth. They were both asked to consider the time required to gather the data, the areas the ship would be tested in and the loads that had to be generated in order that a sensible extrapolation to design and extreme conditions could be achieved reliably. Both studies concluded, independently, that a vessel of between 295 and 361 ft. (90 and 110 m) should provide the data necessary to validate numerical and small-scale physical models. Since the main function of the ship is to be a "structural transducer" it is imperative that the structure is representative of a warship and as light as possible so that induced strains can be measured by the onboard instrumentation with the minimum of error. Obviously the need for the lightest structure could compromise the structural safety of this ocean-going ship. Studies were carried out to investigate the best way of ensuring structural safety. These showed that adopting a structural design code such as the DNV High Speed Light Craft rules, should result in a structure similar in style to a typical Royal Navy frigate. Safety of the vessel will, therefore, be demonstrated by adherence to appropriate Classification Society rules.

To ensure the "structural transducer" role is not compromised the contractor was required to obtain confirmation from DNV that the structural weight of RV Triton was minimized.

As well as providing a mechanism for a safe structural design the adoption of classification society standards was also consistent with the need for the vessel to be operated by QinetiQ using a contracted commercial crew. This approach meant that RV Tritonwould also need to meet the requirements of the Maritime and Coastguard Agency and be registered as a U.K. merchant vessel.

Trials Instrumentation System Clearly, a structural demonstrator is of little use unless the structural responses can be measured and recorded. RV Triton has been fitted with a significant instrumentation package feeding data into a central recording facility. The system records more than 300 channels covering a wide range of data from structural instruments and other sources such as machinery control, ship motion, steering gear, navigation and the environment.

The Trials Instrumentation System (TIS), as it is known, has been developed by the US Navy from the system installed in the SWATH T- AGOS 19 and will be fitted to RV Triton under an inter-governmental agreement.

Hydrodynamic Parameters A number of hydrodynamic studies were carried out at Haslar as part of the trimaran research program. These were used to generate a reference hull form and to define the required hydrodynamic parameters. During the early stages of the program it emerged that under certain stern quartering sea conditions and with the ship having a metacentric height of that expected of a trimaran warship, models exhibited unacceptable dynamic roll characteristics. With a shorter roll period, equivalent to that of a scaled mono-hull, this characteristic improved. Exploration of this phenomena during the trials of RV TRITON is a vital part of trimaran development. Tight control of the predicted roll period of the vessel was included in the specification.

The contract for the design and build of RV Triton was placed with Vosper Thornycroft in July 1998 as the result of a competitive tender process. The ship was launched in May 2000 and delivered to QinetiQ (then DERA) in August of the same year.

Hull Form The main hull is of round bilge form with underwater sections approaching semi-circularity amidships. A gentle rise of buttock lines aft leads to a counter-stern transom with minimal immersion. The sidehulls are of multichine design on the outboard face with a plane inboard face for ease of manufacture.

A parallel section is included extending above and below the waterline to avoid large changes in waterplane as displacement varies. Above this there is a flared section outboard and a haunch inboard to provide additional buoyancy as the opposite side hull emerges when heeling. Both hull and decks are longitudinally stiffened and supported by transverse frames at 1,500 mm spacing.

Frames are fabricated up to two-deck level while above this both frames and deck beams are flanged. The steel grade used has a yield stress of 265 Mpa that is similar to the main structural steel used on frigates.

Propulsion RV Triton is a diesel electric ship with both propulsive and ship service power provided by a pair of diesel generators.

Drive is provided by a single conventional shaftline in the main hull, plus a right angle drive thruster in each side hull. The main shaft is driven by a single 3.5MW AC electric motor through a reduction gearbox. Side hull thrusters are driven by 350 kW electric motors.

The prime movers are two Paxman 12VP185 powered 2MW diesel generators backed up by a 400 kW harbour set and an 80 kW emergency set.

A single fixed pitch propeller and rudder are conventionally arranged under the aft end of the main hull. The capability to fit a larger permanent magnet motor of up to 5 MW power has also been provided so that any related trials could contribute to the business case.

Sea Trials Summary Report Since delivery to QinetiQ in August 2000, RV Triton has been conducting sea trials for the UK Ministry of Defence and the US Department of Defense to evaluate the performance of the trimaran hullform in various operational scenarios and environmental conditions.

All the trials were carried out under a Memorandum of Understanding between the two governments. These trials completed in March 2002.

These trials were conducted in various sea states and at different speeds to ensure the ship encountered a diverse set of sea conditions. One of the perceived advantages of the trimaran hullform is the decrease in resistance and the lower power required to drive the ship when compared against the equivalent displacement monohull. In trials this has been proved as a reduction at high speed.

RV Triton has good directional stability due to its long centre hull and the sea trials have proved that she is far more comfortable in higher sea states than a monohull. Towing operations were conducted in one sea state higher than had previously been conducted for ships of RV Triton's size.

Structural Loading Although considerable research had been successfully carried out, the risk with trimarans was considered to be in determining the structural design of the ship particularly at the junction of the mainhull and sidehulls together with slamming loads on the underside of the cross-deck. This could only be determined by gathering structural strain and load data at ocean going scale. RV Triton conducted a number of specific structures trials where she encountered a wide variety of sea states from sea state 2 through to sea state 8.

The data gathered correlated well with the predicted computer model results with the anticipated results. The ship behaved extremely well in all trials the hull suffered no damage

Maritime Reporter Magazine, page 22,  Sep 2002

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