Subsea Manifold Installation
In deepwater fields the contribution of installation activities to project costs and schedule is higher than for shallower developments. The risks associated with installation are also higher. The metocean condition of deepwater is a key factor for the installation of subsea structures. Relatively gentle conditions in offshore West Africa may not mean installation operations are easier than in the GoM because the persistent swell is likely to result in ideal conditions for vessel motion resonance. High currents in offshore Brazil are likely to be a dominating factor. A subsea development may have more than 30 wells, which requires an extensive installation program of manifolds, PLETs, jumpers, and suction pile foundations. Excluding the flowlines, which are small lift weight components having compact dimensions, we must still consider the large number of individual items that require a long installation program and the large number of heavier manifolds, which also present installation challenges.
Contents
Installation Capability
In the deepwater installation, a number of challenges come up due to the increasing of water depths in the field development. These challenges potentially constrain the installation capacity for subsea structures in deepwater. The capabilities of the subsea installation system are mainly depending on the limitations of the following components of the system:
- Lifting and lowering system, which includes vessel, lift line, and overboarding/lift line deployment system;
- Load control and positioning system, including motion compensation system, buoyancy hook/payload ontrol/positioning, and communications.
The lifting and lowering system directly related to the weight of the loads to be lowered to the deep seabed, the dynamic responses that can augment these loads, and the decrease in the capability of the lifting systems. The load control and positioning system related to placing the load in the desired location, at the correct compass heading, and at a stable attitude on the seabed. The effects of these issues are discussed in following sections.
Lifting and Lowering System
The potential for resonance, associated with the natural period of the payload on the line and with surface vessel excitation, can give rise to very large dynamic loads. It is also difficult to estimate the hydrodynamic added mass for a complex payload shape. Fiber rope is also used in deepwater hardware installations. Fiber options include aramids, polyester, and highmodulus polyethylene for offshore applications. The fiber ropes have several attractive properties such as lower self-weight, small allowable bend radii, lower axial stiffness, a large damping ability to absorb heave created by surface waves, and the ability to be repaired. Petrobas used a polyester rope to successfully install a manifold in deep water of 2000 m (6560ft). However, fiber rope has potential problems related to stretch, creep, and its relatively low melting point.
Load Control and Positioning System
Buoyancy units may have an important role to play in reducing the static lifting line tensions. However, buoyancy units for large subsea components to be installed in deep water are not easy to design in such away that they are manageable and economic. The load control and stability problems of buoyancy are difficult to solve; in particular, the inertia and hydrodynamic loading of the system are increased, which contributes to undesirable dynamic effects. Very significant dynamic effects can result when lowering heavy weights on long lines. The excitation caused by the motions of the surface vessel can be amplified with large oscillations and high dynamic tensile loads in the lifting line. Motions in the heave direction may be only lightly damped, and the added mass of the load can be very significant.
For example, a suction anchor consisting of a flooded cylinder with closed top will have an added mass that is many times its weight in air due to the water trapped inside and entrained around it. When combined with dynamic magnification caused by oscillations due to surface waves, the line tension of 460 tonnes is obtained for a suction anchor with a weight in air of 44 tonnes. The shape of the item to be installed, which in turn determines the added mass, can therefore be crucial to this dynamic response and to the ability to install it. It can be shown that for lowering into deep water there will nearly always be a depth at which a resonant response will occur. It is important for this resonant region to be passed through relatively quickly and for it not to occur at full depth where careful control is required for placement of the payload on the seabed. Modeling methods, as shown in the next section, have been developed to predict the behavior of these dynamic responses so that design and planning of the lowering operations can attempt to minimize and avoid them.
Motion Compensation
For the installation of subsea structures in deep water, installation contractors encounter limitations when using equipment without heave compensation systems. Limitations include:
- Excessive dynamic amplification of the load during lowering, with the risk of overloading or even rupturing of the cable;
- Unstable situations during the landing of the subsea structure and its positioning on the seabed;
- ROV assistance operations with respect to the moving load.
Active and passive motion compensation systems are mainly used in subsea installations. Many cranes for subsea installation are equipped with an active heave compensation (AHC) system to compensate for the vertical heave motion at the crane jib tip by means of a powerful computer system and a high-speed winch system. A passive heave ompensator (PHC), such as a cranemaster, is another kind of heave compensator that can alleviate the high hook loads. In essence, the cranemaster is a damped spring system set above the subsea structure using compacted gas over an oil accumulator acting as an isolator. Two types of PHC systems for subsea structure installation are introduced here because of their impressive heave compensation effects.
