anchor selection and installation for shallow water mooring systems
TRANSCRIPT
Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference Stavanger, Norway, June 17-22, 2001 Copyright © 2001 by The International Society of Offshore and Polar Engineers 1SBN 1-880653-51--6(Set); ISBN 1-880653-53-2 (VoL 11); ISSN 1098-6189 (Set)
Anchor Selection and Installation for Shallow and Deepwater Mooring Systems
Roderick Ruinen and Gijs Degenkamp Vryhof Anchors BV
Krimpen ad Yssel, The Netherlands
ABSTRACT
The exploration and production of hydrocarbons is steadily progressing into deeper and deeper waters. Currently development plans are being made for projects in waterdepths exceeding 3000 m. This large range of waterdepths in which anchors are used, results in the requirements for an anchor being different for shallow and deep water locations. This paper will focus on the selection of anchors for shallow and deep water environments and present a way for the anchors to be installed.
KEY WORDS: Anchor, catenary, taut leg, installation, vertically loaded anchor, VLA.
~TRODUCTION
In shallow water (in this paper selected as 1000 m or less) the most common mooring line arrangement is the catenary mooring using predominantly chain and wire rope. An important feature of such a mooring is that part of the mooring line will generally be laying on the seabed during all loading situations. This results in a requirement for an anchor that can withstand large horizontal loads with small vertical loads. For these applications a wide selection of drag embedment anchors is available, of which a few will be reviewed indicating their area of application and restrictions based on project experience.
In a deep water mooring system (1000 m and deeper) the weight of a catenary mooring system using chain and wire rope will tend to become very large, giving restrictions to the design of the floater. One solution would be to lighten the weight of such a mooring line by including synthetic rope inserts. A more common solution for deep water mooring systems is to use a taut leg mooring, where the mooring lines enter the seabed at a large angle (up to 45°). These large mooring line angles in a taut leg mooring system result in an anchor being required that can withstand both large horizontal and vertical loads. For such an application a new type of drag embedment anchor has been developed, the vertically loaded anchor (VLA). The performance of the VLA will be examined, based on project experience.
One of the most common methods for the installation of anchors, both conventional and VLA, has been to use an anchor handling vessel to generate the required horizontal pulling forces. This system of installation works fine as long as the required installation load is within the capabilities of the anchor handling vessels. With the use of ever larger floaters, mooring in harsh environments and moving to ever deeper waters, the forces acting on the anchor have steadily increased, resulting in installation loads in the order of 6000 kN to 8000 kN becoming more common. For such installation loads a more efficient method of installation is to use a subsea tensioning device, allowing the anchors to be installed with vertical pulling capacity only using the vessels winches. The required vertical pulling capacity is generally in the order of 40% to 50% of the required anchor installation load. The use of the subsea tensioning device will be explained using project experience from both shallow and deep water areas.
DESIGN REQUIREMENTS
To allow an anchor to be used successfully for a certain project, the following information should be available for the anchor design process:
1. The calculated maximum intact and damaged design loads, 2. The soil conditions present at the site, 3. The type of anchor forerunner that is used, 4. The type of mooring system that will be used, 5. The required classification society for approval.
The importance of these parameters will be discussed in more detail below.
Design loads and classification society. The design loads will, in combination with the safety factors according to the specified classification society, determine the required anchor holding capacity. Typically the maximum design loads will be specified for the mooring line with the highest loads, resulting in the anchors all being designed for the same maximum holding capacity. However in a situation where the loading of the anchors is greatly dependent on the anchor location, for instance due to very directional environmental forces, it is possible to base the anchor design on the individual anchor loads. The design loads will also determine the required anchor installation load which is typically equal to the maximum intact design load. When compared to the required anchor holding capacity, the anchor installation load will
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generally be equal to 50% to 70% of the required anchor holding capacity (design loads multiplied by relevant factor of safety).
