In oil and gas terminals the use of quick release hooks to moor vessels during cargo handling is recommended by the Oil Companies International Marine Forum (OCIMF), as they form the basis of a safe mooring system. In addition to the hooks, the instrumentation to monitor and control the speed of approach of a vessel during berthing (especially during contact with the jetty), the drift-off of the vessel whilst moored during cargo transfer, the tensions in the mooring lines, the status of each hook, and the meteorological and oceanographic conditions, can all be incorporated to provide a fully integrated safe mooring and monitoring system (see Figure 1).
Quick release hooks
As stated above the basis of a safe mooring system is the quick release hook. The Strainstall quick release hooks are based on the well respected Stevens design, and are unique as they have been designed to provide full mechanical protection to the release mechanisms and to incorporate load monitoring. Strainstall offer electric, hydraulic, and pneumatic remote release systems for the hooks, with the activation devices completely integrated into the hook assembly. There are no protruding parts that can be damaged by the mooring lines, and there is also a direct connection between the hook release mechanism and the release activation device. Other hooks on the market differ in that they offer release activation devices mounted on top of the hook, on the side of the hook, or at the back of the mooring units with a steel wire connection to the release mechanism. All these types of systems carry a high risk of mechanical damage from the mooring lines or horizontal and vertical swivelling of the mooring hooks.
When using mooring load monitoring, most of the quick release hooks currently available tend to have a load shown when no mooring line is attached, or is attached at slack load. This residual load is caused by the ‘offset load value’, and is induced by the weight of the hook and/or slack mooring line. Strainstall use a ‘vertical active only’ support within the design that compensates for the weight and prevents ‘offset load value’, which can be as much as 16 – 20te in severe conditions. This is particularly important if the slack rope situation is being monitored, as this ‘offset load value’ will be interpreted as a rope tension load, even though the rope may be slack.
The normal configuration for petrochemical installations involve multiple hooks on a common base, generally 2, 3 or 4, with an integrated capstan to assist heaving in the mooring lines from the vessel. Each hook is proof tested to 150% of its rated load, and the release mechanism is tested at full load rating at our own test facility, and is witnessed by a surveyor from an independent classification society. They are supplied with complete material certification and traceability (see Figure 2).
If remote release is incorporated, the hooks can be released from a remote location – generally the control room or jetty office – as well as by activating a button on the hook motor starter enclosure mounted on the rear of the hook. Local displays of the mooring load tensions and audible and visual alarms can also be incorporated into the hooks.
DockAlert – Speed of approach
DockAlert provides protection of the jetty infrastructure during approach of the vessel to the berth. Experience has shown that controlling the speed of impact of the vessel onto the fenders prevents damage to the berth. Once the vessel is safely moored to the berth, the system will monitor any drift-off of the vessel and provide warning to the operators.
The system uses two eye-safe laser units installed on the jetty and aimed perpendicular to the berthing line. These measure the distances and the speeds of the bow and stern from the jetty. The relative angle of the vessel to the berth is also calculated, and the data displayed in a number of ways:
• On large digit displays mounted on the jetty and visible from the vessel at 200+ metres so that the pilot can monitor his approach, and certified for use in hazardous areas
• On handheld pagers/PDA’s carried onto the ship or used on the jetty, both safe area and intrinsically safe types
• On jetty repeat displays generally installed local to the loading arms, and certified for use in hazardous areas