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The constant demand by operators for increased heavy lifting capacity along with the requirement for operations in greater water depths and remote locations is seeing the offshore construction vessel sector innovate like never before.
The range of concepts and designs on the drawing boards of marine construction contractors has always been vast, but the number of multifaceted vessels that have recently entered or are due on the market soon reflects the increasingly flexible approach they are adopting.
One is based on a concept that first came to light back in 1987 but is only now starting to come to fruition – the Allseas Group's giant newbuild vessel Pieter Schelte, currently under construction at Daewoo Shipbuilding and Marine Engineering in South Korea. The vessel is designed for carrying out single-lift installation and removal of large oil and gas platforms and the installation of subsea pipelines.
Another is Subsea 7's newbuild Seven Borealis, the company's flagship pipelay and heavy lift vessel, with its unique 5,000-tonne mast crane. Yet another is Seaway Heavy Lifting's Oleg Strashnov crane vessel, with its own 5,000-tonne lift capacity.
Although very different and innovative designs, the rationale behind each of them is essentially the same: to offer offshore oil and gas operators (and offshore wind farm operators) increasing single-lift capabilities; faster vessel transit speeds between jobs; and a wider range of services, including major pipelay contracts.
A recent presentation about Allseas' Pieter Schelte given at the 2012 Offshore Technology Conference (OTC) in Houston in May gave the audience an update on the progress of this giant vessel for the Swiss-based contractor. The vessel is named after offshore heavy lifting industry legend Pieter Schelte Heerema.
Pieter Schelte on course
According to the paper authored by his son, Edward Heerema (the owner and president of Allseas), the vessel is on course for delivery and commissioning by year-end 2013 from Daewoo, complete with the topsides lift system and the pipelay system, and will be ready for offshore operations in early 2014. By the end of 2014, the jacket lift system will be fitted.
The dynamically positioned platform installation, decommissioning, and pipelay vessel has an estimated total building cost of more than US $1.6 billion. With a length of 382 m (1,253 ft) and a width of 117 m (384 ft), this giant vessel will have a topsides lift capacity of 48,000 metric tonnes and a jacket lift capacity of 25,000 metric tonnes. The pipelay tension capacity will be 2,000 tonnes, doubling the capacity of the company's Solitaire vessel to become the world's largest pipelay vessel.
With a wide slot 122-m long by 52-m wide (400-ft by 171-ft) at its bow to fit around platform substructures, it will be equipped with a hydraulically operated topsides lifting system. For the installation or removal of jackets, the vessel will have a system to two tilting lift beams at the stern.
The ability to carry out very large single lifts will substantially reduce offshore hookup costs in topsides installations, while in topsides decommissioning laborious module removal will be unnecessary. This has significant benefits in terms of shifting more work onshore where it is safer and more cost-effective.
Acknowledging that such a vessel will not always be able to find sufficient heavy lifting work, it will also be a pipelayer with an S-lay capacity of up to 2,000 metric tonnes for installing heavy pipelines in both deep and shallow waters.
The Pieter Schelte will be have eight main diesel generators, with a total installed power of 95 MW, powering 12 azimuth thrusters. With a maximum speed of 14 knots, the vessel will be able to accommodate up to 571 people.
Borealis lights up market
A different kind of vessel is Subsea 7's Seven Borealis, which was delivered to the company earlier this year and which by the end of this year is scheduled to start work for Total on the operator's deepwater CLOV development offshore Angola, laying pipe-in-pipe production flowlines in J-lay mode. It also will lay water injection and gas export lines in J-lay and S-lay mode and install a gas export single hybrid riser and manifolds.
The vessel possesses a 5,000-metric-tonne lift capacity crane installed by Huisman, and with the top of the mast reaching 150 m (492 ft) above the main deck, it is the world's largest offshore mast crane and the first of its kind to be able to lift this amount of weight. With the crane's full heave-compensated deepwater lowering system, it is able to install heavy loads required for deepwater subsea production systems.
In a technical paper presented by Subsea 7 and Huisman Equipment at OTC, the companies state that other than the dedicated heavy lift vessels in the market, most of the subsea, umbilicals, risers, and flowlines (SURF) construction vessels have limited crane capacities while specializing in installation of SURF products in deep water only. "The mast crane onboard Seven Borealis has a capacity normally only found on dedicated heavy lift vessels and combines that with the capability of installing SURF products (rigid pipe, flexibles, and umbilicals) in deep water. As a result, the mast crane onboard Seven Borealis bridges the gap between SURF installation capabilities and heavy lift," the authors said.
The DP vessel, which also can be moored via an eight-point mooring system, is based on Ulstein's SOC 5000 design and has a wide range of operational capabilities, including platform and deck installations, mooring and subsea installations, and platform removals. It also has a dual main hoist that can tilt jackets.
Having cost an estimated $460 million to build, the ship became part of Subsea 7's fleet when the company merged with Acergy in 2010. With an overall length of 181 m (594 ft), a beam of 46 m (151 ft), and a moulded depth of 16 m (52 ft), the vessel has a transit speed of 14 knots.
Power is provided via two 5.5 megawatt (MW) main thrusters and four retractable thrusters, each giving an extra 3.2 MW power during transit. A bow thruster, meanwhile, provides 2.7 MW of thrust.
Oleg Strashnov in action
Already in action has been Seaway Heavy Lifting's own state-of-the-art crane vessel, the Oleg Strashnov, which began operations last year.
With its innovative hull shape, the vessel has a transit speed of 14 knots, a 5,000 metric-tonne fully revolving crane (hinged on an A-frame that allows it to be lowered for bridges when passing through the Bosphorus Strait or Suez Canal), and an 800 metric-tonne auxiliary hook, enabling it to carry out a wide range of tasks from dual hook upending of large jackets to heavy deck installations as well as the installation of large subsea structures.
The DP vessel earlier this year carried out successful lifts offshore India, including hoisting a 4,000-tonne living quarters module for India's state oil company ONGC on its Mumbai High North field after carrying out several jobs offshore Europe in 2011.
The recurring theme among these vessels is that of flexibility and speed. Heavy lifting capacity itself is not a new thing – Heerema Marine Contractors' Thialff and Saipem's S7000 semisubmersible dual-crane vessels have both been around since the mid-1980s and in recent years have both lifted topsides on or around the 12,000-metric-tonne mark.
What separates this new generation of vessels is their ability to multitask in order to gain year-round employment and high sailing speeds that enable them to satisfy demand across the globe in a timely manner that until now has been impossible to achieve.
Editor's Note: Excerpts have been included in this article from OTC 2012 papers 23169 and 23462.