Soil conditions. For the dimensioning and proper installation of drag embedment anchors the availability of site specific soil data is important. The quality of the soil data determines both the choice for the size of the anchor and the accuracy of the installation parameters such as drag and penetration. Basically there are two extremes for the type of soil data that is supplied for the anchor design. The first is the situation where little or no soil data is provided for the intended mooring location. The second is the situation where a soil investigation has been performed at each of the intended anchor locations such as required for piles. The type of soil encountered at the intended mooring location will not only influence the anchor size, but will also determine the optimum setting of the fluke/shank angle. In the case where little or no soil data is available, the anchor design will be based on the worst soil conditions that can be expected at the site. The anchors that are designed for the site will be conservatively sized, to ensure sufficient holding capacity in the expected soil conditions. When soil data is available at all of the intended anchor locations, it is possible to optimise the anchor design for each location. The anchors can be optimised for each anchor location or for a cluster of anchors, depending on the clients wishes. This can lead to 2 or 3 different anchor sizes and even different types being required for the project.
Anchor forerunner. For the forerunner on the anchor either chain or wire rope can be used, although the use of a chain forerunner is more common. The use of a wire forerunner can however have significant benefits on the anchor performance in certain soil conditions, as a wire rope will have less resistance in the soil when compared to a chain forerunner of similar breaking strength. In very soft clay soil conditions, the use of a wire rope forerunner will allow the anchor to penetrate much deeper into the seabed and as a consequence a much smaller anchor is required than when a chain forerunner is used. The wire rope forerunner will also benefit the anchor performance in situations where there is a crust of hard soil overlaying a softer soil. The wire rope will penetrate much easier through the hard crust and thus allow the anchor to penetrate much deeper into the softer soil below.
Mooring system. For offshore mooring applications two different types of mooring system are commonly used, the catenary mooring where the restoring forces are generated by the weight of the mooring line and the taut leg mooring system where the restoring forces are generated by the elasticity of the mooring line. The catenary mooring system is generally used in shallow water while the taut leg mooring system is more appropriate in deep water applications. A catenary mooring system is generally designed in such a way that part of the mooring line is always in contact with the seabed. This results in a requirement for an anchor which is capable of withstanding large horizontal loads but does not need to withstand significant vertical loads. For the catenary mooring system the conventional drag embedment anchor is appropriate. In a taut leg mooring system the mooring lines enter the seabed at a significant angle (generally between 30 ° and 45°). This means that the anchor has to be able to withstand both high vertical and horizontal loads. For this type application the vertically loaded anchor (VLA) has been developed.
ANCHORS FOR SHALLOW W A T E R MOORING SYSTEMS.
For catenary mooring systems in shallow water there are many different anchors available on the market. Some examples are given below.
Stevin Mk3 anchor (see Fig. 1), The Stevin Mk3 anchor is a hinged anchor type (allows movement of the shank to both sides of the fluke), which means that when landing on the seabed it always turns itself into the fight position for burial. It can be simply dropped without assistance from an anchor handling tug. The Stevin Mk3 is still used around the globe on dredgers, drilling rigs, barges, supply vessels and crane barges.
The fluke/shank angle is adjustable into two positions, the mud angle for very soft clay and the sand angle for soil such as medium and hard clay and sand. The Stevin Mk3 is comparable in performance with the Flipper Delta anchor.
li
Fig. 1 - Stevin Mk3 anchor.
Stevpris Mk$ (see Fig. 2). An anchor design with a very high ratio of ultimate holding capacity (UHC) versus anchor weight and a very quick and deep penetration, i.e. a very short drag length. The deep penetration of the Stevpris anchor makes it very suitable to resist uplift forces that might occur in deep water where the anchor lines are relatively short. Most omen the anchors are being used by pipe-laying barges, dredges, dfill-rigs and other vessels that move rather often and that set their anchors also on a regular basis. The Stevpris Mk5 anchor is comparable in performance with the FFTS Mk4 anchor.
Fig. 2 - Stevpris Mk5 anchor
Stevshark Mk5 (see Fig. 3). For difficult soils the Stevpris anchor is adapted to penetrate cemented carbonate soils and renamed Stevshark. The Stevshark anchor can be equipped with serrated shank and cutter-teeth for better penetration. The fluke points are specially reinforced to withstand high point loads.
