The CSS: a new wave in the offshore vessel market

The compact semi-submersible (CSS) concept commenced life in 2005, and now, nine years later, the first unit has been delivered and was the winner of the Support Vessel of the Year Award at the 2014 Annual Offshore Support Journal Conference and Awards.

The concept started at the Offshore Technology Conference in Houston, where the general manager of Doha Marine Services (Qatar), Robin Reeves, stopped by the STX Marine booth to discuss weather downtime and how it could be reduced. On the back of a scrap piece of paper, an idea was sketched out for a new concept that would be stable in rough weather, could maintain position without anchors and would be economical to build, own and operate.

STX Marine began developing general arrangements for a variety of CSS options, which were introduced to the market as they became available. The potential end users provided valuable comments and suggestions, which prompted the rearrangement of the layout on numerous occasions. There was a huge wish list of capabilities, features and requirements, but no money, and the CSS concept as it was then only moved forward in 2007 after Mr Reeves left the Middle East and started a new offshore marine company, MAC Offshore.

Everyone with in-depth knowledge of offshore working conditions recognised the opportunity that the CSS offered, but that didn’t initially mean they bought into the concept. There were a lot of negative comments surrounding the design, and it would have been easy to drop the idea, but experienced offshore operators who saw the benefits it had to offer encouraged them to see it through.

A few key players early on were Captain Mike Mead of M3 Marine, who invested personally in the design, as well as DVB Bank, who provided MAC Offshore loan investments when other international banks would not entertain the thought. Hallin Marine also invested in the design and will take ownership of the second CSS, Derwent, later in 2014.

When the specification and general arrangement were completed, MAC Offshore sought a yard to build the first vessel, and Fujian Mawei Shipbuilding came forward with an offer. The design was by no means complete in April 2010 when the first contract was signed, but MAC Offshore’s belief in STX Marine coupled with Mawei’s support gave them the confidence to progress.

The CSS design developed by STX Marine in co-operation with MAC Offshore and the other CSS design partners integrates the motion benefits of a semi-submersible into a compact mobile platform to ensure low day-charter rates and an industry-leading design in terms of operability, time on station and operational flexibility.

With the objective of creating a platform with superior deadweight, deck cargo area and capacities to comparable designs, with dynamic positioning to DP3 level and flexible mission fit, the STX Marine engineering team and its partners produced a vessel of 84m in length with a single full-length strut per side that is more capable, more configurable and much more cost-effective than its closest competitors.

The CSS provides a configurable platform that can be adapted, and easily re-adapted, to changing customer needs and that outperforms more expensive purpose-built ships in operability and time on station. The wide deck spanning the twin hulls permits versatile use of deck space and a wide variety of options for unit configuration. The CSS also provides an open, stable platform for many applications, allowing owners to quickly reconfigure at little cost based on different charter requirements. The structural configuration of the unit is also optimised for ‘producibility’, making the CSS both rapid and easy to build as well as easy to refit and maintain.

From the outset, achieving the right balance between deadweight and seakeeping was a key factor in selecting the principal dimensions of the CSS and, in particular, the size of the lower pontoon and the length/width of the struts. Extensive resistance and seakeeping model tests were conducted to assess hull resistance, structural loads, unit operability, ship motion characteristics and slamming loads. The resulting platform offers the ability to undertake a wide variety of construction, well intervention, dive support and accommodation roles, all with unrestricted worldwide operations.

STX Marine says the CSS has substantially reduced rolling motions and only a slight increase in pitch and heave motions when compared to a monohull design with equivalent capabilities. Operability studies for the CSS accommodation variant, stationed in the Campos Basin off Brazil, indicate operability ratings that are comparable to a much larger 200m monohull. The accommodation unit can remain connected to a rig by gangway 99 per cent of the time, suggesting that only the most extreme weather events will require the CSS to detach from the rig.

The DP3 capability of the CSS, which can sustain damage and lose one thruster due to a worst-case fire or flood or equipment failure, can still remain connected 89 per cent of the time. The CSS can therefore maintain service and continue to meet charter under nearly all foreseeable conditions, while remaining efficient and effective both in capital and operational costs.

One of the greatest challenges in the CSS project was to design a hull structure that could withstand the prying loads from the multihull configuration and also have an adequate fatigue life based on unrestricted worldwide operations. A comprehensive scope of computational fluid dynamics analysis in combination with the model test load data and other analytical ship motion methods provided the excellent basis on which the structure was developed.

Fundamentally, the structure was designed to be relatively simple with large flat panels and large radius corners to limit high stress concentration areas while maintaining an acceptable steel weight. The critical area that received the greatest attention was the upper haunch, which connected the inner face of the vertical strut to the underside of the main transverse wet deck/hull structure. This location generated the highest structural loads and also required the greatest attention to structural details to ensure the fatigue requirements were met. The use of local insert plates in the shell and optimised framing helped alleviate stress concentrations and extend the fatigue life of the particular structural details.

Specific analysis was performed to ascertain the wave impact loads on the underside of the wet deck, as this is a complex scenario with limited previous experimental or analytical data available on which to compare results. The design has now received full ABS approval with unrestricted service notation based on draught and wave height limitations.

The majority of the machinery and systems on the CSS designs are common to most diesel-electric vessels and have been configured to suit the DP3 designation. Six diesel generators, three in the upper section of each hull, provide power for the hotel and main deck equipment loads as well as the four azimuth thrusters rated at 2,500kW each and located at the end of each pontoon. The azimuth thrusters are capable of being removed without a drydock by soft patch above the pontoons while trimming/heeling the vessel. New CSS designs are also exploring electric podded thrusters that will be underwater demountable while at sea. The power and propulsion plant is designed to compensate for a failure, fire or flood in any one machinery space, and if a complete engineroom space is rendered inoperable, the power plant is still capable of providing DP capabilities but at a reduced capacity.

