Paper Session 12 Iversen r

PIANC World Congress San Francisco USA 2014

1 of 10

STRUCTURAL CONSIDERATIONS FOR SELECTION OF QUICK-RELEASE MOORING HOOKS

By Rune Iversen, P.E.1

This paper presents potential problems and suggested methodologies for selection and installation ofmooring hooks on existing structures, focusing on installations at marine oil terminals. The papergives a presentation of current requirements and guidelines, and for selection of mooring hooks fornew design related to vessels that will call at the terminal and environmental conditions. Theseguidelines are then discussed with relation to when mooring hooks will be installed on existingstructures, with suggestions for how to solve potential problems that might arise. An example case ispresented with the challenges that were encountered during selection and design of the new mooringhooks and what the proposed solutions were.

1. INTRODUCTION

Quick release mooring hook installations in new design will in a best practice scenario adhere to ahierarchy of failure modes that attempts to maximize safety and minimize economic impact if themooring system is overloaded. It has been identified that the successive modes of failure should beas follows: winch brake rendering, mooring line failure, mooring hook failure, mooring structure failure.While the concept is apparently simple, the safety factors and design guidelines provide a less thanclear path when any changes are made to an existing mooring system. These changes can occur atdifferent levels, including change in mooring line strengths or mooring line types that are seen atvessels calling at a terminal, upgrades of mooring hardware at a terminal due to regulatoryrequirements, or upgrade of mooring hardware due to changes of service at a terminal.

It is very important to realize that the requirements for sizing mooring hooks and design and evaluationof their supporting structures can be very different depending on whether the mooring hooks will befitted onto new construction or to an existing structure. In new design, large factors of safety areencouraged recognizing the long operational life of a new structure and the desire to provide ameasured amount of flexibility in the design to accommodate future operational changes. In anevaluation of existing structures one can usually accept much lower factors of safety and limits inoperations are often already in place based on said evaluations.

2. SAFETY FACTORS IN MOORING SYSTEMS AND RELATEDUNCERTAINTIES

There seems to be a common understanding across the marine industry that quick release mooringhook installations in new design should, in a best practice scenario, adhere to a hierarchy of failuremodes that attempts to maximize safety and minimize economic impact in the event of mooringsystem overload. It has been identified that the successive modes of failure should be as follows:

1. Winch brake rendering (not a real failure mode but rather a safety measure that will helpdistribute loads in case of overloading)

2. Mooring line failure3. Mooring hook failure4. Mooring structure failure

While it is often stated that this is an important safety consideration to prevent undesirable andunpredictable failures, the discussion below suggests that this might not be the case. The workingdocument for PIANC MarCom Working Group 153 also suggests that this hierarchy of failure modes isfor design purposes only. In an evaluation performed for installation of mooring hooks, whether it isfor new design or on existing structures, it is important to keep these seemingly contradictingstatements in mind.

It should be noted that none of these failure modes are expected to be reached during normaloperating conditions. As can be seen below, large factors of safety are built into most stages of themooring system.

1Senior Staff II, Simpson Gumpertz & Heger Inc., [email protected]


2 of 10

2.1 Winch brake limit

The winch brake limit on the vessel is strictly speaking not a failure mode, but rather a safety measureto distribute forces more evenly in case of an accidental overload of mooring lines. Winch brakerendering is the anticipated first mode of “failure” in a mooring system. If winches start rendering,loads will be more evenly distributed and better shared between several mooring lines and mooringhooks. Oil Companies International Marine Forum (OCIMF) suggests that winches be designed tohold at least 80% of the Minimum Breaking Load (MBL) of the mooring line, but have the capability tobe adjusted down to about 60% of MBL. Even with the winch set to render at 60% of MBL, the actualload where it might render could be as high as 70 to 80% of MBL.

If mooring lines are changed on a particular vessel to a stronger or weaker line there is no guaranteethat these limits will still be in effect, and care should be taken to adjust the winch break limits if such achange is made.

