Perhaps one of the most challenging features of an integrated design approach, is the heavily multidisciplinary nature of the design work we undertake. There are many examples of how an optimised design can be achieved and the benefits of taking an integrated approach to design. In this blog post, I shall use pipeline protection system as a vehicle to highlight some of the interesting opportunities we can exploit.
Pipeline Protection Systems
Surface laid pipelines have traditionally been protected using crushed rock (rock dump), concrete mattresses or other forms of mechanical protection. This provides a form of restraint on the pipeline, with the corresponding forces generating global stability (buckling) problems and considerable loads at the tie-in structures. This has led to more sophisticated protection methods, e.g. covers in Glass Reinforced Plastic (GRP), steel or concrete, See Figure 1.
Figure 1: A Typical “Dome” GRP Protection Cover
Main Design Considerations
The design considerations for protection systems are similar whether a pipeline, subsea structure, midline tee or end termination are considered. A pipeline protection system will be used as an to illustrate these design considerations.
Figure 2 below presents a typical GRP protection system installed over an expansion loop close to the Tie-in point of a pipeline and a subsea manifold. The system consists of a series of covers interlocked by articulated “domes” at each end and positioned to allow the pipeline to displace freely laterally and axially. In the state illustrated, the covers can be considered to be in a temporary state prior to being stabilised against trawling loads by graded rock fragments (rock dump).
Figure 2: Overview of a Pipeline Protection System
Pipeline System Design
For pipeline system design, it is the flow assurance constraints that often define the operating conditions during the life-cycle of the pipeline system.. In the case of a high temperature or pressure product there may be a requirement to bury the pipeline to provide thermal insulation, protection and to ensure global stability. High temperature and pressure conditions also result in large axial and lateral displacements of the pipeline. Resisting such displacements can generate high loads that may be transferred to tie-in structures. Expansion loops are therefore often utilised to provide a controlled form of displacement and thus limit the forces transferred to tie-in structures. The geometry of such expansion loops, including the maximum axial and lateral displacements of the pipeline are a key consideration when designing the protection system.
Trawl-Gear Interaction and Hydrodynamic Loading
The stability of covers on the seabed can be a major challenge. The high forces from current and wave action in shallow water may lead to sliding and overturning of covers. This is overcome by adding ballast weight. The combination of ballast weight and external forces, such as trawl-gear interaction may again lead to excessive bearing pressure on the seabed, causing soil failure. Selecting the optimum structure geometry and mudmat size to minimise the effect of external forces while satisfying stability constraints is therefore a key step in the design process.
Overview of the Protection System Design Process
A design methodology for subsea protection systems can be idealised as shown in Figure 3. Optimisation is achieved through integration of the structural, geotechnical, naval and fabrication aspects of the protection system design. Use of advanced analysis techniques including computation fluid dynamics (CFD) and non-linear finite element modelling of soil-structure interaction, can be integrated into the design process.
Figure 3: Integrated Design Approach for Protection System Covers
Optimised Installation Procedure
During deployment, covers are subject to forces from hydrodynamic drag and added mass. These forces are dependent on the size of the covers and the motion of the installation vessel. Since GRP covers are light relative to their area, they are subject to high added mass and drag coefficient during deployment. GRP covers can be designed to be “stackable”, this saves deck space and installation time, at the same time as increasing the deployment weight and significantly reducing the risk of slack slings.
A dynamic amplification factor (DAF) of 2 is normally used when designing deployment rigging. A less conservative factor can be justified for GRP covers through the use of CFD analysis to establish the correct drag coefficient. A deployment analysis including predicted crane tip movement can be performed using the correct DAF for a set of stacked covers. A structural design check can also then performed to check the integrity of the covers during deployment. In Figure 4 below a set of eight GRP protection covers are being over-boarded in a deployment frame.
Figure 4: Deployment of a stack of GRP covers
Reduced Trawl Gear Interaction Loads
Subsea installations located in areas with fishing activity need to be designed with protection systems to avoid damage to the installations and, in some areas, to avoid obstruction of fishing activity. Covers outside the safety zone of a platform are normally designed to be over-trawlable. Trawl loads are given in NORSOK U‑001 with dropped object and fishing gear loads tabulated as show in Table 1. While these loads are generic for typical fishing gear in the North and Norwegian seas, the standard also allows the loads to be reduced where model testing demonstrates that a design is subjected to a lower load conditions.
Table 1: Fishing gear interaction loads after Norsok U-001
In Table 1, the factor of two on trawl net friction implies that the 200kN load was measured on each warp line attached to the trawl net, the total over-trawling force associated with net friction is therefore 400 kN. Snagging is normally considered as an abnormal operation (PLS), while impact and frictional loads caused by passing fishing gear shall be regarded as normal operation (ULS). This applies unless the frequency of trawling is low enough to considered such an event as a PLS condition. These loads can be disregarded if the structure geometry can be documented to be overtrawlable/snag free through model tests or geometric evaluations combined with relevant model tests.
