Catamarans, an underrated offshore solution, will eventually dominate
Catamarans need to be seriously considered for offshore operations as they will provide more working deck space, greater deck loads, more accommodation and superior habitability than a mono-hull and for less cost.
They have proved that they can launch and recover underwater vehicles in significantly higher sea states than mono hulls.
Comparably, to provide the same deck space, same deck capacity and accommodation as a mono hull, the capital cost of the catamaran will be 2/3rds that of the equivalent mono hull and fuel consumption on station will be half.
Consider the 50m option designed as an offshore support vessel.
The important features developed by the catamaran concept is 425m2 ( 4570ft2) of useable deck area, 300 tonne deck capacity with practically no restrictions on placement of load, and accommodation for 53 in secure comfortable cabins.
Average fuel consumption, including provision for 8 days transit is approximately 5 tonnes/day.
Capability operating ROVs and Diving Bells
A startling lesson that was leant with the DP catamaran, SeaWorker, was that, despite its relatively small size, a working class ROV could be launched and recovered in the Cook Strait- that notorious stretch of water between the North and South Islands of New Zealand- in a sea state higher than any mono hull had been able to achieve. This was because a mono hull tends to launch and recover either through a moon pool which experiences pistoning of the sea within the pool or over the side with problems arising from roll and wave action. Vehicles launched from A frames at the transom of mono hulls experience high pitch accelerations because of the large distance from the centre of gravity.
The moon pool provided for SeaWorker through the tunnel structure is on the centreline aft of amidships and does not suffer pistoning nor is it influenced by rolling or pitching motion.
Further lessons learnt through the use of SeaWorker were the facts that height of deck load was immaterial, ie the load could have been placed on the flight deck, and that high lift cranes could be distributed anywhere around the periphery of the working deck increasing the versatility of its layout.
Experience has shown that modest sized catamarans can continue to operate in seas up to sea state 6.
The hulls being long and slender are suitable for efficient propulsion at speeds up to 20 knots. The hulls must be designed to minimise lateral as well as fore and aft resistance whilst hovered on station which results in specific solutions.
The bow and transom shapes must be designed not only to reduce ahead resistance but must also be designed to reduce rotational resistance, ie yaw, to permit the vessel to rotate a full 360 degrees in under 200seconds. This rotational speed is also a function of thruster capacity, but minimising the resistance minimises the sizes of the thrusters and reduces on-station and capital costs. These features, if designed correctly, will increase pitch damping and reduce pitch amplitude.
Habitability and Overall Hull Form
To be successful in habitability terms it is important that beam is moderated otherwise unacceptably high roll accelerations are introduced. A moderate beam together with fine hulls leads to reduced pitching motion and acceptable rolling motion and our experience, which is now more than 10 years, is that this combination has proven to be highly successful. JHCL developed a motion prediction program, QSDM, for predicting catamaran motions which has been verified with physical tests and has shown that catamarans can meet the habitability standards of OHSAS 18001. Our research has shown that catamarans can provide a hovered platform where the crew will experience less overall motion than a mono hull.
Accelerations, giving rise to nausea, are a function of the distance of the observer from the centre of gravity of the vessel. In general the height of catamarans is less than mono hulls, length is comparable, but beam will be greater. Accelerations arising from pitch and heave can be shown to be less because of the finer hulls and lower water plane area.
Yaw accelerations are controllable and are not a significant factor but the control of yaw does have a significant impact on rolling motion.
Rolling of catamarans can be reduced by specifically designed dynamic positioning systems that reduce motion arising from oblique waves which result in roll reduction by reducing the RMS yaw amplitude. The catamaran plan form with large transverse distances between thrusters is particularly effective at controlling yaw, but the dynamic positioning software must be a generic catamaran solution such as SeaTamer® and not one adapted from a mono-hull solution.
The layout of a modern DP catamaran permits accommodation closer to the centre of gravity (the centre of motion) both vertically and longitudinally. The lower positioning of accommodation and working spaces possible with the catamaran reduces the transverse component of roll acceleration. This leaves the vertical component which reduces to the heave amplitude of an individual hull arising from an oblique wave system. However this can be largely eliminated with a DP system that can adequately control yaw.
Therefore, provided attention is applied to the acceleration environment, acceptable habitability can be achieved.
Habitability and Noise
A proposal made to the Royal Singaporean Navy for a submarine support vessel included the use of variable AC drives for the propulsion motors which meant that all the motor rooms in the hulls were essentially quiet with a lowering of the noise levels experienced by the crew.
Plant rooms were located at working deck level in the superstructure away from accommodation and control spaces. The sites of plant rooms (two of them were required) permit the optional use of air cooled engines which in turn eliminates the installation of a raw water cooling system. A repair by replacement policy was possible as the main engines, four per side, could be directly railed to the working deck for onward transport to a specialist maintainer.
Experience has shown that retractable azimuth bow thrusters have some drawbacks. However to meet high thrust demands at the bow to control yaw retractable tunnel thrusters are essential, as their greater immersion permits greater thrusts and careful relative angling between the port and starboard tunnels overcomes interference problems.
The further advantage of the tunnel thruster solution is the fact that thruster noise is screened from the vessels underwater transducers.
Non retractable azimuth stern thrusters are essential as they exploit the remarkable facility of a DP catamaran to hold station. It may not be obvious, but two azimuth stern thrusters operating alone can maintain the position and heading of a catamaran because of the transverse separation between them which provides a rotational component as well as a vectored component. Bow thrusters increase performance capability and introduce a high degree of redundancy as the catamaran can fall back onto just two of any thruster combination as long as one of these is an azimuth thruster.
A Diversity of Catamaran Solutions
Designs have been completed for a DP catamaran measuring under 45m and under 500GRT and have been designed with twin moon pools to permit the continuous presence of a dived vehicle or bell.
High transit speed, economic because of the slender hulls, can be provided with alternate conventional propulsion alongside the stern thrusters which permits optimisation of the thrusters for the DP role.
The diverse prospect of catamarans in the offshore role will result in them becoming dominant for offshore support operations, surveying and inspection whether by divers or ROVs.
80m Offshore DP Catamaran- 1100m2 of working area, deck capacity 1250 tonnes, Super Puma helicopter, accommodation for 150, twin moon-pools, 30m Sea Crane and stern H Frame.