2.2 Rope strength

The first real safety measure in mooring systems is the fact that during mooring analyses and mooringsystem design, mooring line loads are limited to between 45 and 55% for lines, and 40 and 43% fortails, depending on the mooring line material. Table 1 gives an overview of these limits as presentedby OCIMF:

Line Material Allowable Load (% of MBL)

Steel 55%

Polyamide (nylon) 45%

Other synthetic lines 50%

Polyamide tail 40%

Other synthetic tail 43%

Table 1 - Allowable mooring line loads

In an accidental load scenario, if the winches do not render properly once the mooring lines aretensioned past the allowable loads, the next failure mode should ideally be breaking of the mooringlines. However, there is considerable uncertainty in the actual capacity of the mooring lines.

The reported mooring line strength might be different depending on whether it is quoted by themanufacturer or the vessel. Vetting services for the vessel such as Q88 will usually report theMinimum Breaking Load (MBL), which is usually used in all mooring analysis. Manufacturers oftenreport both Minimum Breaking Load as well as Mean Breaking Load, where the Minimum BreakingLoad is usually set at 2 standard deviations below the Mean Breaking Load. For practical purposesthis usually translates to a difference in 10-15% between the two.

With an assumed normal distribution of rope strength around the mean value, the Maximum BreakingLoad (2 standard deviations higher than mean) can be 10-15% stronger than the Mean BreakingLoad. The Maximum Breaking Load of rope with a stated MBL can therefore be up to 30% strongerthan the assumed value.

Although most lines can be assumed to be stronger than the stated MBL in new condition, wear andtear on the lines will over time reduce the actual strength of the lines.

2.3 Hook strength

Following the hierarchy of strength as stated above, the hooks are next in line as a failure mode.When comparing the two, the MBL of the mooring lines is usually compared to the Safe Working Load(SWL) of the mooring hooks.

Mooring hooks are manufactured with a stated SWL from the manufacturer. This is usually thecapacity referenced by guidelines and codes when recommendations are made for sizing of mooringhooks. The SWL is usually well below any real failure modes of the mooring hooks, and three morelimit loads are often stated from the manufacturer:


3 of 10

Proof load – 125-150% of the stated SWL Yield load – often about 200% of SWL Ultimate load – 2-300% of SWL

By inspection one can see that the proof load of the hooks will be close in value to the anticipatedMaximum Breaking Load of the mooring lines if the SWL of the hooks are selected to match the MBLof the mooring lines.

As can be seen there is a lot of safety built into the design of the hooks themselves. In addition,depending on the project, hooks usually rated for a certain SWL can be used for lower rated hooks,thereby increasing these safety factors further.

2.4 Base strength

Manufacturers of mooring hooks indicate in their documentation that mooring hook bases aredesigned to be able to withstand all hooks loaded to 100% of their SWL. This can be assumed to be aSafe Working Load for the bases, with additional safety factors for yield and ultimate strength.

2.5 Anchor strength

Anchor patterns and the number of bolts used for mooring hook bases vary between manufacturers.Bolts are typically around one meter in length (40 inches), with diameters between 46 and 90 mm.With increasing hook capacities and numbers, some manufacturers increase the number of bolts whilekeeping the diameter of the bolts constant, other manufacturers increase the bolt diameter whilekeeping the number of bolts the same. In general, bolt capacities are rarely a limiting factor in thecapacity of the mooring hook assemblies.

When anchor bolts are anchored into concrete, two main installation types can be used. The anchorbolts can be embedded into the concrete, relying on concrete breakout strength for uplift capacity. It isrecommended that additional reinforcing is placed around the anchor bolts to ensure that a propercapacity is provided. The anchor bolts can also be extended to go all the way through the concretedeck, in which case the capacity of the anchors in uplift will be determined by the bearing capacity ofwashers and bearing plates in the deck soffit.

