VESSEL REVIEW | MO10 & MO11 – UK's Mainprize Offshore adds crewboats to windfarm support fleet

UK operator Mainprize Offshore recently took delivery of two new crewboats in a series built by Strategic Marine of Singapore. Named MO10 and MO11, each boasts a prominent semi-SWATH catamaran hull design that combines enhanced seakeeping with reduced fuel consumption and emissions.

Stability of a SWATH coupled with improved weight control

Each crewboat has all-aluminium construction, a length of 26 metres (85 feet), a beam of 10 metres (32 feet), a draught of only three metres (9.8 feet) and capacity for 24 technicians and up to 30 tonnes of cargo on fore and aft decks with a total area of 140 square metres (1,500 square feet). The design was launched in December 2022 following development work that included gathering insights from various stakeholders.

“Primarily, the unique aspect of this vessel design is the implementation of a semi-SWATH hull design,” James Lewis, Head of Business Development – Europe at Strategic Marine, told Baird Maritime. “This sits somewhere between a conventional catamaran and a full SWATH design, therefore borrowing the favourable sea-keeping characteristics of a SWATH hull whilst avoiding the compromises that a full SWATH can suffer from, such as complex ride control and ballasting systems, deep draught, and weight sensitivity.

“By reaching this practical middle ground, the vessel can benefit from both cat and SWATH hull characteristics.”

The hull form has been extensively tank-tested using CFD. The result is a hull design that combines class-leading seakeeping that improves operational performance and reduces fuel consumption and emissions. The improved seakeeping ability also significantly reduces heave, pitch, and roll, allowing crew transfers to be safely conducted even in wave heights of two metres.

Two Caterpillar C32B 1,491kW (2,000hp) engines driving Servogear four-bladed, controllable-pitch propellers will deliver a speed of over 30 knots and a bollard push of 25 tonnes. The operator is able to harness the engines’ power across the rpm range by modifying the pitch of the propellers, thus ensuring suitability for high-speed applications as well as zero-speed applications.

Deck equipment layout guaranteeing increased versatility

"As always with vessel building and especially with fast aluminium craft, weight control is extremely important," added Lewis. "This is even more critical with a semi-SWATH design, which is more weight-sensitive than a conventional catamaran. The owner therefore needed a shipyard with experience and extremely high levels of project control and oversight, where weight is tracked forensically in order to meet design criteria.”

To aid in its offshore maintenance duties, each crewboat relies on deck equipment consisting of a Palfinger Marine knuckle boom crane and a 120kg anchor with winch. A fixed-boom slewing davit is meanwhile fitted to the bridge deck and has a manual winch system for recovering individuals from the water during man overboard situations.

“It is fairly uncommon for these crewboats to feature hydraulic crane foundations on both the fore and aft decks whereas crewboats typically operate with cranes on the foredeck,” said Lewis. “This is another example of the versatility of the design in anticipation of a varied and demanding operational life.”

Each crewboat also boasts an extensive electronics suite that includes a Phontech intercom, CCTV cameras, Furuno displays, compasses and radios, a TP-Link wifi router, and a Jotron EPIRB among others. The entertainment system consists of two LCD TV screens in the saloon with satellite connectivity.

The onboard facilities meanwhile include a mess, a fully functioning galley, crew cabins, toilets, showers, and a luggage compartment. All interior spaces are air-conditioned.

Design insights applicable to other vessel types

Lewis explained that lead times for equipment presented a challenge during the construction of the new crewboats.

“In some cases, we had to take matters into our own hands. For example, we recently ordered 100 engines in order to control this aspect of our project.”

The complexity of the vessels was also taken into consideration, as they are high-tech vessels with numerous components from the major machinery items such as the engines to other components such as navigation electronics, steering systems, HVAC, freshwater and blackwater systems, fuel transfer systems, and firefighting gear to name a few.

“Having a focused and dynamic project and procurement team who are able to control lead times for these things is essential,” Lewis told Baird Maritime. “Due to the measures we had put in place, we’re pleased to report the vessels were delivered on budget and on time.”

As SWATH and semi-SWATH vessels are still rare worldwide, work on the crewboats provided Strategic Marine’s construction teams the opportunity to gain experience on vessels of unconventional design.

“This continues our track record of building innovative and unconventional vessels,” Lewis explained. “We are now also building three 35-metre surface effect ship (SES) designs capable of top speeds of over 50 knots.”

MO10 and MO11 have already begun operating in Northern European waters. These are the tenth and eleventh vessels, respectively, to join Mainprize Offshore's current active fleet, and they are being utilised for the daily transits of technicians to and from offshore wind turbines.

“These vessels are part of a general fleet expansion program for the operator,” said Lewis. “They have already been awarded contracts forming part of the owner’s growth plans.”

Design work on the two crewboats was provided by UK-based naval architecture firm Walker Marine Design in compliance with Bureau Veritas class rules.