PLEM Foundation (Mudmat) Sizing and Design
The PLEMs foundation provides a support for the PLEM structure. A preliminary mudmat area or dimensions for the PLEM are estimated based on the soil bearing capacity and the estimated weights of the hubs, valves, piping, jumper loads, and mudmat. The mudmat may be designed with skirts for stability and to reduce settlement and movement on the seabed. The dimensions will be adjusted with consideration given to the limitations imposed by the installation vessel, fabrication shop, and overland and sea transportation situations. The design should be analyzed for structural integrity in terms of:
- In-place operating conditions, structural loads, and fatigue damage;
- Thermal expansion of the pipelines.
Knowledge of the soil properties at the PLEM target area is required to properly size the mudmat, to determine stability, and to estimate short-term and long-term settlement. These soil data are more important for a mudmat than the deep soil sample required for pile design.
General load conditions for mudmat sizing are as follows:
- For the PLEM landing condition, the submerged weight, including the hubs, hub end caps, piping, and the support frame are applied to the foundation.
- For the jumpers installation case, the loads from the jumper, connector tool, and ROV are added.
- For the operation case, the ROV and connector tool are removed and the jumper operating loads introduced.
- For the maintenance case, ROV loads are added to the operation case. For two-hub PLEMs, loads on one hub are kept as operating but ROV loads are added to the second hub.
- For the case of operation and maintenance in the thermal expanded positions, the location of the hubs and loads are transferred to the expanded position on the mudmat foundation. The foundations should be designed to support the PLEMs with the design loads as indicated, within limits of the required safety factors in accordance with API RP2A (WSD) and aminimum factor of safety for soil bearing of 2.0.
The design of mudmat foundations should include the following issues per API RP 2A-WSD:
1. Stability, including failure due to overturning, bearing, sliding, or combinations thereof;
2. Static foundation deformations, including possible damage to components of the structure and its foundation or attached facilities;
3. Dynamic foundation characteristics, including the influence of the foundation on structural response and the performance of the foundation itself under dynamic loading;
4. Hydraulic instability such as scour or piping due to wave pressures, including the potential for damage to the structure and for foundation instability;
5. Installation and removal, including penetration and pull-out of shear skirts or the foundation base itself and the effects of pressure build-up or drawdown of trapped water underneath the base.
The objective of checking the overturning capacity for a shallow foundation is to ensure that, under the applied loads, the foundation will be stable if placed on a hard surface. Overturning could take place either around the longitudinal edge or the transverse edge of the foundation. The governing overturning direction will yield the lowest safety factor. The followingformula is used to calculate the overturning safety factor of the shallow foundation around one of its edges:
in which M is the resultant overturning moment in the overturning direction, Fz is the resultant vertical force acting at the geometric center of the mudmat, and L is the distance from the geometric center to the rotating axis.
Load eccentricity decreases the ultimate vertical load that a footing can withstand. This effect is accounted for in bearing capacity analysis by reducing the effective area of the footing according to empirical guidelines. The calculation method of effective area is detailed in the section title “C6.13 Stability of Shallow Foundations” of API-RP-2A. The recommendations pertaining to the design issues for a shallow foundation are given in Sections 6.13 through 6.17 of API-RP-2A and include:
- Stability of shallow foundations;
- Static deformation of shallow foundations (short-term and long-term deformation);
- Dynamic behavior of shallow foundations;
- Hydraulic instability of shallow foundations;
- Installation and removal of shall foundations.
The following sections provide a detailed design example for a mudmat based on DNV Classification “Foundations”.
Penetration Resistance of Skirts
The height of the skirts is designed to guarantee that with the occurrence of the maximum estimated consolidation settlement, the structures will retain their ability to be displaced freely through the mudline. The skirt penetration resistance in sand, Qp, is estimated according to DNV code:
Bearing Capacity during Installation
Themudmat minimum dimensions required to keep the structure supported at the mudline are a function of the ultimate vertical capacity of the soil, considering the effect of the resultant load eccentricities (related to its length and width), as a reduction of the mudmat area (effective area concept). Assuming that the subsoil contains sand, the vertical bearing capacity is checked according to:
Bearing Capacity during Operation
Potential soil failure modes due to trawler loading and thermal expansion loading include failure from lateral sliding and deep-seated failure. A short description of calculation principles for the different failure modes is given below.
The lateral soil resistance against pure sliding of the foundation may be calculated by:where, r tan is the friction between foundation and seabed and W is the vertical load on the mudmat. If a rock berm is placed on the mudmat, additional lateral capacity (earth pressure) is obtained according to:
Deep-Seated Vertical Failure
A foundation’s stability is based on the limiting equilibrium methods ensuring equilibrium between the driving and the resisting forces, according to Janbu et al. The foundation base has been idealized to account for load eccentricity according to the principle of plastic stress distribution over an effective base area. In addition, a verification of the final rotation (loss of horizontality) of the structures should be done, considering the overturning moment envelope that will act during the entire life of the structure.
The settlement of the structures is analyzed according to Janbu’s method.The soil strains 3 are calculated by:
where H is soil layer thickness.
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