Figure 1 – Example of mooring hooks with circular anchor bolt layout


4 of 10

Figure 2 - Mooring hooks with rectangular anchor bolt layout

3. CURRENT DESIGN STANDARDS AND GUIDELINES

There are very few clear-cut guidelines or regulations pertaining how to do a full new design ofmooring hooks and the supporting structures. The State of California takes one of the morecomprehensive approaches in its Marine Oil Terminal Engineering and Maintenance Standards(MOTEMS), with knowledge and methodologies drawn from several of the other sources mentionedbelow. Common among most publications is to size individual mooring hooks by a comparison of theSafe Working Load (SWL) of the hooks with the Minimum Breaking Load (MBL) of the anticipatedstrongest line to be used at the terminal. The path from there is more unclear with differingsuggestions on how to design the anchorage as well as the supporting structures. A summary ofsome of the most commonly used standards and publications is presented in the following.

3.1 PIANC

PIANC has no currently published document that gives recommendations for mooring hook design butWorking Group MarCom 153 has included guidelines in its working document titled“Recommendations for the Design of Marine Oil Terminals” (Ref. 5). These recommendations aretaking a very comprehensive approach to the subject, while trying to reflect not only existingregulations but also specific company standards for sizing of mooring hooks. At the time of submittalof this paper the following guidelines were given:

SWL of hooks to be larger than 1.0 x MBL of strongest anticipated mooring lines Design of support structures based on SWL of hooks Multiple hooks:

o Normal condition: UL = 0.6 x N x SWLo Extreme condition: UL = SWL + 0.8 x SWL x (N-1)

N = number of mooring hooks

UL = Unfactored Load

A wise step in this document is to somewhat detach the design of the anchorage of the mooring hooksas well as supporting structures from the MBL of the anticipated strongest mooring line. Although thecurrent document strongly suggests that individual hooks should be sized based on the strongest line


5 of 10

concept, it is made clear that any steps after that should be purely based on the MBL of the mooringhooks.

Figure 3 - Example of mooring hooks on new construction

3.2 MOTEMS

The Marine Oil Terminal Engineering and Maintenance Standards, also known as MOTEMS (Ref. 3)has taken one of the more comprehensive approaches to selecting and designing mooring hooksamong the studied references. The standard, now a mandatory part of the California Building Codespecifies the following:

Single hooks should be able to withstand the MBL of the strongest mooring line with a safetyfactor of 1.2

Anchorage of multiple hooks as well as design of supporting structure needs to withstand thefollowing load: Fd = 1.2 x MBL (1+ (n-1) x 0.75)

N = number of mooring hooks

Fd = Factored load

While it is not clearly stated what “should be able to withstand” actually means, recent experiencesuggest this is being interpreted such that the SWL of the mooring hooks should be higher than theMBL of the strongest mooring line, with the mentioned safety factor of 1.2. MOTEMS also states thatno more than one line should be tied to each mooring hook.

3.3 British Standards (BS)

The British Standards (Refs. 1 and 2) specify that mooring points should have a safe working load ofnot less than the maximum breaking load of the largest line. Guidance is given on typical linestrengths for various vessel sizes. These standards also specify typical factors of safety between theSWL and yield and breakage of mooring hooks as 1.25 and 2.5, respectively.

3.4 Japanese standards

The Technical Standards and Commentaries for Port and Harbour Facilities in Japan (Ref. 6) does nothave any specific reference to mooring hooks, but specifies forces to use in analysis of mooringhardware to be equal to the breaking strength of the mooring lines used at the facility in question.


6 of 10

3.5 OCIMF

OCMIC’s Mooring Equipment Guidelines (Ref. 4) are mostly aimed towards mooring equipment on thevessels, but has a very small section on selection of mooring hooks. The recommendation fromOCIMF is that berth mooring points should have a SWL of not less than the MBL of the largest ropeanticipated.

3.6 Recommendations in text books

The Port Designer’s Handbook (Ref. 7) provides good guidelines for design of ports and maritimefacilities. In terms of sizing single mooring points, this reference is kind of an outlier as it suggest tosize mooring hooks based on the strongest line needed to safely moor vessels at the berth. Here it isalso suggested that the design load for each hook support structure should be defined as the totalnumber of hooks times the SWL per hook.