Structure specific over-trawlability tests can be used to investigate the loads experienced during interaction with trawl nets and otter-board (trawl door) impact (Figure 5). From these tests it can be proven whether the structure is “over-trawlable” i.e. not snaggable, and in addition, establish trawl net friction and trawl board over pull loads. These loads can be significantly lower than those found in the NORSOK standard.
Figure 5: Scale model testing of over-trawling events
Structural Design Of GRP Cover
Trawl loads are modelled as a force acting on a small area of the cover. The magnitude of the force depends on the nature of cover, location and water depth. Figure 6 below presents a plot of a segment of a GRP cover being subjected to trawl load. Using a realistic degree of fixity for the mudmats based on the geometry of the rock dump and interface between the mudmat and the underlying seabed soil can yield significant savings in reinforcement.
Figure 6: Trawl load and boundary conditions
Route Selection, Expansion Loop Geometry & The Number of Covers
The tie-in design of a pipeline subject to temperature or pressure induced expansion is often a critical design exercise, the requirements are normally two-fold:
1. Control displacement of the pipeline such that the minimum bend radius or wall stresses are within acceptable limits;
2. Limit axial forces and bending moments to fall within bounds established for tie-in system components.
The optimisation of route alignment and forces transferred to tie-in equipment is achieved through detailed modelling of the route alignment adjacent to the tie-in point. A typical tie-in model is presented in Figure 7 below, for the rock dumped case (A) and with GRP covers (B).
Figure 7: Tie-in modelled with rock dump (A) and with GRP covers (B)
Between the two extreme cases demonstrated in Figure 8 there is a point at which the effective axial force and induced bending moments in the Tie-in hub result in an optimum degree of stress utilisation. For the case in which the pipeline is protected utilising rock dump, the effective axial force and bending moments at the Tie-in hub are high (See Figure 8-A), for a similar temperature and pressure the equivalent force profile for the case in which the pipeline is protected using GRP covers is shown in Figure 8-B. The force is approximately 55% lower in this case.
Figure 8: Effective axial force a vs. distance from Tie-in hub for the fixed case (A) and free case (B).
With the above knowledge, the axial force and bending moment profiles are used to optimise the length of pipeline protected using GRP covers, with the remainder of the line being protected using rock dump or trenching solutions. In this way it is possible to ensure that the length of GRP covers installed is reduced to the minimum possible.
CFD analysis consists of transient viscid CFD computations providing the forces and moments acting on the protection covers. The CFD analysis can be used to obtain an accurate assessment of the drag co-efficient for a structure considering the effect of rock installed over the mudmats.
For all the CFD simulations, the horizontal force, the vertical force and the over turning moment about the downstream mud-mat edge can be monitored. Typically a number of wave oscillations are run for each case to assure that the force times series do not change from one period to the next.
Figure 9: Pressure distribution and velocity vectors on cover
The time series of the horizontal force, the vertical force and the overturning moment from the CFD analysis can be post-processed to find the maximum values and worst load combinations. The force time series of the forces and moment are further used as input to check the criteria for sliding, lifting and overturning.
Figure 10: Time series of horizontal force, lift force and tilting moment
The selection of mudmat width and rock berm geometry are key stages in the protection system design. There is significant scope for optimisation of both elements if the interaction between structure, rock berm and supporting seabed soil is analysed correctly.
The interaction between a GRP cover and the seabed soils can be modelled to confirm the critical failure mode. The failure mode is controlled by the strength of the seabed sediments, stiffness of the GRP cover and any supporting rock installed under the cover to improve stability. Figure 11 presents a brief overview of a structure placed on sand seabed and stabilised utilising rock dump.
Figure 11 : Trawl event and yielding of active footing
From the above Figure it is apparent that the left hand or active side of the cover has deformed (displacements scaled up x100). In a standard limit equilibrium analysis or hand calculation, this would be taken as failure and the footing size would need to be increased or the weight of stabilising rock increased, depending on whether the footing yielded in sliding or bearing.
In this example, as the active footing yields in sliding, load is redistributed to the passive side as shown in Figure 12 below. The load distribution process continues as the mudmat on passive side of the GRP cover mobilises additional soil resistance such that the global stability of the GRP cover is maintained.
The ability to distribute load from a yielding footing to a stable footing depends upon the strength and stiffness of the GRP laminate and therefore an integrated design approach. Having establish such degree of integration in the design it is possible to exploit this and develop novel solutions that would be impractical if elements of the design package were considered separately.
The opportunities for optimising individual elements of a system design are greatly enhanced when using an integrated design approach. This is so for most structural designs, and the dependencies between disciplines need to be well understood and mapped to exploit these opportunities. If a tightly integrated and well motivated design team can be mobilised on a project then the process can be quite easily managed. The benefits for our example case are presented in Figure 13 below: