This invention relates to the construction of towed streamers suitable for use as vibration isolation modules (VIMs) and the method of manufacture of such towed sections.
Streamers are generally of one of two types. One such type are liquid filled and have been used as a prior art, and generally have strength members, the pay load and data transmission means within a liquid core with a jacket encapsulating these components. Such streamers are generally less robust than streamers of a generally solid nature.
Towed streamers usually comprise an elongate buoyant streamer having spaced along its length the required payload. Streamers are generally made from several shorter modules which are joined together to form the full length, and the modules used along the length have varying structures and are designed to perform different functions. The reason for positioning more than one type of module along the length of the streamer is in part because prevailing ambient conditions and forces associated with towing vary along the length.
Towed streamers are usually of considerable length and are subject to quite high towing strains and are consequently provided with rigid, high tensile or non-compliant strain members longitudinally to transmit tension along the streamer.
It is necessary for towed streamers to be constructed to withstand towing tensions whilst also allowing sufficient flexibility for winching onto a bollard or the like and problems are generally encountered in maintaining the spacing and/or position of strength members during such winching.
When using towed streamers consisting of a towing section, active section and tail sections, vibrational energy is generated from both the towing and the tail end. The forward end energy arises from ships' motor, machinery and propeller vibration, and flow induced vibration from normal and tangential flow over the towing section. The tail end energy arises from tangential and normal flow over the tail end sections, especially where the tail end is attached to a surface buoy. Where the tail has a free end, vortex shedding from the tail produces lateral motion which translates into axial vibration.
The vibration energy is picked up by the sensors in the active section of the streamer, from two mechanisms (a) physical movement of the sensor and (b) through pressure generated in the fluid which normally surrounds the sensor.
Towed streamers by their nature are deployed from and recovered into the towing vessel and the usual method is to stow the streamer on a winch. This provides a compact method of stowing the streamer. The recovery cycle can be achieved by directly winching the streamer onto the drum or by detensioning the streamer prior to stowage on the drum. If short recovery times are to be achieved, it is essential that all streamer sections are robust and are capable of being stowed on the winch under tension.
One aspect of this invention is that the towed streamers can be deployed and recovered during streamer operations at a load which permits the towing vessels to proceed slowly, rather than have to stop, during recovery. This is a major advantage if several streamers are towed from a single vessel. Recovery of the VIMs can be achieved by direct winching or by a combination of direct winching and some form of detensioning unit. This is in contrast to other forms of VIM, such as liquid filled VIMs, which have very little capability in regards to mechanical winching.
In addition to the features already mentioned it is necessary to be able to transmit data to the towing vessel from the sensors through the towing section in all cases and where a tall buoy is attached, it may be necessary to communicate data to and from the tall buoy.
Active sections of the towed streamer are generally designed to be towed horizontally, this being achieved by ensuring that the density is close to being equal to that of the water. It is also preferably that the towing sections close to, both fore and aft of the sensor section are also close to neutral buoyancy to reduce the effects of cross flow and thus limit vibrational noise generation.
A further requirement for the tow section may be to depress the active section to the required working depth. This can be achieved by attaching a heavy body to the tail end of the tow section, or body with planes attached which can be angled to provide a downwards force or if necessary an upwards force in special circumstances. Alternatively the depression can be achieved by using a heavy tow section which has a density greater than water.
In some applications it is necessary that the streamer is operated in two modes, an operational mode where the streamer is towed at a relatively slow speed to allow data to be gathered by the active sections, and a sprint mode where the streamer is towed at a high speed to enable the tow ship to transit rapidly between operational areas.
The main problem with the high speed mode is that excessive elongation of the towing sections can occur thus imposing strain on the data carriers.
Generally therefore such towed streamers are provided with high tensile, non-compliant strength members extending longitudinally to transmit tension along the streamer. However a problem with the use of such non-compliant strain members is that whilst their rigidity is satisfactory for taking towing strain, the rigidity amplifies undesirable noise especially during the, operational mode.