Figure 4 - Example of mooring with more than one line per hook

4. SELECTION OF ROPE MBL FOR SELECTION OF MOORING HOOKS

As can be deducted from the above, most references, with the exception of one (Ref. 7), state that fornew construction, mooring hooks should be selected based on the anticipated maximum mooring linestrength for vessels calling at the terminal. At first sight this seems to be a reasonable approach toensure that the mooring lines are the weakest link in the chain of mooring components but there areseveral pitfalls associated with this approach.

First of all it can be difficult to anticipate what this mooring line strength would be. Some of thereferences provide good estimates for typical mooring line strengths depending on vessel size, and asummary is given in Table 2. There will however always be vessels carrying stronger lines than thenormal range. If a vessel with unusually strong lines is used to size the mooring hooks, both thehooks and supporting structures might be severely overdesigned. Structurally this is of no concern,but it has the potential to significantly add cost to a project. It is therefore important to realize that avessel with unusually strong mooring lines should not be used for sizing of mooring hooks.


7 of 10

Vessel Size (DWT) Typical MBL (MT)

15-20,000 35-45

20-40,000 40-50

40-70,000 45-65

70-120,000 55-80

120-150,000 70-100

150-300,000 80-120

Table 2 - Typical Mooring line strengths for tankers

It is also important to consider how a mooring analysis for the terminal in question will be performed.To provide the operator of a terminal with the maximum possible flexibility of vessels they can accept,a mooring analysis should strive to establish the minimum mooring line strength needed to provide forsafe mooring arrangements for the selected vessel types and environmental conditions. There willtherefore be two different mooring line strengths established in the mooring of a vessel; the mooringline strength used for sizing of hooks and the mooring line strength used for establishing safe mooringof a vessel at the terminal. This can easily lead to very conservative designs, especially if there is alarge spread between the two numbers.

As already mentioned, Ref 7 suggests a methodology for selecting mooring hooks that has thepotential to remedy this gap. Instead of using the anticipated strongest mooring line of vessels callingat the terminal, it is suggested to use the mooring line strength needed to safely moor vessels at theterminal. Depending on the terminal, location and expected environmental conditions, this might be aviable and economical solution to avoid overly conservative designs.

5. POSSIBLE PROBLEM SCENARIOS

5.1 Vessel calls with stronger mooring lines

The Terminal Operating Limits (TOLs) for a terminal will usually contain information for the operatorregarding the minimum line strength requirements for safe mooring at the terminal. If a strictadherence to the thought of a hierarchy of strength is followed, there is a potential for a problem if avessel calls at the terminal with stronger lines than the ones used for design of the mooring hooks. Itdoes not seem to be normal practice to have this maximum mooring line strength stated anywhere inthe TOLs.

It needs to be clear under which circumstances a problem might arise from stronger mooring lines.Under any type of normal operating conditions, stronger lines will only benefit the mooring of a vesselas it will allow less movement and higher factors of safety for overloading of the lines. The potentialfor a problem arises when one or several lines are accidentally overloaded at the same time as thewinches on the vessel are not rendering properly. In that case, with a mooring line much strongerthan the hook it is tied to, there is a potential that the mooring hook will fail before the mooring line.On the other hand it is important to remember that the mooring hook will not fail at the SWL, nor at theproof load or the yield load, but at the ultimate load.

The problem in this particular scenario is not that the failure of one single mooring hook necessarily isless safe than the failure of a mooring line, but rather that the failure has a more severe economicaland operational impact on the terminal. A failed mooring line can be replaced quickly by the vesselwhile the failure of a mooring hook might take weeks or months to remedy, depending on theavailability of spare parts.

With the safety factors outlined earlier one can deduct that a small increase in mooring line strengthcompared to the design mooring line strength is of no particular concern. If vessels are the same size,operating loads should not be any higher with the increase in mooring line strength.


