VESSEL REVIEW | Signet Capella & Signet Sirius – Signet Maritime tugs to provide channel escort for large crude tankers

Houston, Texas-based shipping company Signet Maritime Corporation has expanded its towage fleet with the recent acquisition of two new tugs in a series built at the Pascagoula, Mississippi facilities of subsidiary Signet Shipbuilding and Repair (SS&R).

Signet Capella and Signet Sirius were both built to a design developed by Canadian naval architecture firm Robert Allan Ltd (RAL) in conjunction with Netherlands-based Rotortug. The vessels each feature a unique triple drive propulsion layout that permits omnidirectional manoeuvrability in addition to offering the benefits of a fully redundant and precise machinery configuration. This then makes the tugs ideal for a range of roles that include ship handling, terminal support, escort, and general towing even in congested port waters.

"We recognised the need to design a new vessel that will safely and efficiently meet the needs of the Port of Corpus Christi Channel Improvement Project, which would significantly deepen the ship channel," owner Signet Maritime told Baird Maritime. "To overcome the challenges of the deeper channel and the unique environmental conditions experienced at the Port Aransas Jetties, we conducted extensive simulation and analysis to ensure the safe transit of deep-draught very large crude carriers (VLCCs)."

The owner said it worked in conjunction with RAL to perform computational fluid dynamic (CFD) studies to validate the performance of the tug design against the increased hull resistance of the deeper-draught vessels. These studies led to the final design used on the two tugs, with their primary duties being marine operation and protection of the Enbridge Ingleside Energy Center, one of the country's largest crude oil storage and export terminals by volume. In particular, the design of the tugs solved the issues related to increased hydrodynamic effects of deeper-draught vessels and environmental conditions that are experienced at the jetties.

"We were involved throughout the design and simulation process with the engineering and operations teams," added SS&R. "We were tasked with delivering a vessel that could outperform any other tug in its class, with increased redundancy and incorporating lessons learned from our previous builds and the tugs already working in the same application."

The builder added that the completion of the Corpus Christi Channel Improvement Project will mean VLCCs with 51 feet (15.5 metres) of draught will create a greater demand for high-power, highly manoeuvrable tugs that can perform escort at-speed and in unorthodox configurations. The new Signet tugs are therefore purpose-built for such an application.

Optimised deck machinery package

The new tugs each have an LOA of 103.3 feet (31.48 metres), a moulded beam of 45.5 feet (13.86 metres), a maximum draught of 21.5 feet (6.55 metres), a moulded depth of 15.58 feet (4.74 metres), and a gross tonnage of 299. Three MTU 12V4000 M65L 2,575hp (1,920kW) US EPA Tier IV engines drive Kongsberg US205 thrusters housing 2,500mm controllable-pitch propellers to deliver a significant bollard pull of 90.5 tons (92 tonnes). Signet Maritime said the three-thruster design allows for significantly more manoeuvrability, power, and control while being able to exert forces in vectors that are not possible with a traditional two-thruster design.

RAL said extensive analyses and simulations have been performed to confirm the suitability of the design for the proposed operations, including simulations at the Seamen's Church Institute and TDT-Sim analysis, in conjunction with Markey Machinery, to ensure proper winch selection for the tugs to meet the environmental criteria for the escort and docking of the VLCCs. The results are 200hp (149kW) high-speed, high-torque render recovery winches to allow for advanced escort manoeuvres off the bow or the stern.

"The DESF-52 bow winch can perform escort operations in up to a two-metre (6.56-foot), six-second wave period," said SS&R "The winch is integrated into the deck to reduce weight, and Markey Machinery made a number of advancements in lightening and reducing the winch's footprint specifically for the tugs' needs.

"The TESF-52 stern winch matches the design of the bow winch, with a reduction in gearing. The drive systems for the stern winch are interchangeable with the bow winch and fail-over on the fly; if the vessel were to lose the power electronics for the bow in-operation, the stern systems would line up to keep the winch controlled and powered.

"Each tug has a modular polyurethane fender system designed by Buoyant Works to reduce weight and lower waste for replacement," the owner added. "With this modular design, smaller sections can be replaced individually as opposed to having to replace the entire system."

RAL and Signet also collaborated with selected vendors and classification society ABS to ensure the development of the tugs with the combined manoeuvrability of three thrusters and the high-speed, high-torque performance of the winches. The result is significantly improved close-quarters ship handling and constant line "time under tension" to safely manoeuvre laden ships of up to 300,000 DWT.

A more streamlined design process

RAL said the tugs are also notable for being the first commercial vessels in US history to be produced using 3D structural models in combination with key plans in design, approval, and construction. The full cycle of design and construction has been completed in 3D, and due to its proven effectiveness, all future vessels constructed at SS&R will follow the same design regimen.

The tugs are also among the first vessels in the United States to receive ABS' ENVIRO, Low Emissions Vessel (LEV), and CyberSecurity-1 (CS-1) notations.

"A high degree of collaboration allowed RAL, ABS, the US Coast Guard (USCG), and SS&R to work on the same 3D drawings and eliminate the risk of deviation when converting to 2D drawings," said Signet Maritime. "Of special note is the CS-1 notation, as cybersecurity is the coast guard's top security focus and we support its objectives with the aim of protecting our customers, our crews, and the area in which we operate."

Versatile and highly durable

For SS&R, the electrical systems and the complexity of the hull structure posed the biggest challenges for its construction team.

"The electrical systems for these vessels are a step above what is found in any other vessel we have built," SS&R told Baird Maritime. "With approximately 500 alarm and monitoring points all throughout, a motor control centre, an advanced switchboard, and plant power redundancy, these systems are just a few contributing factors to the Automatic Bridge Centralized Control Unmanned (ABCU) notation attained from ABS and to the ease of operation for the crew."

The builder added that the sponson hull design, the shear and cambered main deck, and high-yield strength steel insert plates in way of the forward and aft deck winches contributed to a complex fabrication and assembly process.

"We learned that, when building larger vessels with complex hull designs, there is an advantage to using preform jigs for the larger unit assemblies. Also, taking advantage of the 3D model and generating isometric pipe drawings will save many labour hours when building a vessel with vast amounts of piping systems.

The tugs were constructed with all-LED lighting to ensure high efficiency and low life-cycle waste. Specialised drivers were also added for low interference with radio frequencies.

Solid state navigation and communications equipment such as radar and GMDSS were incorporated to guarantee low energy use and longer lifespan. The GMDSS package also removes any navigational restrictions on the vessels, enabling international ocean service if needed.

Each tug has been certified to ABS Firefighting Vessel-1 notation and has a firefighting capacity of 10,600 gallons (40,125 litres) per minute at a range of 400 feet (121 metres). This is combined with an internal foam storage tank and foam injection capability along with a complete deluge water curtain.

"The tugs were also designed to intact damage stability standards for all below-deck watertight spaces," the owner told Baird Maritime. "This ensures the vessels are survivable for the crew in the event of a breach of the aft or forward thruster rooms, or even the main engine room. Although such a feature is not required by regulation and is difficult to attain on a vessel of this size, it is critical to our standards of safety in design."

This important feature means that, even if there is a hull breach in any space of the tug, it will still remain upright.