Fig. 3 - Stevshark Mk5 anchor
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DIMENSIONING OF CONVENTIONAL DRAG E M B E D M E N T ANCHORS.
A reliable method for the dimensioning of anchors is based on the comparison of anchor behaviour in similar soils. The test data gathered over the years have allowed Vryhof to derive design lines by plotting all the data for tests in similar soil on log-log paper. The design lines that represent these data are shown in Fig. 4 and show a relation between anchor weight and UHC based on a power law formula. Such a formula is used to determine the UHC of other (usually larger) anchors by extrapolation.
UHC = A × W s [ l d ' , I ] (1) The parameters UHC and W in the formula have been expressed in
[kN], the parameter B has no unit, the parameter A has the unit [kNt'a]. Both contain all the unknown factors that are related to scale and other influences. The values of A and B depend on the type of anchor and the type of soil conditions encountered. Fig. 4 shows the holding capacity chart for the Stevpris Mk5 anchor based on the above mentioned formula.
For the Stevin Mk3 and Stevpris Mk5 anchors, the following A and B parameters have been derived for use in formula (1), depending on the soil conditions and the type of forerunner (chain or wire) that is used.
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A Stevpris anchor has been tested in the Gulf of Mexico with gradually increasing pull angle. The maximum resistance was obtained for 180 upli~ at mud line. Although a slightly lower load was expected, this load happened to be equal to the UHC determined at 0 ° uplift. The API RP-2SK on Floating Production Units recognises this effect and allows a 50 uplift in intact condition and 10 ° in damaged condition. These angles appear small, but they can mean an enormous saving on anchor lines in deep water areas.
V L A - T h e S t e v m a n t a (see Fig. 5). To meet industry demands for an anchor suitable for taut leg mooring systems, an extensive testing program has been completed which has led to a new type of anchor, the Stevmanta VLA, where a traditionally rigid shank has been replaced by a system of wires connected to a plate. The anchor is designed to accept vertical (or normal) loads and is installed as a conventional drag embedment anchor with a horizontal load at the mudline to obtain the deepest penetration possible. By changing the point of pulling at the anchor, vertical (or normal) loading of the fluke is obtained thus mobilising the maximum possible soil resistance.
As the Stevmanta VLA is deeply embedded and always loaded in a direction normal to the fluke, the load can be applied in any direction, i.e. between 0 ° and 90 °. Generally the anchor is used for taut-leg mooring systems, where generally the load angle varies from 25 ° to 45 ° . The angle adjuster changes the mode of the anchor from pull-in mode to vertical (or normal) mode.
Currently Petrobras has the Stevmanta VLA in use on 3 production platforms offshore Brazil, being the P-27, the P-36 and the P-40 in waterdepths of 500 m, 1500 m and 1000 m respectively.
Fig. 5 - Stevmanta VLA
STEVMANTA VLA CALCULATION.
Based on geotechnical principles for embedded plates in soil, the ultimate resistance, referred to as the _ultimate l~ull-out capacity (UPC), of the Stevmanta VLA can be calculated with the following formula:
UPC = N c xS u x A (2)
With N¢ the beating capacity factor, Su the undrained shear strength of the soil at the penetration depth of the Stevmanta VLA and A the area of the Stevmanta VLA.
In the soil conditions where the Stevmanta VLA is currently designed for (very soft to soft clay), the undrained shear strength of the soil is generally given as a function of the penetration depth (D) below seabed, i.e. a formula of the following form:
S u = k 0 + k I x D (3) For a plate installed to a certain depth in the soil and subjected to
vertical or inclined loads, two different failure mechanisms can be identified, being:
1. Shallow failure. This failure mechanism occurs when the anchor is not deeply embedded, typically less then 3 fluke lengths. The failure mechanism is characterised by the anchor
being pulled out of the soil with the entire column of soil above it.
2. Deep failure. When the anchor has embedded more than 3 fluke lengths into the soil the deep failure mechanism will occur. The failure mechanism is characterised by the flow of soil from the top of the plate to the bottom of the plate (plastic failure of the soil).