8 of 10

5.2 Mooring hardware upgrade due to change in service

If a terminal wants to increase the vessel sizes they can accept at the terminal it might be necessary toinstall more, or larger mooring hooks. A mooring analysis will need to be done to determine the extentof the necessary upgrades. If the vessel size increase is of a more nominal nature, additional mooringhooks are probably not required. In that case the increase in vessel size might be reduced to an issueof a small increase in mooring line strength, which is described above.

If additional lines are required for safe mooring of the vessel, several solutions are available toaccommodate this:

The operator can choose to moor the vessel with more than one line tied to each hook.Although this is typically not a recommended practice, most mooring hook manufacturers statethat it is ok to tie two lines to each hook provided that the combined diameter of the mooringlines does not exceed their specifications. If line tension monitoring is used at the terminal,this is not a desirable solution as it becomes impossible to monitor the tension of the separatemooring lines, and the tension monitoring is reduced to monitoring of the tension in the hooks.

The mooring hook unit can be replaced with a larger unit, providing additional mooring hooks.A discussion follows in Section 6 regarding selection of upgraded mooring hook units.

5.3 Mooring hardware upgrade due to regulatory requirements

Several marine oil terminals in Northern California are located on land leased from the State ofCalifornia. As part of some lease renewals, terminals have been required to abandon the use of morethan one mooring line per hook. As the replacement of the mooring hooks is under the jurisdiction ofthe State of California and the California State Lands Commission, MOTEMS is the governingdocument for the replacement work.

The goal of this process is to ultimately increase the safety of the mooring operations. It is notexpected that the terminals should be required to build any new structures to support the upgradedmooring hooks. Through the MOTEMS Audit process it has already been established that thesupporting structures can withstand the anticipated mooring loads, and with no upgrade in servicethese loads will not increase with the installation of new hooks. The decision will therefore need to bemade regarding what constitutes the safest path forward in selection of new mooring hooks.

6. APPROACH TO UPDATES OF EXISTING MOORING HOOKS

In the case of upgrades of mooring hooks on existing structures, there is no real problem as long asthe existing structure is strong enough to support stronger or more mooring hooks. In that case, themooring hooks can be selected similarly to the methodology for new design. Care should be taken toapply the proper loads and load factors when evaluating the existing structure.

The issue arises when the mooring structures do not have enough capacity to support an upgrade inmooring hooks according to the requirements for new design, which vary depending on the referenceused. Short of designing and building new supporting structures for the mooring hooks a decision willneed to be made where the break should be in the hierarchy of strength in the mooring system.

6.1 Selection of mooring hooks according to line strength

One possible solution is to select the mooring hooks based on the anticipated strongest line for thevessels that call at the terminal. It is then ensured that the mooring hooks will be stronger than themooring lines, so the weaker link will have to be somewhere farther down the line in the mooringsystem. One possibility is then to design the mooring hook anchorage with weak links so theanchorage will fail before the mooring structure. While this might seem like a viable solution to protectthe mooring structure, failure of the anchor bolts will probably be a violent and non-ductile failurewhere one can anticipate complete loss of the mooring hook assembly.

The next possibility is then to design the anchorage to support the full load of the hooks, pushing thefailure down to the supporting structure itself. This is a very undesirable solution as an accidentaloverloading of the mooring lines might lead to complete loss of the mooring structure. This isundoubtedly a very costly solution to the operator.

With the selection of mooring hooks with a higher SWL than what has been used in the original designone can therefore run into the possibility that the mooring hook anchorage, or supporting structure,


9 of 10

cannot fully support the combined capacity of the mooring hooks. It can therefore be deducted thatextreme care should be taken to install mooring hooks with higher SWLs than the existing ones.While operational limitations can be put in place to remedy this, it is possible that with the passage oftime the details behind the limitations might get lost. Decisions could then be made that will increasethe loads that will be transferred into the mooring structures past safe levels.