Heave-Compensated Landing System
The heave-compensated landing system (HCLS) used by Delmar consists of the following equipment: a compensating buoy system, compensating belly chain, and pendant wire. The buoyancy system suspends the subsea structure, keeping the system close to a neutrally buoyant condition in the vertical direction. The subsea contracture is lowered to the seafloor in a controlled manner by paying out wire from the AHV, which transfers more chain and thus more weight to the buoyancy system. As more chain is added, the added weight overrides the buoyant forces, causing the subsea structure to slowly sink. The length and configuration of the chain are the compensating portion of the system. HCLS has a very good heave compensation effect, but it is difficult and complicated to get the subsea equipment overboard from the barge.
Polyester Rope
Polyester rope is one of the most popular ropes used as anchor and mooring lines in the mooring systems of the offshore industry. It is very close to nylon in strength but stretches very little. Polyester rope has highly elastic properties and a good damping effect.
Installation Equipment and Installation Methods
The installation of a subsea structure/manifold requires careful planning and coordination with workboats, a crane barge or floating drilling vessel, and acoustic and/or electrical location equipment. The choice of installation vessel is based on vessel availability, existing mooring equipment, adequate crane capacity, and suitable deck space for transportation.
Crane Barge
The foundation suction pile is 60.3 tonne, while the manifold is 88.5 tonne. The manifold is landed and locked to the suction pile. A ROV is used to monitor the direction and position of the manifold during the entire installation procedure.
In water depths up to 1000 m (3280.8ft), manifolds are normally installed by cable from a crane barge or drilling riser directly, depending on the wire length and crane capacity. For water depths greater than 1000 m (3280.8ft), for which the length of the crane cable is not long enough to land the subsea structure directly on the seabed, an A&R wire and winch may be used to land the structure, and the load is transferred from the crane to the A&R wire at a water depth of approximately 100 m (328.1m).
DP Rig and Drilling Riser
Because the manifold size was bigger than the moon-pool size on the available rig, the manifold could not be lowered through the moon-pool directly, so a means to transport the manifold from the shipyard to the field was required. The rig was maneuvered so that it was placed over the transportation barge, allowing the riser connection and the lift operation. The manifold was then fixed by six stoppers and prior welded underneath the moon-pool to avoid movement during the transport and installation phases.
The installation methods by crane barge and by DP rig using drilling risers in a water depth of 500 m (1640ft) were compared based on the analysis of Petrobras projects. Analyzing the results and considering the barge’s daily
cost as around 80% of the rig’s daily cost, the following conclusions were made:
- Installation using a crane barge is not economically feasible in crowded areas where a lot of equipment and flowlines have already been installed at the seabed or in water depths where the mooring system demands special resources.
- Time spent waiting on the weather for an operational window and the mooring system deployment task represents around 35% of the total manifold installation cost when the installation is made by a crane barge. The installation of stoppers in the drilling rig, underneath the moonpool, represents 30% of the total manifold installation cost.
- The total cost of the installation by drilling riser is 25% lower when compared with the total cost of the installation by crane barge and there is a potential optimization that can be reached.
- The option of using a crane barge is still attractive because such a resource is promptly available and an installation by rig may impact the drilling schedule and completion of other activities.
Other Installation Methods
Installation of Sheave Manifold
The Roncador manifold project located in the ultradeep Campos Basin offshore from Brazil marks the first time a subsea manifold has been installed at a water depth exceeding 1600 m (5250ft). The Roncador manifold was installed at water depth of 1885 m (6184ft) using steel cables and support vessels.
Pendulous Method for Manifold Installation
The manifold has these dimensions: 16.5 m (54.4ft) (L) 8.5 m (27.9ft) (W) 5.2 m (17.1ft) (H), with a weight in air of 280 tonne at water depth of 1900 m (6233.6ft). Buoyancy sets are used to decrease the load on the rope.
Selection of Installation Method
The installation method and equipment selected for the subsea structure should ensure safe and reliable operation in accordance with the selected intervention strategy. The subsea production system should fulfill the following requirements:
- Installation equipment (temporary and permanent) should not cause obstructions and restrict intervention access.
- Disconnection of lifting slings and lifting beams/frames/arrangements used during installation should be according to the selected intervention strategy. A backup system may be provided.
- The installation system should not represent any hazard to the permanent works during installation, release, reconnection, and removal.