Machinery cooling on the vessel variants has been accomplished using a number of different cooling systems. Machinery cooling can be configured using box coolers, Sperre pleat or rack coolers or central coolers depending on the environmental conditions and the owner’s preference. Throughout the variants, there are also different levels of fire-fighting capability on board with a deck integrated fire-fighting system for the helideck as well as coverage by fire monitors for the vessel’s decks and external fire-fighting. In addition, the domestic water and treatment systems have proven to be easily scalable up to 500 passengers and beyond. The scalability of the various systems on board has proven that the vessel type is extremely flexible.

Most of the CSS designs have large-capacity cranes that require the ship to be ballasted to its wet deck (deck between the legs) to maximise the stability for heavy lifts. As such, the vessel is fitted with large ballast tanks and a comprehensive ballast system to move great volumes of water quickly, reducing the overall time to perform the lift. The vessels were designed to be operable throughout the world and have been fitted with various ballast treatment options to suit the mission and client preferences.

Safety on the vessel is paramount with hydrogen sulphide (H2S)-sensing ventilation systems that will automatically shut down the ventilation to prevent the spread of the poisonous gas to allow the vessel time to escape. Most of the variants are also fitted with large state-of-the-art hospitals with an operating room and multiple wards that are accessible from the main elevator that extends to the helideck. The lifesaving equipment on board is industry leading with 100 per cent lifesaving capacity with any one section of the vessel lost, in a combination of gravity and free-fall lifeboats. They are, in fact, using the largest enclosed lifeboats in the world.

The primary function of the first CSS delivered, Olympia, is to act as a floating accommodation unit for rig personnel. The vessel is fully outfitted with all accommodation, hotel and entertainment services to provide a comfortable and functional environment for the rig workers during their rest time. The accommodator concept is intended to remain connected to the rig via a telescopic gangway system and is capable of accommodating up to 431 people. Other features include a 12.8-tonne rated helideck, DP3 rating and a 150-tonne telescopic boom crane.

The CSS concept has drawn widespread and increasing interest from industry, leading to orders for eight units, four different variants and immediate interest in both repeat units and alternative concepts. The variants currently under construction include a light well intervention unit, two accommodation units and a flexible light well intervention/construction unit with four-point mooring dubbed ‘fit for purpose’. The following paragraphs describe the variants in more detail.

The second CSS, Derwent, a light well stimulation vessel for Hallin Marine, a Superior Energy Services company, was delivered earlier this year. The primary function of this CSS variant is well intervention, well stimulation and subsea installations, which are performed using the same hullform concept as the accommodation variant. The key features of the vessel include a MacGregor 200-tonne module-handling system, 150-tonne knuckleboom crane, two remotely operated vehicle (ROV) launch and recovery systems, a moonpool, a helipad that accommodates 12.8-tonne helicopters, DP3 rating and accommodation for 152 people. This vessel shows the versatility of the concept.

The fit-for-purpose CSS is a multifunction well intervention, supply and light construction vessel. The primary features of the vessel are a 150-tonne lattice boom crane, a telescopic heave-compensated gangway, a moonpool, a 12.8-tonne rated helideck, DP2 rating and accommodation for 199 people. The vessel is equipped to operate as a thruster-assisted four-point moored vessel significantly reducing refuelling requirements. It can also operate in DP2 mode without the use of the mooring system.

The CSS maintainer vessel’s primary function is to act as a floating maintenance support unit for offshore platforms. This version of the design will remain connected to the rig via a telescopic gangway system and is capable of accommodating up to 350 people. Other features include a 12.8-tonne rated helideck, DP3 rating and a 60-tonne knuckleboom crane. Further variants of this platform include a setup and tank configuration for well stimulation equipment, a moonpool and subsea ROV launch and recovery capabilities.

The Next Generation CSS is a concept that grew from industry demand for a highly versatile platform based on the current STX CSS hullform with increased accommodation and cargo-carrying capacities. Designed around the most current ABS and IMO regulations, the result is a DP3-capable platform that can accommodate 300 personnel in fully ILO Maritime Labour Convention (MLC) 2006-compliant single cabins. An enclosed workshop leading directly off the 2,000m2 clear working deck space with a capacity of 5 tonnes/m2, a 9m x 8m moonpool and a versatile, customisable crane configuration lead to an extremely multifunctional platform capable of supporting construction, maintenance, rig support, well intervention and many additional offshore operations.

Work on the Next Generation CSS is still underway, with planned completion of a fully validated design expected by the end of 2014.

Several versions of innovative design under construction

Marine Assets Corporation’s first CSS, Olympia, won OSJ’s Support Vessel of the Year award at the Annual OSJ Conference and Awards on 19 February.

Olympia is a DP3, 84m compact semi-submersible that will operate as a flotel, but as highlighted elsewhere in this article, a number of different versions of the CSS have been developed.

Construction of the various versions of the CSS continues, with the second unit in the series, Belait, a well intervention vessel, undocked at the end of 2013 and due to enter service shortly. Another accommodation vessel version of the CSS design, Venus, was launched in May 2014.

As of mid-June, seven CSS units had been ordered, and MAC continued work with STX Vancouver on the design of a CSS Mk II, which will have more beds and increased crane capacity. OSJ