6.2 Selection of mooring hooks according to actual line loads

The other route that can be taken when mooring hooks are selected is to select the mooring hooksbased on the actual mooring line loads derived from a mooring analysis under survival conditions. Anappropriate load factor should be applied to these loads to arrive at an appropriate SWL for themooring hooks. While using this methodology it is still important to keep an eye on the actual capacityof the mooring structures. It is more likely that this methodology will lead to a sizing of mooring hooksthat gives a good balance between strength of the mooring hooks and the safety of the overallmooring system.

Even if the MBL of the anticipated strongest mooring line is higher than the stated SWL of the mooringhooks, there are still numerous safety factors in play to protect the structure:

- Mooring analysis limits the line loads to 45 to 55% of mooring line MBL, under the mostextreme environmental conditions

- Mooring winches on the vessel are expected to start rendering at mooring line loads of 60% ofmooring line MBL

- If tension monitoring is in place, warnings will be given once the mooring line loads approachlevels close to any of the aforementioned limits

- Mooring hooks will have proof loads, yield loads and ultimate loads well in excess of thestated SWL

The similarities in this methodology can easily be seen when compared to how evaluations aretypically done for existing mooring systems as an evaluation of existing mooring systems will focus onthe actual demands being placed on the system, with appropriate load and safety factors.

7. EXAMPLE

A marine oil terminal in Northern California is in the process of replacing some of their mooring hooksto mitigate mooring arrangements that utilize two lines per hook, as well as to install tensionmonitoring at all mooring points. At the terminal in question, mooring dolphins have double hookmooring hook assemblies, with each hook having a capacity of 45 MT. While this size of mooringhooks can appear to be undersized for the size of vessels this terminal serves, a comprehensivemooring analysis has confirmed that operations are safe even under the combined loads of maximumcurrents and survival level wind speed limits.

While the current mooring arrangements work safely, with additional safety factors on both the SWL ofthe mooring hooks as well as the capacity of the structure, it was important to realize that the goal ofthe upgrade was not to increase capacity in any way, but to avoid two lines per hook mooringarrangements and to install tension monitoring. It was therefore decided that it was important to notinstall mooring hooks with more capacity than the existing ones to avoid driving more loads into thesupporting structures.

The decision was made to install mooring hooks with the same SWL as the existing ones, but toreplace the double hook units with triple hook units. This way the mooring dolphins would see theload from the same number of lines, but with similar safety factors in place when tying up the lines.While the new mooring hooks will not have a SWL larger than the MBL of the anticipated strongestmooring line, their proof load is larger than the MBL, as are both the yield and ultimate loads.

The mooring hardware was also checked against actual loads, with proper load factors applied, withresulting acceptable factors of safety.

8. REFERENCES

1. British Standards Institution (2010). BS 6349-2:2010, Maritime Works – Part 2: Code ofPractice for the Design of Quay Walls, Jetties and Dolphins.

2. British Standards Institution (1994). BS 6349-4:1994, Maritime Structures – Part 4: Code ofPractice for Design of Fendering and Mooring Systems.


10 of 10

3. California Building Standards Commission (2013). California Building Code, Title 24, Part 2,Chapter 31F, Marine Oil Terminal Engineering and Maintenance Standards (MOTEMS).

4. OCIMF (2008). Mooring Equipment Guidelines, 3rd

edition (MEG3). Witherby SeamanshipInternational. ISBN 978 1 905331 32 1.

5. PIANC MarCom 153 (February 2014). Recommendations for the Design of Marine OilTerminals, Working Document.

6. Ports and Harbours Bureau, Ministry of Land, Infrastructure, Transport and Tourism (MLIT,National Institute for Land and Infrastructure Management MLIT, Port and Airport ResearchInstitute (2009). Technical Standards and Commentaries for Port and Harbour Facilities inJapan. The Overseas Coastal Area Development Institute of Japan.

7. Thoresen, C. A. (2010). Port Designer’s Handbook, 2nd

edition. Thomas Telford Limited.ISBN 978-07277-3568-3.

Documents

Paper Session 12 Iversen r