The paper reports recent advances in the installation of pipeline bundles using diverless and ROV-less techniques The paper details the technology used to remotely position bundles, which are typically 6.5km long and weigh approximately 4,000 tons to an accuracy of 1 m or better It includes a description of the system used to perform high accuracy EHF acoustic metrology for rigid spools at a range of 7km The paper also outlines how these methods can be used to reduce the number of vessels and the time taken to install bundles with the associated reduction in cost
Rockwater has been building bundles for many years and has successfully launched 29 bundles The technique used to tow bundles from the construction site to the oil field is known as Controlled Depth Tow Method (CDTM) The principals of this method are well understood and can be summarised as follows
The bundle is designed and built to be positively buoyant by a small known amount
The bundle is designed and built to be positively buoyant by a small known amount
When the bundle is stationary the pipeline floats approximately 5m above the seabed (known as the off bottom mode), with a few links of chain on the seabed
When the bundle is towed the chains produce hydrodynamic lift which allows the bundle to "fly" in mid water
As the installation area is reached the bundle is returned to its off bottom mode, which allows it to be accurately positioned with minimum force
The bundle is then flooded and hooked up
It is clear that systems must be put in place to monitor the bundle, which will assure Rockwater and our clients that the bundle is "safe" during launch, tow and installation and to allow the tow master to control the bundle The principal requirement for control is when the bundle is being towed in mid water During this stage of the installation there is no visual contact with the bundle and a method of measuring and reporting its depth is required
Early bundle tows used a direct acoustic link between the bundle and a vessel sailing alongside the bundle in this method, the depth of the bundle was measured using a pressure transducer and this depth information was encoded In analogue form as the time between two acoustic pulses The information was transmitted to the monitoring vessel In the LF acoustic band (10kHz to 20kHz) which has a range of 2 to 25km It was thus possible to measure the depth at several locations on the bundle from one monitoring vessel
As bundle length Increased to 6km and greater, more than one monitoring vessel was required to provide depth information since the bundle became significantly longer than the acoustic range of the system The cost of the monitoring vessels became the driving force to find alternative methods of communicating with the bundle During 1993 and 1994 several alternative methods were evaluated
