The advantages of using the float-over method to install offshore topsides are listed below:
- Reduced schedule interfaces – for a float-over, only one asset is required, which is able to perform both the sea transport and the float-over topside installation. The float-over topside installation can be executed from load-out to installation by a single contractor. Interfaces between the transport contractor and lifting or float-over contractor can be removed from the project execution plan.
- Reduced risk – risks due to alignment of schedule of transportation vessel and semi-submersible crane vessel are avoided.
- Capacity – the capacity of float-over topside installation exceeds the maximum capacity of the semi-submersible crane vessels. This makes the need for multiple lifts for big production platforms obsolete and gives the operator more flexibility from an installation weight perspective.
- Minimise offshore exposure hours – work done offshore is considered a higher risk as opposed to work done onshore. Therefore, reducing the offshore exposure hours for the workers is desired for companies looking to reduce their risk exposure resulting is safer operations.
- Reduced offshore hookup and commissioning – the capability to install heavy platforms as single-lift instead of multiple lifts reduces the time required to execute offshore hook-up and commissioning. This strength is only applicable to platform sizes that exceed the locally available crane capacity and would otherwise require multiple lifts.
- Safety – the float-over topside installation can be designed such that any single point failure can be dealt with.
- Availability of vessel – since there are more HTVs than crane vessels, the availability of HTVs is an advantage for project managers. In addition, mobilisation of HTVs is generally faster as the transit speed of the vessels is much higher.
- Cost savings – the costs of a float-over topside installation compared to a semi-submersible lift are likely to be lower, firstly for chartering of transport/installation vessel compared to chartering of transport vessel and semi-submersible crane vessel, and secondly for offshore hook-up and commissioning for a ‘single lift’ float-over topside installation compared to a multiple lift installation by semi-submersible crane vessel.
To better understand the float-over concept, a brief introduction breaking down the process into steps may be helpful. There are several phases involved when installing an offshore topside with an HTV.
The load-out phase is the starting point of a float-over topside installation. The integrated topside is build onshore and needs to be loaded onto the installation vessel, by means of self-propelled modular trailers or by the use of skid tracks. Requirements for the load-out stage are governed by the following parameters: integrated topside weight, tidal range, quayside dimensions.
Figure 2. Topside lowered onto jacket.
Following completion of the load-out, the integrated topside has to be seafastened on board the vessel prior to commencing the sea transport phase. Stability of the vessel is critical for the transport. Stability is mainly driven by the width and depth of the vessel.
After completion of the transit, the vessel needs final preparations prior to the commencement of the actual docking operation of the vessel. During the stand-off phase, preparatory works need to be executed such as removal of seafastenings, start-up of mooring/docking/mating winches, start-up of motion and weather monitoring equipment, start-up of active load-transfer system, and pre-ballasting of the vessel. For these preparations, the vessel needs to be moored at a stand-off location. The mooring spread for the vessel will be dependent on the field lay-out and the environmental conditions.
Once the preparatory work is completed, the docking phase can commence. During this phase, the vessel is moved into the jacket. A few conditions need to be safeguarded in this phase such as alignment of the vessel stern with the jacket slot, lateral impact loads on the jacket not to exceed limit loads of jacket and fendering arrangement, no vertical impact loads between topside legs and jacket legs, control over the movement of the vessel in longitudinal and transverse direction as well as control over the alignment of the vessel.
Once the vessel is positioned directly above the jacket structure and docked, the topside legs need to be aligned with the jacket legs. The tolerance for this alignment is to a high extent driven by the diameter of the stabbing cones. During the pre-mating phase, the clearance between the topside legs and the jacket legs will be reduced by ballasting the installation vessel. A few aspects need to be taken into account: limited lateral movement of the vessel relative to the jacket to ensure alignment of the topside and jacket legs; lateral impact loads on the jacket not to exceed limit loads of jacket and fendering arrangement and; vertical impact loads on the jacket not to exceed limit loads of the jacket and LMU design.
The mating phase is the most critical point of the installation leaving no room for uncertainty in the functionality of the load transferring equipment. In this phase, the topside is lowered onto the jacket by the vessel’s rapid ballasting system (RBS) where large quantities of water enter the ballast tanks, which enables the vessel to submerge. During this phase, the topside’s weight is completely transferred onto the jacket. For a successful float-over operation, countless hours of preparation come down to the exact moment when the topside makes contact with the jacket. Custom engineered elastomer is used in the design of the LMUs to dampen the impact during load transfer.
Figure 3. Topside skid operation.
In the post-mating phase, the topside sits secure on top of the jacket and a clearance gap is created between the topside support unit (DSU) and the vessel to ensure limited contact between the two. To further limit the impact loads, the distance between the vessel’s DSU and integrated topside need to increase, while limiting lateral and vertical movement to avoid impact loads.
After the completion of the ballasting operations to increase the clearance between the DSU and the integrated topside, the vessel can be undocked from the jacket. In the exit phase, the vessel departs from the jacket slot on its own propulsion or with assistance from tugs. It is important during this phase that the lateral impact loads on the jacket not to exceed limit loads and no vertical impact loads between the DSU and the integrated topside. In addition, full control of the vessel’s longitudinal and transverse direction as well as the alignment is important.
These eight float-over phases may differ depending on assets, equipment and installation approach used. The float-over vessel plays a critical role as it both transports and installs the integrated topside. While there are different types of HTVs, not all are designed to install topsides offshore.
Written by Jonathan Martinez, Dockwise, the Netherlands. Edited by Ted Monroe
Read the article online at: https://www.oilfieldtechnology.com/exploration/14052014/part_two_jonathan_martinez_dockwise_explains_that_interest_is_growing_for_floatover_installations/