The most economical anchor design occurs when the anchor can be embedded to a depth were the deep failure mechanism occurs, typically more than 20 m below seabed.
Based on the test results of the Stevmanta VLA, the bearing capacity factor Nc is found to be equal to 12 for a Stevmanta VLA which has penetrated deeply into the soil (3 or more fluke lengths). This value of 12 for the bearing capacity factor corresponds well with values found in literature.
With the UPC known for a specified anchor size at a specified depth in specified soil conditions, a method had to be found for determining the attainable depth for this situation. Firstly the different factors affecting the penetration depth of the anchor were inventoried. The following factors were found to be of influence to the attainable penetration depth:
* The undrained shear strength of the soil, basically given by the factor kl in formula (3).
• The diameter of the mooring line (d) connected to the angle adjuster. The larger the diameter becomes, the higher the resistance is to the mooring line and the shallower the penetration becomes.
• The area of the anchor (A). A larger anchor will encounter more resistance in the soil.
• The fluke/shank angle (~) used. Applying the results of the tests with the different Stevmanta VLAs
to the above mentioned factors affecting the anchor penetration, the following formula was derived:
D = 1.5 x kt °'6 x d -0"7 x A °'3 x (tan~) 1"7 (4)
The required Stevmanta VLA area can now be calculated using the previously mentioned formula. The only variable that still needs to be determined is the required installation load (Fi,st). The required installation load is given by:
UPC Finst = (5)
rv/h Based on the test results of the Stevmanta VLA, the ratio between
the applied horizontal load and the vertical load (rye) we use, varies between 2.5 and 3.5. Evaluating the performance of the Stevmanta VLA over the years, it can be concluded that the ratio between the UPC and the installation load has increased from 1.8 : 1 with the early prototypes to 3.5 : I with the current model of the Stevmanta VLA.
Using the above mentioned formulas, the design method for the Stevmanta VLA is as follows (see also Fig. 6):
1. Determine the required Ultimate Pull-out Capacity (UPC) in kN.
2. Determine the undrained shear strength profile of the soil. 3. Determine the anchor penetration depth as function of the fluke
area. 4. From formulas (2), (3) and (4) the required Stevmanta VLA
fluke area (A) is found. 5. The required Stevmanta VLA installation load is determined by
using formula (5).
6 0 3
1 4" (.,
tO
0 = ,
I
6 D..
"5.
2000
1800
1600
1400
1200
1000
SO0
600
4OO
200
0 0
i
/ , ( •
I ~ •~ 0 ! # • ! e •
t l t i
I t
/a .....
/
0 5 10 15 20 25 30
Stevmanta Fluke Area (m ~) - - -~ .
Fig. 6 - Stevmanta VLA typical holding capacity graph Curve A - 76 mm diameter wire rope Curve B - 121 mm diameter wire rope Curve C - 151 mm diameter wire rope
STEVMANTA VLA EXPERIENCE
The Stevmanta VLA has been successfully used on three projects to date, the P-27 (Ruinen and Degenkamp 1999), P-36 (Henriques et al. 2000) and P-40 production units offshore Brazil. A summary of the design and installation data is presented in table 2. The values presented for the installation load, penetration depth and drag length are the avera values encountered.
Required UPC 6852 kN Anchor 11 m z Stevmanta VLA Installation load 2805 kN Penetration depth 23.5 m Drag length 45 m
Required UPC 8600 kN Anchor 13 m ~ Stevmanta VLA Installation load 3442 kN Penetration depth 30 m Drag length 75 m
> ) i : ~ ~,; ~ !~ !, ~ :i:~, i ~ ,, ~:~ ~4S~)P~i Required UPC Anchor Installation load Penetrati0n depth Drag length
9208kN 13m 2 Stevmanta VLA 4609kN 2 0 m 4 3 m
Table 2
A N C H O R INSTALLATION.
For the installation of anchors, various methods are commonly used today. Three of these methods will be discussed in more detail.
