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Offshore Support Journal

Offshore Support Journal

Subsea installation ships are flexible, fuel-efficient and green

Mon 13 Nov 2017

Subsea installation ships are flexible, fuel-efficient and green
Maersk’s subsea support vessels are especially flexible units that can undertake many different types of work

Maersk Supply Service says flexibility was its watchword when designing its Stingray-class subsea support vessels, the first of which has now been delivered

 

When Maersk Supply Service ordered four advanced subsea support vessels three years ago, it could not have anticipated the very different market into which they would be delivered compared to the one that prevailed at the time.

Since the vessels were ordered – on speculation rather than against a contract – the oil price has fallen steeply, stabilised and begun to rise again, oil companies have made swingeing cutbacks but have begun to make money again and the offshore supply vessel market has been through what is probably the worst recession it has experienced in the last 30 years.

Despite the very different market that now exists, it can be argued that the very high level of flexibility built into Maersk Supply Service’s new vessels in anticipation of a high level of demand for all kinds of subsea work means that they are also well placed to do well in a downturn. They are, the company says, purpose built for the subsea market, but they were not built for one purpose and are purpose designed to undertake a wide range of work scopes. The aim, Maersk Supply Service says, is to be able to offer clients vessels that can be adapted to undertake project-specific operations and do so at an attractive price.

Maersk being Maersk, it worked with design and engineering company Marin Teknikk in Norway – which has designed dozens of subsea vessels – to develop a design that is much more than just a standard subsea support ship. The design that the company developed has an extremely high level of redundancy – it has one of only a very few examples of a particularly flexible and adaptable crane from Huisman in the Netherlands, a particularly high level of accommodation and a host of other features that could see it find work in the offshore oil and gas and renewables industries.

Add to that the ability to carry out flexlay operations using a 3,000-tonne capacity carousel and a lay tower, if required, or have a well intervention or top-hole drilling spread installed, and the in-built flexibility of the Stingray-class becomes apparent. Supporting saturation diving operations is also within the capability of the newbuilds, and the high hook height that the specialised crane selected by Maersk Supply Service provides puts the ships in the frame for decommissioning work too.   

COSCO Dalian Shipyard in China delivered the first of what will be a class of four Stingray-class MT 6027 subsea support vessels to Maersk Supply Service this autumn. Following on from this unit, Maersk Installer, another is due to be delivered by the end of 2017, with the third in Q1 2018 and the fourth in Q2.

Although the first three ships are as yet uncontracted, July 2017 saw Maersk Supply Service announce that it had secured a contract with Subtec SA de CV in Mexico, a subsidiary of Blue Marine Group, for the fourth ship in the class – a contract that will see it go to work in the Bay of Campeche, undertaking general support duties including inspection, maintenance and repair of platforms, in addition to well stimulation and enhancement work. This particular contract will commence in Q3 2018 with a firm period lasting until June 2020, and the company is quietly confident that, by then, negotiations around the other vessels in its new subsea fleet will have borne fruit.

Intended for a wide range of operations in deep and shallow water, including in challenging offshore environments, Maersk Supply Service’s new vessels mark its entry into the subsea vessel sector, although modified anchor handlers and platform supply vessels in its fleet have long been active – and highly successful – in this market segment. They combine an energy-efficient propulsion train and class 3 dynamic positioning, and the Huisman crane is a 400-tonne active heave compensated unit.

Complementing this level of capability are two work-class remotely operated vehicles capable of operating in up to 3,000 m of water and with the option of being deployed over the side or through an internal moonpool. The vessels also have a large free deck area (a total of 1,850 m2) with a large number of sea-fastening features for project cargo. The vessels have accommodation for 120 people, all in single cabins, and a secondary 100-tonne crane, supplied by SMST, provides full active heave compensation to a depth of 3,000 m and is capable of lowering a load of 93 tonnes to a depth of 1,000 m, 73 tonnes to 2,000 m and 53 tonnes to 3,000 m.

Huisman’s highly flexible rope luffing knuckleboom crane which has been specified for the newbuilds won an Innovation of the Year Award at the 2015 Annual OSJ Conference, Awards & Exhibition. Of a new hybrid design, it has a number of advantages compared with a conventional unit. Chief among these is the fact that the crane maintains the advantage of the knuckleboom functionality – providing a low suspension point, which is essential for offshore construction activity – without the disadvantages of conventional knuckleboom cranes, such as a heavy boom that affects a crane’s load curves and adversely affects stability.

Another important advantage of the active heave-compensated hybrid crane that appealed to Maersk Supply Service is that it is mainly electrically driven and very energy efficient. As a result, it does not experience the same kind of idling losses that a conventionally powered crane suffers from. Another potential benefit is that there are no hydraulics on deck and no possibility that the crane could cause unwanted environmental effects.

Energy efficiency was also a big driver in the design of the propulsion machinery, which Maersk Supply Service wanted to be as simple and reliable as possible. As a result of the very high level of redundancy built in to the machinery – which takes the form of a conventional diesel-electric arrangement with fixed-pitch propellers – the company says it sees little or no likelihood that the vessel will ever be off hire due to a mechanical failure. This is particularly important given that subsea vessels increasingly work in remote areas, far from service and support.

Six large thrusters provide the Stingray-class vessels with a high level of manoeuvrability, and although classed as dynamic positioning 3 (DP3) units with a four-split switchboard, the flexibility of the machinery enables Maersk Supply Service to operate the vessels as DP2 units, should the client require, or operate them in a particularly flexible, efficient and cost-effective mode with a single engine providing power in a closed bus-tie setup. Doing so reduces fuel consumption and emissions. It also enables a significant reduction in engine hours, with less wear and tear on the other engines, and expanded windows for maintenance.

The large deck described above has large numbers of tie-down points or sockets that, in most cases, will eliminate the need for hot work. Should hot work be required to ensure efficiency and flexibility for client operations, Maersk Supply Service has built-in cofferdams above all of the ship’s tanks in order to ensure that welding can be carried out with minimal adverse effect on coatings, hence minimising the additional cost to a charterer of reinstating these. 

Last but by no means least, the offshore vessel company also specified a high level of comfort class for the new ships, which it describes as almost cruise line standard – it says it wants clients to remember their time on the vessels in a good light – and has ensured that everyone on board has their own highly specified and finished cabin with onboard entertainment and excellent lighting. It even employed a cruise ship specialist to work on aspects of the design of the cabins, of which there are 120 in total.

MAERSK INSTALLER

Owner                 Maersk Supply Service

Builder                COSCO Dalian

Designer             Marin Teknikk

Length                137 m

Breadth              27 m

Crane                 400 tonnes (AHC)

Deck area          1,850 m²

Accommodation 120 persons

DP                      DP3

ROVs                 2 x work-class, 3,000 m

Moonpool 1       8.4 m x 8.4 m

Moonpool 2       4.8 m x 4.8 m

Helideck           CAP 437 certified

Class                +1A1 Subsea Support Vessel Ice Class 1C FS Clean design, LFPL, SPS