Crew Transfer Vessel Product

Overview

A crew transfer vessel (CTV) is a specialised fast catamaran built to deliver offshore wind turbine technicians and maintenance crews from a supply base to a wind farm 50–200 km away in rough seas. Unlike general-purpose passenger ferries, CTVs sacrifice passenger comfort for speed and seaworthiness: they cruise at 25–35 knots in order to complete a round trip in one work shift, even when the sea state is 2–2.5 m significant wave height.

The Hull Structure is a twin-hulled catamaran, typically 24–28 m long with composite or aluminium construction. The Waterjet Propulsion System waterjets deliver superior shallow-draft performance and directional control. The Crew Cabin is heavily shock-isolated and furnished with safety-harness Cabin Seat, anticipating frequent pitching and slamming in a seaway. Most distinctively, the Bow Fender System and Transfer Ladder are engineered to enable crew stepping across a 2–3 m gap between a pitching vessel and a moving, pitching offshore platform in near-real time.

Catamaran hull design

The Hull Structure consists of two long slender tubes bridged by two transverse Cross Connection beams amidships and aft. Each tube is about 2.5–3.5 m diameter and subdivided by internal crew-transfer-vessel-watertight-bulkhead into forward, mid and aft buoyancy chambers, giving redundancy and trim flexibility. The two hulls are typically 6–8 m apart; this spacing minimises pitching moment while providing a wide platform for the cabin structure in between.

The material choice — Port Hull and Starboard Hull — strongly influences whole-life cost. Composite vessels are lighter, stiffer and quieter, reducing structural fatigue and crew fatigue. Aluminium vessels are faster to build, easier to repair, and have a lower initial cost. Most modern CTVs are composite.

Propulsion and fuel efficiency

The Diesel Engine pair, typically 500–750 kW each, drives Waterjet Propulsion System propulsion. Waterjets are high-efficiency propulsors at high speed (22+ knots) because they avoid the cavitation losses of propellers in shallow or aerated water. The Intake Scoop and Pump Impeller accelerate water to high velocity; the Nozzle Steering pivots the jet thrust for precise directional control, eliminating the need for a separate rudder and allowing full 90° turning in place.

Jet steering is crucial during transfer: the master backs the CTV up to the platform, points the nozzles forward to stop the forward motion, then adjusts throttle and angle to hold a stable 1–2 knot approach speed. The Reverse Bucket deflects the jet forward for astern operation.

Cruising speed is typically 90% power, consuming 120–160 L/h of marine diesel, for a range of 150–250 nm depending on sea state and payload.

Cabin and seating

The Crew Cabin is a self-contained compartment, usually mounted on isolating dampers atop the cross beams. Interior dimensions are roughly 6 m × 3.5 m, fitted with two rows of Cabin Seat — heavily cushioned marine seats with integral crew-transfer-vessel-safety-harness and shock-absorbing pedestals. Unlike a passenger ferry where comfort is a service feature, CTV seating is engineered to limit acceleration exposure during slamming: a seat's base reduces vertical acceleration from 0.5–1 g in the hull to 0.2–0.3 g for the occupant's torso. The seats are inward-facing or forward-facing; on some vessels they swivel to face the transfer ladder during operations.

The cabin is heated by a Heating Unit and cooled by a small Air Conditioning system to maintain 15–25 °C. Ventilation is mechanical — opening a window at 30 knots is impossible. An Emergency Hatch in the roof allows rapid evacuation if the vessel capsizes.

Bow fender and transfer gear

The Bow Fender System is the vessel's most complex subsystem. It consists of two articulated Fender Arm tubes, one on each side of the bow, extending outward and forward at the master's command by Fender Hydraulic Cylinder. Each arm is tipped with a Fender Pad of elastomer (typically Korfil or open-cell foam) that contacts the platform to absorb energy and limit lateral acceleration. The fenders are lowered to full extension only when the vessel is 30 cm from the platform, cushioning any remaining closing speed.

The Transfer Ladder is a lightweight stainless steel structure, hinged to the bow and lying flat when underway. During transfer, it is manually erected to form a 1.5 m slope from the vessel to a platform cleat or structure. The Safety Handholds and Lanyard Attachment give crew something to grip and a point to clip a personal flotation device lifeline. A Safety Net extends beneath the ladder to catch anyone who slips.

Transfer in heavy weather is inherently risky: a 2.5 m swell or 25 kt wind is usually the limit. In marginal conditions, transfer is rescheduled; downtime waiting for weather windows is a major constraint on offshore wind productivity.

Stabilisation

The Stabilisation System system — Fin Stabiliser and Trim Tab — work together to minimize motion and keep the Motion Sensor reading acceptable. Fin stabilisers actively reduce roll period and amplitude by generating lift on demand; the Stabiliser Servo hydraulic cylinders adjust fin angle several times per second in response to gyro input. Trim tabs are passive and fixed by the master to trim pitch for a particular weight distribution and speed. In a 2 m sea at 25 knots, a well-stabilised CTV rolls at 15–20° amplitude; an unstabilised CTV might roll 30–40°, causing crew discomfort and increasing transfer risk.

Navigation and weather awareness

The Navigation Suite is compact and robust, integrated onto a Chart Display screen. The GPS Receiver provides position accurate to 1 m; the Radar Transceiver (3 kW X-band) detects a wind turbine tower at 5 nm range and can track sea clutter suppression. The Electronic Compass fuses gyro and fluxgate data for heading accuracy. Wind speed is continuously monitored by the Anemometer — transfer is prohibited above a threshold (usually 20–25 kt apparent wind). A VHF Radio links the CTV to the platform operations centre ashore.

Most CTVs operate on a daily roster, departing at dawn or mid-morning, delivering crews by mid-morning, returning by evening. The 4–6 hour round trip is tight, leaving little margin for weather delays or mechanical slowdowns. Downtime due to weather or equipment failure directly reduces the revenue productivity of a multi-million-euro asset.