- Lifting/installation arrangements should be designed to minimize the lifting height.
The installation of a subsea system should satisfy the following issues:
- Be video -recorded during installation operations.
- Use installation tools with a fail-safe design.
- Allow flushing of hydraulic circuits subsequent to connection of interfaces.
- Where possible, not be dependent on unique installation vessels.
- Have position indicators on all interface connections.
- Be installable utilizing a minimum number of installation vessels.
- Require installation within a defined practical weather window that is consistent with the specific type of installation equipment and vessel to be used.
Installation Analysis
Subsea manifold design should be subjected to a thorough analysis to ensure that the structure can handle the installation, leveling, and lowering forces with proper safety factors. The normal practice is to perform the installation analysis during the final phases of the project in order to establish limiting weather criteria for the marine operations. However, if a preliminary dynamic analysis is conducted earlier, during the conceptual or design phase, a preliminary assessment of the main factors of dynamic loading may allow integration of structural design criteria and operational requirements. Lifting analysis can help to quantify dynamic forces and thus identify the critical stages of the operation. In cases where the hydrodynamic forces will determine the sea state limitation, an analysis can potentially enable an expansion of the operational weather window and optimization of the project schedule. During installation, the lifted structure is exposed to dynamic loading due to the motion of the installation vessel and the direct action of waves.The installation procedure involved in the lifting operations normally includes following steps:
- Barge lift: lifting from the deck of the crane vessel;
- Splash zone: lowering through the splash zone;
- Transferring to A&R wire: load from the manifold is transferred from the crane to A&R wire due to water depth;
- Lowering in deep water: lowering analysis at critical resonance depth;
- Landing on the seabed: the velocity and heave amplitude of the installed manifold should be controlled within the installation criteria to land the manifold safely. The characteristics of these installation steps are summarized in the
following subsections.
Barge Lift
The main purpose of the analysis is to determine the minimum crane lifting speed required to avoid recontact of the manifold with the barge deck while lifting. This minimum crane lifting speed could be obtained by using the criteria that the crane lifting velocity should be greater than the relative vertical velocity between the manifold base and the barge to avoid recontact. The pendulum motion of a structure due to crane tip movement is one of the limiting criteria. Bumper frames and tugger lines can be used to control the motion of the structure.
Splash Zone
The motions of the installation vessel combined with the motion of surface waves will create a significant load on the subsea structure when it is lowered through the splash zone. It is important to determine the wave loads during the passage through the splash zone and the added mass and damping when the subsea structure is submerged to the seabed. However, the added mass and drag coefficients are immersion dependent. The installed structure should be exposed to extreme direct wave loading in the analysis. The added mass associated with the installed structure at different submerged volumes may be estimated using the radiation/diffraction software Diodore, while the drag coefficients are estimated using CFD software. The slamming forces are estimated separately using the DNV code.
Transferring to A&R Wire
After lowering through the splash zone, the installed structure is lowered further to a water depth of about 100 m (328.1ft) to transfer the load from the crane to A&R wire if the crane wire is not long enough for the water depth. Once it reaches the transfer depth, the payout on the crane wire is stopped, the A&R winch wire is deployed and then connected with an ROV to the lowering yoke, and the load transfer is made from the crane main hook wire to the A&R winch. The hydrodynamic coefficients of the installed structure are kept the same as those of the final phase of the splash zone analysis. The tensions on the yoke and the motions of the installed structure during the upending procedure are checked.
Lowering in Deep Water
After transferring the structure load to the A&R winch, A&R wire is used to lower the structure to the seabed. To determine the peak vertical motions of the subsea structure during the lowering procedure, the resonance depth has to be calculated. In a regular wave analysis, the resonance occurs where the wave period corresponding to the maximum heave motions of the vessel in a particular wave period range matches the natural period of the subsea structure lowering system. The natural period of the lowering system depends on the length of the A&R wire, so at a certain length of A&R wire resonance occurs and the corresponding depth is identified as the resonance depth. The hydrodynamic parameters of the structure are the same as for the last step. Dynamic loading is mainly due to crane tip motion.
Landing on the Seabed/Subsea Structure
The hydrodynamic coefficients used in the landing analysis are same as those used in the lowering analysis. The maximum allowable touchdown velocity should be specified in the installation criteria. Installation may be performed in heave compensation mode to satisfy the criteria. The hydrodynamic parameters of the structure may be influenced by the seabed. Dynamic loading is mainly due to crane tip motion.
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