Perdido topsides and spar. (Image courtesy of Shell)

The fleet of vessels capable of carrying out offshore construction projects is limited at a time when the demand for vessels and for extreme capabilities is increasing.

A number of companies are evaluating new uses for traditional proven technologies with smaller vessels, while others are extending fleet capabilities to outfit vessels to carry out even more exacting projects.

Tried and true

A project recently carried out in the Gulf of Mexico (GoM) illustrates how traditional approaches continue to push the limits of offshore construction and installation.

The Perdido field, which lies in Alaminos Canyon Block 857 in 7,817 ft (2,383 m) of water, approximately 200 miles (322 km) from Houston, will become the world’s deepest offshore oil and gas drilling and production facility when it is completed. It will have a peak production of 130,000 boe/d from three fields (Great White, Silver Tip, and Tobago, with Great White responsible for about 80% of the total reserves). First production from Perdido is expected in early 2010.

The field is operated by Shell on behalf of partners BP and Chevron Alliance Wood Group Engineering, part of energy services company John Wood Group plc, designed the topsides for the Perdido Development facility. The company provided the conceptual design, front-end engineering, detailed design, construction support, and installation/ commissioning support for this unique topsides deck.

According to Alliance President Norb Roobaert, the Perdido topsides design was instrumental in the project’s success. “The Alliance lightweight topsides design techniques were a key enabler to permit Shell to advance this project,” he said. “When combined with Shell’s innovative solutions to reduce the riser load, this resulted in significant weight reduction and a significant savings in time and cost, as well as producing a safer facility.”

The design accommodates production from five subsea, direct vertical access separation caissons for this host facility. Alliance demonstrated its ingenuity and flexibility, engineering a fit-for-purpose design in partnership with Shell to successfully develop this challenging project.

The 9,773-short-ton topsides and rigging (a US record for the GoM) provided by Alliance were lifted in place by Heerema Marine Contractors’ Thialf, the company’s largest deepwater construction vessel.

The multifunctional dynamically positioned (DP) Thialf is capable of a tandem lift of 14,200 metric tons (15,600 short tons). Tailored for the installation of foundations, moorings, spars, tension leg platforms, and integrated topsides, as well as pipelines and flowlines. The vessel features dual cranes that provide for depth reach lowering capability and heavy lift capacity to set topsides.

In March 2009, HMC completed installing the 9,500-metric-ton topsides and living quarters on the Perdido spar. In fact, the Perdido project set several records, including installation of the deepest sparmooring system and the deepest pipe line end termination (PLET) at 9,678 ft (2,950 m).

Eric Romijn, project director for HMC, pointed out that the Thialf lift was not the only record set in this project. “Earlier in the project, another HMC vessel, the Balder, carried out the spar and the subsea installation, consisting of nine suction piles and polyester mooring lines, five flowlines, one water injection line, and three steel catenary risers (SCRs) necessary for the project,” he said.

Both vessels have a history of setting offshore installation records.

In 2007, the Thialf set additional records on the Independence Hub project in the GoM, installing the longest mooring lines at 2.4 miles (3.9 km) and the deepest mooring piles at 8,990 ft (2,740 m) and the installation of a flowline initiated by deploying a PLET at the same water depth.

The Thialf was able to perform these milestone projects in part as a result of an enormous upgrade project that concluded in November 2007. The upgrade, which took 14 months from concept to delivery, included outfitting the vessel with a PLET reconnect tower, an abandonment and recovery winch, and four additional six-cylinder engines to increase the vessel’s DP capacity. HMC calls the upgrade one of company’s most important strategic initiatives to maintain its market position in the GoM.

Meanwhile, the Balder set the deepest and heaviest SCRs (20 in. and 24 in.) in 6,037 ft (1,840 m) of water on the Thunder Horse field in 2006 and shortly thereafter carried out an even more challenging pipe-in-pipe (PIP) project on the Atlantis field at greater water depth, 7,136 ft (2,175 m).

As the company’s first PIP project, Atlantis was a challenge. According to Bas Zoon, project manager, the project was extremely exacting. “Since this was HMC’s first pipe-in-pipe job, a lot of new equipment had to be designed and fabricated. Three years of study and engineering and onshore fabrication were required to prepare all equipment and procedures and complete all tests to be able to install the 16 by 10 in. PIP system.”

Most of the systems onboard were loaded to maximum capacity, particularly during the transfer of the riser system to the platform.

Wendell Freeman, project manager, pointed out the realities of this milestone, calling the project, “the most challenging” he had ever worked on. “The success of the Atlantis flowline project and the groundwork it laid for future projects was a result of the close working corporation between HMC and BP.” Freeman attributed the offshore success not only to outstanding execution, but to a solid onshore foundation.

Something old, something new

Though vessels with extensive capabilities are the vessels of choice for some, other companies are willing to investigate alternative approaches.

A search for options on a project offshore Brazil led to an InterMoor initiative that produced a solution to allow an installation normally carried out by a large crane barge or construction vessel to be performed from the back of a small boat.

The project called for two artificial lift manifolds (ALMs), two spacer templates, and a well conductor to be installed using an anchor-handling vessel (AHV) in 5,249 to 6,398 ft (1,600 to 1,950 m) water depth. And the solution had to meet installation requirements without sacrificing safety.

The ALMs were to be installed to provide six slots to laterally control the location of 48-in. diameter foundation conductors. One ALM weighed 55 tons and had four slots. The other weighed 35 tons and had two slots. Six 48-in. diameter foundation conductors had to be installed through the slots on the spacer templates, and eleven 36-in. diameter wellhead conductors would be placed at various locations on the field.

Because the wellhead conductors would be driven to the desired penetration depth with a subsea hydraulic hammer instead of being jetted (the traditional method of installation), they had to be welded onshore to the correct length and installed in a single piece. The plan was to hammer the conductor directly on top of the low-pressure wellhead housing of the subsea wellhead assembly. The top of each ALM conductor was fitted with a housing adapted for hammering, a suction head attachment, and a guide cone.

During installation, each ALM would carry a share of the manifold weight as well as the weight of an electrical submersible pump, which would later be mounted on top of an ALM conductor.

Computer modeling of the operation led to confidence in its practical application. In the end, an AHV was able to support the template from two winch wires connected to the vessel’s 600-ton towing winch and 150- ton secondary winch. The wires were secured to attachment points at the top corners of the template that were designed to counteract rotational forces resulting from subsea currents. With the winch connected to a four-way bridle, the AHV was able to lower the ALMs to the seabed.

With the ALMs in place, the next step was to set all 17 conductors in a single field visit. A specially constructed barge worked in conjunction with the AHV to carry out this segment of the project. When the conductors were installed, the final step in the process was to mobilize the deepwater hammer spread onto the AHV, a challenge due to limited deck space and the absence of a large crane. Part of the solution was specially designed and fabricated equipment, including:

• A stacking system that allowed standard containers to be loaded on the AHV deck two tiers high;

• A deployment skid that would allow safe deployment and recovery over the stern roller;

• A foundation frame to raise the large umbilical winch to allow deployment; and

• An anti-twist clump weight system.

Though the installation was not without incident, it was carried out successfully and presented a test case that provides data for improving the operation. This solution is now field-proven and can be applied in other applications.