New thinking yields benefits for modern OSV designs

Several challenges have confronted the offshore industry as a result of moves into deeper waters and harsher environments, including a need for more ships of advanced design; some novel shipyard friendly solutions are proposed by Guido Perla & Associates*

The total number of drilling rigs has increased 46 per cent worldwide since 1981. First generation OSVs – of which most are approximately 25 years old – may still have an economic advantage in shallow water, due to their lower acquisition cost, but other pressures are quickly replacing these vessels, since they cannot fulfil technical, safety or comfort requirements.

These circumstances have generated design trends yielding more advanced, larger, environmentally friendly and safer PSVs of up to 100m length or more. Coupled with operations in more difficult conditions is the need for increased cargo capacity and greater product variety to ensure satisfactory profits. Deck areas in excess of 900m² facilitate increased deck capacities of 2,500-5,000 tons to meet the requirements for serving deepwater markets by carrying increased amounts of drill pipe, mooring lines and other deck loads.

Integral cargo tanks are also pushing the limits of traditional OSV design, by allowing a greater variety of products to be transported, with capacities in excess of 2,400m³ for liquid mud, in addition to traditional volumes of bulk mud, fuel oil, excess fuel, drilling brine, rig water, cargo fresh water, as well as special products, such as methanol, if required.

These increases in vessel size are also accompanied by stringent environmental regulations, and to further increase cargo transporting flexibility, advanced multipurpose tanks capable of carrying either dry bulk goods, liquid mud, fuel oil or drill cuttings in the same tanks, as well as liquids for Oil Recovery Operations (ORO), are currently under development. While the principle of multipurpose tanks is not new, modern tanks aimed at simplifying cleaning and change of cargo from liquid to dry have been incorporated into future Guido Perla & Associates (GPA) designs.

Incorporating diesel-electric propulsion systems into offshore designs allows vessels to fulfil increased cargo space demands. Ships equipped with such machinery benefit from elimination of the drive shaft and, therefore, the potential for increased tank volumes. GPA designs, such as the 64m GPA 640 PSV and the 70m GPA 670 PSV, are diesel-electric and have been proven to be very efficient solutions for their owners and operators.

Diesel-electric propulsion can even surpass the increased cargo space demands if the design exploits the system’s advantages and flexibility by locating the engineroom above the main deck – a concept pioneered by Guido Perla & Associates several years ago. The first vessels benefiting from this concept are 10 54m GPA 654 PSVs delivered in 2008. The innovative layout, which has been applied in other GPA designs, allows cargo capacities below deck to be increased by 30 per cent, akin to capacities found in much larger vessels.

In 2008, the number of new PSVs/OSVs delivered or under contract reached 378, of which 236 are to be equipped with diesel-electric propulsion systems. Of these, 51 are diesel-electric types of GPA design, not including a 54-vessel GPA 254L AHTS series currently under construction in China.

To support increasing production capacities, existing shipyards have taken measures to expand their facilities while new yards solely focused on offshore vessel construction have been established, such as Sino Pacific Group in China, which invested in two new sites, Zhejiang Shipyard and Dayang City Shipyard. This group has become a key player in offshore vessel construction.

Of the Bourbon Liberty series, totalling 76 vessels, numerous examples of the GPA 654M PSV and GPA 254L AHTS design are currently under construction at both the Zhejiang and Dayang City yards.

To facilitate the rapid building of large numbers of vessels at comparably low capital costs, designs based on yard-friendly construction have played a significant role in these projects. Simplified construction methods, such as single-curvature hulls, flanged plates and transverse framing – combination that is the standard for GPA offshore designs – contribute to decreased construction time and cost.

Single-curvature hulls, while not fully accepted yet by some, have proven to be more efficient, not just during construction but also during operations. In a variety of applications, but typically for medium to large displacement vessels, a well-designed chined hullform has approximately the same resistance characteristics of an equivalent round bilge hullform. Furthermore, locating the engineroom above main deck contributes to simplified construction, as well as offering improved and safer maintenance, since the engineroom can be reached more easily.

Some modern designs include further advances to simplify and expedite construction. A well-proven concept delivering such flexibility is a Modular Electric Propulsion System. Electric Power Design Inc (EPD), along with GPA, brought new thinking in electric propulsion to market by providing a ship’s engine control room (ECR) in a pre-manufactured container.

The ECR, completely tested and designed as an integral part of the ship’s structure, allows for equipment to be installed and tested in a controlled environment. During installation, the ECR is simply lowered onto the vessel, secured and connected to power and control cables externally – an approach that greatly reduces the possibility of equipment damage while in the shipyard.

‘Constructability’ not only facilitates the growing demand for OSVs but also results in reduced man-hours, improved cost figures and shorter build cycles. As an example, a GPA offshore vessel designed for production is delivered approximately every two weeks at one of two purpose-built Sino-Pacific yards in China, and during the recent bidding process for the current Petrobras tenders, quotations by shipyards for GPA designs clearly demonstrated a consistent construction cost advantage in comparison with other designs, as a result of these special features.

Design trends also have addressed heightened concerns over environmental issues. Approaches to minimising the environmental impact of offshore operations are primarily driven by regulation but are also becoming corporate-driven; these factors have had a significant impact on offshore design evolution in recent years. These trends focus on tank segregation to decrease the risk of oil or other hazardous substances spills, and engine emissions, and on obtaining Clean Class notations.

Marpol regulations are the most significant currently affecting offshore designs, to improve the effects of ship operations on the environment. Docket No USCG-2007-27813 of the Federal Registry informed the industry that the “USCG will enforce new Marpol Annex I regulations for US flagged vessels that are required to hold an International Oil Pollution Prevention (IOPP) Certificate, including Regulation 12A of revised Annex I (IMO Resolution MEPC 141 (54)) Oil Fuel Tank Protection.” This being the case, design requirements are established for protectively located fuel tanks for all ships with an aggregate oil fuel capacity of 600m³ and above, and with a building contract on or after 1 August 2007 or delivery on or after 1 August 2010.

Furthermore, the USCG plans to adopt provisions set out by Marpol Annex II (IMO Resolution A 673 (16)) for ‘new’ OSVs regarding the transportation of Noxious Liquid Substances (NLS) in limited quantities, which already applies to vessels on international voyages with liquid mud and other cargoes classified as NLSs. The requirements for vessels intended to carry limited quantities of more than 800m³ of NLSs include tanks segregation and construction requirements. As a result of these double-hull specifications, a 3,000 dwt PSV built just a few years ago will need to have a deadweight of approximately 4,500 tonnes in order to carry the same amount of cargo fuel and liquid mud.

Ship designs must follow increasingly stringent engine emissions regulations focused on reducing the environmental impact of shipping. IMO’s Marine Environment Protection Committee (MEPC) works continuously on improving these standards. For offshore design trends, the most relevant regulations introduce a three-tier structure for nitrogen oxides (NOx) emission standards for new marine engines, depending on the date of their installation, with significant emission reductions (Tier 3) mandated for ships operated within designated Emission Control Areas (ECAs).

Tier 1 applies to a diesel engine, which is installed on a ship constructed on or after 1 January 2000 and prior to 1 January 2011; for Tier 2, NOx emission levels for a diesel engine refer to installation from 1 January 2011; and for Tier 3, NOx emission levels for a diesel engine refer to installation from 1 January 2016. Outside a designated ECA, Tier II limits apply.

In addition to complying with mandatory engine emissions standards established by IMO and US EPA, also with other inevitable regulations in the future, adoption of voluntary Clean Class notations from a classification society is an increasing trend. Class societies have incorporated some compulsory regulations, such as the Marpol Annex I and Annex II rules, as an integral part of their Clean Class requirements.

By incorporating a Clean Class notation into a vessel design, owners demonstrate an added interest in limiting emissions and operational and accidental pollution. By taking pro-active steps and assuming responsibility for the environment, operators improve their corporate image with customers and authorities.

While emissions regulations, as well as Clean Class notations, address and continuously improve the minimisation of measured emissions output, these standards do not enforce limitations on the actual fuel consumption and fuel efficiency of a vessel, and until recently, there was little motivation to consider overall fuel consumption and efficiency for PSV/OSV operations.

The possibilities of improving fuel economy have grown through the use of diesel-electric propulsion configurations, facilitating improvements in consumption and emissions by 30 per cent. Nevertheless, requirements for DP operation are detrimental to fuel economy because engines must operate at reduced demand although (as highlighted on a number of occasions in OSJ), diesel-electric systems can considerably improve fuel consumption by providing greater flexibility in the use of power. Recent comparisons of fuel consumption on traditional OSVs and diesel-electric PSVs demonstrate the efficiency of the latter while at sea. A diesel-electric PSV consumed, on average, 16 per cent less fuel than a mechanical-drive vessel.

A conventional OSV tends to consume less fuel while at the quay, due to smaller diesel generators. As at-sea utilisation increases, the total fuel consumption improves for diesel-electric PSVs. This is most probably due to the smaller generator size typically installed on direct-drive OSVs compared to diesel-electric PSVs. Diesel-electric PSVs could therefore benefit from either installing a harbour generator, using the emergency generator as a harbour generator, or by installing additional gensets as part of a diesel-electric propulsion plant, thereby providing smaller increments of power.

While at sea, diesel-electric PSVs exhibit greater fuel economy, due largely to the amount of time spent in DP operations or in standby. However, to fulfil redundancy requirements for a DP2 classification, a vessel must have reserves of power generation in order to maintain position in the event of loss of a single prime mover.

For instance, a DP2 class ship equipped with two main gensets and one smaller genset must have both main sets running to meet redundancy requirements. This can result in inefficient engine loading that is far less than the optimum of 80 per cent. With over 50 per cent of vessel deployment time being spent at 30 per cent load, the most efficient solution in terms of fuel savings has not yet been reached with diesel-electric plant.

A potential solution to further improve fuel economy in a diesel-electric PSV could be to enhance the configuration of the electric plant by installing more gensets with smaller output ratings, thus providing smaller power increments, which is especially useful during DP mode or other reduced load operations. These smaller gensets, while providing the same total required output, would be running closer to the optimum load. This concept of matching the operational profile more efficiently is not fully embraced as it brings with it a higher capital cost, as well as higher service and maintenance expense.

The current high level of interest in crew comfort takes the form of an emphasis on noise, vibration, indoor climate and lighting levels. Significantly reduced noise and vibration levels are being achieved by a combination of ship layout, material selection and structural design.

Locating the engineroom above the main deck creates an additional deck between the bow thrusters – notorious contributors of noise – and the accommodation spaces, which further reduces noise and vibration levels. Modern diesel-electric vessels also benefit from a significant reduction in vibration due to flexible mounting of the gensets, which is not common with mechanical drive systems. OSJ

* This article is based on an edited version of a paper, The Correlation between Offshore Vessel Design Trends and Operational Challenges, given at the Annual OSJ Conference in London, 24-25 February 2009 by Dan Koch, vice president for engineering at Guido Perla & Associates