Anchor installation using an anchor handling vessel (AHV). The most common method for anchor installation is using an AHV. The AHV deploys the anchor and the mooring line to the seabed and uses the bollard pull to embed the anchor into the seabed to the required installation load (see Fig. 7). The maximum installation load that can be generated depends on the boUard pull that is available. In case the required installation pull exceeds the bollard pull of the AHV, an option is to use two AHVs to pull in tandem.
/
Fig. 7 - Anchor installation with AHV Subsea tensioning device and conventional drag embedment
anchor. A different method to install the anchors is to use a subsea tensioning device. Using this method two anchors are tensioned against each other (see Fig. 8). The installation vessel applies a vertical force to the subsea tensioning device, which results in a larger horizontal force on the anchors. Typically the required vertical force will be in the order of 40% to 50% of the required anchor installation load. This means that using a subsea tensioning device the installation can be performed using an AHV or a crane barge. One of the requirements for conventional drag embedment anchors is that they are installed in a direction that is approximately the same as the direction that the actual mooring load is applied in. This results in the anchor having to be tensioned across the centre of the mooring circle. In situations where there is no anchor present directly opposite the actual mooring anchor (for instance when 9 anchors are used for the mooring in 3 cluster of 3 anchors) a reaction anchor will have to be used to tension the anchor against.
Fig. 8 - anchor installation with subsea tensioner
Subsea tensioning device and vertically loaded anchor (VLA). Like a conventional drag embedment anchor, the VLA can also be installed using a subsea tensioning device. But because the Stevmanta VLA can be installed in any direction compared to the actual mooring direction, the possibilities for the anchor installation are much greater. For example, in a mooring system with the anchors grouped in clusters, it is possible to install the two anchors situated next to each other
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instead of tensioning across the centre of the mooring (see Fig. 9). For the Stevmanta VLA a new installation procedure has been developed using only one AHV to perform the installation of two anchors at the same time. This method uses the newly developed subsea connectors and a subsea tensioning device. Briefly the methods is as follows:
Fig. 9 - alternative installation with subsea tensioner
The AHV deploys the installation system consisting of the Stevmanta VLA 2, a subsea connector, the subsea tensioner with a male portion of a subsea connector on top, a tri-plate with male portion of the subsea connector and a breaking link and Stevmanta VLA 1. The deployment wire is connected though a subsea connector to the tail of Stevmanta VLA 1 (see Fig. t0).
/ / / /
disconnector
Stevmanta 1
disconnector and break link
Stevtensioner and disconnector
disconnector
Stevmanta 2
"////////////
Fig. 10 - anchor deployment
When the entire system is on the seabed, the deployment wire is disconnected from the tail of Stevmanta VLA 1 (position 1 in Fig. 11) and moved to the male part of the subsea connector on the subsea tensioner (position 2 in Fig. 11). The AHV then performs the tensioning procedure. When the tensioning has been completed, the break link will free Stevmanta VLA 1 from the tensioner. The subsea connector on Stevmanta VLA 2 is then also disconnected and the subsea tensioner is recovered to the surface. The mooring lines can then be connected to the male parts of the subsea connectors that are connected to the anchor forerunner of Stevmanta VLA 1 and 2.
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\
I \ 2
Fig. 11 - configuration on seabed (rotated 90 °)
CONCLUSION
The importance of the anchor design parameters such as the design loads and soil conditions has been shown and how they influence the performance of the anchor. The application of different types of anchors in mooring systems has been discussed and how anchors can be used in shallow and deep water mooring systems. Using the presented installation methods for the anchors, it is shown how economical mooring system installation can be performed in any water depth.
REFERENCES
Henriques CCD and Fachetti, MB (2000). "Roncador Field: Transport and Installation of the Mooring System" Offshore Technology Conference, OTC 2000, Houston, USA, OTC 12141.
Ruinen, RM and Degenkamp, G (1999). "First Application of 12 Stevmanta Anchors (VLA) in the P27 Taut Leg Mooring System." Deep Offshore Technology, DOT 1999, Stavanger, Norway, Vol.. 2., session 15.
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