Seismic Hardware Modification

A new crane unit with the ability to reposition the lead-in behind the ship must be developed and installed. This element is introduced in section 1.4 Present work. The goal with this supplement equipment is to reposition the lead-in, so the cable span behind the vessel is reduced as much as possible. This will force the lead-in down into the sea closer to the stern, within the breadth of the vessel. With this solution the lead-in are better protected from the contact forces.

The wake in ice, created by the ship will generate the necessary open water behind the vessel. To assure the structural integrity of the lead-inns, the cables must be coated with a heavy duty plastic material or steel fairings. Buravtsev & Jokat (1996) describe a suitable solution for the threading of lead-inns and umbilical cords.

Deploying the equipment in open water and then guiding the lead-inns down under the surface with a modification that will inflict a minimal transformation to the ships stern. The deployment of equipment can be done following the same procedures as for a conventional configuration.

When the hardware is positioned, the cranes can seize on to the lead-inns and pull/force them down to the preferred position. With this approach no additional modification to the stern is needed. To eliminate the superwide, the deflector is directly connected to the junction point between lead-in number one and streamer number one via the lever arm. Within the seismic industry this is known as ‘direct-towing’. Vertical control of the deflector is required, to fully eliminate the surface equipment.

Neutral buoyancy is accomplished by modification applied to the flotation device and adding of weight to the lower frame. To achieve depth control a horizontal foil is added to the frame of the deflector. By changing the angle on the foils the depth of the deflector can be controlled relative to the surface. Figure 29 shows a design concept of a depth controlled deflector.

Figure 29: Illustration created in OrcaFlex 3D of a deflector with depth control. Lateral control is induced by a horizontal foil shape.

The number of lead-inns must be reduced to compensate for the breadth limitations considering the wacke behind the vessel. Compared to an ordinary¹¹ configuration, the Arctic MSOs will cover a smaller area. The surface lines and surface equipment connected to the junction points are replaced with Submergible Depth Controlled Bodies (SDCBs). This is elongated bodies with foils attached on each side, providing the option of depth control to the configuration. SDCBs are attached in the transition between the lead-inns and the streamers. The intended objective of the SDCBs is to ensure that the cables are within the depth interval specified by the client.

Additionally the units are capable of inflicting enough force to perform emergency dives to avoid contact with ice flows.

SDCBs are currently a product of the authors’ creativity. SDCBs will be subject for the FEM dynamic simulation presented and conducted in chapter four.

11 Ordinary- compared to the vessels maximum of 12 streamer configurations.

Figure 30: OrcaFlex illustration of a Submergible Depth Controlled Body and details.

To control the depth of the configuration, a SDCBs control unit is required to adjust the angle of attack which will correspond to a given depth. The configuration equipment is buoyancy neutral at a given depth h, and the angle of attack represents the manipulating variable¹². Wanted depth is denoted by . The notation is the difference between and . To measure the depth each of the SDCUs must be fitted with a depth gauge. The updating time interval of the depth gauge is a contributing variable to the characteristics of the control unit (Haugen, 2003). The updating interval from the depth gauge and the overall processing time of the regulator will have large influence of the response characteristics of the control unit. Theoretical projection of the control process can be viewed in Figure 31.

Figure 31: Schematic illustration of a basic PID control unit.

12 Manipulating variable - the variable or manipulated variable used to inflict control to the process (Haugen 2003).

This depth control unit is a so-called PID –regulator, for a more information and schematics see Haugen (2003). This section demonstrates the principle for a suitable control unit. Given the high technical specification of the Remus AUV, it is more than likely that an adaptive PID controller will be installed. For more information Haugen (2003, page 99-103) describes the approach.

By elimination of the surface equipment the possibility of positioning the configuration by GPS is compromised. The acoustic system described in section 2.4.3 is dependent on correction input from the GPS data. To counteract the distortion of precision inflicted on the positioning of the deployed hardware, a new towed surface cable is added to the configuration. This cable contains equipment with acoustic positioning systems to interact with the existing acoustic system. To improve the precision of the position, the cable would be equipped with GPS and acoustic transmitter’s equipment. The intention of the cable is to float at the surface and provide GPS input to the acoustic system. The aim is to keep the distortion of precision to a minimum.

Design features incorporated in the cable has to accommodate the possibility of severe contact forces between the ice and the cable. To eliminate as much of the cable friction-forces the coating should be as smooth as possible and be able to withstand the forces occurring when the cable is subjected to ice.

4 Theory, Modelling and Simulations

For the purpose of acquiring data regarding the research question stated in chapter one, multiple simulations has been conducted. Simulations have been conducted using OrcaFlex 3D software package. OrcaFlex is 3D time domain finite element software.

By utilizing a virtual 3D model and the FEM the program is capable of calculating static and dynamical solutions for flexible cable configurations. This involves that the simulations is designed in a three dimensional virtual space. The virtual space are determined by coordinate system with axes x, y, and z.