The ABS classed Day Tide from Tidewater Inc., delivered in 2008, is one of the new offshore tug and supply vessel. (Images courtesy of ABS)

Many of the humble earlier generation workhorse offshore supply vessels (OSVs) continue to serve shallow-water units, but the evolution of the offshore industry has called for a comparable evolution in the nature and function of the support vessel fleet that has provided exciting challenges for the designers who specialize in this sector.

The American Bureau of Shipping (ABS), an offshore classification society, classes more than 1,300 of these vessels worldwide and has nearly 600 currently on order to its standards. It has been in a unique position to see the transformation in their design capabilities over the years.

Categorizing OSVs

“It has become a much more diversified market,” said Mike Sano, senior engineer attached to the energy project development team at ABS. Sano leads the society’s OSV market sector group, which monitors and anticipates advances within this specialized fleet
of vessels.

There are more than a dozen different niches or categories within the OSV market, Sano explained. “The list of acronyms just keeps growing. They all fall under the generic OSV label, but the market now looks at their specific capabilities — are they platform supply boats (PSV), or anchor-handling tug and supply (AHTS) vessels, or well production and test (WT) boats, or oil spill response vessels (OSRV) and so on. Although many of these are still considered ‘multifunction’ craft, they are often very specialized vessels that have been designed to carry out very specific tasks.”

Most of the modern support vessels fall into one of three principal sectors. They transport supplies, support drilling activities, or are equipped for emergency response. “There is crossover,” Sano said, “but usually each vessel has been designed with a primary purpose in mind.”

The basic OSV design continues to exhibit the classic profile of an accommodation and bridge block in the forward part of the vessel and an exposed working deck aft. The differences lie in the details. Most operators have developed proprietary designs that they believe best reflect their operational experience and increase the vessel’s flexibility, and therefore its marketability, within a sector that remains subject to unnerving rate fluctuations.

Hull designs and rudder and thruster systems have also advanced to improve the maneuverability of these vessels for the often close-quarters work in which they are involved. Advanced dynamic positioning (DP) systems for superior station keeping are now common installations. “Most of the designs we see incorporate at least DP2 redundancy,” Sano said, noting that some of the latest designs — particularly those destined to work further offshore in the new deepwater fields — are specifying DP3-classed systems. These vessels must meet all the requirements of DP2 vessels with the additional requirement of redundant compartments and machinery spaces. “This is a major redesign philosophy from conventional OSVs.”

The propulsion systems also have become more sophisticated, with some owners fitting diesel-electric arrangements to provide greater flexibility to the internal arrangement. The smaller footprint of the engine room offers alternatives for optimizing the space for supporting rig operations such as increasing liquid mud tank capacity to around 5,000 bbl.

Fleet status

ABS offices worldwide review OSV designs; however, the ABS New Orleans office in the US has become the epicenter for design review of these specialized vessels. According to Brent Savoie, manager, New Orleans engineering for ABS, his office has been inundated with plan approval of new OSVs, most of which include
a series of sister vessels.

“These OSVs are technologically advanced,” Savoie said. “They feature increased cargo capacity, panoramic navigation bridge visibility, large accommodation spaces, enhanced crew amenities, and state-of-the-art propulsion and automation systems.” Savoie said not only has ABS seen a resurgence of the OSV market but an increase in the market for specialized multipurpose OSVs as well. These OSVs are designed to transport heavy cargoes and carry out maintenance and repair on platforms, facilities, and subsea piping. OSVs designed for inspection, maintenance, and repair (IMRs) are equipped with large accommodation spaces, heavy lift cranes, heliports, and a streamlined bow shape for harsh environment operations.

Design innovations aside, the competency of the crew remains a critical element in the safe operation of these vessels in what remains a hazardous occupation. The capsizing of a newly delivered OSV off the northern UK coastline recently was attributed to human error, demonstrating that even the most sophisticated vessel can still be vulnerable if improperly operated.

Although recent economic news predicts a softening in the offshore market in response to a global economic slowdown, that weakness has not yet surfaced in the OSV sector. It continues to mirror the innovation and expansion in deepwater offshore development and is transforming the fleet into an effective and powerful vessel class.